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An illustrated handbook of DNAPLtransport and fate in the subsurface

R&D Publication 133

3932 DNAPL handbook A/W 10/9/2003 10:36 am


The Environment Agency is the leading public body protecting and

improving the environment in England and Wales.

It’s our job to make sure that air, land and water are looked after by

everyone in today’s society, so that tomorrow’s generations inherit a

cleaner, healthier world.

Our work includes tackling flooding and pollution incidents, reducing

industry’s impacts on the environment, cleaning up rivers, coastal

waters and contaminated land, and improving wildlife habitats.

Published by:

Environment AgencyRio HouseWaterside Drive, Aztec WestAlmondsbury, Bristol BS32 4UDTel: 01454 624400 Fax: 01454 624409

ISBN : 1844320669© Environment Agency June 2003

All rights reserved. This document may be reproduced with

prior permission of the Environment Agency.

This report is printed on Cyclus Print, a 100% recycled stock,

which is 100% post consumer waste and is totally chlorine free.

Water used is treated and in most cases returned to source in

better condition than removed.

Cover photograph: Coal tar DNAPL penetrating synthetic porous medium of glass

beads. Courtesy of Prof S.Leharne, University of Greenwich.

Authors:

Prof B.H.Kueper1, Dr G.P.Wealthall2, J.W.N.Smith3,

Prof S.A.Leharne4 & Prof D.N.Lerner5.

1. Queen’s University, Ontario, Canada

2. British Geological Survey

3. Environment Agency

4. University of Greenwich

5. University of Sheffield

Statement of use:

This report describes current understanding of DNAPL fate

and transport in the subsurface. It applies those concepts to

hydrogeological conditions found in the UK, in order to develop

a series of conceptual models of DNAPL behaviour following its

release into the subsurface environment. It will help practitioners

understand the principles of DNAPL fate and transport in the

subsurface, and allow improved design of investigation and

assessment of DNAPL pollution.

Environment Agency Project Manager:

Johnathan Smith, NGWCLC

Environment Agency R&D Publication 133

3932 DNAPL handbook A/W 10/9/2003 10:36 am

Environment Agency Illustrated handbook of DNAPL transport and fate in the subsurface 1

Executive summaryDense non-aqueous phase liquids (DNAPLs) such as creosote, coal

tar, chlorinated solvents and polychlorinated biphenyl oils represent

a particular class of soil and groundwater contaminant that exist as

a separate liquid phase in the presence of water. DNAPLs come to

rest in the subsurface as disconnected blobs and ganglia of liquid

referred to as residual DNAPL, and in potentially mobile distributions

referred to as pools.

The region of the subsurface containing residualand pooled DNAPL is referred to as the source zone.Groundwater flowing through the source zone slowlydissolves the DNAPL, giving rise to aqueous phaseplumes of contamination hydraulically down-gradientof the source zone. Some DNAPL compounds areresistant to biodegradation and sorb little; they cantherefore give rise to substantial aqueous phaseplumes. Other DNAPL compounds are relativelyimmobile in groundwater and, therefore, are highlyretarded relative to the rate of groundwater flow.In unsaturated media, volatile DNAPLs give rise tovapour phase contamination.

Because DNAPLs are only slightly soluble in water,DNAPL source zones can persist for many decadesand, in some cases, even hundreds of years.Some DNAPLs are highly toxic and even very lowconcentrations in groundwater or the atmospherecan pose an unacceptable risk to human health orthe environment.

The fact that DNAPLs are denser than water allowsthem to migrate to substantial depths below thewater table in both unconsolidated deposits andfractured bedrock. Delineating the spatial extent ofthe DNAPL source zone at a site can be a substantialundertaking, requiring at times several years ofinvestigation and significant financial resources.

Remediation strategies are site-specific, with separateapproaches often warranted for the DNAPL sourcezone and its associated aqueous phase plume. Therehas been limited success in removing all DNAPL frombelow the water table at sites, particularly in a fracturedrock environment. Remediation strategies are thereforeoften directed towards source zone containment orstabilisation, partial mass removal, plume managementor plume interception, within the framework ofappropriate risk-management objectives.

The purpose of this handbook is to provide auser-friendly overview of the nature of DNAPLcontamination in a UK context. It is intended toassist site investigators, site owners and regulatorsin conducting site investigations, conducting riskassessments and selecting remediation approaches.While this handbook reflects the state-of-the-art atthe time of publication, it should be noted that thediscipline of groundwater and soil contamination byhazardous organic liquids is evolving continuouslyand is relatively ‘young’ compared with many otherareas of science and engineering. Readers aretherefore advised to keep abreast of the new advancesin understanding and approaches expected in theforeseeable future.

3932 DNAPL handbook A/W 10/9/2003 10:36 am Page 1

Environment Agency Illustrated handbook of DNAPL transport and fate in the subsurface2

Contents

Executive Summary 1

Chapter 1 Introduction 5

Chapter 2 Types of DNAPLS 6

2.1 Creosote 6

2.2 Coal tar 7

2.3 Polychlorinated biphenyls (PCBs) 7

2.4 Chlorinated solvents 8

2.5 Mixed DNAPLs 9

Chapter 3 DNAPL source zones in unconsolidated deposits 10

Chapter 4 DNAPL dissolution in unconsolidated deposits 14

Chapter 5 DNAPL source zones in fractured rock 19

Chapter 6 DNAPL dissolution in fractured rock 22

Chapter 7 Determination of DNAPL presence and delineation of the DNAPL source zone 26

7.1 Site use/Site history 26

7.2 Aerial photographs, maps and plans 27

7.3 Screening of Soil borings 27

7.4 Screening of rock cores 27

7.5 Laboratory analysis of soil samples 27

7.6 Bailers and interface probes 28

7.7 Contaminant concentrations in groundwater 28

7.8 Presence of contamination in hydraulically anomalous locations 29

7.9 Persistence of contamination 30

7.10 Persistence of alcohols in groundwater 30

Chapter 8 UK Specific conceptual models 31

8.1 Chlorinated solvent release into Cretaceous Chalk 31

8.2 Coal tar release into Cretaceous Chalk 34

8.3 Chlorinated solvent release into Triassic Sandstone 34

8.4 Coal tar release into fractured Triassic Sandstone 36

8.5 DNAPL release into a thin veneer of clay-rich till 38

8.6 DNAPL release into a thick sequence of clay-rich till 38

8.7 DNAPL release into a sand/gravel aquifer 40

8.8 DNAPL release into made ground 40



Chapter 9 Which parameters to measure at a site 44

Chapter 10 Basic remediation strategies 47

10.1 Remediation goals within the DNAPL source zone 47

10.2 Remediation goals downstream of a DNAPL source zone 49

10.3 Remediation technologies 50

10.3.1 Groundwater pump-and-treat 51

10.3.2 Permeable reactive barriers 51

10.3.3 Physical barriers 52

10.3.4 Enhanced biodegradation 52

10.3.5 Thermal technologies 52

10.3.6 Chemical flushing to mobilise contaminants 52

10.3.7 Excavation 52

10.3.8 Chemical flushing to destroy contaminants 53

10.3.9 Soil vacuum extraction 53

10.3.10 Water flooding 53

10.3.11 Air sparging 53

Chapter 11 Glossary of terms and abbreviations 54

Chapter 12 References 57

Acknowledgements 58

Appendix A DNAPL pool height above capillary barrier 59

Appendix B Fracture aperture required to stop migration in bedrock 60

Appendix C Soil concentration calculation in the absence of DNAPL composition analysis

List of Figures

Figure 1 Samples of DNAPL recovered from ground investigations at a solvent recycling facility 9

Figure 2 DNAPL distribution in unconsolidated deposits 10

Figure 3 Residual DNAPL in (a) unsaturated and (b) saturated porous media 11

Figure 4 Maximum DNAPL pool height above various capillary barriers 12

Figure 5 DNAPL migration pathways in unsaturated sands 13

Figure 6a Cross-section depicting spatial variability of groundwater concentrations in a plume 14

Figure 6b Visual appearance of a smoothly varying distribution of concentration following contouring 15

Figure 7 Visual appearance of a smoothly varying distribution of concentration following contouring 15

Figure 8 Aqueous phase concentrations immediately downstream of DNAPL source 17

Figure 9 DNAPL mass versus time 18

Figure10 DNAPL pool at base of burden: unlikely scenario 19

Figure 11 Fracture aperture required to stop migration versus height of accumulated DNAP 20

Figure 12 DNAPL migration to depth in fractured bedrock: likely scenario 21



Figure 13 Development of a steady-state plume 22

Figure 14 Matrix diffusion process 23

Figure 15 DNAPL release and subsequent depletion by dissolution into single fracture 24

Figure 16 Concentration versus time at exit of fracture (from Reynolds and Kueper 2002) 25

Figure 17 Probable and potential DNAPL zones 30

Figure 18 Release of chlorinated solvent DNAPL into cretaceous chalk 32

Figure 19 Release of coal tar DNAPL into cretaceous chalk 33

Figure 20 Release of chlorinated solvent DNAPL into triassic sandstone 35

Figure 21 Release of coal tar DNAPL into triassic sandstone 37

Figure 22 DNAPL release into thin veneer of clay-rich till 39

Figure 23 DNAPL release into a thick sequence of clay hill 41

Figure 24 DNAPL release into an unconsolidated sand/gravel aquifer 42

Figure 25 DNAPL release into made ground and the effects of construction activities and services 43

Figure 26 Site investigation techniques 44

Figure 27a Steady-state plume prior to mass removal and (b) steady-state plume 48

Figure 28 Physical isolation of DNAPL source zone 49

Figure 29 Steady-state plumes (a) without and (b) with biodegradation 50

Figure 30 Groundwater pump-and-treat 51

Figure A1 DNAPL pool above capillary barrier 59

Figure B1 Vertical accumulation of DNAPL in a fracture network 60

List of Tables

Table 1 Possible components of creosote (adapted from Cohen and Mercer, 1993) 6

Table 2 Composition (per cent) and selected properties of various Aroclors excluding

carrier fluids (adapted from Cohen and Mercer, 1993) 7

Table 3 Industries and industrial processes associated with chlorinated solvents 8

Table 4 Physical and chemical properties of selected chlorinated solvents

(from Mackay et al.,1993) 9

Table 5 Component composition of DNAPL sample obtained from a solvent recycling facility 9

Table 6 Industries and industrial processes sometimes associated with DNAPL presence

in the subsurface (modified from US EPA, 1992) 26

Table 7 Example calculation of soil DNAPL threshold concentration: single component DNAPL

below the water table 28

Table 8 Example calculation of cumulative mole fraction in a groundwater sample 29

Table 9 Contaminant characteristics to establish during site investigations 45

Table 10 Unconsolidated deposit characteristics to determine during site investigations 45

Table 11 Bedrock properties to determine during site investigations 46



1

Introduction

Dense non-aqueous phase liquids (DNAPLs) thathave been widely used in industry since the beginningof the 20th century. DNAPLs are only slightly solublein water and therefore exist in the subsurface as aseparate fluid phase immiscible with both water andair. Common types of DNAPLs include timber treatingoils such as creosote, transformer and insulating oilscontaining polychlorinated biphenyls (PCBs), coaltar, and a variety of chlorinated solvents such astrichloroethene (TCE) and tetrachloroethene (PCE).Unlike light non-aqueous phase liquids (LNAPLs) suchas petrol and heating oil (which are less dense thanwater), DNAPLs (which are denser than water) havethe ability to migrate to significant depths below thewater table where they slowly dissolve into flowinggroundwater, giving rise to aqueous phase plumes.A release of DNAPL at the ground surface can thereforelead to long-term contamination of both the unsaturatedand saturated zones at a site.

Although DNAPLs have been produced and utilisedwidely since the beginning of the 20th century, theirimportance as soil and groundwater contaminantswas not recognised until the 1980s. This lack ofrecognition by the industrial, regulatory and researchcommunities was partly due to the fact that theanalytical methods and equipment required todetect low concentrations of organic compoundsin groundwater were not widely available or useduntil relatively recently. In addition, some chemicalmanufacturer material safety data sheets distributedfrom the 1940s until the early 1970s suggested that‘acceptable practice’ for the disposal of wastechlorinated solvents and the residues of distillationwas to spread them onto dry ground to allow themto evaporate. These early material safety data sheetsrecognised the volatile nature of many DNAPL chemicals,but they did not recognise the ability of DNAPLs toinfiltrate rapidly into the subsurface, causing soil andgroundwater pollution. Additional factors contributingto the relatively late awareness of the impact of DNAPLson soil and groundwater quality include society’sgeneral lack of understanding of the importance ofgroundwater as a supply of potable water, and thewidespread use of shallow soil systems as a locationto dispose of unwanted materials.

There are potentially several thousand DNAPL-impacted sites throughout the UK. Many of thesesites are affected by releases of DNAPL that tookplace in the middle of the 20th century (coincidentwith the rise in industrial activity post World War II),as well as by more recent discharges. In addition,there are thousands of DNAPL-impacted sites inNorth America, continental Europe and otherindustrialised areas of the world. Experience from thepast 20 years has demonstrated that DNAPL sites aredifficult to investigate and challenging to remediate.DNAPL can penetrate fractured rock and clay and, inmost hydrogeologicalz environments, many decadesare required for natural groundwater dissolution todissipate DNAPL sources. DNAPL impacted soil andgroundwater are of major concern in the UK; mostDNAPL compounds have been found to be toxic tomammals and other fauna. Certain DNAPL compoundsare highly mobile in the subsurface and groundwaterforms an integral part of the hydrologic cycle as wellas an important resource in its own right.

The purpose of this handbook is to provide a user-friendly overview of the nature of DNAPL contaminationin a UK context. It is intended to assist site investigators,site owners and regulators in conducting siteinvestigations and risk assessments, and in selectingremediation approaches. While this handbook reflectsthe state-of-the-art at the time of publication, itshould be noted that the discipline of groundwaterand soil contamination by hazardous organic liquidsis evolving continuously and is relatively ‘young’ incomparison with many other areas of science andengineering. The reader is therefore advised to keepabreast of the new advances in understanding andapproaches expected in the foreseeable future.



2

In general, a DNAPL is defined as a heavier-than-water organic liquid that is only slightly soluble inwater. The acronym was first introduced in the USAduring litigation proceedings in New York State inthe late 1970s. The primary classes of DNAPLs includecreosote, coal tar, PCB oils and chlorinated solvents.Other, less frequently encountered DNAPLs includemercury and certain crude oils. All DNAPLs can becharacterised by their density, viscosity, interfacialtension with water, component composition,solubility in water, vapour pressure and wettability.These terms are used throughout this handbook; ashort description of each is given in the glossary inSection 11.

Types of DNAPLS

Table 1 Possible components of creosote (adapted from Cohen and Mercer, 1993)

2.1 CreosoteCreosote is composed of various coal tar distillatesand was commonly used to treat wood productssuch as railway sleepers and telegraph poles. It is stillused today in certain timber-treating operations andas a component of roofing and road tars. Creosotecontains many hydrocarbons, primarily polycyclicaromatic hydrocarbons (PAHs) and phenolic compounds.Table 1 lists a number of possible components ofcreosote. Creosote may be blended, however, withup to 50% of a carrier fluid such as diesel fuel priorto use. The density of creosote typically rangesbetween 1,010 and 1,130 kg/m3, depending onthe amount and type of any carrier fluid. Creosoteis therefore one of the least dense DNAPLs ofenvironmental interest. It often takes a long time formovement to cease following initial release into thesubsurface because creosote is only slightly denser

Acid extractable Base/neutral Heterocyclic

phenol naphthalene quinoline

cresols methylnaphthalenes isoquinoline

pentachlorophenol dimethylnaphthalenes carbazole

xylenols biphenyl 2,4-dimethylpyridine

2,3,5-trimethylphenol acenaphthene benzo[b]thiophene

fluorene dibenzothiophene

phenanthrene dibenzofuran

anthracene

fluoranthene

pyrene

chrysene

anthraquinone

2,3-benzo[b]pyrene

methylanthracene

benzo[a]pyrene

diphenyldimethylnaphthylene

diphenyloxide



than water and has a relatively slow downward(gravity-driven) migration.

The relatively high viscosity of creosote, which typicallyranges between 20 and 50 cP, also facilitates the longmigration timescale. It is not uncommon to encountersites where creosote DNAPL is still moving followingits introduction to the subsurface as much as 50 or60 years earlier.

In assessing the impact to groundwater, mostinvestigators select a subset of creosote compoundsto characterise water quality. These may includenaphthalene, benzo[a]pyrene and phenanthrene.Because some of these compounds are typically veryhydrophobic, they tend to sorb strongly to soils androck. This means that aqueous plumes of certaincontaminants associated with creosote sources willbe heavily attenuated relative to the rate of groundwaterflow, and therefore may not have migrated farbeyond the spatial extent of the DNAPL creosote.

2.2 Coal tarLike creosote, coal tar is a complex mixture ofhydrocarbons produced through the gasificationof coal. Coal tar was historically produced as a by-product of manufactured gas operations up untilapproximately 1950, and is currently still producedas a by-product of blast furnace coke production.Coal tar contains hundreds of hydrocarbons, includinglight oil fractions, middle oil fractions, heavy oilfractions, anthracene oil and pitch. The density ofcoal tar typically ranges from 1,010 to 1,100 kg/m3

and the viscosity from 20 to 100 cP. The relatively

low density and high viscosity of coal tar implies thatit may still be migrating as a DNAPL at sites where itwas introduced to the subsurface many decades (oreven a century) earlier. With respect to the impacton groundwater, most investigators typically selecta subset of compounds to assess the impact on waterquality. These may include the suite of BTEX compounds(benzene, toluene, ethylbenzene and xylenes), aswell as PAHs including benzo[a]pyrene, naphthaleneand phenanthrene.

2.3 Polychlorinated biphenyls(PCBs)Polychlorinated biphenyls (PCBs) are a class of 209chemical compounds referred to as congeners, inwhich between one and ten chlorine atoms areattached to a biphenyl molecule. The synthesis ofPCBs was first reported in 1881, with full recognitionof their industrial uses developing in the 1930s. Themajority of PCBs were manufactured by the MonsantoCorporation between 1930 and 1977 for use in capacitors,transformers, printing inks, paints, pesticides and otherapplications. Monsanto marketed PCBs under the tradename Aroclor, distributing a variety of formulationsdiffering from each other with respect to the amountand particular types of congeners present. Each Aroclorcan be identified by a four-digit code. In mostformulations, the first two digits in the code designatethe number of carbon atoms in the biphenyl ring,while the last two digits designate the weight percent chlorine. Aroclor 1254, for example, contains12 carbon atoms in each biphenyl ring and 54 per

Table 2 Composition (per cent) and selected properties of various Aroclors excluding carrier fluids(adapted from Cohen and Mercer, 1993)

Aroclor 1221 Aroclor 1242 Aroclor 1260

biphenyl 11.0 – –

monochlorobiphenyl 51.0 1.0 –

dichlorobiphenyl 32.0 17.0 –

trichlorobiphenyl 4.0 40.0 –

tetrachlorobiphenyl 2.0 32.0 –

pentachlorobiphenyl 0.5 10.0 12.0

hexachlorobiphenyl – 0.5 46.0

heptachlorobiphenyl – – 36.0

octachlorobiphenyl – – 6.0

Density (kg/m3) 1180 1380 1560

Total (aqueous) solubility (µg/l) 200 240 2.7

Vapour pressure (Pa @ 25˚C) 0.893 0.053 5.333 x 10-3

Viscosity (cP) 5 24 resin



cent chlorine by weight. Worldwide productionof PCBs has now ceased, mainly in response torecognition of their toxicity and their tendency tobioaccumulate in animal tissues. However, they remainin limited use and may be present as impurities inlocations where they were used previously.

PCBs were often blended with carrier fluids such aschlorobenzenes and mineral oil before distribution.Depending on the particular combination ofcongeners present and the type of carrier fluid, thedensity of most PCB oils encountered in practiceranges from approximately 1,100 to 1,500 kg/m3,while the viscosity ranges from approximately 10 to50 cP. The relatively high density of PCB oils indicatesthat the timescale of migration may be relativelyshort, but their relatively high viscosity results in anintermediate range of timescales of migration. Thismeans that PCB DNAPLs may still be migrating atsome sites where they were introduced into thesubsurface in the past few decades. As discussedin largely on the viscosity and density of the DNAPL,together with a variety of other, site-specific factors.

With respect to impact on groundwater, mostcongeners are extremely hydrophobic and thereforesorb strongly onto soils and rock. Consequently, ifPCBs are detected in groundwater samples, the DNAPLsource is typically immediately up-gradient of themonitoring location. Exceptions are sites wherecolloid-facilitated transport is occurring or wherethe PCBs are dissolved in other organic contaminantssuch as oils. Carrier organic liquids may be LNAPLs aswell as DNAPLs. PCB DNAPLs are often encounteredat former solvent and waste oil recycling facilitieswhere they have been co-disposed with a varietyof other organic liquids such as chlorinated solventsand aromatic compounds. Table 2 presents thecomposition and selected physical properties of threeparticular Aroclors in the absence of any carrier fluids.

2.4 Chlorinated solventsChlorinated solvents such as trichloroethene (TCE),tetrachloroethene (PCE) and tetrachloromethane(carbon tetrachloride, CT, or CTET) form a class ofDNAPL compounds that have been produced inlarge quantities throughout the world since themiddle of the 20th century. Typical uses of thesechemicals include dry cleaning, metal degreasing,pharmaceutical production, pesticide formulationand chemical intermediates. Chlorinated solventstypically enter the subsurface as a result of pastdisposal directly onto land, storage and disposalinto unlined evaporation ponds and lagoons, leakingstorage tanks and vapour degreasers, leaking pipingand accidental spills during handling and transportation.Chlorinated solvents can be encountered as singlecomponent DNAPLs (for example, as primarily PCEat a dry cleaning facility or as primarily TCE at a metal

Table 3 Industries and industrial processes associated withchlorinated solvents

degreasing facility), or as part of a multi-componentDNAPL containing other organic compounds suchas PCB oils, mineral oils and fuels (for example, ata former solvent or waste oil recycling facility).Table 3 lists industries and industrial processes thathave been associated historically with the presenceof chlorinated solvent DNAPL in the subsurface.

The density of most chlorinated solvent DNAPLsranges from approximately 1,100 to 1,600 kg/m3

and their viscosity from approximately 0.57 to 1.0cP. Chlorinated solvent DNAPLs are therefore denserthan water and typically less viscous than water.This can result in rapid rates of subsurface migrationand means that chlorinated solvent DNAPLs aretypically no longer moving at sites where they wereintroduced to the subsurface even as recently as twoor three years ago. Table 4 lists selected physical andchemical properties of commonly encounteredchlorinated solvents. As can be seen, these compoundsare volatile, indicating that they will give rise tovapour phase contamination in unsaturated media.

These compounds are typically characterised by lowKoc values, indicating that aqueous phase plumes willnot be strongly retarded relative to the rate ofgroundwater flow. Koc describes the distribution ofan organic compound between water and the organiccarbon content of the solid phase. High Koc values arecharacteristic of strongly sorbed compounds. Suchcompounds are significantly retarded with respect togroundwater flow. The relatively rapid rate of chlorinatedsolvent DNAPL migration and the relatively low degreeof sorption are the two primary factors that distinguishthis class of DNAPLs from creosote, coal tar and PCBs.

Industry Industrial process

Electronics manufacturing Metal cleaning

Solvent production Metal machining

Pesticide/herbicide Tool and die operations

manufacturing

Dry cleaning Vapour and liquid degreasers

Instrument manufacturing Paint stripping

Solvent recycling Storage and transfer

of solvents

Engine manufacturing

Steel productmanufacturing

Chemical production

Rocket engine/

fuel manufacturing

Aircraft cleaning/

engine degreasing



2.5 Mixed DNAPLsIn general, a DNAPL that is composed of only onechemical compound is referred to as a single componentDNAPL. Dry cleaning fluid (typically tetrachloroethene)is an example of this (although, strictly, it too containslow concentrations of stabilisers and preservatives).A DNAPL that is composed of two or more chemicalcompounds is referred to as a multi-componentDNAPL. Creosote and coal tar are examples of multi-component DNAPLs. Whether a single componentor a multi-component DNAPL exists at a site dependson past uses of the various compounds at the siteand the methods of disposal. Table 5 presents thecomposition of a multi-component DNAPL obtainedfrom a monitoring well at a former solvent recyclingfacility. Activities at this site resulted in the blendingof various organic liquids prior to disposal.

As seen in Table 5, the DNAPL at this site containschlorinated solvents, PCBs and a variety of aromaticcompounds. Each of these components is availableto dissolve from the DNAPL into groundwater. The density of this particular DNAPL sample wasmeasured as 1,200 kg/m3. It is interesting to notethat the DNAPL contains toluene and xylenes, which

Table 5 Component composition of DNAPL sampleobtained from a solvent recycling facility

are themselves less dense than water, but havecombined here with heavier-than-water componentsto form a DNAPL.

The physical/chemical properties of the DNAPL maybe spatially variable at a site. Seven DNAPL sampleswere obtained from this solvent recycling facility;each sample had different physicochemical properties,including the component composition (Figure 1).The degree of spatial variability that may exist at asite with respect to the physicochemical propertiesof the DNAPL will depend on the site’s use and history.Clearly, a solvent recycling facility with a long periodof operation may exhibit significant spatial variabilityof DNAPL properties in the subsurface, while a smallmetal degreasing operation with a limited period ofoperation may result in a more uniform DNAPLcomposition.

Regardless of site history, however, DNAPLs encounteredin the subsurface may have different physical andchemical properties from reagent grade non-aqueousphase liquids (NAPLs). This may be the result ofindustrial processes in which they were used priorto disposal or as a result of contact with naturallyoccurring substances present in the soil zone.

Figure 1 Field DNAPL penetration into syntheticporous media

Table 4 Physical and chemical properties of selected chlorinated solvents (from Mackay et al., 1993)

Solvent Molecular Aqueous Density Vapour Viscosity Koc

weight solubility (kg/m3) pressure (cP) (l/kg)(mg/l) (Pa@˚C)

trichloroethene 131.4 1,100 1460 9,000 0.57 126

tetrachloroethene 165.8 200 1620 2,600 0.90 364

tetrachloromethane 153.8 790 1590 15,000 0.97 439

trichloromethane 119.4 8,000 1480 26,000 0.56 44

chlorobenzene 112.6 500 1110 1,580 0.80 330

1,1,1-trichloroethane 133.4 1,320 1330 16,000 0.84 152

Compound Percentage mass

1,1,1-TCA 0.7

TCE 3.7

PCE 14.3

toluene 4.7

m-xylene 0.3

o,p-xylene 2.3

1,2,4-TCB 0.10

PCB-1242 40.6

PCB-1254 7.1

Petroleum hydrocarbons >C7 26.2



Upon release at the ground surface, the DNAPL willmigrate both vertically and laterally in the subsurface(Figure 2). Residual DNAPL, in the form of disconnectedblobs and ganglia of organic liquid,is formed at thetrailing end of a migrating DNAPL body. Theformation of residual DNAPL, which occurs inresponse to pore-scale hydrodynamic instabilities,always takes place. The individual blobs and gangliaof organic liquid comprising residual DNAPL aretypically between 1 and 10 grain diameteres in length.Residual DNAPL will form in both unsaturated andsaturated media, and is held in place by capillary forcesthat arise because the interface between the DNAPLand water, and the interface between DNAPL and air,is in a state of tension.

3

DNAPL Source zones inunconsolidated deposits

Figure 2 DNAPL distribution in unconsolidated deposits (after Pankow and Cherry, 1996)

The amount of residual DNAPL retained by a typicalporous medium such as silt, sand and gravel istypically between 5 and 20 per cent of the pore spacein the particular lenses and laminations invaded bythe DNAPL.

Figure 3(b) presents a close-up view of residualDNAPL in saturated porous media. The residualDNAPL forms discrete blobs and ganglia of liquidthat are disconnected from each other. In most typesof porous media, even relatively large hydraulicgradients cannot mobilise residual DNAPL. Siteinvestigation activities such as pumping tests andwell purging will therefore not draw residual DNAPLinto well screens and sand packs.

DNAPL pool in fractures DNAPL residual in fractures

GW flow pool

residual

vapour

release

drift

dissolvedplume

bedrock



The various blobs and ganglia of residual DNAPLdissolve slowly into flowing groundwater, giving riseto aqueous phase plumes. Because the solubility ofmost DNAPLs is relatively low in water and groundwatervelocities are typically low, it can be many decadesbefore all residual DNAPL is depleted due to naturalprocesses.

Figure 3(a) presents a close-up view of residualDNAPL in unsaturated porous media. As in saturatedporous media, the DNAPL forms discrete blobs andganglia of liquid that are disconnected from eachother. The DNAPL blobs are exposed to both air andwater, allowing for both vapourisation into the airphase across DNAPL-air interfaces and dissolutioninto infiltrating water across DNAPL-water interfaces.Once present in soil moisture, dissolved contaminantswill be available for partitioning across air-waterinterfaces (a process referred to as volatilisation).

Because the vapour pressure of many DNAPLcompounds is relatively high, the lifespan of residualDNAPL in the unsaturated zone can be much lessthan the lifespan of residual DNAPL below the watertable. The vapourisation process can deplete residualchlorinated solvent DNAPLs such as TCE and PCEwithin 5-10 years in relatively warm and dry climates.This will not eliminate the presence of vapour phase,absorbed phase and aqueous phase contaminationin the unsaturated zone, but it can lead to an absenceof the DNAPL phase. The absence of chlorinatedsolvent DNAPL in the unsaturated zone at a siteshould not, in general, be used as a basis for

Figure 3 Residual DNAPL in (a) unsaturated and (b) saturated porous media

concluding that past releases of DNAPL did notoccur at that site or that past releases of DNAPLfailed to reach the water table.

As illustrated in Figure 2, DNAPL in unconsolidateddeposits can also come to rest in larger accumulationsreferred to as pools. DNAPL pools tend to formabove finer grained horizons that provide thenecessary capillary resistance to support the DNAPLaccumulation. Unlike residual DNAPL, pools containDNAPL that is continuous between adjacent pores,with local saturations of up to approximately 70 percent of the pore space. The finer grained horizonupon which DNAPL pooling can occur need not bea well-defined, laterally extensive clay unit. DNAPLpooling can occur on silt and fine sand horizons atall elevations within unconsolidated deposits. Themaximum pool height is inversely proportional tothe permeability of the particular horizon upon whichpooling is taking place, with clay and silt units typicallysupporting higher pools than fine sand horizons.

water water

DNAPL DNAPL

air

aquifer/soil grain aquifer/soil grain

(a) (b)



Figure 4 illustrates typical maximum pool heights forcoal tar, creosote, a chlorinated solvent and a mixedDNAPL perched above a variety of capillary barriers.The calculations and assumptions adopted in Figure 4are outlined in Appendix A. Larger pool heights canform for higher DNAPL-water interfacial tension,lower DNAPL density and lower capillary barrierpermeability. For chlorinated solvent and PCB DNAPLs,pool heights typically range from a few centimetres toseveral tens of centimetres. Chlorinated solvent poolsas thick as 2m have been reported at sites in the USA,but this is a relatively rare occurrence. For creosoteand coal tar, DNAPL pool heights are generally largerthan those associated with PCB and chlorinatedsolvent DNAPLs because of the lower density ofthese compounds.

Unlike residual DNAPL, pooled DNAPL is relativelyeasy to mobilise with increases in the hydraulicgradient (this is the basis for water flooding toenhance crude oil recovery in the oil industry).Unless the risk of vertical DNAPL mobilisation isacceptable, care must be taken to avoid performingpumping tests beneath DNAPL source zones. Drillingthrough pooled DNAPL also carries with it a risk ofvertical DNAPL mobilisation and many practitionersadopt an ‘outside-in’ approach to delineating DNAPLsites in order to minimise the chances of directlyencountering pooled DNAPL during site characterisation.

Figure 4 Maximum DNAPL pool height above various capillary barriers

0

1

2

3

4

5

Residual and pooled DNAPL collectively form what isreferred to as the DNAPL source zone. It is within theDNAPL source zone that dissolution into groundwateroccurs and aqueous phase plumes originate. DNAPLwill not migrate downwards through unconsolidatedmedia as a uniform body, but instead will migratealong multiple pathways in a very tortuous manner;this is sometimes referred to as dendritic form due itits resemblance to the branches of a tree.The specific migration pathways will be governed bythe bedding structure of the porous medium, withmigration occurring along pathways on the scale ofmillimetres to metres. In horizontally bedded media,significant amounts of lateral spreading can beexpected, including in directions not coincident withthe direction of groundwater flow. The fieldexperiments reported by Poulsen and Kueper (1992),and Kueper et al. (1993) demonstrated, for example,that the orientation of bedding structures (Figure 5)is the primary factor controlling the directions andspecific pathways of DNAPL migration, which can beseen red due to SUDAN IV dye.

Creosote

Coal tar

Chlorinated Solvent

Mixed DNAPL

DNAPL pool Capillary Barrier

Silt Fine Sand Medium Sand Coarse Sand

DN

APL

Po

ol H

eig

ht

(m)



These experiments also demonstrated that slow,dripping releases of DNAPL are likely to migrate togreater depths than sudden, single event releases.It is therefore not practicable to define all of thespecific DNAPL migration pathways at a typicalindustrial site.

A much more attainable, yet still difficult, goal is todefine the lateral extent of the DNAPL source zone,without specific delineation of residual DNAPL andDNAPL pools within the overall source zone.

Given the selective and tortuous nature of DNAPLmigration, it follows that the majority of porousmedia within a DNAPL source zone will containneither residual nor pooled DNAPL. The probabilityof directly encountering residual or pooled DNAPLwith a conventional drilling programme is thereforerelatively small. It is now commonly accepted thatdirect visual observation of DNAPL does not occurat most DNAPL sites. Instead (as discussed in Section7), the presence of DNAPL is inferred usingalternative lines of evidence. The overall bulkretention capacity of porous media within a DNAPLsource zone is generally thought to range fromapproximately 0.5 to 3 per cent. This retentioncapacity is defined as the volume of DNAPL (as bothresidual DNAPL and pools) divided by the overallbulk volume of the source zone. These values arelower than local-scale residual saturations (5-20 per

cent of the pore space) because they are expressedin relation to the bulk volume impacted and becausenot all lenses and laminations within the impactedzone will have been invaded by the DNAPL.Exceptions will occur at some sites, with some sourcezones containing bedding structures and capillaryproperties capable of retaining higher amounts.

Lower DNAPL density, higher DNAPL viscosity, andhigher DNAPL-water interfacial tension generallylead to larger amounts of lateral DNAPL spreadingboth above and below the water table. Creosote,for example, has been observed to have migratedhundreds of metres from release locations at certainsites in the USA. The extent of lateral migration ofchlorinated solvent DNAPLs tends to be less, but hasbeen observed to be tens to hundreds of metres atmany sites. This had led to a useful rule of thumbthat in horizontally bedded media, ‘DNAPL mustmigrate sideways in order to migrate down’.

Figure 5 DNAPL migration pathways in unsaturated sands. DNAPL presence shows red due to SUDAN IV dye. Bedding dips 30°below horizontal (source: Poulsen and Kueper, 1992). Image is 15 cm from top to base



4

DNAPL dissolution inunconsolidated deposits

Both residual DNAPL and pools will dissolve intogroundwater flowing through the DNAPL source zone,giving rise to aqueous phase plumes. Given the tortuousand sporadic nature of DNAPL occurrence within thesource zone, it follows that the associated aqueousphase plumes will exhibit significant spatial variabilityin terms of concentration.

Figure 6a illustrates a vertical cross-section througha DNAPL source zone along with a depiction of theassociated aqueous phase plumes. Monitoring wellshave been placed at various locations in the cross-section, along with posted concentrations.Figure 6b shows the possible result if the postedconcentrations are contoured; it gives the impressionof a single, smoothly varying distribution of

Figure 6a Cross-section depicting spatial variability of groundwater concentrations in a plume

concentrations but this is an over-simplificationof the true spatial distribution of concentrations.Although discrete sampling devices are commerciallyavailable to profile groundwater plumes at the scaleof centimetres, this level of detail is usually notrequired (or achieved) in site investigations.

Various factors influence the magnitude of contaminantconcentrations obtained from monitoring well samplesrelative to the actual concentrations in the aquifer.Figure 7 depicts a single monitoring boreholedownstream of a zone of residual DNAPL. For thepurposes of this discussion, it is assumed that theDNAPL is TCE, with an aqueous solubility of 1,100mg/l. At point A (immediately down-gradient of thezone of residual DNAPL), one would expect the local

gro

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DNAPL release

DNAPL

dissolved plume

5mg/l

35mg/l

3mg/l N.D

1mg/l N.D



Figure 6b Visual appearance of a smoothly varying distribution of concentration following contouring

Figure 7 Visual appearance of a smoothly varying distribution of concentration following contouring

gro

und

wat

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low

DNAPL release

50 mg/l 5 mg/l0.5 mg/l

DNAPLsource zone

gro

und

wat

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low

A1,100 mg/l

B210 mg/l plume

DNAPL

C20 mg/l



groundwater concentration to be approximately1,100 mg/l. As we move downstream to point B,the concentration will be less due to hydrodynamicdispersion. Dispersion always occurs in the subsurfaceand results in a lowering of concentrations along thecentreline of a plume in the downstream direction.The maximum concentration of 1,100 mg/l can onlybe observed immediately adjacent to the DNAPL andwill not be observed anywhere down-gradient of thesource zone.

Point C represents a groundwater sample obtainedfrom the monitoring borehole after water has beenpurged from the borehole. The concentration in thegroundwater sample is significantly less than that atpoint B due to in-borehole dilution. This refers to thefact that pumping the monitoring borehole drawsin both the local contaminant plume as well assurrounding uncontaminated water. The result isa mixing of clean and contaminated water in themonitoring well, and a resulting lowering ofconcentrations in the obtained sample relative towhat may be present in the aquifer immediatelyadjacent to the well. In addition to mixing duringpurging, this in-borehole dilution effect can occurnaturally if vertical flow gradients exist within theborehole.

It was assumed in the above example that themonitoring well of interest was placed preciselyalong the centre line of the plume, where maximumcontaminant concentrations will exist. If themonitoring well were placed offset from the plumecentre line, sampled concentrations would be evenlower. This is because contaminant concentrationsdecrease in the transverse direction (both horizontallyand vertically) away from the plume centre line.In addition to monitoring well placement and thefactors discussed above, biotic and abiotic degradationcan result in the lowering of concentrations in thedown-gradient direction within a contaminant plume.

The net effect of hydrodynamic dispersion, in-borehole dilution, monitoring well placement andpotential degradation processes is that contaminantconcentrations in a sample obtained from a monitoringwell downstream of a DNAPL source zone may besignificantly less than the aqueous solubility of theDNAPL of interest. Experience has shown that aDNAPL source may be present upstream of a monitoringwell if sample concentrations exceed 1 per cent ofthe effective solubility of the component of interest(US EPA, 1992). The 1 per cent ‘rule of thumb’ hasbeen criticised because it does not provide guidanceon how far upstream the DNAPL source zone islocated. It is clear that a variety of site-specific factorsinfluence the magnitude of sampled contaminant

concentrations and that some of these factors cannotbe determined (for example, the distance a monitoringwell is offset from plume centre line and the amountof in-borehole dilution occurring during purging).In practice, it is common to simply use the 1 percent ‘rule of thumb’ as a means of establishing thatDNAPL may be present upstream of the monitoringwell in question, and therefore as a means ofjustifying the use of additional site investigationtechniques to confirm or refute the presence ofDNAPL. In other words, the 1 per cent ‘rule ofthumb’ should not be used in isolation to establishDNAPL presence at a site, but instead should beused with other converging lines of evidence to bothestablish DNAPL presence and to delineate thespatial extent of the source zone. Site techniques toestablish DNAPL presence and delineate the spatialextent of the source zone are discussed in Section 7.

The above discussion assumed that the DNAPL ofinterest was composed only of TCE. If the DNAPLof interest is composed of a variety of components,these components will not dissolve into groundwaterat their single component, textbook solubility values.Rather, the various components may compete forthe dissolution process. The dissolution of a multi-component NAPL can be described using Raoult’s law.Raoult’s law states that the effective solubility of aNAPL component in (ground)water is equal to theproduct of the mole fraction in the NAPL and thesingle component aqueous solubility of thatcompound:

Equation 1

where:

Ci is the effective solubility of component i;

mi is the mole fraction of component i in the NAPL;

Si is the single component solubility of component i.

Ci = miSi



In practical terms, the effective solubility of acomponent is the maximum concentration thatcould possibly be observed in groundwater. Sampleconcentrations obtained from monitoring wells inthe plume will be less than the effective solubilitydue to in-borehole dilution, hydrodynamic dispersionand other effects. The relative concentration ofindividual components, however, is dictated byRaoult’s law. In terms of the 1 per cent ‘rule of thumb’,it is 1 per cent of the effective solubility that is takenas an indicator of possible upstream DNAPL presence,not 1 per cent of the single component solubility.

To illustrate the effects of Raoult’s law, consider athree-component DNAPL consisting of 50 per centchloromethane (methylene chloride), 25 per centtoluene and 25 per cent of a semi-volatile organiccompound (SVOC) by mass. Chloromethane has arelative molecular mass of 84.93, a density of 1,327kg/m3 and a single component aqueous solubility of20,000 mg/l. Toluene has a relative molecular massof 92.1, a density of 867 kg/m3 and a singlecomponent aqueous solubility of 500 mg/l. TheSVOC is assigned a relative molecular mass of 200,a density of 1,100 kg/m3 and a single componentaqueous solubility of 10 mg/l. Toluene representsthe intermediate molecular weight and intermediatesolubility component, while chloromethane representsthe most soluble and lowest molecular weight

Figure 8 Aqueous phase concentrations immediately downstream of DNAPL source

component. In this example, the DNAPL is assumedto be present at a residual saturation of 20 per centin a 1 m3 volume of fine-grained sand having ahydraulic conductivity of 1 x 10-5 m/s, a porosity of0.30, and to be subject to a hydraulic gradient of0.01. The DNAPL is assumed to be dissolving intogroundwater under equilibrium conditions accordingto Raoult’s law.

Figure 8 presents the aqueous concentrations of thethree compounds of interest exiting the 1 m3 ofaquifer material as a function of time. Figure 8 showsthat the chloromethane concentration declines withtime, while the toluene and SVOC concentrationsdisplay moderate increases. The rapid decline of thechloromethane concentration stems from the factthat it has the highest effective solubility (Ci)according to Raoult’s law. As the chloromethane ispreferentially depleted from the NAPL at an earlytime, the NAPL becomes enriched in the lowereffective solubility compounds, which then showcorresponding increases in their effective solubilities.In addition, the total concentration (sum of the threecompounds) decreases with time. This illustrates thata decrease in total concentration with time does notindicate that DNAPL is not present; it is simply aresult of the fact that the higher solubility compoundsare preferentially depleted from the DNAPL at anearly time.



Figure 9 DNAPL mass versus time

Figure 9 presents a plot of DNAPL mass in the 1 m3

of aquifer material as a function of time. At t = 0,69.3 kg of DNAPL is present in the sample volume.The DNAPL mass decreases with time as the threecomponents dissolve into flowing groundwater,but the rate of mass depletion slows appreciablyafter approximately 400 days once most of thechloromethane has been depleted. The slower rateof mass depletion with increasing time is consistentwith the fact that the total concentration (Figure 8)decreases with time.

This example illustrates some fundamental aspectsof multi-component DNAPL dissolution applicableto all sites. In summary, the dissolution of a multi-component DNAPL into groundwater will becharacterised by the preferential depletion of thehigher effective solubility components at an earlytime. These components will therefore displaydecreasing concentrations with time. The lowereffective solubility components will display slowerrates of concentration decrease with time, with somecomponents displaying moderate increases inconcentration with time. The total concentration ofall components will decrease with time. This shouldnot be mistaken as an indication that DNAPL is notpresent. In addition, DNAPL mass will decreasefastest at an early time (after DNAPL entry), showing

increasingly slower rates of mass depletion as timeprogresses from the initial entry of DNAPL into thegroundwater.

Raoult’s law is based on the premise that the variouscomponents compete for the dissolution process.However, if the DNAPL of interest contains a co-solventsuch as a low molecular weight alcohol, the presenceof the co-solvent may invalidate the use of Raoult’slaw and result in an enhancement of the variouscomponent solubilities in groundwater. This co-solventeffect typically only occurs for relatively highco-solvent concentrations (for example, 20 per centco-solvent or more by mass in the DNAPL) and tendsto be relatively short-lived (the co-solvent will depleteitself quickly from the DNAPL at an early time).



5

DNAPL source zones in fractured rock

DNAPL will enter fractures in bedrock both above andbelow the water table. Analogous to unconsolidateddeposits, both residual DNAPL and pools will form inrock fractures, with a higher likelihood of poolformation in horizontal to sub-horizontal features.Fracture entry pressures are directly proportional tointerfacial tension and inversely proportional tofracture aperture. This results in preferential DNAPLmigration through the larger aperture fractures of afracture network. The strike and dip of the morepermeable fractures will therefore control theprimary directions of DNAPL migration in a fracturenetwork. Figure 2 (see Section 3) illustrates thepresence of DNAPL in fractured rock below the watertable; note that not all fractures of the network havebeen invaded and that substantial amounts of lateralDNAPL migration have taken place throughhorizontal features.

Once DNAPL enters a fracture network, it is probablethat continued downward and lateral migrationoccurs until the source of DNAPL to the bedrock isexhausted. Figure 10 illustrates a pool of DNAPL atthe base of overburden overlying fractured bedrock.The depiction in Figure 10 is unlikely in that theDNAPL has come to rest as a continuous verticaldistribution between the pool in overburden andsome depth denoted as point A. The depiction isunlikely because capillary pressure increases linearlywith depth in a hydrostatic system; this means thatthe fracture aperture at point A would need to beextremely small to support the overlying distributionof DNAPL.

Figure 10 DNAPL pool at base of overburden: unlikely scenario

residual DNAPL

DNAPL pool at baseof overburden (drift)

DNAPL in fractures

DNAPL release

A A A A



Figure 11 presents a plot of the fracture aperturerequired to stop migration versus depth for a varietyof DNAPL types. Appendix B outlines the calculationprocedure used to produce this graph. The fractureaperture required to stop migration denotes thelargest aperture that can exist at the correspondingdepth such that DNAPL migration is arrested,resulting in the depiction illustrated in Figure 10. For chlorinated solvents such as TCE, fractureapertures need to decrease quickly with depth inorder to prevent further downward migration.Figure 11 indicates that even a 1m accumulation ofTCE in fractured rock would require apertures to beno larger than 9 µm at the base of the accumulationin order to prevent further downward migration.(By way of comparison, a human hair is approximately50 µm thick). Experience has shown that fracturesremain open to depths of many hundreds of metresin many rock types, with measured apertures in theorder of hundreds of micrometres at many sites.Figure 11 assumes that water is perfectly wettingwith respect to the DNAPL of interest. If contactangles between the water and DNAPL were greaterthan zero degrees, even smaller fracture apertureswould be required to arrest migration.

Figure 11 shows that less dense DNAPLs such ascreosote and coal tar do not require as drastic areduction in fracture aperture with depth to arrestdownward migration, but that significant reductionsare still required to support an accumulation ofDNAPL. A head of creosote or coal tar of 2 m, forexample, would require all fracture apertures belowthe DNAPL to be less than 41 µm to preventdownward migration. All the fractures are rough-walled and therefore exhibit a range of apertureswithin the fracture plane. In the above example,the largest aperture within the fracture plane needs

to be restricted to 41 µm or less in order to arrestdownward DNAPL migration.

Figure 12 presents the much more likely scenario inwhich DNAPL is only present as residual at the baseof overburden, with a significant migration to depthin the fracture network. The absence of pooled DNAPLat the base of overburden is therefore not an indicationthat DNAPL has not entered fractured bedrock.

For chlorinated solvent DNAPLs with densitiessignificantly greater than water and relatively lowviscosities, it is probable that DNAPL migration toa considerable depth in the fracture network willhave occurred, and that the DNAPL is no longermoving, by the time site investigations begin. Forcreosote and coal tar DNAPLs, however, which arecharacterised by lower densities and higher viscosities,migration in the fracture network may still be occurringtoday in response to releases that occurred manydecades ago. In such cases, DNAPL at the shallowerelevations may have reached a stable configurationof residual DNAPL and pools, but DNAPL at depthmay still be migrating.

Given the fact that DNAPLs are likely to have migratedto considerable depth in bedrock systems, carefulconsideration must be given to drilling activitiesaimed at determining the total depth of migration.At most sites, the total depth of DNAPL migration infractured bedrock is not determined. This stems fromthe fact that drilling through DNAPL source zonescarries with it an associated risk of re-mobilisingDNAPL, allowing it to migrate deeper into thesubsurface. Because DNAPL does not enter all thefractures in a network, a large number of boreholesmay be required to estimate the depth of DNAPLmigration; such an exercise is often prohibited bycost. Furthermore, knowledge of the depth ofmigration is often of little practical use becauseremediation technologies are currently unable toremove DNAPL completely from depth in bedrocksystems. The aqueous phase plume is typically themost mobile form of contamination. Drilling effortsare therefore often focused on determining the rateand direction of plume migration, with particularemphasis on data collection to assess the likely risksto identified receptors.

The overall ability of fractured bedrock to retainresidual and pooled DNAPL is relatively small giventhe low fracture porosity of most rock types.A typical fractured rock, for example, may exhibitfracture porosities in the range of 0.001 to 0.01.Assuming that DNAPL will occupy on average 20 percent of the fracture pore space, this range of fracture

Figure 11 Fracture aperture required to stop migration versusheight of accumulated DNAPL



Figure 12 DNAPL migration to depth in fractured bedrock: likely scenario

porosities corresponds to bulk retention capacitiesranging from 0.0002 m3 DNAPL per m3 of bedrockto 0.002 m3/m3 (that is, between 200 ml and 2 litresof DNAPL per m3 of rock). This implies, for example,that one drum of DNAPL containing 205 litres(0.205 m3) of product will occupy a bulk bedrockvolume of 103-1,025 m3.

It is clear that relatively small volumes of DNAPLhave the potential to impact relatively large volumesof bedrock. This conclusion holds for most rock typesin the UK, including all the major water supplyaquifers. Drilling through an overburden DNAPLsource zone into underlying bedrock should thus beapproached with caution. Care should be taken togrout casings into competent rock before drillingdeeper, and a DNAPL contingency plan to identifyand recover DNAPL during drilling activities shouldbe in place.

It should also be pointed out that downwardgroundwater flow can mobilise DNAPL pools deeperinto the subsurface. The specific mechanism causingthe mobilisation of DNAPL is a manipulation of capillarypressure in response to the imposed hydraulic gradientin the groundwater. Pumping tests in bedrock shouldtherefore generally be avoided beneath overburdenDNAPL source zones where a small amount ofdownward DNAPL mobilisation could bring about

contamination of a large volume of bedrock. De-watering overburden deposits by lowering the watertable into bedrock should also, in general, beavoided.

The discussion above has focused on the migrationof DNAPL through bedrock fractures. In cases wherethe rock matrix is relatively coarse-grained, someentry of DNAPL into the rock matrix may also occur.This is generally not a concern in crystalline rocks,chalk and limestones, but may be a concern in theTriassic sandstones encountered in the UK whererelatively coarse-grained sediments and smallamounts of calcite/dolomite cement characterisethe rock matrix. In cases where the DNAPL is wettingwith respect to water (not common, but possibleespecially with coal tars), spontaneous imbibitionof the DNAPL into the rock matrix can occur.

residual DNAPL

DNAPL residualin fractures

DNAPL pool in fracturesDNAPLmigration to depth

DNAPL release



6

DNAPL dissolution in fractured rock

Once DNAPL is present in bedrock, it will slowlydissolve into groundwater flowing through openfractures, giving rise to aqueous phase plumes.The plumes will generally migrate in the hydraulicallydown-gradient direction subject to advection,dispersion, sorption to fracture walls, possiblebiodegradation and matrix diffusion. As with plumemigration in unconsolidated deposits, the chemicalcomposition of the plume will be a function of thechemical composition of the DNAPL. Therefore, theplume would expect to be dominated by highereffective solubility compounds at an early time,gradually shifting later towards higher concentrationsof the lower solubility compounds. Like plumes inunconsolidated deposits, all plumes in fracturedbedrock will eventually reach a steady-stateconfiguration where the leading and side edges ofthe plume (as defined by a specific concentrationlevel) are no longer expanding. One objective ofmany site investigations is to determine whether theaqueous phase plume has reached its steady-stateconfiguration.

Figure 13 illustrates the development of a steady-state plume downstream of a DNAPL source zonethat is providing a constant concentration source ofcontamination. The concept of a steady-state plumeis applicable to both porous and fractured media,and can result from the dispersion process alone.As a result, all plumes reach a steady-state configurationat some point in time where the leading and sideedges of the plume are stable as defined by aspecified concentration contour and ultimately, oncethe source zone is exhausted, will shrink. Figure 13shows that the monitoring well closest to the DNAPLsource zone reaches a steady-state concentration attime t1 .The next monitoring well reaches a steady-state concentration at t2, which is greater than t1.The monitoring well near the leading edge of theplume reaches a steady-state concentration at t3,which is greater than t2.

Figure 13 Development of a steady-state plume

DNAPLsteady-state plume

monitoring well

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low

Time

Co

nce

ntr

atio

n

C3

C2

C1

Time

Co

nce

ntr

atio

n

C3

C2

C1

Time

Co

nce

ntr

atio

n

C3

C2

C1



This general pattern illustrates that plumes reachsteady-state concentrations first near the source andlast at the leading edge. If the boundaries of theplume were defined by concentration C3 (for example,a regulatory limit equal to C3), then the entire plumewill have reached steady-state by time t3. The precisevalue of t3 is site-specific and is influenced by anumber of factors including hydraulic conductivity,hydraulic gradient, source zone concentration,sorption and degradation. In general, degradationwill lead to shorter steady-state plumes than thosearrived at by dispersion alone. Whether a plume hasreached steady-state is typically determined throughseveral years of groundwater quality monitoringand/or the use of numerical simulation.

With respect to physical processes influencing plumebehaviour, there is one fundamental difference betweenporous and fractured media. Plumes in fractured clayand rock are subject to a process known as matrix

diffusion. Matrix diffusion refers to the process wherebysolutes dissolved in groundwater diffuse into and outof the rock matrix. If concentrations are higher in theopen fracture, the diffusion process will result indissolved contaminants moving into the rock matrix(forward diffusion). This is illustrated schematicallyin Figure 14. If concentrations are higher in the rockmatrix, dissolved contaminants will move out of therock matrix and into water in the open fractures(back diffusion). Matrix diffusion will occur in all rocktypes exhibiting a finite matrix porosity. The diffusionprocess will therefore occur in virtually all chalk andsandstone rock types in the UK, in clay deposits andin some weathered crystalline rock environments.

Figure 14 Matrix diffusion process

DNAPL dissolution intoflowing groundwater

fracture

diffusion halo

advection and dispersionof solutes in fracture

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und

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low

A

A’

decreasing concentrationaway from the fracture

A

A’

concentration

molecular diffusioninto matrix



One manifestation of the matrix diffusion processis that solute plumes in fractured rock and clay willmigrate slower than the rate of groundwater flow.The rate of plume advance in fractured chalk, clayand sandstone can therefore be significantlyattenuated relative to the rate of groundwatermigration, with attenuation rates as high as 100 ormore. The attenuation is greater for smaller aperturefractures, higher matrix porosity and slower movinggroundwater. This explains why solute plumes infractured (dual porosity) environments are oftensmaller in spatial extent than predicted by groundwatervelocity calculations alone.

A second manifestation of the matrix diffusion processis that the timescale of remediation in fractured rocksis often controlled by the back diffusion process andnot by the presence of DNAPL in fractures. Even forrelatively short initial exposure times, the back diffusionprocess can continue for many decades. Considerthe release of a small volume of DNAPL into a singlefracture in clay as illustrated in Figure 15.

As a result of diffusion into the matrix and dissolutioninto groundwater flowing through the fracture, theDNAPL is eventually completely depleted. Groundwaterconcentrations at the exit to the fracture will exhibitmeasurable concentrations for long periods as a resultof back diffusion from the matrix.

Figure 16 (adapted from Reynolds and Kueper,2002) presents the concentrations of five chlorinatedsolvents at the exit of the fracture illustrated inFigure 15 as a function of time.

The fracture is assigned an aperture of 30 µm, a dipof 60˚ below horizontal and a length of 3m. It wasassumed in each case that the DNAPL of interest wasintroduced to the fracture at t = 0(Stage 1 in Figure 15),allowed to redistribute to residual DNAPL (Stage 2),and then allowed to dissolve completely into flowinggroundwater and diffuse into the clay matrix (Stage3). The fracture is subject to clean water injection atthe top of the system after Stage 2, producing flowin a downwards direction.

Figure 16 shows that measurable exit concentrations

Figure 15 DNAPL release and subsequent depletion bydissolution into a single fracture

Stage 1

Stage 2

Stage 3

DNAPL pool

initialinvasionof fracture

redistributionto residual asa result ofcapillary action

diffusion ofaqueous phasepollutant intorock matrix(VH = 0.01)

groundwater flow

diffusive flux

exit



persist for many hundreds of years as a result of theback-diffusion process. Although the residual DNAPLdissolved itself out of existence in less than 21 weeksin all cases, the exit concentrations persist for longperiods because the back-diffusion process is slowerthan the initial forward-diffusion process. This stemsfrom the fact that solutes are continuing to migratefurther into the clay matrix while at the same timediffusing back into the open fracture after the DNAPLhas completely dissolved itself away. In addition, theconcentration gradient driving back-diffusion istypically less than the initial concentration gradientdriving forward-diffusion into the matrix whileresidual DNAPL is present in the fracture.

Figure 16 Concentration versus time at exit of fracture (from Reynolds and Kueper, 2002)

Key: CB = chlorobenzene; DCE = dichloroethene;TCE = trichloroethene; PCE = tetrachloroethene, EDB = 1,2-dibromomethane

The implication of matrix diffusion in fractured clayand rock is that solute plumes, as well as DNAPLsource zones, are difficult to remediate. In fracturedenvironments exhibiting matrix diffusion, conventionaltechnologies such as pump-and-treat should beviewed as either a source zone containmenttechnology, or a plume interception technology, notas a technology capable of restoring groundwater tonear-pristine quality within short periods of time. Theeffectiveness of many remedial techniques is diffusionlimited and this process needs to be consideredduring development of a remedial strategy, andselection of a remedial technique.



7

Determination of DNAPL presenceand delineation of the DNAPL source zoneA primary goal of many site investigations is to establishwhether DNAPL is present in the subsurface and, ifso, to determine the spatial extent of the DNAPLsource zone.

DNAPL presence is often established on the basis ofconverging lines of evidence rather than direct visualobservation. The tortuous and sparse nature ofDNAPL migration pathways, together with the factthat boreholes and other invasive procedurestypically only physically access a minute fraction ofthe subsurface, result in a low probability of actuallyencountering DNAPL in many site investigations.Thus, DNAPL is often not observed directly at manyDNAPL sites. The overall extent of the source zoneshould typically be delineated at a site, and not thespecific locations of individual DNAPL pools and zonesof residual DNAPL within the overall source zone.

The following investigative techniques are typicallyemployed in establishing DNAPL presence at a site.

7.1 Site use/site historySite investigations should always begin with a deskstudy; this may start with employee interviews andrecord searches to establish what operations havetaken place at the site (BSI, 2001). Table 6 lists avariety of industries and industrial processes thatare often, but not necessarily, associated with thepresence of DNAPLs in the subsurface. Furtherguidance can be found in the Industry Profile series(DoE, 1995) produced by former Department of theEnvironment (now the Department for Environment,Food and Rural Affairs).

Table 6 Table 6 Industries and industrial processes sometimes associated with DNAPL presence in the subsurface(modified from US EPA, 1992)

Industries Industrial processes

Timber treatment Metal cleaning and degreasing

Coal gasification (Town gas production) Metal machining and plating

Electronics manufacturing Tool-and-die operations

Solvent or paint production Paint removing

Pesticide/herbicide manufacturing Solvent storage above or below ground

Aeroplane maintenance and engine manufacturing Solvent transmission through piping

Military bases and rocket fuel production Solvent loading and unloading

Dry cleaning Disposal of mixed wastes in landfills

Instrument manufacturing Storage of liquid wastes in lagoons and ponds

Transformer oil production

Vehicle manufacturing

Transformer reprocessing

Steel industry coking

Pipeline compressor stations


fracture in rock core, even when drilling directlythrough fractures that originally contained residualDNAPL or pools. Possible exceptions may be creosoteor coal tar, which tend to be more viscous than otherDNAPLs. In some cases, however, the DNAPL mayhave stained the rock fracture walls, which may bediscernible upon inspection of the core. However,not all DNAPLs will stain fracture walls. Consequently,the absence of staining should not be relied on asunequivocal evidence that the fracture has not beenexposed to DNAPL. Additional screening techniqueswhen drilling in bedrock include inspecting the drillreturn water for visual or olfactory evidence ofhydrocarbons (for example, iridescent sheens orodours) and headspace analysis of rock core thatmay contain high concentrations of contaminantsdiffused into the matrix. Exposure to hazardoussubstances should, however, be controlled and sniffingcores is not appropriate unless it is clear that it is safeto do so. Prior use of calibrated PID/FID equipmentmay be appropriate where there is any doubt.

7.5 Laboratory analysis of soilsamplesSoil characterisation programmes typically involvesending discrete soil samples to the laboratory forquantitative analysis of contaminant composition.The presence of NAPL in a soil sample can beevaluated using:

Equation 2

where:

Ci is the concentration of an organic substance ator above that which may be present in anon-aqueous phase (mg/kg);

Ci is the effective solubility of the substance ingroundwater (mg/l);

Pb is the dry soil bulk density (kg/l);

Kd is the soil-water partition coefficient (l/kg);

w is the water-filled porosity (dimensionless);

H’ is the unitless Henry’s law constant(dimensionless);

a is the air-filled porosity.


7.2 Aerial photographs, mapsand plansIf available, aerial photographs, maps and planscan be examined to determine if drum storage areas,railway unloading areas, above ground storage tanks,evaporation ponds, etc. were present at the site.Such features may represent potential DNAPL entrypoints into the subsurface.

7.3 Screening of soil borings Soil samples can be taken from unconsolidateddeposits both above and below the water table usinga variety of techniques such as hollow stem auguring,push sampling and trial pits. During drilling operations,soil samples can be taken and subjected to visualexamination. If DNAPL is present at high saturations(that is, pooled DNAPL) it will be probably be readilynoticeable. If the DNAPL is present at low saturations(that is, residual saturation) or if the DNAPL has thesame colour as the soil matrix, it may not be visuallyapparent and a dye shake test may be required.A dye shake test involves mixing a small quantity ofa hydrophobic dye such as SUDAN IV with the soilsample in a sealed glass jar. In the case of SUDAN IV,the dye is red in colour and will partition only into aNAPL phase; it will not partition into water. If a NAPLis present in the soil sample, it will manifest itself asred globules interspersed within the soil matrix.Other field screening methods for soil borings includeheadspace analysis using a portable vapour analyserwith photoionisation detection (PID) or flameionisation detection (FID), and the use of fluorescenceanalysis. Fluorescence analysis involves taking theobtained soil sample and subjecting it to a fluorescentlight in a dark space. Many hydrocarbons such asaromatic and polyaromatic hydrocarbons having oneor more benzene rings (for example, coal tar, creosoteand PCBs) will fluoresce, along with DNAPL mixturescontaining petroleum products and certain unsaturatedaliphatic hydrocarbons such TCE and PCE.

7.4 Screening of rock cores Bedrock drilling techniques are typically veryaggressive due to the high temperatures and highwater velocities that are usually generated in theimmediate vicinity of the boring. The DNAPL isgenerally pushed out of rock fractures before thecore is retrieved at ground surface. It is thereforeunreasonable to expect DNAPL to be present in a

Ci = (KdPb + w + H’ a)T Ci

Pb

T

O O

O

O



The soil-water partition coefficient is oftenapproximated by:

Equation 3

where:

Koc is the organic carbon-water partition coefficient(l/kg);

foc is the fraction organic carbon present in the soil(dimensionless).

For structurally similar compounds in a multi-component NAPL, the effective solubility, Ci, canbe estimated using Equation 1 (see Section 4).

Equation 2 represents the maximum amount ofcontaminant that can be present in a soil samplein the sorbed, aqueous and vapour phases withouta NAPL phase being present. If reported soilconcentrations exceed , it can be concluded thata NAPL phase was present in the sample. Table 7 presentsan example calculation of the above procedure fora single component DNAPL composed of TCE locatedbelow the water table. This calculation procedureassumes that the composition of the DNAPL is known,a priori, such that the required mole fractions can bedetermined. If the DNAPL is known to be composedof primarily one component, the mole fraction ofthat component can be assumed to be one. At siteswhere a multi-component DNAPL is suspected and aDNAPL sample has not been obtained for componentcomposition analysis, the required mole fractions willnot be known. In such cases, this calculation procedurecan still be employed, but with a slight modification.

Table 7 Example calculation of soil DNAPL thresholdconcentration: single component DNAPL belowthe water table

For a multi-component DNAPL of unknowncomposition, the sum of the mole fractions mustequal one. DNAPL will therefore be present in a soilsample if the following condition is met:

Equation 4

where:

Cobs is the reported concentration of component i;

CS is the single component soil concentration of component i;

n is the total number of components observed inthe soil sample.

Appendix C presents an example calculation usinga soil sample containing five different compounds.

7.6 Bailers and interface probesIn addition to water level and groundwater qualitymonitoring, monitoring wells and piezometers canbe inspected for the presence of DNAPL usingbottom-loading bailers and weighted oil-waterinterface probes.

7.7 Contaminantconcentrations in groundwaterAs discussed in Section 4, experience has shown thatDNAPL may be present up – gradient of a monitoringwell displaying sampled groundwater concentrationsin excess of 1 per cent of the effective solubility ofthe component of interest. Calculating the 1 percent effective solubility requires a priori knowledgeof the DNAPL composition such that mole fractionscan be established. If the DNAPL is thought to becomposed of primarily one component, the molefraction of that component can be approximated tobe one. If the DNAPL is thought to be composed ofseveral components, however, a modified approachwill be required in cases where a DNAPL compositionanalysis is not available.

For a multi-component DNAPL dissolving intogroundwater, the equilibrium aqueous phaseconcentration can be represented using Raoult’slaw as presented in Equation 1 (see Section 4).

Kd = Kocfoc

TCE solubility in water = 1,100 mg/l

Fraction organic carbon of soil = 0.003

Porosity of soil = 0.27

Koc of TCE = 126 l/kg

Dry bulk density of soil = 1.9 kg/l

Air filled porosity = 0

CTCE = (126(0.0031)1.9 + 0.27) = 572 mg/kg

T

1100

1.9

T

T

∑ CobsT

CST

≥1i = l

n

Ci T



Because groundwater concentrations are typicallyobtained from monitoring well samples, a certaindegree of dilution will occur due to borehole dilution,hydrodynamic dispersion, non-optimal well placementand degradation. In this analysis, the degree ofdilution due to these three processes will berepresented by the parameter a, such that:

Equation 5

where:

Ci is the concentration observed in the monitoringwell;

Ci is the effective solubility given by Equation 1.

For a multi-component DNAPL, the sum of the molefractions must equal one:

Equation 6

Combining Equations 5 and 6 yields:

Equation 7

If there were no effects from borehole dilution,dispersion, degradation or monitoring well placement,the parameter a would equal one in the vicinity ofa multi-component NAPL. However, because theseprocesses all occur to some degree, a will take on avalue less than one. If the 1 per cent ‘rule-of-thumb’is adopted, it follows that DNAPL may be present

upstream of a monitoring well if a >0.01. This assumesthat the degree of borehole dilution, dispersion anddegradation is identical for each component ofinterest. This is likely to be the case for boreholedilution and dispersion, but may not be the case fordegradation. For monitoring wells in close proximityto DNAPL, however, the amount of degradation maybe minimal such that the error introduced by varyingamounts of degradation will be negligible.

Equation 7 can be applied on a sample-by-samplebasis without having to assume that the NAPLcomposition is spatially uniform. In addition, theparameter a can be any value less than one. If it isbelieved that 10 per cent of the effective solubilityindicates NAPL presence, for example, a can be setto 0.1. Table 8 presents an example calculationwhere five components have been detected in amonitoring well. Although each component hasbeen detected at a concentration less than 1 percent of the single component solubility, the cumulativemole fractions add up to 3.4 per cent, providingevidence of possible DNAPL presence upstream ofthe monitoring location.

7.8 Presence of contaminationin hydraulically anomalouslocationsFigure 2 presents a schematic illustration of DNAPLpresence in the subsurface. Given that groundwateris flowing from left to right in the figure, it is notpossible for DNAPL sources in the unsaturated zonealone to give rise to aqueous phase contaminationdeep in the overburden or in the fractured bedrock.If a flow line cannot be drawn between knownshallow sources and deeper, up-gradient or side-gradient occurrences of aqueous phase contamination,a DNAPL source may be present up-gradient of those

Table 8 Example calculation of cumulative mole fraction in a groundwater sample

∑ Ci

Si

=1

obs

Ci

Cia =

obs

∑ Ci

Si

= aobs

Compound Concentration in Single component CiSi

monitoring well (mg/l) solubility (mg/l)

trichloroethene 4.4 1100 0.004

tetrachloroethene 1.8 200 0.009

toluene 3.5 500 0.007

chlorobenzene 4.0 500 0.008

trichloromethane 48.0 8000 0.006

0.034∑ Ci

Si

obs



monitoring locations. A classic example of this is thepresence of groundwater contamination at depth ina flow field characterised by an upward componentto flow. If groundwater is flowing up from depth(for example in a groundwater discharge zone) andaqueous phase contamination is encountered at depth,the source of that contamination will be hydraulicallyup-gradient (that is, deeper) of the monitoring wellin question

7.9 Persistence ofcontaminationIf contamination persists at a particular monitoringlocation, it follows that a stable source of contaminationsuch as DNAPL may be present hydraulically up-gradient of that location. If contaminant releasestook place 20 years ago, for example, and thegroundwater velocity at the site was 50 m/year,one would expect a non-sorbing aqueous phasecontaminant to have travelled approximately 1,000m in the 20-year period. If contamination is stillconnected to the source area with generallydecreasing concentrations in the down-gradientdirection, a stable source of contamination is likelyto be feeding the plume.

7.10 Persistence of alcoholsin groundwaterIf readily degradable co-solvents such as alcohols aredetected in monitoring wells, it is likely that they arederived from a stable source such as residual or pooledDNAPL. Alcohols are known to partition into bothgroundwater and NAPL phases. It is also knownthat alcohols tend to biodegrade readily in manygroundwater environments. If alcohols are detectedregularly in a particular monitoring well, it is possiblethat a DNAPL source is supplying the alcohols togroundwater.

Considerable uncertainty will exist with respect todelineating the DNAPL source zone. Establishingwhether DNAPL is present at a site is typically easierthan delineating the exact spatial extent of theDNAPL source zone. It is advisable to adopt a two-tiered approach.

Figure 17 illustrates both a probable and a potentialDNAPL zone. The probable DNAPL zone encompassesthat volume of the subsurface where many converginglines of evidence suggest DNAPL presence. Thesemay include the locations of former known DNAPL

storage and release areas such as evaporation pondsand drum disposal areas, locations where DNAPL hasbeen visually observed in soil samples, locationswhere soil concentrations are above the calculatedDNAPL threshold, and locations where groundwaterconcentrations are very high (for example, over 10per cent effective solubility). The potential DNAPLsource zone provides a factor of safety, and mayrepresent the volume of the subsurface where fewerconverging lines of evidence suggest DNAPL presence.

Both the estimated extent of the probable andpotential DNAPL source zones should be updated,along with other aspects of the site conceptualmodel, whenever new data become available(Environment Agency, 2001a). It is unreasonable toexpect site investigation techniques to be able todefine the lateral and vertical extent of the DNAPLsource zone exactly; regardless of the level of datacollection, uncertainty will always exist.

With respect to drilling strategies, it is recognisedthat drilling directly through a DNAPL source zonecarries with it a significant risk of mobilising DNAPLdeeper into the subsurface. Many practitioners adoptan outside-in approach in which monitoring wellsare first placed outside the suspected DNAPL sourcezone. Subsequent borings are added closer andwithin the source area on an as-needed basis untilthe source zone location has been defined in sufficientdetail to satisfy the investigation objectives. In somedrilling programmes, a contingency plan is providedin case strong evidence of the presence of DNAPL isgathered during drilling operations. Such a plan mayinvolve terminating boreholes or moving to analternative drilling location whenever mobile DNAPLis encountered. It is also customary to install sumpsat the base of monitoring wells and piezometerssuspected of being in close proximity to DNAPLsources. The sumps facilitate the collection of DNAPLand avoid DNAPL exit from the bottom of the wellfollowing entry into the well at a higher elevation.

possible DNAPL

probable DNAPLdisolved plume

gro

und

wat

er f

low

Figure 17 Probable and potential DNAPL zones



8

UK Specific Conceptual Models

ceased within a relatively short time period followingthe entry of DNAPL into bedrock. Groundwater flowthrough the fractures containing residual and pooledDNAPL has brought about DNAPL dissolution andthe evolution of a dissolved phase plume.

The plume is migrating through the set of interconnectedfractures subject to advection, dispersion, a limitedamount of biodegradation, and matrix diffusion.Because of matrix diffusion, the plume has notmigrated as far as would be predicted by simplegroundwater velocity calculations. At this site, theplume is migrating 50 – 500 times slower than thegroundwater, with greatest attenuation in the smalleraperture fractures. Diffusion of aqueous phasecontaminants into the Chalk matrix is also occurringimmediately adjacent to the residual and pooledDNAPL, where concentration gradients are greatest.With time, a greater and greater portion of the massis transferred from the open fractures to the matrix.In the smaller aperture fractures, the DNAPL hascompletely diffused into the matrix.

With respect to site characterisation, efforts belowthe water table are focused on defining the spatialextent of the aqueous phase plume and establishingwhether it has reached a steady-state configuration.Limited drilling is taking place within the DNAPLsource zone. The extent and depth of the sourcezone in bedrock are being inferred on the basis ofthe spatial distribution of groundwater concentrationsdownstream of the source zone. In the case of a singlecomponent DNAPL, groundwater sampling is focusedon the contaminant of interest and its daughterproducts. In the case of a multi-component DNAPL,groundwater sampling includes the most mobilecomponent(s) of the DNAPL since they will definethe leading edge of the plume, but may also includeless mobile compounds that present a potential riskto human health and the environment. The mostmobile component of the DNAPL is taken to be acomponent that is present at detectable concentrations,sorbs little, and is not subject to appreciable amountsof degradation.

8.1 Chlorinated solvent releaseinto Cretaceous ChalkFigure 18 illustrates a release of chlorinated solventDNAPL at the ground surface into shallow driftdeposits overlying fractured Cretaceous chalk. Inthe drift, residual and pooled DNAPL have migratedlaterally in all directions in response to the beddingstructure of the unconsolidated deposits. Due to thevolatile nature of the solvent DNAPL, a well-definedvapour plume has developed. Depending on thebedding structures, soil sampling to determine DNAPLpresence may be a ‘hit and miss’ proposition becauseof the tortuous and sparse nature of DNAPL migrationpathways. Soil vapour sampling may be a moreefficient way of determining the lateral extent of thesource zone in the drift deposit, where the DNAPLgives rise to vapours.

The volume of the DNAPL release was sufficient toreach the top of bedrock, which is characterised byweathered chalk. DNAPL has entered the fracturedchalk and, because of the low fracture porosity, hasmigrated to considerable depth. The depth of DNAPLmigration cannot be determined easily because ofthe large number of boreholes required, the highcost and the risk of dragging contamination deeperinto bedrock. DNAPL has come to rest primarily aspooled DNAPL in the horizontal fractures and asresidual DNAPL in the vertical and steeply dippingfractures.

DNAPL is restricted to fractures below the watertable and has not entered the chalk matrix as a separateliquid phase because of the high entry pressureassociated with the narrow pore throat size in a chalkmatrix. Above the water table, however, some areasof the chalk matrix have very low water saturationsand limited DNAPL migration into the matrix hasoccurred. DNAPL has migrated preferentially in thelarger aperture fractures and has spread laterally inall directions, including hydraulically up-gradient ofthe release location. Because of the high density andlow viscosity of the chlorinated solvent DNAPL,migration through the subsurface was rapid and


Rel

ease

of

chlo

rin

ated

so

lven

t D

NA

PLin

to C

reta

ceo

us C

hal

kFe

atur

esD

om

ain

1Re

sid

ual a

nd p

oole

dD

NA

PL in

dri

ft

2Va

pou

r

3D

NA

PL m

igra

tion

into

unsa

tura

ted

cha

lk

4M

atri

x d

iffus

ion

and

adve

ctiv

e va

pou

r

5Po

oled

DN

APL

infr

actu

res

6Re

sid

ual D

NA

PL in

frac

ture

s

7Ex

tent

of g

roun

dw

ater

plu

me

8M

atri

x d

iffus

ion

next

to D

NA

PL

9A

que

ous

pha

sed

iffus

ion

into

mat

rix

10A

dve

ctio

n an

d

dis

per

sion

of

cont

amin

ants

in fr

actu

res

11G

roun

dw

ater

flow

dir

ectio

n

12D

NA

PL m

igra

tion

tod

epth

Surf

ace

Dri

ft(u

nsat

urat

ed)

Bed

rock

(uns

atur

ated

)

Bed

rock

(sat

urat

ed)

12

34

7

5

6

8

9

1112

10


Fig

ure

18

Rele

ase

of c

hlor

inat

ed s

olve

nt D

NA

PL in

to C

reta

ceou

s C

halk

DN

APL

rel

ease



Rel

ease

of

coal

tar

DN

APL

into

Cre

tace

ous

Ch

alk

Feat

ures

Do

mai

n

1Re

sid

ual a

nd p

oole

dD

NA

PL in

dri

ft

2Re

sid

ual D

NA

PL in

frac

ture

s

3Po

oled

DN

APL

infr

actu

res

4A

que

ous

pha

sed

iffus

ion

into

mat

rix

5D

iffus

ion

into

mat

rix

next

to

DN

APL

6Pl

ume

exte

nt

7O

n-g

oing

DN

APL

mig

ratio

n (l

ater

al)

8O

ngoi

ng D

NA

PLm

igra

tion

(ver

tical

)

9G

roun

dw

ater

flow

dir

ectio

n

Surf

ace

Dri

ft(u

nsat

urat

ed)

Bed

rock

(uns

atur

ated

)

Bed

rock

(sat

urat

ed)

1

2

34

7

5

6

89

Fig

ure

19

Rele

ase

of c

oal t

ar D

NA

PL in

to C

reta

ceou

s C

halk

77

DN

APL

rel

ease



8.2 Coal tar release intoCretaceous ChalkFigure 19 illustrates a release of coal tar DNAPL atthe ground surface into shallow drift deposits thatoverlie fractured Cretaceous Chalk. The coal tarrelease took place during the early part of the 20thcentury at a coal gas manufacturing facility. In theoverburden, residual and pooled DNAPL havemigrated laterally in all directions in response to thebedding structure of the unconsolidated deposits.A well-defined vapour plume is not present becausethe highly volatile components of the coal tar werepreferentially depleted at an early time, leaving onlylow volatility components of the DNAPL in the drift.

Although soil sampling can be a ‘hit and miss’exercise depending on bedding structures, it is stillthe most practical way to define the lateral extentof the source zone in the unconsolidated deposits,where the DNAPL does not readily give rise to vapours.

The volume of the DNAPL release was sufficient toreach the top of the bedrock, which is characterisedby weathered chalk. DNAPL has entered the fracturedchalk and, because of the low fracture porosity, hasmigrated to a considerable depth. DNAPL migrationis still occurring today due to the relatively lowdensity and high viscosity of the DNAPL, and becauselarge volumes of coal tar were released. The depth ofDNAPL migration cannot be determined easily becauseof the large number of boreholes required, but limiteddrilling will take place in the source zone to determineif the DNAPL is still in a mobile form. Monitoringwells will be completed with sumps below the wellscreen to facilitate the collection of mobile DNAPL.In many places, DNAPL has already come to restprimarily as pooled DNAPL in the horizontal fracturesand as residual DNAPL in the vertical and steeplydipping fractures. DNAPL is restricted to fracturesbelow the water table and has not entered the chalkmatrix as a separate liquid phase because of the highentry pressure. Above the water table, however,some areas of the chalk matrix are very dry andlimited DNAPL migration into the matrix has occurred.DNAPL has migrated primarily in the larger aperturefractures and has spread laterally in all directions,including hydraulically up-gradient of the releaselocation. The amount of lateral spreading is greaterthan that for the chlorinated solvent DNAPL releasedepicted in Figure 18 because of the lower densityof the DNAPL.

Groundwater flow through the fractures containingresidual and pooled DNAPL has brought aboutDNAPL dissolution and the evolution of a dissolvedphase plume. The plume is migrating through the

set of interconnected fractures subject to advection,dispersion, biodegradation and matrix diffusion.Because of the matrix diffusion process, the plumehas not migrated as far as would be predicted bygroundwater velocity calculations. At this site, theplume is migrating 50 – 500 times slower than thegroundwater with higher amounts of attenuation inthe smaller aperture fractures. Diffusion of aqueousphase contaminants into the chalk matrix is alsooccurring immediately adjacent to the residual andpooled DNAPL. With time, a greater and greaterportion of the mass is transferred from the openfractures to the matrix.

With respect to site characterisation, efforts belowthe water table are focused on defining the spatialextent of the aqueous phase plume and establishingwhether it has reached a steady-state configuration.Although some drilling is taking place within theDNAPL source zone, the extent and depth of thesource zone in the chalk is being inferred largely onthe basis of the spatial distribution of groundwaterconcentrations downstream of the source zone.Because the coal tar release occurred several decadesago, the more mobile, low molecular weightcomponents of the DNAPL no longer represent themajor constituents of the plume. A few indicatorcompounds are selected to define the extent of theplume; because of their low mobility, the dissolvedplume is not expected to have advanced far aheadof the DNAPL source zone.

8.3 Chlorinated solvent releaseinto Triassic SandstoneFigure 20 depicts a release of chlorinated solventDNAPL at the ground surface into shallow drift depositsoverlying fractured sandstone. In the overburden,residual and pooled DNAPL have migrated laterally inall directions in response to the bedding structure ofthe unconsolidated deposits. Because of the volatilenature of the solvent DNAPL, a well-defined vapourplume has developed. Depending on the beddingstructures, soil sampling to determine DNAPL presencemay be a ‘hit and miss’ proposition due to the tortuousand sparse nature of DNAPL migration pathways.Soil vapour sampling may be a more efficient way todetermine the lateral extent of the chlorinated solventsource zone in the drift deposit.

The volume of the DNAPL release was sufficient toreach the top of bedrock. DNAPL has entered thefractured sandstone and, due to the low fractureporosity, has migrated to a considerable depth.The depth of DNAPL migration cannot be determinedeasily because of the large number of boreholes



Rel

ease

of

chlo

rin

ated

so

lven

t D

NA

PLin

to T

rias

sic

San

dst

on

eFe

atur

esD

om

ain

1Re

sid

ual a

nd p

oole

dD

NA

PL in

dri

ft

2Va

pou

r

3A

que

ous

pha

sep

lum

e m

igra

tion

4A

que

ous

pha

se p

lum

em

igra

tion

in fr

actu

re

5Re

sid

ual D

NA

PL in

frac

ture

s

6A

que

ous

pha

sem

atri

x d

iffus

ion

7D

NA

PL p

enet

ratio

n in

toco

arse

gra

ined

mat

rix

8Po

oled

DN

APL

infr

actu

res

9M

atri

x d

iffus

ion

adja

cent

to

DN

APL

10G

roun

dw

ater

flow

dir

ectio

n

11D

NA

PL m

igra

tion

to d

epth

Surf

ace

Dri

ft(u

nsat

urat

ed)

Dri

ft(s

atur

ated

)

Bed

rock

(sat

urat

ed)

1

2

3

4

7

56

8

9

11

10

Fig

ure

20

Rele

ase

of c

hlor

inat

ed s

olve

nt D

NA

PL in

to T

riass

ic S

ands

tone

DN

APL

rel

ease



needed, the high cost and the risk of draggingcontamination deeper into bedrock and creatingnew conduits. DNAPL has come to rest primarily aspooled DNAPL in the horizontal fractures and primarilyas residual DNAPL in the vertical and steeply dippingfractures. DNAPL is not necessarily restricted tofractures below the water table and has entered thesandstone matrix as a separate liquid phase wherethe matrix is composed of weakly cemented, coarse-grained sediments. Above the water table, DNAPLhas entered those regions of the rock matrix thatexhibit continuous air pathways and those composedof coarse-grained sediments.

DNAPL has migrated primarily in the larger aperturefractures and has spread laterally in all directions,including hydraulically up-gradient of the releaselocation. The angled nature of the fractures in someregions has led to preferential migration in thedown-dip direction. Because of the high densityand low viscosity of the chlorinated solvent DNAPL,migration ceased within a relatively short timefollowing entry of DNAPL into the bedrock.

Groundwater flow through the fractures containingresidual and pooled DNAPL has brought aboutDNAPL dissolution and the evolution of a dissolvedphase plume. The plume is migrating through theset of interconnected fractures subject to advection,dispersion, a limited amount of biodegradation, andmatrix diffusion. Because of the matrix diffusionprocess, the plume has not migrated as far as wouldbe predicted by groundwater velocity calculations.At this site, the plume is migrating 50 – 500 timesslower than the groundwater with higher amountsof attenuation in the smaller aperture fractures.Diffusion of aqueous phase contaminants into thesandstone matrix is also occurring immediatelyadjacent to the residual and pooled DNAPL. Withtime, a greater and greater portion of the mass istransferred from the open fractures to the matrix.

With respect to site characterisation, efforts belowthe water table are focused on defining the spatialextent of the aqueous phase plume and establishingwhether it has reached a steady-state configuration.Limited drilling is taking place within the DNAPLsource zone. The extent and depth of the sourcezone in the sandstone is being inferred on thebasis of the spatial distribution of groundwaterconcentrations downstream of the source zone.In the case of a single component DNAPL, groundwatersampling is focused on the contaminant of interestand its daughter products. In the case of a multi-component DNAPL, groundwater sampling is focusedon the most mobile component(s) of the DNAPLsince this will define the leading edge of the plume.

The most mobile component of the DNAPL is takento be a component that is present at detectableconcentrations, sorbs little and is not subject toappreciable amounts of degradation.

8.4 Coal tar release intofractured Triassic SandstoneFigure 21 illustrates a release of coal tar DNAPL atground surface into shallow drift deposits overlyingfractured sandstone.

The coal tar release took place in the early part of the20th century at a coal gas manufacturing facility. Inoverburden, residual and pooled DNAPL have migratedlaterally in all directions in response to the beddingstructure of the unconsolidated deposits. A well-defined vapour plume is not present because thehighly volatile components of the coal tar werepreferentially depleted at an early time, leaving onlythe low volatility components of the DNAPL remainingin overburden. Although soil sampling can be a ‘hitand miss’ exercise depending on bedding structures,it is still the most practical way to define the lateralextent of the source zone in the unconsolidateddeposits, where the DNAPL does not give riseto vapours.

DNAPL has entered the fractured sandstone and,because of the low fracture porosity, has migratedto a considerable depth. DNAPL migration is stilloccurring today due to the low density and highviscosity of the DNAPL, and because large volumesof coal tar were released. The depth of DNAPLmigration cannot be determined easily because ofthe large number of boreholes needed, but limiteddrilling will take place in the source zone to determineif the DNAPL is still in a mobile form. Monitoringwells will be completed with sumps below the wellscreen to facilitate the collection of mobile DNAPL.In many places, DNAPL has already come to restprimarily as pooled DNAPL in the horizontal fracturesand primarily as residual DNAPL in the vertical andsteeply dipping fractures. DNAPL is present primarilyin open fractures below the water table, but has alsoentered the sandstone matrix where it is composedof coarse-grained sediments that exhibit a relativelylow entry pressure. Above the water table, someareas of the sandstone matrix are either also coarsegrained or very dry, and DNAPL migration into thematrix has occurred.

Below the water table, DNAPL has migrated primarilyin the larger aperture fractures and has spread laterallyin all directions, including hydraulically up-gradient of



Fig

ure

21

Rele

ase

of c

oal t

ar D

NA

PL in

to T

riass

ic S

ands

tone

Rel

ease

of

coal

tar

DN

APL

into

Tri

assi

c Sa

nd

sto

ne

Feat

ures

Do

mai

n

1D

NA

PL p

ool s

low

lyen

teri

ng fr

actu

res

2A

que

ous

pha

se p

lum

efo

rmat

ion

3D

NA

PL d

isch

arg

e to

surf

ace

wat

er

4Po

oled

DN

APL

5M

atri

x d

iffus

ion

6Re

sid

ual D

NA

PL

7Pl

ume

in fr

actu

re

8G

roun

dw

ater

flow

dir

ectio

n

9O

n-g

oing

DN

APL

mig

ratio

n to

dep

th

Surf

ace

Dri

ft(u

nsat

urat

ed)

Dri

ft(s

atur

ated

)

Bed

rock

(sat

urat

ed)

12

3

4

7

5

6

8

9

DN

APL

rel

ease



the release location. The amount of lateral spreadingis greater than that for the chlorinated solvent DNAPLrelease depicted in Figure 20 because of the lowerdensity of the DNAPL. In Figure 21, lateral spreadinghas carried coal tar to a receiving water body.Evidence of DNAPL entry into the water body includessurface sheens and the presence of DNAPL blobs inbottom sediments. The discharge of coal tar to thereceiving water body is facilitated by the upwardcomponent to groundwater flow in this area. Theupward groundwater flow is capable of carrying thecoal tar DNAPL upwards against gravity because theDNAPL is only marginally denser than water.

Groundwater flow through the fractures containingresidual and pooled DNAPL has brought aboutDNAPL dissolution and the evolution of a dissolvedphase plume. The plume is migrating through theset of interconnected fractures subject to advection,dispersion, biodegradation and matrix diffusion.Because of the matrix diffusion process, the plumehas not migrated as far as would be predicted bygroundwater velocity calculations. At this site, theplume is migrating 50 – 500 times slower than thegroundwater with higher amounts of attenuation inthe smaller aperture fractures. Diffusion of aqueousphase contaminants into the sandstone matrix is alsooccurring immediately adjacent to the residual andpooled DNAPL. With time, a greater and greaterportion of the mass is transferred from the openfractures to the matrix.

With respect to site characterisation, efforts belowthe water table are focused on defining the spatialextent of the aqueous phase plume and establishingwhether it has reached a steady-state configuration.Although some drilling is taking place within theDNAPL source zone, the extent and depth of thesource zone in the sandstone is being inferred largelyon the basis of the spatial distribution of groundwaterconcentrations downstream of the source zone.Because the coal tar release occurred several decadesago, the more mobile, low molecular weightcomponents of the DNAPL no longer represent themajor constituents of the plume. A few indicatorcompounds are selected to define the extent of theplume and, because of their low mobility, the plumeis not expected to have advanced far ahead of theDNAPL source zone.

8.5 DNAPL release into a thinveneer of clay-rich tillFigure 22 illustrates a surface release of DNAPL intoclay-rich till overlying Cretaceous Chalk.

The relatively thin veneer of clay till contains verticalfractures, root holes and sand seams that providepreferential pathways for DNAPL migration. Residualand pooled DNAPL is retained in these pathways andDNAPL entry into the underlying fractured bedrockoccurs. Their sparse nature means that DNAPL migrationpathways may not be encountered directly by sitecharacterisation efforts conducted in the clay till.

The porous nature of the till matrix allows for adegree of migration of vapours in unsaturated regionsof the till, dependant on site-specific conditions, andfor aqueous phase diffusion of contaminants inwater-saturated regions. Soil sampling conductedin the till may reveal low levels of contaminationassociated with the diffusion halos originating fromthe fractures and sand seams. The use of angledborings can increase the probability of encounteringvertical fractures. Significant amounts of lateralDNAPL migration can occur in all directions ifhorizontal pathways such as sand seams are laterallyextensive and connected. The degree of lateralDNAPL migration is greater for lower density DNAPLssuch as creosote and coal tar, and less for highdensity DNAPLs such as chlorinated solvents. Withrespect to plume migration, matrix diffusion into theclay matrix will significantly retard the rate of aqueousphase contaminant migration relative to the rate ofgroundwater flow.

8.6 DNAPL release into a thicksequence of clay-rich tillFigure 23 illustrates a surface release of DNAPL intoa thick sequence of clay rich till. The till unit containsboth vertical fractures and sand seams which providepathways for lateral and vertical DNAPL migration.The frequency of fracturing decreases with depth,causing greater amounts of lateral DNAPL spreadingat shallow depth. As in Figure 22, the directions oflateral DNAPL spreading are dictated primarilyby geological structure and not the direction ofgroundwater flow. In cases where the till unit isexceptionally thick, it is possible that DNAPL couldnot penetrate the sequence. In practice, however,it should be assumed that DNAPL has traversed claytill sequences unless groundwater sampling beneaththe till unit (with appropriate seals around boreholesto prevent migration down the hole) reveals anabsence of contamination.

Like Figure 22, residual and pooled DNAPL areretained in the fractures and sand seams that transmittedDNAPL. Given their sparse nature, DNAPL migrationpathways may not be encountered directly by sitecharacterisation efforts.



DN

APL

rel

ease

into

th

in v

enee

r o

f cl

ay r

ich

till

Feat

ures

Do

mai

n

1Re

sid

ual D

NA

PLin

san

d le

ns

2Re

sid

ual D

NA

PL in

frac

ture

s

3D

NA

PL in

san

d le

ns

4D

iffus

ion

halo

5A

que

ous

pha

se p

lum

e

6Re

sid

ual D

NA

PL in

bed

rock

frac

ture

s

7A

que

ous

pha

se d

iffus

ion

in b

edro

ck m

atri

x

8Po

oled

DN

APL

inb

edro

ck fr

actu

re

Surf

ace

Dri

ftfr

actu

red

till

(sat

urat

ed)

Bed

rock

(sat

urat

ed)

1

23

4

7

5

6

8

Fig

ure

22

DN

APL

rel

ease

into

a t

hin

vene

er o

f cla

y-ric

h til

l

DN

APL

rel

ease



The porous nature of the till matrix allows migrationof vapours in unsaturated regions of the till and foraqueous phase diffusion of contaminants in water-saturated regions.

Soil sampling conducted in the till may reveal lowlevels of contamination associated with the diffusionhalos originating from the fractures and sand seams.The use of angled borings can increase the probabilityof encountering vertical fractures.

The degree of lateral DNAPL migration is greater forlower density DNAPLs such as creosote and coal tar,and less for higher density DNAPLs such as chlorinatedsolvents. With respect to plume migration, matrixdiffusion into the clay matrix will significantly retardthe rate of aqueous phase contaminant migrationrelative to the rate of groundwater flow. This meansthat the plume may not be far ahead of the DNAPLsource zone, particularly for compounds that exhibita high degree of sorption and moderate-to-highamounts of biodegradation.

8.7 DNAPL release into asand/gravel aquiferFigure 24 illustrates two DNAPL releases into a sand/gravel aquifer; the DNAPL on the left is creosote andthe DNAPL on the right is a chlorinated solvent. Forboth releases, residual and pooled DNAPL are presentin the unsaturated zone as well as below the watertable. There is a general lack of vapour presenceabove the water table for the creosote release, butsubstantial vapour phase contamination for thechlorinated solvent. In addition, in this example,much of the chlorinated solvent DNAPL in theunsaturated zone has vapourised itself out of existencebecause of its volatile nature. Characterisation activitiesin the unsaturated zone include soil sampling forboth releases, with the addition of vapour samplingfor the solvent release. The chlorinated solvent vapourswill have a much longer lifespan in the unsaturatedzone than residual and pooled DNAPL.

The volume of release was sufficient in each caseto reach a basal clay layer overlying bedrock. Thesolvent DNAPL encountered an erosional window inthe clay layer and has entered the fractured bedrockbelow. The creosote DNAPL did not encounter anywindows in the clay and, therefore, is restricted tothe sand/gravel aquifer. A well-defined plume hasevolved below the water table for both types ofDNAPL. The solvent plume exhibits greater spatialextent in the down-gradient direction as a result ofadvection and dispersion processes, combined withthe fact that the solvent of interest does not readilydegrade or sorb to aquifer minerals.

Site characterisation activities below the water tableinclude the use of conventional hollow-stem auguringtechniques to obtain soil samples and install monitoringwells, as well as a variety of push-device probesdesigned to obtain grab samples.

The grab groundwater sample results are used todecide the sites of permanent monitoring wells.Drilling into bedrock is approached with caution andproceeds only in areas where it has been shown thatthe sand/gravel aquifer is void of residual and pooledDNAPL. This is facilitated by taking continuous soilcores in the sand/gravel aquifer at the location ofeach proposed drilling location in the bedrock.

8.8 DNAPL release into madegroundFigure 25 illustrates a DNAPL release into madeground containing a variety of subsurface structures.DNAPL has leaked from an underground storagetank and found its way into the gravel backfill ofa building footing. Preferential migration of DNAPLtakes place along the gravel because of its lowcapillary resistance.

Figure 25 DNAPL release into made ground and theeffects of construction activities and services DNAPLalso migrates to depth along abandoned piles, whichalso provide a pathway of low capillary resistance.The presence of residual and pooled DNAPL abovethe water table outside the building enclosuresupports the potential for vapour diffusion and/oradvection into the interior of the structure. Indoorair sampling is carried out to evaluate the presenceand magnitude of vapour phase contamination.

Figure 25 also illustrates a second DNAPL releasethat has encountered the gravel backfill surroundingunderground piping. The backfill provides a preferentialpathway for DNAPL migration. DNAPL has alsoencountered an abandoned water supply well andhas short-circuited to depth along the well. Siteinvestigation techniques include conventional drillingand vapour sampling methods, but also include theuse of geophysics to locate the buried structures.



Fig

ure

23

DN

APL

rel

ease

into

a t

hick

seq

uenc

e of

cla

y til

l

DN

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rel

ease

into

th

ick

seq

uen

ce o

f cl

ay r

ich

till

Feat

ures

Do

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1Re

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PL in

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halo

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PL fl

ow t

erm

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o co

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uous

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the

till

Surf

ace

Dri

ftfr

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(sat

urat

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(sat

urat

ed)

1

2

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APL

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ease



Fig

ure

24

DN

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ease

into

an

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and/

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fer

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APL

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ated

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d g

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l aq

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atur

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ain

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e

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ater

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dir

ectio

n

3Re

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PL

4A

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se p

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5St

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rap

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(i.e

. lat

eral

dis

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ract

ured

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ock

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atri

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2

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ft(u

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ed)

Un

frac

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ent

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6

7



Figure 25 DNAPL release into made ground and the effects of construction activities and services

DNAPLrelease

Undergroundstorage tank

BuildingInterior

vapourdiffusion

andadvection

DNAPL leakagealong pile DNAPL short-circuiting

down abandoned well

vapourplume

DNAPL ingravel backfill

LEAK

DNAPL migrationalong pipe/service

corridor



9

Which parameters to measure ata site?

Objectives for site investigations are site-specificand depend on the potential source-pathway-receptorlinkages (called ‘pollutant linkages’ under Part IIAof EPA 1990), the initial conceptual site model (anduncertainties therein), and the potential risks thatneed to be assessed and managed. The objectivesof many site investigations may need to include:determining the presence of DNAPL; estimating thespatial extent of the DNAPL source zone; determiningthe presence of an aqueous phase plume; and,estimating the spatial extent of that plume, in orderto allow a robust assessment of the risks that thepollution poses. Additional objectives may include

estimating the rate of plume migration, assessingwhether the plume is at steady-state, and assessingthe degree of matrix diffusion that has occurred, alongwith a variety of other objectives that will form thebasis for selecting a remediation strategy for the site.

Figure 26, in conjunction with the tables below,provide a brief summary of the parameters that canbe measured at a site, along with a brief descriptionof how the information can be used.

Table 9 lists a variety of contaminant characteristicsthat are useful in establishing a site conceptualmodel and in selecting a remediation strategy.

Figure 26 Site investigation techniques

Other approaches

● employee interviews

● aerial photographanalysis

● outcrop mapping

● drilling

● down-hole logging

● laboratory analysis

● on-site analysis

rock

matr

ix

samples

DNAPL

sample

soil v

apour

surve

y soil b

orings

(saturat

ed zo

ne)

soil b

orings

(unsat

urated

zone)

soil s

amples

groundwate

r

sedim

ent

samples

surfa

ce w

ater

samples

monitorin

g

wells



Table 9 Contaminant characteristics to establish during site investigations

Parameter Example use of information

DNAPL density DNAPL mobility and pool height calculations

DNAPL viscosity Determine if DNAPL could still be moving

Design of NAPL recovery system

DNAPL component composition Effective solubility calculations

Predict future composition of plume

DNAPL-water interfacial tension Determine importance of capillary forces

Pool height calculations

Organic carbon partition coefficient Determine degree of aqueous phase sorption

and rate of plume migration

Contaminant half-life Determine degree of degradation and rate

of plume migration

DNAPL vapour pressure Determine if vapour migration is a potential issue;

Estimate lifespan of DNAPL above water table

Date and volume of DNAPL release Estimate of depth of DNAPL migration.

Is DNAPL still moving?

Potential DNAPL release locations Help guide monitoring well placement

Table 10 Unconsolidated deposit characteristics to determine during site investigations


Porosity Plume velocity calculation; Diffusion calculations

Dry bulk density DNAPL threshold concentration calculation

Fraction organic carbon Plume velocity calculation; DNAPL threshold calculation

Hydraulic conductivity Plume velocity calculation; Design of extraction wells

Displacement pressure Pool height calculations

Bulk retention capacity DNAPL mass estimate

Contact angle Refinement of conceptual model on DNAPL mobility

Hydraulic head distribution Directions of groundwater flow and velocity of groundwater

Bedding structures Directions of DNAPL migration

Spatial extent of DNAPL source zone Guide remedy selection and design

Spatial extent of plume Guide remedy selection; risk analysis

Measurement of the DNAPL properties requires therecovery of a sample of DNAPL from the subsurface.If this is not possible but the composition of theDNAPL is known, its density and viscosity can beestimated from literature sources.

DNAPL-water interfacial tension should not be estimatedfrom handbooks, however, as this is a site-specificparameter which is influenced strongly by even smallamounts of impurities. The organic carbon partitioncoefficient is typically obtained from literature sources,along with the DNAPL vapour pressure. The contaminanthalf-life depends on site-specific geochemical

conditions and, therefore, should not generally betaken from handbooks or the literature; thisparameter is typically determined through modelcalibration.

Information about the date and volume of DNAPLrelease is often not available, but efforts can bemade to gain information from employees and byexamining purchase records. Employee interviews,facility building plans and aerial photographs canbe used to help determine the locations of potentialDNAPL releases.



Table 10 lists a variety of parameters that may beneeded at the various stages of site investigation, riskassessment and selection of remedial or managementoptions and can be defined during the investigationof unconsolidated deposits such as sands, silts andgravels. The porosity, dry bulk density and fractionorganic carbon are tests that can be conducted onsamples of retrieved core.

Hydraulic conductivity is typically determined ona field-scale using pumping tests and slug tests. Thedisplacement pressure of capillary barriers is usuallynot measured on a site-specific basis, but can beestimated from values of interfacial tension andhydraulic conductivity. The contact angle is usuallyassumed to be such that the system is water wet inthe presence of DNAPL, but exceptions can occur.The hydrolic head distribution is typically determinedthrough quarterly water level measurements inpiezometers and monitoring wells. Bedding structuresare determined from test pits, while the spatial extentof the DNAPL source zone and plume are determinedusing the techniques discussed in Section 7.

Table 11 lists a variety of parameters that can bedetermined as part of fractured rock and clay siteinvestigations.

The matrix porosity, matrix dry bulk density andmatrix fraction organic carbon need to be measuredon samples obtained from a core. Each major rocktype should be sampled as these parameters exhibitspatial variability. Although no specific number of

Table 11 Bedrock properties to determine during site investigations


Matrix porosity Diffusion calculations

Matrix dry bulk density Estimate of remediation timeframe

Matrix fraction organic carbon Estimate of (retarded) plume velocity

Orientation of major fracture sets Determine direction of plume migration

Directions of DNAPL migration

Fracture spacing Diffusion calculations

Fracture porosity Plume velocity calculation

Bulk rock hydraulic conductivity Plume velocity calculation

Design of extraction wells

Hydraulic head distribution Directions of groundwater flow and velocity of groundwater

Bulk retention capacity DNAPL mass estimate

Contact angle DNAPL-rock-water wetting relationship

Spatial extent of DNAPL source zone Guide remedy selection

Spatial extent of plume Guide remedy selection; risk analysis

samples can be dictated, a general rule of thumbis that 5-10 samples should be obtained duringa site investigation.

The orientation of major fracture sets is typicallydetermined through outcrop mapping and down-hole geophysical surveys. The fracture spacing istypically determined using the results of down-holehydraulic testing in conjunction with core logs andan optical televiewer. It should be noted that coremay contain mechanical breaks that can be difficultto distinguish from natural fractures.

The fracture porosity is typically determined fromestimates of fracture spacing and estimates of fractureaperture determined from hydraulic testing. The bulkhydraulic conductivity is generally determined throughpumping tests and slug tests, as well as hydraulictesting using straddle packer assemblies.

The bulk retention capacity of the rock mass forDNAPL is typically estimated from fracture porosity.The contact angle is usually assumed to providea water-wet system, but site-specific measurementsmay be required if this assumption is questioned.The spatial extent of the DNAPL source zone andthe aqueous phase plume are determined usingthe approaches discussed in Section 7.



10

Basic Remediation Strategies

Remediation goals vary from site to site dependingon regulatory jurisdiction, perceived and actual risks,site use, zoning issues, and the level of fundingavailable. In the UK, remediation is typically undertakento manage unacceptable risks to human health orthe environment (Environment Agency, 1999). Thatis to say, remediation goals are risk-based. Remedialactions are usually selected taking account of theeffectiveness, practicability, durability and likelycosts and benefits of the various remediation options.The Environment Agency seeks to ensure that, withinthis framework, the most sustainable approach toremediation is selected. The area of contaminationthat is the focus of remediation may vary from siteto site, but typically involves one or both of thefollowing:

● DNAPL present within the source zone alongwith the associated aqueous and sorbed phasecontamination in the source zone. If unsaturatedmedia is involved, vapour phase contaminationmay also be addressed.

● Aqueous phase contamination present downstreamof the DNAPL source zone. This will typically havesorbed phase contamination (on the soil or aquifermaterials) associated with it and may includevapour phase contamination in unsaturated media.

These two categories distinguish between thatregion of the subsurface containing residual andpooled DNAPL (the source zone) and the associatedaqueous phase plume present down-gradient of thesource zone. This distinction is useful in that manyremediation technologies are applicable only to oneof these two zones. The most mobile form ofcontamination is often the aqueous phase plumeoriginating from a stable configuration of residualand pooled DNAPL. If DNAPL is no longer movingat a particular site, only the aqueous phase plumemigrating in groundwater and vapours in theunsaturated zone presents a means of spreadingthe extent of contamination.

10.1 Remediation goals withinthe DNAPL source zoneRemediation goals within the DNAPL source zoneshould aim to achieve risk-based objectives. Theymay include one or more of the following:

● Complete removal of all DNAPL, aqueous phase,sorbed phase and vapour phase contaminationwithin the source zone. This remediation goal istypically referred to as source zone restoration.

● Removal of sufficient DNAPL mass from the sourcezone such that the length of the resulting aqueousphase plume downstream of the source zone willbe subject to effective natural attenuation processesthat stabilise and subsequently reduce the plume.This remediation goal is typically referred to aspartial mass removal.

● Reduction of saturations within DNAPL pools toresidual levels such that the DNAPL is stabilised.This remediation goal may be appropriate at siteswhere the DNAPL is still moving or where DNAPLpools may start moving as a result of drilling,excavation or pumping activities. This remediationgoal is typically referred to as stabilisation ofmobile DNAPL.

● Hydraulic or physical containment of the DNAPLsource zone such that aqueous phase plumes canno longer expand. This remediation goal istypically referred to as source zone containment.

Source zone restoration has not yet been achievedat any sites where appreciable quantities of DNAPLare present below the water table. Source zonerestoration is an extremely challenging goal that islikely to be attainable only at well-characterisedsites exhibiting simple geology, shallow depthsof contamination and simple contaminants such assingle component NAPLs. Many of the UK geologicalsettings of contamination (such as those that includefractured rock) are likely to mean that source zonerestoration is (currently) infeasible, although incompletesource zone restoration may still bring about theachievement of risk-management objectives.



Partial mass removal is an attainable goal at somesites. However, great uncertainty remains regardingthe amount of NAPL mass removal required toachieve a specified reduction in groundwaterconcentrations down-gradient of the source zone.Because the concentration exiting the source zoneis a primary factor influencing the length of steady-state plume that will develop, there is uncertaintyregarding the relationship between mass removaland the likelihood of achieving an effective naturalattenuation remedy for the site. It has been establishedthat there is not a linear relationship between massremoval and end-point groundwater concentrations.Removing 50 per cent of the NAPL mass, forexample, does not lead to a 50 per cent reductionin groundwater concentrations. This stems from thefact that the concentration derived from residual andpooled NAPL is not related to the mass of NAPLpresent, but rather to many complicated factors suchas NAPL-water contact area, the configuration ofresidual and pools, and groundwater velocity.

The relationship between mass removal and lengthof steady-state plume is illustrated in Figure 27.Figure 27a depicts a current situation where theaqueous phase plume extends beyond the complianceboundary. The compliance boundary may be set at

a property boundary or an off-site receptor such asa river, lake or public supply well. Figure 27b illustratesa shorter steady-state plume that has been affectedas a result of NAPL mass removal within the sourcezone. The steady-state plume no longer extends tothe compliance boundary, implying that a sufficientamount of NAPL mass has been removed from thesource zone.

While partial mass removal may not bring abouta significant short-term reduction in groundwatercontaminant concentrations, it is likely to reducethe duration over which the source and plumepersist. This may be an important in achievinga sustainable solution that avoids bequeathingthe burden of pollution to future generations.

Stabilisation of mobile NAPL has been achieved at avariety of sites and can be an appropriate remediationgoal where NAPL is currently migrating or may start tomigrate in the future. This remediation goal is typicallyconsidered at sites that have had very large releases ofNAPL or at sites where the DNAPL has low mobility(for example, coal tar and creosote sites where thehigh viscosity and low density of the DNAPL leads tolong timescales for migration). The reduction of saturationswithin NAPL pools can render them immobile, but will

Figure 27 (a) Steady-state plume prior to mass removal and (b) steady-state plume

DNAPLsource zone

complianceboundary

complianceboundary

DNAPLsource zoneafter partial

mass removal

plume

plume

gro

und

wat

er fl

owg

roun

dw

ater

flow

(a)

(b)



generally not lead to a reduction of groundwaterconcentrations downstream of the source zone.

Source zone containment is often an achievableremediation goal that is implemented at sites wheresource zone restoration and partial mass removalare unlikely to be effective or will not achieve risk-management objectives. As illustrated in Figure 28,hydraulic or physical isolation of the NAPL sourcezone severs the down-gradient contaminant plume. Source zone containment typically requires long-termmaintenance activities such as groundwater extractionfrom within physical barriers or continued operationand maintenance of a pump-and-treat system.

10.2 Remediation goalsdownstream of a DNAPLsource zoneRemediation downstream of the DNAPL source zoneshould seek to manage risks to human health andthe environment, and may include one or more ofthe following goals:

● Complete elimination of the groundwater plume,including removal of all aqueous and sorbed phasecontamination. This remediation goal is typicallyreferred to as aquifer restoration.

● Interception of the groundwater plume usingeither groundwater extraction wells or permeablereactive barriers (PRBs) such that contaminationis eliminated down-gradient of the interceptionsystem. This approach is typically referred to asplume interception.

● Monitoring of groundwater concentrations andintrinsic processes to ensure that a steady-state(or ideally shrinking) plume has been achieved.This remediation approach is typically referred toas monitored natural attenuation and does notinvolve any physical human intervention (otherthan monitoring), but relies on naturally occurringprocesses to degrade and retard contaminants.

Aquifer restoration is rarely achieved at sites due tothe long periods of time required to desorb contaminantsfrom aquifer solids and the long periods of timeassociated with back-diffusion from the rock matrixand other low permeability features present in thesubsurface. Aquifer restoration requires that theNAPL source zone be either completely removed orisolated from the groundwater flow system.

Plume interception is a commonly employed approachat sites where the presence of the groundwater plumeis unacceptable. The design and construction ofgroundwater pump-and-treat systems, for example,is commonplace in many countries and has aconsiderable experience base (CIRIA, 1995).

Figure 28 Physical isolation of DNAPL source zone

physical(low permeability)

barrier rechargegroundwaterabstraction

physical(low permeability)

barrier

severedplume

gro

und

wat

er fl

ow



Monitored natural attenuation recognises the factthat, after an initial period of expansion, all groundwaterplumes will either reach a steady-state length orrecede with time as a result of natural processes.Biological degradation is not a necessary requirementfor natural attenuation, since dispersion alone willresult in a steady-state plume. Because dispersionalways occurs in the subsurface, all plumes willeventually reach a steady-state configuration. Thesuperposition of biodegradation and dispersion,however, will lead to a shorter steady-state plumelength (as illustrated in Figure 29). If degradationaccelerates later, plumes can recede and becomeshorter. For sites where the source zone containsa multi-component NAPL, the progressive depletionof higher solubility, more mobile components canalso lead to plume recession as the NAPL becomesenriched in lower solubility components.

Monitored natural attenuation typically requires goodknowledge of the current extent of contamination, agood understanding of the groundwater flow system,an appropriate monitoring well network and severalyears of groundwater quality data to establish trendsin concentration with time (Environment Agency, 2000).

10.3 Remediation technologiesThe relatively recent awareness of soil and groundwatercontamination by DNAPLs has resulted in theintroduction of several innovative remediationtechnologies during the past ten years. While sometechnologies are relatively mature, others are still atthe experimental stage and do not have a largeexperience base.

Some technologies target the DNAPL source zoneonly, while others can target both the source zoneand the aqueous phase plume. This handbook isnot intended to provide a detailed evaluation ofremediation technologies. The aim of the followingdiscussion, which provides a general overview of thevarious technologies currently available, is to helpthe reader decide which to examine further. Furtherdiscussion of DNAPL remediation technologies canbe found in Environment Agency (2001b) and ITRC(2000).

Figure 29 Steady-state plumes (a) without and (b) with biodegradation

DNAPL

DNAPL

steady-state plumewithout attenuationg

roun

dw

ater

flow

gro

und

wat

er fl

ow

(a)

(b)

steady-state plumewith attenuation

(e.g. biodegradation)



10.3.1 Groundwaterpump-and-treatThe use of groundwater extraction wells to removeaqueous phase contamination and/or contain theDNAPL source zone hydraulically is a maturetechnology with a large base of experience (see forexample CIRIA, 1998). The technology is generallysuitable for all types of groundwater contaminants.Pumping groundwater creates a capture zone,within which all groundwater eventually flows intothe extraction wells. This is depicted in Figure 30.

The extracted groundwater is typically treated exsitu in a treatment plant before being discharged toa watercourse, sewerage system or back to theground. Because of the long time required to desorbcontaminants from aquifer solids, the long timescalesassociated with back-diffusion from the rock matrixand the long time required to dissolve residual andpooled NAPL, most pump-and-treat systems operatefor many decades.

10.3.2 Permeable reactivebarriersAt sites where groundwater plumes are shallow andreadily accessible, a trench or ‘funnel and gate’ typeof PRB can be constructed and filled with suitablepermeable reactive material(s). The groundwaterplume flows naturally through the permeable barrier,within which degradation processes occur. If theresidence time in the barrier is sufficient, groundwaterconcentrations exiting the downstream side of thebarrier can achieve risk-based standards; otherwise,a PRB may be used in conjunction with monitorednatural attenuation or an active remediation technique.

Permeable reactive barriers are a passive means ofplume interception, which do not accelerate theremoval of DNAPL from the source zone. Reactiveiron is currently the most commonly employed barriermaterial. Reactive iron is only suitable for certaintypes of contaminants, however, and properscreening studies should be carried out before full-scale implementation (Environment Agency, 2002).

Figure 30 Groundwater pump-and-treat

DNAPL

DNAPL

steady-state plumewithout attenuation

gro

und

wat

er fl

owg

roun

dw

ater

flow

(a)

(b)

detached plume

pum

p-a

nd-t

reat

wel

ls



10.3.3 Physical barriersPhysical containment of the DNAPL source zoneusing sheet piling, injection grouting, secant pilingand freeze walls is a means of physically isolatingresidual and pooled DNAPL from the groundwaterflow system. A considerable base of experience existswithin the geotechnical community with respect tothe construction of such systems (see for example,CIRIA, 1996). Limited groundwater extraction istypically required from within the physical enclosureto offset infiltration and to ensure that leakage isinwards. The use of physical barriers is typicallylimited to unconsolidated deposits and requiresgood knowledge of the spatial extent of the DNAPLsource zone.

10.3.4 EnhancedbiodegradationThe injection of nutrients and other geochemicalagents to stimulate biological activity is a means ofdegrading some contaminants in situ. If these agentsare injected upstream of the DNAPL source zone,the accelerated degradation of contaminants in theaqueous phase will lead to an accelerated rate ofDNAPL dissolution and can lower concentrationsexiting the source zone.

Enhanced biodegradation is typically employed as apartial mass removal technology in order to achievea shorter steady-state plume length down-gradientof the DNAPL source zone. There is a reasonablylarge base of experience associated with thistechnology, but its application is highly dependenton site-specific conditions (for example ambientgeochemistry and contaminant composition) andproper screening tests and field pilot testing shouldbe carried out before full-scale implementation.

10.3.5 Thermal technologiesThermal technologies such as steam flooding, insitu thermal desorption (resistive heating), six-phaseheating, radio frequency heating and microwaveheating are relatively new technologies that rely onheat to vapourise and mobilise contaminants. Thermaltechnologies are applied within the DNAPL sourcezone and can be considered as a means of partial massremoval. They can be applied both above and belowthe water table, and require a soil vapour extractionsystem to contain and extract contaminant vapours.

Thermal technologies are still in the experimentalstage, but several field trials and a limited numberof full-scale applications have been completed. It iscurrently difficult to predict the amount of massremoval that can be achieved using thermal technologiesat a particular site. The technologies are generallyvery aggressive and can carry a risk of mobilisingcontamination to previously un-impacted areas.

10.3.6 Chemical flushing tomobilise contaminantsTechnologies such as surfactant flooding and alcoholflooding are relatively new technologies that can beused to mobilise and remove residual and pooledDNAPL within the source zone through either loweringinterfacial tension or enhancing DNAPL solubility.

Chemical flooding is a means of partial mass removalthat has been evaluated in several field trials with awide range of results. It is currently difficult to predictthe amount of mass removal that can be achievedusing chemical flushing at a particular site. Chemicalflushing is an aggressive means of mass removal,which can carry a risk of mobilising contaminantsinto previously un-impacted areas. In fractured rock,enhancing DNAPL solubility in fractures can lead toincreased contaminant loading to the rock matrix.

10.3.7 ExcavationThe physical removal of residual and pooled DNAPLfrom the source zone through excavation is oftenconsidered at sites where the extent of contaminationis restricted primarily to unconsolidated deposits inthe unsaturated zone. Factors to consider include theshort-term risks of exposure, cost, accessibility andwhether the completed excavation will resultin risk reduction and an improvement in air orgroundwater quality. If DNAPL is present belowthe water table, for example, the removal ofcontaminants from above the water table throughexcavation may not result in any improvement ingroundwater quality, although it may reducemigration of vapours into the atmosphere or buildings.



10.3.8 Chemical flushing todestroy contaminantsChemical agents such as oxidants can be used todestroy aqueous phase contaminants in situ. Oxidantsare typically injected upstream of the DNAPL sourcezone such that the in situ destruction (oxidation) inthe aqueous phase accelerates the rate of DNAPLdissolution.

Oxidant flooding is a relatively new technology thatcan be considered as a means of partial mass removalfrom the source zone. Several field trials and limitedfull-scale applications have been carried out witha wide range of results. When applied to fracturedrock, oxidants have the potential to diffuse into therock matrix and to destroy contaminants in situ.Common oxidants include Fenton’s reagent,potassium/sodium permanganate and sodiumpersulphate. Because the oxidant demand of anynaturally occurring organic carbon present in thesubsurface can be high, screening studies and fieldpilot testing should be carried out before consideringfull-scale application (ITRC, 2001).

10.3.9 Soil vacuum extractionSoil vacuum extraction (SVE) involves withdrawal ofair from the unsaturated zone to:

● accelerate the vapourisation of residual andpooled DNAPL;

● the volatilisation of contaminants from soilmoisture;

● the desorption of contaminants from aquifer solids.

SVE is a relatively mature technology that has seenwidespread application at sites impacted by volatileorganic compounds (VOCs). It is commonly used incombination with other techniques such as thermaltechnologies or air sparging. SVE (in isolation or incombination with other technologies) can meet soil-based remediation goals and tends to work best atsites characterised by low moisture contents andmoderate to high permeability.

Although de-watering has been considered to extendthe depth of application of SVE, this is generally notwarranted because de-watered soils are likely to exhibitlow air permeability. In addition, the removal ofcontamination from deep within the unsaturatedzone is unlikely to lead to a significant improvementin groundwater quality if DNAPLs are present belowthe water table. De-watering also carries with it a riskof mobilising DNAPL pools deeper into the subsurface.

10.3.10 Water floodingThe mobilisation and removal of pooled DNAPLthrough increases in the hydraulic gradient is referredto as water flooding, and is a means of halting DNAPLmigration and removing potentially mobile pools.The petroleum industry has considerable experienceof water flooding and the technique has beenapplied at some hazardous waste sites where largevolumes of DNAPL are present in the subsurface.

Water flooding is a source zone stabilisation technologythat is relatively straightforward to implement. It doesnot remove residual DNAPL, however, implying thatgroundwater concentration reduction may not beassociated with the mass removal activity.

10.3.11 Air spargingAir sparging refers to the injection of air below thewater table within the DNAPL source zone such thatcontaminants partition into the rising stream of airand thus accelerate DNAPL dissolution.

Air sparging is a partial mass removal technologythat has not seen widespread success at DNAPL sites,although it has been used with greater success incombination with SVE at petrol spill sites. While somemass removal has been demonstrated, intimate contactbetween residual and pooled DNAPL and the risingair is difficult to achieve because of heterogeneity.



11

Glossary

BiodegradationThe degradation of contaminants in either theunsaturated or saturated zones as a result of biologicalactivity. The rate of biodegradation depends onfactors such as the presence of micro-organismscapable of degrading the contaminant(s), availabilityof electron acceptors, temperature and the specificcontaminant of interest. Biodegradation typicallyresults in the formation of daughter products, whichmay or may not biodegrade in the system of interest.Biodegradation manifests itself as lower contaminant(parent) concentrations in groundwater and a shortersteady-state plume. If oxygen is the primary electronacceptor, the degradation process is referred to asaerobic. Anaerobic degradation occurs when oxygenhas been depleted and other electron acceptors suchas nitrate, sulphate, iron or manganese facilitatedegradation.

Capillary pressureThe pressure difference between the non-wettingfluid and the wetting fluid. Capillary pressure arisesbecause of interfacial tension. The capillary pressureis directly proportional to the interfacial tension, andinversely proportional to the radius of curvature of thefluid-fluid interface. Usually expressed in Pascals (Pa).

DensityMass per unit volume. Usually expressed in kg/m3 forliquids such as DNAPLs.

DispersionThe spreading of aqueous phase contaminants dueto small-scale velocity variations in both porous andfractured media. Because of dispersion, concentrationsdecrease towards the leading and side edges ofa plume.

DNAPL (dense non-aqueous phase liquid)A liquid that is denser than water and only slightlysoluble in water. DNAPLs exist in the subsurface asa separate fluid phase in the presence of either airor water, and can both vapourise into air and slowlydissolve into flowing groundwater. Examples includechlorinated solvents, creosote, coal tar and PCB oils.

DNAPL component compositionThe composition of a DNAPL. The various componentsthat combine to form the DNAPL phase are eachpresent at a specific mass fraction. In some cases,the DNAPL of interest may be a single componentliquid (for example, pure trichloroethene) and inother cases it may be composed of many differentchemical constituents (for example, creosote).

DNAPL dissolutionThe transfer of components present in the DNAPL tothe water phase. Over time, the DNAPL compositionwill change as certain components dissolve out ofthe DNAPL earlier than other components. Theeffective solubility of these other components willtherefore increase later.

Effective solubilityThe aqueous solubility of a compound in(ground)water, where that compound is derivedfrom a multi-component DNAPL. The effectivesolubility is proportional to the molar fraction of thatcompound in the DNAPL and the compound’s single-component solubility (as described by Raoult’s law).

Fracture entry pressureThe threshold capillary pressure required for a non-wetting fluid to enter a wetting-fluid saturated fracture.Fracture entry pressures are directly proportional tothe interfacial tension and inversely proportional tothe fracture aperture. Usually expressed in Pascals (Pa).

Fracture porosityVolume of open fractures per unit volume of bulkrock. Typical values range between 0.001 and 0.01(that is, 0.1-1 per cent).



Interfacial tensionA tensile force that exists in the interface betweenimmiscible fluids. Without interfacial tension, DNAPLswould be fully miscible (infinitely soluble) in water.The fact that interfacial tension exists between aDNAPL and water is a defining feature of a DNAPL.Interfacial tension can be measured in the laboratory;typical units are N/m and dynes/cm (1,000 dynes/cm= 1 N/m). Interfacial tension exists between any pairof immiscible fluids such as air and water, DNAPLand water, and DNAPL and air.

LNAPL (light, non-aqueous phase liquid)A liquid that is denser than water and only slightlysoluble in water. LNAPLs exist in the subsurface asa separate fluid phase in the presence of either airor water, and can both vapourise into air and slowlydissolve into flowing groundwater. Examples includefuel oils such as diesel, petrol and heating oil.

Matrix diffusionThe transfer of contaminants dissolved in groundwaterfrom open fractures to the rock or clay matrix.If concentrations are higher in the open fractures,diffusion will occur into the rock or clay matrix(forward diffusion). If concentrations are higher inthe matrix, diffusion will occur out of the rock orclay matrix into water in the fractures (back-diffusion).As a consequence of matrix diffusion, contaminantsin fractures will migrate more slowly than thegroundwater.

Porous media displacement pressureThe threshold capillary pressure required for anon-wetting fluid to enter a wetting-fluid saturatedporous medium. Lower permeability media such assilts and clays exhibit higher displacement pressuresthan more permeable media such as coarse sandsand gravels. Usually expressed in Pascals (Pa).

Residual DNAPLDisconnected blobs and ganglia of organic liquid(DNAPL) trapped by capillary forces in either porousor fractured media. Residual DNAPL forms at thetrailing end of a migrating DNAPL body as a resultof pore-scale hydrodynamic instabilities. ResidualDNAPL saturations are typically between 5 per centand 20 per cent of pore space for both porous mediaand fractures. Residual DNAPL is difficult to mobilisethrough increases in the hydraulic gradient (forexample, aggressive groundwater pumping).

PlumeA contiguous region of groundwater containingdissolved contaminants. Plumes are typically formedby the dissolution of DNAPL into groundwater andtherefore occur hydraulically down-gradient of theDNAPL source zone. Plume migration is subject toadvection and dispersion, and may also be subject tosorption, biodegradation and matrix diffusion.

Pooled DNAPLA continuous distribution of DNAPL in either porousmedia or fractures. DNAPL pools in porous mediaform above capillary barriers and typically range inboth length and thickness from several centimetresto several metres. Pooled DNAPL is potentiallymobile and is relatively easy to mobilise throughincreases in the hydraulic gradient (for example, asbrought about by groundwater pumping). DNAPLpools in fractured media tend to form in horizontaland sub-horizontal fractures rather than vertical orsteeply dipping fractures.

SorptionThe transfer of contaminants dissolved in water tothe solid phase (typically fracture walls, the surfacesof sand/silt/clay grains or the surfaces of the solidportion of the rock matrix). Sorption is typically higherfor more hydrophobic contaminants, and higherwhere greater amounts of naturally occurring organiccarbon are present on the solid surfaces of interest.

Source zoneThat region of the subsurface containing residualand/or pooled DNAPL.

Steady-state plumeThe term applied to a contaminant plume that is nolonger advancing in flowing groundwater. The timerequired to reach a steady-state configuration andthe resulting length of the steady-state plume dependon factors such as groundwater velocity and thedegree of dispersion, sorption and biodegradationoccurring.



VapourisationThe transfer of mass from the DNAPL phase to theair phase (often referred to as evaporation). The rateof vapourisation is proportional to the vapour pressureof the DNAPL, which in turn is temperature-dependent. Highly volatile DNAPLs such as somechlorinated solvents will vapourise quicker than lowvolatility DNAPLs such as PCB oils. In a multi-componentDNAPL, the individual compounds with high vapourpressures will vapourise more quickly than those withlower vapour pressures, resulting in an enrichment ofthe DNAPL in the low vapour pressure compoundsover time (referred to as weathering).

ViscosityThe shear resistance to flow of a fluid. Higherviscosity (thicker) fluids migrate more slowly inthe subsurface than lower viscosity (thinner) fluids.Viscosity is temperature-dependent and should bemeasured in the laboratory at the subsurfacetemperature of interest. Typical units include Pascalseconds (Pa s), centipoises (cP), and centistokes (cSt).

VolatilisationThe transfer of contaminants dissolved in water tothe air phase. Volatilisation is characterised by theHenry’s law constant of the dissolved contaminantof interest.

WettabilityDescribes the affinity of one fluid for a solid surfacein the presence of a second fluid. The fluid thatpreferentially wets the solid surface is referred to asthe wetting fluid and the other as the non-wettingfluid. A perfectly wetting fluid spreads spontaneouslyto coat the solid surface. A perfectly non-wettingfluid repels the solid surface and typically forms aspherical (beaded) shape on the solid surface. In manysubsurface systems, water is wetting with respect toair, DNAPL is wetting with respect to air, and water iswetting with respect to DNAPL. Wettability is quantifiedby the contact angle, which is the angle measuredbetween the fluid-fluid interface and the solid surfaceat the point of contact with the solid. Wettabilityis dependent on the chemical composition of thegroundwater, the chemical composition of theDNAPL and the chemical composition of the solidsurface of interest.



References

British Standards Institution (BSI), 2001. Investigationof potentially contaminated sites: code of practice.BS 10175:2001. BSI, London.

Cohen, R.M. and Mercer, J.W., 1993. DNAPL siteevaluation. C.K. Smoley/CRC Press, Boca Raton,Florida. ISBN 0873719778.

Construction Industry Research and InformationAssociation (CIRIA), 1996. Remedial treatment ofcontaminated land using in-ground barriers, linersand covers. Special Publication 124. CIRIA, London.

Construction Industry Research and InformationAssociation (CIRIA), 1998. Hydraulic measures for thecontrol and treatment of groundwater pollution. CIRIAReport 186. CIRIA, London.

Department of the Environment (DoE), 1995.Industry profile series. Series of 47 reports. DoE(now Department for Environment, Food and RuralAffairs), London.

Environment Agency, 1999. Methodology for thederivation of remedial targets for soil and groundwaterto protect water resources. R&D Publication 20.Environment Agency, Bristol.

Environment Agency, 2000. Guidance on theassessment and monitoring of natural attenuation ofcontaminants in groundwater. R&D Publication 95.Environment Agency, Bristol.

Environment Agency, 2001a. Guidance on thedevelopment of conceptual models and the selectionand application of mathematical models ofcontaminant transport processes in the subsurface.National Groundwater and Contaminated LandCentre Report NC/99/38/2. Environment Agency,Bristol.

Environment Agency, 2001b. Source treatment fordense non-aqueous phase liquids. R&D TechnicalReport P5-051/TR. Environment Agency, Bristol.

Environment Agency, 2002. Guidance on the use ofpermeable reactive barriers for remediatingcontaminated groundwater. National Groundwaterand Contaminated Land Centre Report NC/01/51.Environment Agency, Bristol.

Interstate Technology and Regulatory Council (ITRC),2000. Dense non-aqueous phase liquids (DNAPLs):review of emerging characterisation and remediationtechnologies. http://www.itrcweb.org/DNAPL-1.pdf.

Interstate Technology and Regulatory Council (ITRC),2001. Technical and regulatory guidance for in situchemical oxidation of contaminated soil andgroundwater. http://www.itrcweb.org/ISCO-1.pdf.

Kueper, B.H., Redman, J.D., Starr, R.C., Reitsma, S.and Mah, M., 1993. A field experiment to study thebehaviour of tetrachloroethylene below the water table:spatial distribution of residual and pooled DNAPL.Journal of Ground Water, Vol. 31, No. 5, pp. 756-766.

Mackay, D., Shiu, W.Y. and Ma, K.C., 1993.Illustrated handbook of physical-chemical propertiesand environmental fate for organic chemicals. Vol. 3Volatile organic chemicals. Lewis Publishers, BocaRaton, Florida. ISBN 0873719735.

Pankow, J.F. and Cherry, J.A.,1996. Dense chlorinatedsolvents and other DNAPLs in Groundwater. WaterlooPress, pp. 522.

Poulsen, M. and Kueper, B.H., 1992. A fieldexperiment to study the behavior of tetrachloroethylenein unsaturated porous media. Environmental Scienceand Technology, Vol. 26, No. 5, pp. 889-895.

Reynolds, D.A. and Kueper, B.H., 2002. Numericalexamination of the factors controlling DNAPL migrationthrough a single fracture. Journal of Ground Water,Vol. 40, No. 4, pp. 368-377.

US Environmental Protection Agency (US EPA), 1992.Estimating the potential for occurrence of DNAPL atSuperfund sites. US Environmental Protection AgencyOffice of Solid Waste and Emergency ResponsePublication 9355.4-07FS. US EPA, Washington DC.

12



AcknowledgementsThis handbook arose from a joint Engineeringand Physical Sciences Research Council (EPSRC)/Environment Agency funded project dealing withthe penetration of organic liquids in deep fracturedUK aquifers. In February 2000, the project team metat ICI Halochemicals in Runcorn to discuss what hadbeen learnt about the behaviour of DNAPLs in UKaquifers and what might be the behaviour of DNAPLsin a variety of geological settings. This handbook isthe product of that initial meeting. The project teamconsisted of: David Lerner (project leader), GaryWealthall, Adrian Steele (Sheffield University), GavinHarrold, Nigel Tait, Jose Valcarcel, Steve Leharne(University of Greenwich), Bernard Kueper (projectadvisor, Queen’s University, Ontario), Jonathan Smith(Environment Agency) and Richard Moss (ICI).

The authors wish to thank:

● EPSRC for the funding the project under its WastePollution Management Initiative (grant numberGR/L59191);

● the Environment Agency for co-funding the projectand for its financial support for the creation of thishandbook;

● Professor John Cherry (University of Waterloo,Ontario) for regularly inviting team members tomeetings of the University Solvents in GroundwaterConsortium which did much to bring the teamup to speed on the very latest issues of DNAPLsin aquifers;

● ICI for making available facilities for our initialmeetings and for supporting the project frominception through to completion.

The Agency acknowledges those individuals whoreviewed the final report and provided helpfulcomments.



The height of DNAPL that can accumulate abovea capillary barrier below the water table can beestimated using Equation 8 (Kueper et al., 1993):

Equation 8

where:

H is the height of pooled DNAPL;

Pc is the capillary pressure at the base of the pool;

Pc is the capillary pressure at the top of the pool;

PD is the DNAPL density;

PW is the groundwater density;

g is the acceleration due to gravity.

Equation 8 assumes that a hydrostatic system exists;for the pool height in a flowing groundwater system,see Longino and Kueper (1995). The maximum stablepool height is obtained by setting Pc equal to thedisplacement pressure of the capillary barrier. Thecapillary pressure at the top of the pool, Pc , willdepend on how the pool was formed. A reasonableassumption is to set the capillary pressure at the topof the pool equal to the terminal pressure of themedium containing the pool.

Equation 8 also assumes that water is wetting withrespect to DNAPL. The influence of a non-zerocontact angle is incorporated into the choice of Pcand Pc . Equation 8 represents a one-dimensionalforce balance and therefore assumes that the lateralsides of the pool are constrained by media withappropriate displacement pressures. Shorter equilibriumpool heights will arise in media that do not providelateral capillary resistance of the pool. The calculationsused to create Figure 4 (see Section 3) are summarisedin Table A1.

References

Kueper, B.H., Redman, J.D., Starr, R.C., Reitsma, S.and Mah, M., 1993. A field experiment to study thebehaviour of tetrachloroethylene below the water table:Spatial distribution of residual and pooled DNAPL.Journal of Ground Water, Vol. 31, No. 5, pp. 756-766.

Longino, B.L. and Kueper, B.H., 1995. The use ofupward gradients to arrest downward DNAPL migrationin the presence of solubilizing surfactants. CanadianGeotechnical Journal, Vol. 32, No. 2, pp. 296-308.

DNAPL poolcapillary barrier

Pc – Pc

(PD - PW )gH =

“

‘

“

“ ‘

‘

“

‘Figure A1 DNAPL pool above capillary barrier. The capillary

barrier is assumed to be able to provide both verticaland lateral resistance to pool movement in ahydrostatic system.

Appendix A:

DNAPL pool height abovecapillary barrier

A



Table A1 Maximum DNAPL pool heights (PW = 1,000 kg/m3 and g = 9.806 m/s2 for all calculations)

Creosote/ Chlorinated Mixed DNAPL

coal tar solvent 3c

Silt PD (kg/m3) 1050 1460 1100

(K=1x10-6m/s) Pc (Pa) 4750 6650 2375

Pc (Pa) 2375 3325 1188

IFT (N/m) 0.020 0.028 0.010

Pool height (m) 4.844 0.737 1.211

Fine sand PD (kg/m3) 1050 1460 1100

(K=1x10-5m/s) Pc (Pa) 2400 3360 1200

Pc (Pa) 1200 1680 600

IFT (N/m) 0.020 0.028 0.010

Pool height (m) 2.447 0.372 0.612

Medium sand PD (kg/m3) 1050 1460 1100

(K=1x10-4m/s) Pc (Pa) 800 1120 400

Pc (Pa) 400 560 200

IFT (N/m) 0.020 0.028 0.010

Pool height (m) 0.816 0.124 0.203

Course sand PD (kg/m3) 1050 1460 1100

(K=1x10-3m/s) Pc (Pa) 100 140 50

Pc (Pa) 50 70 25

IFT (N/m) 0.020 0.028 0.010

Pool height (m) 0.102 0.016 0.025

IFT = DNAPL-water interfacial tension

“

‘

“

‘

“

‘

“

‘



Appendix B:

Fracture aperture required to stopmigration in bedrock

Consider the vertical accumulation of DNAPL in afracture network depicted in Figure B1. It is assumedthat there are no vertical components to groundwaterflow and that the DNAPL has come to rest becauseof a narrowing of fracture apertures in the vicinity ofpoint A. The narrowing of fracture aperture providesthe capillary resistance to support the overlyingdistribution of DNAPL. The graph on the right handside of Figure B1 shows that both the groundwaterand DNAPL pressures increase linearly with depth;thus, the capillary pressure also increases linearlywith depth. The relationship between the verticalheight of accumulated DNAPL (H) and the fractureaperture at A required to support the accumulationof DNAPL is given by:

Equation 9

where:

H is the vertical height of accumulated DNAPL;

is the DNAPL-water interfacial tension;

O is the contact angle;

PN is the DNAPL density;

PW is the groundwater density;

g is the acceleration due to gravity;

e is the fracture aperture.

Equation 9 assumes that the top of the DNAPLaccumulation exists at a capillary pressure of zero.In cases where the top of the accumulation is at anon-zero capillary pressure, a simple adjustment canbe made to the equation. Table B1 summarises thecalculations used to produce Figure 11 (see Section 5).

Figure B1 Vertical accumulation of DNAPL in a fracture network. Assuming a hydrostatic system, capillary pressure increases linearlywith depth (modified after Kueper and McWhorter, 1991).

2 cosO

(PN - PW )geH =

B



IFT = DNAPL-water interfacial tension. All three DNAPLs are assigned IFT = 0.020 N/m.

DNAPL densities are: 1,100 kg/m3 (mixed composition); 1,050 kg/m3 (coal tar/creosote) and 1,460 kg/m3 (TCE).

References

Kueper, B.H. and McWhorter, D.B., 1991. Thebehaviour of dense, non-aqueous phase liquids infractured clay and rock. Journal of Ground Water,Vol. 29, No. 5, pp. 716-728.

Height of Mixed composition Coal tar/creosote (µm) TCE (µm)accumulation (m) DNAPL (µm)

0.1 407.9 815.8 88.7

0.2 203.9 407.9 44.3

0.5 81.6 163.2 17.7

1.0 40.8 81.6 8.9

2.0 20.4 40.8 4.4

3.0 13.6 27.2 2.9

4.0 10.2 20.4 2.2

Table B1 Vertical accumulation of DNAPL in a fracture network. The fracture aperture (in µm) required to supportaccumulation of DNAPL is given (PW = 1000 kg/m3 and g = 9.806 m/s2 for all calculations).



Appendix C:

Soil concentration calculation inthe absence of DNAPL compositionanalysisFor a multi-component DNAPL of unknowncomposition, the sum of the mole fractions mustequal one. DNAPL will therefore be present in a soilsample if the following condition is met:

Equation 10

where:

is the reported concentration of component i;

is the single component soil concentration ofcomponent i;

n is the total number of components observed inthe soil sample.

As an example, consider a soil sample that has beenshown by the laboratory to contain trichloroetheneat a concentration of 145 mg/kg, tetrachloroetheneat a concentration of 155 mg/kg, tetrachloromethaneat a concentration of 200 mg/kg, chlorobenzene ata concentration of 177 mg/kg and 1,1,1-trichloroethaneat a concentration of 213 mg/kg. Table C1 illustratesimplementation of Equation 10 to determine whetherthese concentrations correspond to the presence ofa multi-component DNAPL in the soil sample. Thesoil sample has been collected from below the watertable in a sand aquifer characterised by a porosity of25 per cent, a fraction organic carbon of 0.003 anda dry bulk density of 1,990 kg/m3. The last columnof Table C1 sums to greater than 1.0, indicating thatDNAPL was present in the soil sample.

∑ CobsT

CST

≤1i = l

n

CobsT

CST

Table C1 Soil concentration calculation for multi-component DNAPL

Compound Koc Solubility(mg/kg) (l/kg) (mg/l) (mg/kg)

trichloroethene 145 126 1,100 554 0.262

tetrachloroethene 155 364 200 244 0.636

tetrachloromethane 200 439 790 1,140 0.175

chlorobenzene 177 330 500 558 0.317

1,1,1-trichloroethane 213 152 1320 768 0.277

SUM = 1.668

CobsT

CST

CSTCobs

T

C


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