2012-12-12_HYDROLOGY - M1983176 - Laserfiche WebLink

Home Browse Search

Mr. Rob Laird Speer Mining Resources SWSP December 12, 2012 Page 2of4 Depletions The depletions at the site consist of evaporation from exposed groundwater, water used for dust suppression, lagged depletions resulting from the cessation of dewatering, and lagged depletions resulting from the `first fill' of the pit. The total exposed groundwater surface area at this site as of January 2011 was 26 acres. As of November 2012 the site has been backfilled so that 18.5 acres of groundwater are exposed. For the purposes of this SWSP, it is assumed that the pit will be backfilled at a steady rate of 5 acres per year, or 0.42 acres per month so that by December 2014 the exposed groundwater at the site will be 7.14 acres. Net evaporative depletions were calculated using a gross annual evaporation of 44.5 inches from the exposed water surface, with a credit of 9.93 inches for effective precipitation. The net depletion of ground water due to evaporation from the ground water exposed at the site was calculated to be 43.22 acre -feet for 2013, and 27.51 acre - feet in 2014 as shown on the attached Tables AI.3 and AI.4. The Applicant has estimated that 0.33 acre -feet of water will be used for dust suppression at the site per year, as shown on the attached. There will be no material mined at the site during this plan period and no other operations that would consume groundwater. The Alluvial Water Accounting System ( "AWAS ") model was used with the alluvial aquifer boundary condition option to lag depletions to the South Platte River. The following parameters were used in the model: transmissivity (T) = 96,043.2 gallons per day per foot, distance (X) from the centroid of the surface of the exposed ground water to the river = 578 feet, distance (W) from the aquifer boundary through the exposed ground water to the river channel = 6,800 feet, and specific yield (SY) = 0.2. The location of the stream depletion is assumed to be perpendicular to the river. The lagged depletions due to evaporation and dust control during the SWSP period are shown in Column 4 of Tables AI.7 and AI.8 and are estimated to total 44.37 acre -feet for 2013 and 29.89 acre -feet for 2014. Additional lagged depletions due to previous pumping for dewatering purposes at the site were calculated using the AWAS program. Following shutdown of the dewatering pumps on August 1, 2010, the pit subsequently filled with approximately 156 acre -feet of water over a period of 60 days. The depletions resulting from this "first fill" were included in the determination of lagged depletions. Column 5 of Tables AI.7 & AI.8 shows lagged depletions due to past pumping for dewatering purposes, pump shutdown, and the subsequent filling of the pit, equaling 6.93 acre -feet in 2013 and 1.98 acre -feet in 2014. Total lagged depletions at the site are estimated to be 51.3 acre -feet for 2013 and 31.87 acre -feet for 2014. These depletions are based the projections of constant backfilling at a rate of 5 acres per year. Should the pit not be backfilled, or be backfilled at a rate that is Tess than 5 acres per year this SWSP will become invalid and an amendment will be required. Replacements The proposed source of replacement water for this plan is fully consumable effluent leased from the City of Aurora. The applicant has entered into a five -year lease agreement with the City of Aurora ( "Aurora ") whereby Aurora will deliver fully consumable water to the South Platte River at the outfall of the Metro Wastewater Treatment Plant (WDID 0200700). The 5 -year lease requires an annual notice by the applicant to determine the final water schedule for the upcoming water year. For water year 2013 (November 2012 through October 2013) Aurora will deliver 57.05 acre -feet of water. The point of replacement is located approximately 10.62 miles upstream of the Speer Mining Resources Pit. A 0.5 percent per mile (5.31 percent total) transit loss has been accounted for in the replacement schedule. The