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INTERIM PROBLEM ASSESSMENT REPORTmnmRW REMEDIAL ACTION PLAN -3- Former Beacon Station No. 494, 2185 East Fremont Street Stockton, CA--AMV Project No. 19005.07 ' eth lbenzene and xylenes Y y (BTEX) using U.S. Environmental Protection Agency (EPA) Method 8020, and total petroleum hydrocarbons as gasoline (TPHg) and total petroleum hydrocarbons as diesel (TPHd) using modified EPA Method 8015. ' Table 1 includes the analytical results for the 18 soil boring samples submitted for laboratory analysis from the April 1992 drilling event. All samples were nondetectable for TPHd, and only samples from the boring for monitoring well MW-3 had TPHg concentrations greater than ' 1 milligram per kilogram (mg/kg). The highest TPHg results in monitoring well MW-3 were 910 mg/kg in the sample from 35 feet and 730 mg/kg in the sample from 50 feet. Appendix C contains copies of the certified analytical reports. ' 3.1.3 Monitoring Wells and Ground Water Level Measurements ' The three soil borings now designated as MW-1, MW-2, and MW-3 were completed as 4-inch- diameter monitoring wells, each to a depth of approximately 83 feet below surface grade. Construction details for each monitoring well are contained in Appendix D. AMV surveyed the casing riser of each monitoring well relative to a San Joaquin County bench mark located at the northeast corner of the intersection of "D" and Fremont Streets. The ' railroad spike bench mark, located in the side of a telephone pole 2 feet above ground surface, is at an elevation of 21.43 feet above mean sea level (MSL). Ground water was present at depths ranging from 71.29 to 72.02 feet below the top of the respective monitoring well casings ' at the time of water level measurements on April 17, 1992 (Table 2). Calculated ground water elevations based on depth to water measurements were within 3/100ths of 1 foot, indicating a flat ground water gradient. The April 1992 measurements indicated an inferred direction of ' ground water flow generally toward the southeast. The estimated ground water gradient was approximately 0.001 foot per foot. Because the ground water gradient is so flat, a minor change in the measurements could result in a major shift in apparent ground water flow direction (see Section 3.2.3). 3.1.4 Ground Water Sample Collection and Analytical Results ' On April 17, 1992, AMV collectedround water ter samples from monitoring wells MW-1 through MW-3 using procedures described in Appendix A. The laboratory analysis for each ground ' water sample included EPA Method 602 for BTEX and modified EPA Method 8015 for TPHg and TPHd. r rPtW5.n3j