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boulder riprap. The upper portion of the bank was spread with topsoil and revegetated with a <br />grass seed mixture. During the project, large boulders were placed in the riverbed to im prove <br />habitat. As a result of this project, much of the north riverbank in the study area upstream andf sh <br />downstream of Sanborn Creek is covered with boulder riprap. <br />The North Fork of the Gunnison River is a 4th order stream where it flows through the study <br />area. Typical flows measured during spring runoff in May and June exceed 1000 cfs at a gaging <br />station 2.5 miles east of Somerset. During this period, the current is swift and water depths may <br />range from 0.5 to 1.5 m. The highest recorded peak flow of 9,220 cfs occurred on May 24, <br />1984, and the lowest minimum flow was 21 cfs, recorded during the winter of 1978 (USGS, <br />1992). The Paonia Dam influences river flow in the study area typically by reducing peak flow <br />during spring runoff periods but extending moderate flows through the summer irrigation period. <br />This flow regime may be at least partly responsible for the bottom characteristics of the river, <br />which has a nearly uniform coverage of large cobbles and small boulders. <br />Water Quality of the Mine discharge and Receiving Waters: <br />Table 1 shows typical water quality data for the discharge and for the receiving water. The <br />mine permit allows a discharge of 425 gpm (0.95 cfs). Assuming an effluent TDS of 2940 <br />mg /l and a receiving water TDS of 150 mg /l, the resulting TDS of water downstream due to <br />mixing of the effluent would be 272 mg /l, assuming maximum discharge and 21 cfs river <br />flow. This also represents a dilution of the effluent to 4.5 percent. Current discharge volume is <br />approximately 150 gpm (0.33 cfs), which would produce a final TDS of 195 mg /l and a <br />dilution to 1.5 percent, assuming 21 cfs river flow. During the period of study, mine discharge <br />was intermittent. <br />Toxicity Studies: <br />Several whole effluent toxicity (WET) tests have been performed using water fleas <br />(Ceriodaphnia dubia) and fathead minnows (Pimephelas promelas) . All WET tests using <br />Ceriodaphnia have failed, with calculated 48 hr LC -50 values ranging from 68 to 89.3 <br />percent. Dilutions to 50 percent or less of the effluent when mixed with samples of the <br />receiving water consistently show 100 percent survival of Ceriodaphnia, according to <br />Commercial Testing and Engineering (CT &E) laboratory results. All tests using fathead <br />minnows have passed, with LC -50s exceeding 100 percent, as did a single WET test using <br />Daphnia magna. Phase 1 toxicity identification studies on the mine effluent performed by <br />CT &E implicated alkalinity and sodium ions as the major causes of toxicity. Subsequent <br />toxicity identification evaluations completed in October, 1993, indicated that the toxicity is <br />most likely due to TDS in the form of bicarbonates. <br />"a, 4 <br />