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NOW" `Site Assessment Work Plan 1022 Frontage Road, Ripon, California CONESTOGA-ROVERS March 7, 2008 &ASSOCIATES 2.2 Regional Geology and Hydrogeology The site is located within the San Joaquin Valley Groundwater Basin(Basin No. 5-22). The Modesto, Riverbank,and Turlock Formations,as well as Recent Alluvium are the primary sources of groundwater production within the San Joaquin Valley Groundwater Basin. 2.3 Local Geology and Hydrogeology Soils encountered during drilling consist primarily of interbedded sand,silty sand,sandy silt,silt,and clay to a total explored depth of 60 ft bgs(boring B-18). Saturated silty sand is encountered across the site at depths between 25 and 35 feet bgs. The shallow groundwater aquifer is detected within this silty sand bed. _ Boring and well construction logs detailing the subsurface findings are included in Appendix B.—0-ye? i Since May 1998, groundwater monitoring activities have been ongoing at the site. Since 1998, the groundwater table has decreased in elevation from approximately 15 ft bgs to as deep as approximately 27 ft bgs. The decrease in the groundwater table elevation is presumed to be related to groundwater pumping at nearby municipal and domestic wells. Groundwater levels measured on November 13,2007 indicate that groundwater beneath the site flows primarily towards the southeast at a gradient of approximately d-SYJ 3 0.0001 ft/ft with a slight depression around MW-1 (Figure 2). A rose diagram on Figure 2 represents the 0 historical groundwater flow directions. Groundwater flow direction has fluctuated since groundwater �S i s monitoring began in August 1999; however,the overall trend is toward the west/no hwest. The varied groundwater flow direction is likely due to the flat groundwater gradient observe at the site and/or nearby groundwater pumping. 4.ate(R 2 0 0'7 Rkr 2.4 Sensitive Receptors In 2003, Advanced GeoEnvironmental, Inc. (AGE) of Stockton, California conducted a well survey to identify any domestic,municipal,irrigation,or monitoring wells within a 2,000-foot radius of the site.The well survey identified one municipal well and several domestic wells within the search radius. The municipal well is located 750 west of the site and is screened from 170 to 195 ft bgs, with additional screened intervals below this depth. Well DW-1 is located onsite near well MW-8 and is screened from 160 to 200 ft bgs.`Well DW-1 is used to operate the truck wash onsite. A domestic or commercial well is located 175 ft southeast of the site on Ripon Farm Service property and is screened from 176 to 196 ft bgs. A third domestic well is located at 23066 South Highway 99 and is screened from 180 to 200 ft bgs. Detailed findings of the well survey are presented in AGE's Well Survey and Site Model dated June 6, 2003. The closest water body to the site is the Stanislaus River which is located approximately 9,000 ft south of the site. 2 Worldwide Engineering,Environmental,Construction,and IT Services