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i : Short-Term Effectiveness Remediation progress will be better documented in the short term with Option A. Long-Term Effectiveness Option A is inherently more reliable because no mechanical systems are involved. Option B actively controls or reverses migration of gasoline constituents by extracting groundwater while Option A allows migration to be passively controlled by natural confinement and biodegradation processes. Although capture of the contaminated zone is easily achieved by pumping wells in Option B, the low permeability of the materials in the shallow aquifer will severely limit the rate of groundwater cleanup. One of the processes that cleans up an aquifer is flushing the I contaminated zone with clean water. Clean groundwater moves inward from the edge of the contaminated area, thereby flushing the contaminants from the aquifer. However,,this process of aquifer flushing is slow, typically taking many pore volumes of flushing to achieve remedial goals. The process is analogous to cleaning a sponge that is saturated with soap. It takes many squeezes of the sponge before soap suds are no longer present in the discharge. Based on the forecasts from the groundwater model, it would take about 6 years to flush 1 pore volume from the contaminated zone by groundwater pumping. To achieve complete remediation may take 10 pore volumes (60 years) or more. Remediation by flushing may I take even longer if contaminants that occur within the target area do not move freely with the. groundwater, either because-of sorption, inefficient flushing, or the presence of free-phase product, remediation by flushing may take even longer. Implementation Both options are technically feasible. Option B is more challenging administratively because of the permitting and approvals required, The public may perceive Option B to be more proactive than Option A,but Option B will have offsetting noise, traffic, and visual impacts. Cost Effectiveness Option B's estimated present worth is more than double the cost of Option A as presented in Tables 4-2 and 4-3. It is assumed that the sampling items presented in the cost estimate for Option A (Table 4-2) would replace the cost of conventional groundwater monitoring for intrinsic bioremediation demonstration wells. The annual cost of long-term groundwater monitoring of 10 wells is included in the Option B estimate (Table.4-3). f Evaluation Summary Both options are expected to provide human health and environmental protection and to remove contamination. Three key issues differentiate these options: long-term effectiveness; administrative feasibility; and cost effectiveness. There are significant Iimitations to I SF010030963.DOC 4-9