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Former Beacon Station No. 12474 January 27,2003 Stockton,California Quarterly Groundwater Monitoring Report w 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) and the fuel oxygenate methyl-t-butyl ether (MTBE) by Environmental Protection Agency (EPA) Method 8260B. Results Depth-to-groundwater measurements were subtracted from the surveyed TOC elevations 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. The groundwater generally flows southward to southeastward across the site under a gradient of approximately 0.002. Groundwater analytical results are summarized in Table 1. Benzene concentration data from this sampling event were used to construct a benzene iso cone entrati on map as shown on Figure 3. 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 I and 2 in Attachment D. 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 sheet for this quarter is contained in Attachment E. Recommendations Horizon conducted fieldwork on October 8 and 9, 2002 to further delineate the extent of petroleum hydrocarbons in groundwater. The report describing the results of the additional delineation work is in preparation. At the request of Ultramar, Horizon proposes to modify the quarterly monitoring program. The proposed monitoring program modifications will continue 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 analytes to establish baseline concentrations for those particular wells. It is proposed to decrease the sampling frequency of monitoring wells MW-4, MW-5, MW-6, MW-9, MW-12 and air sparge well AS-I to a semi-annual (first and third quarters) schedule. Monitoring wells MW-7, MW-8, MW-10, MW-11 and vapor extraction well VW-1 shall continue to be sampled on a quarterly schedule. Monitoring wells MW-1, MW- 474q m r02Q4.doc Project No. 1474.45 2 HORIZON ENVIRONMENTAL INC.