2019-01-29_HYDROLOGY - M1983090 - Laserfiche WebLink

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Poudre Pits SWSP January 29, 2019 Plan IDs 3218 lt. 4585 Page 4 of 8 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 through July 2014 was returned directly to the river through an unnamed slough located above the Whitney Ditch. In the past, lagged depletions resulting from dewatering at the sites were calculated using AWAS with the infinite aquifer boundary condition. The following parameters were used in the model: the distance (X) from the centroid of the exposed ground water surface to the river; the transmissivity (T); and the specific yield (S). The Glover parameters used for each pit are shown in Table D below. Table D- Glover Parameters Dewaterin Site Name X (ft) T (gpd/ft) S La Poudre 1,500 50,000 0.20 North La Poudre 2,500 50,000 0.20 Beginning with the 2019 plan year, you have proposed to lag dewatering depletions using the alluvial aquifer boundary condition, consistent with the methodology and aquifer width parameter for lagging operational depletions, which is believed to be more appropriate given the close proximity of the pits to the Cache la Poudre River. In order to ensure that this change in methodology does not result in unreplaced depletions to the river, you have proposed to continue lagging dewatering depletions resulting from dewatering that occurred in 2018 and earlier using the infinite aquifer boundary condition, and to lag depletions resulting from dewatering that occurs in 2019 and beyond using the alluvial aquifer boundary condition. Because the remaining depletions resulting from dewatering prior to 2019 as calculated using the infinite aquifer method lag out for a long time period in relatively small quantities, you have proposed to "wrap" the lagged depletions remaining after 2028, which represent less than 10% of the total volume pumped for dewatering purposes, and redistribute and replace these depletions during the time period of 2019 through 2018. Depletions resulting from post-2019 dewatering are not proposed to be wrapped at this time. Based on discussions with the mine operator, dewatering during 2019 is projected to occur on the Lee Parcel portion of the La Poudre site at a rate of 330 gpm, with 220 gpm delivered directly to the river and 110 gpm delivered to recharge in the Wildlife Pond. Actual monthly meter readings must be used to determine the true dewatering rate, and to determine how much water was delivered to recharge versus being directly returned to the river. Table E below summarizes the impacts past and projected dewatering operations at each pit will have on the stream system during this SWSP period of January 1, 2019 through December 31, 2019, as calculated using the methodologies and assumptions described above.