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EPA COMMENTS ON <br /> DDRW-TRACY' S DRAFT QUARTERLY MONITORING REPORT <br /> (AUGUST 1991 DATA) <br /> EXECUTIVE SUMMARY AND INTRODUCTION <br /> 1. In both of these sections and in a few places throughout the <br /> text, TCE and PCE are inaccurately described as <br /> trichloroethane and tetrachloroethane respectively. This <br /> should be corrected to read trichloroethene (TCE) and <br /> tetrachloroethene (PCE) . Later in the text, analytical <br /> results showing other constituents, such as 1, 1, 1- <br /> trichloroethane (1, 1, 1-TCA) , are detailed in the same text <br /> where the TCE and PCE are again misspelled. It becomes <br /> difficult to follow the results as described in the text <br /> with these inaccuracies. <br /> • Correct the text accordingly. <br /> 2 . The document states in the Executive Summary (ES. 3 . 3) that <br /> "of the 89 existing monitoring wells, 86 were sampled in the <br /> August 1991 quarterly sampling round. " Throughout the text, <br /> reference is made to 1186" total sampled wells. There are <br /> actually 92 existing monitoring wells, of which 89 were <br /> sampled in the August 1991 sampling round. <br /> • This inconsistency in total existing wells and total <br /> number of wells sampled should be corrected. <br /> EVALUATION OF GROUNDWATER DATA, AUGUST 1991 <br /> 3 . The hydrogeology of the DDRW-Tracy site is described in <br /> terms of an Upper, Middle and Lower Horizon of the Upper <br /> Tulare Formation. According to boring and well construction <br /> logs from previous documents (WCC, October 1991) , all three <br /> of these horizons are apparently located within or above the <br /> Upper Tulare Formation. A cross-section showing the <br /> relationship between the "horizons" and the Upper Tulare, <br /> Corcoran Clay, and the Lower Tulare Members would be helpful <br /> in clarifying this terminology. EPA's interpretation is <br /> taken from the Radian Corporation August 1986 report which <br /> cites "Geology, Hydrology, and Water Quality of the Tracy- <br /> Dos Palos Area, San Joaquin Valley, California" , USGS Open <br /> File Report (1971) and "Geohydrologic Study No. 38-26-0488- <br /> 85, Defense Deport Tracy, Tracy, California" , by USAEHA <br /> (1985) . Figures from these publications are included as <br /> Appendix A of this review. <br /> 4 . The horizontal hydraulic gradient across the site was <br /> estimated to be 0. 0021 feet per foot (ft/ft) for the Upper <br /> Horizon, 0. 0022 ft/ft for the Middle Horizon and 0. 0021 <br /> 1 of 7 <br />