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J� 1 Quarterly Ground-Water Monitoring May 10, 1990 j Beacon Station No. 502, Lodi, California AGS 86091-7 Fuel 'yanks (LUFT) Method and for benzene, toluene, ethylbenzene, and total xylene isomers (BTEX) by Environmental Protection Agency Method 602. Copies of the laboratory analysis reports are attached. The results of these analyses and previous laboratory analyses are summarized in Table 1. The results of the laboratory analyses indicate that hydrocarbon constituents were not detected in the five ground-water samples. Hydrocarbon concentrations in the samples from the five monitoring wells have steadily decreased since the October 19$6 monitoring event, as shown in Table 1. The hydrocarbon concentrations from this monitoring event continue that downward trend. Applied GeoSystems evaluated the ground-water gradient across the site. The elevation of the top of the casing in each well was surveyed by Moldenhauer Engineering Company, a licensed land surveyor from Davis, California, on November 27, 1989. Elevations were a" measured in feet relative to mean sea level, using a local U.S. Geological Survey benchmark as a datum_ The measurements of the depths to ground water were combined with the casing elevations to calculate the ground-water elevations. These data are summarized in Table 2 and were used to evaluate the ground-water gradient at the time of this quarterly monitoring event (Plate P-3). The average ground-water gradient assuming isotropic conditions was evaluated to be 0.010, with a flow direction towards the south-southeast; the magnitude of the gradient appears to vary across the site. The direction of the ground- water gradient is similar to that evaluated in previous sampling events. .j 1 U 2 • i