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Mr.Nuel Henderson,P.G., SJCEHD September 21,2005 ARCO Station 6100,Tracy,California Project No.U6100-15-1 Additional Site Assessment Work Plan Page 7 monitoring wells will be installed off-site. Stratus anticipates that five additional monitoring wells (MW-16 through MW-20), screened across sand zone 2, will be necessary to adequately characterize the lateral extent of petroleum hydrocarbon impact to groundwater. Wells MW-16 through MW-20 will be installed in relatively close proximity to borings B-44 through B-48. The precise location, screening interval, and number of monitoring wells will be coordinated with SJCEHD personnel prior to installation. SCOPE OF WORK The objectives of the proposed work activities are to: • Further assess the lateral and vertical extent of petroleum hydrocarbon impact to soil. • Further characterize the extent of free product and dissolved petroleum hydrocarbon impact within sand zone 2 south-southeast of the site. • Additionally characterize geologic relationships south-southeast of the site. • Further investigate sand zone 2 for the presence of groundwater off-site. To accomplish these objectives, Stratus will implement the following work activities: • Advance five (5) exploratory soil borings (13-44 through B-48) to depths ranging from approximately 120 to 135 feet bgs using mud rotary coring methods. • Collect soil samples at desired intervals from soil cores recovered from borings B-44 through B-48. • Drill and install five (5) 4-inch diameter groundwater monitoring wells (MW-16 through MW-20) using 10-inch diameter hollow stem augers. Soil samples will not be collected during the advancement of the well borings. • Develop and sample the newly installed groundwater monitoring wells. • Survey the location and elevation of each newly completed soil boring and monitoring well. The proposed scope of work has been subdivided into tasks 1 through 4. All work will be conducted under the direct supervision of a State of California Professional Geologist, and will be conducted in accordance with standards established by the Tri-Regional Board Guidelines for Investigation of Underground Tank Sites (CRWQCB, April 2004) and SJCEHD. A C-57 licensed contractor will perform all drilling and well installation activities. Information regarding field practices and procedures is presented in Appendix A. KABp-Arco for URS\6100\Work Plans\arco6100-workp1an 09-2005.doc