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Quarterly Groundwater Monitoring Results This report summarizes results of the quarterly groundwater monitoring conducted on August 12, 1997,December 22, 1997,April 16,1998, and July 23, 1998 at the Disco property in Stockton,California.The results include water level measurements and laboratory analytical results.Water level measurements and groundwater sample collection were performed by Del Tech of Oakdale,California.Groundwater samples were also collected on May 20, 1998 and analyzed for tert-Butyl alcohol (TBA),because the groundwater samples collecting during the April 16 and 17,1998 sampling event were not analyzed for TBA. Field data sheets,chain-of-custody and laboratory analysis reports for these sampling events have been previously provided,therefore,they are not included as attachments to this summary. Figure 1 shows a map of the Disco property and vicinity.The Disco property is located southeast of the intersection of Myrtle Avenue and Filbert Street in Stockton, California. Groundwater Elevation Data-Groundwater elevation data for the period between September 1991 and August 1998 are summarized in Table 1.Shallow groundwater elevation contours for the August and December, 1997,April,May,and July 1998 sampling events are presented on Figures 2,3,4,5,and 6,respectively.Table 2 presents the groundwater gradient direction and magnitude for each quarter since July 1993. Figure 7 graphically presents the variation in flow directions at this site since July 1993.Groundwater contour maps from previous quarterly monitoring events are also presented in Appendix 1 of this report. Groundwater elevations measured within monitoring well MW-DIS-8S were not used for drawing groundwater contours from August 1997 through July 1998 because they were unusually high. This may be due to changes in the elevation of the top of the well casing,i.e. the well casing may have settled.Monitoring well MW-DIS-3 may have also settled,since recent groundwater elevations measured in this well have also been unusually high. Construction on the street and sidewalk is currently occurring around wells MW-DIS-8S and—8D, and the dirt area surrounding MW-DIS-3 may have become compacted due to heavy truck traffic.Also,groundwater elevations measured in AS/SVE wells and nested piezometers were not used in drawing these gradient maps because these wells have not yet been surveyed.It is recommended that all wells at this site be re-surveyed to ensure that the — groundwater elevation measured in each well is accurate. The groundwater levels have risen about 30 feet at this site since measurements began in September 1991. Groundwater flow directions were west to northwest in 1993 and 1994. Groundwater flow directions were not calculated during 1995 due to SVE system operation (Flow directions were determined in October 1995 when the system was temporarily turned off). The groundwater flow direction has been consistently towards the southeast at a magnitude of approximately 0.001 ft/ft since October 1995(Figure 7). Analytical Results-Analytical results are presented in Tables 3 and 4. Table 3 presents groundwater sample analysis for TPH-gasoline and benzene,toluene,ethyl benzene,and ( xylenes(BTEX).Table 4 presents analytical results for MTBE and other oxygenate compounds. QTA-REP 9.98MC