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Mr. Maurice Benson <br /> February 26, 2010 <br /> Page 4 <br /> potable wells on private residences. While the report proposes reductions in the <br /> sampling frequencies, there is no evaluation presented on the impact to human health <br /> or safety considerations for implementing the reduced sampling. Additionally, the <br /> Warning Levels are equivalent to the MCL for many of the contaminants, which does <br /> not allow for any advanced notice to the agencies until the contaminant concentrations <br /> are at the MCL level or higher. The Plan should consider either reducing the Warning <br /> Levels to one-half the MCL where equivalent, or incorporate Public Health Goals as <br /> Warning Levels for increased protectiveness, monitoring, and reporting. <br /> The concept of using guard wells as an early warning system is an acceptable <br /> approach, but is best applied when the plumes' flow paths are well characterized and <br /> documented, and when there is representative monitoring from guard wells up-gradient <br /> from the at-risk potable wells. Typically, at least three different lines of evidence should <br /> be used to determine plume stability and guard well protectiveness. Accordingly, <br /> DESJC should apply the three-dimensional groundwater model predictions as another <br /> tool to help determine whether plume migration is likely or occurring, and if the guard <br /> wells are providing an adequate "early warning." <br /> DTSC recommends that DESJC provide the regulatory agencies with the <br /> comprehensive hydrologic analysis demonstrating that the guard well cluster locations <br /> are appropriately screened in the specific hydrologic zones represented by the PWs. <br /> The hydrologic analysis would allow the regulatory stakeholders to individually assess <br /> the current monitoring well/guard well locations to help gain a higher degree of comfort <br /> with the proposed monitoring network, but also allow feedback for areas where <br /> monitoring coverage is deemed deficient for the protection of down-gradient potable <br /> wells. <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.aov <br /> Sincerely, <br /> �O /,/4?1 ;;/� <br /> Peter MacNicholl, P.E. <br /> Remedial Project Manager <br /> Brownsfield and San Joaquin Cleanup Program <br /> cc: See next page. <br />