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t . THIRD AND FOURTH QUARTER, 1999 GROUNDWATER MONITORING REPORT USA STATION#110 - STOCKTON DECEMBER, 1999 22 Monitoring Well Purging The mointoring wells were purged using a 2-inch diameter, submersible Rediflo groundwater ' pump or disposable baiter, until a minimum of three equivalent well volumes of water were removed from each well Groundwater purged from the wells was placed into Department of Transportation approved 55-gallon drums and stored on-site pnor to disposal by USA To reduce the potential for cross contamination between the wells, all purging and sampling equipment was washed in a trisodium phosphate solution and rinsed in potable water, prior to each usage During the groundwater purging operations, physical parameters such as pH, temperature, and conductivity were monitored and recorded on field data sheets, which are included in Appendix A Groundwater purging was discontinued when the physical parameters appeared to stabilize in the purged groundwater 2.3 Groundwater Sampling Prior to sampling, the wells were allowed to recharge to a minimum of 80 percent of their initial g static water levels Groundwater samples were collected from MW-1, MW-2, MW-1-P through MW-6-P using new disposable bailers The samples were placed into the appropriate laboratory prepared containers, using proper sample handling and chain-of=custody (COC) protocol issued ' by the USEPA The samples were labeled with the date, time, identifying well number, then stored in a cooler at 4° Centigrade or less, and transported to a state certified laboratory under completed COC documentation 2.4 Groundwater Analysis ' Groundwater samples were analyzed for TPH G, BTEX, and MTBE using EPA Methods 8015M and 8020, respectively The analysis were conducted by Sierra Laboratones (Sierra), a California State certified laboratory, in accordance with state guideUnes and EPA protocol ' 3.0 SUMMARY AND CONCLUSIONS The following sections discuss findings from the groundwater gauging and sampling activities conducted on July 7, and October 21, 1999 31 Groundwater Flow and Gradient Groundwater data collected at the site on July 7, 1999 indicates that depths to groundwater ranged from 55 76 to 56 61-feet below ground surface (bgs) for the third quarter gauging event Depths to groundwater on October 21. 1999 ranged from 57 97 to 59 04-feet bgs for the fourth i� quarter gauging event 2 1