SITE INFORMATION AND CORRESPONDENCE

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W.T.Nickerson July 14, 1997 Page 6 We agree. While a formal conceptual site model has not yet been developed for this facility, much is known about the characteristics of these aquifers from regional data and from the behavior of the monitoring wells during purging. We also have more than two years of ground water level measurements which reflect how the aquifers respond to seasonal inflow and recharge. Aquifer testing is planned as apart of the feasibility study process. "6.4 Air Sparging and Vapor Extraction Air sparging and vapor extraction are being used successfully to remediate a groundwater plume in Yolo County that contains, among other chemicals, 1,2-DCP. The Yolo County site contains 1,2-DCP in similar concentrations to this site. The report's statement that"a specific drawback to air sparging and vapor extraction is the low concentrations of contaminants present" is misleading and apparently inaccurate." We intend to obtain information on the project mentioned from Yolo County staff in the near future. We recognize the viability of in-situ methods such as air sparging and vapor extraction. All sites, however, are not amenable to such solutions. All in-situ remediation technologies which rely upon or enhance volatilization and/or biodegradation are subject to the limitations of the physical system in which they are operating. Such factors as soil porosity and pore communication, adsorption coefficients, interactions with authigenic mineral and organic compounds, soil pH, hydraulic characteristics, the suitability of the subsurface environment for support of adequate populations of beneficial organisms to all serve to limit how effective the in-situ method will be for a given site. If regulatory levels of"not detected"are going to be enforced at this site, a great deal of effort could be expended installing such systems, which might be prevented from achieving the mandated levels by any number of dii ferentprevailing conditions. In particular, where concentrations are at a fewparts per billion, persistent and seemingly stubborn residue often remain at contaminated sites despite the best designs and most diligent system maintenance. The statement was included to provide valid empirical information from other sites, as at low initial concentrations it is often difficult to establish an effective microbial population, due to lack of available food. We mention these potential problems only for purposes of ensuring the efficient expenditure of remediation funds. The issues raised in this report are to be resolved in future pilot-scale or bench-scale testing, aquifer testing, further site characterization, and research. Other contaminants now recognized as needing mitigation can almost all be treated in ways similar to those discussed in this report. We appreciate the comments by the Board, and look forward to working with the Board in the development of the most appropriate course of action for this site, in accordance with policy.