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7.0 PROPOSED CORRECTIVE ACTION PLAN Based on the nature of the soil impacts beneath the site, and based on the results of the VET as described herein, Geocon recommends soil vapor extraction as the most feasible and cost effective interim corrective action option. Test data indicates that vapor extraction has the ability to remove hydrocarbon mass from beneath the site without further excavation and can utilize the existing extraction wells. The soil remediation goal for the subject site is to reduce the concentrations of TPHg in soil to below I 100 mg/kg. This soil remediation goal is supported by the following discussion. Geocon's previous experience in the operation of VES's indicates that the degree of soil contaminant reduction varies in accordance with the length of extraction and the mass removal decay rate. Typically, if a soil VES is operated until the mass removal rate has dropped to approximately less than 5 ppd, resultant post- remediation soil concentrations are usually less than 100 mg/kg. The operation of vapor extraction systems at mass removal rates significantly less than 5 ppd is not considered cost-effective because the residual contaminant mass consists of the heavier end range of the petroleum hydrocarbon product,which are difficult to extract due to decreased volatility. 1 i This section outlines the proposed Corrective Action Plan (CAP) by providing preliminary design criteria and scheduling information relative to the design, permitting and operation of the VES. In addition, this section presents a site specific VES layout which includes proposed subsurface trench routes and the aboveground remediation equipment compound location. 7_1 Preliminary Design Criteria I A VET was performed to simulate future vapor extraction and obtain preliminary design criteria for a VES. Data obtained from the VET indicated that a vapor extraction per well flowrate of approximately 50 scfm can be achieved for wells VW-1 and VW-2. The VET data also indicated that a TPHg vapor concentration of approximately 33,000 mg/m3, as limited by the processing capacity of the therm-ox, with a corresponding mass extraction rate of approximately 300 ppd can be expected at the time of VES start-up. The results of the VET indicated that vapor extraction technology is a feasible remedial alternative for the site. The preliminary design criteria for the VES is summarized below and a VES Capture Zone and Layout Map for the subject site is depicted on Figure 7. • Number of vapor extraction wells=2 I • Recommended extraction flowrate per extraction well=50 scfm • VES Maximum Process Capacity Specification= 150 scfm • Estimated TPHg vapor process concentration at start-up=33,000 mg/m3 • VES TPHg mass extraction rate at 100 scfin =300 ppd i i Project No.58100-06-11C -9- December 17,1996