Sensitivity analysis of a soil leachability model for petroleum fate and transport in the vadose zone

Advances in Environmental Research 7 Ž2002. 59᎐72

Sensitivity analysis of a soil leachability model for petroleum fate and transport in the vadose zone Molly M. Gribb a,U , Katalin J. Bene b, Arthur Shrader c a

b

Department of Ci¨ il Engineering, Boise State Uni¨ ersity, 1910 Uni¨ ersity Dri¨ e, Boise, ID 83725, USA Department of Ci¨ il and En¨ ironmental Engineering, Uni¨ ersity of South Carolina, 300 Main Street, Columbia, SC 29208, USA c Assessment and Correcti¨ e Action Branch, Di¨ ision of Underground Storage Tank Management, South Carolina Department of Health and En¨ ironmental Control, Columbia, SC 29201-1708, USA Accepted 9 July 2001

Abstract Risk-based assessment methods are commonly used at petroleum-contaminated sites. In South Carolina, the Soil Leachability Model ŽSLM. is used to calculate site-specific target levels ŽSSTLs. for soils that may leach contaminants to groundwater. The SLM is a series of analytical equations that is based upon the Green and Ampt equation to predict infiltration rates and accounts for equilibrium partitioning and first-order biodegradation of the contaminant as it travels to groundwater. To reduce costs, many soil property inputs to the model are estimated using regression equations that relate textural classification to these physical properties. It was not known what effect errors in these inputs might have on the SSTLs computed with the SLM. Thus, a sensitivity analysis was performed to determine the influence of parameter variability on benzene and naphthalene SSTLs computed for three soil types and two groundwater depths. The results of this study indicate that SSTLs computed with the SLM are very sensitive to organic carbon content and biodegradation half-lives for sand, loam, and clay, saturated hydraulic conductivity for loam and clay and most soil input parameters for clay. Overestimation of organic carbon content or underestimation of biodegradation half-lives in sand, loam or clay, or underestimation of the saturated hydraulic conductivity in loam or clay can result in SSTLs that are orders of magnitude too large, and therefore, potentially unconservative. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Petroleum-contaminated soil; Risk-based corrective action; Site assessment; Underground storage tank; Contaminant transport

U

Corresponding author. Fax: q1-208-426-4800. E-mail address: [email protected] ŽM.M. Gribb..

1093-0191r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 1 . 0 0 1 0 7 - 1

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M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

1. Introduction In 1995, there were over 3800 confirmed releases of petroleum products from underground storage tanks in South Carolina. Historically, the state specified soil contaminant levels that were used to determine which contaminated sites would require clean up. Since the risk to human health and the environment associated with such sites depends on the potential pathways of exposure and proximity to receptors, many states, including South Carolina, have developed tiered, riskbased procedures for assessing petroleum-contaminated sites ŽUS EPA, 1998.. The basis for the tiered approach is detailed in American Society for Testing and Materials ŽASTM. Standard E 1739-95: Risk-Based Correcti¨ e Action Applied at Petroleum Release Sites ŽASTM, 1995.. In South Carolina, Tier 1 consists of comparing concentrations of contaminants at or near the source to risk-based screening levels ŽRBSLs. predetermined by the state. If these RBSLs are exceeded, remedial action is taken using the RBSLs as target values for clean up. If this is impractical, then interim remedial action Žpartial source removal or other action. may be taken to allow reassessment of the site under the tiered approach. Further evaluation is carried out if the site conditions vary significantly from those used to develop the RBSL values, or if it is thought that site-specific target levels ŽSSTLs. could differ substantially from the Tier 1 RBSLs, and allow for more cost-effective clean up. In Tier 2, SSTLs are based on site-specific inputs and relatively simple analytical solution methods. Remediation to Tier 2 SSTLs, interim remedial action or Tier 3 evaluation may follow Tier 2. In Tier 3, more sophisticated fate and transport models are used with site-specific data. A complete description of the South Carolina tiered approach is presented in the South Carolina Department of Health and Environmental Control ŽSC DHEC. guidance document for risk-based assessment of petroleum-contaminated sites ŽSC DHEC, 1998.. SC DHEC uses a series of analytical equations called the Soil Leachability Model ŽSLM. to calculate Tier 2 SSTLs based on field measurement of some soil and contaminant properties, and estimation of others. It was not known what effect errors in these inputs might have on the SSTLs computed with the SLM. Thus, a sensitivity analysis was performed to determine the influence of parameter variability on computed benzene and naphthalene SSTLs. Three soil types Žsand, loam and clay., and two distances between the source of contamination and groundwater, 152.4 Ž5 ft. and 914.4 cm Ž30 ft., were considered. SSTLs were first calculated with average soil property values, published contaminant biodegradation half-lives, and organic carbon and total petroleum hydrocarbon contents typical

of South Carolina sites. The soil and contaminant inputs were then varied and the SSTLs were recalculated and compared to the SSTLs obtained with average or typical input values to determine the effect of variability in the input values on calculated SSTLs. In this way, the effects of measurement andror estimation errors in the inputs to the SLM were determined.

2. The Soil Leachability Model (SLM) Menatti et al. Ž1994. first proposed use of the SLM to predict flow and transport of contaminants through unsaturated soil to groundwater at petroleum-contaminated sites. This set of analytical equations accounts for flow of the contaminated water from the source location through the unsaturated soil to the groundwater table, equilibrium partitioning of contaminant between the gas, liquid and solid phases, and first-order biodegradation. The rate at which the soil pore water moves toward groundwater is estimated using the Green and Ampt Ž1911. equation. A horizontal wetting front is assumed to exist, below which the soil is assumed to have constant moisture content. The soil above the wetting front is assumed to be saturated with a constant hydraulic conductivity and moisture content. The time it takes for the wetting front to reach a given depth, t w Žsec., is: tw s

Hw q Lf y h cr f Lf y Ž Hw y h cr . ln K Hw y h cr

ž

/

Ž1.

where f Žcm3rcm3 . is the air-filled porosity of the soil, K Žcmrs. is the hydraulic conductivity of the wetted zone, Hw Žcm. is the ponding depth of water at the surface, and Lf Žcm. is the depth of the wetting front from the surface. For a conservative approach, the saturated hydraulic conductivity value, K s , is used instead of a lower unsaturated value. In the SLM, Lf is defined as the distance from the source of soil contamination in the vadose zone to the top of the groundwater table. The capillarity of the dry soil ahead of the wetting front increases the hydraulic gradient. Green and Ampt Ž1911. accounted for this additional driving force with the critical pressure head parameter, h cr Žcm.. The contaminant travel time is assumed to be retarded with respect to that of bulk water due to equilibrium, linear sorption of the contaminant to organic matter in the soil. The retardation coefficient is defined as ŽFreeze and Cherry, 1979.: Rs

tgw ␳ s 1 q b ⭈ Kd tw n

Ž2.

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

where tgw and t w are the contaminant and bulk water travel times, respectively, for a given distance, ␳ b Žgrcm3 . is the bulk density of the soil, and n Žcm3rcm3 . is the porosity. K d Žcm3rg. is the soilrwater partitioning coefficient, which Karickhoff et al. Ž1979. and others have shown to be equivalent to the product of the partitioning coefficient for the contaminant between water and organic carbon, K oc Žcm3rg., and the fraction of organic carbon in the porous medium, f oc Žmgrkg.: K d s K oc ⭈ f oc ⭈ 10y6

Ž3.

In petroleum-contaminated soils, organic carbon associated with the total petroleum hydrocarbon content, TPH Žmgrkg., may be present. Residual heavy hydrocarbons can be effective sorbents for the lighter hydrocarbons ŽBoyd and Sun, 1990.. To account for this effect, Menatti et al. Ž1994. replaced f oc in Eq. Ž3. with the total organic carbon content of contaminated soil, f cs Žmgrkg.: f cs s f oc q

ž

TPH 1.724

/

Ž4.

where 1.724 is the conversion factor for calculating the amount of organic carbon from the total petroleum hydrocarbon content ŽLyman et al., 1990.. The time it takes for the contaminant dissolved in the aqueous phase to reach groundwater, tgw , is determined by multiplying the time it takes for water to travel from the source to groundwater by the value of R calculated using Eq. Ž2.. The concentration of contaminant dissolved in the pore water due to equilibrium partitioning between the gaseous, solid and liquid phases, C w Žmgrl., is given by ŽFeenstra et al., 1991.: C w s CS ⭈

␪r ⭈ ␳ w q ␳ b ␳ b ⭈ K d q ␪r q f ⭈ H

Cgw s C w ⭈ ey

0.693 t gw t 1r 2

where Cgw Žmgrl. is the contaminant concentration in the pore water when it reaches the groundwater table at time tgw Ždays., C w Žmgrl. is the contaminant concentration in the pore water at the source calculated from Eq. Ž5., and t 1r2 Ždays. is the first-order biodegradation half-life of the contaminant. A number of simplifying assumptions are inherent in the SLM. First, it is assumed that the contaminant emanates from a single point in the unsaturated zone where the maximum concentration of contaminant is discovered. It does not account for the volume of contaminant released, or the time that has elapsed prior to discovery. The model only accounts for vertical flow due to a constant head of water at the location of the source. Diffusion and dispersion are neglected. Finally, the SLM does not account for the presence or flow of non-aqueous phase liquids.

3. Use of the SLM in South Carolina In South Carolina, the SLM model was used by SC DHEC Ž1998. to calculate and publish Tier 1 RBSLs. If soil contaminant concentrations at a site are above Tier 1 RBSLs, Tier 2 SSTLs are calculated with the SLM to determine if remediation to less stringent target levels is possible. The RBSLs and SSTLs are the soil contaminant concentrations predicted to be protective of groundwater at the source location, and these values are compared to the measured soil sample total concentrations Ž CS . to determine the need for remedial action. SC DHEC modified the equations of the SLM as previously presented for these purposes. In addition, SC DHEC elected to account for dilution and attenuation of contaminant when it mixes with the groundwater. The maximum contaminant concentration in the soil pore water Žat the source. protective of groundwater, C w,p Žmgrl., is calculated as:

Ž5. C w,p s CGWsstl ⭈ DAF ⭈ e

where CS Žmgrkg. is the mass-based total concentration of contaminant in the solid, liquid and gaseous phases of the soil sample, ␪ r is the residual moisture content of the soil Žcm3rcm3 ., ␳ w is the density of water Žgrcm3 ., and H is the dimensionless Henry’s law constant for the contaminant. This equation does not account for any non-aqueous phase liquid that may be present in heavily contaminated soils. Finally, a first-order rate equation is used to account for biodegradation of the contaminant as it travels through the unsaturated soil to groundwater: Ž6.

61

0.693 t gw t 1r 2

Ž7.

where CGWsstl Žmgrl. is the SSTL for the contaminant in groundwater as predicted from the RBSL for groundwater, or from a fate and transport model, and DAF is the dimensionless dilutionrattenuation factor. Site-specific target levels for soil contamination Ž CS sstl Žmgrkg.., at the source location are calculated with the following equation ŽSC DHEC, 1998.: CS sstl s C w ,p ⭈

␳ b ⭈ K d q ␪r q f ⭈ H ␪r ⭈ ␳ w q ␳ b

Ž8.

where the parameters are as defined previously. Using this approach, SC DHEC developed Tier 1 RBSLs for

62

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

two broad classes of soils with input values typical of South Carolina sites for separation distances ranging from Lf s 152.4 to 914.4 cm. DAF values of 8.0 and 2.0 were used for soils with K s values greater than 10y4 cmrs, and soils with K s values less than or equal to 10y4 cmrs, respectively. The US EPA Ž2001. maximum contaminant level ŽMCL. of 0.005 mgrl for benzene, and 0.025 mgrl for naphthalene were used as the values of CGWsstl ŽSC DHEC, 1998.. For Tier 2 assessment, SC DHEC requires measurement of TPH, f oc , Lf , and the weight fractions of sandand clay-sized particles of the soil at each site. The weight fractions according to the US Department of Agriculture ŽUSDA. classification system ŽUSDA, 1951. are used to estimate K s , ␳ b , h cr , ␪ r , and n via graphical relationships developed by Rawls and Brakensiek Ž1989.. As an example, the Rawls and Brakensiek Ž1989. K s contour plot as modified by SC DHEC Ž1998. is shown in Fig. 1. These graphs are based on regression

equations that are valid for soils containing sand-sized fractions between 5 and 70%, and the clay-sized fractions between 5 and 60%. An organic matter content of 1.5% was assumed. Other assumptions include setting the air-filled porosity equal to the difference between the porosity and residual moisture content Ž f s n᎐␪ r ., and setting Hw equal to the South Carolina annual recharge depth of 25 cm. Inputs such as the Henry’s law constant, organic carbonrwater partitioning coefficients, and first-order biodegradation half-lives of the contaminants are set equal to published values. Lf is taken as the distance from the location of the highest soil contaminant measurement to the groundwater table. The CGWsstl values are set equal to groundwater RBSLs as determined by SC DHEC Ž1998., or SSTLs as calculated with a fate and transport model to compute maximum acceptable concentrations in the pore water Žat the source location. that will not result in unacceptable groundwater contamination.

Fig. 1. Rawls and Brakensiek Ž1989. graphical method for determining K s Žcmrs. for soils based on percentages of clay- and sand-sized particles by weight, as modified by SC DHEC Ž1998..

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

63

Table 1 Mean, low and high soil hydraulic input values Soil type

Parameter n Žcm3 rcm3 .

␪r Žcm3 rcm3 .

␳b Žgrcm3 .

Ks Žcmrs.

Sand Mean Low High

0.437 0.311 0.563

0.02 0.0001 0.058

1.49 1.16 1.83

5.8= 10y3 1.4= 10y3 1.0= 10y2

y11 y20 y0.1

Loam Mean Low High

0.463 0.287 0.639

0.03 0.0001 0.124

1.42 0.80 2.03

1.9= 10y4 1.4= 10y5 2.8= 10y3

y32 y73 0.1

Clay Mean Low High

0.475 0.379 0.571

0.09 0.0001 0.3

1.39 1.14 1.65

1.7= 10y5 1.7= 10y7 2.8= 10y4

y72 y135 y0.1

4. Methods The sensitivity analysis of the SC DHEC SLM was performed to determine the potential effects of measurement or estimation errors in soil and contaminant transport input parameters on CS sstl values predicted with the model. Each input parameter value was varied from its mean or typical value to high and low values, and the resulting changes in the CS sstl values were calculated. Three soil types Žsand, loam and clay., two contaminants Žbenzene and naphthalene ., and two separation distances between the source and groundwater Ž152.4 and 914.4 cm. were studied. Tables 1 and 2 give the ranges of soil and contaminant transport parameters that were varied, and Table 3 lists the soil, contaminant and site parameter values investigated. The mean values of n, ␪ r and K s for sand, loam and clay were set equal to mean values published by Rawls and Brakensiek Ž1989. for each soil type in their Table 3. The high and low values of n were set equal to the Table 2 Additional soil and contaminant input values varied in the sensitivity analysis Parameter

Typical Low High

TPH Žmgrkg. a

300 1 1000

foc Žmgrkg. a

1000 100 10000

t1r2 Ždays. Benzene b

16 1 1000

Naphthalene 48b 1 1000

mean value reported by Rawls and Brakensiek Ž1989., plus or minus two standard deviations Žalso obtained from Table 3.. The high value of ␪r was set equal to the mean plus two standard deviations, while the low value was arbitrarily set close to zero Ž0.0001. to avoid unrealistic negative values. The high K s for the sand class was arbitrarily set equal to 1.0= 10y2 cmrs and the low value equal to the lowest contour value Ž1.4= 10y3 cmrs. for sandy soil in Fig. 1. High and low values of K s for loam were set equal to the high and low contour values for loamy soils in Fig. 1. The high value for clay was set equal to the high contour value for clay soil Ž2.8= 10y4 cmrs. in Fig. 1 and the low value was arbitrarily set equal to 1.7= 10y7 cmrs. Rawls and Brakensiek Ž1989. did not publish means or standard deviations for ␳ b . However, they did pub-

Table 3 Additional soil, site and contaminant input values Parameter

Value

CGW sstl Žmgrl.

0.005 Žbenzene. 0.025 Žnaphthalene .a 152.4 Ž5 ft. and 914.4 Ž30 ft. 25 cm 81 Žbenzene.b 1543 Žnaphthalene .b 0.226 Žbenzene.b 0.002 Žnaphthalene .b 1.0c

Lf Žcm. Hw Žcm. Koc Žcm3rg. H Ž᎐. DAF Ž ᎐ .

a

a

b

b

Representative values selected for this study. Values used in SC DHEC approach ŽHoward et al., 1991..

hcr Žcm.

SC DHEC Ž1998.. Montgomery and Welkom Ž1991.. c Value used in this work.

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

64

lish n values, which were used to calculate mean ␳ b values for the soils from the equation: ␳ b s 2.65 Ž 1 y n .

Ž9.

where 2.65 is the assumed soil particle density in grcm3. High and low ␳ b values were obtained with mean n values, plus or minus two standard deviations, for each soil type. Mean, high and low h cr values were computed from the equation ŽRawls and Brakensiek, 1989.: h cr s

2 q 3␭ 1 q 3␭

ž h2 / b

Ž 10.

where ␭ is the dimensionless Brooks and Corey Ž1964. pore size index, and h b Žcm. is the Brooks and Corey bubbling pressure head. The high and low values for h cr were calculated using the high and low values of ␭ and h b calculated from mean values reported by Rawls and Brakensiek Ž1989., plus or minus one standard deviation. If an unrealistic positive value of h cr was obtained, it was replaced by a value of y0.1 cm. The values of TPH and f oc investigated were selected to reflect possible conditions at petroleum-contaminated sites in South Carolina. TPH and f oc values are generally equal to or less than 300 and 1000 mgrkg at such sites, respectively ŽMiner, 1998.. The high and low values for TPH in Table 2 reflect measured high and low values observed around the state at contaminated sites. High and low f oc values were arbitrarily set one order of magnitude larger and smaller, respectively, than the assumed mean value of 1000 mgrkg that was used by SC DHEC Ž1998. to establish RBSLs. Mean t 1r2 values for benzene and naphthalene were set equal to the default values used in the SC DHEC SLM Ž16 and 48 days, respectively; Howard et al., 1991.. Since t 1r2 values vary significantly in the field, 1

and 1000 days were selected to represent the low and high values for both contaminants, respectively. CGWsstl values were set equal to 0.005 mgrl for benzene and 0.025 mgrl for naphthalene. The effects of the dilutionrattenuation factors were not considered in this work, so a DAF value of 1.0 was used. Likewise, the effects of variable soilrorganic carbon partitioning coefficients, Henry’s Law constants for the contaminants, or ponding depths were not investigated. The mean or typical input values from Tables 1 and 2 were used to calculate the site-specific target levels of soil contamination Ž CS sstl values. for the two separation distances and contaminants. If these CS sstl values are considered to be the ‘true’ values for the prescribed sets of site conditions, then effects of measurement or estimation errors in the various inputs to the SLM can be determined by examining changes in the calculated CS sstl values when the high and low parameter values are used. As some CS sstl values were several orders of magnitude larger than those calculated with mean or typical inputs, the ratios of the CS sstl values computed with high or low values with respect to the mean or typical CS sstl values were also calculated. Ratios less than 1.0 correspond to CS sstl values that are smaller than those calculated with mean or typical input values, while ratios greater than 1.0 correspond to CS sstl values that are greater than what would be predicted with a mean or typical parameter values. Ratios greater than 1.5 or less than 0.5 Ž"50% change from the CS sstl values calculated with the mean or typical inputs. were considered significant in the discussion that follows. Calculated CS sstl values that were greater than the saturation concentration Že.g. the soil contaminant concentration for which the gaseous and dissolved pore water phases are saturated with the contaminant. represent conditions for which the specified risk level cannot be reached or exceeded for the given exposure pathway ŽASTM, 1995..

Table 4 CS s st l Žmgrkg. values calculated for benzene in sand with low, mean, and high parameter values for a 152.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

7.56= 10y4 8.12= 10y4 9.55= 10y4 8.51= 10y4 8.63= 10y4 7.81 = 10y4 4.89= 10y4 9.08= 10y4

9.47= 10y4 9.25= 10y4 7.83= 10y4 8.52= 10y4 8.50= 10y4 1.01= 10y3 4.50= 10y3 8.48= 10y4

0.89 0.95 1.12 1.00 1.01 0.92 0.58 1.07

1.11 1.09 0.92 1.00 1.00 1.19 5.29 1.00

CSsstl mean

8.51= 10y4

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

65

Table 5 CS s st l Žmgrkg. values calculated for benzene in sand with low, mean, and high parameter values for a 914.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

7.74= 10y4 8.40= 10y4 9.85= 10y4 8.78= 10y4 9.87= 10y4 8.07 = 10y4 5.02= 10y4 1.53= 10y3

9.86= 10y4 9.53= 10y4 8.10= 10y4 8.80= 10y4 8.66= 10y4 1.05= 10y3 4.95= 10y3 8.48= 10y4

0.88 0.95 1.12 1.00 1.12 0.92 0.57 1.74

1.12 1.08 0.92 1.00 0.98 1.19 5.63 0.96

8.79= 10y4

CSsstl mean

5. Results The CS sstl values resulting from varying parameters from their mean or typical values to high or low values Žpresented previously in Tables 1 and 2., and the ratios

of these CS sstl values with respect to values calculated with mean or typical inputs Ž CSsstl mean values. are shown for sand in Tables 4᎐7, for loam in Tables 8᎐11 and for clay in Tables 12᎐15. For benzene in sand with separation distances of

Table 6 CS s st l Žmgrkg. values calculated for naphthalene in sand with low, mean, and high parameter values for a 152.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

4.53= 10y2 4.56= 10y2 4.52= 10y2 4.53= 10y2 4.64= 10y2 3.87 = 10y2 1.08= 10y2 6.30= 10y2

4.54= 10y2 4.48= 10y2 4.55= 10y2 4.54= 10y2 4.52= 10y2 6.09= 10y2 4.12= 10y1 4.50= 10y2

1.00 1.01 1.00 1.00 1.02 0.85 0.24 1.22

1.00 0.99 1.00 1.00 1.00 1.34 9.08 0.99

4.53= 10y2

CSsstl mean

Table 7 CS s st l Žmgrkg. values calculated for naphthalene in sand with low, mean, and high parameter values for a 914.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

4.76= 10y2 4.82= 10y2 4.72= 10y2 4.77= 10y2 5.78= 10y2 4.08 = 10y2 1.09= 10y2 8.04= 10y1

4.80= 10y2 4.70= 10y2 4.84= 10y2 4.79= 10y2 4.66= 10y2 6.42= 10y2 6.48= 10y1 4.52= 10y2

1.00 1.01 0.99 1.00 1.21 0.85 0.23 16.8

1.00 0.98 1.01 1.00 0.98 1.34 13.6 0.94

CSsstl mean

4.78= 10y2

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

66

Table 8 CS s st l Žmgrkg. values calculated for benzene in loam with low, mean, and high parameter values for a 152.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

8.29= 10y4 9.51= 10y4 1.36= 10y3 9.88= 10y4 4.14= 10y3 9.37 = 10y4 6.02= 10y4 8.53= 10y4

1.21= 10y3 1.19= 10y3 8.79= 10y4 1.05= 10y3 9.13= 10y4 1.19= 10y3 6.12= 10y3 8.48= 10y4

0.82 0.94 1.34 0.97 4.08 0.92 0.59 0.84

1.19 1.17 0.87 1.04 0.90 1.18 6.03 0.84

1.01= 10y3

CSsstl mean

Table 9 CS s st l Žmgrkg. values calculated for benzene in loam with low, mean, and high parameter values for a 914.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

1.58= 10y3 2.67= 10y3 3.28= 10y3 2.42= 10y3 1.99= 103 2.46 = 10y3 1.33= 10y3 ᎐a

4.36= 10y3 2.52= 10y3 2.50= 10y3 2.94= 10y3 9.75= 10y4 3.13= 10y3 9.02= 10y2 9.22= 10y4

0.59 1.00 1.23 0.91 7.50= 105 0.92 0.50 ᎐

1.64 0.95 0.94 1.11 0.37 1.18 33.9 0.35

2.66= 10y3

CSsstl mean a

CS s st l value greater than saturation concentration.

152.4 and 914.4 cm, the high and low values for all variables investigated result in CS sstl values that differ from those predicted with typical or mean values Ž CSsstl mean . by less than one order of magnitude. Increasing

the separation distance has little to no effect on calculated CS sstl values. The high value of f oc Ž10 000 mgrkg. results in CS sstl values that are over five-fold greater than the CSsstl mean obtained with the typical f oc

Table 10 CS s st l Žmgrkg. values calculated for naphthalene in loam with low, mean, and high parameter values for a 152.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

5.23= 10y2 5.44= 10y2 4.97= 10y2 5.12= 10y2 4.58= 10y1 4.55 = 10y2 1.14= 10y2 1.66= 102

5.41= 10y2 4.99= 10y2 5.67= 10y2 5.63= 10y2 4.54= 10y2 7.15= 10y2 1.63 4.53= 10y2

0.98 1.02 0.93 0.96 8.60 0.85 0.21 3.12= 103

1.02 0.94 1.07 1.06 0.85 1.34 30.6 0.85

CSsstl mean

5.33= 10y2

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67

Table 11 CS s st l Žmgrkg. values calculated for naphthalene in loam with low, mean, and high parameter values for a 914.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

1.96= 10y1 2.64= 10y1 1.28= 10y1 2.01= 10y1 ᎐a 1.99= 10y1 1.66= 10y2 U ᎐

2.69= 10y1 1.59= 10y1 4.19= 10y1 2.72= 10y1 5.02= 10y2 3.13= 10y1 ᎐a 4.86= 10y2

0.84 1.13 0.55 0.86 ᎐ 0.85 0.07 ᎐

1.15 0.68 1.80 1.17 0.22 1.34 ᎐ 0.21

2.33= 10y1

CSsstl mean a

Calculated CS s st l value greater than saturation concentration.

value Ž1000 mgrkg. for both separation distances. Decreasing t 1r2 from its typical Ž16 days. to its low value Ž1 day. results in a CS sstl that is 1.74-fold greater than

the CSsstl mean for the 914.4-cm separation distance. Increasing t 1r2 to 1000 days has little effect at either separation distance.

Table 12 CS s st l Žmgrkg. values calculated for benzene in clay with low, mean, and high parameter values for a 152.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

1.90= 10y3 2.48= 10y3 2.65= 10y3 1.98= 10y3 ᎐a 2.30= 10y3 1.43= 10y3 9.42= 102

3.15= 10y3 2.06= 10y3 2.33= 10y3 4.47= 10y3 1.10= 10y3 2.82= 10y3 4.60= 10y2 1.06= 10y3

0.77 1.01 1.08 0.80 ᎐ 0.94 0.58 3.83= 10y5

1.28 0.84 0.95 1.82 0.45 1.15 18.7 0.43

2.46= 10y3

CSsstl mean a

Calculated CS s st l value greater than saturation concentration.

Table 13 CS s st l Žmgrkg. values calculated for benzene in clay with low, mean, and high parameter values for a 914.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

1.96 1.31= 102 1.32= 101 5.57 ᎐a 1.55= 101 2.10 ᎐a

1.34= 102 1.13= 10y1 2.15= 101 1.13= 102 1.88= 10y3 1.90= 101 ᎐a 1.22= 10y3

0.12 7.89 0.80 0.34 ᎐ 0.94 0.13 ᎐

8.08 0.01 1.30 6.83 1.14= 10y4 1.15 ᎐ 7.39= 10y4

CSsstl mean a

1.65= 101

CS s st l greater than saturation concentration.

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

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Table 14 CS s st l Žmgrkg. values calculated for naphthalene in clay with low, mean, and high parameter values for a 152.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b hcr Ks TPH foc t1r 2

1.40= 10y1 2.18= 10y1 1.30= 10y1 1.14= 10y1 ᎐a 1.35= 10y1 1.60= 10y2 U ᎐

1.73= 10y1 7.43= 10y2 1.92= 10y1 3.82= 10y1 4.76= 10y2 2.10= 10y1 ᎐a 4.68= 10y2

0.89 1.38 0.82 0.72 ᎐ 0.86 0.10 ᎐

1.10 0.47 1.22 2.43 0.30 1.33 ᎐ 0.30

1.57= 10y1

CSsstl mean a

CS s st l value greater than saturation concentration.

For naphthalene in sand, the effect of varying the input value for f oc is more significant than for benzene. The low value of f oc results in CS s st l values that are 0.24- and 0.23-fold the CSsstl mean for 152.4 and 914.4 cm, respectively. Use of the high value of f oc results in CS sstl values that are 9.08- and 13.6-fold greater than the CSsstl mean for separation distances of 152.4 and 914.4 cm, respectively. Decreasing the t 1r2 value for naphthalene from its typical Ž48 days. to its low value Ž1 day. results in a CS sstl that is 16.8-fold greater than the CSsstl mean for the 914.4-cm separation distance. As with benzene, increasing the separation distance has little effect on CS sstl values calculated for naphthalene in sand, except for that calculated with the low value of t 1r2 , which increased by over 16-fold from its value at 152.4 to that at 914.4 cm. CS sstl values for benzene in loam for a separation distance of 152.4 cm are 4.08- and 6.03-fold greater

than the CSsstl mean when the low value of K s Ž1.40= 10y3 cmrs. and the high value of f oc are used, respectively. Varying all other parameters results in changes in the CS sstl for benzene that are less than "50% of the CSsstl mean . When the separation distance is 914.4 cm, the high value of n Ž0.639., and the high and low values of K s , f oc , and t 1r2 result in changes in the CS sstl value that are greater than or equal to "50% of the CSsstl mean . Use of the low value of K s Ž1.4= 10y5 cmrs. results in a CS sstl that is 7.5= 10 5-fold times greater than the CSsstl mean and approaches the saturation concentration of the soil, while the high value of K s Ž2.8= 10y3 cmrs. results in a CS s st l that is just 0.37-fold the CSsstl mean . Changing f oc from its typical value to low and high values results in CS sstl values that are 0.5- and 33.9-fold, respectively, the CSsstl mean for benzene in loam. Decreasing t 1r2 from its typical value of 48 to 1 day results in a CS sstl that is greater

Table 15 CS s st l Žmgrkg. values calculated for naphthalene in clay with low, mean, and high parameter values for a 914.4-cm separation distance from groundwater Parameter varied

CSsstl low Žmgrkg.

CSsstl high Žmgrkg.

Csstl low r Csstl mean

Csstl high r Csstl mean

n ␪r ␳b Hcr Ks TPH foc t1r 2

᎐a ᎐a ᎐a ᎐a ᎐a ᎐a 5.03= 10y1 ᎐a

᎐a 2.86= 101 ᎐a ᎐a 1.05= 10y1 ᎐a ᎐a 8.78= 10y2

᎐ ᎐ ᎐ ᎐ ᎐ ᎐ 6.54= 10y6 ᎐

᎐ 3.75= 10y4 ᎐ ᎐ 1.37= 10y6 ᎐ ᎐ 1.14= 10y6

CSsstl mean a

7.68= 104a

CS s st l value greater than saturation concentration.

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

than the saturation concentration for the soil, which means that the selected risk level cannot be reached, while increasing t 1r2 to 1000 days results in a decrease in the CS sstl to 0.35-fold the CSsstl mean . With a separation distance of 152.4 cm, CS sstl values for naphthalene in loam are 8.6-fold greater for low K s , 0.21-fold the CSsstl mean value for low f oc , 30.6-fold greater for high f oc , and 3120-fold greater for low t 1r2 than for when typical or mean input values are used. High and low values of K s , f oc and t 1r2 also significantly impact naphthalene CS sstl values in loam for a separation distance of 914.4 cm. For the low K s , the CS sstl calculated is greater than the saturation concentration of the soil for naphthalene, while the high K s yields a CS sstl that is 0.22-fold the CSsstl mean value. The low f oc results in a CS s st l that is 0.07-fold the CSsstl mean value, while the high f oc results in a CS s st l that is greater than the saturation concentration of the soil for naphthalene. The low and high values of t 1r2 result in CS sstl values that are greater than the saturation concentration of the soil for naphthalene, and 0.21-fold CSsstl mean value, respectively. All other low and high parameter values result in changes that are less than "50% for both contaminants and separation distances. For benzene in clay with a separation distance of 152.4 cm, the low K s value Ž1.7= 10y7 cmrs. yields a CS sstl that is greater than the saturation concentration for benzene in clay, while the high K s value Ž2.8= 10y4 cmrs. yields a CS sstl that is 0.45-fold the CSsstl mean obtained with a mean K s value of 1.7= 10y5 cmrs. The high value of f oc results in a CS s st l that is 18.7-fold the CSsstl mean . Varying t 1r2 from its mean of 16 days results in CS sstl values that are 3.83= 10 5-fold the CS sstl mean for the low value of 1 day, and 0.43-fold the CS sstl mean for the high value of 1000 days. For benzene in clay with a separation distance of 914.4 cm, the effects of changing parameter values from their mean or typical values are greater than "50% of the CSsstl mean for all parameters except ␳ b and TPH. The low Ž0.379. and high Ž0.571. values of n yield CS sstl values that are 0.12- and 8.08-fold the CSsstl mean value, respectively. The low Ž0.001. and high Ž0.3. values of ␪r yield CS s st l values that are 7.89- and 0.01-fold the CSsstl mean value, respectively. The low Žy135 cm. and high Žy1 cm. values of h cr yield CS s st l values that are 0.34- and 6.83-fold the CSsstl mean obtained with the mean h cr value of y72 cm, respectively. The low K s value again yields a CS s st l that is greater than the saturation concentration of the soil for benzene, while the high K s results in a CS s st l value that is four orders of magnitude smaller than the CSsstl mean . The low f oc results in a CS s st l that is 0.13-fold the CSsstl mean , while the high f oc results in a CS s st l value that is greater than the saturation concentration of the soil for benzene. Varying t 1r2 from its typical value of

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16 days results in CS sstl values that are greater than the saturation concentration of the soil for the low value of 1 day, and approximately four orders of magnitude smaller than the CSsstl mean for the high value of 1000 days. For naphthalene in clay with a separation distance of 152.4 cm, the high value of ␪ r yields a CS s st l that is 0.47-fold the CSsstl mean , while the high value of h cr yields a CS sstl that is 2.43-fold the CSsstl mean . The low K s again yields a CS s st l that is greater than the saturation concentration of the soil, while the high value results in a CS sstl that is 0.30-fold the CSsstl mean . The low f oc results in a CS s st l that is one order of magnitude smaller than the CSsstl mean , while the high f oc yields a CS sstl that is greater than the saturation concentration for naphthalene in clay. Varying t 1r2 from its mean of 48 days to the low value of 1 day also results in a CS sstl that is greater than the saturation concentration, while increasing t 1r2 to the high value of 1000 days results in a CS sstl that is 0.30-fold the CSsstl mean . For naphthalene in clay with a separation distance of 914.4 cm, greater changes in the CS sstl values are obtained by varying the inputs than is the case for sand or loam, or for clay with a 152.4-cm separation distance. In this case, only the low value of f oc and the high values of ␪ r , K s and t 1r2 yield CS s st l values that are less than the saturation concentration for naphthalene in clay. Changing parameter values from their mean or typical values results in changes in the CS sstl values that are greater than "50% for all parameters, except TPH. The high value of ␪r yields a CS s st l that is over four orders of magnitude smaller than CSsstl mean . The high K s , low f oc and high t 1r2 all result in CS s st l values that are approximately six orders of magnitude smaller than the CSsstl mean . In these analyses, the dilution and attenuation factor Ž DAF . was set equal to 1.0 in Eq. Ž7.. All of the benzene CS sstl values, including the computed CSsstl mean values, for sand at both separation distances are below the SC DHEC Ž1998. reporting limit of 5 = 10y3 mgrkg. All of the benzene CS sstl values computed for loam with low parameter inputs, and all of the high parameter inputs, except for f oc , are also below the reporting limits for the 152.4-cm separation distance. At the separation distance of 914.4 cm, the benzene CS sstl values computed for loam with all of the low parameter values, except for K s and t 1r2 , and all of the high parameter input values, except f oc , are below the reporting limit. The benzene CSsstl mean values for loam at both separation distances are also below the reporting limit. In these cases, the computed CS sstl values would not be enforceable, and any variability in CS sstl values below the reporting limits is not significant from a regulatory point of view. Recalculating the CS sstl values with SC DHEC recommended DAF values of

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

70

2.0 for clay and 8.0 for sand increases the CS sstl values, so that only the benzene CS sstl values calculated for sand with the low value of f oc at both separation distances, and for loam with the low value of f oc at the 152.4-cm separation distance, are below the reporting limit.

䢇

6. Conclusions The Soil Leachability Model is used in South Carolina to predict site-specific target levels for soils that may leach contaminants to groundwater. We investigated the sensitivity of the model to changes in various soil and contaminant input values by determining the changes in the calculated CS sstl values for benzene and naphthalene in sand, loam and clay, for separation distances between the location of the greatest contamination in the unsaturated zone and groundwater of 152.4 and 914.4 cm. The main conclusions that can be drawn from this work are as follows: 䢇

䢇

䢇

䢇

䢇

In all cases, the CS sstl values calculated for benzene are smaller than those for naphthalene, due to its smaller partitioning coefficient and the value used for the RBSL for groundwater. The shorter separation distance of 152.4 cm generally yields smaller CS sstl values than the separation distance of 914.4 cm for the soils and contaminants investigated. Increasing the separation distance from 152.4 to 914.4 cm has little effect on CS sstl values calculated for sand, but results in increasingly large changes in the CS sstl values for loam and clay. The effects of TPH variations on CS sstl values are small, vary little with soil type, and are unaffected by increasing separation distance for the range of values investigated Ž1᎐1000 mgrkg.. In sand, increasing f oc from its typical value for benzene at either separation distance, or decreasing the t 1r2 at 914.4 cm, results in changes that are greater than "50% of the CSsstl mean . For naphthalene in sand, varying f oc to its higher or lower value at either separation distance, or decreasing t 1r2 at 914.4 cm results in changes that are greater than "50% of the CSsstl mean . In loam, changing the mean K s and f oc to their low and high values, respectively, has the only significant impact on CS sstl values for benzene at a separation distance of 152.4 cm. For a separation distance of 914.4 cm, changing K s , t 1r2 or f oc from the mean value, or increasing n, results in significant changes in the benzene CS sstl . For naphthalene at separation distance of 152.4 cm, the low K s, low and high f oc , and low t 1r2 values have the

䢇

greatest impact on calculated loam CS sstl values. For a separation distance of 914.4 cm, high and low values of K s , f oc and t 1r2 result in even greater deviations in the CS sstl values for both contaminants than at 152.4 cm. CS sstl values for benzene in sand and most benzene CS sstl values in loam are below the SC DHEC Ž1998. reporting limit of 5 = 10y3 mgrkg when a dilutionrattenuation factor of 1.0 is used, resulting in unenforceable target levels. The use of greater dilutionrattenuation factors results in CS sstl values that exceed the SC DHEC reporting limit for benzene for the set of conditions investigated. CS sstl values are most sensitive to changes in the input parameters for clay, and this sensitivity increases with separation distance. For a separation distance of 152.4 cm, the high and low K s , high f oc , and high and low t 1r2 values for benzene result in the largest changes in the calculated CS sstl values. The same is true for naphthalene at 152.4 cm, but the CS sstl values are also significantly impacted by the high values of ␪ r , h cr and t 1r2 , and the low value of f oc . For benzene at a separation distance of 914.4 cm, all of the high and low parameter values impact the CS sstl values by more than "50%, except for ␳ b and TPH. Only the low value of f oc and the high values of ␪ r , K s and t 1r2 yield CS s st l values that are less than the saturation concentration for naphthalene in clay for a separation distance of 914.4 cm.

The results of this sensitivity analysis provide a rational means for determining which soil property values are more critical, and which can be estimated knowing that errors in the estimates will not greatly influence the calculated CS sstl values. Since CS sstl values are most sensitive to f oc for sandy soils, this study suggests that once the soil is classified as such, only this value need be measured on site, and representative soil property values based on soil type Žsuch as those calculated with the Rawls and Brakensiek relationships. can be used to obtain reasonable estimates for other soil properties. For loam, the values of K s , f oc and t 1r2 significantly influence CS sstl values calculated with the SLM, especially at the 914.4-cm separation distance. For the conditions investigated, underestimation of K s or t 1r2 , or overestimation of f oc , can result in calculation of CS sstl values that are several times to several orders of magnitude greater than those calculated with mean or typical inputs, and potentially unconservative. For clay, the values of K s , f oc and t 1r2 significantly change CS sstl values calculated with the SLM for both contaminants and separation distances. The CS sstl values for both contaminants at a separation distance of 914.4 cm are significantly influenced by all inputs, except TPH Žand ␳ b for benzene.. This suggests that the

M.M. Gribb et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 59᎐72

values selected for inputs for most soil properties have a great impact on calculated CS sstl values for clay soils. As is the case for loam, underestimation of K s or t 1r2 , or overestimation of f oc , can result in calculation of CS sstl values for clay that are several times to several orders of magnitude larger than those calculated with mean or typical inputs. However, only the low value of f oc and the high values of ␪r , K s and t 1r2 yield CS s st l values that are less than the saturation concentration for naphthalene, making the effect of variability in most parameters on the calculated CS sstl values at 914.4 cm inconsequential from a regulatory point of view. It is important to recall that the Soil Leachability Model is used in South Carolina to obtain conservative screening and site-specific target levels. Many of the simplifying assumptions employed in the Soil Leachability Model likely limit the accuracy of its predictions. For example, use of the Green and Ampt Ž1911. analytical approach can result in over-prediction of infiltration rates. Conversely, this approach cannot account for fingering or other non-Darcian behavior that may result in faster transport of contaminants to groundwater. In the South Carolina approach, constant t 1r2 values of 16 and 48 days ŽHoward et al., 1991. are used to account for the assumed first-order biodegradation of benzene and naphthalene. These values may be too short for typical field conditions, where oxygen availability can severely limit biodegradation ŽLyman et al., 1992.. Bekins et al. Ž1998. found that first-order rates may only be applicable if benzene concentrations are - 1 mgrl, and Barker et al. Ž1987. found that a zeroorder rate was more appropriate for describing the biodegradation rate of benzene, toluene and xylene in their study of a sandy aquifer. Recent studies have suggested that the first-order degradation assumption may not be appropriate for petroleum-contaminated sites, as degradation processes and microbial populations vary significantly at such sites ŽOdermatt, 1997.. Given the sensitivity of the site-specific target levels to the input values selected for the biodegradation t 1r2 value in sand for the longer separation distance, and loam and clay soils for both separation distances, it may be prudent to neglect potential biodegradation in the analysis, rather than overestimate it and risk calculating non-conservative site-specific target levels. More research on this topic is needed. State agencies responsible for the regulation of leaking underground storage tanks may find the results of this study useful in setting or revising data collection requirements for use in models similar to the Soil Leachability Model. Given the importance of the soil parameter input values on the calculated site-specific target levels, the performance of the Rawls and Brakensiek Ž1989. regression equations for relating soil classification to soil properties should be quantified via

71

comparison with soil property measurements for the range of soils typically encountered at these sites. Finally, given the uncertainty about the model itself, the performance of the Soil Leachability Model for predicting site-specific target levels should be verified via field studies for a wide range of site and source conditions.

Acknowledgements The writers wish to acknowledge the financial support of the SC DHEC Division of Underground Storage Tank Management and National Science Foundation CAREER Grant No CMS-9501772. The helpful comments of the reviewers are also acknowledged. References American Society for Testing and Materials, 1995, Risk-Based Corrective Action Applied to Petroleum Release Sites, E 1739-95, ASTM Annual Book of Standards, 11.04: 1᎐51. Barker, J.F., Patrick, G.C., Major, D., 1987. Natural attenuation of aromatic hydrocarbons in a shallow aquifer. Ground Water Monit. Rev. Winter, 64᎐71. Bekins, B.A., Warren, E., Godsey, E.M., 1998. A comparison of zero-order, first-order, and Monod biotransformation models. Ground Water 36 Ž2., 261᎐268. Boyd, S.A., Sun, S., 1990. Residual petroleum and polychlorobiphenyl oils as sorptive phases for organic contaminants in soils. Environ. Sci. Technol. 24, 142᎐144. Brooks, R.H., Corey, A.T., 1964. Hydraulic Properties of Porous Media. Colorado State University. Hydrology Paper 3, 27 pp. Feenstra, S., Mackay, D.M., Cherry, J.A., 1991. Presence of residual NAPL based on organic chemical concentrations in soil samples. Ground Water Monit. Rev. 11, 128᎐136. Freeze, R.A., Cherry, J.A., 1979. Groundwater. Prentice-Hall Inc, Englewood Cliffs, NJ. 404 pp. Green, W.H., Ampt, G.A., 1911. Studies on soil physics, part 1. The flow of air and water through soils. J. Agric. Sci. 4, 1᎐24. Howard, P.H., Boethling, R.S., Jarvis, W.F., Meylan, W.M., Michalenko, E.M., 1991. Handbook of Environmental Degradation Rates. Lewis Publishers, Chelsea, MI. 776 pp. Lyman, W.J., Reidey, P.J., Levy, B., 1992. Mobility and Degradation of Organic Contaminants in Subsurface Environments. C.K. Smoley, Chelsea, MI. 395 pp. Lyman, W.J., Reehl, W.F., Rosenblatt, D.H., 1990. Handbook of Chemical Property Estimation Methods. American Chemical Society, Washington, DC. 960 pp. Karickhoff, S.W., Brown, D.S., Scott, T.A., 1979. Sorption of hydrophobic pollutants on natural sediments. Water Resour. Res. 13, 241᎐248. Menatti, J.A., Marrin, D.L., Anderson, M.D., 1994. Fate and transport modeling of diesel fuel contamination in the vadose zone. In: Calabrese, E.J., Kostecki, P.T. ŽEds.., Proceedings of the 1993 4th Annual West Coast Confer-

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ence on Hydrocarbon Contaminated Soils and Groundwater, Hydrocarbon Contaminated Soils and Groundwater, Long Beach, CA, 4. Association for the Environmental Health of Soils, Amherst, MA, pp. 97᎐111. Miner, R., 1998, personal communication ŽProject Manager, SC DHEC, Division of Underground Storage Tank Management, Columbia, SC.. Montgomery, J.H., Welkom, L.M., 1991. Groundwater Chemicals Desk Reference. Lewis Publishers, Chelsea, MI. 640 pp. Odermatt, J.R., 1997. Simulations of intrinsic biodegradation using a non-linear modification of first-order reaction kinetics’. J. Soil Contam. 6 Ž5., 495᎐508. Rawls, W.J., Brakensiek, D.L., 1989. Estimation of soil water retention and hydraulic properties. In: Morel-Seytoux, H.J. ŽEd.., Proceedings of the NATO Advanced Research Workshop on Unsaturated Flow in Hydrologic Modeling

Theory and Practice. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 275᎐300. South Carolina Department of Health and Environmental Control, 1998, South Carolina Risk-Based Corrective Action for Petroleum Releases, Bureau of Underground Storage Tank Management, Columbia, SC Ž57 pp... USDA Soil Conservation Service, Soil Survey Staff, 1951. Soil Survey Manual. US Government Printing Office, Washington, DC. 503 pp. US EPA Office of Underground Storage Tanks, 1998, Map Of Status Of ASTM RBCA Training in the UST Program: July 7, 1998 ŽInternet: http:rrwww.epa.govrswerust1rrbdmr rbcamap.htm, accessed 2r5r01.. US EPA, 2001, Current Drinking Water Standards, Office of Groundwater and Drinking Water ŽInternet: http:rr www.epa.govrsafewaterrmcl.html, accessed 2r5r01..