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<br />., <br /> <br />2017 <br /> <br />pre-snowmelt runoff. dissolved Zn concentrations are larger, but alkalinity and pH are also larger. thus more <br /> <br />. <br /> <br />dissolved zinc can be complexed. About 62 percent of the dissolved Zn was predicted to occur as complexed <br /> <br />species during low-flow conditions when alkalinity and pH were typically largest. <br /> <br />Figure 9 near here. <br /> <br />Sensitivity testing of the model was conducted to detennine the impact of modeling assumptions on Cd. <br /> <br />Cu. Pb. and Zn speciation results. Dissolved Fe was assumed to exist as Fe+3 based on the large dissolved-oxygen <br /> <br />concentrations. However, previous studies have shown that Fe+2 and Fe+3 are both present in the dissolved phase <br /> <br />of the Arkansas River (Kimball el al. 1992). Sensitivity tests were run with dissolved Fe entered in several <br /> <br />oxidation states. The activities predicted for species of Cd. Cu. Pb. and Zn were not sensitive to the changes in the <br /> <br />redox state of Fe. Another assumption that was made in the modelling was that ferrihydrite (Fe(OH)3) was <br /> <br />allowed to precipitate from solutiun. Sensitivity tests were run where ferrihydrite was not allowed to precipitate. <br /> <br />. <br /> <br />The results of this testing indicated again that the activities predicted for species of Cd. Cu. Pb, and Zn were not <br /> <br />sensitive to the solid phase defmed for Fe. <br /> <br />For this study, the dissolved phase was operationally defined as water passing through 0.45 micron filter. <br /> <br />However. previous studies have shown that some of the dissolved Fe passing through a 0.45 micron filter exists in <br /> <br />the colloidal fonn (Kimball el al. 1995). To detennine the impact of adsorption on the activities predicted for <br /> <br />species of Cd. Cu. Pb. and Zn. a diffuse double-layer adsorption model was specified for reactions involving <br /> <br />colloidal ferrihydrite. The amorphous structure of ferrihydrite creates a large surface area for binding sites. The <br /> <br />estimated surface area specified in the modeling was 600 meters-squared per gram of ferrihydrite (Dzombak and <br /> <br />Morel. 1990). Surface densities for sorption sites on ferrihydrite were divided into two types: I) type-I sites <br /> <br />correspond to high-affInity cation-binding sites and 2) total-reactive sites available for sorption of cations, anions. <br /> <br />and protons. The site densities for type 1 and type 2 sites are based on empirical data and were assumed to be <br /> <br />. 0.005 mole/mole of Fe and 0.23 mole/mole of Fe, respectively (Dzombak and Morel, 1990). <br /> <br />16 <br />