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• • Page 3 GeologicalTeckxicslnc. Geoprobe Investigation Project No. 425.2 September 18,2008 GTI per San Joaquin County Environmental Health Department (SJCEHD) directive submitted a work plan for two sentinel wells installation at the site to protect the domestic wells from contamination on September 26, 2007. SJCEHD approved the work plan on January 16, 2008 and the two wells were installed on June 25 and 26, 2008. Geological Technics Inc. (GTI) submitted the remedial action plan: Air Sparging, Soil Vapor Extraction and Oxygen Releasing Compound Application, on November 29, 2007 followed by 3 addenda dated February 5, 25 and March 19, 2008. San Joaquin County Environmental Health Department (SJCEHD) approved the air sparging and vapor extraction portion of the remedial action plan subject to positive results from a limited 2-week pilot test in their correspondence dated April 30, 2008. The limited 2-week pilot test is outlined in the addendum prepared by GTI dated August 13, 2008. However, the ORC injection design and application at the site per SJCEHD directive requires further site investigation to clarify the horizontal and vertical extent of the plume toward northern portion of the site (under Durham Ferry Road). The present work plan details the geo-probe investigation to better understand the vertical and horizontal extent of the plume on the north side of the site. 1.3 Site Geology and Hydrogeology The following section discusses the geology and hydrogeology as documented during this investigation based on the interpretations of the field geologists: • Figure 4A shows the cross-sections of the site geology. • The cross sections indicate that the site geology consists of thin layers/lenses of different soil types. • Most of the contamination plume exists in the shallow well zone (screen intervals 5 feet to 22 feet bgs). • The site geology is heterogeneous for this site. There is different depth versus soil type divisions for every cross-section. However, typically the site geology appears as sand or silty-sand units that are sandwiched between finer-grained clay units. This may be the reason that contamination has not moved to the deep zones. • Depths to groundwater are typically around 12 feet bgs. • Groundwater direction has ranged from northeast to northwest with a greater trend to the east in the shallow zone (Zone A), and to the northwest in the deeper zone (Zone B) in the last five quarterly monitoring events. • The average gradient is 0.0013 ft/ft in Zone A and 0.0032 in Zone B. • Vertical gradient between shallow and deep well pairs have been predominantly positive in the quarterly monitoring events. • Soil samples (in the bench scale sampling) have been analyzed for geotechnical data that are summarized in the following table: