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CvvATc A S S O C I A T E S I N C . ti.. 5�1 Alternative 1 -Natural Attenuation with Groundwater Monitoring ♦ Criterion ]: The natural attenuation alternative will not have immediate health based risks. Monitoring results have shown that off-site migration of petroleum hydrocarbon impacted groundwater is not occurring and the potential to impact sensitive receptors that may be located nearby is very low. The site is surfaced with asphalt, so the possibility for exposure to humans by volatilization, dust, or dennal contact with impacted soil and groundwater is minimal,with little or no fire or explosion hazard. ♦ Criterion 2: This alternative would reduce the existing levels and volume of impacted soil and groundwater over time by natural degradation and attenuation, but the rate of reduction is not known. The L possible migration of petroleum hydrocarbons in soil caused by surface water flushing contaminants into the groundwater is low due to the asphalt surface on site. The groundwater gradient is flat and the direction is generally consistent to southwest. ♦ Criterion 3: This alternative would not immediately remediate soil or groundwater to levels acceptable to regulatory agencies. Natural degradation of residual hydrocarbons would be expected to occur over time. _" ♦ Criterion 4: Additional costs for implementing this alternative are related to the costs associated with the continued monitoring to ensure natural attenuation is occurring and the permitting and destruction of existing wells at project completion. The cost of this alternative is estimated to be about $20,000 per year for monitoring, $15,000 to $20,000 for enhancing biodegradation with introduction of oxygen if warranted, and $30,000 for a closure plan and well destruction. ♦ Criterion 5: LW This alternative is effective in minimizing the health-based risks in the short term. Exposure to humans by contaminant releases to the air due to dust, or through ingestion or dermal exposure to impacted soil or groundwater is minimal during groundwater monitoring and well destruction activities but is otherwise nonexistent. ' ♦ Criterion 6: The long term effectiveness of this alternative is not acceptable due to impacted soil remaining beneath the site. If groundwater rises to historical levels, it could come in contact with impacted soil. This alternative would reduce the toxicity and volume of the contaminants in the soil and groundwater by natural biodegradation, but the rate of reduction is not known. The ..� possibility for health-based risks in the long term is low to moderate because of the potential of impacting groundwater with higher concentrations of petroleum hydrocarbons. S:1Environmenta11625771SCMCAPAdddoc 7