This article was downloaded by: [University of Western Ontario]On: 12 November 2014, At: 09:47Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK

Soil and Sediment Contamination: An InternationalJournalPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/bssc20

Risk-Based Corrective Action of HydrocarbonContamination at a Former Major Urban PetroleumStorage Site in the U.K.Jason Clay a & Mark E. Harris aa URS Dames & Moore, Blackfriars House, St. Mary's Parsonage, Manchester, M3 2JA, EnglandTel: +44 161 832 0166, Fax: +44 161 832 1493, Email: Jason Clay@urscorp.comPublished online: 24 Jun 2010.

To cite this article: Jason Clay & Mark E. Harris (2002) Risk-Based Corrective Action of Hydrocarbon Contamination at aFormer Major Urban Petroleum Storage Site in the U.K., Soil and Sediment Contamination: An International Journal, 11:5,701-718, DOI: 10.1080/20025891107050

To link to this article: http://dx.doi.org/10.1080/20025891107050

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of theContent. Any opinions and views expressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon andshould be independently verified with primary sources of information. Taylor and Francis shall not be liable forany losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoeveror howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use ofthe Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

701

Soil and Sediment Contamination, 11(5):701-718 (2002)

1532-0383/02/$.50© 2002 by AEHS

Risk-Based Corrective Action ofHydrocarbon Contamination at a FormerMajor Urban Petroleum Storage Site in

the U.K.

KEY WORDS: risk based corrective action, petroleum soil and groundwater contamination,bioremediation.

Jason Clay and Mark E. Harris

URS Dames & Moore, Blackfriars House, St.Mary’s Parsonage, Manchester, M3 2JA,England Tel: +44 161 832 0166, Fax: +44 161832 1493, Email: Jason_Clay@urscorp.com

The former site of a major petroleum stor-age facility adjacent to a major urban wa-tercourse was found to have potentiallysignificant concentrations of hydrocarbonsin soil and groundwater that needed to beaddressed prior to redevelopment.

A series of intrusive investigations wereundertaken to collect physical and chemi-cal data for a Quantitative Risk Assess-ment (QRA) of potential impacts on humanhealth and the wider environment, in orderto derive a remedial strategy for redevelop-ment of the site for light industrial use. Asite-specific QRA methodology was devel-

oped using both U.K. and U.S. guidance toproduce Risk-Based Clean-up Levels(RBCLs) for benzene, and other petroleumhydrocarbons. The U.K. has no nationallybased guidance on risk assessment andstudies are designed by the consultant forsubmission to the U.K. Environment Agency(EA) for their approval. It is the EA’s role todetermine whether the work has been un-dertaken satisfactorily.

To achieve these RBCLs, ex situbioremediation was identified as the bestpracticable remedial option. This was car-ried out in windrows using mechanical aera-tion (to achieve oxygenation with ammonianitrate granule and woodchip addition) fora total of approximately 5291 m3 of soil.The bioremediation process was success-ful in achieving the site specific RBCLs forbenzene and for other hydrocarbons withinan average of 5 to 6 weeks.

This article describes the successfulimplementation of Risk-Based CorrectiveAction (RBCA) at this petroleum releasesite as a demonstration of how risk-basedremedial standards for contaminated sitescan be achieved with regulatory approval.

340349.pgs 9/16/02, 2:55 PM701

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

702

SITE SETTING

Introduction

his former urban industrial oil terminal site covered an area of approxi-mately 4.1 ha close to the center of a major city in the north of England,

adjacent to a canalized water course of reasonably low quality. The main use of thesite since 1932 has been as a fuel distribution terminal. These operations ceasedwhen the last depot closed in 1996. The majority of the structures, gantries, tanks,and pipework had been decommissioned and demolished after closure. (Refer toFigure 1 for the site layout.)

The site had known historical releases/contamination incidents and had beenheavily investigated (refer to Figure 1 for investigation locations). These investi-gations provided a relatively good understanding of the prevalent geologicalconditions, which were found to consist of man-made ground (industrial waste anddemolition rubble) over alluvial silt, over sand and gravels with clay or weatheredmudstone forming the underlying bedrock of the Lower Coal Measures sequence.

Groundwater was encountered beneath the site in localized perched water tablesabove the alluvial silts and within the made ground and as a semiconfined horizonwithin the sands and gravels (refer to Figure 1 for inferred groundwater elevations).Dames and Moore undertook three phases of investigation at the depot, includinga Phase I desk-based study summarizing all the available information and outliningdata gaps. A Phase II soil vapor survey was undertaken to target the investigationmore thoroughly and 17 soil and groundwater monitoring wells were installed. Amore focused Phase III investigation was performed to further assess contamina-tion and speciated ground gas.

The investigation’s chemical analyses followed the UK Environment Agency-approved protcol and focused on the major known contaminants associated withpetroleum hydrocarbons. These included:

• Total petroleum hydrocarbons (TPH)

• Volatiles (benzene, toluene, ethylbenzene and xylene (BTEX) and methyltert-butyl ether (MTBE).

Free product constituents were also analyzed, including a breakdown of aro-matic and aliphatic carbon chain bandings: C10 – C16, C17 – C24, and C25 – C35.

TPH contamination in soils was observed at concentrations up to 22,000 mg/kg;benzene at up to 2834 mg/kg, ethyl benzene at a maximum of 254 mg/kg, tolueneat up to 171 mg/kg, and xylene at a maximum of 482 mg/kg.

Analysis found MTBE at up to 1.4 mg/kg in the silt and 1.14 mg/kg in the madeground. Free product was observed at the water table within approximately 20 ofthe monitoring wells installed at the site. This allowed contours of ‘apparent’ freeproduct thickness to be inferred.

T

340349.pgs 9/16/02, 2:55 PM702

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

703

FIG

UR

E 1

Site

layo

ut a

nd a

reas

iden

tifie

d fo

r re

med

iatio

n.

340349.pgs 9/16/02, 2:55 PM703

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

704

Each of the 20 product samples collected during this study was subjected toanalysis for BTEX and MTBE. The results obtained indicated significant variationin the volatile aromatic component with benzene concentrations varying betweennondetect up to 5800 mg/l. The lack of a clear pattern in benzene concentrationsin product indicated that multiple points of release sources or incidents were likelyto be responsible for the observed impacts. In addition, earlier releases may havedegraded/migrated to a greater extent than later ones.

Groundwater Analytical Results

Groundwater was found to be heavily impacted by TPH and its constituents in apattern approximately coincident with the plumes of free phase product.

QUANTITATIVE RISK ASSESSMENT

Introduction

The U.K. is currently researching the risk-based approach to contaminated land,and no national guidance or prescribed approach is currently available. The regu-lator, the U.K. Environment Agency, must assess each case on its own merits anddetermine whether the investigative approach was appropriate. The invesitgationand risk assessment described here were designed with this in mind.

The next stage of the investigation was to determine, using quantitative riskassessment, whether measured contaminant levels in soil and groundwater pre-sented an unacceptable health risk to on-site workers and nearby off-site residents,and whether the measured contaminants in soil and groundwater presented anunacceptable risk to controlled waters (Controlled Waters are defined under U.K.legislation as any environmental water, including groundwater, surface waters,rivers lakes, etc. and some marine/estuarine areas).

QRA uses mathematical modeling techniques to produce site-specific numericalestimates of risk. These allow preliminary estimates of the need for, and scope of,remediation to be identified in a robust and scientifically defensible manner by thesetting of remedial objectives, risk-based clean-up levels (RBCLs).

Because the previous investigations were carried out by different companies, atdifferent dates, on different parts of the site, using different investigative andanalytical techniques, there were discrepancies in the quality and detail of availabledata. In particular, there were discrepancies between the descriptions of TotalPetroleum Hydrocarbon (TPH) contamination. Whereas earlier reports expresshydrocarbon contamination as total TPH or Diesel Range Organic (DRO) fraction,latter investigations broke the TPH fraction down into constituent chain lengthsdescribed above.

340349.pgs 9/16/02, 2:55 PM704

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

705

The distinction between the two classifications of TPH (as DRO or by constitu-ent chain lengths) is important. The large variation in molecular size within the C10

to C35 range is reflected by a similarly large variation in mobility and toxicity. Froma risk assessment perspective, therefore it is inappropriate to consider the exposureto the entire DRO fraction. Therefore, the QRA used only the constituent chainlength TPH data.

Human Health Risk Assessment

The assessment comprises five stages as follows:

• Calculation of exposure point concentrations

• Human dose estimation

• Effects (toxicity) assessment

• Risk estimation

• Risk evaluation

Exposure point concentrations are chemical concentrations in environmentalmedia at the points at which exposure is assumed to occur. They include vapourconcentrations in ambient air and in buildings, arising from both soil and ground-water contamination, and concentrations of chemicals in dust. Simple algorithms(ASTM, 1995), and the site physical characteristics, are used to calculate theseconcentrations because no field data are available for these media.The human dose estimation step involves the use of assumptions regardinghuman physiology and behavior to calculate the chemical dose that each of theidentified receptors could be exposed to as a result of performing their assumedactivities. Assumptions regarding human behavior and physiology are taken froma variety of sources, including the ASTM document referenced above, and aredesigned to be health conservative, that is, they refer to the likely behavior, andtherefore exposure patterns, of a hypothetical Maximally Exposed Individual(MEI).

Simple equations are used to calculate the dose that the potentially exposedreceptors could sustain, resulting in the calculation of a Maximum Daily Intake(MDI) for each contaminant. This dose estimate is then used in the calculation ofrisk for noncarcinogenic compounds. Carcinogens are assessed via the calculationof a Chronic Daily Intake (CDI) by averaging the total daily dose (calculated usingthe MDI and exposure duration) over the assumed lifetime (70 years):

MDI = Daily Chemical Intake × Bioavailability × Exposure FrequencyBody Weight

340349.pgs 9/16/02, 2:55 PM705

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

706

CDI = (MDI child × 5 years) + (MDIadult × 25 years)70 years lifetime

The Chronic Daily Intake for an employee is based on an assumed 30 yearexposure.The effects assessment consists of an expert review of available (published)toxicity data in the form of Tolerable Daily Intake (TDI) or Slope Factor (SF)values, and the identification of suitable effects criteria for the chemicals ofpotential concern. A TDI is defined as the maximum daily intake of aparticular chemical that could be sustained indefinitely without adverse ef-fect. A Slope Factor relates the dosage of a carcinogen to the resultantincreased cancer risk.The risk estimation step mathematically compares the MDI with the TolerableDaily Intake (TDI) for noncarcinogenic contaminants and/or the CDIs with theSlope Factors (SF) for carcinogenic contaminants, in order to calculate HumanHazard Indices (HHIs) and increased lifetime cancer risk estimates (increasedprobability of tumor formation over an assumed lifetime):

HHI = MDI/TDI

Carcinogenic Risk = Slope Factor (risk per mg/kg-d) × CDI(mg/kg-day)

The risk evaluation step compares the risk estimates with acceptability criteriain order to determine their significance. In common with general risk assessmentpractice, a Maximum Allowable Risk Level (MARL) of one (1) is used in theassessment of noncarcinogenic risks (USDoE, 1996) from individual contami-nants. A MARL of 10–4 (corresponding to an increased lifetime cancer risksgreater than one in ten thousand) has been used for carcinogenic contaminants.This is in line with our understanding of the U.K. government’s unpublishedexposure assessment methodology, which recommends the MARL currentlyused by the U.K. Health and Safety Executive of 10–6 per year (corresponding toa life risk of 7 in 100,000). This value has also been used in the Netherlands inthe development of the Dutch Intervention Values (RVM, 1994) and is advo-cated in a study of cancer risk MARLs accepted by the USEPA in regulatoryactions (USEPA, 1996).

For contaminants with a significant pollutant linkage site-specific Risk-BasedClean-Up Levels (RBCLs) are calculated. These are theoretical soil or waterconcentrations below which pollutant linkages cease to be significant. Siteremediation to these specifications therefore would theoretically negate potentialrisks to identified receptors.

340349.pgs 9/16/02, 2:55 PM706

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

707

The Conceptual Site Model (CSM) — Human Health Assessment

The CSM provides a description of the site in terms of the potential pollutantlinkages. Pollutant linkages are defined as complete source-pathway-receptor re-lationships. These being receptors that may be potentially exposed to contamina-tion by a defined pathway such as dust or vapor inhalation. Receptors in this casewere workers on-site and residents (both adult and child) living close to the site.The CSM involves consideration of site conditions and results in the identificationof pollutant linkages that are to be quantitatively assessed.

Elevated concentrations of contaminants present above the screening concentra-tions provided by the Dutch Intervention Values are subject to modelling. For thepurposes of assessing likely exposure, on-site soil is divided in to shallow and deepas each has distinct exposure pathways. The contaminant migration pathwaysconsidered for each receptor are shown in Table 1.

Results of Human Risk Assessment

Unacceptable risks were identified to on-site employees from the following non-carcinogenic compounds:

• Toluene (from free product only) — indoor vapor inhalation

• MTBE (from free product only) — indoor vapor inhalation

• TPH C10 - C16 aliphatic (from shallow soil only) — dermal contact, inges-tion, and indoor vapor inhalation (combined risk)

• TPH C10 to C16 aromatic (from shallow soil only) — dermal contact,ingestion, and indoor vapor inhalation (combined risk)

Risks to on-site employees were also identified from the following carcinogens:

• Benzene (from soil, groundwater and free product) — dermal contact,ingestion, and indoor vapor inhalation (combined risk)

Those receptors living away from but adjacent to the site were not found to bepresented with an unacceptable level of risk.

Site-specific Risk-Based Clean-Up Levels (RBCLs) calculated for all observedcontaminants in shallow and deep soil and in groundwater are listed in Table 2.

Recommendations for the Protection of Human Health Receptors

The human health risk assessment did show potentially significant risks topeople directly exposed to the site, that is, employees working on-site, and there-

340349.pgs 9/16/02, 2:55 PM707

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

708

TABLE 1CSM Pollutant Linkages

TABLE 2RBCLs Based on the Potential Risk to Human Health

340349.pgs 9/16/02, 2:55 PM708

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

709

fore remedial action was necessary. The predominant pathways being from theinhalation of vapors derived from soil and free product.

Controlled Waters Risk Assessment

A separate but similar procedure (EA, 1996) assessed the potential risk to con-trolled waters from the site. Again, maximum contaminant concentrations thatexceeded the screening values were used as the source term in the conceptual sitemodel.

The potential migration pathways by which contamination could impact areceptor were considered to be:

• Lateral off-site migration of contaminants via the localised perched ground-water system toward the river

• Vertical migration of contamination through the silt deposits into the under-lying sand and gravel aquifer

• Lateral off-site migration of dissolved phase contamination within the sandand gravel aquifer.

A regulatory search was undertaken, and no licensed groundwater abstractions fordrinking water or industrial use were found within a 1-km radius of the site.Therefore, potential receptors were assumed to be the point at which contaminatedgroundwater would effect local surface water systems and comprised:

• The water drainage system at a distance of 120 m and at a depth within theSand and Gravel aquifer

• The nearby river

Assumptions and Parameters

DegradationBiological contaminant degradation was simulated using field studies availablewithin the literature (BP Publication; Anon, 1994) regarding the degradation ofBTEX compounds. Table 3 lists the half-lives used within this study, all of whichrelate to aerobic degradation.

All BTEX compounds are readily degraded under aerobic conditions, whileunder anaerobic conditions the rate of degradation is only slightly reduced fortoluene, ethylbenzene, and xylenes. In the total absence of oxygen, the rate ofdegradation for benzene is very slow; however, even minute amounts of oxygen

340349.pgs 9/16/02, 2:55 PM709

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

710

can produce aerobic degradation of benzene (Williams et al., 1997). Dissolvedoxygen at low levels has been measured within BTEX-contaminated groundwaterobtained from the site. Therefore, suggesting the possibility of aerobic degradation.

Results of the Controlled Water Assessment

The controlled water risk assessment identified potentially adverse impacts towater quality at:

• The water drainage system at a distance of 120 m via deep groundwatermigration of Benzene and MTBE

• The river via infiltration of perched groundwater from the southeast cornerof the site. The contaminants of potential concern for this pathway wasbenzene, both dissolved phase and in free product.

Table 4 lists the RBCLs generated from the controlled water simulations. They arebased on meeting a Water Target Value (WTV) in the river. The benzene WTV waschanged from the World Health Organization (WHO, 1993) value of 10 µg/l byusing an acceptable lifetime cancer risk of 10–4 rather than 10–5 used by WHO. Thisresulted in a target value of 100 µg/l for benzene.

Quantitative Risk Assessment Conclusions

Potentially significant risks were identified to both human health and controlledwaters and therefore remedial action was deemed to be necessary. Table 5 providesa combined list of RBCLs.

TABLE 3Calculated average half lives within aquifer materials

340349.pgs 9/16/02, 2:55 PM710

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

711

TABLE 4Results for Controlled Waters Receptors

TABLE 5Combined RBCLs

340349.pgs 9/16/02, 2:55 PM711

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

712

Groundwater Quality After Source Remediation

Groundwater modeling was used to support the view that no active groundwaterremediation or long-term monitoring would be required once the source of thecontamination was removed. The groundwater quality was predicted to be less thanthe target concentrations at the drainage sump (the nominal compliance point 120m from the site boundary) within the following timeframe:

• Three years for benzene

• Four years for MTBE

Groundwater quality was predicted to improve over a relatively short period,following remediation of the site, and therefore the Environment Agency agreedthat continued monitoring was unnecessary.

Estimation of Residual TPH Concentrations

The QRA was used to develop RBCLs for benzene, and MTBE, and TPH. Thelowest RBCL for TPH in soil was 6410 mg/kg for the hydrocarbon fraction C10 toC16 (aliphatic) based on usage by on-site workers. It was expected, however, thatTPH concentrations of this magnitude in soils could reestablish as free product atthe water table (TPH Criteria Working Group, 1996) following remediation.

Therefore, the concentrations of TPH in soil that could be considered to repre-sent free phase was determined from estimates of the bulk soil free pore volume(USEPA, 1996). The results of this study suggested that a conservative TPHremdial criteria of 3550 mg/kg would prevent free phase product from migratingto the water table.

Derivation of Remedial Targets

The site-specific RBCLs adopted as the remedial targets in soils are summarizedin Table 6.

An empirical clean-up criteria of 10 mm for apparent free product thickness wasproposed and agreed to by the Environment Agency on the basis that benzene andMTBE exceeded the RBCLs in areas of the site where the apparent thickness offree product equalled or exceeded 10 mm.

Furthermore, it was determined that the actual thickness of product within thesilty formation was actually lower that the product thickness found within wells inthe formation due to the difference in capillary pressure in the silty formation andthe well.

340349.pgs 9/16/02, 2:55 PM712

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

713

Remediation Areas

On the basis of the results of previous site investigations and the QRA, the depthand lateral extent of 10 areas were determined to exceed the RBCLs. The areasidentified for remedial excavation are shown in Figure 1.

SITE REMEDIATION

Design Philosophy

Previous site investigations suggested that the majority of the free product andcontaminated soils are contained within the silty horizons that underlay theman-made ground at the site, and which form a confining layer to the gravelsbeneath.

We anticipated that the removal of the primary source of the contaminationwould result in a gradual dissipation of residual groundwater contamination.Therefore, we did not propose to specifically remediate groundwater contamina-tion identified within perched groundwater or in the gravel unit. However,groundwater pump and treatment was undertaken as part of the excavationworks, and this effectively improved groundwater quality within and adjacent tothe areas of excavation.

The silty horizon was acting as an aquitard across the site; therefore, it wasnecessary to replace excavated material from this horizon with material of equiva-lent permeability and thickness.

The preferred option of treating the organic contamination was ex situ biologicaltreatment after consideration of the level of contamination, soil structure, and, inparticular, the volume to be treated together with the available area at the site.

Scope of Remediation Works

The scope of remediation works comprised the following:

TABLE 6Remedial targets in soils

340349.pgs 9/16/02, 2:55 PM713

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

714

• Demolition — required in areas where the stability of existing structures on-site would be compromised by the remedial works

• Excavation — soils in the contamination source areas were excavated andgroundwater that entered the excavation was removed for treatment prior todischarge to the foul sewer

• Stockpiling — uncontaminated soils suitable for re-use as backfill werestockpiled on site. Excavated soils that were unsuitable for use as backfillwere removed for disposal off site

• Ex situ bioremediation — contaminated soils were treated on site and usedas backfill to reinstate the excavations

• Validation — excavations and treated soils were validated prior to backfill-ing in order to confirm that remedial targets had been met

• Backfilling — uncontaminated and treated soils were used as backfill andplaced using compaction methods in accordance with the Specification forHighway Works (HA, 1998)

• Reinstatement of aquitard — low permeable material was used to replacematerial excavated from the silty horizon to ensure uniform permeability

Excavation

Excavated soils were stockpiled, tested and treated, and reinstated or disposed. Thefollowing categories were considered:

• Contaminated soils for off-site disposal

• Contaminated soils for remediation and reinstatement

• Uncontaminated soils for reinstatement

• Uncontaminated, but geotechnically unsuitable soils for off-site disposal

Three levels of checks were used to segregate the excavated soils into the classeslisted above; visual inspection, field testing using a photo-ionization detector (PID)and validation sampling. Validation sampling and analysis was carried out on 1sample per 100 m3 to confirm whether each 100 m3 of soil was contaminated andrequired remediation or offsite disposal.

During the site works, a system for tracking excavated soil was developed torecord the origin, the stockpile, the bio-bed windrow, and the final destination ofthe reinstated soil.

340349.pgs 9/16/02, 2:55 PM714

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

715

Treated Groundwater

To facilitate the excavation and backfilling operations, it was necessary to dewaterthe site. All waters removed from the excavations were treated and tested on siteprior to discharge to the foul sewer, in accordance with the consent to dischargeagreed with the water company. Results confirmed that water discharge operationscomplied with the consent to discharge.

The on-site water treatment plant was constructed to allow contaminated waterpumped out of excavations to be cleaned to a level suitable for consented dischargeto sewer. A multiphase design was put in place, using a large receptor tank, a twin-chamber oil/product separator, and a carbon filter to remove residual organiccontaminants. The receptor tank was designed to act as a holding tank and silt trap.The water then passed through the separator and into the main carbon filterassembly before discharging to sewer. The separated oil was recovered to a producttank for off-site disposal.

Ex situ Bioremediation

The contractor was responsible for the detailed design of the bioremediationprocess, obtaining an Environment Agency Mobile Plant License for the sitetreatment and meeting Environment Agency representatives during site visits.

The bioremediation process comprised the treatment of the contaminated soil bynutrient addition and physical aeration to optimize the degradation by indigenousmicrobial populations. Most soil systems contain large numbers and diversity ofmicroorganisms capable of biodegradation. However, a range of environmentalfactors limit degradation, including oxygen and nutrient content (principally ni-trate, phosphate, and potassium), pH, temperature, and moisture content. Theaddition and/or control of these limiting factors were carried out to optimisebioremediation potential.

The remediation process at the site comprised:

• Construction of the biopiles with the approximate dimensions of 4 m wide,50 m long, and 1.5 m high

• Conditioning of treatment materials by disaggregation, homogenization,and the addition of bulking material

• The addition of nutrients comprising phosphate and nitrate containing fer-tilizer in pellet form at variable rates, dependent on contaminant concentra-tions

• Treatment (physical aeration) by turning the biopiles approximately twicea week

340349.pgs 9/16/02, 2:55 PM715

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

716

• Covering the beds with water repellent, insulating, woven polypropylenefleeces to control temperature and moisture content (and dust)

• Monitoring pH and temperature

• Sampling and validation

During the bioremediation process, coarse woodchip was added, at a concentra-tion of 1% by volume as a bulking agent to assist with the breakdown of thecohesive properties of the contaminated silt materials and to aid disaggregation andhomogenization of the biopile. To demonstrate that the addition of such materialsdid not increase the potential for methane generation beyond that which may beanticipated from similar soils not contaminated with petroleum hydrocarbons,samples were submitted for laboratory analysis.

The test results indicated that there was no increase in the potential for methanegeneration with both samples showing results less than the in-house methoddetection limit. The lack of methane production precluded the need for any long-term ground gas monitoring on the site.

Validation Testing

To confirm that the remedial targets had been achieved, the vertical and lateral extentof the excavations and soils from the biopiles were tested. Treated soils were used forthe reinstatement of the excavations. Table 7 summarizes the reduction in concentra-tion achieved during the biotreatment process for each of the target contaminants.

Backfilling Operations

A volumetric survey was undertaken to establish the depths and thicknesses ofmaterials that had been excavated. In all but one area, the excavation was reinstated

TABLE 7Reduction in contaminant concentrations in soils after biotreatment

340349.pgs 9/16/02, 2:55 PM716

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

717

with similar materials recovered from elsewhere on-site. One excavation wasbackfilled with an imported inert clay to act as an aquitard. The clay was obtainedfrom a local source. Permeability tests carried out on samples of this clay materialconfirmed its suitability for use as aquitard.

In all areas, the excavations were capped with a layer of compacted granular materialobtained by crushing concrete recovered from the demolition and excavation works.

Monitoring

After completion of the remedial works, routine monitoring was carried out toconfirm that residual free product was not present on the groundwater outside theremediated areas. Monitoring comprised a total of five weekly rounds of wellmeasurements at 30 selected wells. Wells immediately adjacent and mainly down-gradient all areas of excavation were chosen.

The post-works monitoring schedule recorded a zero or negligible productthickness in 28 of the wells for the 5-week period, the exceptions were two wellswhere free product thickness exceeded 10 mm during week 1 of the monitoringprogram. The original excavations were extended to remove a relatively smallvolume of contaminated soil from the close proximity of both monitoring wells toremediate the areas further. The well monitoring results, following these activities,demonstrated that these additional remedial actions were successful in removingthe product sources from the vicinity of both wells.

Well Decommissioning

All monitoring wells were decommissioned using a bentonite seal to eliminate apotential pathway to the groundwater.

CONCLUSION

The areas of the site excavated were remediated successfully, with soils sampledfrom the bioremediation pile having concentrations below the agreed RBCLs forthe designated light industrial end-use.

Any localized residual contamination on the site is expected to dissipate overtime. This approach is considered to be consistent with the requirements of theEnvironment Act 1995 and has been completed to the satisfaction of the Environ-ment Agency.

It should be noted that should a more sensitive end-use for the site be proposed, forexample, residential housing, then it is recommended that a further Quantitative RiskAssessment be undertaken and the need for further remediation at the site be considered.

340349.pgs 9/16/02, 2:55 PM717

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14

718

REFERENCES

ASTM (American Society for Testing and Materiels). 1995. Standard Guide for Risk Based Correc-tive Action Applied at Petroleum Release Sites. E1739-95.

BP Publication. Anon. 1994. Natural Attenuation. Groundwater Remed. Environ. Eng. News. Issue2, third quarter.

EA (Environment Agency). 1996. Methodology to Determine the Degreee of Soils Clean-up Re-quired to Protect Water Resources. Technical Report 13.

HA (Highways Agency). 1998. Manual of Contract Documents for Highway Works. Volume 1,Specification for Highway Works. March, 1998.

TPH Criteria Working Group 1996. Selection of Representative TPH Fractions Based on Fate andTransport Considerations. Amherst Scientific Publishing.

USDoE (U.S. Department of Energy). 1996. Criteria for Establishing De Minimis Levels of Radio-nuclides and Hazardous Chemicals in the Environment (ES/ER/TM-187). RAP TechnicalMemoranda. Office of Environmental Management. 1996.

USEPA (U.S. Environmental Protection Agency). 1996. Soil Screening Guidance: Technical Back-ground Document. EPA 9355.4-17A.

RVM (Rikinstituut voor Volksgezondheid en Milieu). 1994. Human Exposure to Soil Contamina-tion: A Qualitative and Quantitative Analysis towards proposals for Human ToxicologicalIntervention Values. (Modified from the 1991 report). The Hague. 1991.

WHO (World Health Organisation) 1993. Guidelines for Drinking Water Quality. Geneva, 2nd ed.1993.

Williams, R.A., Shuttle, K.A., Kunkler, J.L., and Hooper, S.W. 1997. Intrinsic bioremediation in asolvent-contaminated alluvial groundwater. J. Industr. Microbiol. Technol., v.18, pp. 177–188.

340349.pgs 9/16/02, 2:55 PM718

Dow

nloa

ded

by [

Uni

vers

ity o

f W

este

rn O

ntar

io]

at 0

9:47

12

Nov

embe

r 20

14