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June 2, 1992 ��nA Beacon Station No. 492, Manteca, California '-' Evaluation of Ground-Water Gradient Moldenhauer Engineering of Davis, California, registered civil engineering firm and State- licensed land surveyors, surveyed the elevations and locations of monitoring wells MW-1, MW-2, MW-3, and MW-4 on February 26, 1992. The elevations of the wellheads with respect to mean sea level and the groundwater elevations are shown on Table 3. The groundwater gradient, evaluated from water-level measurements obtained on February 26, 1992, and the surveyed elevations of the wells,was 0.0005, and the direction of ground-water flow was towards the west, as shown on Plate 6. Results of Water Analyse Groundwater samples collected from the wells were analyzed for TPHg and TPHd using EPA Method 8015, BTEX using EPA Method 602, total lead using EPA Method 7000, and �. EDB using EPA Method 8010. Additionally, a water sample collected from well MW-4 was analyzed for fecal coliform bacteria, as this well is located within 20 feet of a public sewer line. Water samples were analyzed at Applied Analytical Environmental Laboratories, Western Environmental in Davis, California, Sequoia Analytical Laboratories, and FGL Environmental in Stockton, California. Results of laboratory analyses of the ground-water samples collected from monitoring wells MW-1 through MW-4 indicated TPHg, TPHd, total lead, EDB, and BTEX concentrations were below minimum detection limits established by the California Regional Water Quality Control Board. Results of these analyses are shown on Table 4 and Table 5. Results of fecal coliform bacteria analysis of the ground-water sample collected from well MW-4 indicated that concentrations of fecal coliform bacteria were detected in that sample at a concentration of 2.2 mean predicted number per 100 milliliters. DISCUSSION AND RECOMMENDATIONS Results of this investigation indicate that no free-phase, residual or dissolved concentrations of petroleum hydrocarbons were detected in the soil and ground-water samples submitted w.. for laboratory analysis. Based on the groundwater gradient data evaluated from depth-to- water measurements collected on February 26, 1992, monitoring well MW-4 provides upgradient monitoring, and well MW-1 provides downgradient monitoring. Based on the �- gradient value, the ground-water gradient is essentially flat. It is our understanding that Ultramar will reinstall and operate a soil vapor-extraction system at the site. A 100 cubic- foot-per-minute catalytic unit is to be installed in June 1992, and is anticipated to operate for 2 months. We recommend no further assessment, remediation or monitoring of ground water at the site. We further recommend that signed copies of this report be forwarded to 0316kear 86102-7 4