2016-02-02_HYDROLOGY - M2011049 - Laserfiche WebLink

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f Partners Combined SWSP Page 2 of 6 • January 27, 2016 water stored in the Overland Trail Reservoirs pursuant to case no. 00CW251. Depletions Pursuant to § 37-90-137(11)(b), C.R.S. and 2009CW49, a gravel pit operator or property owner does not need to replace depletions that occur due to evaporation from ground water exposed prior to January 1, 1981 as a result of open mining of sand and gravel ("pre-81 areas"), regardless of whether mining continued after December 31, 1980. Per our "General Guidelines for Substitute Water Supply Plans for Sand and Gravel Pits" updated April 1, 2011, pre-81 areas are tied to the physical location at which the groundwater was exposed prior to January 1, 1981 with the exception for areas whose reallocation was approved by the State Engineer prior to January 1, 2011. Previous SWSPs (prior to January 1, 2011) approved the pre-81 area without specific mention of its location. Therefore, the State Engineer's Office allowed the pre-81 area to be re-allocated and memorialized under the May 11, 2011 SWSP approval. The applicant provided a map (Figure 1 - Home Office Pit) showing the specific location of the pre-81 area, which includes the 22.1 acres of ground water exposed in Lamb Lake A. The credits for the pre-81 area are tied to the location identified on Figure 1 - Home Office Pit and may not be re-allocated to other areas of ground water exposure within the gravel pit boundary. Evaporative depletions for the remaining lakes were calculated using a gross annual evaporation of 38 inches, with a credit of 10.58 inches for effective precipitation (based on an average annual precipitation of 15.12 inches for the Fort Collins weather station). You have estimated the surface area of Treiber Lake B as 25.6 acres which results in a net evaporative depletion of 58.49 acre-feet for this plan period. You have estimated the surface area of ground water exposed in the Lamb Lakes after December 31, 1980 as 61.8 acres which results in a net evaporative depletion of 141.19 for this plan period. You have assumed that no ground water will be exposed in Treiber Lake B after completion of the slurry wall by the end of May, 2016. Lagged depletions resulting from exposure of ground water in Treiber Lake B prior to June of 2016 will continue to be replaced under this SWSP. Please note that until the liner for Treiber Lake B is approved by this office, any water exposed within the lined perimeter must be treated as ground water with evaporation losses assessed. The construction of a slurry wall around Treiber Lake B will require approximately 20 acre- feet of water which will be consumed in mixing the bentonite slurry mix. No water was pumped for this purpose during 2015. You have estimated that 2.5 acre-feet will be required for the slurry wall in March 2016, 7.5 acre-feet in April 2016, and 10 acre-feet in May 2016. This water will be pumped from Treiber Lake B and is considered to be 100% consumed. The monthly evaporative depletions to the Cache la Poudre River were lagged using the AWAS program developed by the IDS Group at Colorado State University. The parameters used in the model were an average distance from the centroid of each lake to the river (X) of 744 feet, an estimated distance from the site to the no flow aquifer boundary (W, measured perpendicular to the point of impact) of 6,500 feet, an aquifer transmissivity (T) of 160,000 gallons per day per foot, and a specific yield (S) of 0.2. Since the water required for slurry wall construction will be pumped from Treiber Lake B, those depletions were lagged using a distance (X) of 640 feet from the centroid of Treiber Lake B to the river. All other parameters remained the same. Lagged depletions due to past and projected evaporation and operations at the site will total 181.7 acre-feet for 2016 and 143.1 acre-feet for 2017. Should the slurry wall around Treiber Lake B not be completed and pass the leak test as contemplated in this SWSP, an amended SWSP must be submitted to this office.