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. Monitoring Welland Sparge Point Installation Report—Former Unocal Service Station#0187
<br /> July 15,2002
<br /> PREVIOUS ENVIRONMENTAL WORK
<br /> Two 1,000-gallon gasoline and one 550-gallon waste oil USTs were removed from the southeast corner of
<br /> the subject site in 1967 Data related to soil conditions beneath the USTs at the time of removal is not
<br /> available These USTs were replaced with two 10,000-gallon gasoline and one 280-gallon waste oil
<br /> USTs situated in a common excavation in front of the station building The two 10,000-gallon gasoline
<br /> and one 280-gallon waste oil USTs, associated product piping, hydraulic lifts, and oil/water separator
<br /> were removed and the station facilities demolished in January 1992 The locations of pertinent site
<br /> features are shown on Figure 2
<br /> From 1993 through 1997, GeoStrategies Inc (GSI) installed six on-site(U-I through U-6) and three off-
<br /> site (U-7 through U-9) groundwater monitoring wells, installed one on-site vapor extraction well (UV-1),
<br /> and drilled fifteen on-site soil borings (U-A through U-O) The groundwater monitoring wells installed
<br /> by GSI are screened at varying depths between approximately 20 and 35 feet below ground surface (bgs)
<br /> In November 1998, ARCADIS Geraghty & Miller installed one on-site nested piezometer well (NP-1),
<br /> and in January 1999 installed one off-site nested piezometer well (NP-2) Each nested piezometer well
<br /> consists of three 1-inch diameter wells screened at varying depths between approximately 20 to 45 feet,
<br /> 60 to 75 feet, and 110 to 120 feet bgs, which have been designated Zone A, Zone B, and Zone C,
<br /> respectively From May through November 2000, GR installed two additional on-site (U-14 and U-15)
<br /> and fourteen additional off-site (U-10 through U-13, U-16 through U-25) groundwater monitoring wells,
<br /> two additional on-site vapor extraction wells (UV-2 and UV-3), and eight on-site ozone microsparge
<br /> wells (SP-IA and B through SP-4A and B) Wells U-10 through U-13 were installed and screened at
<br /> varying depths within the three zones Wells U-14 through U-25 were installed in six sets of two wells
<br /> each, with the wells in each set screened from approximately 65 to 75 feet (Zone B), and 110 to 120 feet
<br /> (Zone C) bgs The ozone microsparge well points were installed in four sets of two wells each, with each
<br /> set consisting of wells set at 75 and 120 feet bgs The locations of the wells are shown on Figures 2 and
<br /> 3 A more detailed description of previous environmental activities is presented in GR report
<br /> #240042 09-1, Interim Remedial Action Test Report and Additional Subsurface Assessment Work Plan,
<br /> dated August 28, 2001
<br /> The lack of a significant difference in groundwater elevation between wells screened in the three zones
<br /> (Zones A through C) suggests these zones are all part of the same hydrogeologic unit, and there is
<br /> hydrostatic communication between the uppermost and the deepest depths explored Previous
<br /> investigations have not encountered any fine-grained units that appear to act as a confining layer for
<br /> groundwater, and all of the fine-grained units encountered below first-encountered water are saturated,
<br /> suggesting that none of these units would act as a complete barrier to the downward migration of
<br /> hydrocarbons
<br /> Based on groundwater analytical results from the most recent groundwater monitoring and sampling
<br /> event of February 25 through 27,2002,the lateral extent of the dissolved hydrocarbon plume is delineated
<br /> in Zone A, but not in Zone B and Zone C In Zone A, impact is limited to the vicinity of wells U-1
<br /> through U-3,NP-1-22, and NP-2-22 Dissolved hydrocarbon concentrations in Zone A drop significantly
<br /> from the source area near the southeast corner of the site, to downgradient well NP-2-22 The dissolved
<br /> hydrocarbon plumes in zones B and C are delineated to the west (upgradient), north, and southeast, but
<br /> not downgradient(east to northeast) of the Unocal site In Zone B,dissolved hydrocarbons were reported
<br /> in wells NP-1-65, NP-2-60, U-13, U-18 and U-20 In well U-20, the furthest downgradient Zone B well,
<br /> 24004211-1 2
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