SITE INFORMATION AND CORRESPONDENCE

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0 0 Irrigation Well -2- 6 July 1995 DDRW Tracy Source areas which contributed to VOC contamination in ground water are approximately 1600 to 1800 feet northeast and downgradient of the proposed irrigation well location. The source area closest to the irrigation well is SWMU 27 (former Area 1). No other potential source areas have been reported upgradient of this area at DDRW Tracy. Monitor wells LM117 A and LM24 A are closest to this source area and are located 1500 feet from the proposed irrigation well. Monitor well LM24A and MW 117A have detected PCE at concentrations of 28 ppb and 9.8 ppb, respectively. General Comments: 1. I am concerned that the on-site contaminant plume could be affected by the proposed well because the apparent interconnectiveness of the paleochannel deposits shown in the isopach maps and the geologic cross-sections. Although the ground water flow direction is northeast,the orientation of the paleochannel deposits has been attributed to be the preferred pathway for off- site contaminant migration. 2. Aquifer test results indicate that the four water bearing zones are interconnected and that irrigation wells have had a major impact on contaminant transport at DDRW Tracy. Aquifer test results indicate that irrigation wells have influenced the upper horizon, middle horizon and lower horizon water bearing zones. Agricultural well AG-2, partially completed in these aquifer zones, appears to have had the most influence on contaminant transport. During operation of well AG- 2, the cone of depression for the upper horizon extended 1400 feet upgradient in the Upper Horizon, 1400 to 1700 feet upgradient in the Middle Horizon and, 800 feet upgradient in the Lower Horizon. Some effects were also reported downgradient of well AG-2 in the Lower horizon. Also, water chemistry data indicate that agricultural wells appear to be the primary mechanism for vertical gradients and downward contaminant transport into the Lower Horizon. Recommendations: 1. If the irrigation well is constructed in the Upper Tulare Formation, the long-tern impacts it may, have on the remedial design should be evaluated. DDRW Tracy's Ground Water Flow and Chemical Transport Model would be readily available and could be used for such an evaluation. 2. It would be useful for DDRW Tracy to evaluate the influence the irrigation well has on the contaminant plume on an annual basis. I recommend that the long-term ground water monitoring program include,at a minimum, annual sampling for VOC's at monitoring points that are closest to the irrigation well. DDRW Tracy should also consider conducting monthly water elevation measurements for these on-site monitor wells, during the irrigation season, to determine the capture zone of the irrigation well. 3. DDRW Tracy is currently operating an interim ground water extraction and treatment system that is pumping approximately 470 gpm. DDRW Tracy should evaluate the feasibility of exporting treated ground water to the subject property instead of disposal into the on-site Storm Drain Pond.