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i Biosparging up gradient of hydrocarbon sources allows oxygenated water to flow across the source area and encourage source area hydrocarbon degradation Biosparging down gradient of hydrocarbon source areas remediates hydrocarbons as they migrate from the source area, thereby containing the hydrocarbon source area Since bioventing and biosparging may present an explosion risk if oxygen is mixed with high vapor concentrations, source area bioventing is typically implemented if hydrocarbon vapor concentrations are well below the lower explosion limit (LEL) Infected air can also be vented back to the remediation pad to minimize air infection into the vadose zone In some cases, SVE is performed to reduce vapor-phase hydrocarbon concentrations before implementing a i bioventing/biosparging approach Cost Effectiveness Bioventmg and biosparging is a cost effective method of remediating site soil and ground water This alternative has not been shown to be effective against MTBE Operation and maintenance costs for this alternative are typically significantly lower with a biosparging system than SVE or GWE systems Recommendation Cambria recommends considering bioventing/biosparging as a potential contingency plan if other remedial techniques are not effective Before a long-term bioventing approach is implemented, short-term testing should be performed to evaluate the potential for hydrocarbon migration and the development of explosive conditions • CORRECTIVE ACTION PLAN Cambria's proposed interim CAP involves the following Performing periodic GWE from the former tank complex for period of approximately six months, Installing ORCs in wells around the perimeter of the site, Analyzing ground water samples for intrinsic bioremediation parameters to further assess the ground water potential for natural attenuation and for RBCA parameters, Assessing the effectiveness of the corrective action, Implementation of a contingency plan The plan calls for GWE in the source area and installation of ORCs at the perimeter of the site This combination should maintain hydraulic control over the source area, remove petroleum hydrocarbon and MTBE mass from the subsurface, and move oxygen-rich ground water from the perimeter toward the source area The details of our proposed CAP are presented below 240-0783 25