ARCHIVED REPORTS_FIRST QUARTER 2010 GROUNDWATER MONITORING REPORT

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URS ' Mr.James Barton Page 2 of 8 California Regional Water Quality Control Board, Central Valley Region ' April 29, 2010 ' (MTBE)was identified in soil and groundwater samples. The highest hydrocarbon concentrations were detected in soil and groundwater samples collected near the USTs and dispenser islands. In May 1998,Remedy, Inc. drilled three soil borings, one using mud rotary(B-11) and two using cone penetrometer testing(CPT) (CPT-11 and CPT-12). In addition to TPH-G, TPH-D, BTEX, and MTBE, tetrachloroethene (PCE) was reported in soil samples collected from B-11 and CPT-11. In June 1998,Remedy Construction removed two 10,000-gallon diesel USTs and four 10,000-gallon gasoline USTs. Staining and hydrocarbon odors were present in soil beneath the USTs. Soil samples were collected from the UST excavations and along product pipelines. Grab groundwater samples were ' collected from the UST excavation. Approximately 726 tons of impacted soil were over-excavated from the former tank excavations and disposed of at the Altamont Landfill in Livermore, California. ' In August 1998, Remedy,Inc. conducted an area well survey for the site and identified 46 drinking water wells within 0.5 mile of the site, including a drinking water well on site. Groundwater samples were collected from the on-site drinking water well in July 1997,January 1998,April 1998, and July 1998. ' In April 1999,ASE drilled two CPT borings (CPT-13 and CPT-14). Depth-discrete groundwater samples were collected at 40, 70, and 117 feet below ground surface(bgs)using a SimulProbe sampler. The only contaminant of concern that exceeded the California Department of Health Services (DHS)maximum contaminant level (MCL)for drinking water was benzene at 4.8 parts per billion(ppb), detected in the groundwater sample collected from 70 feet bgs in boring CPT-13. ' In May 1999, ASE installed monitoring wells MW-1 though MW-10 at the site. Groundwater was encountered at approximately 14 feet bgs during drilling and installation of all six on-site groundwater monitoring wells (MW-1 through MW-6). The groundwater flow direction in the first zone was to the northwest at a gradient of between 0.008 to 0.01 feet per foot. In August 1999, ASE collected samples from groundwater monitoring wells, as well as the on-site drinking water well. Free-floating hydrocarbons,believed to be gasoline,were present on the ' groundwater surface in monitoring well MW-5. Hydrocarbon sheen was present on the groundwater surface in monitoring well MW-6. ASE began bi-weekly measurement and bailing of the free-floating hydrocarbons in monitoring wells MW-5 and MW-6. Measurement and free-floating hydrocarbon bailing was reduced to monthly after January 2000. In December 1999, ASE conducted a more detailed well survey and identified 57 wells within a 2,000-foot radius. Of these 57 wells, 39 are domestic, 3 are irrigation only, and 15 are of unknown usage. The closest properties to the site with wells are directly to the east and southeast and across Airport Way to the west. Based on groundwater flow in the shallow zone,these wells are located in upgradient or cross-gradient directions from the site.The closest wells potentially downgradient of the site are approximately 120 feet northwest. The closest domestic well directly downgradient of the site's USTs is located approximately 300 feet north of the site. H:\Wprocess\RESD\Franks One Stop\QTR 10\1QMR10 Report.dm