mathematical modeling using the new epa soil screening ...enhanced soil vapor extraction and soil...

TECHNICAL MEMORANDUM

Mathematical Modeling Using The New EPA SoilScreening Level Method To Establish PreliminaryRemediation Goals For Soil Contamination At theMalvern TCE Superfund Site

PREPARED FOR:Barbara Rudnick/USEPA

PREPARED BY: Mark Lucas

DATE: December 20, 1996

Introduction

Remediation at the Malvern TCE Superfund may involve addressing the contaminated soil andgroundwater at the two separate areas of concern (main plant area, former disposal moundedarea). Methods of soil remediation under evaluation for the feasibility study include excavationand ex-situ treatment along with common insitu methods such as soil vapor extraction, thermal-enhanced soil vapor extraction and soil washing. The public health risk assessment for the sitehas indicated that contaminated subsurface soils pose a risk to construction workers involved inexcavation activities. More important however, contaminated soils represent a continuingsource of contamination to groundwater beneath the site. In the area around the Malvern site,there are approximately 55 residences using wells that withdraw water from the water tableaquifer within the Cambrian-Age Ledger Formation, a carbonate bedrock unit that underlies thesite. Subsequently, the primary of objective of implementing soil remediation at both areas ofconcern at the Malvern TCE site is to prevent further leaching of contaminants in soil to theunderlying groundwater.

The objective of the work described in this memorandum was to use the EPA Soil ScreeningLevel (SSL) method to quantify the potential transfer of contaminants from subsurface soil togroundwater and to establish preliminary remediation goals (PRGs) for the subsurface soil thatwould be protective of human health. The SSL method for the partitioning of contaminantsfrom soil to groundwater was issued by USEPA in April 1996 as a portion of the Soil ScreeningGuidance. The Soil Screening Guidance was developed to help standardize and accelerated theevaluation and remediation of contaminated soils at National Priorities List (NPL) withpotential future residential land use. The guidance provides methodology for calculating riskbased, site-specific, SSLs for contaminants in soil that may be used to identify areas requiringfurther investigation at NPL sites.

The guidance also indicates that SSLs can be used as PRGs provided appropriate conditions arecompatible with the assumptions applied to developing the SSLs. PRGs may then be used as abasis for developing final clean-up levels based on the nine-criteria analysis described in theNational Contingency Plan [Section 300.430 (3) (2) (I) (A)]. Similar to other methods used for

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MATHEMATICAL MODELING USING THE NEW EPA SOIL SCREENING LEVEL METHOD TO ESTABLISH PRELIMINARY REMEDIATION GOALSFOR SOIL CONTAMINATION AT THE MALVERN TCE SUPERFUND SITE

developing PRGs, SSLs should only be used as cleanup levels when a site-specific nine-criteriaevaluation of the SSLs as PRGs for soil indicates that a selected remedy achieving the SSLs isprotective, complies with Applicable or Relevant and Appropriate Requirements (ARARs)andappropriately balances the other criteria, including cost (USEPA, 1996).

Prior to initiating this study, CH2M HILL evaluated vadose zone transport models in regard totheir suitability in establishing PRGs and presented the results of the evaluation in a technicalmemorandum to EPA. These model codes ranged in complexity from a simple series ofanalytical equations developed by Summers (1980) to relatively complex numerical, finitedifference and finite element codes. The capability of these codes and their salient features *andshortcomings were compared against subsurface conditions at the Main Plant area (MPA) andformer disposal mounded area (FDA/MA) in the technical memorandum. Upon review byEPA, it was agreed to test the new SSL method for determining PRGs at the Malvern TCE site.The underlying assumptions of the SSL methodology is well suited to the subsurface conditionsat both the MPA and FDA/MA.

Site Conceptual Model

Developing a conceptual site model is a critical initial step in applying mathematical modelingmethods such as the SSLs to determining PRGs. A conceptual site model is a three-dimensionalportrait of site conditions that illustrates contaminant distributions, release mechanism, exposurepathways, migration routes and potential receptors (USEPA, 1996). The conceptual site modeldocuments current site conditions and is supported by maps, cross sections, and site diagramsthat illustrate contaminant release and migrations to potential receptors. After developing theconceptual site model, the model is compared to underlying assumptions used for developingthe SSLs. If most of the conditions portrayed in the conceptual site model are not compatiblewith the assumptions for the SSL, the analyst should consider selecting a different method fordetermining PRGs.

The Malvern TCE site consists of two areas of concern that are separated by 1,900 feet alongthe Transcontinental Gas Pipeline in East Whiteland Township, Chester County, Pennsylvania(Figure A-l). The site is owned and operated by Chemclene who continues to sell hydraulic oiland hydrogen peroxide from the location. Combined site acreage for the MPA and FDA/MAtotals 5 acres. Brief simplified conceptual site models are developed for each area of concernbelow. More detailed site information on the hydrogeology, contaminant distribution andcontaminant fate and transport appears in the Draft RI Report (CH2M HILL, 1996).

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" is- :MATHEMATICAL MODELING USING THE NEW EPA SOIL SCREENING LEVEL METHOD TO ESTABLISH PRELIMINARY REMEDIATION GOALS

FOR SOIL CONTAMINATION AT THE MALVERN TCE SUPERFUND SITE

MPAi

The MPA is approximately one acre in size and consists of a former solvent recycling facilitythat processed TCE, PCE, 1,1,1-TCA and MEG waste solvents. The MPA contains adistillation building where solvents were recycled in two 220 gallon stills, a storage building, aconcrete pad area, and an above ground storage tank area. Prior to 1986, four undergroundstorage tanks were located end to end on the east side of the MPA. Three tanks were used tostore MEG, and the remaining tank stored gasoline. Collection of surface and subsurface soilsamples over several episodes of sampling at the MPA indicates that the greatest soilcontamination is associated with the former underground storage tank (UST) area, the aboveground storage tank area (AST), and an area beneath the present loading dock where wastesolvent distillate was disposed on the ground surface and allowed to seep into the subsurface.Analytical data from subsurface soil samples indicates that contamination is distributed throughthe soil section to the top bedrock at an average depth of 60 feet below grade.

The subsurface beneath the MPA consists of 40 to 100 feet of heterogeneous soils overlying theLedger Formation, a thick bedded dolomite that defines the bedrock basement beneath the site(Figure A-2). Typical of a mature karst terrain, the top of the Ledger Formation exhibitspinnacle topography that varies in elevation by up to 40 feet in a lateral distance of less than100 feet. Because of the pinnacle bedrock surface, the thickness of the unconsolidated sectioncan vary significantly. Unconsolidated sediments consist of red-brown silts and silty claysinterbedded with well sorted medium to fine grained sands (Figure A-3). Contacts betweenlithology types are often gradational and appear to grade both vertically and laterally. Becauseof the oxidized nature of the unconsolidated sediments, the organic content of soils appears low.Organic debris such as ligntized stems and root traces were observed concentrated in occasionalthin laminae.

The water table occurs at or below the average elevation of the soil bedrock interface (elevation300 feet MSL). Subsequently, the unconsolidated section comprises a relatively thick ( 60 to 80feet) vadose zone beneath the MPA. Furthermore, because of the position of the water table,the moisture content of unconsolidated materials is generally low with the exception of soilsnear the ground surface that are saturated from precipitation events.

At the MPA, the Ledger Formation is often mantled by a thin zone of weathered rock thatranges in thickness from 2 to 10 feet. The Ledger Formation consists of tan to gray thickbedded dolomite. Generally the rock matrix is competent with no primary porosity.Groundwater flow in the Ledger Formation at the MPA occurs along solution channels, andmesocopic fracture fabrics such as joints, cleavage, faults and bed partings.

Water level monitoring at wells screened in the Ledger Formation indicates that water levelsrespond quickly to precipitation events suggesting that hydraulic conditions in the LedgerFormation are unconfined. Potentiometric surface maps were generated from monthly waterlevel collection episodes by USGS and indicate that groundwater flows to the northeast towardthe Catanach Quarry along a hydraulic gradient controlled by quarry dewatering.Groundwater beneath the MPA flows to the northeast at gradient of 0.014 ft/ft. The MPAappears to lie on the distal tip of the regional cone of depression caused by pumping at thequarry complex. Equipotential lines are elliptical and open to the northeast. Inferred flowlinesconverge to the northeast (Figure A-4).

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Testing of extraction wells installed by USGS has shown that well yields at the MPA varysignificantly ranging from 2 to 20 gpm in wells screened across approximately the same zoneand located only 50 to 100 feet apart. Two 24-hour constant rate aquifer tests at the MPAyielded an average transmissivity of 566 ftVday.

Soil contamination at the MPA consisted of common chlorinated aliphatic hydrocarbonsdisposed at the site including PCE, TCE and 1,1,1-TCA along with less halogenated andchlorinated forms (presumably attenuation byproducts) such as 1,2-DCE (total), 1,1-DCE, and1,1-DCA. Aromatic hydrocarbons were also encountered in samples collected below the formerunderground storage tank area. These compounds consisted of benzene, ethylbenzene, tolueneand xylene encountered in soil samples below the former UST area.

Beneath the former UST area, the AST area and distillate condensate disposal area, elevatedconcentrations of CAHs were distributed from near the ground surface to the top of bedrockor the water table surface. Unlike the CAHs, aromatic hydrocarbons and SVOCs were notpersistent in the subsurface soil to the water table. Additional contaminant sources were notencountered during the RI. Contaminant distribution in the vadose zone is often a importantissue in selecting a mathematical method for determining PRGs. Many vadose zone modelssimulate a contaminant source at the ground surface only, while others can simulatecontamination distributed throughout the vadose zone.

Groundwater contamination beneath the MPA consists primarily of CAHs including PCE,TCE, cis 1,2-DCE, trans-l,2-DCE, 1,1-DCE , 1,1-DCA and vinyl chloride. With the exceptionof toluene, aromatic hydrocarbons have not been detected in groundwater samples from theMPA. The highest total VOC concentrations occur on the east boundary of the site, butdegrade quickly to less than 10 ug/1 in downgradient monitor wells located 150 feet away(Figure A-5). Concentrations in the core of the contaminant plume beneath the former USTarea are elevated above the one percent solubility limit for TCE and PCE suggesting that densenonaqueous phase liquids (DNAPL) may be present. However, DNAPLs were not observedduring conventional groundwater sampling or dye testing of samples collected from the bottomof monitor wells.

The most halogenated and chlorinated CAHs, PCE and TCE predominate at the MPAindicating that contaminants in soil continue to enter the groundwater system. Contaminationextends to 180 feet below the water table in the central portion of the contaminant plume, butappears to exhibit an average plume thickness of 150 feet across the rest of the site.

In summary, contaminants at the MPA consist of several classes of VOC and SVOCcompounds. These contaminants have migrated from spills at surface or near-surface sourcesthrough a thick heterogeneous vadose zone of unconsolidated sediments to the water table thatlies within a carbonate bedrock unit. Conceptually, surface sources are sufficiently closetogether to be considered a single, composite source. At present, contaminants are welldistributed through the soil column in the vadose zone.

Upon reaching groundwater, contaminants migrated vertically and horizontally with theambient hydraulic gradient. With the exception of toluene, an aromatic hydrocarbon, onlyCAHs were able to migrate through the vadose zone into the groundwater. Of these CAHs,the most chlorinated and halogenated forms, TCE and PCE are predominant in groundwater.Although concentrations of CAHs are sufficiently elevated in the central portion of the plumeto suggest DNAPL is present, no DNAPL has been observed during groundwater sampling or

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specifically designed DNAPL studies. Concentrations within the contaminant plume declinequickly with distance from the site boundary (and plume core). The contaminant plumeappears to extend only 150 feet downgradient.

FDA/MA

The FDA/MA is approximately 5 acres in size and lies north of the Transcontinental GasPipeline. The solvent distillation process produced a moderate amount of sludge which wasshipped to an approved disposal facility by a licensed transporter. In the past, drums containingstill bottom sludges were buried onsite in a wooded area located 1,900 feet southwest of theMPA. Both areas lie adjacent to the Transcontinental Gas Pipeline. During construction of thepipeline in 1952, a large area was excavated at the site of the present FDA. Over time, theexcavations were filled with discarded drums, derelict equipment, assorted rubbish, andexcavated soil. The excavations were approximately 30 feet by 50 feet by 15 feet deep. TheMA is located on the western edge of the FDA and was originally a depressed area formed as astormwater runoff trench which was filled with drums containing still bottoms fromChemclene's distillation process. The FDA/MA lies on the side of Bacton Hill, subsequentlyground surface elevation change from 380 to 360 feet MSL moving north to south across thesite.

The subsurface framework beneath the FDA/MA is similar to the MPA (Figure A-2) with athick vadose zone comprised of unconsolidated materials overlying the pinnacle topography ofthe top of the Ledger Formation. Similar to the MPA, soils are very heterogeneous, howeversilt and clay content is generally higher at the FDA/MA. In contrast to the relativelycompetent bedrock at the MPA, the Ledger Formation at the FDA/MA is extensivelyweathered. Immediately beneath the FDA/MA, differentially weathered bedrock ranges up to140 feet thick.

Groundwater beneath the FDA/MA flows to the south-southwest along a gradient that rapidlydecreases from 0.026 ft/ft north of the excavations to 0.001 ft/ft in the area south of theexcavations (Figure A-3). The geometry of the potentiometric surface does not appear relatedto the topography of the ground surface. This geometry could be related to the highlytransmissive character of the Ledger Formation in the area of the FDA/MA. Usually hydraulicgradients are higher in lower permeability media than in high permeability environments. Theunusual geometry of the potentiometric surface suggests that the area of elevated transmissivitymay be limited to the immediate area around the FDA/MA where underlying bedrock isdeeply weathered (Figure A-2). The average transmissivity of the Ledger Formation asdetermined from two 24-hour constant rate aquifer tests was 62,500 ftVday (Sloto, 1996).

Similar to the MPA, soil contamination at the FDA/MA consisted of VOCs including CAHsand aromatic hydrocarbons, and SVOCs like phenantlirene, napthalene, pyrene, chrysene, andphthalates. Contaminants were distributed throughout the vadose zone, althoughconcentrations appear to decline rapidly to residual levels below 25 feet below grade. TotalVOC concentrations decline below 10 ug/kg below 25 feet below grade. Contaminants insubsurface soil were detected to 42 feet below grade in a boring at the MA. The distributionpattern of contaminant concentrations at the MA suggests that contaminants persist in thesubsurface soil at low concentrations (several ug/kg) to the soil-bedrock interface.

Groundwater beneath the FDA/MA is contaminated with a number of CAHs including TCE,PCE, 1,1,1-TCA, cis 1, 2-DCE, 1,1-DCE and 1,1-DCA at concentrations exceeding USEPA

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MCLs. A monitor well immediately downgradient of the FDA/MA exhibited the mostelevated concentrations, with total VOC concentrations greater than 1,000 ug/1. In manymonitor wells at the FDA/MA, the concentrations of less chlorinated and halogenated forms ofCAHs, particularly cis 1,2-DCE, are equivalent to more chlorinated forms like TCE and PCE.As these compounds were not disposed at the FDA/MA, elevated concentrations of thesedegradation products suggests that attenuation of CAHs is fairly advanced at the FDA/MA.

A contaminant plume extends into.Hillbrook Circle development downgradient from theFDA/MA. The contaminant plume in the development appears discontinuous in map view(Figure A-7). Downgradient areas of elevated concentrations are adjacent to areas where -contaminants have not been detected. Areas of elevated concentrations may represent outliersof residual contamination from a once contiguous contaminant plume that is now receding.Evaluation of historical total VOC concentrations along the centerline of the contaminantplume have shown declining concentrations over several episodes of monitor and residentialwell sampling since 1990. Significant contamination (total VOCs greater than 10 ug/1) ingroundwater at the FDA/MA appears to extend to 150 feet below the water table, similar tothe MPA. Contaminant concentrations decline rapidly below 100 feet below the water table.

In summary, the site conceptual model for the FDA/MA is similar to the MPA. Contaminantsleaked from buried drums buried in two adjacent areas. Contaminants have migrated through athick vadose zone of heterogeneous, unconsolidated sediments to the water table that lies withincarbonate bedrock. At present, contaminants are dispersed throughout the vadose zone. At theFDA/MA, the carbonate aquifer has undergone extensive, localized, pervasive weathering and asa result is very transmissive. Upon reaching the water table, contaminants migrated bothlaterally and vertically with the ambient hydraulic gradients. Contamination migrated beneaththe Hillbrook Circle Development and contaminated local residential wells. To date, thecontaminant plume appears to have migrated 1,200 feet downgradient from the site.

The downgradient portion of the plume is discontinuous. Outliners of elevated contaminationlie adjacent to wells where no contamination was detected. The distribution of CAHs in theplume and concentration ratio of specific chemicals indicates that less halogenated andchlorinated forms like cis 1,2-DCE and 1,1-DCE are abundant as known parent forms such asPCE and TCE. Concentration ratios, the discontinuous nature of the downgradient portion ofthe plume and historical concentration trends indicate that natural attenuation of CAHs isoccurring in the contaminant plume at the FDA/MA.

Mathematical Modeling

Analytical Solution (SSL Method)

The Soil Screening Guidance (EPA, 1996) uses a simple linear equilibrium soil/water partitionequation to estimate contaminant release in soil leachate. The method also uses a water balanceequation to calculate a dilution factor to consider the reduction of soil leachate concentrationsfrom mixing in an aquifer. The SSL method however does not simulate flow or transport inthe vadose or saturated zones. Although the method is being used to determine PRGs, the SSLmethod was originally developed to screen sites or specific areas of sites for the need for furtherinvestigation. Results of SSL (surface soil, subsurface soil, soil to groundwater partitioning)equations are compared to risk-based concentrations. If the SSLs exceed risk-based

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concentrations, additional investigation and eventually remediation is required at the site.When SSLs are lower than calculated risk-based concentrations, an area of the site can beignored for further investigation.

The soil/water partition equation (Equation 1; Table 1) relates the concentrations ofcontaminants absorbed by organic carbon to soil leachate concentrations in the zone ofcontamination. The equation estimates SSLs corresponding to target leachate contaminantconcentrations (Cw). An adjustment has been added to the equation to relate sorbedconcentrations in the soil to the measured total soil concentration. This adjustment assumesthat soil-pore water, contaminants adsorbed to solids and soil gas are conserved during samplecollection, and that concentrations for a soil sample represent these three phases of contaminantdistribution in the vadose zone.

The use of the soil/water partition equation to calculate PRGs assumes an infinite source ofcontaminants extending to the top of the aquifer. At the MPA, this mode of contaminantdistribution is valid beneath the former underground storage tanks, the condensate distillationarea, and the above ground storage tanks where relatively elevated contaminant concentrationsare encountered from less than 5 feet below grade to the top of the water table. At theFDA/MA, the contaminant distribution also appears to extend from near the ground surface tothe top of the water table, although contaminant concentrations decline at depths greater than25 feet below grade.

As leachate moves through the vadose zone, contaminant concentrations are attenuated bysorption. Sorption and volatilization (accounted for with Henry's Law Constant) are the onlyattenuation mechanisms accounted for in the SSL equations. In the aquifer, dilution by cleangroundwater further reduces concentrations before contaminants reach a receptor. In the SSLmethod, the reduction in concentrations is addressed by a dilution attenuation factor (DAF).DAF is defined as the ratio of leachate concentration to receptor point concentration. Thereceptor point concentration is set at the MCL. The minimal DAF is 1.0 corresponding to nodilution or attenuation, and the concentration at a receptor equals the soil-leachateconcentration. The SSL considers only one groundwater attenuation processes: contaminantdilution in groundwater. A simple mixing zone equation derived from the water balancerelationship (Equation 3; Table 1) is used to calculate a site specific dilution factor.

Mixing zone depth is estimated from a third equation (Equation 3; Table 2) that relates mixingto aquifer zone thickness, along with other hydraulic coefficients used in Equation 2. Theresulting DAF at each site was multiplied by the MCL to determine the target soil leachateconcentration (CW) for a number of site contaminants. Assumptions to this SSL Method alsoaccommodate using a default DAF of 20 if site conditions are not well defined.

Application to Subsurface Soil PRGs

MPA

The SSL method was applied to a list of ubiquotas VOC contaminants in subsurface soil at theMPA as shown in Table A-2. These contaminants were common in a number of subsurfacesoil samples collected during the RI and during previous sampling episodes (CH2M HELL,1996). Many of these VOC compounds were also encountered in groundwater samples. PRGswere also estimated for aromatic hydrocarbons, although these compounds with the exceptionof toluene, were absent in groundwater samples. Although SVOCs were encountered in

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subsurface soil samples at the MPA, they were not encountered in groundwater samples, andsubsequently were not considered for this PRG evaluation.

Physiochemical coefficients such as the organic partition coefficient (Koc) and Henry's LawConstant (H') (Table A-3) for each compound were obtained from tables in the Soil ScreeningGuidance (USEPA, 1996). A default value of 0.002 was used as the organic fraction for soil atthe MPA based on the Soil Screening Guidance. This value is relatively high, given the absenceof organic material in the soil matrix at the MPA and was selected in the absence of actual totalorganic carbon (TOG) data for soil. Use of this value is probably not as conservative asutilizing lower actual values that result in less sorption and ultimately lower PRGs. Actual"organic fraction values, if lower than the default value would generate more conservative PRGs.Default values for soil particle density (Ps) and dry bulk density (Pb) were taken from the SoilScreening Guidance and appear in Table A-4.

Site specific DAF and mixing depth coefficients were estimated using Equations 2 and 3 inTable A-l. The length of the contaminant source (L) at the MPA comprised the AST, distillatecondensate disposal, and the former UST areas and was 250 feet long. Saturated thickness ofthe aquifer was assumed to be 150 feet based on the average depth of groundwatercontamination. Aquifer hydraulic conductivity was determined by dividing the averagetransmissivity value generated from the aquifer tests (566 ftVd) by the assumed saturatedthickness (150 feet) for an input value of 3.8 ft/day. Average infiltration rate was assumed to be14 inches per year based on an average yearly precipitation rate of 42 inches per year, with 1/3of the precipitation infiltrating the ground surface in a relatively open, unpaved area. Thehydraulic gradient at the MPA was 0.02 ft/ft.

Following a conservative approach, the contaminant receptor was assumed to be a residentialwell screened at the top of die aquifer at the core of the plume. In this scenario, the watertable serves as the potential receptor, and resulting SSLs will protect a groundwater userwithdrawing water at the site after soil and groundwater remediation is completed.

FDA/MA

Contaminants in soil at the FDA/MA were modeled in a similar manner as at the MPA. Asthe same contaminants are present at both sites, the only difference in model input parametersbetween the two sites were source length (150 feet), aquifer hydraulic conductivity (417 ft/day)and hydraulic gradient (0.001 ft/ft; Table A-3).

Preliminary Remediation Goals

Two sets of PRGs were calculated for contaminants at the MPA and FDA/MA based on sitespecific DAF and the USEPA recommended default DAF value of 20 (Tables A-4 and A-5).PRG values are displayed in Tables A-4 and A-5 along with the maximum contaminantconcentrations detected in subsurface soils at both sites and the Pennsylvania (PA) Act 2 Levels.Maximum contaminant concentrations and PA Act 2 Levels are displayed to help gauge thereasonableness of PRGs.

At the MPA ,PRGs estimated for the site-specific DAF ranged from 0.01 for vinyl chloride to8790 mg/kg for xylene. PRGs for 1,1-DCE and vinyl chloride based on the site specific values

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were less than conventional (non low detection) analytical detection limits. With the exceptionof toluene and xylene, PRGs estimated using the site specific DAF were less than PennsylvaniaAct 2 Levels by one to several orders of magnitude (Table A-4).

Estimating PRGs with a default DAF value of 20 for both sites results in a generating identicalPRGs for both the MPA and FDA/MA. With this approach PRGs range from 0.01 mg/kg to879 mg/kg. With the exception of vinyl chloride and 1,1 DCA, PRGs are less than one orderof magnitude less than PA Act 2 levels using the DAF default value. The PRGs for xyleneusing both the site specific and default DAF values are greater than PA Act 2 Levels.

PRGs calculated at the FDA/MA using a site specific DAF are only slightly lower than PRGsestimated using the default value (Table A-5). The site specific DAF at the FDA was 12.35 incomparison to 2.61 at the MPA. This difference appears mostly attributable to the differencebetween hydraulic conductivity coefficients for the two sites. The highly transmissive LedgerFormation beneath the FDA\MA accommodates a smaller mixing zone and subsequently ahigher, site specific DAF. With more dilution occurring in the aquifer, at the FDA/MA,higher soil leachate values are accommodated before exceeding the MCL at a potential receptor.

The USEPA SSL method appears to provide reasonable PRGs using both site specific anddefault DAF values. In only several cases, were PRGs lower than actual analytical detectionlimits. Using the EPA default value of 20 appears to provide the most reasonable values for theMPA. At the FDA/MA, there were only moderate differences between the site specific PRGsand PRGs estimated using a default DAF. To maintain a level of consistency in applying PRGsto the Malvern TCE Feasibility Study, CH2M HILL proposes to use the uniform set of PRGsestimated using the EPA recommended default: value of 20. The default generated valuesrepresent significantly more reasonable and applicable standards for the MPA. At theFDA/MA, the difference between the values generated by the two methods were generallyminimal.

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Table 1Summary of SSL Equations for Determining FRGs

at Malvern Superfund Site

Equation 1. Partitioning Equation for Migration to Ground Water

Screening Level in Soil (mg/kg) = Cw [Kd + —2—^———]

Default

- target soil leachate concentration (mg/1) MCL x DAF- soil-water partition coefficient (L/Kg) Kocxfx- carbon/water partition coefficient (L/Kg) chemical-specificb

fx - fraction organic carbon in soil (g/g) 0.002 (0.2%)8W - water-filled soil porosity (L ter/Lsoii) 0.39a - air-filled soil porosity (L /L ) n-9wPb - dry soil bulk density (Kg/1) 1.5n - soil porosity (Lp /L ) l-(Pb/Ps)Ps - soil particle density (Kg/1) 2.65H' - dimension less Henry's Law constant chemical-specific11

(assume to be zerofor inorganiccontaminantsexcept mercury)

DAF Dilution Factor

Table 1 (continued)Summary of SSL Equations for Determining FRGs

at Malvern Superfund Site

Equation 2. Derivation of Dilution Factor

KidDilutionFactor = 1 + IL

K - aquifer hydraulic conductivity (m/hr)i - hydraulic gradient (m/m)I - infiltration rate (m/yr)d - mixing zone (m)L - source length parallel to groundwater flow (m)

Equation 3. Estimation of mixing zone depth

d = (0.0112L2) 0.5 + da {1 - exp [(U)/(Kida)]}

d - mixing zone depth (m) - from top of water tableL - source length parallel to groundwater flow (m)i - infiltration rate (m/yr)K - aquifer hydraulic conductivity (m/yr)i - hydraulic gradient (m/m)da - aquifer thickness (m)

TABLE B-2Summary of Physiochemical Parameters for Contaminants Used for PRG Analysis

Chemical Koc(1> H(2)Parameter (Kg/I) Dimensionless

PCE 155 75.4TCE 166 42.21,1,1-TCA 110 70.5Cis, 1,2-DCE 35.5 16.7Trans, 1,2-DCE 52.5 38.51,1-DCE 58.9 1.11,1-DCA 31.6 23Vinyl Chloride 18.6 1.1Benzene 58.9 22.8Ethylbenzene 36.3 32.3Toluene 182 27.2Xylene 386 276

Source: USEPA Soil Screening Guidance

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TABLE B-3Summary of Model Input Parameters for SSL Method

Input Parameter

Foe

Ow

Oa

Ps

Pb

n

K

i

1

L

da

d

DAF

Description

Organic fraction in soil

Water filled porosity

Air filled porosity

Soil particle density

Dry bulk density

1-(Pb/Ps)

Hydraulic conductivity

Infiltration rate

Hydraulic gradient

Length of source

Aquifer thickness

mixing zone depth

Dilution factor*9

Units'"

fraction

fraction

fraction

Kg/I

Kg/I

unitless

ft/d

in/yr

ft/ft

ft

ft

ft

unitless

MPA

0.002

0.2

0.23

2.65

1.5

0.43

3.7

14

0.02

250

150

36.6

2.61

FDA/MA

0.002

0.2

0.23

2.65

1.5

0.43

417

14

0.001

150

150

17.1

12.35

(1) Units presented as common American units to benefit reader.(2) Site specific dilution factor. An EPA recommended default value of 20 was also tested as an inputparameter.

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mathematical modeling using the new epa soil screening ...enhanced soil vapor extraction and soil...

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