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Third Quarter 2000 Groundwater Monitoring Results <br /> Introduction <br /> This report summarizes results of the third quarter 2000 groundwater monitoring <br /> conducted on November 15 and 16,2000 at the Disco property,located at 110 North Filbert <br /> Street in Stockton,California. Groundwater level measurements and groundwater sample <br /> collection were performed by Del Tech Geotechnical Services of Oakdale,California. <br /> Groundwater level measurements have been collected 36 times and groundwater samples <br /> have been collected up to 31 times at this site since July 1991. <br /> Field data sheets,chain-of-custody and laboratory analysis reports for this sampling event <br /> are included as attachments to this summary. Figure 1 shows a map of the Disco property <br /> and vicinity.The Disco property is located southeast of the intersection of Myrtle Avenue <br /> and Filbert Street in Stockton,California. There are currently 6 monitoring wells located <br /> upgradient of the former Disco gas station(MW-DIS-5,-6,-7S,-713,-8S,and-8D),3 <br /> monitoring wells located cross-gradient(MW-DIS-3,-9,and-10),and 7 wells located within <br /> the plume(MW-DIS-1,-2,NP-1-70',-80'and 90',SVE-North,and SVE-South). Currently, <br /> there are no monitoring wells located in the downgradient direction of the plume. <br /> This monitoring report presents groundwater elevation data,analytical results,conclusions, <br /> and recommendations for future sampling. <br /> Groundwater Elevation Data <br /> Groundwater elevation data for the period between September 1991 and November 2000 are <br /> summarized in Table 1.Shallow groundwater elevation contours for this sampling event are <br /> presented on Figure 1. Table 2 presents a summary of the groundwater gradient direction <br /> and magnitude and Table 3 presents the calculated vertical gradients for each quarter since <br /> September 1991. Figure 2 graphically presents the variation in flow directions at this site <br /> since September 1991:Figure 3 is a hydrograph showing water levels versus time in all <br /> monitoring wells. <br /> The groundwater flow direction and gradient magnitude for this monitoring event are <br /> approximately 146°and 0.0004 ft/ft,respectively.Groundwater elevations measured in <br /> shallow monitoring wells were used to determine the groundwater contours and <br /> groundwater flow direction for this sampling event and are based on the general <br /> groundwater contours shown on Figure 1.The groundwater flow direction has been <br /> consistently towards the southeast,at an average direction and magnitude of 1431 and <br /> 0.0008 ft/ft since October 1995 (see Figure 2).As illustrated in Figure 3,groundwater <br /> i <br /> elevations have risen approximately 33 feet(froze 77 ft bgs to 44 ft bgs)since 1991. <br /> Groundwater elevations measured in the deep wells MW-DIS-7D and MW-DIS-8D are not <br /> used to interpret the horizontal groundwater gradient because these wells are screened in <br /> different stratigraphic intervals from the shallow wells.However, these wells are used to <br /> calculate vertical gradients using the MW-DIS-7S/7D,and MW-DIS-8S/-8D well pairs. The <br /> calculated vertical gradients for this quarter were 0.0000 and 0.0003 (downward),for <br /> MW-DIS-7S/7D,and MW-DIS-8S/8D,respectively.The vertical gradient measured at <br /> MW-DIS-7S/7D was consistently downward previous to February 1999,however,it was <br /> �. upward for the five events previous to this one. <br /> 1 <br />