2022-03-25_HYDROLOGY - M2017036 - Laserfiche WebLink

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Knox Pit SWSP January 19, 2022 Plan ID 5958 Page 2 of 8 be placed around the perimeter of the mine to seal the post-mining pits from the surrounding ground water system. A perimeter drain will be instatled between the compacted liner and the alluvial aquifer concurrent with the placement of the compacted liner to allow for groundwater flow around the site. Upon completion of mining, the site is proposed to be reclaimed to pasture/rangeland. During this plan period, replacement water will be provided pursuant to an agreement with the North Weld County Water District. Water Management You have devetoped a numerical groundwater model, calibrated to water level measurements taken from wells tocated adjacent to and around the proposed Knox Pit, to establish time- and mining-phase-dependent inflow rates, and the relationship between the water level in the Water Management Pond and anticipated groundwater inflows or outflows. The Water Management Pond will effectively act as a recharge pond when receiving dewatering water and/or storm water runoff in excess of the amount needed for operations at the site. Dewatering at the site is anticipated to commence in July 2022 at an initial rate of over 5,000 gpm. The dewatering rate is anticipated to swiftly decrease to approximately 700 gpm during the irrigation season and 400 gpm during the non-irrigation season. Once the compacted backfilt is installed, irrigation season dewatering rates are anticipated to decrease further to approximately 500 gpm. For the purposes of this SWSP, you have used an estimated dewatering rate of 700 gpm for both the irrigation and non-irrigation season. The dewatering discharge from the active mine sump to the Water Management Pond witl be measured using a totalizing flow meter. Storm water falling on the active mine site will be collected in a settling basin and will gravity-flow to the Water Management Pond. A tipping bucket rain gage will be installed onsite to collect daily precipitation data. Storm water inflows into the Water Management Pond must either be directly measured or calculated based on the drainage area and on-site precipitation. Depletions Currently, 2.08 acres of groundwater surtace area are exposed in the Water Management Pond. The amount of surface area is estimated to increase to 3.40 acres when construction of the Water Management Pond recommences, and will reach a final surface area of 4.80 acres after comptetion of construction. Be�inning in June 2022, groundwater will also be exposed in the Reclaim Pond and dewatering trenches. In the first month, you have estimated that the area of groundwater exposed in the Reclaim Pond and dewaterin� trenches will total 35 acres; for every month thereafter the amount of groundwater exposed in the Reclaim Pond and dewatering trenches will total 1.30 acres. You have estimated evaporation from the exposed groundwater surfaces using a gross annual evaporation of 38.28 inches based on the "free water surface" (FWS) evaporation from NOAA Technical Report NWS 33. The applicant may alternately calculate gross evaporation at the site based on the local evapotranspiration (ET) data pubtished by the Northern Colorado Water Conservancy District for the Fort Collins Central (228) weather station when such data is available. The reported ET for alfalfa will be multiplied by the reported daily pan coefficient to catculate pan evaporation. Pan evaporation will be multiplied by 0.7 to calcutate pond evaporation. A factor of 0.7 should not be applied to the "free water surface" (FWS) evaporation obtained from NOAA Technical Report NWS 33. For the purposes of this SWSP, you have assumed that no precipitation will occur at the site. Based on the projected operations and assumptions, the depletion due to