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C LEARWATE R (; I, (1 L 1" 1 N C. ` Environmental Srrviccs V 3.0 WORKPLAN FOR CONFIRMATION SOIL BORINGS 3.1 Proposed Scope of Work The purpose of the proposed work is to demonstrate that the SVE/AS system operation and removal of 7,377 lbs of gasoline from the subsurface resulted in a significant reduction in gasoline contaminants in soil beneath the site. To accomplish this goal, four soil borings will be drilled in the area of the former UST system and in areas of previously known vadose zone/capillary fringe contamination (Figure 5). Soil samples collected from zones of previously known contamination will be retained for laboratory analysis. Analytical results for these samples will be compared to those collected during UST removal and initial site investigation phase to determine efficacy of clean-up. All work will be conducted in accordance with the LUFT Field Manual, the Tri-Regional Board Staff Recommendations, and Article 11 Corrective Action Regulations of Chapter 16 Under Storage Tanks Regulations. Specific project tasks are summarized below. 3.2 Field Methods Prior to drilling, Clearwater will prepare site specific Health and Safety Plan for drilling activities. Prior to conducting and drilling, the site will be marked by Underground Service Alert (USA) to identify utilities leading to the site. Appropriate drilling permits will be obtained from the SJCPHS/EHD prior to drilling. All fieldwork will be conducted in accordance with Clearwater's Field Protocols (attached). The borings will be advanced in locations where contamination was previously observed or documented, in order to compare pre- and post-remediation contaminant levels. Confirmation borings CB-1 and CB-2 will be drilled in the locations of the former south UST and the former dispenser island, respectively (Figure 5). Due to access constraints (low overhead), a boring cannot be drilled exactly in the location the second former UST. However, boring CB-2 is located with several feet from the side of the former north UST, and therefore will suffice for reassessment of that area. TPHg levels were reported to have been greater than 10.000 mg/kg in soil samples collected from the base of the excavations during UST removal at a depth of 10 feet bgs. Additionally,previous assessment data from MW-2, RW-1, and VW-3 indicated that TPHg concentrations at a depth of approximately 15 feet bgs ranged between 450 and 700 mg/kg (Figure 6). The remaining two confirmation borings,CB-3 and CB-4, will be located near VW-4 and MW-1,respectively. Although no soil sample data exist for this area,drilling observations/OVM readings and the previous presence of SPH in the area of MW-1 suggest that TPHg concentrations were elevated at depths between 10 and 15 feet bgs, the historical range for groundwater fluctuation. For the sake of comparison, it is assumed that pre-remediation TPHg concentrations in this area were also likely between 450 to 700 mg/kg(Figure 6). A direct push drill rig will be used to advance the confirmation borings. Drilling will be performed by a C-57 licensed driller under the supervision of a Clearwater geologist. It is assumed that the four borings will be drilled to a maximum depth of 15 feet bgs. During drilling,soil samples will be collected at five-foot intervals. Soil types will be documented, as well as depth to groundwater, if encountered. Soil samples will be collected in accordance with local regulatory D-107 4Q99 GW&WP Confirmation Borings 4 [november 15. 1999