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Mc Alex Schatz <br />Mazch 3Q 2007 <br />Page 6 of 7 <br />The shape of the drawdown contours due to mounding and shadowing (Figure 7) is <br />reflected in the drawdown contours produced from mining at Cells 7 and 8 (Figures 8 and 9). <br />The alluvium generally thins as you move left from Fountain Creek across the model domain. <br />This is likely why the dewatering affects a large area on the ]eft of the model. As there do not <br />appear to be wells west of the site, these drawdowns do not appeaz to pose awell-related concern <br />at this time. <br />Note that the predicted drawdown contours and water table elevations reflect the <br />dewatering effects of the gravel pits alone and not the actual head contours at any given point. <br />Pumping of the nearby wells, as well as variations in climate (dry or wet), and changes in river <br />flow and stage conditions will affect water table levels in the area. Similarly, evaporative effects <br />of the filled, unlined mined pits were not modeled, but are considered minimal compared with <br />the dewatering effects of the active gravel pit. The model, as constructed, is classified as a <br />"difference" model reflecting only the impacts to the system resulting from the dewatered pits. <br />This is an accepted methodology for such determinations, especially, and as is the case in this <br />study, where limited or no data are available on other stresses known to be affecting the system. <br />PIEZOMETER LOCATIONS <br />Based on the modeling results and Lafarge's goal of reducing impacts to the off-site well <br />users, we recommend that at least four piezometers are monitored before and during the <br />dewatering and mining operations. Baseline pre-mining data should be collected and compared <br />to data collected during operations to quantify the impact and verify the site is responding as <br />predicted by the model. We recommend piezometers in the locations presented on Figure 11. <br />Installation of PZ-3 may not be necessazy if water levels in We119 are monitored instead (verify <br />the well is actually on the property, is no longer being used, and that construction information is <br />available). <br />SUMMARY 8r CONCLUSIONS <br />The dewatering of the Sundance gravel pits will have localized effects on the water table <br />in the alluvium. As previously stated, model-predicted drawdown in the nearby wells ranged <br />from 1 foot to approximately 5 feet (Tables 1 and 2). Given that the saturated thickness of the <br />alluvium in the area is fairly low, the wells south of the site could be significantly impacted and <br />their ability to continue to pump as in the past could decrease. If well effects are noticed south of <br />the site, the recharge system should be modified to minimize the impacts to off-site wells. We <br />again stress the lack of empirical data the level of interconnection between the flowing stream <br />and the alluvial aquifer. As the modeling strongly indicates high sensitivity to this <br />interconnection (especially with respect to drawdown predictions for the areas east of the creek), <br />we reiterate our recommendation that further field testing be carried out to develop defensible <br />data on the nature of this interconnection and the model be recalibrated and further predictive <br />runs to either validate the current predictions or to develop new drawdown predictions as <br />appropriate. <br />5:536 -Banks and Gesso, LLC~536.4 - Sundance Pit~Report~Final Letter Report.doc <br />MaAin and Wood Water Consultants, Inc. <br /> <br />