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Sierra Nevada that interfinger with the poorly sorted clays and silts of the Coastal Ranges. Cretaceous and Tertiary strata contain laterally continuous beds of mudstone, siltstone, conglomerate, sandstone, and limestone. Quaternary to recent units consist of lake and alluvium deposits. Soil encountered beneath the site generally consists of interbedded fine grained sand and clay with silty sand and sand from ground surface to the total explored depth of approximately 115 feet below grade (fbg). The interbedded layers range in thickness from approximately 5 to 20 feet thick. 2.3 Regional and Site Hydrogeology The site lies within the Eastern San Joaquin Groundwater Basin. The basin is bound to the north by the Mokelumne River, to the west by the San Joaquin River, to the south by the Stanislaus River, and to the east by consolidated bedrock. The nearest major surface water body is Yosemite Lake, approximately 3,500 feet to the west of the site. Groundwater flow within the basin is dominated by a large cone of depression located east of Stockton due to agricultural pumping. Wells at the site have been installed in four water-bearing zones. The shallow A Zone extends from approximately 30 to 45 fbg, the B' and B Zones extend from approximately 55 to 70 fbg and 75 to 85 fbg, respectively, and the C Zone extends from approximately 100 to 120 fbg. The aquitards between each water-bearing zone primarily consist of silts, which allow some hydraulic communication between the water-bearing sand zones. 2.4 Environmental History In 1999, three 10,000-gallon capacity underground storage tanks (USTs) used for gasoline and diesel fuel were removed from the site. The UST pit area was over-excavated with excavation dimensions of approximately 35 feet by 40 feet by 15 feet deep (Stantec Consulting Corporation [Stantec] 2011). During the excavation of the impacted soil, unidentified fiberglass piping was discovered (Stantec 2011). Additional excavation work and sampling were completed on April 8, 1999 to remove the unidentified piping. A brief overview of environmental assessment and remediation activities conducted at the site are presented below and more details for each assessment and remediation event are presented in Attachment B. 2.4.1 Previous Environmental Assessment Previous environmental activities at the site include the installation of twenty-three groundwater monitoring wells, six remediation wells, eight soil borings, and six soil vapor probes. Four of the six soil vapor probes were temporary and were destroyed after they were sampled. 2.4.2 Previous Environmental Remediation Remedial activities at the site began in 1999 with the removal of all site infrastructure, including USTs, dispenser islands and product piping, and over-excavating hydrocarbon-impacted soil (approximately 1,707 tons) in the vicinity of the USTs and dispenser islands. From March 2010 to June 2012, mobile oxygen injection events were conducted every two weeks and reportedly GHD I Well Installation Work Plan Addendum 111193145(5)1 Page 2