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0 <br /> Well Monitoring Program -5- 11 September 1992 <br /> DDRW, Tracy <br /> the five wells selected for background for the Operable Unit No. 1 (OU-1) Risk <br /> Assessment, may be impacted by off-site sources of ground water contamination. Off- <br /> site wells No. 14 (8820 Valpico Road) and No. 15 (8747 Almendra Way) had low <br /> concentrations of chloroform (about 3 µg/l ) in the ground water samples. These <br /> wells are down gradient from Triple E Produce which is a confirmed off-site source <br /> of chloroform ground water contamination. <br /> Therefore, Tracy should consider other methods for analyzing the chemical <br /> characteristics of the water bearing zones such as ratios or graphical methods (eg. , <br /> trilinear diagrams) . The analytical results for general minerals collected during <br /> the August 1991 quarter should be used in these evaluations. Tracy should consider <br /> installation of a monitor well cluster in the most up gradient location on the base <br /> and out of the influence of any on-site sources to determine the concentrations of <br /> metals and minerals in each of the horizons. The installation of these wells is <br /> also needed because of the two off-site, up gradient sources of VOC ground water <br /> contamination (chloroform) . <br /> SPECIFIC COMMENTS <br /> 1 . The Draft Report provided an evaluation of specific ground water fluctuations, <br /> anomalous points on the hydrographs (Appendix B) and discussions on the <br /> horizontal and vertical gradients. However, the Draft Report could have been <br /> improved by providing an analysis of the general quarterly changes in the water <br /> level contour maps for each of the horizons and the general trends in the <br /> hydrographs. The Draft Report indicates that the TCE and PCE plumes trend in <br /> the direction of ground water flow. Therefore, general trends in the hydraulic <br /> aspects of the ground water system are integral to contaminant transport. <br /> Changes in the ground water flow from horizon to horizon, from season to season <br /> and over time are critical to understanding the plume migration. Dramatic <br /> changes in ground water elevations and local changes in the ground water flow <br /> direction in each horizon may reveal pumping influences from nearby supply <br /> wells. Impacts from the continuing drought may be observed on the hydrographs. <br /> General findings such as these should be presented in the Final Engineering <br /> Report on the Well Monitoring Program. <br /> 2. Specific proposals for on-going monitoring were presented in Table 7-4 of the <br /> Draft Report. Some additional specific comments on the proposals are as <br /> follows: <br /> a. It is unclear why two wells are needed for quarterly monitoring if both <br /> wells are relative close to each other, monitor the same horizon and have <br /> relatively similar, stable concentrations. An example of this potentially <br /> unnecessary monitoring is LM-28 and LM-62. Only one of these wells needs <br /> to be monitored on a quarterly basis. The other monitor well could be <br /> monitored on a semi-annual basis. <br /> b. It is unclear why two monitor well clusters are needed for quarterly <br /> monitoring if a nearby cluster, which has similar contaminant <br /> concentrations, is also monitored quarterly. Specifically, it is unclear <br /> why the cluster site LM-49, LM-50 and LM-51 needs to be monitored quarterly <br /> when it is between two other cluster sites (LM-30, LM-40, LM-41 and LM-48; <br /> and LM-55, LM-56 and LM-57) . The majority of the wells in these other two <br />