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Beacon Station No 3641 January 23, 2003 Stockton, California _Quarterly Groundwater Monitoring Report xylenes (BTEX), and the fuel oxygenate methyl-t-butyl ether (MTBE) by Environmental • Protection Agency (EPA) Method 8260B Analytical reports and the chain-of-custody document are contained in Attachment C 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 contour map (Figure 2) The groundwater flow direction for this sampling event was southeastward under a gradient of 0 002 Analytical data are summarized in Table 1 Benzene concentration data were used to construct a benzene isoconcentration map (Figure 3) Geo-Tracker Electronic Data Deliverables The analytical electronic data deliverable (EDD) is uploaded by Kiff The groundwater well measurement file EDD for this quarterly monitoring event has been uploaded by Horizon The GEO_WELL file upload confirmation sheets for this and the previous four quarters are contained in Attachment D Recommendations At the request of Ultramar, Horizon proposes to modify e quarterly monitoring program • The propos c modified monitoring program 0 nue to provide adequate data to monitor the plume at the site and improve cost effectiveness of the monitoring program Results of previous quarterly monitoring events have confirmed or ruled out the presence of the petroleum hydrocarbons analytes in each monitoring well Annual analyses for the full complement of analytes is adequate to monitor their occurrence The groundwater monitoring frequency may be modified to consider site requirements depending on the investigative or remedial status of the site Groundwater samples from future well installations will initially be evaluated for all site analytes to establish baseline concentrations for those particular wells It is proposed that we decrease the sampng frequency of monitoring well MW-3, sparge well SW-1 and vapor wells VW-1 and VW-3 to a semi-annual (first and third quarters) schedule and monitoring wells MW-1, MW-5, MW-7 and MW-8 to an annual schedule (first quarter) The remaining monitoring wells MW-2, MWA UW-6 and vapor wells Vf1-5 and VW-7 shall continue to be sampled on a�quarte�Tschedule The proposed quarterly monitoring schedule would take effect starting the first quarter of 2003 641gmr02Q4 doc 164145 2 HORIZON ENVIRONMENTAL INC