Reactive uptake of trace metals in the hyporheic zone of a mining-contaminated stream, Pinal Creek, Arizona

Significant uptake of dissolved metals occurred by interaction of groundwater and surface water with hyporheic-zone sediments during transport in Final Creek, AZ. The extent of trace metal uptake was calculated by mass balance measurements made directly within the hyporheic zone. A conservative salute tracer injected into the stream was used to quantify hydrologic exchange with the stream and groundwater. Fractional reactive uptake of dissolved metals entering the hyporheic zone was determined at 29 sites and averaged 52 +/- 25, 27 +/- 19, and 36 +/-: 24% for Co, Ni, and Zn, compared with Mn uptake of 22 +/- 19%. First-order rate constants (lambda(h)) Of metal uptake in the hyporheic zone were determined at seven sites using the exchange rate of water derived from tracer arrival in the streambed. Reaction-time constants (1/lambda(h)) averaged 0.41, 0.84, and 0.38 h for Co, Ni, and Zn, respectively, and 1.3 h for Mn. In laboratory experiments with streambed sediments, metal uptake increased with preexisting Mn oxide concentration, supporting our interpretation that Mn oxides in the hyporheic zone enhance trace metal uptake. Reach-scale mass-balance calculations that include groundwater metal inputs indicated that decreases in metal loads ranged from 12 to 68% over the 7-km perennial reach depending on the metal. The decreases in metal loads are attributed to uptake of trace metals by Mn oxides in the hyporheic zone that is enhanced because of ongoing Mn oxide formation. Analysis of dissolved-metal streambed profiles and conservative solute tracers provide a valuable tool for quantifying metal uptake or release in the hyporheic zone of contaminated streams.