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Quarterly Ground-Water Monitoring June 30, 1989 Beacon Station No. 3-492, Manteca, California AGS 86102-3 The samples collected from ground-water monitoring wells MW-1, MW-2, and MW-3 were analyzed at the Applied GeoSystems laboratory in Fremont, California (Hazardous Waste Testing Laboratory No. 153) , for total petroleum hydrocarbons (TPH as gasoline using modified Environmental Protection Agency (EPA) Method 8015 and benzene, toluene, ethylbenzene, and total xylene isomers (BTEX) using EPA Method 8020. The results of these and previous analyses are summarized in Table 2. Detectable concentrations of BTEX were present in the sample collected from well MW-1. BTEX concentrations in the samples collected from wells MW-2 and MW-3 were generally below the detection limits of the analytical methods used, although xylene isomers were present in detectable concentrations in the sample from well MW-3. Copies of the laboratory Analysis Reports are attached to this letter report. The ground-water gradient across the site was evaluated by combining measurements of the static water level with the relative elevation of the top of the casing in each monitoring well. The relative elevation of the top of the casing in each well was previously evaluated with a Lietz/Sokkisha C3E Automatic Level. The static water levels were adjusted using the top of + the casing in well MW-2 as a datum. These data are summarized in Table 3. On March 2, 1989, the maximum difference in elevation Of the ground--water surface measured in the wells was 0.01-foot. The ground-water gradient, therefore, was inferred to be essentially flat; similar results have been obtained during ' previous evaluations. Applied GeoSystems recommends that monitoring of ground water at the site be continued on a quarterly basis. The next sampling has been scheduled for the week of June. 4, 1989. F _ A 3 e