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r1restone Store#3573 400 North EI Dorado Street, Stockton, California October 25, 1995 VES Quarterly Report and Request for Closure-4589 Page 10 of 14 r 3.5 Estimated Remaining Phase-Partitioning Concentrations Phase-partitioning calculations have been performed to estimate the concentrations of both TPH-G and benzene remaining in the pore water and the soil on a dry weight bass These concentration estimates can be useful for making regulatory compliance decisions, and are provided as an alternative to a verification drilling program which is believed to be unwarranted at this site. These phase-partitioning calculations have been performed based on presumed equilibrium conditions with respect to hydrocarbon compounds in the subsurface environment. This presumption is supported by the relatively unchanged TPH-G off- vapor concentrations observed following the one-month shut down period m which actual equilibrium conditions existed. In addition, the subsurface temperature and pressure gradients created by SVES operation are minor and should not significantly affect the chemical equilibrium behavior of the existing hydrocarbon compounds. The equations describing the subsurface phase partitioning are included on the Phase Partitioning Worksheets in Appendix 7.8. The worksheets identify assumed soil properties, define the specific calculations, present the associated units, and resolve the numerical equations. SThere are three phases within the subsurface zone where petroleum hydrocarbons can exist. These phases include: soil vapor—in which hydrocarbons can volatilize, soil pore water—in which hydrocarbons can dissolve, and soil particles—on which hydrocarbons can adsorb Hydrocarbon concentrations reach a natural equilibrium between these three phases. These equilibrium relationships can be described by several equations commonly used to establish the phase partitioning characteristics of various chemicals. Based on the known levels of TPH-G and benzene in the soil vapor phase (analytical results), concentrations of these constituents can be calculated for the pore water phase and the soil phase Based on presumed equilibrium conditions, the estimated average concentrations of TPH-G and benzene remaining in pore water at this site are 0 364-mg/L and 0.0024-mg/L, respectively The estimated average concentrations of TPH-G and benzene remaining in soil at this site are 1 .3-mg/kg (ppm) and 0.12-ug/kg (ppb), respectively. i RYAMMURPHY, INCORPOBAZM De'e'r POW CoostroCOM AU029100011t Lm Angelis