SITE INFORMATION AND CORRESPONDENCE_2006-2007

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Mr. Mark Adams - 2 - May 11, 2007 Second, staff disagrees with the conclusion reached for the B zone aquifer. Based on the fact that PCE is breaking down primarily to cis-1,2-Dichloroethene (cis-1,2-DCE), LFR proposes that natural anaerobic dechlorination is effectively remediating the B zone aquifer, and recommends monitoring only for this zone. On the contrary, it appears that the dechlorination reaction may be stalled at 1,2-DCE, as shown by low levels of vinyl chloride relative to the concentrations of cis-1,2-DCE, TCE, and/or PCE in MW-205B. In addition, staff disagrees with the statement on page 2 (paragraph 2) that total hazardous substances in B-zone well MW-2058 have "generally" decreased since 2001. We do not see a clear downward trend in either the tabulated data, or the time versus concentration data shown on Figure A-2. Furthermore, no other evidence has been presented to document that 1,2-DCE is degrading to harmless end products of ethene gas, carbon dioxide and water. Clear evidence must be presented that the declorination reaction is proceeding completely to harmless end products. Therefore, ERD technology should also be proposed for the B-zone aquifer. Third, LFR recommends the use of ERD technology to remediate the Lower A-zone groundwater. They will prepare a work plan for this zone to be submitted in the form of an Interim Remedial Action (IRA) Detailed Plan. The Plan will contain the design of the ERD delivery system, including number of injection and observation wells, and screen intervals and depths. Staff recommends that the entire A-zone aquifer, upper and lower, and the B-zone, be included in the Plan and system design. As stated above, staff is concerned about the lack of existing data on competing biological and chemical processes that must be collected before implementing a successful ERD. The Detailed Plan should include, but not be limited to, the following components: a. An evaluation phase to collect detailed aquifer parameters needed for ERD. b. A cost and benefit analysis of the different potential carbon donor sources that could be used, and a recommended selection for carbon donor; C. An evaluation of the presence and concentration of microbial terminal electron- accepting processes such as Fe III, sulfate, and nitrate; d. An evaluation of the presence and concentration of methane, and the potential for competitive inhibition of complete dechlorination; and e. An evaluation of the presence and concentration of Dehalococoides ethanogenesis for breakdown of cis-1,2-DCE, or similar naturally occurring microbes capable of completing the dechlorination process; Fourth, in the proposed IRA Detailed Plan, a complete discussion and cost benefit analysis should be provided describing the different substrates being considered for use in the ERD pilot test. The Regional Water Board must concur with the final selection of substrate for ERD before the pilot test can proceed. In summary, the Regional Water Board recommends that the entire A-zone aquifer, upper and lower, and the B-zone, be included in the IRA Detailed Plan and ERD system design proposal, and a detailed aquifer evaluation be conducted as specified above.