SITE INFORMATION AND CORRESPONDENCE_2004-2014

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Mr. Maurice Benson - 2 - 23 December 2009 Defense Logistics Agency 3. Has the potable well screen interval been determined? (Note: If the potable well screen interval is unknown, it must be assumed that the screen is in the flow path of all up- gradient plumes. If the well construction data are not available, a down-hole video log may help to isolate the plume flow paths of concern.) 4. How far is the .potable well from the leading edge of the plume? 5. Is the guard well screened appropriately to detect COCs that may threaten the potable well? The Draft Reports do not provide the necessary documentation (Items 1 through 4, above) to support any potable well sampling frequency reductions. Potable Well Contingency Plan DESJC's contingency plan proposes a potable well Warning Level concentration for each COC; the Warning Level concentration would be used to indicate when or if additional evaluation or monitoring would be warranted. However, DESJC has not also proposed levels that would trigger evaluation of alternative water supplies or that would trigger water supply replacement. Furthermore, the schedule and actions for replacement of impacted potable well water supply, such as supply of bottled water, wellhead treatment, well replacement, or alternative water supply hookup, is not provided in the contingency plan. In some cases, DESJC's proposes to use maximum contaminant levels (MCLs) as the COC Warning Levels that would trigger additional evaluation or increased monitoring. However, by the time concentrations have reached MCLs, contingencies for water supply replacement should have already been implemented. The Warning Level concentration of a COC should be the detection limit for purposes of reporting (DLR) published by the California Department of Public Health. The DESJC contingency plan must establish COC concentrations that require immediate supply of alternate water for users of potable wells. These concentrations could be called Impacted Levels and used to indicate that immediate water replacement or wellhead treatment is necessary. The Impacted Level concentration should be established by determining risk to well users. Public Health Goals established by the California Environmental Protection Agency, Office of Environmental Health Hazard Assessment, are established in this way. The contingency plan should also set a realistic time schedule for DESJC to supply bottled water, and provide an alternate water source or wellhead treatment. DESJC should also establish contingency concentrations for guard wells that would trigger more frequent monitoring of both guard and potable wells, and would trigger an evaluation study for water supply replacement in the event that the potable well becomes impacted in the future. For example, a COC concentration in a guard well at the Warning Level could trigger increased monitoring of the guard well; a COC concentration in a guard well at the Impacted Level could trigger a water supply replacement evaluation for the potable well.