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Mr. Werner _ 2 _ � 16 September 1999 Sicvol Remedial Alternatives Evaluation Report ies which 1. I have recently attended presentations on innovative ermal desorptiont in cleaning up VOCsin soil and effectiveness of in-situ chemical oxidation and ate these alternatives and compare them with technologies groundwater. BP may wish to evalu evaluated in the report. 2. The report states that the contaminant concentrations which are protective of thaee beneficial Cleanup will be groundwater have not been determined. Our 13 October 1998 letterp achieved when it is demonstrated that background groundwater quality zero)S' Background or zero been reached (since he CDCs are not naturally occurring, their background concentration concentrations typically mean less than the detection limit for that constituent. Due to economic or technologic reasons, sometimes a background concentration cannot be achieved. e W ch an nn1icaO that is _ technically and economically feasible. For example, 1,1-DCE's detection limit and maximum contaminant level for drinking are 0.5 and 6 µg/l, respectively. Therefore, the goal is to clean up 1,1-DCE to less than 0.5 µg/l, but if that is not possible, it must be cleaned up to at least 6 µg/l. In an instance where the beneficial use protective level is less than the detection limit, the former becomes the cleanup goal and any trace detection means exceedance of that goal. Table 1 shows the beneficial use protective levels (i.e., WQOs) for 1,1-DCE and the other VOCs being detected at the site. Table 1— Water Quality Objectives for Constituents of Concern Reference Constituents Detection Limit WQOI Maximum Contaminant Level 1,1-dichloroethene 0.5 a 6 µ 1,1-dichloroethene 0.5 µ 1 5 Agll Maximum Contaminant Level 1,1,1 -trichloroethane 0.5 µa 1 200 a 1 Maximum Contaminant Level CAL EPA Cancer Potency Factor 1,1,2 -trichloroethane 0.5 µg/1 0.49 µg/1 as a Drinking Water Level Chloroform 0.5 U.S. EPA Integrated Risk µg/1 6 µ Information System CAL EPA Cancer Potency Factor Tetrachloroethylene 0.5 µg/1 0.69 µ g/1 as a Drinking Water Level California Public Health Goal for Trichloroethylene 0.5 µg/1 0.8 µg/1 DrinkingWater 3. The report states that Alternative A, which consists of continued operation of the existing system, is the most cost-effective option, allows migration of groundwater containing up to 50 µg/1 of VOCs, and would eventually achieve maximum contaminant levels for drinking; Alternative B, which consists of adding to Alternative A three extraction wells in the downgradient direction, would shorten cleanup time and abate offsite migration of contaminants; and Alternative C, air sparging with soil vapor extraction, is approximately 70% more expensive than Alternative B. The report recommends Alternative A as the chosen alternative, but also states that if additional remedial actions are warranted in the downgradient sections of the plume, addition of extraction wells is