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- Former Beacon Station No. 12474 May 23,2001 Stockton-California „ „„. S?uartgrlyy Groundwater Monitoring Refit# w were submitted, under chain-of-custody documentation, to Kiff Analytical LLC (Kiff), a California Department of Health Services-certified analytical laboratory (FLAP No. 2236) located in Davis, California. The samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg); the volatile aromatic compounds benzene, toluene, ethylbenzene and total xylenes (BTEX); the fuel oxygenate methyl-tert-butyl ether (MTBE) by Environmental Protection Agency (EPA) Method 8260B. Results Depth-to-groundwater measurements were subtracted from the surveyed TOC elevations (reported by previous consultants) to calculate groundwater elevations. Groundwater elevation data, as summarized in Table 1, were used to construct the groundwater contour map as shown on Figure 2. Groundwater analytical results are summarized in Table 1. Analytical reports and chain-of- custody documents are contained in Attachment C. Historical groundwater level and analytical data collected prior to 1998 are summarized in Tables 1 and 2 in Attachment D. Benzene concentration data from this sampling event were used to construct a benzene isoconcentration map as shown on Figure 3. • Recommendations Horizon recommends continued quarterly monitoring at the site. A Problem Assessment Report/Remedial Action Plan should be developed to describe the problem and propose corrective actions. Report Distribution We recommend a copy of this report be forwarded to: Mr. Michael J. Inf ima Mr. Gordon Boggs Environmental Health Division CRWQCB - Central Valley Region San Joaquin County Public Health Services 3443 Routier Road, Suite A P.O. Box 2009 Sacramento, California 95827-3098 Stockton, California 95201 Mr. Sohail Bafaiz 392 West Harder Road Hayward, California 94544 474gmr01 Q 1.doc 1474.44 2 HORIZON ENVIRONMENTAL INC.