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August 2, 1991 WNSMA Former Texaco Station, Manteca, California ground water and (2) 1 year of quarterly ground-water monitc.ing to evaluate the fluctuations of hydrocarbon concentrations in the ground water at the site (Applied GeoSystems, August S, 1988, Report No.38053-1). In October 1989, Applied GeoSystems observed the excavation of approximately 1,100 cubic yards of soil frcvn V.-r3 site to the depth we had previously inoted ground water in the onsite wells ane the lateral limits of photoionization detector readings and subjective evidence. Result,, of laboratory analyses of soil samples collected tiom the sidewalls of the excavation indicated hydrocarbons were not present (Applied GeoSysi:.ms, October 26, 1989,Report No 38053-2). 0 In December 1989, an Applied GeaS' tems geologist conducted the first quarterly 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 :f ground water samples collected during the subsequent year of monitoring events indicate the concentrations of dissolved hydrocarbons had decreased to below laboratory detection limits(Applied GeoSystems,December 18, 1990,Report No.38053-2). Therefore,Applied wells be sampled semi-annually during 1991 to verify GeoSystems recommended the hydrocarbon concentrations had decreased to below Maximum Contaminant Levels established by the California Department of Health Services. Present Monitorin Event On June S, 1991, an Applied GeoSystems technician measured the depth to the ground- water surface in each monitoring well and sampled ground water from the wells. The field procedures used are presented in an attachment to this report. The differences in height among the well casing tops 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 Ground-Water Surface Map (Plate 3). Data used to compile Plate 3 are shown in Table 1. The direction of ground-water flow at the site was slightly west of north with an average gradient of 0.0058. Samples of ground water were taken from the monitoring wells for subjective analysis. No floating product or product sheen was noted in samples from the monitoring wells. Results of the subjective analyses are presented in Table 2. < + After performing subjective analyses, the technician purged the three monitoring wells before collecting samples. Disposal of the purged water remains your responsibility. After purging each well, the technician collected groundwater samples for laboratory analysis. The samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg) using modified Environmental Protection Agency (EPA) Method 8015, total petroleum 0626gbro d 38053 +A3