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F <br /> T ya 92. <br /> � F <br /> � �2Y <br /> x <br /> A S S O C I A T E S I N C <br /> • Criterion 2 <br />' This alternative would reduce the existing levels and volume of impacted soil and <br /> groundwater over time by natural degradation and attenuation, but the rate of reduction is <br /> not known The possible migration of petroleum hydrocarbons in soil caused by surface <br /> water flushing contaminants into the groundwater is minimal due to the asphalt and <br /> concrete surface on site The groundwater gradient direction has generally been towards the <br />' northeast with a magnitude of 0 004 ft/ft The plume has already migrated off-site to the <br /> east <br /> ♦ Criterion 3 <br /> This alternative would not immediately remediate soil or groundwater to levels acceptable <br /> to regulatory agencies Natural degradation of residual hydrocarbons would occur over <br />' time but generally occurs at slower rates in lower permeability soils such as the silts and <br /> clays found beneath the site <br />' ♦ Criterion 4 <br /> Additional costs for implementing this alternative are related to the costs associated with the <br /> continued monitoring to ensure natural attenuation is occurring and the permitting and <br /> destruction of existing wells at project completion The cost of this alternative is estimated <br /> to be about $11,000 per year for monitoring, $7,000 to $15,000 for enhancing <br /> biodegradation with introduction of oxygen if warranted, and $15,000 for a closure plan and <br />' well destruction <br />' ♦ Criterion 5 <br /> This alternative is effective in minimizing the health-based risks in the short term <br /> Exposure to humans by contamination releases to the air due to dust, or through ingestion or <br /> dermal exposure to impacted soil or groundwater is minimal during groundwater <br /> monitoring and well destruction activities but is otherwise nonexistent <br /> ♦ Criterion d <br /> The long-term effectiveness of this alternative may be unacceptable This alternative would <br /> reduce the toxicity and volume of the contaminants in the soil by natural biodegradation, <br /> but the rate of reduction is not known but is likely poor The possibility for health-based <br /> risks in the long term is low to moderate as long as the site remains surfaced and exposure <br /> of humans to impacted soil does not occur during any type of site excavation or structure <br /> demolition activities Impacted groundwater has already migrated off-site <br /> ♦ Criterion 7 <br /> Of all the alternatives the passive remediation approach is the easiest to implement <br /> Groundwater monitoring activities would continue to ensure natural attenuation is <br />' occurring, but monitoring may continue for an extended period of time When <br /> W 1625961reportslpilotcap doe 10 <br />