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Section 4 Groundwater Remediation Options This section evaluates options for groundwater remediation.at the Disco site. As discussed in t Section 2 of this report, the NAPL gasoline constituents in the smear zone beneath the former service station, which has been feeding the groundwater gasoline plume continually through percolation and dispersion, are currently being removed by AS/SVE. The remedial options discussed in this section target gasoline constituents that have dissolved into the groundwater and migrated away from the source. Characteristics (extent and chemical-concentrations) of. the dissolved-phase plume are described in Section 3. Following are the objectives of groundwater remediation at the Disco site: + Control the migration of gasoline constituents. • Demonstrate progressive constituent mass removal from the confined plume. v • Minimize direct or indirect exposure of humans and the environment to gasoline constituents. Because of the extreme depth to groundwater (70 feet) at the Disco site, cost effective remediation options are limited to those that do not involve excavation or extensive drilling. The two remediation options discussed in this section address the objectives listed above and cost effectiveness with respect to the depth to groundwater. , This section includes: a brief summary of hydrogeological conditions at the site that set the stage for groundwater remediation discussions; an assessment of risks associated with the dissolved plume of gasoline constituents; a description of two groundwater treatment options; and an evaluation of those options. Hydrogeological Conditions Summary Groundwater movement and contaminant transport on site are naturally limited by a flat groundwater surface gradient, very slow movement in the horizontal direction (5 to 10 feet/year), and even slower movement in the vertical direction(less than 1.0 footlyear). Water supply wells in the area are all screened well below the A and B Zones because of the low yield of these formations, as demonstrated in the pump tests and subsequent groundwater modeling described in Section 3. Assessment of Risk- Source and Extent of Contamination The source of contamination, NAPL gasoline in the smear zone, is being addressed by the AS/SVE,,system presently in operation. The extent of NAPL contamination is likely to be in a narrow conically shaped smear zone extending from 30 feet to'90 feet bgs. The extent of aqueous- or dissolved-phase contamination, as presented in Section 3, is a triangularly shaped SF010030983.DOC 4-1