SITE INFORMATION AND CORRESPONDENCE

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u. 110 Sonora, Stockton Page 3 for direct reaction appears to be limited. Although an increase in DO is potentially ' I beneficial to the microbial community,which may assist in the natural attenuation process of the contaminants, it is currently too early to tell. The reduction oxidation potential (redox) of VW-2d has increased, becoming a less reducing (-153 mV) to a more oxidizing environment (-95.1 mV),which is less conducive for anaerobic microbial activities: Anaerobic activities are among the important components of long-term natural attenuation of the contamination. Regardless of the supposed benefits of ozone injection on enhancing natural attenuation, the fact there has been no significant reduction trend in the contaminant " concentrations of VW-2d over the length of operation of the ozone injection system brings in the question of cost-effectivenessin continuing the operation of the ozone system at this t site. CVRWQB agreed with EHD's recommendation to terminate the OZ system to monitor for post-remediation effect beginning November 11, 2005 far a period of one year, with the condition subject to change based on quarterly sampling results. Currently only 3 wells,VW-2d,VW-5d (screened from 33 to 40 feet bsg), and MW-6 t. (screened from 60 to 65 feet bsg) are being sampled on a quarterly basis. At this time, EHD directs you to sample all the historically sampled wells including MW-1, MW-3, MW-5,MW- 6,VW-1,VW-Zd VW-3,AND VW-5d in the next quarter. The next quarterly meeting with the City of Stockton to discuss the status of this site is scheduled for 10:00 a.m.on January 25, 2006 at the offices of EHD. u Donna Heran,REHS, Director Environmental Health Department Jeffrey Wong, Senior REHS Nuel C. Henderson Jr., PG LOP/Site Mitigation Unit LOP/Site Mitigation Unit c: CVRWQCB—James L. L. Barton,PG c: Advanced GeoEnvironmental, Inc. -Bill Little