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<br />2006 <br /> <br />Samples were all analyzed by the U.S. Geological Survey's National Water-Quality Laboratory in Arvada. <br /> <br /> <br />. Colorado. Dissolved constituents were analyzed using methods described by Fishman and Friedman (1989) and <br /> <br />Fishman (1993). Dissolved calcium. magnesium, sodium, manganese, and iron were analyzed using atomic <br /> <br />emission spectrometry, inductively coupled plasma. Dissolved cadmium, copper, and lead were analyzed using <br /> <br />atomic absorption spectrometry in conjunction with a graphite furnace containing a graphite platform. Dissolved <br /> <br />sulfate and chloride were analyzed using ion-exchange chromatography. Alkalinity was determined on <br /> <br />electrometric titration using a standard solution of a strong acid to a fixed.endpoint of pH 4.5 (Fishman and <br /> <br />Friedman, 1989). <br /> <br />Species distribution of dissolved Cd, Cu, Pb, and Zn in samples collected from the Arkansas River was <br /> <br />determined using the metal speciation equilibrium model M1NTEQA2 (U.S. Environmental Protection Agency. <br /> <br />1990). M1NTEQA2 uses simultaneous solution of the nonlinear mass action expressions and linear mass balance <br /> <br />relationships to predict the thermodynamic propenies of electrolyte solutions. Successive approximations and <br /> <br />. nested iterations are performed using equilibrium constants to converge on solution properties. For large, complex <br /> <br />systems, like the Arkansas River, the equilibrium constant approach is the preferred modeling approach <br /> <br />(Nordstrom el al.. 1979). An initial estimate for the activity of each component is made to calculate the <br /> <br />concentration of each species according to mass action expressions written in terms of component activities; the <br /> <br />total mass of each component is then calculated from the concentrations of every species containing that <br /> <br />component. The calculated total mass for each component is then compared with the known input for total mass for <br /> <br />each component. If the calculated total mass and the known input total mass differ by more than a pre-set tolerance <br /> <br />level, then a new estimate of the component activity is made and the entire procedure is repeated (U.S. <br /> <br />Environmental Protection Agency, 1990). <br /> <br />The use of chemical-equilibrium models requires certain assumptions to be made about the modeled system. <br /> <br />The field constituents of temperature and pH were used to describe equilibrium field conditions. The oxidation! <br /> <br />. reduction ion pair of Fe+3/Fe+2 was entered as the redox couple that is most likely to exist in oxidized natural waters <br /> <br />7 <br />