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EPA COMMENTS ON <br /> DRAFT WORK PLAN FOR WELL EVALUATION <br /> AND ABANDONMENT AT <br /> DEFENSE DISTRIBUTION REGION WEST <br /> TRACY, CALIFORNIA <br /> (OCTOBER 1991) <br /> 1. DLA identifies and refers to three water-bearing "horizons" , <br /> the Upper (approximately 20 to 40 feet below ground surface <br /> [bgs] ) , Middle (approximately 65 to 90 feet bgs) , and Lower- <br /> Horizons (approximately 120 to 140 feet bgs) . These more <br /> permeable, water-bearing horizons are typically where the <br /> groundwater monitoring wells for the groundwater <br /> investigation of DDRW-Tracy have been screened. From the <br /> boring and well construction logs presented with the Work <br /> Plan, all three of these horizons are apparently located <br /> within or above the Upper Tulare Formation. This <br /> terminology can be confusing. It would be helpful if the <br /> DLA provided a figure illustrating these "horizons. " <br /> • Add a figure (e.g. , a cross section) to the document <br /> which shows the locations of each well to be abandoned <br /> relative to the three "horizons" referenced in the Work <br /> Plan, the Upper Tulare, the Corcoran Clay, and the <br /> Lower Tulare. Differentiate between these aquifers. <br /> Include well depths and screening intervals. <br /> 2 . For ease in locating the wells in relation to the <br /> contaminated areas at DDRW-Tracy, it would be helpful to <br /> have the surveyed locations of Wells No. 1, 2 , 3 , 4 and 7 <br /> represented on Figure 1-3 , Plan View of Site with Monitoring <br /> Well Locations. <br /> • Revise Figure 1-3 to include this information. <br /> Page 2-5 , Section 2 . 3 . 5 <br /> 3 . Reference is made to concentration contour maps in the Draft <br /> RI/FS Report (WCC, 1991a) , that reportedly show tetrachloro- <br /> ethene (PCE) and trichloroethene (TCE) concentrations in <br /> groundwater that are thought to be nondetectable [ND] in the <br /> Middle and Lower Horizons in the vicinity of Well No. LM-60 . <br /> Concentrations in the Upper Horizon are reportedly <br /> anticipated to be within the range of ND to 5µg/L. This <br /> prediction of a range of ND to 5 Ag/L in the Upper Horizon <br /> in the vicinity of LM-60 does not appear to be supported by <br /> the data (see Table 1 in these comments) . <br /> (1) According to the Draft Quarterly Groundwater Monitoring <br /> 1 of 12 <br />