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Well Sampling Frequency -5- 12 July 1991 <br /> DDRW, Sharpe Site <br /> hydraulic data which can be correlated to the analytical data from these <br /> wells. Due to the proximity of this cluster to other monitor wells, <br /> Sharpe should consider deletion of wells such as 429, A-9 and DW-4 <br /> instead of this cluster. <br /> Page 3-9. Table 6 lists those wells recommended for quarterly sampling. In <br /> general , we believe that too many wells are recommended for quarterly <br /> sampling and that the number of well clusters for quarterly sampling may <br /> be reduced. We recommend the following wells be monitored no more <br /> frequently than annually due to the upgradient locations and/or to the <br /> relatively low concentrations of VOCs. <br /> 401 (full cluster) 405A 414A 415A <br /> 429A 447 (full cluster) 452A 459A <br /> 501 (full cluster) 503 (full cluster) 506 (full cluster) <br /> Page 3-10. Table 7 lists those wells recommended for annual sampling. In general , <br /> we believe that many of the private supply wells may not need to be <br /> sampled on an annual basis but may be reduced to a lower monitoring <br /> frequency and/or monitored by request. Those private supply wells which <br /> may not need annual sampling are those wells which are not down gradient <br /> of the plumes, have historically not had VOC contamination and have <br /> monitor wells between the plumes and the private supply well which can <br /> serve as a early warning detection wells. For example those wells west <br /> of 12644 Harlan Road, wells south of 427 Roth Road and wells to the <br /> northwest of AGW-13 technically may not need annual monitoring. <br /> Page 3-12. Table 8 lists those wells recommended for monthly monitoring. We <br /> recommend that some of the wells not be monitored monthly but rather <br /> quarterly due to the well locations relative to the plume and the VOC <br /> concentrations in the well samples. <br /> We recommend that well clusters 420, 447 and 522 be monitored only <br /> quarterly. Monitor well 432A should be monitored only annually. <br /> Monitor well cluster 420 is near the northern edge of the plume and the <br /> concentration of TCE was below the DWS in the January to March 1991 <br /> Quarterly Monitoring Report. Monitor well cluster 447 is upgradient of <br /> the plume and VOCs were not detected in the samples collected during the <br /> last quarterly sampling round. Monitor well cluster 522 is off-site and <br /> is about 700 feet downgradient from the plume. Concentrations of VOCs <br /> in samples from this cluster were less than the DWS in the January to <br /> March 1991 Quarterly Monitoring Report. Because of the low VOC <br /> concentrations in these wells, monthly water quality samples may be <br /> excessive. Samples from monitor well 432A had concentrations of TCE of <br /> below 1 µg/l in the January to March 1991 Quarterly Monitoring Report. <br /> Because this well is in the center of the plume, it is assumed that the <br /> highest concentrations of VOCs may be found deeper than in the A zone. <br /> It is not clear why monthly water quality data would be useful from this <br /> well . <br /> This table includes wells to be monitored monthly in the South Balloon <br /> Area near extraction well C-3. Monitor wells 450C and 451CD are close <br /> to each other to the west of C-3. In the January to March 1991 <br />