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---------- C,�;7, vj%Tc 3 A S S O C I A T E S I N C well has historically contained .free product that has been removed. The monitoring wells on the perimeter of the site, MW1, MW2, MW3, MW5, and MW6, have reduced concentrations of TPHd which are generally near the method detection limit. Groundwater samples were most recently collected on October 18, 2000 from all wells, MWl through MW6. Prior to sample collection, each sampled well was purged of a minimum of three ,J well casing volumes of water. Groundwater samples were collected after physical parameters (pH, temperature, and conductivity) were monitored and had stabilized. Dedicated disposable bailers with bottom-emptying devices were used to collect samples from the wells. Floating petroleum product has previously been observed in well MW4. On October 18, k=; 2000, only a slight, unmeasurable sheen was observed in well MW4. No free product was recovered from the well because the quantity was too small to be collected into a bailer. Table F 4 summarizes the product thickness measurements and volume of product removed from previous monitoring events. Laboratory analysis of samples indicates that TPHd was detected in groundwater samples =- collected from well MW4 at a concentration of 9,640 parts per billion (ppb) and in well MW3 at 1,600 ppb. TPHd was reported at nondetect or just above the method detection limits in wells MW 1,MW2, MW5, and MW6. L KID), 4.2.3 Mass Balance Calculations-Groundwater .`' Using groundwater analytical results presented in Table 2, the most recent data available, ATC estimated the lateral extent of dissolved TPHd concentrations beneath the site and was then used by ATC to perform mass balance calculations to estimate the mass of residual contamination in `f the site's saturated zone. The total mass of dissolved TPHd present in the saturated zone is estimated to be approximately 5.6 pounds. Mass balance calculations are included in Appendix D. 5.0 GROUNDWATER RECEPTOR SURVEY 76/0.3 In e II Z l t,IJ /as !i44 `-r ATC performed a well survey for the site at the request of PHS/EHD in their correspondence w ll dated July 14, 1999. ATC reviewed Department of Water Resources (DWR) groundwater well 1.Y construction logs and San Joaquin County well installation permits to identify any groundwater 714 k wells located within a 2000-foot radius of the site. The search revealed twenty-two wells within approximately 2,000 feet of the site. None of the ��d ` wells are located close enough to the site to be considered potential receptors. The well locations are shown on Figure 4. The closest well is approximately 1,000 feet from the site. Smith's Canal J F` is located approximately 100 feet to the south of the site. Based on historical groundwater gradients measured at the site the groundwater gradient direction is to the north. Based on .::., g information in the San Joaquin County Flood Control and Water Conservation District's Spring 1997 Groundwater Report, the inferred regional groundwater gradient is to the northeast. Based w:162574.021reporislclosure-smith.doc 6