a case study of the management and remediation of soil contaminated with polychlorinated biphenyls

Engineering Geology 53 (1999) 177–185

A case study of the management and remediation of soilcontaminated with polychlorinated biphenyls

G. Norris a, Z. Al-Dhahir b, J. Birnstingl c, S.J. Plant b,*, S. Cui b, P. Mayell ca Nortel Networks, Oakleigh Road South, New Southgate, London N11 1HB, UK

b Golder Associates (UK) Ltd, 54–70 Moorbridge Road, Maidenhead, Berkshire SL6 8BN, UKc R.O. Environmental Services Group, Plc, Westcott Venture Park, Aylesbury, Buckinghamshire, HP18 0NP, UK

Abstract

During environmental investigations at a communications manufacturing facility in South West England, polychlori-nated biphenyl (PCB) and chlorinated solvent contamination was identified in soil at a number of locations acrossthe site. The presence of the PCB contamination is known to have been caused during capacitor manufacture andgeneral storage of PCBs on the site during the sixties and seventies.

In general, the PCB contamination was relatively low and the affected soil was excavated and disposed of directlyto a licensed landfill. However, a small area was identified under a roadway in which the PCB contamination exceededthe figure governing disposal to landfill. As a result, an alternative treatment/disposal technique was required for thismore heavily contaminated material. The contaminated soil contained a high clay fraction which made treatmentmore difficult.

A number of remediation technologies/disposal routes were investigated including bioremediation, solvent washingand incineration. However, research work and laboratory bench scale studies into soil washing and bioremediationindicated that these techniques would not be viable/effective. Also, solvent washing trials indicated that the timerequired to complete the trials and the relatively high associated development costs made this option unattractive.The most effective and commercially attractive solution to the problem was found to be low temperature thermaldesorption (LTTD).

An advantage of LTTD is that it not only removes the organic contaminants, including PCBs, but also allows thesoil to retain its essential characteristics so that the treated material can be re-used.

There were other areas of the site where access for investigation was difficult. These areas were evaluated usingdetailed probabilistic risk assessments to evaluate both any potential risk to humans and the potential costs associatedwith any remediation works required. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Alternative technologies; High clay content; Incineration; Low temperature thermal desorption; Polychlorinated biphenyls(PCBs); Success

* Corresponding author. Fax: +44-1628-770-699.E-mail addresses: [email protected] (G. Norris),

[email protected] (S.J. Plant)

0013-7952/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S0013-7952 ( 99 ) 00031-9

178 G. Norris et al. / Engineering Geology 53 (1999) 177–185

1. Introduction listic risk assessments will be published in duecourse.

The PCB contaminated soil discussed in thepresent study arose from a 20 ha telecommunica-tions manufacturing facility built in the 1950s. 2. Geology and hydrogeologyCapacitor manufacture, and retrofilling of capaci-tors and transformers, led to contamination of the The site is underlain by a series of partly

weathered Devonian slates and tuffs, followed bysoil in a number of small areas on the site. It isbelieved that subsequent cut and fill construction a thick sequence of fractured, and in some places

karstic, Devonian limestone.operations spread the contamination over largeareas of the site. The soil under the roadway pavement area

consisted of fill material up to 2 m in thickness,Three remediation strategies were adopted.(i) Where the contamination was light ( less overlying weathered mudstone and tuffaceous soil.

The fill included brick, concrete and wood frag-than 20 mg kg−1), the soil was excavated and laterdisposed of to a licensed landfill. Extensive use ments. Gradings of the fill and in situ soils to a

depth of 2 m below the roadway (the zone treated,was made of immunoassay field testing techniqueto control the extent of the excavation. as discussed later) indicated sandy silty clay soil

with an average moisture content of 20%, liquid(ii) Where there was heavier contamination (upto 1300 mg kg−1) of around 1200 m3 of soil under limit of 46% and plastic limit of 29%. Fig. 1 shows

a plan of the roadway area (with contours of PCBa small roadway area where, historically, filling ofcapacitors had taken place, the excavated soil was contamination). Fig. 2 shows typical gradings for

the treated soil.treated using low temperature thermal desorption(LTTD). The exploration of alternative techniques The groundwater elevation under the site is

about 35 m below the ground surface. There wasand the final successful treatment using LTTD arethe subject of this case study. no evidence of PCB contamination of the ground-

water in any of the boreholes drilled on the site.(iii) In the case of much of the remainder ofthe site, field investigations revealed no significant This is not surprising due to the low water solubil-

ity and low mobility of PCBs and the relativelyPCB contamination. However, there remainedsome uncertainty about relatively large areas of deep groundwater table.the site where access was difficult; for example,under factory buildings where the current manu-facturing process is extremely sensitive to any 3. Contamination in the roadway areavibrations or dust associated with soil boring andsampling. For these areas, their status was eval- There have been several investigations in the

roadway area over the period 1993 to 1994, whichuated using detailed probabilistic risk assessments,to evaluate both any potential risk to humans and indicated PCB and chlorinated solvent contamina-

tion to a depth of 11 m (the maximum depth ofthe potential costs associated with any remediationworks required if the buildings were to be demol- the boreholes in those investigations). The results

of a later PCB investigation at a grid spacing ofished as part of a future development programme.In addition to the PCB contamination, there 4 m (Fig. 1) gave a peak value of 1300 mg kg−1

and an average of around 120 mg kg−1. The depthwas also evidence of chlorinated solvent contami-nation of the groundwater which necessitated a for that later investigation was restricted to the

top 2 m, in accordance with the remediation strat-separate extensive site investigation and tracertesting programme to assess the potential impact egy for the roadway area as discussed in the

next section.on nearby sensitive receptors. So far, no such effecthas been identified. It is interesting to note that the earlier investiga-

tions, which included 37 boreholes/probes in totalIt is anticipated that the results of the chlori-nated solvent investigation and the PCB probabi- in the relatively small roadway area of around 7 m

179G. Norris et al. / Engineering Geology 53 (1999) 177–185

Fig. 1. CB contamination of upper 2 m of soil under roadway area.

Fig. 2. Grading analysis for treated soil.


by 60 m, recorded a peak PCB value of 1254, and this was the general depth limit decidedupon for the excavation. Where possible, the con-120 mg kg−1 only, which is less than 10% of thetaminated soil containing PCBs greater thanpeak value recorded using the much closer grid50 mg kg−1 was removed, but some contaminationspacing (a further 50 probes) based on statisticalwas present at depths greater than 2.5 m and alsosampling requirements (Anon, 1992).in small areas at the roadway boundary whereIn addition to the PCBs, chlorinated solventsaccess, and hence removal, was not possible dueat concentrations of up to 2300 mg kg−1 were alsoto the presence of site services and ducts.encountered. These included trichloroethene

The excavated area was reinstated with a 1 m(TCE), tetrachloroethene (PCE), trichloroethanethick moisture conditioned compacted clay liner(TCA) and 1,1,2-trichloro-1,2,2-trifluoroethanewith a permeability of less than 1×10−9 m s−1. A(Freon 113). It is believed that the roadway areawhite 2 mm thick HDPE geomembrane was placedwas also used for temporary storage of spenton top of the clay liner to form a compositesolvents. It is possible that the relatively immobilebarrier. After some backfilling (400 mm) withPCBs were able to penetrate the soil to the depthsclean, Marldon Grit, a separate, thin sacrificialencountered as a result of dissolution into thegeotextile sheet was emplaced to act as a warningmore mobile chlorinated solvents.marker about 600 mm below grade, for any futureexcavations. Further clean, sub-base roadwaymaterials were then placed and compacted and the4. Remediation strategy for the roadway areareinstated surface was sealed with rolled asphalt.

The above remediation strategy has significantlyCurrently there are no specific regulatory guide-reduced the risk to human health associated withlines available in the UK with respect to PCB soilany future excavation activities in the roadwaycontamination. The Dutch Intervention Standardsarea, since any excavation for future services, etc.(Dutch Ministry of Housing, 1994) suggest a figurewill generally be restricted to a depth of 1 m. Also,

of 1 mg kg−1 of total PCBs as a maximum limit. and as indicated earlier, the risk of PCB contami-However, the Dutch Standards are considered nation of the deep groundwater table is consideredconservative for industrial sites. In comparison, minimal and the remediation and sealing of thethe Interim Canadian Environmental Quality roadway area will further reduce any such risk.Criteria for Contaminated Sites (Canadian Councilof Ministers of the Environment, 1991), whichwere based on generic assessment at the time of 5. Temporary storage of the contaminated soilthe work rather than the risk based criteria whichare more commonly used today, offer the following An ongoing development programme for thelimits: site required the rapid removal of contaminated

soil and reinstatement of the roadway area. HenceLand use Maximum permissiblePCB concentration (mg kg−1) it was necessary to stockpile the contaminated soil

in a temporary landfill cell constructed in a cornerIndustrial/commercial 50 of the site. The possible requirement for planningParkland/play area 5

permission and site licence was discussed with theAgriculture 0.5local County Council who advised that, because

The location of the roadway within the site indi- of the small quantity involved (1200 m3), the tem-cates that the area is likely to be used solely for porary nature of the landfill and the even smallerindustrial/commercial purposes and hence the quantities required for the envisaged remediationCanadian limit of 50 mg kg−1 was identified as the field trials, such permits would not be required.intervention figure most suitable for this area. They indicated, however, that they would review

The majority of the contamination was found the situation after assessing the results from anyto be restricted to the upper 2.5 m of the road- field trials carried out.

The temporary landfill cell was a rectangle,way, the main PCB contaminant being Aroclor


some 40 m long, 30 m wide and 1 m to 3 m deep. the clean coarser particles from the contaminatedfiner fraction of the soil was studied and dis-It was lined with 2 mm double textured HDPE

geomembrane resting on 150 mm layer of sand counted. The small volume of soil did not makethis option commercially viable. Also, this processand overlain by a 1200 g m−2 protection geotextile.

A 1 mm very low density polyethylene (VLDPE) would have resulted in around 40% of the finefraction of the soil still requiring further treatment.liner cap was then placed on the contaminated fill.

Clean run off from the cap was collected in two The next option considered was bioremediation.Two specialist companies were engaged indepen-small lined lagoons attached to the cell. After

quality assurance testing and with the approval of dently to carry out laboratory scale experimentson samples obtained from the site. The teststhe local water company, the water was discharged

to the sewer on site. allowed for two steps; the first being the reductionof the chlorine content of the PCBs using chemicalGreat care was taken in the excavation of the

contaminated soil from the temporary landfill cell, reagents, the second step being the degradation ofthe low chlorine PCB congeners by special strainsto avoid any possible damage to the basal liner

and also to minimise the volume of contaminated of bacteria imported from either the USA orGermany. Whilst some progress was made withrun off ( leachate) generated, which was collected

in one of the lagoons. Using a small excavator, the second step on non-site low chlorine PCBcongeners, it was found that the chemicalthe protective geotextile was peeled off the basal

liner which, after checking that the PCB concen- by-products produced in the first step on the sitePCBs were toxic to the bacteria. Both companiestration was within authorised limits, was then

disposed of with the geotextile to a licensed landfill. independently decided to abandon the bioremedia-tion route and to recommend solvent washingContaminated run off water was collected and

treated using chemical flocculants to reduce the technique instead.The solvent washing technique has been usedsuspended solids. The water was then passed

through a sand filter and two canisters of granular in the USA since 1994 and there is a considerablebody of experience there. In the UK, the techniqueactivated carbon. The maximum recorded PCB

concentration before treatment was 0.3 mg l−1 is in its development and field trial stages. A localcompany with patented slurry mixer was engagedwhich reduced to below the detection limit of

0.02 mg l−1 after treatment and prior to disposal to carry out selection of an appropriate solventand laboratory scale trials. Some progress wasto sewer.made on the solvent selection but it soon becameapparent that the time required to complete thetrials and the relatively high associated develop-6. Disposal routes for the contaminated soilment costs made this option unattractive.

It is possible that, in other circumstances, whereAs indicated earlier, the average PCB concen-tration measured in the ground was 120 mg kg−1, larger quantities of soil are to be treated and where

sufficient time is available for development andwith a peak of 1300 mg kg−1. These concentrationsare well outside the permissible limits in landfills, field trials, this technique may prove to be viable.

The final and successful option considered waswhich currently stand at 20 mg kg−1. Hence dis-posal to a licensed landfill was not an option. low temperature thermal desorption, developed by

British Aerospace, and discussed in the nextA remaining option was high temperature incin-eration. However, incineration costs are very high, section.at around £770 per tonne, including drumming thewaste and hauling it to a licensed incinerator. Thetotal cost would have been around £1.8M for the 7. Low temperature thermal desorptiontreatment of the 1200 m3 of soil, and hence othermore cost effective solutions were investigated. Low temperature thermal desorption at around

400°C is a proven remediation technology suitableThe soil washing option using water to separate


for the treatment of soils contaminated with low a few hundred mg/kg are seldom exceeded in feeds-tock to the SRU.and middle distillate organic compounds such as

solvents, gasoline, diesel and lubricating oils.Contaminated material is continuously fed 7.1. Pilot thermal remediation trials on the

excavated soilsthrough a rotary kiln where it is heated to temper-atures sufficient to evaporate/combust the contam-inants, effectively stripping them from the soil The principal soil contaminant was PCB

Aroclor 1254. From vapour pressure curves, theThe exhaust gases and any non-combustedvapours then pass through dust filters into a ther- boiling point of this Arochlor has been estimated

by Uzgiris et al. (1995) to be 335°C. In the samemal oxidiser unit or after burner, where controlledoxidation at a minimum temperature of 850°C study, the authors observe that recalcitrant PCB

residues that remain in certain clays following theensures extremely high destruction efficiencies ofthe contaminant vapours. The treated soil passes initial desorption kinetic phases can be eliminated

through soil treatment at temperatures in excessout of the plant, and is available for re-use. Itlooks much the same as the original soil that of the contaminants’ boiling points. Thus, thermal

treatment with the SRU at temperatures in excessentered the plant, except that it is free of thecontaminants and is virtually sterile. of 400°C was expected to be successful. Mass

balance calculations indicated that thermal treat-It may therefore be used as an engineering fill,or for resale. Treatment is, of course, exempt from ment of the soil would not result in breach of the

stringent emissions consents stipulated in thethe Landfill Tax. The process is summarised inFig. 3. plant’s authorisation. These assumptions were

tested in a sequence of small pilot trials, treatingThe British Aerospace Royal Ordnance(BAeRO) thermal soil remediation unit (SRU ) is one truck load of material (ca. 20 tonnes). Results

are presented in Table 1. The soil treatment ratebased on a standard design used in the USA, andwas manufactured by Gencor Beverley. The plant for the specific trial was 18 tonnes per hour.

Wendt (1994) describes the reaction chemistryhas been substantially modified to comply withUK Integrated Pollution Control (IPC) legislation and kinetic paths in the destruction of chlorinated

hydrocarbons (CxClyHz), stating that their princi-in accordance with its classification as a hazardous

wastes incinerator. pal influences on the combustion process are accel-eration of high hydrocarbon and carbon monoxideArticle 6 of the EC Directive on the Incineration

of Hazardous Wastes states in paragraph 2 that (CO) production, and the formation of significantquantities of hydrochloric acid vapour (HCl ),wastes with a halogenated organic content of up

to 10 000 mg kg−1 (1%) can be incinerated at a which in turn retards CO burnout. Measurementof products of incomplete combustion (totalminimum temperature of 850°C with a minimum

residence time of 2 s. These are the conditions in hydrocarbons [VOCs] and CO) has been proposedby the United States Environmental Protectionthe oxidiser unit of the SRU. However, for treat-

ment of PCBs, soil contaminant concentrations of Agency ( US EPA) as a means of controlling stack

Fig. 3. Simplified overview of thermal desorption unit.


Table 1Summary of PCB thermal treatment trial results

Material Initial PCB (mg kg−1)a Treated PCB (mg kg−1)a HCl (mg m−3) VOC (mg m−3) COb (mg m−3)PCB soil 24.5±4.6c 0.16±0.055c 2 5 6Clean soil <0.1 <0.1 <2.7 8.4 2

a Sum of congeners: 28, 52, 101, 118, 138, 153, 180. PCB analysis by GCMS in single ion mode (SIM).b Hourly average of continuous monitoring.c Sample standard deviation.

emissions of reformation products of concern, such material was delivered to the BAeRO facility inChorley, Lancashire. Up to 20 loads per day wereas polychlorinated dibenzo-p-dioxins (PCDDs)

and polychlorinated dibenzofurans (PCDFs), delivered between 7th and 25th November 1996,and transferred to undercover, leachate controlledoften collectively referred to as dioxins (Ancharya

et al., 1991). storage on arrival.Pre-treatment crushing and screening of theLevels of HCl, VOC and CO observed in the

pilot study were comfortably within the IPC SRU contaminated soil was required to reduce rock andclay lump size to below 50 mm (2 in) for treatmentemission consents, set at 10 mg m−3, 20 mg m−3

and 100 mg m−3, respectively. Direct measurement by the SRU. It was necessary to add quantities ofconcrete, crushed to below 100 mm, to the materialof dioxins was not possible within the limited

treatment period of the pilot study. However, fed into the crusher/screener to prevent caked clayfrom building up and blocking the pre-treatmenttreatment of the PCB contaminated soil was not

found to adversely influence the indicator stack system. The pre-treatment process also providedan effective means of homogenising the contami-emissions, in the context of typical variability

observed in daily measurements. There was conse- nated material, reducing contaminant hot spots,facilitating thermal treatment, and enhancing uni-quently no evidence to support enhanced forma-

tion of dioxins. Direct dioxin measurement was formity of plant output. Material was treated atan average rate of 12 tonnes per hour.undertaken, however, on commencement of full

scale treatment. All prescribed emissions were monitored at thecommencement of full scale treatment, byPCB levels achieved by thermal remediation

were well within the target value of 1 mg kg−1 Environmental Safety & Analytical Services(ESAS), University of Sunderland, UK. This pro-(sum of congeners 28, 52, 101, 118, 138, 153 and

180). Remediation of the full soil consignment vided a succinct measurement of all relevant emis-sion parameters, and confirmation of correctusing the BAeRO SRU was therefore proposed.

The target value of 1 mg kg−1 was adopted for the calibration of on-line monitoring equipment. Soilanalysis for routine quality control was undertakentreated soil as it was intended that the treated

material would be used for landscaping purposes by BAeRO Environmental Services Group (ESG)laboratories. A typical GCMS chromatogramon the BAeRO site in Chorley.illustrating clean-up efficiency for PCBs is pre-sented in Fig. 4. Results of emissions tests and7.2. Full scale thermal remediation of PCB

contaminated clay PCB clean-up levels achieved are presented inTables 2 and 3.

The above results demonstrate effective remedi-Procedures developed in the pilot remediationtrials were employed for thermal treatment of the ation of PCB contaminated soil using low temper-

ature thermal desorption. Thermal treatment hasfull consignment of 2300 tonnes of soil contami-nated with PCB Aroclor 1254. Since the total been shown to remove PCBs from a difficult soil

matrix sufficiently for the soil to pass stringentquantity of material was too small for transportof the SRU to the site, to be cost effective, the residual contamination values, without unaccept-


Fig. 4. GCMS SIM chromatogram from PCB contaminated soil before and after thermal treatment.

Table 2Average gaseous emissions during thermal PCB treatment

CO SO2 NOx

VOCa HCl HF Dioxinsb

Valuec 9.4 not detected 165 15 5 1.5 0.013Consent 100 50 190 20 10 2 1

a Total organic carbon.b Toxicity equivalent (TEQ) ng m−3 corrected to 11% O2, dry at STP.c All data except for dioxins are expressed in mg m−3 corrected to 11% O2, dry at STP.

able impacts on the environment and at a cost pertonne significantly lower than that of high temper-Table 3

Average non-gaseous emissions during thermal PCB treatment ature incineration.

Total Total PCBs in outputparticulate metals soila

Valueb 14.5 0.15 0.1±0.1 8. Discussion and conclusionsConsent/target 20 1 1

Three remediation strategies were adopted ina Sum of congeners 28, 52, 101, 118, 153 and 180.relation to the PCB contamination at the site.b All data are expressed in mg m−3 corrected to 11% O2, dry

at STP. (i) Where the contamination was light ( less


than 20 mg kg−1) the soil was excavated and dis- costs, and the treated soil retained its essentialcharacteristics and was re-used.posed of to a licensed landfill.

(ii) Where there was heavier contamination (upto 1300 mg kg−1) of soil which contained a rela-tively high clay fraction, four techniques wereconsidered. Soil washing and bioremediation were

Referencesfound not to be viable/effective. Solvent washingtrials and the relatively high associated develop-

Anon, 1992. Soil Sampling and Analysis – Practices and Pitfalls.ment costs made this option unattractive. TheSpecial Feature, The Hazardous Waste Consultant.most effective and commercially attractive solution

Dutch Ministry of Housing, 1994. Dutch Intervention Valueswas found to be low temperature thermal desorp- and Target Values – Soil Quality Standards. Dutch Ministrytion (LTTD). of Housing, Spatial Planning and Environment.

Canadian Council of Ministers of the Environment, 1991.(iii) In other areas of the site where access forReport CCME EPC-CS34, Interim Canadian Environmen-investigation was difficult, a probabilistic risktal Quality Criteria for Contaminated Sites, September.assessment was adopted to evaluate the potential

Uzgiris, E.E., Edelstein, W.A., Phillipp, H.R., Iben, I.E.T.,risk to humans and potential costs associated with 1995. Complex thermal desorption of PCBs from soil. Che-any remediation works required. mosphere 30 (2), 377–388.

Wendt, J.O.L., 1994. Combustion science for incinerator tech-The above case study demonstrates the success-nology. 25th International Symposium on Combustion, Theful use of different remediation/assessmentCombustion Institute, pp. 277–289.approaches to deal with the PCB contamination

Ancharya, P., DeCicco, S.G., Novak, R.G., 1991. Factors thatat the site. LTTD proved to be the preferred option can influence and control the emissions of dioxins and furansfor the more heavily contaminated soil with a cost from hazardous waste incinerators. J. Air Waste Manag.

Assoc. 41 (12), 1605–1615.saving of roughly 75% compared with incineration

a case study of the management and remediation of soil contaminated with polychlorinated biphenyls

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