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Site History 14000 Highway 88,Lockeford,CA Time Oil Property No.04-049 2. Soil samples would be collected from each boring at 5-foot intervals and submitted for analyses based on visual observation and field instrumentation readings. A minimum of three samples would be submitted from each boring. 3. In the event that groundwater is encountered, the boring to be placed near W-8 would be installed as a 2-inch diameter groundwater monitoring well. If needed, up to three additional groundwater monitoring wells might be installed to assist in determining the extent of impacted groundwater. All wells would be properly developed and purged prior to the collection of groundwater samples. Additional Environmental Investigation - December 1994 While conducting field activities during the weeks of December 20 and 27, 1994, hydrocarbon-impacted groundwater was encountered while attempting to define the vertical extent of soil contamination. Therefore, four groundwater monitoring wells (MW-1 through MW-4) were installed and one soil boring (SB-11) was advanced in an attempt to define the extent of contamination in both media. As in previous site activities, soils encountered during drilling consisted of fine silty sands to approximately 15 feet below ground surface (bgs); this strata was underlain by fine to coarse sands to approximately 30 feet bgs. Clayey silt was generally encountered at 30 feet to 35 feet while fine to coarse sands and gravelly sands were encountered to 70 feet bgs. Silty clay with sandy and gravelly stringers was encountered from 70 feet bgs to the extent of each boring. Groundwater was first encountered at depth 85 feet bgs. _rf MW-1 was installed to the northeast of the currently operating pump islands; MW-2 was installed to the east of MW-1; MW-3 was installed to the southeast of the former tank bed; and MW-4 was installed to the southwest of the former tank bed (presumably downgradient). Each of the wells was constructed of 2-inch diameter PVC and screened from a depth of 65 feet bgs to 100 feet bgs. Soil boring SB-11, placed to the west of the former tank bed, could only be advanced to a depth of 78 feet bgs due to difficult drilling conditions. Because the extent of shallow soil contamination was defined during earlier investigative activities, soil samples were collected only from depths of 25 feet and greater. Based on field instrumentation readings, four to five soil samples from each drilling location were submitted to a laboratory and analyzed for total petroleum hydrocarbons as gasoline (TPH) and also for benzene, toluene, ethylbenzene, and xylenes (BTEX). Gasoline was found in the samples collected from MW-1 at 35 feet (3.4 ppm), 45 feet (680 ppm), 83 feet (3.2 ppm), and 100 feet (1.6 ppm); varying concentrations of BTEX were also found in these samples. Trace concentrations of benzene, ethylbenzene, and xylenes (none exceeding 0.01 ppm) were also found in the samples collected from MW-2 at 45 feet, 55 feet, and 60 feet, and in the sample collected from MW-5 at 71 feet. Hydrocarbons were not identified in any of the remaining samples. On January 19, 1995, the groundwater monitoring wells were purged and sampled. Prior to purging, measurements were collected from each well to evaluate groundwater elevation, flow direction, and gradient. During field activities, groundwater with a southeasterly flow and a hydraulic gradient of 0.13 ft./ft. was encountered at 75.96 feet bgs to 86.80 feet bgs. Hydrocarbons were found in groundwater samples collected from each of the monitoring wells. The sample collected from MW-1 contained 5,200 ppb TPH, 200 ppb benzene, 1,100 ppb toluene, 160 ppb ethylbenzene, and 620 ppb xylenes. The sample collected from MW-2 was found to contain 460 ppb TPH, 30 ppb benzene, 48 ppb toluene, 5 ppb ethylbenzene, and 30 ppb xylenes. Analysis of the sample collected from MW-3 indicated 430 ppb TPH, 18 ppb benzene, 48 ppb toluene, 13 ppb ethylbenzene, 4