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F S E C 0 R Conceptual Site Model March 24, 2004 Page 8 • Concentrations of petroleum hydrocarbons were assumed to be distributed linearly with depth, and, • A porosity of 35 percent, which corresponds to "silty-sand" (Amenean Society for Testing Materials —D2488) was used to calculate the approximate mass remaining in soil Analytical data from soil samples collected at the site indicate that typically, the maximum € concentrations of hydrocarbons,ris found at approximately 4 feet bgs Degradation rates were used to estimate the mass of TPHd, TPHg, BTEX, and MtBE remaining in soil as of September 1996 A given mass of TPHd, TPHg, BTEX, and MtBE is known to decrease (decay) at a rate proportional to the amount originally present The equation used for estimating the mass of hydrocarbons d remaining in sod is presented in Appendix E Half life coefficients were obtained from Environmental Degradation Rates, (Howard et al 1991) The mass decay rate calculation is presented in Appendix F Mass balance calculations completed for TPHg, TPHd, BTEX, and MtBE indicate that the mass of petroleum hydrocarbons in soil on September 1996 was 2762 0 pounds (lbs) 720 5 Ib, 12 9 lbs, 18 4 lbs, 43 3 lbs, 221 0 lbs, and 25 8 lbs, respectively Based on known decay rates for TPHd, TPHg, BTEX, and MtBE calculations indicate that approximately 16 2 pounds of TPHg remains in soil 5.0 GROUNDWATER MONITORING AND SAMPLING • In March 1995, following the installation of MW-1 through MW-3 groundwater monitoring and sampling was initiated Groundwater elevations have risen approximately 10 feet since monitoring and sampling began Although the groundwater flow direction has been shown to vary, the principal a groundwater flow direction at the Site appears to be to the northeast (Figures 6 and 7) 5.1 Groundwater Chemical Concentration Trends t Groundwater analytical results were obtained from groundwater monitoring reports and the 61 GeoTracker database for the site and surrounding properties An evaluation of onsite and offsite groundwater analytical data is summarized in sections 51 1 and 5 1 2, respectively Clean up levels for TPHg and MtBE were derived from the following documents N • MtBE/Benzene Water Quality Goals were derived from the RWQCB-CVR, A Compilation of Water Quality Goals, August 2000 maximum contaminant level goals (MCL) Secondary MCL 5 0 micrograms per liter (µg/L) and primary MCL 1 0 µg/l_ for MtBE and benzene, respectively, • TPHg CRWQCB - San Francisco Bay Region's (CRWQCB-SFBR) Volume 2 Background Documentation for the Development of Tier 1 Soil and Groundwater Screening Levels, dated August 2000, risk based screening levels (RBSL) RBSL (groundwater that is not a current or potential source of drinking water) 5,000 pg/L - ddt "W,�� A chemical concentration map is presented as Figure 9 Historical groundwater monitoring and sampling data and data obtained from the GeoTracker database is presented in Appendix G I IClievrori1920331REPORTSICSMICSM 92033 (REV B) doc )