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Beacon Station No 3641 August 14, 2002 <br /> ( 1 Stockton California Quarterly Groundwater Monitoring Report <br /> total xylenes (BTEX), and the fuel oxygenate methyl-t-butyl ether (MTBE) by <br /> Environmental Protection Agency (EPA) Method 8260B Analytical reports and the chain- <br /> of-custody <br /> hain- <br /> ofcustody document are contained in Attachment C <br /> Results of Groundwater Monitoring <br /> Depth-to-groundwater measurements were subtracted from the surveyed TOC elevations to <br /> calculate groundwater elevations Groundwater elevation data, summarized in Table 1, were <br /> used to construct a groundwater contour map (Figure 2) The groundwater flow direction in <br /> the area of the USTs and the western dispenser island varied from northwestward to <br /> southward under gradients ranging from approximately 0 02-0 006 <br /> Analytical data are summarized in Table 1 Benzene concentration data were used to <br /> construct a benzene isoconcentration map (Figure 3) <br /> Recommendations <br /> It is recommended that quarterly groundwater monitoring be continued to evaluate <br /> groundwater quality trends Horizon is reviewing site data to evaluate the need for an <br /> additional investigations to delineate impacted groundwater <br /> Distribution <br /> A copy of this report should be forwarded to <br /> Mr Michael Infurna Ir R E H S <br /> San Joaquin County Environmental Health Department <br /> 304 East Weber Avenue, Third Floor <br /> P O Boa 388 <br /> Stockton California 95201-0388 <br /> Mr Goi don Boggs <br /> Regional Water Quality Control Board <br /> Central Valley Region <br /> 3443 Routier Road, Suite A <br /> Sacramento, California 95827-3098 <br /> 641 gmr02Q2 doc <br /> 1641 45 2 HORIZON ENVIRONMENTAL INC. <br />