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• • Work Plan for Additional Subsurface Assessment and Remedial Action—Former Unocal Station#0187 November 27,2001 PREVIOUS ENVIRONMENTAL WORK Two 1,000-gallon gasoline and one 550-gallon waste oil USTs were removed from the southeast corner of the subject site in 1967. Data related to soil conditions beneath the USTs at the time of removal is not available. These USTs were replaced with two 10,000-gallon gasoline and one 280-gallon waste oil USTs situated in a common excavation in front of the station building. The two 10,000-gallon gasoline and one 280-gallon waste oil USTs, associated product piping, hydraulic lifts, and oil/water separator were removed and the station facilities demolished in January 1992. The locations of pertinent site features are shown on Figure 2. From 1993 through 1997, GeoStrategies Inc. (GSI) installed six on-site (U-1 through U-6) and three off- site (U-7 through U-9) groundwater monitoring wells, installed one on-site vapor extraction well (LTV-1), and drilled fifteen on-site soil borings (U-A through U-O). The groundwater monitoring wells installed by GSI are screened at varying depths between approximately 20 and 35 feet below ground surface(bgs). In November 1998, ARCADIS Geraghty & Miller installed one on-site nested piezometer well (NP-1), and in January 1999 installed one off-site nested piezometer well (NP-2). Each nested piezometer well consists of three 1-inch diameter wells screened at varying depths between approximately 20 to 45 feet, 60 to 75 feet, and 110 to 120 feet bgs, which have been designated Zone A, Zone B, and Zone C, respectively. From May through November 2000, GR installed two additional on-site (U-14 and U-15) and fourteen additional off-site (U-]0 through U-13, U-16 through U-25) groundwater monitoring wells, two additional on-site vapor extraction wells (UV-2 and UV-3), and eight on-site ozone microsparge wel (SP-lA and B through SP-4A and B). Wells U-10 through U-13 were installed and screened at varying depths within the three zones. Wells U-14 through U-25 were installed in six sets of two wells each, with the wells in each set screened from approximately 65 to 75 feet (Zone B), and 110 to 120 feet (Zone C)bgs. The ozone microsparge well points were installed in four sets of two wells each, with each set consisting of wells set at 75 and 120 feet bgs. The locations of the wells are shown on Figures 2 and 3. A more detailed description of previous environmental activities is presented in GR report #240042.09-1, Interim Remedial Action Test Report and Additional Subsurface Assessment Work Plan, dated August 28, 2001. The lack of a significant difference in groundwater elevation between wells screened in the three zones (Zones A through C) suggests these zones are all part of the same hydrogeologic unit, and there is hydrostatic communication between the uppermost and the deepest depths explored. Previous investigations have not encountered any fine-grained units that appear to act as a confining layer for groundwater, and all of the fine-grained units encountered below first-encountered water are saturated, suggesting that none of these units would act as a complete barrier to the downward migration of hydrocarbons. Based on groundwater analytical results from the most recent groundwater monitoring and sampling event on August 13 through 15, 2001, the lateral extent of the dissolved hydrocarbon plume is delineated in Zone A and Zone B,but not in Zone C. In Zone A, impact is limited to the vicinity of wells U-1, U-2, NP-1-22, and NP-2-22. Dissolved hydrocarbon concentrations in Zone A drop significantly from the source area near the southeast corner of the site, to downgradient well NP-2-22. In Zone B, dissolved hydrocarbons were detected in wells NP-1-65, NP-2-65, and U-13. Again, there is a drop in concentrations from the source area to downgradient well U-13.Well locations are shown on Figure 3. 240042.11 2