2016-02-02_HYDROLOGY - M2010071 - Laserfiche WebLink

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Mr. Wayland Page 2 of 6 Goose Haven #2 Expansion SWSP February 1, 2016 material will be in a dewatered state and washed and therefore, pursuant to paragraph 13 of the General Guidelines for Substitute Water Supply Plans for Sand and Gravel Pit (April 2, 2011), a 4% moisture content by weight is charged as a groundwater diversion. This relates to a groundwater loss from mined product of 5.37 acre-feet in 2015 and up to 11.77 acre-feet during 2016. The Alluvial Water Accounting System ("AWAS") model was used with the alluvial aquifer boundary condition option to lag depletions to Boulder Creek. The following parameters were used in the model: transmissivity (T) = 44,883 gallons per day per foot, distance (X) from the centroid of the surface of the exposed ground water to the river = 3,709 feet, distance (W) from the aquifer boundary through the exposed ground water to the river channel = 4,400 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 mining operations during the SWSP period were 6.97 acre-feet for 2015 and are estimated to be 14.36 acre-feet in 2016. The depletions to occur in this plan period are summarized below. The attached Table A1.1, Al.2 and AI.3 shows the monthly breakdown of these values for 2016 and attached Table 1 shows the actual operations in 2015. Table A— Depletion Summary Evaporative Mined Dust Control Total Lagged Loss Product Loss Consumptive Depletions Loss Use 2015 3.73 5.37 0.17 9.27 6.97 2016 3.92 11.77 3.75 19.45 14.36 Total 7.65 17.14 3.92 28.72 21.33 Dewatering Dewatering will occur through two hydraulically separated trenches. Water in the southern dewatering trench system will be discharged into sediment ponds and ultimately discharged into Boulder Creek via Lafayette's return flow canal from the Boulder and Weld County Ditch Headgate. The return flow canal is a concrete lined canal that discharges to Boulder Creek into the initial reach of the Boulder & Weld County Ditch downstream of the diversion structure, but upstream of the ditch headgate and overflow canal back to Boulder Creek. Dewatering is projected to remain continuous throughout the duration of this SWSP approval period. For dewatering analysis it is assumed that the sediment ponds do not allow infiltration and that the dewatered water is returned to Boulder Creek the same month the dewatering occurs. The applicant proposed to dewater the north dewatering trench, which is adjacent to Lower Boulder Ditch, by pumping directly to the Lower Boulder Ditch per an agreement with the ditch company. The Applicant claims that north dewatering trench is filled exclusively by water seeping from the ditch. However, the Applicant has not shown that the Lower Boulder Ditch has a legal right to recapture water that has seeped from the ditch. Therefore, all water from the north dewatering trench must be returned to Boulder Creek through the same method used for the southern dewatering trench. As long as dewatering in the trench systems remains continuous, the net accretions should be sufficient to replace the lagged depletions. At least three years prior to the planned cessation of dewatering, the operator must submit a dewatering analysis that shows how post