Conclusions

To determine soil cleanup goals for the cPAHs protective of underlying groundwater, a conceptual model of potential constituent migration was developed. The site conceptual model consists of two elements: (1) leaching of constituents from the potentially affected soils and (2) migration and dispersion of the constituents with respect to groundwater flow direction.

The OLM was used to describe the leaching potential of the cPAHs present in the soil (Element I), while the VHS model was selected to describe dilution associated with dispersion of the cPAHs in groundwater (Element II). The conceptual model is conservative (from a modeling standpoint) in that biodégradation and retardation in soils and groundwaters are not considered.

Figure 11 Southeastern source area—total cPAH analytical results. (•) Soil sampling location (approximate). Hatching indicates estimated area with £ soils exceeding 700 mg/kg total cPAHs. Total cPAH concentrations: 0-2 ft, 500; 2-4 ft, 685; 4-6 ft, 191.6. ND, not detected. 10
SCALE (FEET)

Figure 12 Southeastern source area—total cPAH analytical results. (•) Soil sampling location. Hatching in- § dicates estimated area with soils exceeding 700 mg/kg total cPAHs. Total cPAH concentrations: 0-2 ft, 10.03; £ 2-4 ft, 216.2; 4-6 ft, 1.259. ND, not detected.

Figure 13 Northern source area—total cPAH analytical results. (•) Soil sampling location. Hatching indicates soils exceeding 700 mg/kg total cPAHs. Total cPAH concentrations: 0-2 ft, 102; 2-4 ft, 81.8; 4-6 ft, 188.9. ND, not detected.

A semiquantitative sensitivity analysis was performed on the results of the site conceptual model. Element I, leaching as predicted by the OLM, was the most influential portion of the modeling effort. Element II had little impact on the final modeled results because of the shallowness of the uppermost water-bearing zone and the proximity of the potential source areas to the alternative point of compliance (APC). As a result, leachate testing was performed to determine whether the use of the OLM was representative of site conditions. EPA suggested the use of TCLP analyses of soil samples taken from each of the potential source areas.

Element I, represented by the OLM, is a multiple regression equation:

where C; is the estimated leachate concentration, Cs is soil concentration, and Ws is water solubility.

TCLP data collected to confirm the validity of Equation (23) with respect to site leaching potential indicates that for cPAH soil concentrations below 1200 ppm (Cs < 1200 mg/kg) the leachate concentration is below the limit of detection. Ct can therefore be expressed as

The first element of the conceptual model is therefore undefined from a practical standpoint using site-specific leaching data, given that where {*} is any number.

Site soils containing in excess of 1200 mg/kg total cPAHs (C^) did not produce detectable concentrations in their respective leachate (C,), and mathematical correlation between soil concentration and site-specific cPAH leachate values beyond the previously stated relationship could not be established. Soils with cPAH levels greater than 1200 mg/kg were not submitted for TCLP analysis.

Based on the site-specific conditions, the use of the OLM to describe leaching in the site conceptual model of constituent transport was a conservative assumption in lieu of site-specific information regarding constituent leaching. Therefore, the ROD-specified direct-contact cleanup goal of 700 mg/kg total cPAHs is also protective of the no leaching potential for this particular site.

In this particular case, spending the additional money to derive a site-specific estimate of leaching potential resulted in a significant cost savings for the potentially responsible party as opposed to removal of soils based solely on the modeled results. However, it should not be overlooked that the modeled results were also adequately protective of the no leaching potential established in the site ROD. Using the analytical solutions in determining a soil cleanup goal at sites having smaller source areas or where less costly remedial alternatives are an option will produce protective and cost-effective results. In any case, the development of soil cleanup goals using analytical solutions protective of underlying groundwater quality is necessary to focus additional field investigations and data collection efforts when the protection of groundwater quality from continued soil releases is a priority.

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