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.� <br /> U SECOR � <br /> I <br /> Work Plan for Additional Site Assessment and Interim Remediation Addendum <br /> 7-Eleven Store#32262 <br /> March 31, 2005 <br /> I <br /> Page 4 <br /> i <br /> I <br /> Groundwater samples collected in 2003 from the Valero station wells MW-1(V), <br /> I MW-2(V), and MW-4(V) contained MtBE concentrations of 1.5 pg/L , <0.5 pg/L, and <br /> <0.5 pg/L respectively (Attachment C). In 2003, onsite 7-11 well MW-5 contained <br /> 120,000 pg/L of MtBE. The relationship between the Valero and 7-11 wells clearly <br /> defined the western limits of the dissolved MtBE plume in 2003. Since 2003, <br /> concentrations of MtBE in the center of the plume (measured in MW-5) have <br /> I decreased significantly to 4,300 pg/Lin January 2005 (Table 1). <br /> 1 <br /> Due to the favorable decreases in these wells, SECOR recommends continuing the <br /> current monitoring and sampling schedule to confirm groundwater concentrations and <br /> plume stability west and northwest of the site. <br /> Please provide a demonstration to SJC/EHD that the observed drawdown <br /> [during pump test] was an effect of"earth tides". <br /> In the initial pump test conducted August 11-12, 2004, maximum drawdown <br /> measured in S-1 and MW-5 was observed to be .09 feet and .07 feet respectively. <br /> These very low drawdown values were considered to be due to earth tides <br /> according to similar values given in the following references that were cited in the <br /> report: <br /> o Fetter, C.W., 1994. Applied Hydrogeology, Third Edition <br />' o Javandel, I and C.F. Tsang, 1986. Capture-Zone Type Curves: A Tool for Aquifer <br /> Cleanup, Ground Water, Vol. 24, No.5, pp 616-625. <br /> The following is an additional reference regarding the affects of earth tides in <br /> groundwater: <br /> o Todd, D.K. 1980. Groundwater Hydrology, John Wiley &Sons, pp. 247-248. <br /> Should you have any questions regarding this site, please contact the undersigned at (916) <br /> j 861-0400. <br /> Sincerely, <br />' SECOR International Incorporated <br />' Damon Brown ,`EF` GEoC0Jaff Auchterlonie <br /> Staff Geologist ��� G'i� Project Manager <br /> DANIEL J. , 'A <br /> A E)Hv1S <br /> Daniel J. D is, R.G. <br /> Senior Geologist No.6435 <br /> O CAN, <br /> 1:17-Elevenl7-11 Stores1322621Workplans132262 Addendum to Offsite well install and interim remediation workplan.doc <br /> i <br /> I <br />