2020-04-06_HYDROLOGY - M1977151 - Laserfiche WebLink

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Greeley Pit SWSP April 6, 2020 Plan ID 2965 Page 2 of 10 due to evaporation from groundwater 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. This office recognizes a total of 40.2 acres at the Greeley Pit as being pre-81 area. Accordingly, for the 71.8 acres of groundwater currently exposed at the Greeley Pit, replacement of evaporative depletions is only required from the 31.6 acres exposed after December 31, 1980. Per the State Engineer's General Guidelines for Substitute Supply Plans for Sand and Gravel Pits as updated April 1, 2011, pre-81 areas are tied to the physical location at which the groundwater was exposed prior to January 1, 1981. The applicant has provided a map showing the specific location of the pre-81 area. The credits for the pre-81 area are tied to the location identified in the 1980 Aerial Photograph shown in the attached Figure 2 (Pre/Post 1981 Exposed Area) and may not be re-allocated to other areas of groundwater exposure within the gravel pit boundary. The current exposed surface area is shown on the attached Figure 2 (Exposed Area Map). The net depletion of groundwater due to evaporation from the surface area of the Greeley Pit exposed after December 31, 1980 was calculated to be 87.19 acre-feet per year, as shown on the attached Table 1. Based on monthly average temperatures reported for the Greeley UNC weather station, ice cover was assumed for the months of December and January. The ice covered periods may be used to reduce the amount of evaporative losses that need to be replaced; however, for the purpose of this SWSP, the Applicant shall replace the net evaporation depletions from the exposed groundwater surface area that may occur during the assumed ice covered period (December and January) 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 Applicant has estimated that a total of 1.00 acre-foot of water will be used for dust control purposes each year from April through September. A monthly breakdown of the estimated dust control use is shown in the attached Table 2. The total consumptive use of groundwater at the Greeley Pit is estimated to be 88.19 acre-feet per year for the approval period of this plan. The Alluvial Water Accounting System ("AWAS"), which uses the Glover method, was used to determine the lagged depletions to the Cache la Poudre River from past and projected evaporation and operational losses at the site. The infinite aquifer boundary condition was selected because a parallel no-flow boundary could not be identified based on information from the USGS Hydrologic Atlas HA-746A. The following parameters were used in the model: a distance (X) of 1,120 feet from the centroid of the 39.7-acre exposed groundwater surface to the river; a transmissivity (T) of 120,000 gallons per day per foot; and a specific yield (S) of 0.2. The estimated lagged stream depletions due to past and projected operations at the Greeley Pit will total 86.24 acre-feet for the first year of operation of this SWSP and 86.35 acre-feet for the second year of operation, as shown on the attached Table 3.