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J � � <br /> Review of the Draft Dieldrin Natural Attenuation <br /> Investigation Results, DDJC Tracy Site, <br /> Tracy, California <br /> December 2006 <br /> GENERAL COMMENT <br /> 1. The vertical extent of the dieldrin plume in the northwest comer of the DDJC-Tracy site <br /> is not clearly explained. The text on page 2-1 states soil and groundwater sampling were <br /> collected at depths from 17 to 65 feet below ground surface (bgs). However, on page 4-1 <br /> the text states the injection wells, the cause of the dieldrin in the northwest corner, were <br /> "constructed with multiple screened intervals from 20 to 100 feet bgs." In addition, the <br /> geological cross section provided as Figure 3-1 stops at 65 feet bgs, and the deepest <br /> groundwater samples collected at cone penetrometer test (CPT) locations CPT235/254, <br /> CPT244/255, CPT257, and CPT259 imply that dieldrin could be deeper than 65 feet bgs. <br /> Please state in the text how the vertical extent of dieldrin is known for the northwest <br /> corner groundwater plume and if the vertical extent is not known please state in the text <br /> what will be done to resolve this data gap. <br /> SPECIFIC COMMENT <br /> 1. Figure 2-1,Dieldrin in Groundwater, Page 2-3: It appears that all of the groundwater <br /> concentrations of dieldrin that are equal to or greater than the aquifer cleanup level of <br /> 0.05 micrograms per liter(ug/L) were not highlighted in bold type as stated in the legend <br /> of Figure 2-1. In addition, for a 2005 CPT hydropunch sample location, CPT242, the <br /> abbreviation"NR" is used for the sample results at 32 feet bgs, however, the meaning of <br /> this abbreviation is not provided in the legend. For Figure 2-1, please show all of the <br /> dieldrin groundwater concentrations that are equal to or greater than 0.05 ug/L in bold <br /> type. In addition, please add to the legend the meaning of the abbreviation "NR". <br /> MINOR COMMENT <br /> 1. Appendix A,Dieldrin Confirmation Sampling Northwest Corner Area, DDJC- <br /> Tracy, Pages A-1 through A-3: The text and tables in Appendix A appear to use two <br /> names for a single monitoring well. It appears the names LM106A and LM106AU are <br /> used interchangeably in the text and tables for a single monitoring well in the northwest <br /> comer area. To prevent confusion,please revise the text and tables in Appendix A to use <br /> a single name for the monitoring well or state in the text the reason for using both names <br /> to represent the same monitoring well. <br />