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Mr. Alex Schatz <br />Mazch 27, 2006 <br />Page 5 of 6 <br />calculated as the difference between the effect on the river with the gravel pits simulated and <br />dewatered, less the natural water balance of the river when modeled without any pits. The effects <br />(depletions) of mining any given pit range from 87 acre-feet per day to 112 acre-feet per day. As is <br />expected, Pits 1 and 3 have the greater effect on the river due to their proximity to the river. These <br />results are interpreted as relatively significant impacts that could have equally significant water <br />rights implications. <br />Note that the predicted drawdown contours and water table elevations reflect the dewatering <br />effects of the gravel pits alone and not the actual head contours at any given point. Pumping of the <br />nearby wells, as well as variations in climate (dry or wet), and changes in river flow and stage <br />conditions will also affect water table levels in the area. Similarly, evaporative effects of the filled, <br />unlined mined pits were not modeled, but would be minimal compazed with the dewatering effects <br />of the active gravel pit. The model, as constructed, is classified as a "difference" model reflecting <br />only the impacts to the system resulting from the dewatered pits. This is an accepted methodology <br />for such determinations, especially, and as is the case in this study, where limited or no data are <br />available on other stresses known to be affecting the system. <br />SUMMARY & CONCLUSIONS <br />The dewatering of the four gravel pits in the Silt area will have localized effects on the water <br />table in the alluvium as well as on the flow in the Colorado River. <br />As previously stated, model-predicted drawdown in the nearby wells ranged from 1.0 feet to <br />approximately 16.5 feet. The depths of the wells in the area generally range from 58 to 190 feet, <br />with one well measuring only a 15-foot depth. Given these depths and the shallow water table in <br />the area, the relatively small amounts of predicted drawdown due to dewatering should not prevent <br />the majority of the deeper wells from continuing to pump as in the past, though pumping costs may <br />increase slightly. For those wells incurring greater levels of drawdown, especially those reported to <br />be shallower, there maybe significant impacts on their ability to continue to pump as in the past. <br />The depletive effects of the mining operations on the Colorado River will require a <br />substitute water supply plan from the State Engineer, to ensure replacement of all depletions in <br />time, place and amount sufficient to prevent injury to other water rights. It is possible to use the <br />water drained from the active pit as a replacement water source, but depending on the location of <br />the return, the shape of the cone of depression surrounding the dewatered pit may change. <br />Depletions to the river due to mining operations (dust suppression, moisture loss in material, <br />concrete or asphalt production, evaporation, etc.) will also require augmentation and should be <br />included in any substitute water supply plan request. Please let us know if we can assist you in the <br />preparation of a substitute water supply plan application. <br />Martin and Wood Water Consultants, Inc. <br />