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THIRD QUARTER 1996 ' GROUNDWATER MONITORING REPORT • USA STATION#110-STOCKTON ' AUGUST, 1996 2.2 Monitoring Well Purging The monitonng wells were purged using a 2-inch diameter, submersible Rediflo groundwater pump, 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(DOT) approved 55-gallon drums and stored on-site prior to disposal by USA To reduce the potential for cross contamination between wells, all purging and samphng 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 80 percent of their initial static water ' levels (SWLs) Groundwater samples were collected from MW-1-P through MW-6-P using a new disposable bailer Samples were placed into the appropriate laboratory prepared containers, using proper sample handling and chain-of-custody(COC)protocol established under applicable SW-846 procedures issued by the USEPA The samples were labeled with the date, time and 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 Analyses I 1 Groundwater samples were analyzed for TPH G and D, BTEX, MTBE and total lead using EPA Methods 8015K 8020 and 239 2, respectively The analyses were conducted by Sierra ' Laboratories, Inc (Sierra) a California State certified laboratory in accordance with state guidelines and EPA protocol 3.0 SUMMARY AND CONCLUSIONS The following sections discuss findings from the groundwater gauging and sampling activities conducted on July 19, 1996 3.1 Groundwater Flow and Gradient Groundwater data collected at the site indicates that depths to groundwater range from 67 31 to ' 68 23-feet below ground surface(bgs) The groundwater elevations range from 42 62 to 4187- feet below mean sea level (MSL) 2