Toxic Substances Hydrology Program
U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12, 1999--Volume 1 of 3--Contamination From Hard-Rock Mining, Water-Resources Investigations Report 99-4018A
The Effect of Trace-Metal Reactive Uptake in the Hyporheic Zone on Reach-Scale Metal Transport in Pinal Creek, Arizona
By Christopher C. Fuller and Judson W. Harvey
The extent of hydrologic exchange between surface water and the streambed and the rate of trace metal uptake by hyporheic sediments was evaluated in Pinal Creek, Arizona. Trace metal uptake was quantified by measuring a conservative tracer injection into the stream and metal concentrations in the hyporheic zone. Fractional reactive uptake of metals entering the hyporheic zone averaged 55, 27, and 39 percent for cobalt (Co), nickel (Ni), and zinc (Zn), respectively, at 29 sites. Manganese (Mn) uptake averaged 24 percent at the same sites. First-order rate constants (_h) of metal uptake in the hyporheic zone were determined at seven sites. Reaction-time constants (1/_h) averaged 0.41, 0.84, and 0.38 hours for Co, Ni, and Zn, respectively, and 1.3 hours for Mn. Overall a trend of increased metal uptake with increasing Mn uptake was observed. Laboratory metal uptake experiments with streambed sediments indicate that metal removal increased with pre-existing Mn oxide concentration, suggesting that the enhanced Mn oxidation in the hyporheic zone contributed to trace metal uptake. Surface-water metal concentrations were simulated over a 2.8-km reach using the average _h coupled with hydrologic parameters derived from modeling in-stream tracer experiments. Simulations of Mn and Ni using the average _h indicated that reactive uptake in the hyporheic zone could account for the net uptake of Mn and Ni downstream over this reach. Simulations of Co and Zn indicated that the extent of uptake in the hyporheic zone could not be accurately distinguished from conservative transport. Uncertainties in defining metal inflows, however, limited the accuracy of reach-scale simulations for Co and Zn.