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Mr. Gerald A. Stauffer - 3 - 15 February 2006 <br /> Valero Logistic Operations, LP <br /> 3. Decreasing contaminant concentration trends: Although there is evidence of overall <br /> decreasing concentration trends at wells PS/P-13 and PS/MW-15 shown in Figures 13 <br /> and 14, respectively, there is little evidence to support this claim at wells PS/P-11 and <br /> PS/P-12 shown in Figures 11 and 12, respectively. Concentration trend graphs for wells <br /> PS/P-11 and PS/P-12 do not exhibit decreasing trends nor do they have a long enough <br /> monitoring history to support this claim. Continued monitoring of these two wells is <br /> needed to evaluate decreasing concentration trends. <br /> While there is some evidence that natural attenuation of contaminants may be occurring at the <br /> site, more complete characterization of the plume and a demonstration of decreasing <br /> concentration trends are necessary before the recommendation for MNA can be properly <br /> evaluated. The Central Valley Regional Board therefore requests that Valero submit a work plan <br /> by 15 March 2006 to characterize groundwater contaminant concentrations at the downgradient <br /> fringe of the plume. Resubmittal of the cleanup plan should be deferred pending an evaluation of <br /> future groundwater monitoring data for plume stability and decreasing concentration trends to <br /> support the recommendation for MNA or other appropriate remedial measures, if warranted. <br /> The next monitoring report for the first quarter 2006 for this site is due by 1 May 2006. If you <br /> have any questions or comments regarding this letter, you may contact me at (916) 464-4719 or <br /> mclardy@waterboards.ca.gov. <br /> 2 <br /> MARK W. CLARDY, P.G. <br /> Engineering Geologist <br /> cc: Ms. Margaret Lagorio, San Joaquin County Environmental Health Department, Stockton <br /> Ms. Rita Koehnen, Port of Stockton, Stockton <br /> Ms. Amanda Spencer, Ash Creek Associates, Inc., Portland, Oregon <br />