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Problem Assessment Work Plan CSLJS—Multi-Campus Regional Center January 23,2003 I 1 Page-8- 6.3 EVALUATION OF ALTERNATIVES While the costs for the no action alternative are the lowest, there is no source reduction and no remediation of the contamination threatening the beneficial use of groundwater. This is the least desirable response because it does not substantially protect groundwater quality from degradation. The excavation alternative is the most costly and the most disruptive alternative, though it will require the least amount of time to complete. The .SVE with groundwater pump and treat remediation alternative is less disruptive to the site than the over-excavation, most cost effective, and will likely result in a significant source reduction with f concomitant reduction in risk of groundwater degradation. Therefore, the selected preferred alternative is l remediation with soil vapor extraction and groundwater pump and treat. ,. 6.4 PROPOSED TARGET CLEAN-UP LEVELS AND VERIFICATION PROCEDURES I The proposed clean-up levels for soil and groundwater at the site are the risk based contaminant levels for I benzene at a cancer risk level of one in one million froth.the ASTM Standard Guide for RBCA Applied at I Petroleum Release Sites (E 1739-95), Table X2.1 Example Tier 1 Risk Based Screening Level (RBSL) Look-up Table (Appendix D). These levels provide a significant level of source reduction and are based on minimizingp otential for human health risks in the vicinity of the site. The completed contaminant transport { pathways are inhalation of indoor air volatilizing from soil and groundwater in a commercial setting. The. proposed clean-up levels are: tJa Soil benzene—0.01 mg/kg Groundwater benzene—74 M/L Attainment of soil clean-up levels will be verified by collecting verification-soil samples in the area of the SVE wells following an evaluation of the SVE effectiveness. The effectiveness evaluation will document the extracted soil vapor concentrations of benzene over a period of at least six months. Performance will be ` evaluated by comparison to the evaluation level of 0.5 Wjm"which represents a risk based screening level for inhalation of outdoor air at a commercial site for a cancer risk level of one in one million: If the extracted concentrations have reached a stable level below 0.5 gg/m3 or have been below detectable levels two months in a row, the system may be pulsed(shut down for short periods of several days and restarted j in different configurations). If low concentrations of extracted vapors persist, soil samples may be collected. Attainment of groundwater clean up will be assessed if benzene levels of extracted groundwater at the extraction wells are below 74 gg/L for three successive months. The system will be shut down and the groundwater colurrm in the well allowed to recover to within 80% of the pre-pumping height. Then the system will be activated again and sampled twice, at start-up and again after thirty days. If groundwater benzene is above the 74 µg/L concentration level upon restarting the system and remains higher after 30 days, remediation will continue as before until two successive months of low concentrations are achieved i and the evaluation method described here will be repeated. If groundwater benzene is above the 74 pg/L concentration level upon restarting the system but drops below that level within thirty days, a written report will be submitted evaluating options for increasing the efficiency of the system, or shutting down. If groundwater benzene concentrations are less than 74 µg/L upon restarting the system and increase to above that Ievel after thirty days, remediation will continue as above. If groundwater benzene concentrations are less than 74 gg/L upon restarting the system and after thirty days, the clean-up levels will have been -reached and pumping may stop. • ow i ilk CONDOR