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• Ozone Microsparge System Pilot Test Status Report,Former Unocal Service Station No. 0187, Stockton,CA
<br /> November 19,2003
<br /> These USTs were replaced with two 10,000-gallon gasoline and one 280-gallon waste oil USTs situated
<br /> in a common excavation in front of the station building. The two 10,000-gallon gasoline and one 280-
<br /> gallon waste oil USTs, associated product piping, hydraulic lifts, and oil/water separator were removed
<br /> and the station facilities demolished in January 1992. The locations of pertinent site features are shown
<br /> on Figure 2.
<br /> From 1993 through 1997, GeoStrategies Inc. (GSI) installed six on-site (U-1 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 (NNI),
<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.
<br /> From May through November 2000, GR installed two additional on-site (U-14 and U-15) and fourteen
<br /> additional off-site (U-10 through U-13, U-16 through U-25) groundwater monitoring wells, two
<br /> additional on-site vapor extraction wells (LTV-2 and UV-3), and eight on-site ozone microsparge wells
<br /> (SP-lA and B through SP-4A and B). Wells U-10 through U-13 were installed and screened at varying
<br /> depths within the three zones. Wells U-14 through U-25 were installed in six sets of two wells each,with
<br /> the wells in each set screened from approximately 65 to 75 feet (Zone B), and 110 to 120 feet (Zone C)
<br /> • bgs. The ozone microsparge well points were installed in four sets of two wells each, with each set
<br /> consisting of wells set at 75 and 120 feet bgs. The locations of the wells are shown on Figures 2 and 3. A
<br /> more detailed description of previous environmental activities is presented in GR report #240042.09-1,
<br /> Interim Remedial Action Test Report and Additional Subsurface Assessment Work Plan,dated August 28,
<br /> 2001.
<br /> GR personnel performed a five-day vapor extraction(VE)pilot test at the site from November 27 through
<br /> December 1, 2000. The VE system operated for a total of 40 hours, and soil vapors were extracted from
<br /> wells UV-1, UV-2, and UV-3. The purpose of the VE pilot test was to evaluate the feasibility of the VE
<br /> technology and collect needed site specific data for the design of a VE system. The results of the VE
<br /> pilot test indicated that VE technology was not an appropriate remedial technology for this site.
<br /> On February 11 through 19, 2002, GR installed one groundwater monitoring well(U-26)and eight ozone
<br /> microsparge wells (SP-1C, SP73C, SP-5A, B, C and SP-6A, B, C). On March 18 through 22, 2002, GR
<br /> installed a second ozone generating panel and connected the newly-installed microsparge welts to the
<br /> system. The new system was placed into service on April 26, 2002.
<br /> Quarterly monitoring and sampling of the groundwater monitoring wells began in August 1994. During
<br /> the most recent sampling event on February 26 through 29, 2003, TPHg were detected at concentrations
<br /> ranging from 330 to 1,600 ppb, and benzene concentrations ranged from 4.3 to 110 ppb. The highest
<br /> concentrations of TPHg and benzene were detected in wells NP_1-65 and U-21, respectively. MtBE was
<br /> not detected in any of the wells. Quarterly monitoring and sampling of the wells is performed by GR.
<br /> Historical groundwater chemical analytical data are summarized in Appendix C.
<br /> 240042.11-3 2
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