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APPY&NIDIX BRISK BASED CORRECTIVE ACTION TIER 1 AND TIER 2 ANALYSIS SITE CLOSURE EVALUATION REPORT . Former Beacon Station No 695 900 South Cherokee Lane Lodi. Caldotmm Delta Project No D093-915 Page 7 The approximate extent of benzene in ground water as of October 1988 is illustrated in a figure prepared by Du Pont Biosystems (Du Pont) included in Appendix A Concentrations of petroleum hydrocarbons in ground water have decreased with tune Tlus decrease occurs in conjunction with decreases in the water table elevation As ground water subsided, the volume of the vadose zone increased allowing the SVE system to remove petroleum hydrocarbons previously in ground water Figure 17 illustrates the approximate extent of benzene in ground water in June 1996 4.0 STPE REL1Z IATION Petroleum hydrocarbons have been removed from the subsurface during the period of August 1989 through February 1996 by the use of a free product recovery system, the excavation of soil during tank and product line removal activities, and the operation of a SVE system i4.1 Free Product Recovery A program of free product recovery was implemented by Du Pont in August 1989 and continued until October 1991 Du Pont documented in a report dated November 12, 1990, that approximately 194 gallons of free product was recovered from monitoring wells MW-8, MW-10 and MW-17 during the above referenced period. Subsequent to October 1991, quarterly ground water monitoring data indicates that monitoring wells MW-10 and MW-17 did not contain measurable free product or were considered dry Data from monitoring well MW-8 indicates that approximately 0 06 feet of free product was present during the March 1992 sampling event Subsequent events indicated that monitoring well MW-8 did not contain measurable free product or was considered dry, however, when ground water samples were collected after March 1992 analytical results did not contain benzene higher than 32 micrograms per liter (µg/L) Subsequent to March 1992, no measurable free product was reported in the monitoring wells 4.2 Tank and Product Lme Removal Activities In .January 1994, USTs and product line piping removal in addition to over-excavation activities were conducted at the site Analytical results and details of the UST removal and over-excavation activities are previously discussed in Section 3 1 Approximately 2,641 cubic yards of soil was excavated and RPT001 915