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Appendix B:Previous Environmental Activities SAIC collected one soil sample from SB-15 at 47 feet bgs to define the vertical extent of affected soil off site. TPHc, TPHg, BTEX, and MTBE were not detected in this soil sample, and thus the vertical extent of affected soil was defined at this depth. SAIC searched DWR files in April 2004 and identified three domestic wells located within 2,000 feet of the site (now defined as the area of the former HPP pipelines adjacent to the Hunter facility). One of the domestic wells was located up gradient of the former pipeline alignments, and two were located cross gradient, approximately 1,000 feet and 1,500 feet away. The well completion reports indicated two domestic wells were constructed with minimum sanitary seals extending to 20 feet bgs. The well completion report of the third well did not state a seal depth. One industrial well may be located in the vicinity of the site, but the exact location was unspecified. Municipal or agricultural water supply wells were not identified during the DWR search,but two wells were described on well completion reports as public wells and were located at the site. One well is sealed from the surface to 150 feet, and the other to 200 feet. There are no domestic wells located down gradient(east) of the site. SAIC concluded that the data collected during this and previous investigations indicated that residual petroleum had affected soil between 5 and 42 feet bgs in the vicinity of SB-17, MW-1, SB-1, SB-2, SB-3, B-1, B-4 through B-7 (MW-2), GMX-3, GMX-8, GMX-1B, and GMX-12. In SB-17, clay and sandy layers below 37 feet bgs appeared to be the most affected. The lateral extent of affected soil appeared to be delineated towards the south, west, north, northeast, and east. However, the lateral extent of affected soil southeast of the former OVP and TAOC alignments and SB-17 was not fully characterized. The area northeast of GMX-6 had likely been affected by a refined-fuel plume as a result of a leaking UST at the former Union Oil Service Station at the eastern corner of Welty Road and Highway 33. The service station operated from 1972 to the mid 1980s. SAIC 2007 MONITORING WELL DESTRUCTION EVALUATION In September 2007, SAIC evaluated historical groundwater monitoring well sampling analytical results,using the following criteria, to determine whether monitoring wells may be destroyed: 1. COPCs in a well must be less than WQOs11 and San Francisco Bay RWQCB Environmental Screening Levels (ESLs) 12 for at least last four sampling events; or 2. COPCs in a well must be stable or declining for at least eight consecutive events where the overall plume is defined. SAIC's review indicated that MW4 and MW-5 should be destroyed. No COPCs were historically detected above ESLs or WQOs in MW-5. Laboratory analytical results for all COPCs were non-detect in MW-4 for the previous four sampling events. SAIC also recommended the destruction of MW-3,based on declining TPHd concentrations. The RWQCB agreed that MW-3 and MW-4 should be destroyed,provided that the results of one additional groundwater sampling event satisfied the monitoring well destruction criteria. The RWQCB also recommended a final sampling event for MW-5. However, SAIC was allowed to 11.RWQCB,2007. Water Quality Control Plan(Basin Plan)for the Sacramento River Basin and the San Joaquin River Basin, 4th ed. October. 12.RWQCB,2008(San Francisco Bay)Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater(Interim Final). May. B-5 jM as = Fmm Science t0 Solutions