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Beacon Station No. 3641 July 30,2004 Stockton California Quarterly Groundwater Monitoring Report fel. butyl ether (ETBE), tert-amyl methyl ether (TAME), tert-butanol (TBA), methanol, ethanol Fr r and 1,2-dichloroethane (DCA) by Environmental Protection Agency (EPA) Method 8260B. Results of Groundwater Monitoring Depth-to-groundwater measurements were subtracted from the surveyed TOC elevations to calculate groundwater elevations. Groundwater elevation data, summarized in Table 1, were used to construct a groundwater elevation contour map (Figure 2). Groundwater level data indicate flow in the immediate underground storage tank area was variable from northwestward to southward and southeastward under gradients up to approximately 0.02. Analytical data are summarized in Table 1. Benzene concentration data were used to construct a benzene isoconcentration map (Figure 3). Analytical reports and the chain-of- custody document are contained in Attachment C. GeoTracker Electronic Data Deliverables The analytical electronic data deliverable (EDD) is uploaded by Kif£ The groundwater well measurement file EDD (GEO_WELL) for this quarterly monitoring event has been uploaded W by Horizon. The GEO_ ELL file upload confirmation sheet is contained in Attachment D. Recommendations 4 _s Horizon recommends continued quarterly monitoring during the ongoing investigation and remediation phases of the project. Horizon performed an additional groundwater investigation in February 2003. The investigation report was submitted on July 10, 2003 and included recommendations, as appropriate, for additional fieldwork to delineate the extent of petroleum hydrocarbons in soil and groundwater. In June 2004, Ultramar secured right-of-entry agreements with the off-site property owners to allow installation of additional off-site groundwater monitoring wells. Installation of the monitoring wells is scheduled for the week of August 23, 2004. A site interim remediation system has been installed at the site. The electrical utility connection is the last remaining item to be completed prior to startup. The remediation system is expected to be started during the third quarter 2004. Distribution A copy of this report should be forwarded to: Mr. Michael Infurna Jr., Senior R. E. H. S. San Joaquin County Environmental Health Department 304 East Weber Avenue, Third Floor Stockton, California 95202-2708 641 gmrQ204.doc Project No. 1641.47 2 HORIZON ENVIRONMENTAL INC.