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1991 Annual Report Memorandum -2- 26 June 1992 <br /> DDRW, Sharpe <br /> sampling of monitor wells for those wells that have historically shown non- <br /> detect for aromatic VOCs is acceptable. <br /> b. Because of the recent findings of polychlorinated biphenyls (PCBs) and heavy <br /> metals in ground water above the Primary MCLS, the monitoring frequency for <br /> those wells with these contaminants (specifically, MW-472A and MW-474A) and <br /> surrounding monitoring wells will need to be monitored quarterly until the <br /> lateral and vertical extent of contamination is defined. <br /> c. The proposal for semi-annual and annual sampling does not specify when the <br /> sampling rounds will occur. Semi-annual sampling should correspond to those <br /> quarterly rounds closest to the height of the wet and dry seasons. Annual <br /> sampling may occur either at the height of the wet or dry seasons. The <br /> advantage to conducting annual sampling during the wet season is that this <br /> quarter would tend to be the most quiescent due to the lack of agricultural <br /> supply well pumpage. However, the advantage to sampling during the height <br /> of the dry season is that it would tend to represent worst case with respect <br /> to the hydraulic conditions. We recommend that the annual sampling occur <br /> during the wet season. <br /> 4. The Annual Report proposes that some of the monitor wells do not require <br /> sampling. Because there was inadequate justification in the report for changes <br /> in the monitoring frequency, we cannot concur at this time with the proposal to <br /> drop wells from the monitoring program. Because the contaminant concentrations <br /> and plume configuration will change over time, monitor wells that are currently <br /> not monitored may need monitoring in the future. The Annual Report should have <br /> provided rationale for deleting wells from the monitoring program and should <br /> note those wells that may need to be reincorporated back into the monitoring <br /> program sometime in the future. We concur with Sharpe's proposal (page 4-1) to <br /> continue to annually review wells to determine if a return to the monitoring <br /> program is needed. <br /> 5. The Annual Report did not propose monitoring of any of the off-site agricultural <br /> supply wells. These wells need to be monitored on an on-going basis because of <br /> their detrimental hydraulic affects on the contaminant plume movement. However, <br /> the monitoring of these wells does not need to be done on a quarterly basis as <br /> the wells are normally used only seasonally. Annual monitoring of these wells <br /> may be adequate. We are available to discuss the most optimal quarter for <br /> monitoring of the off-site agricultural supply wells. <br /> 6. One of the off-site private supply wells (707 Roth Road) was reported to <br /> consistently have concentrations of 1,2 dichloroethane above the Primary MCL of <br /> 0.5 µg/1 . This well is located northeast of Plumes 7&8 in the North Balloon <br /> Area. Because the well is not down gradient, it is unnecessary for Sharpe to <br /> supply an alternative source of water at this time. However, if it is <br /> determined that on-site monitor wells which are south of 707 Roth Road are found <br /> to have similar contaminants and concentrations, then Sharpe will be required to <br /> supply an alternative water supply to this location. Sharpe should continue to <br /> monitor this off-site supply well . <br />