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Mr. Maurice Benson <br /> February 11, 2010 <br /> Page 5 <br /> down-gradient potable wells as no formal evaluation has been presented to the <br /> regulatory agencies demonstrating the safety or protectiveness with the proposed <br /> actions. <br /> DESJC and the contractor URS should provide a more concise depiction in the Annual <br /> Progress Report of the contaminant distributions in groundwater at DDJC-Sharpe and <br /> not present the biased interpretation of data by suggesting contaminant plumes are fully <br /> captured, when data shows incomplete capture west of the Central Area base boundary <br /> and to the northwest. Moreover, off-site monitoring well (MW) cluster MW537C, located <br /> west of the Central Area boundary is showing the highest trichloroethene (TCE) <br /> concentration recorded in an off-site well at 412 pg/L. The off-site monitoring network <br /> results combined with recent Hydropunch® data shows that TCE concentrations above <br /> 25 pg/L in groundwater is 1,000 feet farther north than previous data identified, implying <br /> that the off-site groundwater plumes have migrated further off-site while the DDJC- <br /> Sharpe extraction well network was operating. These data support the lack of <br /> containment determination for DDJC-Sharpe off-site groundwater contaminant plumes. <br /> DTSC recommends that DESJC consider corrective action activities to replace the <br /> DDJC-Sharpe operating water supply wells or decommission the wells responsible for <br /> pulling the North Balloon TCE plume both eastward and downward. DDJC-Sharpe's <br /> potable water supply wells are responsible for pulling the North Balloon TCE plume into <br /> deeper uncontaminated aquifers as a result of the vertical downward hydraulic gradients <br /> caused by the well(s) operations. DESJC needs to evaluate the necessary criteria for <br /> monitored natural attenuation to be considered as a viable component to the pump and <br /> treat remedy. Specifically, USAF MNA Guidance should be consulted prior to <br /> implementing and adopting MNA as a component of the selected remedy. In order to <br /> meet the MNA determination, plume stability in the absence of pumping must be shown, <br /> which is contradictory to DDJC-Sharpe with 15 operating extraction wells in order to <br /> create a hydraulic influence to achieve plume capture. <br /> If you have any questions or comments on this letter, please feel free to contact me at <br /> (916) 255-6514 or e-mail at pmacnich(a)dtsc.ca.gov. <br /> Si erely, <br /> eter acNicholl, P.E. <br /> Remedial Project Manager <br /> cc: See next page. <br />