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Most of the solvent drums brought to the depot in the past were in transit for distribution to <br /> Department of Defense facilities elsewhere. A smaller quantity was also formerly used on <br /> the depot for degreasing and cleaning machinery and parts in the course of maintenance and <br /> repair of depot equipment. Presently, solvents and other chemicals are stored safely in <br /> conformance with EPA regulations. <br /> EXTENT OF CONTAMINATION <br /> TCE and PCE have been found persistently as contaminants in the soil gas, soil, and <br /> groundwater at the depot. Other volatile organic, inorganic, and pesticide compounds have <br /> D 11call <br /> been found The highest levels of contamination in soil and <br /> groundwater are found in maintenance areas and in the former open drum storage areas along <br /> the northeast boundary of the depot. <br /> (0-r aml oawd <br /> TheAgroundwaterOtr at DDRW-Tracy is contained in an aquifer from approximately <br /> 15 feet to 210 feet below the ground surface. ("Aquifer" is a geologic term meaning bedrock <br /> or sed' ents below the ground surface that are saturated with groundwater.) The aquifer ts <br /> dividerinto three parts, or "horizons": the upper horizon, the middle horizon, and the lower <br /> horizon. There is an clay barrier, or "aquitard," below this aquifer that separates <br /> the shallower groundwater of concern from another, lower, water supply aquifer. <br /> The groundwater at DDRW-Tracy is contaminated primarily with the solvents TCE and PCE <br /> and locally with other chlorinated organics, inorganics, and pesticides. TCE and PCE are <br /> organic compounds that vaporize readily at room temperature. In general, such compounds <br /> are called volatile organic compounds (VOCs). Observed concentrations of TCE and PCE <br /> in groundwater exceed state and federal health-based protective levels (Maximum <br /> Contaminant Levels or MCLS) on site and in a plume extending from the depot up to 2500 <br /> feet to the northeast as indicated in Figure 2. (A Maximum Contaminant Level specifies the <br /> concentration of a particular chemical that may not be exceeded in a public water supply.) <br /> b 1r1nm <br /> A remedial investigation/feasibility study (RI/FS) effort began in 198,6and is continuing�a� � �`� <br /> in accordance with CERCLA and SARA guidance. As a result of early <br /> investigations, an interim groundwater extraction and treatment system for on-site <br /> groundwater, or Interim Remedial Measure (IRM), was constructed in 1991. The IRM is <br /> Q190\7063.1(90390C)\7 7 M1007922139 I <br />