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<br />2013 <br /> <br />Soatial and Temooral Variations in Metal Soeciation <br /> <br />. <br /> <br />The uptake of metals from the dissolved phase of the water column generally is the most toxic pathway for <br />aquatic life (Gerhardt. 1993).Genernlly. complexed fonus of metals are considered less toxic. because the metal <br />has effectively combined with another substance. In the upper Arkansas River. the potential for metal <br /> <br /> <br />complexation may vary tempornlly and spatially, due to the influence of streamflow regime on water chemistry. <br /> <br /> <br />Therefore. metal toxicity to aquatic life may be a function of dissolved-metal concentration and streamflow <br /> <br /> <br />regime. The metal speciation equilibrium model. MINTEQA2. was used to estimate the temporal and spatial <br /> <br /> <br />distributions. in relation to streamflow regime. of dissolved Cd. Cu. Pb. and Zrispecies: which are particular metals <br /> <br /> <br />of concern because of their toxicity to aquatic life (U.S. Environmental Protection Agency. 1986). <br /> <br />The metal species predicted for water samples collected from the Arkansas River include the free cations. <br /> <br /> <br />hydroxide complexes. sulfate complexes. carbonate complexes, and chloride complexes. The species predicted for <br /> <br /> <br />. each metal and their relative predominance is shown in table 3. <br /> <br />Table 3 near here. <br /> <br />The predicted dissolved-cadmium species exhibited substantial spatial variability (Figure 6). The highest <br /> <br /> <br />median activities of Cd+2 were predicted for samples collected below Empire Gulch and tended to decrease <br /> <br /> <br />downstream to Ponland. where the lowest median activity was predicted for Cd+2. The more toxic Cd+2 species <br /> <br /> <br />accounted for over half of the cadmium species present at Leadville (56%). Empire Gulch (53%). Granite (59%). <br /> <br /> <br />Buena Vista (69%), and Nathrop (55%). For one sample collected at Buena Vista, 90 percent of the dissolved <br /> <br /> <br />cadmium was predicted to exist in the free-cation species. Low complexing capabilities of water input from Lake <br /> <br /> <br />Creek may contribute to the elevated Cd+2 species activity at Granite and Buena Vista. The degree of cadmium <br /> <br /> <br />complexation increased downstream from Buena Vista to a maximum of about 70 per-ent of the dissolved- <br /> <br />. cadmium species as CdC03 at Ponland. <br /> <br />13 <br />