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' Diamond Manteca CA-June 1993 Groundwater Monttonng Page 4 The static water level elevations collected June 29-30, 1993, were on the average one foot lower than the previous sampling event (February 5, 1993), and also one foot lower than the levels recorded during the April 7, 1992, sampling event The static water levels collected February 5, 1993, were on the average 4 feet higher than the other sampling events SRK contacted the Western Regional Climate Center to obtain precipitation data for the Manteca area Precipitation data was obtained from the nearest station (Stockton, California), located approximately 13 miles north of the site A copy of this information gathered from 1985 through 1992, is provided in Appendix B A review of available precipitation data indicates that this area was experiencing a drought which started in 1988 and began to abate in 1991 The BEI series ' monitoring wells were installed during August 1989 No precipitation was recorded during this month which typically is one of the driest months The SRK series monitoring wells were installed during November 1991 under the direct guidance of the PHS/EHD, and during that month 0 34 inches of ' precipitation was recorded Pre-1988 precipitation levels for November were between 1 and 2 inches By comparing monthly precipitation data in Appendix A with static water level information in Table 1, it can be deduced that static water levels are influenced by seasonal precipitation patterns An estimate of the groundwater gradient was made using the most recent static water elevations to complete an orthographic construction known as a 3-point problem (Davis, G H , 1984, Structural Geology of Rocks and Regions John Wiley & Sons,pp 210) Solving a 3-point problem requires elevation control for at least three points that lie on a common plane, in this case the groundwater surface The elevation control was ' obtained by subtracting the static water levels (measured from the top of well casings)frorn the elevations of the well heads(referenced to the USGS Bench-Mark D-83, Brass Cap on top step of Legion Hail, South side of Yosemite Avenue Elevation 38 051 feet) It should be noted that the groundwater elevations varied by less than 0 5 feet Based on calculations using data collected during the June 1993, sampling event, the groundwater gradient has a very slight gradient in a northerly direction It should be noted that the ' groundwater gradient is extremely flat (the difference in static groundwater elevations between the highest and lowest point, 150 feet laterally is approximately 0 02 feet Copies of the hand calculations and cross- sections used in determining the groundwater gradient are provided in Appendix B of our April 1993 ' monitoring report Registered well information (2,000 feet radius of the site) is also included in our previous report dated April 1993, and qualifies the groundwater gradient to be in a northerly direction r 4 0 GROUNDWATER SAMPLING 4 1 SRK Series Monitoring wells Approximately five casing volumes were purged from each of the three SRK monitoring wells(SRK-MW 1, ' SRK-MW2 and SRK-MW3) Initially a 2-inch submersible pump equipped with variable flow control and braided PVC tubing was used to purge approximately 35 gallons from SRK-MW1 Because of equipment malfunction associated with the 2-inch submersible pump, purging activities;were performed using a 4-inch submersible pump and braided PVC tubing Both the 2-inch and 4-inch submersible pumps, tubing, and electrical leads were carefully steam cleaned before and after each use A hot water solution containing ' non-phosphate soap was also circulated through the pump and tubing prior to flushing and use Static water levels were recorded before and immediately after the three SRK series monitoring wells were purged The static water levels after purging were on the average comparable to levels prior to purging ' Siwa 220-Wy M— Steffen Robertson and Kirsten