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January 27, 1992 - � F�� Former Texaco Station, Manteca, California �3 fluctuations of hydrocarbon concentrations in the ground water at the site (Applied GeoSystems, August 8, 1988, Report No. 38053-1) In October 1989, Applied GeoSystems observed the excavation of approximately 1,100 cubic yards of soil from the site to the depth we had noted ground water and to the lateral limits of photoionization detector readings and subjective evidence_ Results of laboratory an lyses of soil samples collected from the sidewalls of the excavation indicated hydrocarbons were not present (Applied GeoSystems, October 26, 1989, Report No. 38053-2). In December 1989, an Applied GeoSystems geolo ist conducted the first quarterly ground- water monitoring event. The direction of ground water flow across the site was evaluated to be to the northwest, and the average gradient was evaluated to be 0.0012. Laboratory analyses of ground-water samples collected during the subsequent year of monitoring (1990) indicated the concentrations of dissolved hydrocarbons had decreased to below Maximum Contaminant Levels (MCL's) established by the California Department of Health Services for drinking water. In addition, the groundwater I low direction and gradient had remained fairly consistent (Applied GeoSystems, December 8, 1990, Report No.38053-2). Therefore, Applied GeoSystems recommended the wells be sa pled semi-annually during 1991 to verify hydrocarbon concentrations remained below M L's for over a period of 1 year. Data F ' collected during the June 1991 monitoring event indicated hydrocarbons were not detected - in ground-water samples and the groundwater flow direction remained constant. PRESENT GROUNDWATER SAMPLING On December 3, 1991, a RESNA technician measured the depth to the ground-water surface in each monitoring well,performed subjective analyses of ground water, and sampled ground water from the wells. The field procedures used are presented in an attachment to this report. Ground-Water Flow Direction and Gradient The differences in height among the well casing t ps relative to an arbitrary datum (top of casing of well MW-3) were combined with water level data collected during this sampling to construct a Potentiometric Map (Plate 3). Data used to compile Plate 3 are shown in Table 1. The direction of ground-water flow at the site was northwest with a gradient of 0.005. 1231gbro 38053-4 2