SITE INFORMATION AND CORRESPONDENCE_2004-2014

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Mr. Maurice Benson December 15, 2009 Page 2 on recommendations in the Treatment Technology Investigation Results Technical Memorandum. The pilot study location was selected based on groundwater sampling results conducted as part of a cone penetrometer (CPT) investigation in 2006, 2007, and 2008. The CPT-investigation detected TCE and its breakdown products, cis-1, 2- dichloroethene and vinyl chloride present in groundwater in the North Balloon. The CPT investigation indicated TCE concentrations up to 420 micrograms per liter (Ng/L) were found in fine-grained layers of the saturated zone between depths of 50 and 70 feet below ground surface (bgs). Site characterization data showed that large portions of the TCE concentrations were present in fine-grained layers. DTSC has reviewed the work plan and has the following comments: General Comments 1.) The results from the radius of injection/influence calculations indicate that the effective ROI in the High Permeability Test Area was lower than the measured ROI for the Low Permeability Test Area. Specifically, the ROI for the High Permeability areas measured approximately 8 feet, with low permeability locations resulting in a 10 foot ROI. The results of the two permeability tests are counter intuitive in the sense that the lower permeability units should have demonstrated a lower ROI in comparison to the higher permeability units. The report does not discuss this discrepancy, leaving one to conclude that perhaps the High Permeability ROI was solely based on the existing monitoring well (MW) MW308 and the fact that there may have been under-sampling of the radial distances away from the injection points, missing the actual distance the oil substrate traveled under injection. 2.) EOS concentrations decreased significantly during the 12-month monitoring event. EOS concentrations decreased to less than 20 mg/L indicating that the EOS was successful in enhancing the reductive dechlorination of TCE and its breakdown products. TCE rebound was noted during the 12-month monitoring event, acknowledging that additional EOS injections may be necessary as TCE desorbs and diffuses from the tighter grain formations where EOS was distributed. 3.) The reductive dechlorination process created by the injection of EOS in the subsurface appears to have increased concentrations of iron, dissolved manganese and arsenic in saturated sub-surface, more noticeably in the lower permeability areas of monitoring wells MW305AB1, and MW306AB. High permeability areas also had observed increases in iron, manganese and arsenic.