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T ' Dtamond Lumber Manteca Quarterly Groundwater Monitoring Page 4 The static water level elevations collected February 5, 1993, were on the average 4 feet higher than the previous sampling event (August 1992) but are comparable to the levels recorded during early 1992, during the April monitoring event 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 A- The SRK series monitoring wells were installed during November 1991 under the direct guidance of the PHS/EHD 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 in November 1991 when 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 Ievel 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 by 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) from the elevations of the well heads (referenced to the USGS Bench-Mark D-83, Brass Cap on top step of Legion Hall, South side of Yosemite Avenue Elevation 38 051 feet) It should be noted that the groundwater elevations varied by only a few tenths of feet Based an calculations using data during the February sampling event, the groundwater 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 2 feet Copies of the hand calculations and cross-sections used in determining the groundwater gradient are provided in Appendix B SRK contacted Mr Jim Podesta with the Manteca Public Works Department regarding subsurface groundwater information from private, municipal and irrigation wells located within 2,000 feet of the I site Mr Podesta cited the Spring 1992 Semi-Annual Groundwater Report, produced by the San Joaquin County Floor Control and Water Conservation District This document contains groundwater contour maps which include the Diamond Lumber facility Mr Podesta reviewed the relevant maps 1 contained in the Spring 1992 Semi-Annual Groundwater Report, and stated that the groundwater is in a northerly direction 4 0 GROUNDWATER SAMPLING Approximately five casing volumes were purged from each of the three SRK monitoring wells (SRK- MW1,SRK-MW2 and SRK-MW3) using a 4-inch submersible pump and braided PVC tubing Purging causes a localized cone of depression around each monitoring well In essence, the cone of depression ' causes a localized drop in the static water level Special care was taken to keep the exterior of the Steffen Robertson and Kirsten