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Second Quarter 1999 Groundwater Monitoring Results Introduction This report summarizes results of the second quarter 1999 groundwater monitoring conducted on July 15 and 16, 1999 at the Disco property in Stockton,California. Groundwater level measurements and groundwater sample collection were performed by Del Tech Geotechnical Services of Oakdale,California. Groundwater level measurements have been collected 32 times and groundwater samples have been collected up to 27 times at this site since July 1991. Field data sheets,chain-of-custody and laboratory analysis reports for this sampling event are 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. There are currently 6 monitoring wells located upgradient of the former Disco gas station(MW-DIS-S,-6,-7S,-71),-8S,and-8D),3 monitoring wells located cross-gradient(MW-DIS-3,-9,and-10),and 5 wells located within the plume(MW-DIS-1,-2,NP-1-70',-80' and-90'). Currently, there are no monitoring wells located in the downgradient direction of the plume. This monitoring report presents groundwater elevation data,analytical results,conclusions, and recommendations for future sampling. Groundwater Elevation Data Groundwater elevation data for the period between September 1991 and July 1999 are summarized in Table 1.Shallow groundwater elevation contours for the July 1999 sampling event are presented on Figure 1. Table 2 presents the groundwater gradient direction and magnitude and Table 3 presents the calculated vertical gradients for each quarter since t September 1991. Figure 2 graphically presents the variation in flow directions at this site since September 1991. Figure 3 is a hydrograph showing water levels versus time in all monitoring wells. The groundwater flow direction and gradient magnitude for this monitoring event are approximately 150°and 0.0008 ft/ft,respectively.Groundwater elevations measured in ? shallow monitoring wells were used to determine the groundwater contours and groundwater flow direction for this sampling event and are based on the general groundwater contours shown on Figure 1.The groundwater flow direction has been consistently towards the southeast,at an average direction and magnitude of 1431 and 0.0008 ft/ft since October 1995 (see Figure 2).As illustrated in Figure 3,groundwater elevations have risen approximately 32 feet(from 77 ft bgs to 45 ft bgs) since 1991. Groundwater elevations measured in the deep wells MW-DIS-7D and MW-DIS-SD are not used to interpret the horizontal groundwater gradient because these wells are screened in different stratigraphic intervals from the shallow wells. However, these wells are used to calculate vertical gradients using the MW-DIS-7S/7D,and MW-DIS-SS/-8D well pairs.The calculated vertical gradients for this quarter were-0.0005 (upward)and 0.0014 (downward), for MW-DIS-7S/7D,and MW-DIS-8S/8D,respectively.The vertical gradient measured at MW-DIS-7S/7D was consistently downward previous to February 1999,however, it has been upward for the past two events. i i i 1