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JRfo�„Fs UNITED STATES ENVIRONMENTAL PROTECTION AGENCY <br /> Region 9 <br /> Y 75 Hawthorne Street <br /> 6 �PR San Francisco CA 94105-3901 <br /> n <br /> c� <br /> May 25, 2000 y <br /> Mr. Dale Clemens 3= <br /> Environmental Protection Office <br /> DD-Tracy N -- <br /> P.O. Box 960001 `O - <br /> Stockton, CA 95296-0250 <br /> RE: Tracy Defense Depot "Well Monitoring Program, Quarterly Monitoring <br /> Report, First Quarter 2000 Sampling Round” dated May 2000 <br /> Dear Mr Clemens, <br /> Thank you for the opportunity to review the Tracy Defense Depot "Well <br /> Monitoring Program, Quarterly Monitoring Report, First Quarter 2000 Sampling <br /> Round" dated May 2000. Our comments are attached. <br /> The fact that no chemical concentration figures are presented in the <br /> Report make it difficult to evaluate plume migration and extraction system <br /> effectiveness. Therefore, it is recommended that chemical concentration maps <br /> for key contaminants in groundwater (i.e. , TCE and PCE) be included in each <br /> quarterly report. It should be noted that TCE concentrations increased <br /> significantly between the current and the previous sampling event at wells <br /> LM032AU (from 41.4 ug/1 to 80 ug/1) screened in the above upper horizon, <br /> LM017AA (from 1.24 ug/l to 14 ug/1) screened in the upper horizon, and LM056C <br /> (from 8.57 ug/l to 22 ug/1) screened in the lower horizon. Each of these well <br /> locations is associated with a small TCE plume centered around the respective <br /> monitoring location (i.e. , is not part of an existing larger plume) . <br /> Trichloroethene (TCE) concentrations above the Aquifer Cleanup Level <br /> (ACL) of 5.0 µg/L were detected again in the Upper Horizon plume which <br /> currently extends northeast of the Annex onto the Robertson property. This <br /> TCE plume is currently over 2,200 feet (ft. ) downgradient of any extraction , <br /> wells established for groundwater containment and removal in the Upper <br /> Horizon. No evaluation of the apparent ineffectiveness of the Upper Horizon <br /> extraction wells in containing this plume is provided, nor is any <br /> consideration given for additional monitoring wells downgradient of the <br /> Robertson property. Although, the quarterly monitoring report may not be the <br /> appropriate vehicle to address this concern, the effectiveness of the Upper <br /> Horizon extractions wells should be evaluated. <br /> In the third quarter (3Q) 1999 sampling event, Tetrachloroethene (PCE) <br /> was detected above the 5.0 {cg/L ACL in LM059A in the Upper Horizon. PCE was <br />