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RITERJM PROBLEM ASSESSMENT REM)RTMffEPJM REMM AL ACTION PLAN -2- Former Beacon Station No. 494, 2185 East Fremont Street Stockton, CA--AMV Project No. 19005.07 ' 3.0 PREVIOUS INVESTIGATIVE WORK AT THE SITE ' Previous investigative work at the site includes two drilling phases conducted by AMV in April ' and December of 1992, soil sampling conducted by AMV during tank removal activities in March of 1993, and a soil gas screening performed by another consulting firm in March of 1993. ' 3.1 April 1992 Drilling In April 1992, AMV advanced five soil borings and converted three to ground water monitoring ' wells. The work included testing of soils and ground water as described below. ' 3.1.1 Drilling and Soil Conditions On April 17, 1992, AMV advanced three 10-inch-diameter soil borings to a depth of approximately 83 feet below grade and completed each as a monitoring well (MW-1 through MW-3). In addition, AMV advanced two 8-inch-diameter soil borings (B-1 and B-2) to a depth of approximately 30 feet below grade (B-1) and 20 feet below grade (B-2). These borings were ' later abandoned by backfilling with a cement/bentonite mixture to the surface. Figure 2 shows the locations of monitoring wells MW-1 through MW-3 and soil borings B-1 and B-2. ' As borings were advanced, AMV collected soil samples at vertical intervals of 5 feet. Appendix A describes methods used to advance the soil borings and collect soil samples. Soil samples collected from the borings consisted of silty clay, sandy silt, clayey silt, sand, silt, ' clayey sand, and sandy clay. For example, sediment encountered in the boring for monitoring well MW-3 consists of silty clay from the surface to 13 feet below grade, sandy silt from 13 to 22.5 feet, clayey silt from 22.5 to 28.5 feet, sand from 28.5 to 35 feet, silty clay from 25 to 50 ' feet, sand from 50 to 57 feet, clayey silt from 57 to 63 feet, silty sand from 63 to 68.5 feet, clayey silt from 68.5 to 73 feet, sandy silt from 73.5 to 79 feet, and silty clay from 79 to 83 feet, the termination of the boring. Appendix B contains the soil boring logs. The boring logs ' indicate that contacts between soil strata vary between sharp and gradational, and that soil units are not laterally continuous across the site. 3.1_2 Soil Sample Collection and„Analy-tical Results Based on photoionization detector (PID) readings and consideration of sample location with ' respect to possible hydrocarbon source, AMV selected 18 of the soil samples collected in April 1992 for laboratory analysis. The samples were analyzed for the compounds benzene, toluene, rptoo5.mi