2011-02-24_HYDROLOGY - M1978056 - Laserfiche WebLink

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Cheryl Signs, P.E. Page 2 Durham Pit, DRMS-1978-056 February 14, 2011 (44.6 acres exposed prior to January 1, 1981 and 9.0 exposed after December 31, 1980) of ground water currently exposed at the Durham Pit site, replacement of evaporative depletions is only required for the 9.0 acres exposed after December 31, 1980. The area exposed prior to 1981 is shown on the attached map (outlined in purple and numbered 1 through 11). The exception to the requirement to replace evaporative depletions for the pre-1981 area is tied to the location identified on the map and may not be applied to other areas of ground water exposure within the gravel pit permit boundary. Although some of the pre-1981 ponds identified in the map are currently dry because of the dewatering operations at the site, if the dewatering stops the original pre-1981 ponds will fill, once filled the pre-1981 area will be consistent with the attached map. You have provided a monthly breakdown of the annual depletions made up of 22.7 acre-feet of net evaporative loss, 20.60 acre-feet of water lost with the mined product (which represents 700,000 tons of mined product), 2.76 acre-feet of water used for dust control, and 3.50 acre-feet used for concrete batching. In the previous SWSP approval, phreatophyte credit for pasture grass was credited to this plan, which was based on the 6-feet depth to ground water, pending better site-specific information. The Applicant was required to provide site-specific information regarding the depth to the ground water table at this location. You indicated that no credit for phreatophyte is claimed in this SWSP until the depth to the ground water can be established. Therefore for the purpose of this SWSP the net evaporation from the 9.0 acres was determined based on an effective precipitation credit of 70 percent from the average precipitation. Computation of evaporation under this plan was also reduced during the ice covered period. You have assumed the ice covered period to occur during the months of January and December, based on the average temperatures of 26.0°F for January and 27.4°F for December; taken from the Greeley weather station (053533). However, for the purpose of this SWSP, the Applicant shall replace the net evaporation depletions from the exposed ground water surface area that may occur during the assumed ice covered period (the months of January and December) for any time that the pit is not completely covered by ice. Computation of the net evaporation during any time that the pit is not completely covered by ice shall be determined as the pro-rata amount of the monthly gross evaporation rate distribution amount identified in the State Engineer's General Guidelines for Substitute Supply Plans for Sand and Gravel Pits, subtracting the pro-rata amount of the effective precipitation for that period. The monthly depletions to the Cache La Poudre River were lagged from the pit site using the AWAS program developed by the IDS Group at Colorado State University. The parameters used in the model were an estimated distance from the exposed water surface area to the Cache La Poudre River (X) of 150 feet, an estimated distance from the site to the no flow aquifer boundary (W, measured perpendicular to the point of impact) of 2,600 feet, an aquifer transmissivity (T) of 100,000 gallons per day per foot, and a specific yield (S) of 0.2. According to the Applicant, the distance from the surface of the exposed ground water area to the Cache La Poudre River was measured based on the down gradient extent of the exposed ground water surface (nearest point). For purposes of this SWSP, the method used to measure the distance to estimate the lagged depletions will be accepted, however any future SWSP must measure the distance from the exposed ground water surface area to the Cache La Poudre River based on the centroid of the exposed ground water surface. The net depletions from the Durham Pit site during the period of this SWSP are 49.56 acre-feet.