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ment determined that there is an unacceptable to employees working on site inside Building potential excess risk of cancer and noncancer 875. The baseline risk assessment also deter- adverse health effects. mined that no other Site 300 workers or nearby The calculated excess cancer risks for po- residents are affected or potentially affected by tential residential use of ground water in the contaminated soil or soil vapor in the Building vicinity of the eastern GSA debris burial 875 area. trenches or at off-site wells is 10-5 (one in one Elevated excess human cancer risk from in- hundred thousand). The potential excess can- halation of TCE vapor at Building 875 in the cer risk for use of ground water from a hypo- GSA was calculated to be 10-5 indoors. This thetical well that could potentially be installed means that a person spending 8 hours a day, 5 at the site boundary near Building 875 was cal- days a week, for 25 years inside Building 875 culated to be 7 x 10 (seven in one hundred). may have one in one hundred thousand in- These values indicate that if ground water from creased chance of contracting cancer. How- a well at this location was consumed on a regu- ever,this excess cancer risk was based on VOC lar basis, it may present an unacceptable risk. concentrations from soil samples collected at However, no water-supply wells currently ex- the dry well release site prior to the startup of ist at the site boundary location, and ground the soil vapor extraction system. It is likely, water in the area is not used for drinking water. due to ongoing soil vapor remediation activi- In addition, Removal Actions are underway to ties,that current VOC concentrations are lower remove ground water contaminants. Existing than those used to calculate the 10'excess can- off-site water-supply wells are monitored cer risk inside Building 875. monthly for VOCs, but none have ever been U.S. EPA requires that excess cancer risks detected in these wells above Federal or State above one in one million must be addressed by drinking water standards (MCLs). These wells the various risk management measures and/or are located approximately 400 ft south-south- remedial actions presented in the FS. The east from the eastern GSA debris burial trench baseline ecological assessment, conducted to source area. The predominate ground water evaluate the potential for adverse impact to flow and plume migration direction in this area plants and animals from long-term exposure to is to the north-northeast. chemicals in the GSA, determined that VOCs The baseline risk assessment also deter- do not pose ecological risk in this area. This mined that there is an unacceptable potential determination was based on estimates of po- excess risk of cancer and noncancer adverse tential hazard from exposure to contaminants health effects to workers inside Building 875 that were calculated for mammal and aquatic who may inhale TCE vapors possibly evapo- species that could potentially inhabit this area, rating from subsurface soil. This conservatively as well as biological surveys conducted to de- assumes that a person works inside Building termine which species actually inhabit or mi- 875 for 8 hours a day, 5 days a week for 25 grate through the GSA. years. This potential exposure route is limited Summary of Cleanup Alternatives The FS presents a detailed discussion and remedial alternatives evaluated for the GSA are analysis of remedial alternatives for addressing summarized below. contamination and potential risk at the GSA. It also evaluates each alternative based on find- ing Alternative 1 the best balance among these criteria. The Alternative 1, no action, is required by 9