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ATC Associates Inc. vv 1117 Lone Palm Avenue, Suite 201 Modesto,California 95351 209-579-2221 ASSOCIATES i N c . fax: 209-579-2225 bgs, and 65 to 75 feet bgs, respectively. Figure 3 is a cross section base map depicting transects lines for A-A' (Figure 4)and B-B' (Figure 5). V HYDROLOGY AND DISSOLVED CONTAMINANT SUMMARY �.• The groundwaterflo direction beneath the site has historically been variable, but generally trended in a norther y direction. The reported groundwater flow direction associated with the last three quarterly groundwater monitoring events conducted in 2007 and the first event in 2008, ranged from northwest to west-southwest/ The groundwater table generally lies between approximately 5 to 10 feet bgs. Groundwater contour maps for June 5, 2007, August 20, 2007, October 31, 2007, and March 27, 2008, are shown as Figures 6, 7, 8, and 9, respectively. A summary of groundwater monitoring data is presented in Table 1. Analytical results of groundwater samples collected from the monitoring wells are shown in Table 2. lur r i GROUNDWATER ZONES Data collected during the 2004 CPT subsurface investigation, in conjunction with the groundwater laboratory results, suggest the possibility of multiple, distinct groundwater zones beneath the site. Cross Sections A — A' and B — B' (Figures 4 and 5, respectively) were developed from the data derived from the 2004 CPT investigation. Using CPT data, it is not always possible to clearly identify a soil type solely on cone bearing, sleeve friction, and pore water pressure, and as such, the lithology depicted in the cross sections is inferred. The CPT data do not provide conclusive evidence as to the precise relationship between lithologic and hydraulic characteristics beneath the site. However, inferences were made based on the data to develop the lithologic cross sections. During the CPT investigation and previous subsurface investigations, groundwater samples were collected from three selected depths of: 5 to 25 feet 41 feet, and 67 feet bgs,'inferred as silts, sands, or gravelly sand units from the CPT logs. Clay and/or silty clay, based on the CPT soil profiles or visual observations, separated each of these 6.0 water bearing zones. However, this does not imply that there are three distinct and ubiquitous water zones (A, B, and C) beneath the site. It could be concluded, that one or more of these identified zones pinches out laterally, or that two or more units may in fact be hydraulically 60 connected. Groundwater Zone Intervals Three possible water-bearing zones have been identified for activities described in this work V plan. Inferred groundwater Zones A, B, and C and their respective approximate depths bgs are depicted on Figures 4 and 5 and described below. • Zone A: from 5 to 25 feet bgs, 5 ATC Associates Inc. S:\Environmental\25847.0037\Work Plan additional vert-hor delineationTH2.doc v