ARCHIVED REPORTS XR0012292 - Laserfiche WebLink

Home Browse Search

t� 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 16-1/2 feet. First encountered groundwater occurred at approximately 9 to 10 feet below grade between March 10 through 12, 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 with a gradient ranging from 0.003 to 0.006 on March 31, 1992. The highest hydrocarbon 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 concentrations in the soil attenuate with depth and appear to be localized near the coarse-grained sediments underlying the above-ground storage tanks. The horizontal extent of petroleum hydrocarbons in the soil has been defined to non- detectable levels to the east and west, to below 100 ppm to the north, and below 500 ppm to the south. TPH-gasoline was detected in groundwater samples from Monitoring Wells U-2, U-5, U-6, and U-8 at concentrations ranging from 71 ppb in Well U-8 to 980 ppb in Well U-6. Benzene was detected in groundwater samples from Wells U-1, U-2, U-5 and U-6 at concentrations ranging from 2.1 ppb in Well U-1 to 330 ppb in Well U-6. In general, TPH-gasoline concentrations have decreased significantly from the previous sampling event. Petroleum,hydrocarbons in the groundwater appear to be primarily localized ,r 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 Railroad at the adjacent site be included into the characterization and history of this site. This research will provide additional information on hydrogeologic conditions, subsurface lithology and distribution, and location, extent, and concentrations of petroleum 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 a - 3100103/REPORTS 7 June 11, 1992