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SECOND QUARTER, 1999 r GROUNDWATER MONITORING REPORT USA STATION#110-STOCKTON JUNE, 1999 2.2 - Monitoring Well Purging The monitoring wells were purged using a 2-inch diameter, submersible Rediflo groundwater pump or disposable bailer, 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 prior to disposal by USA To reduce the potential for cross contamination between the wells, all purging and sampling equipment were_washed_m a_tnsodium_phosphate_solution and-rinsed-in-potable_water;prior---to each usage Dunng the groundwater purging operations, physical parameters such as pH and conductivity were monitored and recorded on field data sheets, which are included in Appendix A Temperature was not measured this quarter due to equipment malfunction 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 muumurn of 80 percent of their initial static water levels Groundwater samples were collected from MW-1, MW-2, MW-1-P through MW-4-P, and MW-6-P using new disposable bailers The samples were placed into the appropriate laboratory prepared containers, using proper sample handling and cham-of-custody (COC) protocol issued by the USEPA The samples were labeled with the date, time, identifymg i 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 Groundwater samples were analyzed for TPH G and BTEX using EPA Methods 8015M and 8020, respectively The analyses were conducted by Sierra Laboratories (Sierra), a California State certified laboratory, in accordance with state guidelines and EPA protocol 3.0 SUMMARY AND CONCLUSIONS r The following sections discuss findings from the groundwater gauging and sampling activities — conducted ori April 30;-1999-� - -- - - -- --- -- - - - - — ---- -3.1 -Groundwater-Flow-and Gradient- Groundwater radient Groundwater data collected at the site indicates that depths to groundwater ranged from 55 08 to 56 16-feet below ground surface (bgs) The groundwater elevations ranged from 30 03 to 30 55- feet below mean sea level 2