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iQ sr TIRS Ms Lori Duncan Page 3 San Joaquin County Environmental Health Department July 26, 2004 In December 2003, one deep groundwater monitoring well (MW-12D) was installed under the direction of URS in the planter west of North Center Street to assess the vertical extent of petroleum hydrocarbon contamination in groundwater immediately downgradient from the former ARCO station pump islands (Figure 2) ASSESSMENT ACIVITIES AND FINDINGS On November 25, 2003, URS and its drilling subcontractor attempted to drill a boring to Install a dual- completion groundwater monitoring well set to the northeast of the site The drilling subcontractor attempted to hand clear three locations for drilling that were within the area allowed under a City of Stockton encroachment permit and thought to be free of utilities The area had been cleared earlier by a private utility locator Undocumented utilities were encountered in all three locations Alternative locations were not available, so URS Initiated negotiations with the State of California to install the well set in the state parking lot at 31 East Channel Street On May 3, 2004, one dual-completion groundwater monitoring well set (MW-13M[D) was installed • under the direction of URS in the State of California parking lot at 31 East Channel Street, west of El Dorado Street, to assess the lateral and vertical extent of petroleum hydrocarbon contamination in groundwater downgradient from the former ARCO station pump islands (Figure 2) The following paragraphs discuss the drilling and monitoring well installation, the subsurface conditions encountered, monitoring well development, sampling, groundwater analytical data for the newly installed well set, waste disposal, and conclusions and recommendations Drilling and Monitoring Well Installation On May 3, 2004, WDC Exploration and Wells (WDC) of Woodland, California, drilled one soil boring (SO-3) to install one dual-completion groundwater monitoring well set in the State of California parking lot west of El Dorado Street (Figure 2) The location is downgradient from the former site source area The boring was drilled using a hollow stem auger (HSA) drill rig with 8 25-inch outside diameter (OD) augers and then over-drilled with 10-inch OD augers to accommodate the installation of a dual- completion set of 2-inch inside diameter(ID)groundwater monitoring wells A dual-completion groundwater monitoring well set was installed in boring SO-3 upon completion of over-drilling activities First groundwater was encountered at approximately 20 feet below ground surface (bgs) The total depth of the boring was approximately 56 5 feet bgs The boring collapsed to a depth of 51 feet bgs prior to well installation The boring log for SO-3 is Included in Attachment C Monitoring wells MW-13M/D were constructed using 2-Inch ID, schedule 40, flush-threaded polyvinyl chloride (PVC) well casing Five feet of 0 020-inch machine-slotted well screen were set in the depth • K\WprocessV5 BP ARCO\04493\May Well 0414493 well installation reportil4b doc