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<br />.' <br /> <br />.. <br /> <br />2000 <br />INTRODUCTION <br /> <br />. <br /> <br />. <br /> <br />The upper Arkansas River basin is a high-altitude. semiarid basin that extends from Leadville to Pueblo, <br /> <br />Colorado. Historic mining of sulfide-ore deposits in the upper basin near Leadville. Colorado has yielded <br /> <br /> <br />economic quantities of copper (Cu), gold. iron (Fe). lead (Pb). silver. zinc (Zn). and (Tweto. 1968) and has also <br /> <br /> <br />substantially degraded water quality in the upper basin. Water flowing through the numerous abandoned mines <br /> <br /> <br />and tailings piles in the area has contributed large concentrations of cadmium (Cd), Cu. Fe, manganese (Mn). and <br /> <br />Zn to the upper Arkansas River (Moran and Wentz. 1974; Wentz. 1974). Metal concentrations in the rivertend to <br /> <br />exhibit substantial spatial and temporal variability, largely due to the influence of streamflow regime and <br /> <br />streamflow sources. Stream/low is derived primarily from snowmelt in the mountains that border the basin. but <br /> <br />the natural discharge of the river is supplemented by transmountain diversions of water from the Colorado Ri ver <br /> <br />basin. The U.S. Fish and Wildlife Service (1993) has expressed concern that the water imported into the Arkansas <br /> <br />River basin from the Colorado River basin could increase metal toxicity to aquatic life. particularly brown trout. <br /> <br />because the imported water has lower pH and alkalinity than the basin's native runoff; toxicity for many metals is <br /> <br />inversely proportional to pH and alkalinity. Both pH and alkalinity affect the speciation of metals between the <br /> <br />free-metal ions, which are considered the more toxic fonn, and less toxic metal complexes. Unfortunately, an <br /> <br />analysis of free-metal ion concentrations is not routinely possible in most sampling programs and metal <br /> <br />concentrations are typically reported as dissolved (filtered) or as total-recoverable (not filtered) metals. Although <br /> <br />aquatic-life standards for metals are typically based on dissolved-metal concentrations, dissolved metals, as <br /> <br />currently analyzed, contain both dissolved free-metal ions and dissolved-metal complexes. Chemical-equilibrium <br /> <br />models are available which predict effective concentrations. or activities, of free-metal ions and complexed metal <br /> <br />species. based on physical parameters and dissolved aqueous components. Chemical-equilibrium models have <br /> <br />been used successfully in studies to interpret metal speciation in surface waters impacted by acid mine drainage <br /> <br />(Nordstrom. 1991; Smith etaJ., 1991). An analysis of the spatial and temporal distributions of dissolved-metal <br /> <br />. species is necessary to more accurately assess the relative toxicities of these metals in the Arkansas River. <br /> <br />2 <br />