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
ABSTRACT
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.
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