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URS Ms Lori Duncan Page 3 San Joaquin County Environmental Health Department March 25, 2004 In November of 2002, URS supervised installation of dual-completion groundwater monitoring well set MW-1IMID down gradient of the site Groundwater analytical data suggest that groundwater in the vicinity of MW-11M is impacted with TPHg and MtBE MtBE has been detected in well MW-11D during four of five sampling events since the fourth quarter of 2002, but none of the detections have been above the California Secondary Maximum Contaminant Level (MCL) of 5 tg/L ASSESSMENT ACIVITIES AND FINDINGS On December 4 and 5, 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 of 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 deep well, fourth quarter 2003 site-wide groundwater monitoring results, waste disposal, and conclusions and recommendations Drilling and Monitoring Well Installation On December 4, 2003, Woodward Drilling (Woodward) of Rio Vista, California, drilled one deep soil boring (SO-1) for the purpose of installing one deep groundwater monitoring well in the planter west of North Center Street (Figure 2) The location is immediately downgradient of 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 to accommodate the installation of a 2-inch-diameter groundwater monitoring well A single-completion groundwater monitoring well was installed in boring SO-1 upon completion of drilling First groundwater was encountered at 20 feet below ground surface (bgs) The total depth of the boring was 84 5 feet bgs The boring was backfilled to a depth of 81 feet bgs with bentonite pellets to sea] off silty clay and silt strata encountered in the depth interval from 81 to 84 5 feet bgs The clay and silt strata were below mixed saturated sands encountered from approximately 70 to 81 feet bgs Prior to completion of well construction in boring SO-1, the well casing broke off at approximately 10 feet bgs The well subsequently filled with grout and had to be abandoned The driller was able to pull the entire casing and screen from the borehole before the grout could set The borehole was covered until the next day when Ms Lori Duncan of the SJCEHD could be informed of the situation The grout settled to approximately 12 feet below surface grade After discussion of the situation with SJCEHD, URS received approval from SJCEHD to top off the grout to the ground surface After grouting to the ground surface, abandonment activities for SO-1 were complete The boring log for SO-1 is included in Attachment C K 1WprocessU5 BP ARCO\044931Well Install14493 well installation report04 doc