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GAS LTD <br /> BEST CALIFORNIA <br /> March 5, 2009 0-94795 <br /> Mr. Michael J. Infiuna Jr. Site Code: 1299 <br /> Senior REHS r RO#: 0552 i <br /> County of San Joaquin it <br /> Site Mitigation Unit IV/LOP <br /> 600 East Main Street <br /> Stockton, CA 95202 <br /> RE: Former Best Station 4171 <br /> (ARCO #9600) <br /> 1250 North Wilson Way <br /> Stockton, CA 95205 <br /> Subject:Request to Reduce Groundwater Sampling Frequency in Selected Monitoring Wells <br /> Dear Mr. Infurna: <br /> f q <br /> o <br /> This letter is written as a request to reduce the groundwater sampling frequency for several <br /> monitoring wells displaying non-detectable to trace hydrocarbon concentrations, at the above- <br /> referenced Thrifty Oil Co. (Thrifty) site in Stockton, California (Figure 1). The purpose of this <br /> request is to reduce costs and energy expenditure, so that more resources are available for higher <br /> priority tasks such as site remediation. <br /> It is proposed that wells MW-3R,MW-4,MW-6,TB-1,TB-8 and T8 10 be reduced from a quarterly <br /> to annual sampling schedule. Hydrocarbon constituent concentrations in these wells have been at <br /> f non-detectable to very low concentrations for at least the last six quarterly sampling events. The <br /> E current and historical groundwater sampling and gauging data is summarized in the Summary <br /> Table,Table 1,and Table 2;the sampling results of the wells proposed to be sampled on an annual <br /> basis are shaded in the tables. The fourth quarter 2008 groundwater elevation map and TPHg,. <br /> benzene,MTBE, and TBA distributions in groundwater are.presented in Figures 2,3,4,5,and 6, <br /> respectively. As shown in these figures,these six wells are peripheral from the remaining core of the <br /> dissolved hydrocarbon plume,and thus concentrations in these, ;are not expected to significantly <br /> change (as demonstrated by data from the past six sampling events). Adequate control of the <br /> dissolved hydrocarbon plume configuration is therefore expected through annual sampling of these <br /> J peripheral wells, and quarterly monitoring of the remaining wells which display higher dissolved <br /> concentrations. By implementing these sampling reduction measures, a cost savings of <br /> approximately $2,118 per year is estimated for this site. <br /> 13116 Imperial Highway,Santa Fe Springs,CA 90670-0138 (562)921-3581 <br /> i <br /> i <br /> i� <br />