2019-05-29_REVISION - C1981018 (11)

Home Browse Search

29 impacts which will occur as a result of surface facilities appurtenant to the under- ground mine include reduction of water yield from the mine area to the White River, increased sediment loads from disturbed areas, and water quality degradation from the refuse disposal area and mine water discharge. Long-term impacts will center on the effects of subsidence under Scullion Gulch and Red Wash. See the subsidence section of this document for further discussion of subsidence impacts. Small, short-term surface water losses will occur as a result of two factors: retention of water in ponds and removal of water from the White River Lagoon. Runoff from disturbed areas will be routed through sedimentation ponds, thereby reducing the amount of flow to the White River. Evaporation losses will occur because the ponds will not discharge runoff until some of the available storage volume is filled. Very little of the impounded water will re-enter the ground water system due to the clay subsoils which comprise the pond bottoms. Losses from the D-Portal pond will be insignificant when compared to total runoff from the Scullion Gulch watershed. The pond is designed to store runoff from 44 acres, while Scullion Gulch has a contributing drainage area of 7,552 acres at its confluence with the White River. The refuse disposal area ponds receive runoff from approximately 98 acres, which is a very small portion of the 2,950 acres drained by the Red Wash tributary through the northern sector of the permit area. Based on this information, the net loss of surface water quantity to Red Wash and the White River will be negligible. Water supply for the mine is supplied by the White River. Combined water use and evaporative loss at full production is projected to consume 552 acre-feet per year at a withdrawal rate of .763 cubic feet per second. This translates to less than 0.2 percent of the average annual flow in the river of 672 cfs. Even during low flow conditions, when the average flow is 245 cfs, the withdrawal rate will only be 0.31 percent of the available flow. Therefore, adverse impacts on flow in the White River as a result of proposed withdrawal should be minimal. The construction of surface facilities in the permit area will cause temporary increases in the amount of suspended solids in the surface water system. Furthermore, concentration of chemicals associated with increased sediment load, such as iron and manganese, will also increase. At the Deserado Mine, mitigation of sediment load will be accomplished primarily by sedimentation ponds. Although sediment increases will occur in the early phases of construction, the long-term sediment yield may actually decrease below the already high natural occurrence of sediment concentrations due to the installation of control measures. Effects on the surface water system from the refuse disposal area should be minimal. Analysis of the waste material does not indicate the presence of toxic or acid-forming characteristics. The waste will be covered with 30 inches of non- toxic, non-combustible material, thereby reducing the potential for water quality