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Mr. Maurice Benson <br /> December 15, 2009 <br /> Page 3 <br /> Arsenic concentrations at MW307AB jumped from 12 pg/L during injection to 120 <br /> pg/L six months later, with the 12-month performance sampling indicating a <br /> concentration of 110 pg/L. MW308AB, located approximately 8 feet from the <br /> injection well showed a baseline concentration for arsenic of approximately 8pg/L <br /> but increased to a high of 28 pg/L during the 3-month sampling event. The 12- <br /> month sampling event detected a remaining dissolved arsenic concentration of <br /> 17 pg/L, more than twice the baseline concentration. <br /> Conclusions <br /> EOS injections in the North Balloon appear to have successfully reduced VOC <br /> concentrations in the saturated sub-surface through injection into the test area. The <br /> decreasing concentrations of EOS and TCE coupled with increases in dechlorination <br /> products and methane in the test area indicate that EOS was successful in enhancing <br /> the reductive dechlorination process for TCE and its breakdown products. TCE <br /> rebound was noted during the 12-month sampling events indicating that additional EOS <br /> injections may be necessary to remediate the remaining VOC mass as it desorbs and <br /> defuses from the fine-grained soil matrix in areas where EOS was not distributed. <br /> Modest to significant increases in metals were observed after injection of EOS for iron, <br /> dissolved manganese, and dissolved arsenic. The increasing concentration trends for <br /> dissolved arsenic could potentially be an issue in itself considering the beneficial <br /> drinking water uses of the aquifer and the Federal arsenic standard of 10 pg/L. <br /> If you have any questions or comments on this letter, please feel free to contact me at <br /> (916) 255-3713 or e-mail at pmacnich(a)dtsc.ca.gov <br /> Sincerely,, <br /> � e <br /> Peter MacNicholl, P.E. <br /> Remedial Project Manager <br /> Brownsfield and San Joaquin Cleanup Program <br /> cc: Mr. Mike Thomas <br /> URS Corporation <br /> Crown Corporate Center <br /> 2870 Gateway Oaks Drive, Suite 300 <br /> Sacramento, California 95833 <br />