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Jeff Whitworth is <br /> - 2 - is <br /> 4 December 2006 <br /> to be having limited success at reducing groundwater chemical concentrations in part due to <br /> the high concentration source area and low pumping rate. <br /> There is some evidence that natural attenuation of dissolved phase constituents is occurring. <br /> Elevated ferrous iron and methane concentrations in the plume interior indicate biodegradation <br /> is occurring under reducing conditions. In addition, sulfate concentrations in the plume interior <br /> are depleted relative to the perimeter well locations. The 2005 Annual Report prepared by <br /> RETEC concluded that the groundwater plume naturally attenuates within a few hundred feet <br /> of the site boundary. A timetable for groundwater chemical concentrations to reach <br /> background conditions or water quality objectives beneath the site was not provided in the <br /> 2005 Annual Report or the Revised First Semiannual Report. <br /> Shell in the Revised First Semiannual Report recommends MNA as the preferred remedial <br /> alternative and considers this approach to be adequately protective of human health and the <br /> environment. Protection of the groundwater resource should be considered as part of this <br /> discussion as it is one of the goals of groundwater remediation. While Regional Water Board <br /> staff agrees that operation of the GWE system in its existing configuration will not likely reduce <br /> groundwater pollution to background conditions or water quality objectives, more aggressive <br /> remedial approaches should be considered for this site. <br /> At this point, Regional Water Board staff does not concur with the recommendation for MNA <br /> as the preferred alternative for groundwater remediation. Shell should evaluate other remedial <br /> options to cleanup groundwater pollution at the site. The analysis of remedial alternatives <br /> should be provided in a feasibility study due to this office by 1 February 2006. <br /> If you have any questions regarding this letter, you may contact me at (916) 464-4719 or <br /> mclardy@waterboards.ca.gov. <br /> Mark W. Clardy, P.G. <br /> Engineering Geologist <br /> Private Sites Cleanup Unit <br /> cc: Margaret Lagorio, San Joaquin County Environmental Health Department, Stockton <br /> Rita Koehnen, Port of Stockton, Stockton <br /> Ron Orallo, Shell OPUS, Stockton <br /> Mark O'Brien, ERS, Walnut Creek <br /> Yuanyuan Hui, Earthtech, San Jose <br /> Thomas Kosel, ConocoPhillips, Sacramento <br /> MWC/W:\slic doMAustmStaft\Mark\Shell Stockton\10.27-06 Shell Stockton 1at semiannual rev.doc <br />