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Work Plan f r Additional Site Assessment 1022 Frontage Road,Ripon, California C A M B R I A July 25, 2006 a 2.3 Local Geology and Hydrogeology �x Soils encountered during drilling consist primarily of interbedded sand,silty sand,sandy silt,silt,and I+ -r�cl2y Ore clay to a total explored depth of 60 ft bgs(boring B-18). Saturated silty sande encountered across the lives 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. `/I1 1 YyrMO�o�rblr 'i P 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 o be related to groundwater pumping at nearby municipal and domestic wells. On May 15, 20g5;groundwater 8 j flowed away the source area toward the northwest at a gradient of 0.0003 ft/ft (Figure 2). A rose diagram on Figure 2 represents the historical groundwater flow directions. Since 1998, the groundwater flow direction has varied.significantly with groundwater flow predominately toward the northwest. The varied groundwater flow direction is likely due to the flat groundwater gradient observed at the site and nearby groundwater pumping. 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. ' 3.0 PREVIOUS INVESTIGATIONS Below is the chronology of site assessment, feasibility testing and interim remedial activities at the site. Historical soil and groundwater analytical data are presented in Tables 1 and 2, respectively. ' October 1996: In October 1996,AGE drilled borings*B 14 through*B 16 along the southern border of the site. These borings were part of AGE's delineation of a hydrocarbon plume originating from 2