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CERCLA to provide a basis from which to de- Alternative 3 velop and evaluate the other remedial altema- tives. Under the no-action alternative, all re- Alternative 3 includes all the elements of Alternatives 1 and 2 and adds ground water and medial activities in the GSA would cease. Sam- pling and analysis of ground water from moni- soil vapor extraction to remove TCE and other VOCs from ground water, soil and rock. Alter- for wells in the area would continue to monitor TCE and other VOCs in the subsurface. Ad- native 3 is divided into two scenarios: Alterna- ministrative controls would provide a degree tives 3a and 3b. Both are the same with respect of protection to human health by restricting ac- to the objective and method of subsurface soil/ cess to or activities in certain areas of contami- rock remediation but differ in their ultimate ob- nation. Modeling indicates that ground water jectives for ground water remediation. Both Alternatives 3a and 3b include: VOC concentrations would be reduced to drinking water standards through natural at- All elements of Alternatives 1 and 2, plus tenuation and degradation after 75 years under the Alternative 1 scenario. Groundwater moni- Soil vapor extraction and treatment by toring will be conducted for the 75-year period carbon adsorption in the central GSA dry well source area. plus Syears of post-"remediation"monitoring. Estimated 80-year present-worth cost = Extraction and treatment of ground wa- $4,270,000. ter by air stripping and/or carbon ad- sorption in the central GSA dry well Alternative 2 area and the eastern GSA debris burial trenches area. Alternative 2 focuses on preventing human Under Alternatives 3a and 3b, DOE/LLNL exposure to TCE and other contaminants will continue to operate the existing soil vapor through ingestion of ground water from exist- extraction system at the central GSA dry well ing water-supply wells. area to reduce VOC concentrations in soil va- Alternative 2 includes: por to levels protective of ground water (i.e., • Monitoring. MCLs) and to mitigate VOC inhalation risk in- • Contingency point-of-use treatment us- side Building 875. DOE/LLNL expect that soil ing aqueous-phase carbon adsorption vapor extraction will reduce soil vapor concen- for three off-site water-supply wells. trations in the Building 875 area to the • Restrictions on access and procedures remediation goal of 360 ppbv�v within 10 years. for construction in the area. Modeling indicates that this soil vapor concen- As with Alternative 1, reduction of VOC tration is protective of ground water by prevent- concentrations in ground water through natu- ing contamination of ground water at concen- ral attenuation and degradation would take ap- trations above drinking water standards. Alter- proximately 75 years under the Alternative 2 natives 3a and 3b are discussed further below. scenario. Ground water monitoring will be con- ducted for the 75 year period plus 5 years of Alternative 3a post-"remediation" monitoring. Estimated 80-year present-worth cost —_ Under Alternative 3a,DOE/LLNL will ex- $4,570,000. pand the existing ground water extraction and treatment system in the central GSA dry well 10