SITE INFORMATION AND CORRESPONDENCE

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W.T. Nickerson • • July 14, 1997 Page 4 Based on the above, if 1,2-DCP were the only chemical in groundwater beneath the site,then the cleanup level would initially be set at background(non-detect) and if necessary, would rise to 0.5 µg/l (the human health protective level). However, 1,2-DCP is not the only chemical in the groundwater. A human health risk assessment in necessary to determine the concentration of the chemicals which will not pose a significant risk to human health." The Board's position is noted. "5.0 Alternatives for Removal of 1.2-DCP from Groundwater The report contains a quote"...an aquifer receiving a continuous supply of contaminants is extremely difficult to renovate;decontamination of groundwaters is often uneconomical and in some cases technically infeasible." This quote is not germane to the discussion because State and Regional Board policies require a discharger to remove the source of contamination." The initial source of the contaminants has been removed, in that no usage of these compounds, and no usage of the unlined former evaporation ponds at the facility is now permitted. Since contaminated soils are present in the vadose zone beneath the two former pond areas, and since ground water levels are known to rise seasonally into contact with these contaminated soils, a supply of contaminants is still effectively present on site. To date the Board has not required the removal of these contaminated soils, but has instead approved the construction of a liner and cap for the pond areas. For these reasons, we felt that the comment warranted inclusion in the report. "5.1 Pumving and Treating with Activated Carbon The document contains a quote about DNAPLs, and the failure of pump and treat systems when DNAPLs are present. 1,2-DCP will behave as a Dense Non-Aqueous Phase Liquid when it is present at such a high concentration that it is not dissolved throughout the water column, but instead is found as a liquid which has sunk to the bottom of the water-bearing zone. No evidence has been presented to suggest that the 1,2-DCP found at this site is behaving in such a manner." No direct investigation has been undertaken to determine whether 1,2-DCP is not, or was not, present as a DNAPL at the site. If liquid 1,2-DCP was at least present at one time, the difficulties posed to remediation are significant. Thus, we felt the inclusion of this comment was warranted. "5.2 Pumping and Air Stripping Depending on the Air Quality Management District, and the pounds of gas to be vented, it is still possible to vent air strippers directly to the atmosphere." This regulatory position is noted. "5.3 Hydraulic Containment Again the report contains a quote about DNAPLs. However, 1,2-DCP is not behaving as a DNAPL, so this quote is unnecessary." Please refer to our response to the comment on section 5.1 of the report, above.