2011-12-20_HYDROLOGY - M1999069 - Laserfiche WebLink

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Todd Williams Page 2 December 19, 2011 Depletions Evaporation and Operations The exposed ground water surface area is 0.16 acres. Net evaporative depletions were calculated using a gross annual evaporation of 38 inches from the exposed ground water surface, with a credit of 9.5 inches for effective precipitation. The net depletions of ground water due to evaporation from the 0.16 acres at the W -K #3 site totals 0.36 acre -feet per year, as shown on the attached Table 1. There will be no aggregate production, and with the exception of post pumping depletions described below, there will be no other ground water consuming activities at the site. A stream depletion model was used to determine the timing of lagged depletions to the Big Thompson River. The following parameters were used in the stream depletion model for 2012: transmissivity (T) _ 50,000 gallons per day per foot, specific yield (SY) = 0.2 (which is typical for an alluvial aquifer), the distance from the centroid of the exposed ground water to the river (X) = 2,400 feet, and the location of the parallel impermeable boundary was estimated to be 4,000 feet from the stream. To be consistent with previous approved SWSPs for this site, lagged depletions caused by mining operations prior to 2012 were determined using the previous approved distance parameter (X) of 200 feet. Dewatering The mining operation has continuously dewatered directly back to the Big Thompson River since 2000. The estimated dewatering rate at the W -K #3 site had been assumed to be 1000 gallons per minute ( "gpm ") in previous SWSP approvals. Depletions caused by dewatering were assumed to be offset by the accretions caused by dewatering as it was assumed dewatering was continuous and had reached steady state conditions. To do the post pumping depletion analysis, the Applicant used the actual dewatered volume as reported by the operator to the Colorado Department of Public Health and Environment as a part of the operator's discharge permit which was found to average 0.086 million gallons per day (MGD), or 60 gallons per minute (gpm) when converted to a constant flow rate. The Applicant then applied this continuous flow rate starting in January 2000 and ending in November simulating a continuous dewatering operation. A stream depletion model was used to determine the timing of lagged depletions to the Big Thompson River. The following parameters were used in the stream depletion model for 2012: transmissivity (T) = 50,000 gallons per day per foot, specific yield (SY) = 0.2 (which is typical for an alluvial aquifer), the distance from the centroid of the exposed ground water to the river (X) = 2,400 feet, and the location of the parallel impermeable boundary was estimated to be 4,000 feet from the stream. During steady state conditions the annual lagged depletions to the river (Table 4) were approximately 97 acre -feet which is offset by the annual accretions (Table 3). Table 4 for the time period of December 2012 to December 2015 represents the post pumping depletions affecting the river. For the purposes of this SWSP, it is projected that 7.85 acre -feet of post pumping depletions will impact the river in December. This impact will hit the river starting the first month that dewatering ceases. Total Depletions The total lagged depletions that hit the river during this SWSP approval is 8.77 acre -feet. See Table 5 for these depletions monthly distribution. The depletions are heavily dependent on the dewatering schedule. Should dewatering cease earlier than projected, then this SWSP must be amended and updated to show the true timing of post - pumping depletions, and show how they will be replaced in time and amount. Replacements Replacement water for this pit will continue to be made available throughout the year from a lease of 65 acre -feet of fully consumable water from the City of Loveland ( "Loveland "). This lease also