WSP11726

• File Number: 8170
• Description: Arkansas Basin Water Quality Issues
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 3/18/1996
• Author: M Clark and M Lewis
• Title: Metal Speciation in the Upper Arkansas River Basin
• Water Supply Pro - Doc Type: Report/Study

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CONCLUSIONS

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From an aquatic-life toxicity perspective, the MINTEQA2 modeling results of conditions in 1990-93

indicate that dissolved Cd and Zn concentrations may represent a greater cause for concern in the upper Arkansas

River than concentrations of dissolved Cu and Pb. About 50 percent of the dissolved Cd and Zn were predicted to

occur as the more toxic free cations, while less than 5 percent of the dissolved Cu and Pb occurred as free cations.

The most dominant complexed species for Cd and Zn were the carbonates, CdCO) and ZnCO). Dissolved Cu and

Pb complexes were dominated by Cu(OHh and PbCO). Streamflow regime and location within the basin had

little influence on the speciation of Cu and Pb, but Cd and Zn speciation were very sensitive to both. TIle

maximum activities of Cd+2 and Zn+2 occurred at Empire Gulch, just downstream from the Leadville Mining

District, which is the source area for most of the metal-loading to the river. Downstream from Empire Gulch, at

Granite and Buena Vista, the low ionic strength water that enters the AIKansas River through Lake Creek in

addition to poor buffering capacities of local geology, appears to have resulted in a larger percentage of dissolved
. Cd and Zn existing as the free cations. Cd+2 and Zn+2. The largest percentage of dissolved Cd and Zn represented
by their free cations occured during snowmelt runoff. The occurrence of Cd+2 and Zn+2 decreased substantially

downstream from Buena Vista. Based on the average percent of dissolved species represented by the free cations

of Cd and Zn, the free cation occurrence sequence is:

Ponland < Parkdale < Wellsville < Empire Gulch <Nathrop < Leadville < Granite < Buena Vista

The U.S. Fish and Wildlife Service (1993) has expressed concern that metal toxicity to aquatic life.

particularly from Cd. may be increased by the inflow of low ionic strength water through Lake Creek. The model

results do indicate that a larger percentage of dissolved Cd and Zn occurred as the more toxic free cations in the

reach immediately downstream from the confluence of Lake Creek and the Arkansas River. TItis relation appears

to be caused by the Lake Creek inflows. It is imponant to note that while the percentage of the dissolved Cd and

Zn represented by their free cations increased below Lake Creek, the actual activity or concentration of both the

. free and complexed Cd and Zn species decreased in the reach largely due to dilution by Lake Creek. The

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