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