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INTERIM PROBLEM ASSESSMENT REPORT/INTERIM REMEDIAL ACTION PLAN -7- Former Beacon Station No. 494, 2185 East Fremont Street Stockton, CA--AMV Project No. 19005.07 .4.2 Tes Result ' Table 6 contains the field results of the soil gas screening. Of the nine vapor probes installed, field screening indicated that air samples collected from only two (P-7 and P-9) contained detectable concentrations of volatile hydrocarbons at concentrations of 1,200 and 140 parts per i million by volume [ppmv] (as methane), respectively. Field screening indicated concentrations in vapor samples collected from monitoring wells and vapor extraction wells ranged from 50 to greater than 10,000 ppmv (as methane). Wells with concentrations greater than 10,000 ppmv (as methane) included monitoring well MW-3, and extraction wells VW-la, VW-lb, VW-2a, VW-2b, and VW-4b. Table 7 contains the analytical results for vapor samples submitted for laboratory analysis. The laboratory results ranged from 140 ppmv TPHg for monitoring well MW-3 to 23,000 ppmv ' TPHg for VW-lb. Appendix J contains copies of the certified analytical reports for the soil vapor samples. 4.0 CURRENT PHASE OF WORK--GROUND WATER MONTTORING AND SAMPLING Quarterly ground water monitoring and sampling initiated after April of 1992 is continuing. ' This section discusses results from the July 1993 monitoring and sampling performed by AMV. 4.1 July 1993 Ground Water Level Measurements After resurveying all casing risers to the same bench mark described In Section 3.1.3, AMV measured ground water levels on July 27, 1993. The measurements indicated ground water was present at depths ranging from 72.48 to 74.04 feet below the top of the respective monitoring well casings. The elevations determined from these measurements indicate an inferred direction of ground water flow generally toward the northeast (Figure 5). The estimated gradient from ' this data is approximately 0.008 foot per foot. 4.2 July 1993 Ground Water Sample Collection and Analytical Results On July 27, 1993round water samples pies were collected from monitoring wells MW-1 through MW-5. Appendix A describes the procedures used to develop, purge, and sample the ' monitoring wells. Each ground water sample was analyzed for BTEX and TPHg. In addition, samples from monitoring wells MW-2 and MW-5 were also analyzed for halogenated volatile organic compounds (VOCs) using EPA Method 601.