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area to prevent migration of VOCs above drink- post-remediation monitoring. ing water standards (MCLs) into the regional Estimated 70-year present-worth cost for aquifer. In addition, ground water in the east- Alternative 3a = $18,050,000. ern GSA debris burial trenches area and the de- bris burial trench area west of the sewage treat- Alternative 3b ment pond will be extracted and treated to re- Alternative 3b consists of all components duce VOC concentration to drinking water stan- of Alternative 3a but continues active ground dards(MCLs)in the alluvial and regional aqui- fers. Modeling indicates that ground water TCE GSA dry well area until drinking water stan- dards in the shallow aquifer in the cen- dards (MCLs) are reached in both the regional tral GSA dry well area need to be reduced to and shallow aquifer. Modeling indicates that 100 ppb to ensure long-term protection of the regional aquifer. After the 100 ppb remedia- ground water extraction in the central GSA dry tion goal is achieved, ground water extraction well area will reduce VOC concentrations tocurrent MCLs in 55 years. Ground water moni- is discontinued and natural attenuation is relied toring will be conducted throughout the 55 upon to reduce VOC concentrations in the shal- years of remediation plus 5 years of post-re- low water-bearing zone to the remediation goal mediation monitoring. of drinking water standards. The existingEstimated 60-year present-worth cost for ground water extraction and Alternative 3b = $19,750,000. treatment system in the eastern GSA debris burial trenches area will continue to operate to Evaluation of Alternatives reduce VOC concentrations in ground water to MCLs in the shallow and regional aquifers. Nine EPA-specified criteria are used for Modeling indicates that ground water extrac- evaluating remedial alternatives under CER- tion will reduce ground water VOC concentra- CLA. Figure 8 presents an overview of these tions in the Building 875 and debris burial criteria. The FS compares the remedial alter- trenches areas to the remediation goal within natives by analyzing each alternative against 30 years and 10 years, respectively. Modeling the evaluation criteria except for state and pub- also indicates that an additional 35 years may lic acceptance criteria which will be evaluated be required to reduce VOC concentrations to following the public review period. Table I MCLs in the shallow aquifer through natural summarizes the results of that analysis. As attenuation and degradation. Ground water specified by EPA, the two most important cri- monitoring will be conducted throughout this teria are adequate protection of public health 65-year period to achieve MCLs in both the and environment, and compliance with all fed- shallow and regional aquifer plus 5 years of eral and state ARARs. 11