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August 5, 1997 38737 001 Harding Lawson Associates Ms Mary Meays San Joaquin County Public Health Services Page 5 1 gradient position from the former USTs and is also located near the offsite former Lopez UST location known to be impacted with TPHg With one exception, none of the other wells (MW-1, MW-2, MW-3, MW-5, or MW-6) have contained detectable petroleum hydrocarbons in any sample collected during the three years of monitoring after June 1991. The single exception was 0 08 mg/L of TPHg detected in well MW-1 during the September 1993 sampling event(this detection was qualified as a non-gasoline mixture, see laboratory analytical reports, (WaterWork, 1993)) The presence of detectable petroleum hydrocarbons in wells other than MW-4 prior to September 1993 is summarized as follows Monitoring well MW-1 samples have contained detectable TPHg in only 4 of 22 sampling events, MW-3 samples have contained detectable TPHg in only 3 sampling events, and samples from wells MW-2, MW-5, and MW-6 have contained detectable TPHg only once Based on the analytical results of the second quarter 1994 monitoring event (most recent) and the groundwater analytical results (RAH, 1995a and 1995b, and SECOR draft) which indicated nondetectable TPHg concentrations from open borehole samples, a map depicting the lateral extent of the TPHg groundwater plume is presented to Plate 5 The results of detected TPHg concentrations from open boreholes are considered not reliable and have not been included on Plate 5 These data indicate that ' groundwater with concentrations greater than 1 0 mg/l is restricted to the vicinity of the former UST pit The Lopez property adjacent to the Site on the north includes an irrigation well (Plate 3) Although this well is located in an upgradient position, it has been sampled twice to accommodate requests from the PHS/EHD The well was sampled once in March 1989 and again in September 1993 TPHg and BTEX were not detected at concentrations above their respective reporting limits in either sampling event Soil Analytical Data Based on analytical data from previous investigations, soil with concentrations of TPHg (greater than 100 mg/kg) appears to be located within the saturated zone west of the UST location, and extends at least 20 feat beneath the forklift building (Plate 4) As shown on Plate 4, the lateral extent of these soils has been delineated ' RECOMMENDED ADDITIONAL WORK Based on the above described site conditions, it appears that the site may meet the RWQCB criteria for classification as a "low risk' groundwater designation in accordance with Appendix B to the Tri Regional Board Staff Recommendations(RWQCB, 1996) In addition to site characterization to determine the extent of contamination, and demonstration of the effectiveness of whatever remedial measure(s) are selected (RWQCB, 1996), these criteria include the following 1 Contaminants in the vadose zone must not reverse or threaten to reverse the mass reduction rate of groundwater pollutants discussed to Criteria No 4 below 2 Separate phase product has been removed to the extent practicable 3 No existing water supply wells, deeper aquifers, surface waters or other receptors are threatened by pollutants remaining in the aquifer 4 The total pollutant mass remaining in the ground water is decreasing at predicted rates and neither creates, nor threatens to create, a risk to human health and safety or future beneficial uses of the aquifer