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Mr. Maurice Benson <br /> February 11, 2010 <br /> Page 2 <br /> eliminated completely from the DDJC-Sharpe groundwater monitoring program. <br /> According to DESJC staff, the rationale for sampling reductions was to reduce <br /> operational and maintenance costs and liability associated with sampling private <br /> residences potable wells. Although the report proposes significant reductions in <br /> the off-site potable wells there is no evaluation presented on the impact to human <br /> health or safety considerations with implementing the reduced sampling. <br /> Moreover, DESJC has not presented any comprehensive analysis determining <br /> the placement of the guard well cluster relative to the contaminant of concern <br /> (COC) flow paths and whether these wells are screened in the appropriate <br /> hydrologic zone to detect and monitor off-site COC groundwater plumes. In <br /> many cases off-site potable well construction logs do not exist or have not been <br /> procured by DESJC or the County, making it difficult to ensure that the proposed <br /> guard wells are providing a high degree of comparability. <br /> DTSC is not supportive of the proposed reductions in off-site potable well <br /> sampling frequencies for the following reasons: uncertainties in protectiveness <br /> with guard wells location and screen intervals; seasonal groundwater level <br /> fluctuations and flow directions; unknown and variable potable well screen <br /> intervals; and, the likelihood of unknown hydraulic pathways or imbedded <br /> channels in the soil matrix. <br /> DTSC and the regulatory agencies should be provided with a thorough and <br /> comprehensive hydrologic analysis identifying how the guard well clusters in <br /> combination with the reduced sampling frequencies provide a high degree of <br /> protectiveness with the discrepancies noted above. DESJC should consider a <br /> more phased approach for sampling reductions in off-site potable wells along <br /> with multiple lines of evidence that the proposed actions are protective of human <br /> health. <br /> 2.) Page 5-8, section 5.5.4, Recommendations: The section states that <br /> potentiometric surfaces and capture zones are generally consistent throughout <br /> the calendar year even though groundwater levels change between wet and dry <br /> seasons. This statement taken in consideration with the physics of hydrology <br /> would lead one to believe that if water levels were changing this would <br /> correspond to a change in hydraulic gradients and potentially the groundwater <br /> flow directions. The report suggests that the groundwater flow directions do not <br /> change much with seasonal fluctuations in water level which doesn't seem to be <br /> true. Seasonal variability in water levels does seem to dictate groundwater flow <br /> directions at DDJC-Sharpe and should be more clearly identified in the report. <br /> 3.) Page 5-1, section 5.2.1: A statement of"COC plumes in the A, B, and C zones <br /> appear to be contained by extraction and natural attenuation processes..." <br />