Laserfiche WebLink
HAHN WATER RESOURCES, LLC <br /> As the mining plans have been refined, the dewatering concept was modified such that the <br /> dewatering will be accomplished with a series of wells. Whereas the dewatering concept has not <br /> yet been finalized, for purposes of my investigation the concept now includes a series of wells <br /> surrounding the pit. These wells would be augmented with perimeter drains as needed to capture <br /> seepage that may enter the pit in areas at seepage faces, or other areas where the seepage cannot <br /> be intercepted by wells. The perimeter drains would extend a short distance below the floor of <br /> the pit. Locations of the drains cannot be predicted in advance —these would likely be installed <br /> on an as-needed basis as the mining advances and would be removed when no longer needed for <br /> dewatering. In some instances, water collecting on the floor of the mine may be removed with <br /> portable pumps that are repositioned as needed to remove seepage water. Discharge from the <br /> perimeter drains and portable pumps would be combined with discharge from the wells prior to <br /> conveyance to the river. Although the Application identified a single well located near the <br /> western edge of the pit to accomplish the dewatering, the actual dewatering system would likely <br /> consist of multiple wells combined with a perimeter drain. <br /> It is important to note that the dewatering system described herein represents a concept only and <br /> should not be relied upon for design/construction of the dewatering system. The dewatering <br /> concept was developed to allow a "planning-level" investigation of probable water level impacts <br /> and dewatering flow rates. I recommend engaging a dewatering contractor who would make <br /> further investigation of subsurface conditions, design the dewatering system, and refine estimates <br /> of the timing and rates of inflow to be expected during dewatering activities. <br /> Supplemental Findings <br /> Performance of the conceptual dewatering system was evaluated in terms of expected discharge <br /> rates during dewatering and the effects on groundwater levels during the mining period and post- <br /> mining. This investigation made use of the numerical model developed during my initial <br /> investigation, modified to simulate wells rather than a fully penetrating perimeter drain. I also <br /> made use of an analytical model as an independent check on the solution provided by the <br /> numerical model. <br /> My analysis assumed that dewatering would be accomplished with 8 wells approximately evenly <br /> spaced around the perimeter of the mine. I adjusted pumping rates to achieve near-complete <br /> dewatering of the sediments to be mined. I did not attempt to balance the number of wells and <br /> the discharge rates of those wells relative to the efficiency of the dewatering scheme. <br /> The current investigation suggests that dewatering could be accomplished with wells operating at <br /> a combined discharge rate of about 7 cfs or about 3,000 gpm. These results agree with findings <br /> of my earlier investigation. For this analysis, it was assumed that dewatering would be <br /> continuous over an estimated 5-year life of the mine. Figure I shows the predicted water levels at <br /> a location near the western edge of the pit. As seen in this figure, groundwater levels drop <br /> relatively quickly, approaching steady-state water levels in about 6 months. Water levels are <br /> predicted to stabilize in about 12 months. This figure also shows that groundwater levels recover <br /> 2 <br /> 6589 Elaine Road -Evergreen,Colorado 80439 - office 720.242.8639 - cell: 303.870.5757 <br /> hahnwaterresources@gmail.com • www.hahnwaterresources.com <br />