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Valley. The Sierra Nevada mountains are to the and groundwater are found in maintenance areas east and the Diablo range to the west. Areas and in the former open drum storage areas along surrounding the depot are and have been primarily the northeast boundary of the depot. agricultural: orchards,pasture lands,and scattered rural residential land. The contaminated groundwater at DDRW- Tracy is contained in an aquifer from approxi- The depot began operating in 1942 as a mately 15 feet to 210 feet below the ground surface. subdepot. In 1963,it was renamed Defense Depot ("Aquifer"is a geologic term meaning bedrock or Tracy California(DDTC),and,in 1990,was again sediments below the ground surface that are Batu- renamed, this time as the Defense Distribution rated with groundwater.) The aquifer is divided Region West (DDRW) Tracy. into three parts,or"horizons": the upper horizon, the middle horizon,and the lower horizon. There At present, the depot stores and distributes is an impermeable clay barrier, or "aquitard," food, medical supplies, construction materials, below this aquifer that separates the shallower clothing, and electrical, industrial, and general groundwater of concern from another, lower,wa- supplies common to military services located within ter supply aquifer. the western U.S. and throughout the Pacific over- seas area. Much of the depot's area is occupied by The groundwater at DDRW-Tracy is con- 26 large warehouses. There are approximately 75 taminated primarily with the solvents TCE and acres of open storage area at the depot. These areas PCE and locally with other chlorinated organics, were used in the past to store 55-gallon drums of inorganics, and pesticides. TCE and PCE are solvents (including TCE and PCE), petroleum organic compounds that vaporize readily at room products,and antifreeze,as well as compressed gas temperature. In general, such compounds are cylinders, drums, pallets, and steel products. In- called volatile organic compounds (VOCs). Ob- dustrial activities currently occupy approximately served concentrations of TCE and PCE in ground- 28 acres of the depot property. Included within water exceed state and federal health-basedprotec- these 28 acres are the vehicle, railroad,carpentry, tive levels (Maximum Contaminant Levels or and medical equipment maintenance facilities and MCLs)on base and in a plume extending from the the surrounding service areas. depot up to 2500 feet to the northeast as indicated in Figure 2(page 12). (A Maximum Contaminant Most of the solvent drums brought to the Level specifies the concentration of a particular depot in the past were in transit for distribution to chemical that may not be exceeded in a public Department of Defense facilities elsewhere. A water supply.) smaller quantity was also formerly used on the depot for degreasing and cleaning machinery and The compound 1,1-dichloroethene(DCE)also parts in the course of maintenance and repair of has been found sporadically in groundwater at depot equipment. Presently, solvents and other concentrations exceeding its MCL. Aquifercleanup chemicals are stored safely in conformance with levels have been established for TCE, PCE, and EPA regulations. DCE. Other chemicals of concern that have spo- radically been found that do not have cleanup EX'T'ENT OF CONTAMINATION levels include chloroform, carbon tetrachloride, 1,2-dichloroethane,and dieldrin. These chemicals TCE and PCE have been found persistently as either have known off-base sources (chloroform) contaminants in the soil,soil gas,and groundwater or their sources are unknown at this time and at the depot. Other volatile organic,inorganic,and plumes of these constituents have not been identi- pesticide compounds have been found sporadi- fied. These chemicals will be addressed in the cally. The highest levels of contamination in soil future site-wide comprehensive RDFS. 3