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1 i pOar� g�e.a��.s(�s� a1 Site Assessment j uL`1 '— 3b Jimco Truck Plaza C A M B R I A )22 Frontage Road 6;)t@ 35 %`Ripon, California Si 1+j sa J al'44 May 18, 2005 2.3 Geology I Soils encountered during drilling consist primarily of interbedded sand, silty sand,silty clay and clay gg5 to a total explored depth of 40"ft bgs (B-17). A saturated, poorly graded, fine-grained sand was encountered during drilling at depths from 20 to 30 feet bgs, in the vicinity of the former USTs. Boring logs detailing the subsurface findings are included in Appendix B. 2.4 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. Groundwater monitoring activities have been ongoing at the site since May 1998. Groundwater levels have dropped steadily since 1998,having decreased from approximately 15 to 27 ft bgs. The drop in groundwater levels is presumed to be related to groundwater pumping of nearby municipal wells. On February 2,2005,groundwater flowed towards the southwest with a gradient of approximately 0.0006 feet per foot(Figure 3). The groundwater flow direction has fluctuated over the years but generally ranges from southwest to northwest. Due to incorrect well locations plotted by the previous consultant, all monitoring wells and related site features were resurveyed on April 1, 2005. A revised site plan is presented in Figure 2. 2.4 Sensitive Receptor Survey 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 municippal well and several domestic wells within the search radius. The municipal well is located 750Awe t of the site and is screened from 170 to 195 ft bgs, with additional screened intervals below this depth. A domestic well is located onsite near MW-8 and is screened from 160 to 200 ft bgs. A second domestic well is located 175 ft southeast of the site on i 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 I