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I I I I i r l � M%TC 1117 Lone Palm Ave,Suite 201 rr Modesto,CA 95351 Phone:209-579-2221 Fax: 209-579-2225 4.; In July and December 2003, ATC conducted additional subsurface investigation activities at the site that included cone penetration testing, advancing a soil boring (SB8), .and installation of monitoring wells MW4 through MW7. Petroleum hydrocarbons were detected in soil samples collected from the boreholes of MW5 and MW6, and in the groundwater samples collected from MW4 through MW7. On November 17 and 18, and December 5 and 28, 2005, an ATC geologist supervised the advancement of five soil borings, S139 through SB 13: The borings were advanced for the purposes of evaluating the horizontal extent of petroleum impacted soil and groundwater surrounding the site. Details of these activities are summarized in ATC's Summary Report of Additional Subsurface Investigation,dated July 12, 2006. On September 24 and 25, 2007,an ATC geologist supervised the installation of monitoring wells MW8, MW9, and MW 10. Monitoring well MW 11 was installed on June 8, 2007. Wells MW8 and MW 11 were installed off-site to the northeast and north adjacent to 1139 and 1107 East Harding Way, respectively, to evaluate the extent of petroleum impacted groundwater downgradient of the subject site. These activities are summarized in ATC's Subsurface Investigation Report,dated November 19, 2007. 2.0 DISCUSSION 2.1 Hydrogeology The site is located within the Great Central Valley geomorphic province. Groundwater in the Stockton area is present in Recent alluvium. Groundwater enters the Great Central Valley via surface and subsurface flow and by direct percolation of precipitation, stream flow, and applied water. Replenishment of the groundwater occurs by percolation of precipitation, stream flow, and applied water occurs in the forebay areas where permeable sediments are exposed at ground surface. According to the United States Geological Survey (USGS) 7.5-Minute Series Topographic Map of the Stockton West, California Quadrangle, the site is located at an approximate elevation of 24 feet above mean sea level. The regional surface topographic gradient in the vicinity of the site trends towards the west. The groundwater flow direction beneath the site is generally towards the east-northeast with a r gradient of approximately 0.0039 feet per foot (ft/ft) (as recorded in January 2008). Since 2001, La depth to groundwater from the monitoring well network has varied from approximately 36 to 53 feet bgs. Historical groundwater elevation data are included in Table 1. The subsurface rlithology beneath the study area generally consists of silt with subordinate lenses of sand and {r� clay to depths ranging from the ground surface to approximately 43 to 48 feet bgs. Ls:\environmental\22493\reports\feasibility.doc 3 Ir' -