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-23- Description of Hydrologic En vironmei~t - Cround Water Regime (Cont'd) Hawk's Nest Creek. The rates of inflow beneath this drainage flucturate season- ally with a slight lag time in response to the flows in the overlying stream. In the Somerset Mine, inflows beneath Hubbard Creek are continuous and reflect the perennial nature of this stream. Faults and fractures Located outside stream valleys generally dry up with time. Those which continue to flow have flow rates which diminish to a relative trickle. Such inflows may represent the dewatering of lenticular sandstone units with Iimited recharge areas, or may represent flows through fracture zones extending to the surface which have narrow recharge zones on steep slopes. Description of the surface Water Regime The North Fork of the Gunnison River is the major drainage for the general area. Smaller streams in or adjacent to the life-of-mine areas are all tributary to the North Fork (Figure 1). Figure 4 contains the drainage map of the study area with the locations of streams and reservoirs which are covered in the following discussion. The North Fork has an annual streamflow at Somerset of approximately 313,500 acre-feet per year (1962-I 979). The flow is regulated by the Paonia Reservoir 5 miles upstream, which became operational in 1962, Water yields during that period have ranged from a high of 451,300 acre-feet per year in water year 1962 to a low of 82,270 acre-feet in water year 1977. Flow records for the North Fork of the Gunnison are given in Table 2. Water quality parameters for the North Fork are given in Table 3. As indicated, waters in the North Fork are a calcium bicarbonate type. s'here are moderate levels of sulfate. Salinity averages less than 100 mg/1. Just below Paonia Reservoir, Muddy Creek and Anthracite Creek join to form the North Fork of the Gunnison. Anthracite Creek contributes about two-thirds of the flow in the North Fork, Anthracite Creek is a perennial stream with an average annual yield of 157,313 acre-feet for the period of record between July, 2977 to October, 1980 (U.S.G.S., water Research Data). This corresponds to an average flow rate of 217 cfs. The highest discharge recorded was 4,200 cfs on flay 27, 1979. Low flows of 12 cfs have been recorded several times during the period of record. The water of Anthracite Creek is an alkaline, moderately hard, calcium-bicarbonate type water with an average total dissolved solids cpncentration of 71.G mg/1, average pN is 7.8. Total suspended solids average ~.pproximately 25 mg/1. Dissolved metal concentrations are Zow throughout the year, and total metal concen- trations are related to total suspended solids concentrations. The Hawk's Nest life-of-mine area is drained by nine ephemeral streams for which no flow records exist. Two of the gulches have had two water samples collected during 5980 when tJ~ey were flowing during snowmelt and a thunderstorm event. The water is alkaline with an average total dissolved solids concentration of 121 mg/1. Total suspended solids concentrations are low. The Somerset Mine has mine workings below and surface facilities along both EIk and Bear Creeks. These streams are ephemeral and intermittent respectively, and drain directly to the North Fork. Somerset also maintains ventilation facilities on:Hubbard Creek.