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CG <br />July 1, 2010 <br />Mr. Garrett Varra <br />Varra Companies, Inc. <br />8120 Gage Street <br />Frederick, Colorado 80203 <br />Re: Water Impoundment Hydrology <br />CGRS No. 1-135-10807ab <br />Dear Garrett: <br />ENVIRONMENTAL 9MRWD@R9 <br />This letter and attachments address comments presented by Colorado Division <br />of Reclamation, Mining and Safety regarding mounding and shadowing related to <br />water impoundments. <br />Any impoundment or pond that holds water at an elevation above the <br />surrounding water table will have the potential of increasing the water table <br />elevation (mounding) at some distance from the pond, which is determined by <br />the total groundwater-pond head difference. For ponds that are large in areal <br />extent the distance to no influence on the upstream side is usually less than the <br />downstream side as the head difference will be less based on the water table's <br />hydraulic gradient. The influence imposed by the pond can be estimated by one <br />dimensional unconfined flow solutions presented by McWhorter and Sunada, <br />1977. Numerical models can be used but we have found model results generally <br />agree well with analytical solutions. An example calculation using a one <br />dimensional unconfined flow equation is presented as Attachment A. <br />In most unconfined flow regimes we consider the distance to no influence (or <br />measurable influence) generally to be on the order of 600 feet. For your <br />operations it appears the pond and groundwater head difference is on the order <br />of five feet. Given this scenario at 100 and 300 feet from the pond the resulting <br />mounding would be four and two feet, respectively. We advise that great care be <br />given when constructing water impoundments (lined or unlined) within 100 feet <br />of any subsurface structures. <br />We have evaluated the affects of impermeable barriers on groundwater <br />hydrology by using the analytical model TWODAN. We simulated a uniform flow <br />field using aquifer properties typically associated with sand and gravel deposits. <br />We then simulated an impermeable barrier within the flow field. The results <br />indicate that the groundwater hydrology is modified but only severely in very <br />close proximity to the barrier. Upstream of the barrier the hydraulic gradient will <br />increase and will decrease downstream of the barrier. Hydraulic gradient <br />changes of between one and two feet are observed within 15 feet of the barrier <br />wall but are negligible at any sigriificant distance from the hydraulic structure. A <br />P.O. Box 1489 Fort Collins, CO 80522 T 800-288-2657 F 970-493-7986 www.cgrs.com