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First Quarter 1999 Groundwater Monitoring Results Introduction This report summarizes results of the first quarter 1999 groundwater monitoring conducted on February 11, 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 31 times and groundwater samples have been collected up to 26 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,-713,-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 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 September 1991. Figure 2 graphically presents the variation in flow directions at this site since September 1991. Minor changes in elevations of the monitoring wells have occurred due to settling of the wells. Therefore, the elevations of the top of casing of all monitoring wells at the Disco site were resurveyed in 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 elevation measurements were not recorded for piezometers NP-1-70',-80',or -90' during the November, 11, 1998 or February 11, 1999 monitoring events due to the recently installed dedicated Waterra purging/sampling system. Due to the small diameter of the piezometers, there was not sufficient room for both the Waterra sample tubing and the water level sounder. The Waterra sampling system was recommended for these 1-inch diameter piezometers by San Joaquin County Public Health Services/Environmental Health Division (SJC PHS/EHD)staff. This system provides a marked improvement in the purging and sample collection for these small diameter piezometers. A smaller diameter water level sounder has been purchased that will fit down the piezometers, therefore,water levels in the piezometers will be measured in all future groundwater monitoring events,Please note, nested piezometers NP-1-70',-80',and -90'were installed to monitor water levels during the operation of the AS/SVE system,and were not intended or designed for collection of r groundwater samples. i