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URS Mr. Jeffrey Wong San Joaquin County Environmental Health Department October 4, 2002 Page 4 i Groundwater Monitoring Well Development,Sampling,and Surveying On September 9, 2002 Blaine Tech Services, Inc. developed the groundwater monitoring wells by rigorously surging over the length of the screen interval and by pumping approximately 60-gallons of water from each well. The turbidity and conductivity of the development water was measured and recorded periodically during the development process. Well development data sheets are included in Attachment IV. Groundwater samples were collected from the three groundwater monitoring wells by Blaine Tech on September 11, 2002. Groundwater samples were analyzed for TPH-g and BTEX by EPA Method 8015/8020 and fuel by EPA Method 8260B. Field purge sheets are included in Attachment N. Attachment V presents detailed groundwater sampling and analysis procedures. Purge water generated during well development was pumped into a B1ainTech truck-mounted water tank. The water was then transported to the B1ainTech Services holding facility in Roseville, California. Water was then transported by Dillard Environmental Services to the Altamont Landfill and Disposal Facility in Altamont, California for disposal. The monitoring wells were surveyed on September 9, 2002 by Tom Morrow Surveying of West Sacramento, California. The well locations were surveyed with respect to both latitude and longitude • and northing and easting positions. The top-of-casing elevations were surveyed with respect to mean sea level (msl). The survey data were used to determine the groundwater gradient at the site. Well survey data are presented in Attachment V1. Groundwater Analytical Data Analytical results for the three groundwater monitoring well samples collected on September 11, 2002, indicate that TPHg and MtBE were detected in well MW-2 at concentrations of 98 gg/L and 62 gg/L, respectively. MtBE was also reported in the sample from MW-3 at 0.77 µg/L. Groundwater data are summarized in Table 2 and Table 3. Laboratory Analytical Reports are included in Attachment VII. A site map showing the concentrations of TPHg,benzene, and MtBE detected in the groundwater samples from the three monitoring wells is presented as Figure 3. Groundwater Flow Direction Depth to groundwater at the site ranged from 11.34 to 11.37 feet below top of casing (btc), as shown in Table 2. Based on monitoring data collected on September 11, 2002, groundwater flow was to the southwest at an approximate gradient of 0.003 ft/ft (Table 4). The groundwater gradient is relatively flat and may be tidally influenced due to the proximity of the Stockton Channel to the south of the site. A groundwater elevation contour map is included as Figure 2. Conduit and Well Survey URS contracted Subtronics Corporation of Concord, California, to map the locations of underground utilities at the site that could potentially act as a conduit for any petroleum-impacted groundwater. A map showing the locations and depths of the underground utilities at the site is presented as Figure 4. Several utility lines and a storm drain are present between the AM/PM Mini-Mart and the pump • islands. The depth to the bottom of the storm drain was measured at 3 feet 11 inches bgs. It can be reasonably assumed that the utilities identified at the site were installed are less than 5 feet bgs, which is well above the water table. Therefore none of the utility trenches identified in the survey could act as 7G600-022-02/Site Investigation ReportK IJ5 BP ARCO\0545Mite Investigation Report.doc