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APPENDIX BRISK BASED CORRECTIVE ACTION TIER I AND TIER 2 ANALYSIS STE CLOSURE EVALUATION REPORT Poriner Beacon Station No 695 900 South Cherokee Lane Lodi, California Delta Project No D093-915 Page 17 SSTLs were evaluated by conservatively assuming that a drinking water well was located 250 feet directly downgradient A conservative natural attenuation factor (NAF) was estimated from site data Using the NAF, a SSTL was estimated for ground water near the source using the analysis provided in the Tier 2 Guidance Manual (Conner, et al, 1995) A SSTL was estimated for subsurface soils by using the ground water SSTL and the conservative leaching model from the Tier 2 Guidance Manual (Appendix N) 9 2 1 . Natural Attenuation Factor (NAF) Using an empirical NAF requires that the plume either exhibits a steady state or diminishing condition The NAF was determined from the March 4, 1992 data MW-1 had a concentration of 260 µg/L and MW-20 had a benzene concentration of less than 0 5 Ag/L MW-1 is located 10 feet downgradient of the source and MW-20 is located 300 feet downgradient of the source. By conservatively assuming a source concentration of 260 µg/L and a concentration of 0 5 Ag/L for the downgradient MW-20, the • RBCA Tier 2 estimated a NAF of 310 9 2 2 Ground Water SSTL By conservatively assuming that a drinking water well exists within 250 feet and using the NAF estimated in section 5 1.1, a SSTL was estimated for ground water at the source. The benzene SSTL if a drmkmg water well is installed at 250 feet from the source is 2,000 Ag/L for a residential well and 6,600 µg/L for a commercial well Using the California correction factor of 0 29 results in a SSTL of 580 tig/L The model inputs and results are summarized in Appendix O 9 2 3 Subsurface Soil SSTL The RBCA Tier 2 program utilizes a conservative leaching model to estimate the appropriate SSTL for subsurface soils that are protective of ground water receptors The model assumes equilibrium between soil and water, no benzene decay, infinite source of benzene, and conservative ground water dilution With consideration of these conservative assumptions, the subsurface soil benzene SSTL was estimated to be 23 mg/kg for a residential well and 78 mg/kg for a commercial well at 250 feet Using the RP'T001 915