SITE INFORMATION AND CORRESPONDENCE

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• Confirm Capture Area — Water levels will continue to be monitored semiannually in all available site wells to confirm that the capture area from the on-site groundwater extraction system is being maintained similar to that predicted by contaminant fate and transport analysis (Parsons, 1999) to prevent additional offsite migration of contaminants. The rationale for these requests and the proposed monitoring schedule are summarized in Table 1. Groundwater Extraction Wells The seven groundwater extraction wells have been monitored voluntarily in order to track chemical mass removal rates during their operation and contribute to understanding of chemical migration in groundwater. Wells GW -1 through GW -6 have been in place for 10 years. Prior to 2003 the extraction wells were sampled bi-monthly. Since 2003 they have been sampled quarterly. GW -7 began operating in April 2006 and has been sampled quarterly since that time. The analytical data indicate clear decreasing concentration trends for GW -1 through GW -6 and likely stabilized at GW -7 for two years (Attachment A). Since October 2007, groundwater extraction has been conducted using extraction wells GW -2R, GW -4, and GW -7. Evaluations of the capture area conducted in each semiannual report show that this pumping configuration effectively intercepts all on-site contamination sources. For efficiency, it is now proposed to sample the groundwater extraction wells semiannually concurrent with the monitoring well sampling events. SUMMARY Subject to CVRWQCB approval, the semiannual groundwater sampling schedule will be implemented in February 2010 and consist of sampling ten monitoring wells as indicated in Table 1. Three monitoring wells will be sampled annually (MW -7, MW -13, and MW -14); the August event will therefore consist of sampling thirteen wells. Water level measurements will be collected in all available monitoring and observation wells on a semiannual schedule and monitoring reports will continue to be submitted on a semiannual basis. The analytical method for groundwater samples will continue to be USEPA Method SW8260B for halogenated volatile organic compounds. Standard treatment system and extraction well O&M inspections and sampling of the GWETS will continue to be performed bi-weekly and reported monthly in compliance with the City of Stockton discharge permit. Results of this O&M reporting will continue to be summarized in the semiannual reports to the CVRWQCB. Please feel free to call me at (925) 941-3723 or Mr. Sergio Morescalchi of Atlantic Richfield at (925) 275-3807 for any clarification. LKJ Page 4 of 6