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' GEOMATFIIX Consultants (Weston), also on behalf of PureGro, removed two 1,000-gallon underground gasoline storage tanks located along the southeastern side of the warehouse. Four soil samples collected from the bottom of the excavation showed elevated concentrations of petroleum hydrocarbon constituents. In March 1993, Geological Audit Services (GAS) installed four groundwater monitoring wells, designated MW-I through MW-4, and one additional soil boring in the vicinity of the excavation (Figure 2). GAS continued to monitor wells MW-1 through MW-5 in January 1994, November 1995, and May 1996. ' PureGro installed a sixth shallow groundwater monitoring well, designated MW-6, along the western property boundary in the vicinity of the evaporation ditch; however, the date when this well was installed is unknown. Due to the detection of the volatile organic compounds (VOCs), specifically 1,2- ' dichloropropane(1,2-DCP), in groundwater in the vicinity of the gasoline storage tanks, all past tenants were requested by the RWQCB to investigate the use of this compound. It was found that 1,2-DCP had ' been stored on site and all tenants were directed to conduct a historical site use survey and to develop an investigation program. ' In March 1995, TreaTek-CRAT" conducted an investigation at the Facility on behalf of OCC. Results of the investigation, which will hereafter be referred to as the Initial Site Investigation (Initial SI), were ' presented in the Site Investigation Report and submitted to the RWQCB in May 1995 (TreaTek, 1995). A Secondary Site Investigation (Secondary SI) was conducted between September 1995 and January ' 1996, the results of which were submitted to the RWQCB in May 1996 (TreaTek, 1996b). As part of these investigations, nine additional groundwater monitoring wells (MW-7 through MW-15) were installed at and immediately downgradient of the Facility (Figure 2). In addition, a Supplemental Site ' Investigation (Supplemental SI) was conducted in June 1996 to characterize surface soil at the Facility (TreaTek, 1996d). These data were used to conduct a Human Health Risk Assessment. ' During May and June 1997, Geomatrix conducted an additional subsurface characterization to further ' evaluate subsurface conditions beneath and in the vicinity of the Facility (Geomatrix, 1997b). This investigation included the drilling and sampling of six exploration borings to a depth of about 150 feet below ground surface (bgs) and the installation of four monitoring wells, MW-16 through MW-19 ' (Figure 2). In addition, monitoring well MW-5a was installed as a replacement for damaged well MW-5. Two monitoring wells (MW-16 and MW-17) were installed in an upgradient position to ' evaluate background groundwater conditions. Groundwater has been monitored on a quarterly or trimesterly (three times per year) basis since March ' 1995; the monitoring program currently entails collecting samples from monitoring wells MW-1 through MW-19. The results of these monitoring events were submitted to the RWQCB either as part ' of the aforementioned investigation programs or as quarterly/trimesterly monitoring reports. Geomatrix Consultants has been conducting groundwater monitoring activities for the Facility since June 1996 on ' behalf of GSHI. ' cwaorernc�umoocuNr�xsra>aoc 3