SITE INFORMATION AND CORRESPONDENCE

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APR -2 2 - 9 3 T H Ll 8 = 0 0 P _ 0 :3 A 4.0 CONCLUSIONS AND RECOMMENDATIONS The soils encountered in the borings for Wells U-7 through U-10 consisted primarily of silt and silty clay to the maximum depth explored of 1&1/2 feet. First encountered &roundwater occurr�d at approrimately 9 to 10 feet below grade between March 10 through 1; 1992 and stabilized at 9 feet below grade on March 31, 1992. Groundwater flow direction has been consistently north-northwesterly at the site and was northwesterly wiO a gradient ranging from 0.003 to 0.006 on March 31, 1992- The highest .1ydroca rt concentrations in the soils were found west and adjacent to the above-ground storage tanks, and lower concentrations to the south, with no detectable concentrations occurring in the soils that were examined off-site. In general, hydrocarbon concent{ations in the soil attenuate with depth and appear to be localized near the coarse-grained sediments underlying the above-ground storage tanks. The horizontal gAte of petro drncarhnnc ,n a sated has been defined to non- detectable levels to der anrt w ar to, below 100 ppm to the north, and below 500 ppm to the south 17H-gasoline was detected in groundwater samples from Monitoring Wells U-2, U-S, U-6, and U-8 at concentrations ranging from 71 ppb in Well U-8 to 980 ppb in Weil U-6. Benzene was detected in groundwater samples from Wells U-1, U-2,U-5 and U-6 at concentrations ranmg from 2.1 ppb in Weil U-1 to 330 ppb in Weil U-6. In general, TPH-gasoline concentrations have decreased significantly from the previous sampling event. Petroleum hyllrocarbons in the groundwater appear to be primarily localized beneath and adjacent;to the above-ground storage tanks and piping. PACIFIC recommends that research of the results of the investigation performed by Southern Pacific Rail;oad at the adjacent site be inciuded into the characterization and history of this site. T'bis research will provide additional information on hydrogeologic conditions, subsurfarte lithology and distribution, and location, extent, and concentrations of pettoleum hydrocarbons in the soil and groundwater to the north of the site. A soil vapor extraction feasibility study is recommended in the areas of elevated hydrocarbon concentrations to determine if this is a viable remedial option. PACIFIC further recommends that the quarterly groundwater sampling program be 3100103/1REPORT-, 7 luno 11, 1992