2008-09-29_PERMIT FILE - C1981019A (6)

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RULE 2 PERMITS watershed. These calculations assist the mine operations personnel in designing runoff detention ponds, diversion conveyances and channel stabilization structures. The method used to estimate these flow values for each watershed was the United States Department of Agriculture, Soil Conservation Services' Triangular Unit Hydrograph Procedure. Flow records for Milk Creek, near the general area, from 1952 were analyzed for exceedance values. This information and the design storm, a 100-year, 1-hour precipitation event of 1.5 inches was applied to each basin to- yield peak flow estimates for each drainage. Table 2.04.7-13 is a summary of the flow estimates for each-basin. Average 7-day low flow for the 10-year return period (Q7_1o) ranges from 0. cfs for Taylor Creek to 0.29 cfs for Goodspring Creek. These estimates are substantiated by the data presented earlier in Tables 2.04.7- 10, 2.04.7-11 and 2.04.7-12. High flow values for each basin, the 1-day high flow 100-year return period, ranged from 8.0 cfs for Taylor Creek to 37 cfs for Wilson Creek. Peak flow estimates are calculated at 120 cfs for Taylor Creek, 190 cfs for Wilson Creek and 270 cfs for Goodspring Creek. Tables 2.04.7-14, 2.04.7-15 and 2.04.7-16 present the flow estimates for Taylor Creek, Wilson Creek and Goodspring Creek, respectively. These data give the 24-hour and 7-day high flows for return periods of 1, 10, 50 and 100-years and the probability of these flow values being exceeded. Local factors which minimally affect streamflow in the area are several small, man-made impoundments. These reservoirs have a capacity of less than 1-AF and are used to provide normal water supplies for livestock. These structures do not significantly affect the streamflows in the vicinity of the mine. Surface Water Quality General Area Surface water quality in the general area is affected at the baseline level by irrigation, farming, existing roads, previous underground mining and, potentially, the wildlife in the surrounding area. The surface water quality in the general area is extremely variable and is a function of the climate patterns (see Section 2.04.8 for climatological information) and geology (see Section 2.04.6 for geological information) as well as past human-caused disturbances. Water quality constituents show seasonal and monthly fluctuations and yearly fluctuations due to all of these factors. Permit Area Water quality sampling programs have been conducted in and around the mine area since 1974 by various teams of investigators. The BIM EMRIA program set up sampling stations on Taylor, Wilson and Goodspring Creeks to collect continuous data for a great number of quality constituents. Colowyo and its consultants have sampled these same stations as well as a separate series of stations from 1974 to present. All of these stations are located on Map 32 and a table of the site numbers and descriptions is presented earlier in the discussion Occurrence of Surface Water, Table 2.04.7-7. Surface water in the area can be characterized as extremely hard containing an abundance of dissolved solids and variable levels of other constituents. Data collected by Colowyo in 1974 and 1975 showed that total hardness exceeded 300 mg/1 as CaC03 for all samples. Positively charged cations were predominately calcium and magnesium with sodium present in lesser quantities. Potassium was present only in very low quantities and heavy metals were nearly absent. Negatively charged anions were comprised primarily of bicarbonate and sulfate. Chloride was present in low levels. Fluoride, carbonate, nitrate and silica were present in very low levels or were nearly absent. Rule 2 Permits 2.04.7-19 Revision Date: 6/23/08 Revision No.: MR-91