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Additional Subsurface Investigation and Well Installation Work Plan San Joaquin County Public Works June 19,2017 Page 2 of installation and operation of an ozone injection system. Quarterly groundwater monitoring has been ongoing since April 1999. An extensive background summary, historical Site maps showing soil boring locations, and historical data tables are included in Appendix B — Site Background. The locations of existing groundwater monitoring wells and remediation test wells are shown on Figure 2 in Appendix A. 3.0 SITE GEOLOGY AND HYDROGEOLOGY The Site is located in the southeastern portion of downtown Stockton, San Joaquin County, California. The Site is located in the Great Valley Geomorphic Province in the northern San Joaquin Valley. The San Joaquin Valley is a northwest-trending structural depression, filled with up to six vertical miles of lithified non-marine and marine sediments and unlithified non-marine sediments. Regionally, the lithology of the upper 3,000 feet of sediments is indicative of uplift and erosion of the Sierra Nevada to the east and, to lesser degree, the Coast Range Mountains to the west. The Coast Range Mountains generally consist of northwest trending ridges with Franciscan Assemblage and granitic basement rocks. The Sierra Nevada Province is an asymmetric range, with a steep fault- bounded eastern front and gentle western slope that dips under the sediments of the Great Valley to the west. The bedrock complex of the Sierra Nevada Mountains generally consists of metamorphosed sedimentary and volcanic rocks of Paleozoic and Mesozoic age (150 to 300 million years old) and plutonic rocks (chiefly granitic types) of Mesozoic age (80 to 150 million years old). The Site soil type, as mapped by the US Department of Agriculture, is Yellowlark gravelly loam with Class C hydrologic group (slow infiltration rates). Observations during drilling by various consultants suggest that soil beneath the former diesel UST (UST #3) generally consists of silty sand with silt and clay to a depth of approximately 60 feet bgs. Logs of borings and monitoring wells drilled north, east, south, and west of the USTs generally consist of interbedded silt and clay to approximately 60 feet bgs. The depth to groundwater beneath the Site has varied from approximately 37 feet bgs to 50 feet bgs, and flows toward the northeast. Lithologic cross sections showing shallow soil beneath the Site are presented in Figures 3 and 4 in Appendix A. A groundwater gradient map from Site data collected in December 2016 is shown on Figure 5. 4.0 PROPOSED SCOPE OF WORK This section includes a description of the proposed scope of work, including site preparation, drilling and sampling activities, well installation, well development, wellhead elevation surveying, and sampling procedures. The proposed soil boring and monitoring well locations are shown on Figure 6 (Appendix A). Condor will supervise the following scope of work: Drilling and sampling two shallow soil borings in the area where hydrocarbon impact was noted during drilling of ozone injection wells in May 2016; Advancing two deeper borings (CPT-1 and CPT-2) to investigate lithology and hydraulic characteristics beneath the Site and to evaluate the vertical extent of soil and groundwater contamination near the location of the unauthorized release of petroleum hydrocarbons from the former gasoline UST (UST #3); Installing three shallow groundwater monitoring wells (proposed compliance well MW-8 and proposed transition wells MW-9 and MW-10); and CONDOR