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Workplan for Vertical Groundwater Investigation And Bench Test for In-Situ Oxidation 7835 Thornton Road, Stockton, CA Page 3 of 8 feet bgs. Groundwater has been as high as 29.5-31.5 feet bgs in May 2000. Groundwater beneath the site generally flows to the east with an average gradient ranging of approximately 12 to 20 feet per mile. The groundwater gradient calculated from data collected in November 2005 is depicted on Figure 8. Residual Hydrocarbons in Soil Analytical results of soil samples collected during subsurface investigations to date indicated gasoline hydrocarbon impacted soil was initially present in the vicinity of the former underground fuel storage tanks and fuel dispensers from approximately five feet below grade to the water table (at least 55 feet below grade). Subsequent excavation activities removed impacted soils to a depth of approximately 35 to 40 feet below grade, the maximum practical depth for excavation. Laboratory analyses of confirmation soil samples collected from the excavation indicated that soil contaminants were removed laterally, but impacted soils remained in the interval from approximately 35 feet bgs to at least 45 feet bgs in borings SB 1, SB 12, and SB 14 and to 50 feet bgs in boring MW5. The analytical results of soil samples collected during drilling of the deep borings in 1998 suggest that very low levels of gasoline constituents may persist to a depth of at least 98 feet bgs. Soil contamination appeared to attenuate significantly with depth, however, as the highest hydrocarbon concentrations in the soil were detected between 35 and 55 feet bgs, which corresponds to the zone of historic groundwater fluctuation and consists predominantly of fine- grained soils such as clays with some silts and clayey sand. An estimated 1,600 pounds of residual gasoline remains in saturated soil, approximately 1,500 pounds of which resides between the depths of approximately 35 and 55 feet. Attachment 2 contains a table summarizing calculations for residual gasoline remaining in soil. Dissolved Hydrocarbons in Groundwater Subjective analyses of groundwater in site monitoring wells indicated that no wells exhibited any free-floating hydrocarbons since December 1990 (well MW5), but petroleum product sheen was noted in December 1993. Analytical results of groundwater samples collected from site wells in 2005 indicate that TPHg and benzene is present in two site wells. Consistently low levels of dissolved hydrocarbon constituents have been detected in well MW1, located at the edge of the plume. Elevated levels of dissolved hydrocarbon constituents persist in well MW5, located approximately 30 northeast of the location of the nearest former product dispenser. Historical groundwater analytical results are presented in Attachment 1. Based upon the extent of dissolved petroleum hydrocarbons in groundwater beneath the site (November 2005) and assuming a soil porosity of 25% within the saturated zone (35-55 feet bgs), the mass of dissolved gasoline in groundwater beneath the site was calculated to be approximately 22 pounds. Attachment 2 contains a table summarizing calculations for residual gasoline remaining in groundwater. G:\GROLTNDZE\KNOWLES.HAM\REPORTS\WKPLAN—deepwell_bench2.doc