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0 0 A remedial investigation/feasibility study ways" described in the above paragraph. Using a (RDFS) effort began in 1980 and since 1991 has hypothetical future scenario in which a well may continued in accordance with CERCLA and SARA be installed into any one of the aquifers,the risk to guidance. As a result of early investigations, an any resident again will be greater than the mini- interim groundwater extraction and treatment sys- mum protective levels recommended by the U.S. tem or Interim Remedial Measure (IRM), was EPA in the National Oil and Hazardous Sub- constructed in 1991. The IRM is discussed later in stances Pollution Contingency Plan (NCP). this Proposed Plan. The purpose of the IRM is to capture and treat the volatile organic "hot spot" Plants and animals in and around DDRW- contamination in the aquifer and prevent it from Tracy include terrestrial vegetation; soil inverte- migrating off base. The IRM currently operates brates;small mammals;birds;reptiles;and aquatic under Waste Discharge Requirement No.90-275, plants, invertebrates, and vertebrates associated issued by the RWQCB. with the stormwater runoff pond and flood-irri- gated fields downgradient of the site. OU#1 is not SUMMARY OF SITE RISKS believed to presently pose an unacceptable envi- ronmental risk to plants and animals. Environmental studies at DDRW-Tracy have indicated that groundwater contaminated princi- CLEANUP ALTERNATIVES WILL pally by TCE and PCE flows from the northeast PROTECT PUBLIC HEALTH section of the site to off-base private land. Other potential chemicals of concern detected inDDRW- The presently preferred cleanup alternative Tracy groundwater,including boron,nitrate,diel- for groundwater at DDRW-Tracy will reduce any drin,carbon tetrachloride,chloroform,and metals health risks of exposure (based on the scenarios may be naturally occurring or have known off-base presented above)posed by the site contaminants to sources. These and other potential chemicals of "acceptable" levels after an operating period esti- concern will be addressed in the site-wide RI/FS. mated to be close to 30 years. A clean-up period of at least 28 years is estimated based on engineering Arisk assessmentwas performed to define the calculations of the amount of groundwater neces- riskposed to public health and the environmentdue sary to be extracted and treated and the limits of the to the presence of TCE,PCE,and the other chemi- treatment system. A 30-year period is used by EPA cals of concern present in the groundwater. The as the maximum length of time for estimating the assessment focused on the estimated risk an off- costs of cleanups. EPA considers an acceptable base resident would face from being exposed to risk range to be between one in 10,000 and one in contaminated groundwater. Several potential ways 1,000;000 excess cancers. Once the groundwater of being exposed (called "exposure pathways") cleanup is complete,a person's risk of developing that were evaluated included: the resident drinks cancer as a result of exposure to groundwater from a domestic well placed within the off-base would fall within or below that range. With the contamination plume; the resident inhales vapor exception of the no-action and institutional con- while showering with such water; the resident trols alternatives, all alternatives are designed to absorbs such water through the skin while shower- reduce risks of the contaminant plume to accept- ing orwashing;and the residenteats vegetables and able levels. Actual or threatened releases of haz- crops irrigated with the water. ardous substances from the groundwater in OU#1, if not addressed by the preferred alternative or one The groundwater risk assessment concluded of the other activemeasuresconsidered,may present that the cancer risks and other health threats asso- a current or potential future threat to public health, ciated with groundwater off-base exceed recom- welfare, or the environment. mended protective levels for the "exposure path- 4