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No Further Action Request 640 N.San Joaquin Street,Stockton,CA October 9,2002 Page 5 It is Condor's opinion that the low concentration of motor oil range petroleum hydrocarbons detected in the groundwater sample collected from boring B-3 may be the result of trace cross-contamination of the sampling tool from the soil above. As the groundwater in B-3 entered the groundwater sampler, it is • possible that a trace of contamination on the sampler could have mobilized into the sampler and been collected. The following support this possibility: &W • The depth to groundwater at the site is greater than 30 feet bgs. • The laboratory analytical results indicate that no soil samples collected at the site from deeper than 15.5 feet bgs were impacted by hydrocarbon contamination. a The local soil types at the site are tight clays, restricting the movement of contaminants through the subsurface. 0 Heavy hydrocarbon constituents characterize the soil contamination and are prone to adhering to sampling equipment, potentially resulting in cross-contamination of water samples. Groundwater was included in the risk evaluation performed in Section 3.0 in order to provide the most conservative risk analysis. The mass of soil containing hydrocarbons was estimated using an aerial extent of 108 ft2 and a maximum thickness of six feet. The volume of hydrocarbon contaminated soil is approximated to have a lobate LW shape. Condor estimates the mass of soil containing detectable concentrations of hydrocarbons in the subsurface at the site is approximately 32,332 pounds. The geometric mean concentration of 8,536.5 mg/kg of hydrocarbons remaining in the soil was calculated using the analytical results of four Soil samples collected from the volume of contaminated soil. Calculations indicate the total mass of hydrocarbons remaining in the soil at the site is approximately 608 pounds. A calculation worksheet is presented in Appendix B. 5.0 SENSITIVE RECEPTOR SURVEY Condor conducted a sensitive receptor survey within a 2,000-foot radius of the site(study area) to identify L" potential areas of groundwater recharge or discharge (surface water bodies), schools, and potential water well users that could be impacted by migration of groundwater from the site. The sensitive receptor survey consisted of inquiries to the California Department of Water Resources (DWR) Central District in Sacramento, California, contacting public water purveyors, visually inspecting the area within 500 feet of the site, and interviewing residents located in the site vicinity. DWR well records were cross-referenced with Assessor's Parcel Maps and city street maps. This process helped identify well records pertaining to Lo wells estimated to be within the study area. DWR well records may include the well owner's name, a well location map, the address where the well is located, and a log of the well. Lo 5.1 DWR RECORDS WITHIN 2,000 FEET Documents provided by the DWR, associated with Section 2, Township 1 North, and Range 6 East, include information pertaining to 19 water wells. The DWR records were reviewed based on Assessor's V Parcel Maps, area reconnaissance, a topographic quadrangle map, and a city street map. Thirteen of the well records pertain to wells located outside of the study area. Six well records pertain to rive wells that were verified to be within the study area. The information provided by the DWR indicated that two of the i.. five wells located within the study area had been destroyed. ' According to DWR records correlated with Assessor's Parcel Number (APN) and mapping information, l,r six well records pertain to five wells located with the study area. L A f i� CONDOR