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<br />~0~~ <br />~J~E <br /> <br />sludge was discharged from the Yak Tunnel. The color change from this occur- <br />rence was visible downstream to Salida or about 60 to 70 mi downstream (Gary <br />Soldano, Colorado Department of Health, Pueblo, Colo., oral commun., 1986). <br />Mine drainage in the Leadville area has resulted in increased concentrations <br />of iron, manganese, and dissolved solids in the Arkansas River between <br />Leadville and Malta. As the acidic water discharged from the mine becomes <br />neutralized in the river, dissolved metals precipitate and coat the stream <br />bottom and decrease the concentrations of dissolved solids and metals in the <br />stream water. However, as the metal coatings are dislodged from the stream <br />bottom and resuspended, the metals are transported farther downstream, pos- <br />sibly as far as Pueblo Reservoir. As the effects of mine drainage are diluted <br />by tributary inflows, the chemical quality of the stream becomes more similar <br />to its original quality. Cain (1987) noted that the smallest mean specific <br />conductance for the Arkansas River occurs at Buena Vista. Specific conduct- <br />ance of water in the Arkansas River is small from Buena Vista to Canon City <br />because: (I) Only small quantities of municipal and industrial wastewater are <br />discharged to the stream as a result of a sparse population and a lightly <br />industrialized area, (2) streams are in contact with igneous and metamorphic <br />rocks that resist chemical weathering (and are relatively insoluble), and <br />(3) only small quantities of irrigation-return flows enter the Arkansas River <br />upstream from Canon City. Between Canon City and downstream from Portland, <br />the mean specific conductance of water in the Arkansas River almost doubles, <br />primarily as a result of large irrigation-return flows and inflows of saline <br />ground water (Cain, 1987). <br /> <br />Chemical quality of water entering Pueblo Reservoir can be summarized <br />using data collected at station 07097000, Arkansas River at Portland <br />(tahle 1). The U.S. Geological Survey has been analyzing water-quality sam- <br />ples collected at this station since 1977, and daily measurements of water <br />temperature and specific conductance have been made since 1979. Large vari- <br />ations in water temperature occurred during the 1985 water year (fig. 6). <br />During the winter months, water temperatures of 0 Oc (freezing) occurred <br />during many days; during the summer, water temperatures as high as 24 oC were <br />measured. <br /> <br />Cain (1987) determined that the mean specific conductance of the water in <br />the Arkansas River at Portland usually is largest from January to March when <br />streamflows are the smallest, and the smallest specific-conductance values <br />occur during June Or July when streamflows are the largest. During the 1985 <br />water year, the maximum daily mean specific conductance was 540 ~S/cm, which <br />occurred during January 1985 when streamflow was small; the minimum daily mean <br />specific conductance of 140 ~S/cm occurred during June 1985 (fig. 6). Cain <br />(1987) determined the relations between: (1) Specific conductance and stream- <br />flow, (2) specific conductance and concentrations of dissolved solids, and <br />(3) specific conductance and concentrations of major ions. The relation of <br />specific conductance to streamflow for the Arkansas River at Portland can be <br />expressed using the following equation: <br /> <br />log specific conductance (microsiemens per centimeter at 25 OC) <br />= 3.61 - 0.37 log streamflow (cubic feet per second). <br /> <br />14 <br />