SITE INFORMATION AND CORRESPONDENCE

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Mr. Welch 3 6 November 1996 Evaluation of Biological Activity_ Based on apparent exhaustion of the existing oxygen-releasing compound(ORC) socks, and as discussed with SECOR staff in July, staff concurs with SECOR's decision to remove the socks from wells MW-2, MW-3, and MW-4. However, as noted in that conversation and in the 14 August 1996 letter from SECOR, the increasing hydrocarbon concentrations combined with competition for oxygen demand from the peaty, organic-rich soils may have overshadowed the effects of oxygen released by the socks. Evaluation of bioparameters by SECOR's subcontractor, CEM Lab,found that the site has sufficient levels of bacteria for hydrocarbon degradation to occur even at the highest concentrations found at the site.The lab report stated that oxygen addition would be the optimum approach to biological enhancement under the conditions of this site. Staff therefore suggests that future re-application of the socks or other in-well oxygen-enhancing methods be considered. Remedial Approaches and Risk-Based Analysis: The Second Quarter Report stated that evaluation of site parameters for the possible performance of Risk-Based Corrective Action (RBCA) would take place during the third quarter of 1996. This was not done. SECOR's 14 August 1996 letter states, correctly,that to date the Regional Board has not produced a guidance document for RBCA approaches to site remediation. However,the Designated Level Methodology (DLM)has been in use for a long time and is an approach that would be appropriate for use with petroleum hydrocarbon compounds. Additionally,the American Society for Testing and Materials (ASTM) has developed a standard for RBCA that would be usable if adapted to consider all beneficial uses of ground and surface waters. In effect,this approach would treat the groundwater as a receptor of concern. There is nothing inherent in the ASTM RBCA methodology--or any other RBCA model--that requires consideration only of human receptors or human carcinogens. The approach may be used for any receptor (such as aquatic species) and for any beneficial use or water quality goal of interest. Our letter of 11 July 1996 requested a feasibility study examining remedial alternatives for corrective actions at this site. Questions about stability of the levee have raised concerns about extensive application of invasive engineering techniques at the Tower Park site. While staff concurs that there may be some limitations on the type of approaches that can be attempted, we do not believe that there are no reasonable remedial actions that can be taken. Questions of potential threat to the levee should be addressed to specifically-proposed methods or approaches, not to a general rejection of any additional engineered site activities. The Third Quarter Report states, again, that evaluation of site data for possible performance of Risk-based Corrective Action will be conducted. We request that this be done during the fourth quarter 1996 and that a feasibility report with analysis of proposed alternatives be included. CONCLUSIONS: The past two quarterly monitoring reports contain results that may indicate an additional release of diesel at this site, and/or that the full extent of the dissolved plume has not been defined. Alternatively, residual concentrations potentially subject to redissolving when groundwater level rises may remain in the soil profile. Staff also is concerned that gasoline may be present as well as diesel, that monitoring for MTBE has not been performed as requested, and a requested remedial