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Monitoring Well Destruction and Soil and Groundwater Investigation Report US Can—Welty Road December 28,2010 gravels. The topography in the area is relatively flat and at approximately 105 feet above mean sea level. The depth to groundwater generally ranges from approximately 31 feet to 39 feet below ground surface(bgs). Historical data for the site indicates that the groundwater flow direction is inferred to be to the east at 0.001 feet per foot. Regional groundwater flow-direction information and local topography on the relevant United States Geological Survey(USGS) topographic map indicates that groundwater flow in the vicinity of the site is to the east (California Department of Water Resources [DWR], 2003; USGS, 1980). 2.5 PREVIOUS ENVIRONMENTAL ACTIVITIES Several environmental investigations have been conducted at the site and in the site area; a summary of these site investigation activities and results are included as Appendix C. The Central Valley RWQCB reviewed the results of previous investigations and the recommendations in the September 2009 investigation report, and concurred that additional lateral delineation of affected soil and groundwater was necessary to the southwest(inferred up- gradient direction) of affected Areas 1 and 2, and northeast of affected Area 3 (inferred down- gradient direction) near borings SB-23 and SB-28 (Central Valley RWQCB, 2010a). 3. FIELD INVESTIGATION ACTIVITIES 3.1 WELL DESTRUCTIONS SAIC-Benham performed well destruction activities at the site on July 14 through 16, 2010. Each well location was cleared to 8 feet bgs using an air knife. SAIC-Benham subcontracted WDC Wells and Exploration (C57#283326) to properly destroy the wells by removing the existing well casing and over-drilling each monitoring well to the total completed depth of 40 feet bgs using a CME 85 hollow-stem auger drill rig. The destroyed monitoring well locations are shown on Figure 2. Permitting procedures are included as Appendix A. Detailed field methods are included as a component of Appendix D. 3.2 SOIL AND GROUNDWATER INVESTIGATION SAIC-Benham advanced four soil borings at the site on July 20 through 22, 2010 to further evaluate the extent of HPP-BTR affected soil and groundwater. Each boring was cleared to 8 feet bgs using a hand auger. SAIC-Benham subcontracted PeneCore Drilling(C57#906899) to advance the soil borings using a Geoprobe 7822 DT direct-push drill rig. Soil and groundwater samples were collected from each boring for laboratory analysis. The boring locations are shown on Figure 2. Permitting procedures are included as a component of Appendix A. Detailed field methods are included as a component of Appendix D. All work was performed under the supervision of a California Professional Geologist. 3.2.1 Subsurface Conditions SAIC-Benham advanced four soil borings (SB-44 to SB-47) to 45 feet bgs on July 20 through 22, 2010. Sediments encountered consisted of laterally discontinuous strata of clay, silt, sand, and gravel. Field observations indicated hydrocarbon-affected soil in SB-46 from 34.5 to 44 feet bgs. SAIC-Benham collected soil samples for laboratory analysis within and beneath the affected zone at 38 and 44.5 feet bgs,respectively. 3 - ?' The Benham Companies,LLC awmiyow„res�n,;dc,, From Suence to SoMwa