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k �! <br /> CONESTOGA-ROVERS <br /> &ASSOCIATES <br /> February 17, 2010 w Reference No-241661 <br /> -3- <br /> Based.-.on data from the 7-Eleven site and the Shell site,it appears that the MTBE plume from <br /> the 7-Eleven site migrated approximately 100 feet downgradient of the 7-Eleven site. <br /> Former Shell Service Station=Stockton,California,- Site Data <br /> Base on our experience ai other fuel release sites in San Joaquin County, MTBE concentrations <br /> typically decline by greater than one order of magnitude over a distance of 400 feet. The former <br /> Shell service station located at 3011 West Benjamin Holt Drive in Stockton was a case where the <br /> ar <br /> constituent of concerri'was MTBE. The release at this site appeared to originate at the <br /> Well MW-2 was located immediately down adient of <br /> under ound fuel store e tanks USTs). We y $r' <br /> $r' g ( <br /> the USTs!:and the maximum MTBE concentration detected in groundwater samples from this <br /> well was 13,000 µg/L '! A series of three groundwater monitoring wells (MW-1,MW-5,and <br /> MW-12)were located:dolwngradient of MW72-along the axis of the MTBE plume. .Figure 3 <br /> shows the location of the monitoring wells and the historical maximum MTBE concentration at <br />` each well. The historical maximum MTBE concentrations,and the distance of MW-1,MW-5 <br /> ' <br /> and MW-12 from MW72if,are also listed below. <br /> Maximum MTBE .`Approximate'Distance <br /> Well downgradient from <br /> Concentration(µg/L) . MW-2 feet <br /> <r P <br /> MW-2 13,000 -- _ <br /> MW-1 11,200, ,70 <br /> MW-5 790,.;. :140 <br /> MW-1294 1250 - <br /> The decline in MTBE concentrations observed at this site represents a decline in concentration <br /> of three orders of magnitude over a distance of approximately 250 feet. <br /> Discussion <br /> E The above data summaries compare two other San Joaquin County environmental cases with <br /> the subject site. One of the comparison sites (7-Eleven) is located immediately south of the <br /> subject site and is the':best site for comparison due to its proximity. Both comparison sites are <br />�. similar to the subject site in that MTBE is the primary constituent of concern for the site. To be <br />! conservative in this analysis,only the maximum MTBE.concentrations for each well were <br /> considered. - <br /> - - <br /> Services <br /> Woridwide Engineering, Construction,and IT <br /> ` li � <br />