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�� t"� .�. ' - '/ '+6��tfi�i��'`�'n-i �T'+��•dK,&e69�Y 1Ii��. � M�' � F ` - ft i fH safety or groundwater resources exists. This is further supported by the non-detectable levels of gasoline hydrocarbons in a groundwater sample (Mw1) collected beneath the area of highest soil contamination. The 38 foot depth to water provides a large zone of atteE;uation (28 feet uncontaminated soil) which greatly reduces the potential of the shallow soil contamination from adversely affecting groundwater quality. The relatively low permeability silty soil types occurring to a depth of 28 feet should limit the r rate of vertical migration of the residual soil contamination. In addition, the surface above and surrounding this area is graded to prevent ponding and covered by impermeable surfaces (asphalt, concrete and the Police station structure) which greatly limits infiltration of precipitation. This will further seduce the already.,' limited for':vertical migration of contaminants As such, the on-site well IM-11E, completed in a deeper water affected. strata (> 170 feet) should not be adversely Y. 6.0 REMEDIAL ALTERNATIVE Soil remediation alternatives include cap/monitoring, excavation and in-situ treatment approaches. A summary of each alternative is presented below. 6.1 Alternative 1 The area of documented soil contamination as, well as 95t of the project site as a whole is covered by impermeable surfaces. Because this significantly reduces the infiltration of moisture to the subsurface the potential for migration (driven by percolation of moisture through the contaminated soil horizon) is limited. Natural degradation of the residual hydrocarbon contamination should occur over time. Periodic monitoring/sampling of well MW1 would Provide assurance that groundwater quality is not baing adversely affected. 6.2 Alternative 2 Excavation of the shallow soil contamination and treatment on-site (aeration) or disposal to a certified waste disposal facility would remove the potential source of groundwater contamination. As discussed in the previous section, a circular area with a 15 foot radius to a depth Of' 10 feet should define the limits of contamination. The '- excavation of contaminated material would be limited to the north, due to the existing police station structure. On- site aeration, if possible, would be preferred to disposal because of the continued long term liability associated with f disposal of contaminated material at an off-sits facility. e - Confirmaticn soil sampling should be conducted on the walls ---and -floor--of excavation--to--assure--all -of-- the------. .-- - -- - w: