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IIV M M PROBLEM ASSESSMENT REPORTIMERIM REM MIAL ACTION PLAN -5- Former Beacon Station No. 494, 2185 East Fremont Street Stockton, CA--AMV Project No. 19005.07 ' 3 2 3 Momtonn Wells and Qround Water Level Measurement ' The two soil borings now designated as MW-4 and MW-5 were completed as 4-inch-diameter monitoring wells, MW-4 to a depth of approximately 87 feet and MW-5 to approximately 87.5 feet. Monitoring well construction specifications are contained in Appendix D. ' AMV surveyed the casing g riser of each new monitoring well relative to the same San Joaquin County bench mark described in Section 3.1.3. Measurements made in both the new and ' existing wells on December 14, 1992 (Table 2), indicated ground water was present at depths ranging from 75.28 to 77.41 feet below the top of the respective monitoring well casings. This set of measurements indicated an inferred direction of ground water flow generally toward the northwest, almost directly opposite the inferred direction from measurements in April of 1992. This apparent switch in ground water flow direction may be attributable to the flat gradient ' inferred from measurements made in April 1992 (Section 3.1.3); a minor change in measurements resulted in a major change in the apparent direction of ground water flow. The ground water gradient estimated from measurements made in December 1992 was approximately 0.035 foot per foot. Because of the apparent contradiction in ground water flow direction, AMV resurveyed all ' casing risers before taking ground water level measurements on July 27, 1993 (Section 4.1). 3.2.4 Ground Water Sample Collection and Analytical Results ' On December 14, 1992, AMV collected ground water samples from monitoringwells MW-4 and MW-5 using procedures described in Appendix A. Collected ground water samples were ' analyzed for BTEX and TPHg. Table 3 includes the analytical results from this sampling event. The TPHg concentration was ' 860 µg11 in the sample from monitoring well MW-4 but nondetectable in the sample from monitoring well MW-5. Benzene concentration was 40 µgll in the sample from monitoring well MW-4 and nondetectable in the sample from monitoring well MW-5. Appendix G contains copies of the certified analytical reports. 3.3 Tank Removal Activities ' Petroleum product storage at the site consisted of three - 10,000 gallon, single-wall, steel underground storage tanks. Underground storage tank removal occurred on March 8, 1993, with ' Mr. Mike Infurna and Ms. Carol Oz of the San Joaquin County Environmental Health Department (San Joaquin County) in attendance. None of the tanks had any noticeable holes. ' rptws.w