design, construction and operation of a funnel and gate in-situ permeable reactive barrier for...

DESIGN, CONSTRUCTION AND OPERATION OF A FUNNEL ANDGATE IN-SITU PERMEABLE REACTIVE BARRIER FORREMEDIATION OF PETROLEUM HYDROCARBONS IN

GROUNDWATER

TERRY McGOVERN1, TURLOUGH F. GUERIN2∗, STUART HORNER1

and BRENT DAVEY3

1 SRS Australia Pty. Ltd., Environmental Consultants, Werribee, Victoria, Australia; 2 ShellEngineering, NSW State Office, P.O. Box 26, Rosehilly, New South Wales 2142, Australia; 3 Prpic

Davey Consulting, Murrumbeena, Victoria, Australia(∗ author for correspondence, e-mail: [email protected], phone: 61-417-124453)

(Received 27 November 2000; accepted 4 April 2001)

Abstract. A white spirit spill at a factory site located in a residential area of south eastern Australialed to contamination of shallow groundwater that fed into a nearby river. The contaminated ground-water contained toluene, ethyl benzene, and xylene and n-alkanes in the C6–C36 fraction range. Afunnel and gate permeable reactive barrier was designed and built, based on preliminary pilot scaletests using peat as the medium for the gate and the work conducted is presented as a case study.The technical effectiveness of the funnel and gate, over the 10 month operating period in which datawas collected, indicates that peat represents an effective material for use in the gate component offunnel and gate remedial systems. The application of the funnel and gate technology represented asubstantial saving to the client and was effective in preventing ongoing pollution of the nearby river.The construction of the funnel and gate system also incurred the minimum disturbance to the publicaccess areas between the facility and the river.

Keywords: case study, dissolved phase, efficiency, groundwater, passive remediation, peat, per-meable barriers, petroleum hydrocarbons, remediation, sparging

1. Introduction

Treatment walls, or permeable reactive barriers, first reported by McMurty andElton (1985), involve construction of permanent, semi-permanent, or replaceableunits across the flow path of a dissolved phase contaminant plume (Starr andCherry, 1994; Vidic and Pohland, 1996). As the contaminated groundwater movespassively through the treatment wall, contaminants are removed by physical, chem-ical and/or biological processes, including precipitation, sorption, oxidation/reduc-tion, fixation, or degradation. These barriers may contain agents that are placedeither in the path of contaminant plumes to prevent further migration or immedi-ately downgradient of the contaminant source to prevent plume formation (Vidicand Pohland, 1996).

Water, Air, and Soil Pollution 136: 11–31, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

12 T. McGOVERN ET AL.

Several methods have been developed for the installation of permeable treat-ment walls (Eykholt and Sivavec, 1995; Steimle, 1995). The majority of experiencewith installation of these walls is with relatively shallow emplacements (<10 m)using standard geotechnical design and construction approaches, although a fewtechnologies for deeper installations have been identified. In the simplest case, atrench of the appropriate width can be excavated to intercept the contaminatedstrata and backfilled with reactive material. This method would normally be limitedto shallow depths in stable geologic materials.

The major advantages of permeable treatment wall technology over other ground-water remediation approaches are the reduced operation and maintenance (O&M)costs and the enhanced technical efficacy, particularly compared with pump-and-treat approaches (Thomas and Ward, 1995; Clark et al., 1997). Other than ground-water monitoring, the major factor affecting O&M costs is the need for periodicremoval of precipitates from reactive media or periodic replacement-rejuvenationof the affected sections of the permeable wall. The key issues associated with thedesign of a treatment wall include the selection of the reactive media (chemistry,particle size distribution, proportion and composition of admixtures), residencetime in the reaction zone, and the reaction zone size for optimum life span, aswell as the effect of the reaction zone medium on groundwater quality and theultimate fate of a treatment wall, which may impact upon its disposal. Selectionof the reactive media is based on the type (i.e., organic vs. inorganic) and concen-tration of groundwater contaminants to be treated, groundwater flow velocity andwater quality parameters, and the available reaction mechanisms for the removalof contaminants (i.e., sorption, precipitation, and biodegradation).

There are disadvantages in using permeable treatment wall technology. A criti-cism that the life expectancy of the reactive media in a treatment wall may degradewith time has been addressed by developing a construction approach whereby thereactive media is placed in the subsurface in removable cassettes (Day et al., 1999).Another criticism is the high capital costs in setting up such systems and the highlyspecificity of barrier wall materials towards particular contaminants.

Typically, treatment wall system design is based on the results of treatability oreffectiveness studies that can incorporate both batch reaction tests and laboratory-or field-scale column experiments (Rael et al., 1995). Batch tests are conductedto obtain initial measures of media reactivity (i.e. degradation half-life, sorptionkinetics and capacity) that form the basis of the treatment wall design. Column testsare typically conducted by packing a vertical column with the reactive mediumand passing the contaminated groundwater through the column until steady-stateperformance of the reactor is obtained. Flow velocities are adjusted to simulategroundwater velocity and reactor residence time. In addition, the impact of thetreatment wall on groundwater quality can be assessed from these studies. The lifespan of sorption and precipitation barriers is limited by the ultimate capacity ofthe medium to facilitate appropriate removal reactions. Once the ultimate capacityof the medium is exhausted, contaminant breakthrough will occur, and this has

DESIGN, CONSTRUCTION AND OPERATION OF A FUNNEL AND GATE 13

previously been observed with peat used to remove dissolved and free phase andpetroleum hydrocarbons from water (Stehmeier, 1989). In addition, contaminantrelease or resolubilisation may occur.

The funnel and gate system is an application of a permeable reactive barrierfor in situ treatment of dissolved phase contamination (Starr and Cherry, 1994).Such systems consist of low hydraulic conductivity cut-off walls (e.g. 1 × 10−6 cms−1) with one or more gaps that contain permeable reaction zones. Cutoff walls(the funnel) modify flow patterns so that groundwater primarily flows through highconductivity gaps (the gates). The type of cutoff walls most likely to be used in cur-rent practice are slurry walls, sheet piles, or soil admixtures applied by soil mixingor jet grouting. At the current time there are relatively few published reports on thefull-scale design and operation of funnel and gate systems and related systems fordissolved phase petroleum hydrocarbons (Bowles et al., 2000; Schad and Schulze,2000) with even fewer reporting the use of peat (Kao and Wang, 2000). The currentarticle presents the findings of a case study on the use of a funnel and gate systemin Australia.

2. Background

A white spirit petroleum hydrocarbon spill occurred at a factory facility in SouthEastern Australia in December 1997. This volatile petroleum hydrocarbon wasused routinely as a solvent at the facility (Table I). A leaking underground storagetank (UST) caused a quantity of the white spirit (∼2000–3000 L) to leak throughthe scoria fill material in which the UST was located, into the soil of the upperembankment at the rear of the facility. A quantity of the white spirit emerged atthe base of the slope, discharging into a spoon drain, and then via a culvert, flowedinto the nearby river. A proportion of the leaked white spirit was absorbed by thesoil in the embankment and an amount entered the groundwater underlying the site(Figures 1 and 2).

3. Study Objective

The aim of the remedial works conducted in this study was to intercept the dis-solved phase plume of petroleum hydrocarbons present in the shallow aquifermoving toward the river, using a funnel and gate permeable barrier, and reduce con-centrations to less than regulatory limits. A funnel and gate system was consideredto be the most cost-effective way for treating petroleum hydrocarbon contaminatedgroundwater and preventing further contamination of the river. In the proposed re-mediation strategy, contaminated groundwater flow would be directed towards thegate which would then be subject to two treatment processes, applied in sequence,within the funnel and gate system. The processes applied were (1) air sparging, as


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TABLE I

Composition of White Spirita

Component Name % (v/v)

C6–C9 Benzene 0.07%

Toluene 0.04%

Ethyl benzene 1.10%

Total xylene 20%

Total C6–C9 ∼23%

C10–C14 n-Alkanes 77%

C15–C28 n-Alkanes <1%

C29–C36 n-Alkanes <1%

a CAS number is 8052-41-3. White spirit isa dry cleaning solvent, also known as mineralspirits or Stoddard solvent and is a colorlessliquid with a kerosene-like odor.

previously described (Pankow et al., 1993), and (2) absorption and biodegradationwithin a mixture of blended peats as previously highlighted (Cohen et al., 1991;Couillard, 1994).

The development of the remediation strategy for the site and treatment of thecontaminated groundwater after the white spirit spill is outlined in Table II.

4. Site Description and Methodology

4.1. SITE LAYOUT AND GEOLOGY

The area between the rear of the facility and the nearby river consists of a relativelysteep (∼45◦) embankment, which is bisected by a cycle path of approximately2 m width. A spoon drain runs parallel to the western side of the access track,terminating approximately 20 m north of the northern boundary of the site in aculvert, which runs under the cycle path and discharges directly to the nearby river.The facility occupies relatively level ground, exhibiting a lesser incline towards theriver. On site structures consist of a large 2 storey building, the lower floor of whichis used for hide tanning activities. A tank farm is located in the north eastern cornerof the premises, consisting of four underground storage tanks, which are devotedto either fresh or used white spirit (Figure 1).

The geology of the site consists of basement siltstones and sandstones of theupper Silurian Dargile Formation, the upper profile of which has weathered to atan-orange mottled sandy clay. These sediments are visible in outcrops at severallocations along the banks of the nearby river. Variably weathered lavas of the Qua-ternary Newer Basalts formation overlie the Silurian sediments in the eastern half


TABLE II

Development of the remediation strategy after the White Spirit spill

Key activity Date Tasks

Control and containment December 1997 Boom and sorbents placed in river to capture

of spill spilled solvent leaving the site

Removal of leaking tanks

Initial characterization December 1997– Sampling of soil and groundwater

of soil and groundwater February 1998 Developed model of contamination

contamination and its at the site

extent

Selection of remediation March 1998 Selection of funnel and gate option

strategy

April 1998 Feasibility study to determine optimum

configuration of peat

Identified options for remediating

petroleum hydrocarbon contaminated soil

Implementation of March–May 1998 Installation of funnel and gate

remediation strategy

Remediation of contaminated soil excavated

from the site

Monitoring July 1998– Initial 10 month groundwater

May 1999 sampling and analysis program was

conducted to assess effectives of funnel

and gate system

Review of effectiveness March 1998– Soil remediation was validated by sampling

of remediation strategy August 1998 and analysis of the recirculating air during

bioventing, and sampling and analysis of the

soil in the biovented piles for BTEX and

n-alkanes

September 2000– Three monthly groundwater sampling and

Onwards analysis program is planned

of the site, as identified in monitoring wells (MW) 9 and 10. Varying thicknesses offill comprise the upper and lower embankments which is of a highly heterogeneousnature.


4.2. SITE HYDROGEOLOGY

A highly localised, shallow perched water table is present in the fill material whichcomprises the embankment between the factory and the nearby river. The regionalwatertable is expected to be located at approximately the same depth as the levelof the river. Residual clays from the weathered Silurian siltstones tend to be imper-meable, and as such perched water tends to be located at the fill-natural sandy clayinterface.

4.3. AQUIFER TESTING

Aquifer recovery tests were carried out on monitoring wells MW1, MW2, MW3,MW4, MW5, MW6, MW7, and MW8 on 20 November 1998. The procedureinvolved the removal of a known volume of groundwater from the well and mon-itoring the subsequent recovery with time. The hydraulic properties of the fillmaterial were investigated (by slug test) to determine the hydraulic conductivityof this material. The Hvorslev method (Fetter, 1988) was used to calculate thehydraulic conductivity in the immediate proximity to each well tested.

4.4. CONSTRUCTION OF FUNNEL AND GATE SYSTEM

A volume of ∼1600 m3 of petroleum hydrocarbon contaminated soil was excavatedas part of establishing the funnel and gate system. This was placed in 3 bioventedpiles at the facility (refer to biobanks in Figure 1). Details of the remediation of thissoil are not given in this report. The funnel and gate system was comprised of animpervious barrier membrane (i.e. the funnel), directing groundwater into the treat-ment area (i.e. the gate). The gate consisted of a sparging unit upgradient of a blendof peat materials. The funnel component, designed to intercept the contaminantplume before it entered the nearby river, consisted of a 0.75 mm thick High-Density Polyethylene (HDPE) impervious barrier membrane positioned verticallyin the cut-off trench to capture and redirect incident groundwater over the lengthof the spill area, parallel to the nearby river. The gate was composed of sequentialtreatment systems comprising a sparging unit emplaced in basaltic scoria, followedby blended peat materials. The funnel was designed to intercept groundwater flowfrom areas directly downgradient of the spill site, as well as adjacent areas in whichlateral migration of the plume may be occurring. The dimensions of the funneltrench are 27 m long × 5 m deep × 0.6 m wide, excavated to a level of 0.5 mbelow the fill-natural siltstone interface.

4.4.1. Gate ConstructionA stepwise procedure was undertaken so as to ensure the final integrity of thegate and prevent the collapse of the riverbank. This was achieved in the followingmanner. Chain mesh fencing (2 m × 15 m long section) was laid flat along theriver-embankment interface and fastened to the ground using 1.5 m long steel star


Figure 3. Transect through the funnel and gate system (as built) (refer to Section A–A, Figure 2).

pickets to aid long term stability. Sheet piling was inserted into the embankmentparallel to the river bank, at a distance of approximately 4.0 m from the river,dividing the areas to be excavated for both components of the gate system. The areadesignated for the peat (between the river bank and the sheet piling) was excavatedto a depth of 0.5 m below the siltstone-fill interface, and infilled with peat to a depthof 0.5 m below surface. The remaining volume was filled with natural soil overlyinga layer of A15 Type Bidim Geofabric to prevent fine soil particles mixing with andclogging the peat and scoria filtration media. The area was then sown with grassseed in order to promote bank stability. Excavated soil was removed for bioventing.Subsequent to the emplacement of the peat, soil adjacent to the opposite side of thesheet piling was excavated to a depth of 0.5 m below the fill-natural siltstone stratainterface, and a horizontal sparging tube and scoria was emplaced. Thirty cubicmeters of peat (on a wet volume basis of estimated at 40% moisture) was used inthe gate.

4.4.2. Sparging Component of GateThe section of the gate that initially encounters influent groundwater consists of avolume (∼5 m3) of porous basaltic scoria. The scoria was emplaced over a sub-merged, perforated air sparging pipe, creating fine air bubbles, which permeate thewater column within the gate, delivering oxygen to enhance biodegradation. Airwas supplied to the system by a 12-cfm compressor, supplying air at 0.9 m3 hr−1.Soil gas and air concentrations of petroleum hydrocarbons were monitored at thesite using a Foxboro TVA 1000A Flame Ionization Detector (FID) and a Photo-Ionization Detector (PID) unit. A transect diagram through the gate is given inFigure 3.


4.4.3. Peat Composition and Implications for Contaminant RemediationAfter the groundwater was sparged (as it passed through the scoria as describedabove), the groundwater then passed through a peat mixture, immediately down-gradient of the sparging system. Previous studies indicate the high sorptive ca-pacities of humic materials (such as peat) for petroleum hydrocarbons (Kao andBorden, 1997). Peats with low fibre contents and high lignin pyrolysis materialand ash content have been shown to be the most effective peats to remove free anddissolved phase petroleum hydrocarbons from groundwater (Cohen et al., 1991).The ability to both adsorb dissolved and free phase petroleum hydrocarbons fromgroundwater, and provide a catalytic surface on which microbial activity can occur,indicated the suitability of peat for use in groundwater treatment (Cohen et al.,1991). Cohen et al., (1991) have indicated that a wide range of peats is effectivein removal of petroleum hydrocarbon contamination from water, with rates of re-moval efficiency between 63 and 97%. Using this information, a locally availablepeat, known commercially as Biogreen Humic Reed Sedge, with low fibre and highhumic content was selected for use onsite.

4.4.4. Peat Blending and Implications for Hydraulic ConductivityAn important consideration in the design and construction of funnel and gate sys-tems is that the treatment media be of an equal or greater hydraulic conductiv-ity than that of the surrounding substrate, to allow incident groundwater to bechanneled through the treatment aperture. This is particularly important as pre-vious studies have shown that the variability of the saturated hydraulic conduct-ivity of peat can be as high as 10 orders of magnitude, depending on the typeof peat. Accordingly, hydraulic conductivity tests were performed on a varietyof peat/sphagnum peat/cocoa fibre blends, in order that an optimal mixture, withrespect to sorptive capacity, air circulation and hydraulic conductivity, be achieved.Tests were conducted using the ‘falling head test’ method (Schwartz and Domen-ico, 1997). Equipment consisted of a column of packed, saturated peat. The fall inhydraulic head over time (K) was measured from a manometer tube, and substi-tuted in the following equation:

K = 2.3(aL/A(t1 − t0)) log10 h0/h1 ,

where

a = cross sectional area of the stand pipe,

L = length of sample,

A = cross sectional area of sample tube,

(t1 − t0) = elapsed time required for a head fall from h1 to h0,

h0 = starting height of water in the manometer tube,

h1 = end height of water in the manometer tube.


A blend of 70% peat (Biogreen Humic Reed Sedge), 20% sphagnum peat and 10%cocoa fibre was selected for use. This mix contained the maximum percentage ofthe peat with the highest sorptive capacity (Biogreen Humic Reed Sedge), withoutreducing the hydraulic conductivity to below that encountered in the surroundingfill materials.

5. Monitoring Well Installation, Sampling and Analysis

Effective funnel and gate design ensures sufficient residence time of contaminantswithin the treatment zone as well as downgradient substrata permeability, whichprevents contaminated water from ‘banking up’ and circumventing the funnel andgate. Testing of the funnel and gate was achieved via two groundwater monitoringwells, one emplaced immediately upgradient of the gate, and another downgradientof the gate area infilled with peat. By comparing influent and effluent concentra-tions of n-alkanes and BTEX compounds, it has been possible to determine theeffectiveness of the funnel and gate system for removing dissolved phase petro-leum hydrocarbon contamination. The monitoring system was set up such that anybreak-throughs of petroleum hydrocarbons (through the gate) could be detected.Furthermore the bores at the site were arranged so that any petroleum hydrocarbonsescaping the gate (by going around the funnel) would be detected.

A total of 14 monitoring wells were installed at the site. All monitoring wellswere fully screened from the watertable to the end of the boreholes. Since the estab-lishment of the monitoring network in May 1998, the wells have been sampled on afortnightly basis for a period of 5 months, followed by a less intensive sampling re-gime up to September 2000, as agreed to by the local regulatory authorities. Stand-ing water levels were measured prior to well purging and sampling on all occasions.All groundwater samples were analysed for n-alkanes (by GC-FID method, USEPA 8021B), and BTEX using the purge and trap method (US EPA Method 5030).All soil and groundwater samples were collected in Teflon (PTFE)-lined screw capglass jars and keep on ice in coolers (∼4 ◦C) and sent for analysis within 24 hr ofcollection. Recoveries for spiked n-alkanes varied from 75–100% in groundwatersamples. Recoveries for BTEX were 85–100% in groundwater samples. Laborat-ory duplicates reported variation for n-alkane fractions and BTEX of 5–25% ingroundwater samples.


TABLE III

Volumetric budget summary for the steady state calibrationsimulation

Flux Description/origin Volume

(L day−1)

Into model Recharge 76

Bank seepage 3539

Out of model Constant heads (river-seepage) 3591

% Discrepancy 0.65

6. Results and Discussion

6.1. WATER TABLE LEVEL AND HYDRAULIC CONDUCTIVITY TESTS

The water table configuration remained relatively constant throughout the monit-oring period. The groundwater flow lines were observed to flow toward the east,approximately perpendicular to the bank of the river. Figure 4 illustrates the config-uration of the water table (m AHD) on 7 October 1998. Variations in the water tablewere observed with monitoring wells MW1, MW2, and MW3 illustrating similartemporal changes. These wells are furthest from the nearby river, located at thebase of the slope. Closer to the river the water table variation is significantly less,indicating that the water table close to the river is being controlled by the waterlevel in the river itself. Those wells located furthest from the river were expectedto reflect rainfall (recharge) water table response relationships. As expected, thewells nearest the slope fluctuate the most in response to rainfall events, suggestingrecharge is occurring from the slope to the west, most likely as interface drainage.

The results of the subsurface conductivity tests are presented in Table IV. Asexpected the material was found to have extremely variable hydraulic properties.Measurement of the conductivity of the fill material was in some instances erro-neous as the well screens extended through two lithologies, the clay overlying thesiltstone and the fill material itself. The large variability of the fill material suggeststhat the majority of groundwater and contaminant migration through this materialwill be occurring along preferential flow paths. This is likely to result in fastermigration times and a larger spread of the plume.


Figure 4. Measured and interpolated piezometric surface on 7 October 1998 (units in m).

6.2. ESTIMATES OF GROUNDWATER VOLUMES AND PETROLEUM

HYDROCARBON MASS ENTERING FUNNEL AND GATE SYSTEM

From modelling conducted, the concentrations of ethyl benzene and xylene in thegroundwater discharging to the river (ignoring mixing zones) would exceed therelevant criteria, 250 and 620 µg L−1, respectively, within 6–12 months and wouldcontinue to exceed these criteria for a period in excess of 6 yr. The modellingestimated that the width of the discharge face (capture zone) was estimated at∼20 m.


TABLE IV

Measured hydraulic conductivity ofsubsurface

Monitoring well Values

(m day−1)

MW01 0.18

MW02 1.63 × 10−8

MW03 1.26 × 10−5

MW04 0.0001

MW05 0.0046

MW06 0.0001

MW07 0.025

MW08 0.68

The maximum concentrations of n-alkanes and the total of benzene, toluene,ethyl benzene and xylenes (BTEX) present in groundwater from bore wells acrossthe site were 1000 and 25 mg L−1, respectively. These values were reported betweenDecember 1997 and December 1998.

The average concentration of n-alkanes in groundwater over the period fromDecember 1997 to August 1999, was 26 mg L−1 of n-alkanes, and the value forBTEX was considerably lower at <0.05 mg L−1. The predicted amount of petro-leum hydrocarbons likely to be intercepted by the proposed funnel and gate systemare conservative estimates designed to encompass the worst case scenario. Basedon a median hydraulic conductivity of 0.4 m day−1 (estimated from the site data),the median volume of groundwater entering the gate was estimated to be ∼6 m3

day−1. The maximum volume of water entering the gate, based on a conservativeestimate, was expected to be ∼43 m3 day−1. Assuming the average concentrationof petroleum hydrocarbons in groundwater was 26 mg L−1, the maximum mass ofpetroleum hydrocarbons entering the funnel and gate was calculated to be 1.1 kgday−1, and a median value of 160 g day−1.

Assuming a median removal capacity of 20 kg t−1 peat (dry weight basis), asdetermined from previous studies with a range of peats tested for absorbing monoaromatic hydrocarbons (Cohen et al., 1991), it was expected that the petroleumhydrocarbons in the groundwater entering into the funnel and gate (i.e. 1.1 kgday−1), would be treated by the system (as designed) for up to ∼210 days beforebreak through of petroleum hydrocarbons. This period would be longer at 1448days, assuming 160 g day−1 of petroleum hydrocarbons in the influent, entering thegate. These estimates, derived from Cohen et al. (1991), are conservative becausethey do not take into account a highly active microbial population which would


be contributing to the apparent absorption (i.e. removal) process in the gate atthe current site. Previous studies have shown that such microbial populations arereadily established in peat when oil is added (Jack et al., 1994). These estimatesdo, however, provide some indication of the size and design of the gate requiredfor removing the dissolved phase petroleum hydrocarbons. At 306 days, the overallperformance of the funnel and gate has been satisfactory. Further monitoring willreveal the effectiveness of the design (see Implications section of this article).

6.3. PETROLEUM HYDROCARBON REMOVAL EFFICIENCIES

Initial contaminant concentrations determined from samples collected on 1 July1999, i.e day ‘zero’, in the well upgradient of the funnel and gate (referred to asMW 13) were equivalent to those downgradient (of the funnel and gate treatment)(referred to as MW 14) (Table V). The expected decrease of n-alkanes and toluene,ethyl benzene, and xylene concentrations across the funnel and gate system was notapparent in this initial round of sampling and analysis. No benzene was detectedin either MW 13 or MW 14 over the 10 month monitoring period. Subsequentmonitoring from groundwater monitoring wells MW 13 and MW 14, conductedover the following 10 month period that has been reported, indicated a substantialand consistent reduction of petroleum hydrocarbon contaminants in the ground-water due to passage through the funnel and gate system. Concentrations of bothn-alkanes and toluene, ethyl benzene, and xylene, have substantially decreased inconcentration between MW 13 (upgradient) and MW 14 (downgradient) of thefunnel and gate.

The removal efficiencies for the as-built funnel and gate system varied from63 to 96% for the monoaromatic hydrocarbons, toluene, ethyl benzene, and xy-lene. Removal efficiencies for C6–C9, C10–C14, and C15–C28 n-alkane fractionswere 69.7, 81.1 and 67.2%, respectively. The lowest removal efficiencies were forthe C29–C36 n-alkane fraction at 54%. The overall efficiency for the funnel andgate system was 72% during the 10 month period over which data was collected(Table VI). Removal efficiencies tended to decrease at 188 and 306 days withrespect to toluene, ethyl benzene, and xylene and the C6–C9 n-alkane fraction.Further sampling and analysis in the future will ascertain whether this represents atrend in decreasing performance of the funnel and gate. The apparent increase inconcentrations of both the n-alkanes and toluene, ethyl benzene and xylenes in theupgradient well at day 245 correlate with a preceding heavy rain in days before thesamples were taken at that monitoring time.

Previous studies have shown that peats from various sources can remove 63–97% dissolved phase benzene, toluene, and xylenes from contaminated ground-water in laboratory studies and this data was used in the selection of peat for thecurrent study (Cohen et al., 1991), and the same range has been obtained in thecurrent study. Kao and Borden (1997), reporting efficiencies of BTEX removalfrom laboratory groundwater undergoing remediation with a briquet-peat barrier


TAB

LE

V

BT

EX

grou

ndw

ater

mon

itor

ing

data

from

upgr

adie

ntan

ddo

wng

radi

ent

ofdi

ssol

ved

phas

epl

ume

(µg

L−1

)

Day

sTo

luen

eE

thyl

benz

ene

Xyl

ene

C6–

C9

C10

–C14

C15

–C28

C29

–C36

UG

DG

UG

DG

aU

Gb

DG

UG

DG

UG

DG

UG

DG

UG

DG

01

16

280

990

250

410

1100

370

100

190

1012

015

0

81

115

310

2600

110

2900

5010

020

010

04

6317

341

118

530

7400

6022

0023

0095

019

0071

04

120

32

4144

21

320

1900

130

780

560

440

100

100

117

062

4831

03

117

031

0080

3500

400

100

320

100

215

126

5840

21

780

3200

110

6600

340

630

5080

39

1

153

151

123

020

0060

580

150

500

180

120

138

16

188

21

120

2400

2075

078

048

010

010

02

216

215

171

127

027

0030

3500

270

410

140

01

<1

<1

245

11

110

940

2800

270

3000

440

510

150

120

1684

025

0

274

11

134

014

0030

1700

530

730

150

01

7511

306

11

510

022

0080

3100

410

740

100

100

610

18

Mea

n36

.21.

313

.435

827

2411

024

1861

149

730

814

34.

314

554

.5

U95

%C

.I.

50.7

1.6

1677

336

4217

343

9711

7682

341

616

45.

538

865

.3

aR

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ring

toup

grad

ient

bore

(Bor

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W13

).b

Ref

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nt(B

ore

MW

14).


TAB

LE

VI

Effi

cien

cyof

the

funn

elan

dga

tesy

stem

Sta

tist

icTo

luen

eE

thyl

Xyl

ene

C6–

C9

bC

10–C

14b

C15

–C28

bC

29–C

36b

Ove

rall

benz

ene

effi

cien

cy

(%)c

Upp

er95

%C

.I.

96.9

65.4

83.2

77.6

67.7

81.3

60.5

76.1

Ave

rage

96.3

68.3

62.5

69.2

77.6

79.5

53.5

72.4

Mod

alav

erag

e95

.464

.175

.069

.781

.167

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system, stated values of 86, 71, 43 and 28% for toluene, ethylbenzene, m-xyleneand o-xylene, respectively. The removal efficiencies reported in the current fieldstudy were therefore considerably higher than those reported in previous laboratorystudies. In another laboratory study, a peat column decreased petroleum hydro-carbon concentrations in an oily water mixture by 100 fold at 30 000–70 000 mgL−1 (Stehmeier, 1989). However, with a much lower dissolved phase concentrationof 5 mg L−1, the peat column was ineffective at removing the petroleum hydro-carbons (Stehmeier, 1989). The literature values therefore vary considerably forpetroleum hydrocarbon removal effectiveness. These previous studies reinforce theimportance of testing local peat types prior to deploying peat at full-scale.

The downgradient monitoring well reported concentrations of toluene, ethylbenzene, and xylene and n-alkanes in the range C6–C36 below the surface waterguidelines for these pollutants demonstrating that the funnel and gate system isalso meeting the regulatory requirements, for the shallow groundwater entering theriver (Table VII; see row with SEPP values). All the guideline values that werereviewed were met with respect to the specific contaminants of concern in thegroundwater from the site. The remediation will be considered completed when thedowngradient effluent is consistently reporting toluene, ethyl benzene, and xyleneand n-alkanes in the range C6–C36 below the surface water guidelines, over a periodof approximately 1 yr (with quarterly reporting).

7. Implications

This case study has described the effectiveness of an application of the funnel andgate technology at a contaminated site in South Eastern Australia. The implicationsfrom the success of the funnel and gate permeable barrier reported in the currentstudy are:

1. Technical effectiveness. The technical effectiveness of the funnel and gate, overthe 10 month operating period in which data was collected, indicates that peatrepresents an effective material for use in the gate component of funnel andgate remedial systems. Though efficiencies were not high for all contaminants,further ongoing monitoring will allow more definite conclusions to be drawnregarding the selective degradation capacities of the peat for each of the con-taminants tested. The use of sparging apparatus at the base of the gate wouldhave contributed to the technical effectiveness of the funnel and gate system.

2. Cost effectiveness. The costs incurred for this project were ∼71 500 USD (equiv-alent to ∼130 000 AUD) which included the construction of the funnel and gatesystem and bioremediation of the contaminated soil. Alternatively, the costs oftreating the groundwater by a pump and treat method were estimated at ∼USD83 500 or 160 000 AUD (capital costs) and ∼USD 0.2 m−3 (∼5500 USD or∼10 000 AUD) for annual operational costs and a total cost of ∼95 000 USD


TABLE VII

Summary of investigation levels of contaminants in surface waters (µg L−1)

Beneficial use Benzene Toluene Ethyl Xylene C6–C9 C10–C14

benzene

Production of edible 120a, 140b 250 250 380b – –

fish and crustaceaa

Recreation 470c – – – 10000d 10000d

Aesthetic objectives – – – – 10000d 10000d

SEPP groundwater – 250 250 – – –

qualityg

Protection of aquatic 300 300 140 380 –e –e

ecosystems

Groundwater ingestion – 5 1000 700 10000 –f –f

residentialf

Freshwater aquatic 300 300 – – – –

ecosystemsh

a US Code of Federal Register (Anonymous, 1994a).b Dutch guidelines (RIVM, 1994).c US Code of Federal Register (Anonymous, 1994b).d Historical Australian and New Zealand guidelines (ANZECC, 1992).e A dash indicates that ‘Information needed to select threshold concentrations is incomplete’. Thisdocument states that the NSW Clean Waters Act (1970) and Clean Waters Regulations (1972)require licenced discharges to be visually free of oil and grease (∼10 000 µg L−1) (Anonymous,1994c).f ASTM look up tables (ASTM, 1995).g SEPP or State Environmental Protection Policy is the legal instrument by which the en-vironmental regulators in the State of Victoria (Australia) can enforce clean up targets (referhttp://www.epa.vic.gov.au/wp/wp_sepp.htm).h These are the current Australian groundwater quality guidelines and are not enforceable targetsper se (NEPC, 1999).

(170 000 AUD). The application of the funnel and gate technology thereforerepresented a substantial saving to the client and was effective in prevent-ing ongoing pollution of the nearby river. The construction of the funnel andgate system also incurred the minimum disturbance to the public access areasbetween the facility and the river.

3. Further research. Ongoing technical effectiveness of the funnel and gate bar-rier will be conducted over the longer term, by monitoring at 3 month intervalsto ensure continued effectiveness of the funnel and gate system. This is import-ant as previous laboratory studies have shown that BTEX removal efficiencies


decrease with time (Stehmeier, 1989; Cohen et al., 1991). The key areas re-commended for further research are as follows:

• Determining the effect of peat on the quality of the groundwater could alsobe checked to ensure that there are no deleterious effects of the peat on theeffluent groundwater.

• Further research could be conducted to determine the mechanisms by whichthe petroleum hydrocarbons are removed from the groundwater and tomonitor any deleterious effects on removal efficiencies from any potentialincrease in the biomass associated with any microbial growth in the peatwithin the gate.

• The bulk density of peat has also previously been shown to influence theeffectiveness of petroleum hydrocarbon removal from water (Stehmeier,1989) and further research could be conducted to optimise this effect forfull-scale studies.

• Disposal of peat after the project is completed. Testing of the peat fromthe gate should be conducted at completion of remediation. This shouldbe designed to determine if the petroleum hydrocarbons are irreversibly orreversibly bound to the peat, and to determine any risks associated with theused peat. This will have implications for disposal options.

References

Anonymous: 1994a, ‘Water Quality Criteria: Human Health Partial Body Contact’, U.S. Govern-ment.

Anonymous: 1994b, ‘Water Quality Criteria Human Health Fish Consumption’, U.S. Government.Anonymous: 1994c, ‘Guidelines for Assessing Service Station Sites’, Sydney, NSW, Australia, NSW

EPA, 31 pp.ANZECC: 1992, Australian Water Quality Guidelines for Fresh and Marine Waters, Summary

of Water Quality Guidelines for Recreational Waters, Australian New Zealand EnvironmentConservation Council and National Health and Medical Research Council.

ASTM: 1995, ‘Guide for Risk Based Corrective Action Applied at Petroleum Release Sites’, WestConshohocksen, PA, ASTM Committee E50 on Environmental Assessment, 51 pp.

Bowles, M. W., Bently, L. R, Hoyne, B. and Thomas, D. A.: 2000, ‘In situ ground water remediationusing the trench and gate system’, Ground Water 38, 172–181.

Clark, D. K., Darling, D. F., Hineline, T. L. and Hayden, P. H.: 1997, Field Trial of the Biowall Tech-nology at a Former Manufactured Gas Plant Site. 29th Mid-Atlantic Industrial and HazardousWaste Conference, Presentation by Stearns and Wheler, LLC, Cazenovia, NY, U.S.A.

Cohen, A. D., Rollings, M. S., Zunic, W. M. and Durig, J. D.: 1991, ‘Effects of chemical and phys-ical differences in peats on their ability to extract hydrocarbons from water’, Water ResourcesResearch 25, 1047–1060.

Couillard, D.: 1994, ‘The use of peat in wastewater treatment’ Water Research 28, 1261–1274.Day, S. R., O’Hannesin and Marsden, L.: 1999, ‘Geotechnical techniques for the construction of

reactive barriers, J. Hazard. Mat. B, 285–297.Eykholt, G. R. and Sivavec, T. M.: 1995, Contaminant Transport Issues for Reactive-Permeable

Barriers, New York, NY, ASCE, pp. 1608–1621.


Fetter, C. W.: 1988, Applied Hydrology, Oshkosh, Wisconsin, Merill Publishing Company, 592 pp.Jack, T. R., Francis, M. M. and Stehmeier, L. G.: 1994, ‘Disposal of slop oil and sludges by

biodegradation’, Res. Microbiol. 145, 49–52.Kao, C. M. and Borden, R. C.: 1997, ‘Enhanced TEX biodegradation in nutrient briquet-peat barrier

system’, J. Environ. Engin. (New York) 123, 18–24.Kao, C. M. and Wang, C. C.: 2000, ‘Control of BTEX migration by intrinsic bioremediation at a

gasoline spill site’, Water Res. 34, 3413–3423.McMurty, D. C. and Elton, R. O.: 1985, ‘New approach to in situ treatment of contaminated

groundwaters’, Environ. Progr. 4, 168–170.NEPC: 1999, National Environmental Protection Measure – Schedule B, (1) Guidelines on the Invest-

igations Levels for Soil and Groundwater, Adelaide, Australia, NEPC (National EnvironmentalProtection Council), 11 pp.

Pankow, J., Johnson, R. L. and Cherry, J. A.: 1993, ‘Air sparging in gate wells in cutoff walls andtrenches for control of plumes of volatile organic compounds (VOCs)’, Ground Water 31, 654–663.

Rael, J., Shelton, S. and Dayaye, R.: 1995, ‘Permeable barriers to remove benzene: Candidate mediaevaluation’, J. Environ. Engin. 121, 411–415.

RIVM: 1994, Environmental Quality Objectives in The Netherlands – A Review of EnvironmentalQuality Objectives and Their Policy Frame Work in The Netherlands, Amsterdam, RIVM (RiskAssessment and Environmental Quality Division, Directorate for Chemicals, External Safety andRadiation Protection, Ministry of Housing, Spatial Planning and Environment, The Netherlands).

Schad, H. and Schulze, B.: 2000, Funnel and Gate at a Former Manufactured Gas Plant Site inKarlesruhe, Germany: Design and Construction. Remediation of Chlorinated and RecalcitrantCompounds – Chemical Oxidation and Reactive Barriers, Monterey, California, Battelle Press,pp. 315–322.

Schwartz, F. A. and Domenico, P. A.: 1997, Physical and Chemical Hydrogeology, New York, NY,John Wiley & Sons, 640 pp.

Starr, R. C. and Cherry, J.: 1994, ‘In situ remediation of contaminated groundwater: The funnel andgate system’, Ground Water 32, 465–476.

Stehmeier, L. G.: 1989, Development of an Oclanosorb (TM) (peat) Filter for Separating Hydrocar-bons from Bilge Water, Calgary, CA, Nova Husky Research Corporation, 12 pp.

Steimle, R.: 1995, In Situ Remediation Technology Status Report: Treatment Walls, Washington, DC,US EPA, 31 pp.

Thomas, J. M. and Ward, C. H.: 1995, Ground Water for Bioremediation, New York, NY,Geotechnical Special Publication, ASCE, pp. 1456–1466.

Vidic, R. D. and Pohland, F. G.: 1996, Treatment Walls, Pittsburgh, PA, Ground Water RemediationTechnologies Analysis Center, 38 pp.

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