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P0.- -- �-�- &V� 1 Ihamond Manteca,CA-November 1993 Groundwater Monuonng Pae 4 " An estimate of the groundwater gradient was made using the most recent static water level 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 he 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 casing) from the ' elevations of the well heads(referenced to the USGS Bench-Mark D-83, Brass Cap on top 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 on calculations using data during the November sampling event, v the groundwater gradient has a very slight gradient in a northerly direction The difference in static groundwater elevations between the highest (BEI-MW6) and lowest point (SRK-MW1), 150 feet laterally, is approximately 0 17 feet Copies of the hand calculations and cross-sections used in determining the ' groundwater gradient are provided in Appendix B Registered well information (2,000 feet radius of the site) is included in the previous monitoring report dated April 1993, and qualifies the groundwater gradient ' to be in a northerly direction ' 4 0 GROUNDWATER SAMPLING 4.1 SRK Series Monitoring wells More than five casing volumes were purged from each of the SRK monitoring wells (SRK-MW1, SRK- MW2 and SRK-MW3) using a 4-inch submersible pump and braided PVC tubing This procedure is designed to assure that water extending to the top of the groundwater column, slightly above the screened interval, is drawn into the monitor well The submersible pump, 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 approximately five casing volumes (roughly 55 gallons) from each SRK series wells Purging causes a ' localized cone of depression around each monitoring well Because the cone of depression causes a localized drop in the static water level, the lighter, volatile contaminant constituents (typically located on ' the static water surface) are allowed to be drawn into the screened interval of each monitoring well The purged volumes (approximately 55 gallons from each SRK series monitoring well) were pumped into ' new 55-gallon drums The drum lids were securely fastened after each well was purged and sampled, and each drum was correctly labelled to aid in disposal activities following completion of analyses ' Analytical groundwater samples were collected using dedicated disposable bailers equipped with nylon cord These samples were collected after the pH, temperature and conductivity had stabilized "Stabilized" is defined as three consecutive reading within 15% on one another The analytical groundwater samples were collected in laboratory grade sampling containers and placed in a cooler containing frozen ice and blue ice packets, and shipped following strict chain-of-custody procedures to a California certified laboratory (ACZ Laboratories) for analysis Copies of the Geologic Logs and Monitoring Well Construction Diagrams are provided in earlier SRK monitoring reports Copies of the purging/sampling logs are provided in Appendix C 3250422Wgflym- Steffen Robertson and Kirsten