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Geological reckkid/a. Page 11 Additional Site Characterization &4"Quarter Groundwater Monitoring Project No. 1030.2 February 20,2004 • Intermediate depth contamination was detected in GP-1, GP-2 GP-3 GP-4 GP-5 GP-6 and SB-1. ' • Intermediate depth contamination between 11 and 14 feet bgs occupies an area of approximately 3,000 square feet or approximately 325 cubic yards (Figure 6) Groundwater • MW-1 shows a marked decrease in all analyzed constituents. The decrease corresponds to changes in groundwater elevation. ' • A plot of TPH-G and Groundwater Elevation versus Time for MW-1 (Chart 1) shows that petroleum hydrocarbon concentrations increase with high groundwater elevations, further validating the results from MW-1. • MW-2, MW-3, MW-101 and the domestic well were non-detect for all analyzed constituents. Increased groundwater elevations do not impact the concentration of contaminants in these wells suggesting that there is no residual LNAPL near these wells. o MW-2, MW-3 and the domestic well have been non-detect for all analyzed ' constituents since January of 1997, the first monitoring event at the site. o MW-101 has been non-detect for all analyzed constituents since installation and initial testing in May of 2002. ' • Trace levels of benzene, ethyl benzene and total xylenes were detected in MW-4. MW- 4 was non-detect for all other analyzed constituents. (� Site Characteristics • The plume is defined vertically by MW-101 and laterally by MW-5 to the north, MW-2 to the southeast and MW-3 to the southwest. ' • MW-4 appears to be located near the leading edge of the plume. • The clay layer at 19 feet bgs provides a significant barrier to vertical plume migration. • Groundwater slope is gradual (<0.005 ft/ft) which impedes the migration of the plume. • Vertical groundwater gradient is historically positive (upward) making vertical migration unlikely. 1 • Measurements of dissolved oxygen in the shallow monitoring wells range between 0.37 and 0.45 ppm, suggesting the site is not favorable to biodegradation. • A gradual groundwater gradient slope, a dominant upward vertical groundwater ' gradient, fine-grained soils, characterized by silts and clays, and a thick clay unit at approximately 19 feet bgs may have a positive contribution to protection of the deeper aquifers and retardation contaminant migration at this site. Recommendations • Using data acquired from the GeoProbe investigation and the installation of the new monitoring well at the site, GTI proposes to correct and resubmit the Feasibility Study I and Corrective Action Plan submitted on June 25, 2003. • After review and adaptations to the June 25, 2003 report are completed, it will be resubmitted to SJC PHS/EHD for consideration. • Continue quarterly groundwater monitoring.