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Second and Third Quarter 2001 Groundwater Monitoring <br /> Results <br /> Introduction <br /> This report summarizes results of the Second and Third Quarter 2001 groundwater <br /> monitoring conducted on July 13 and October 17,2001,respectively,at the Disco property, <br /> located at 110 North Filbert Street in Stockton,California. Groundwater level measurements <br /> and sample collection were performed by Del Tech Geotechnical Services of Oakdale, <br /> California.In accordance with the 2001 Groundwater Monitoring Plan,the Second Quarter <br /> 2001 monitoring event consisted of collection of water level measurements only,and <br /> groundwater samples and water level measurements were collected during the Third <br /> Quarter monitoring.event. Groundwater level measurements have been collected 39 times <br /> and groundwater samples have been collected up to 33 times at this site since July 1991. <br /> s Field data sheets and laboratory analysis reports for this sampling event are included as <br /> attachments to this summary. Figure 1 shows a map of the Disco property and vicinity. The. . <br /> Disco property is located southeast of the intersection of Myrtle Avenue and Filbert Street in <br /> Stockton,California. There are currently 6 monitoring wells locatedd-upgradient of the <br /> former Disco gas station(MW-DIS-S,-6,-7S,-7D,-8S,and-8D),,3 monitoring wells located <br /> cross-gradient(MW-DIS-3,-9,and-10),and 7 wells located within the plume(MW-DIS-1, <br /> -2,NP-1-70',-80' and-90',SVE-North,and SVE-South). Five monitoring wells(MW-DIS-11, <br /> -12,-13,-14,and-14D)were installed downgradient of the site in November 2001.An <br /> additional deep monitoring well,MW-DIS-11D is scheduled to be installed in December, <br /> 2001.The downgradient wells were not sampled during this monitoring event. <br /> Groundwater Elevation Data <br /> Groundwater elevation data for the period between September 1991 and October 2001 are <br /> summarized in Table 1.Shallow groundwater elevation contours for July and October 2001 <br /> are presented on Figures 1 and 2.Groundwater elevations measured in shallow monitoring <br /> wells were used to determine the groundwater contours and groundwater flow direction. <br /> Between 1991 and July 1994,the groundwater flow direction was predominantly towards <br /> the northwest at an average groundwater flow direction and magnitude of 312°and <br /> 0.0006 ft/ft.From October 1995,through present,the groundwater flow direction has been <br /> consistently towards the southeast,at an average direction and magnitude of 143°and <br /> 0.0008 ft/ft.The groundwater flow direction has been toward the southeast for the past 18 <br /> sampling events,indicating a stable flow regime. <br /> Table 2 presents a summary of the groundwater gradient direction and magnitude and <br /> Table 3 presents a summary of calculated vertical gradients,based on MW-DIS-7S/7D,and <br /> MW-DIS-8S/8D well pairs.Figure 3 graphically presents the variation in flow directions at <br /> this site since September 1991. Figure 4 is a hydrograph showing water levels versus time in <br /> all monitoring wells. As illustrated in Figure 4,groundwater elevations were on an <br /> increasing trend from October 1992 through June 2000,and have been decreasing slightly <br /> f since then.As of October 2001,water levels are currently approximately 27 feet higher than <br /> ' k they were in 1991. <br /> i <br /> i <br />