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i C\77AT � ASS O C I ATE S I N C . • There are apparent discrepancies with site data. For example, UNOCAL reported 1,200 cubic yards were excavated at the site and PHS/EHD believes the volume is more likely to be approximately 300 cubic yards. Groundwater may be continuing to rise in the area of the subject site, which could result in increasing groundwater concentrations due to possible residual soil contamination. • Work on the former UNOCAL site was at a standstill. Additional File Review In order to monitor current activities at the former UNOCAL site and evaluate current site subsurface conditions, ATC submitted a request to PHS/EHD to review the mitigation file for 1665 Pacific Avenue, Stockton, California. The file was made available to ATC for review. Below is a summary of selected points of interest which have occurred since the March 1998 ESA. • On January 4 through 8, 1999 two additional monitoring wells (MW-10 and NP-1) were installed along the eastern portion of the former UNOCAL site. These wells were installed approximately five feet from one another, and completed at different depths, in order to evaluate the hydrologic conditions and the vertical extent of petroleum hydrocarbons beneath the site. Furthermore, an intrinsic bioremediation study was evaluated as to whether the mass of petroleum hydrocarbons beneath the site is being reduced by microbial biodegradation (ARCADIS Geraghty & Miller, February 1999). • Groundwater at the site has been consistently northeast at an approximate gradient of 0.002 ft/ft. The maximum groundwater fluctuation of 17.5 feet was measured in MW-1 between 1990 and 1992 (ARCADIS Geraghty&Miller, February 1999). • UNOCAL's consultant, ARCADIS Geraghty & Miller, reported in-part that: 1) the lateral extent of groundwater impaction has identified however the vertical extent of groundwater impacts has not been precisely defined; 2) vertical extents of soil impacts to the site have been identified and do not exceed below 70 feet bgs; 3) nine years of continuous groundwater monitoring shows that the plume front is stable and has not moved substantially; and 4) the groundwater plume appears to be undergoing microbially mediated biodegradation (ARCADIS Geraghty & Miller, February 1999). • Groundwater samples collected on February 4, 1999 from wells MW-8 and MW- 9, located on the Bank of America property and immediately east respectively, contained detectable concentrations of gasoline constituents. The MW-8 sample W A17292.051sumnk-ayrptdoc 7