SITE INFORMATION AND CORRESPONDENCE

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i • Mr. Maurice Benson February 26, 2010 Page 2 General Comments 1.) The potable well Contingency Plan proposes significant reductions in sampling frequencies for off-site potable wells. The basis for the significant reductions in sampling for off-site potable wells is to reduce operation and maintenance costs and to minimize personal liability from sampling private residences' potable wells. Off-site guard well clusters are proposed to be installed in flow paths located between known COC plumes and off-site PWs. The guard well clusters are proposed to be sampled at least twice per year to help provide an early warning if COC plumes migrate further off-site. DTSC is unaware of any comprehensive analysis determining the guard well cluster locations relative to the known COC plume pathways and if the guard wells are appropriately screened in the specific hydrologic zones represented by the PWs. In many cases PW logs do not exist or have not been procured by DESJC or the County, making it difficult to assure that the guard wells provide a high degree of comparability. Due to the uncertainties in representative guard well location and screen intervals, uncertainties and variability in off-site potable well intervals, and the likelihood of unknown preferential groundwater pathways, DTSC is not supportive of the significant reduction in sampling frequencies proposed in the Contingency Plan. DTSC requests that DESJC provide a more comprehensive analysis identifying how the proposed off-site guard well locations and screen intervals are representative of the off-site potable wells known to be in the COC plume flow path. Specific Comments 1.) Page ES-1, ES.3: The report should be clear in the text (excluding the small print underneath Table ES-1) that the Warning Levels for three of the four constituents are identical to the federal maximum contaminant level (MCL) and provide no protection or response actions for receptors until the MCL is achieved. 2.) Page 2-1, Table 2-1, Warning Levels: Similar to comment #1, the stakeholders should discuss the concept of lowering the warning level concentrations for COCs with identical warning and MCL values. DTSC recommends reducing the Warning Levels to one-half the MCL as a more stringent standard that truly provides protection to human health and creates increased response efforts by DESJC and notification to the regulatory agencies. Reducing the warning levels to one-half the MCL when applicable is also consistent with the agreements made on the January 28, 2010 meeting regarding the draft DDJC-Sharpe Off-Depot Potable Well Contingency Plan.