2019-12-03_HYDROLOGY - M1983090 - Laserfiche WebLink

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Poudre Pits SWSP December 3, 2019 Plan IDs 3218 >t 4585 Page 3 of 9 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 (since 1990) and projected evaporation and operational losses at each site. The following parameters were used in the model with the alluvial aquifer boundary condition: the distance (X) from the centroid of the exposed groundwater surface to the river; the width (W) of the aquifer on the side of the river where the pit is located; the transmissivity (T); and the specific yield (S). The Glover parameters used for each pit site are shown in the table below. Table C - Glover Parameters (Evaporative/Operational Losses Site Name X (ft) W (ft) T (gpd/ft) S West 512 4,800 50,000 0.20 La Poudre Middle 399 4,800 50,000 0.20 East 626 4,800 50,000 0.20 North La Poudre 2,500 4,800 50,000 0.20 The lagged evaporative and operational depletions for 2020 will total 79.35 acre-feet for the La Poudre and North La Poudre sites. DewaterinQ Dewatering has occurred at each pit over varying intervals from 1995 through 2019. The Lee Parcel is the only portion of either site that is proposed to be actively dewatered during this SWSP period. In the past, it was assumed that the pits were dewatered at a rate of 500 gpm with all water released directly to the river. These assumptions have been revised to reflect the historic average dewatering rate of 100 gpm where the majority (75%) of the water was recharged in the adjacent pits with only 25% discharged directly to the river. This is based on a reevaluation of historic operations by the operator's consultant and a December 20, 2012 letter from Mark Johnson, Compliance Manager with Bestway Concrete and Aggregates as well as submitted pump specifications and data showing the 100 gpm is a more accurate estimate than the previously accepted 500 gpm. Mark Johnson described the recharge operation as occurring simultaneously with dewatering in that the operator would discharge the water into an adjacent, previously mined, cell. Once the cell being recharged exceeded its hydraulic capacity the water would then be diverted directly to the river. Dewatering operations at the La Poudre Pit occurred from January 1995 through December 2002, and during October and November of 2014. As stated above, 75% of the water was recharged in the adjacent pits with 25% discharged directly to the river. The Lee Parcel has been dewatered since April 2018 at an average rate of 212 gallons per minute with one-third (33%) of the dewatering water delivered to recharge in the West Lake and two-thirds (66%) delivered directly to the river. At the North La Poudre site, dewatering operations occurred from September 2002 through March 2003 and recommenced from April 2012 through July 2014. The 2012 dewatering was not metered therefore no recharge credit was given. Instead, all 2012 dewatering was assumed to be discharged directly to the river creating no lagged accretions. The operator installed meters at this site by January 2013 and starting January 2013 the actual meter readings are used in the dewatering analysis. Dewatering that occurred from January 2013 through November 2013 and March 2014