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Dimond Lodi,CA-June 1993 Groundwater Monitoring Page 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 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 casing) from the elevations of the well heads (referenced to the City of Lodi Bench-Mark 710: 49.74 feet). It should be ' noted that the groundwater elevations varied by only a few tenths of feet. Based on calculations using data during the June sampling event, the groundwater gradient has a very slight gradient in a southerly direction. ' It should also be noted that the groundwater gradient is extremely flat (the difference in static groundwater elevations between the highest and lowest point, 140 feet laterally, is approximately 0.25 feet). Copies of the hand calculations and cross-sections used in determining the groundwater gradient are provided in ' Appendix B. ' 4.0 GROUNDWATER SAMPLING ' Three casing volumes were purged from each of the SRK monitoring wells using dedicated disposable bailers equipped with nylon cord. The purged volumes were measured in a calibrated bucket, then emptied collectively into a 55-gallon drum located in the covered material storage area that extends along the south ' wall of the former lumber storage area. The drum lid was securely fastened after the three wells were sampled. The analytical groundwater samples were collected after more than 3 casing volumes were purged from each well, and after the pH, temperature and conductivity had stabilized. "Stabilized" is defined as three consecutive readings within 15% of 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. Because lead levels had been detected from each of the three monitoring wells above San Joaquin County PHSIEHD drinking water samples, additional groundwater samples were collected from each monitoring well in laboratory grade containers. These samples were filtered to compare total lead levels in the ' unfiltered samples with the filtered samples, and to determine if lead was a constituent of the soils or the groundwater. The filtration was performed by using a peristaltic Geotech Pump 2, nalgene tubing and 0.5 ' micron filters. Due to equipment malfunction in the field, the PHSIEHD inspector granted approval for the samples to be filtered later that afternoon. The SRK field geologist placed these samples in the cooler with the other samples, and maintained custody of all water samples. Later that afternoon, the SRK geologist tfiltered the samples in Rancho Cordova, California, using the wall plug in adaptor as a power source for the Geotech Pump 2. A separate filter was used for each water sample, and the nalgene tubing was thoroughly decontaminated before and after each filtration using a solution of non-phosphate soap and a deionized water rinse. ' 12701,2155-Wy Lad Steffen Robertson and Kirsten