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A S S O C I A T E S I N C possible migration of petroleum hydrocarbons in soil caused by surface water flushing contaminants into the groundwater is minimal due to the asphalt and concrete surface on site ' The groundwater gradient direction has generally been towards the east with a magnitude of 0 002 ft/ft which makes it unlikely that the plume will migrate off site in the short term ♦ Cntenon 3 This alternative would not immediately remediate soil or groundwater to levels acceptable to regulatory agencies Natural degradation of residual hydrocarbons would occur over time but ' generally occurs at slower rates in lower permeabihty soils such as the silts and clays found beneath the site ♦ Cntenon 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 $11,000 per year for monitoring, $7,000 to $15,000 for enhancing biodegradation ' with introduction of oxygen if warranted, and$15,000 for a closure plan and well destruction ♦ Cntenon 5 This alternative is effective in minimizing the health-based risks in the short term Exposure to humans by contamination 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 ♦ Cntenon 6 The long-term effectiveness of this alternative may be unacceptable This alternative would reduce the toxicity and volume of the contaminants in the soil by natural biodegradation, but ' the rate of reduction is not known but is likely poor The possibility for health-based risks in the long term is low to moderate as long as the site remains surfaced and exposure of humans to impacted soil does not occur during any type of site excavation or structure demolition ' activities With a poor rate of reduction, impacted groundwater may migrate off-site in the long term ♦ Cntenon 7 Of all the alternatives the passive remediation approach is the easiest to implement Groundwater monitoring activities would continue to ensure natural attenuation is occurring, ' but monitoring may continue for an extended period of time When concentrations have reduced to levels acceptable to the regulatory agencies or an apparent decreasing trend is evident, a closure plan will be submitted Well destruction permits would be obtained from the ' appropriate agencies and the wells would be destroyed by overdnlling the well casing and sand • pack and pressure grouting the boring to the surface when concentrations have been reduced to an acceptable level W 162574 OlVilot-fa report doc 11