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Problem Assessment Report Page 13 Ranch Market 23569 South Santa Fe Road,Riverbank CA November 8 2000 Figure 11 presents the layout of an SVE/AS well array that would be required to remove the • gasoline range petroleum hydrocarbons adsorbed to soil and dissolved in groundwater 8.0 EXTENT OF PETROLEUM HYDROCARBONS ' 81 Extent of Petroleum Hydrocarbons Adsorbed to Soil ' Referring to the Cross Sections A-A' and B-B'(Figures 5 and 6), the release of gasoline 1n the area of the former USTs appears to have migrated vertically downward to the groundwater, and then down gradient creating a 10 foot thick smear zone from 20 to 30 feet bgs (Table 2) ' The highest concentrations in soil of benzene, MtBE, and TPHg were found in the soil sample collected from AS-1 at 20 feet bgs, underlying the former UST pit The lateral and vertical extent of gasoline range petroleum hydrocarbons adsorbed to soil has not been completely ' defined to the south by the current data set (Figure 9) 8.2 Extent of Petroleum Hydrocarbons Dissolved in Groundwater ' The highest concentration of dissolved benzene, MtBE, TPHg, DIPE, EDB, TBA, and 1,2- DCA were found in the water sample collected from MW-1, located within the former UST ' pit (Table 3) Referrmng to Figure 10, the grab water sample data and quarterly groundwater samples define the upgradient and cross gradient extent of the dissolved petroleum . hydrocarbons In groundwater The extent of BTEX and TPHg dissolved in groundwater ' declines down gradient generally from MW-1 toward MW-5, but Increases further down gradient at MW-7 Based on the analyses of the water sample collected from MW-7 on September 1, 2000, the downgradient limits of the dissolved MtBE are defined (Figure 10) Based on the water sample collected from MW-5D, a well screen from 65 to 70 feet bgs, the ' vertical limits of benzene and MtBE have not been defined However, the concentration of benzene and MtBE, 10 and 14 ug/L respectively, are at or below the State of California Department of Health Services Primary Drinking Water Maximum Contaminate Levels (MCL) for drinking water of 10 ug/L for benzene and 13 ug/L for MtBE In addition, the concentration of MtBE is below the 5 0 ug/L Secondary MCL Drinking Water Standard for MtBE ' 8.3 Estimated Mass of Petroleum Hydrocarbons Using the soil sample analytical data in Table 1 and the third quarter 2000 groundwater analytical data In Table 3, SECOR calculated the estimated mass of hydrocarbons adsorbed to soil and dissolved in groundwater The assumptions and calculations used are presented in ' Appendix E The estimated mass of hydrocarbons adsorbed to soil was calculated for the vadose zone from ' 15 to 25 feet bgs and within the capillary fringe or smear zone from 25 feet to 35 feet bgs The area, thickness, and average concentrations used to calculate the mass of hydrocarbons Is 1 File PAR-report doe 58COR International Incorporated