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7UJOUD-JJ + l.Vi1-JJ <br /> 4.3.5.3.3 The samples collected in the April/May 1988 sampling round <br /> were analyzed for EPA Method 601 (purgeable halogenated volatile organics) <br /> and EPA Method 602 (purgeable aromatic volatile organics) by ENSECO <br /> California Analytical Laboratories. Groundwater collected from LM 50, LM <br /> 53, LM 54, LM 55, LM 58, and LM 61 was also analyzed by EPA Method 625 <br /> (base/neutral and acid extractable compounds) . Groundwater samples were <br /> collected and analyzed by EPA-Modified GC/FID Method (TPHs) from each of <br /> the following 16 monitoring wells: LM 1, LM 3, LM 4, LM 27A, LM 28, LM 29, <br /> LM 30, LM 32, LM 40, LM 41, LM 49, LM 50, LM 51, LM 52, LM 53, LM 54, and <br /> LM 62. Details of the groundwater sampling and analytical results were <br /> presented in Letter Report Number 4 issued by WCC on June 23, 1988, and are <br /> discussed later in this section. <br /> 4.3.5.3.4 The chemical results from groundwater analyses completed <br /> during the WCC groundwater sampling rounds are found in Tables 4.0-1 <br /> through 4.0-7. Table 4.3-3 contains the chemical concentrations of TCE and <br /> PCE detected in groundwater at the Depot by EPA Method 601 for both the <br /> 1986 Radian analytical results and all subsequent WCC analytical results. <br /> Analytical results collected by Radian, as well as earlier results included <br /> in the Radian report, are included as Appendix H. <br /> 4.3.5.3.5 All the accompanying groundwater quality maps use the most <br /> recently analyzed chemical data. <br /> 4.3.5.4 Extent of Groundwater Contamination at the DDRW Tracy <br /> 4.3.5.4.1 Because of both the hydrogeology and the kinds of <br /> contaminants present in the groundwater at the Depot, it is necessary to <br /> evaluate the extent of contamination at a number of horizons beneath the <br /> facility. The hydrogeology influences this decision because of the <br /> downward groundwater gradient which is present beneath most parts of the <br /> facility. The downward gradient tends to cause contaminants to move <br /> downward rather than stay at the water table. In addition, both TCE and <br /> 4-173 <br />