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I l i ECEIVED Quarterly Groundwater Monitoring Results JUN 171999 This report summarizes results of the quarterly groundwater monitoringEeNMW EWOU-ltIA TH { 1999 at the Disco property in Stockton, California.The results include wate MMa�@EVMtEr jj laboratory analytical results. Water level measurements and groundwater sample collection were performed by Del Tech Geotechnical Services of Oakdale,California. Field data sheets, chain-of-custody and laboratory analysis reports for this sampling event are included as attachments to this summary. Figure I 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 February 1999 are summarized in Table 1. Shallow groundwater elevation contours for the February 1999 sampling event are presented on Figure 1.Table 2 presents the groundwater gradient _ direction and magnitude for each quarter since July 1993.Figure 2 graphically presents the variation iin flow directions at this site since July 1993. The elevation of the top of casing to all monitoring wells at the Disco site were resurveyed in I February 1999. Groundwater elevations measured in all shallow monitoring wells were used to determine the groundwater contours and groundwater flow direction for this sampling event. Groundwater elevations measured in the nested piezometers and the deep wells (MW-DIS-7D and MW-DIS-8D)were not used to interpret the groundwater gradient because those wells are screened in .1 different stratigraphic intervals from the shallow wells. 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. 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. I ' i I , 4i XR-IEP 2.99.000 4 ,