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c 1999 Metal Speciation in the Upper Arkansas Rive[j3asin, . I ----- ,.:....1('('1; ~.I,. I ,~I , I :- ~~-"'--.Ji.'- Colorado, 1990-93 i '. .'Iii.; 10 [9% L. ~ ~-"". By Melanie L. Clark and Michael E. Lewis i , . ! : -;'-,Wrlll":L:tr::rl Cl_'!CP"D .v"'t . i !"" ~.... .Q ~11 j_-=.:::~::;<:jv.:incy [:b~;ict I ----.- Abstract The effects of streamflow regime on metal speciation in the upper Arkansas River were evaluated with the metal speciation equilibrium model, MINTEQA2. Model results of 1990-93 conditions at eight mainstem sites indi- cate that dissolved copper and lead occurred primarily as hydroxide and carbonate complexes, respectively, at all I sites during all streamflow regimes. Conversely, about 50 percent of the dissolved cadmium and zinc, on average, occurred as the more toxic and bioavailable free-cation species, Cd+2 and 20+2. The largest percentage of dissolved . cadmium and zinc represented by the free cations occurred during the snowmelt-runoff flow regime, a period when alkalinity concentrations and pH are lowest in the river. As a result, cadmium and zinc have less opportunity to form , complexes during this period. The maximum percentages ofCd+2 and 20+2 occurred at two sites immediately down- stream from the confluence of Lake Creek and the Arkansas River, Granite and Buena Vista. Low ionic strength water is imported into the Arkansas River basin from the ColoradO River basin through Lake Creek and results in less cadmium and zinc complexation at these sites than at other sites. At Buena Vista about 69 percent of the dis- solved cadmium and about 67 percent of the dissolved zinc occurred as the free cations. This relation and its impli- cation to aquatic-life toxicity is partially offset by the dilutional effects of Lake Creek. which typically has metal concentrations that are significant! y smaller than in the Arkansas River. . 1