U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Interactions between Shallow Ground Water and Surface Water that Affect Metal Transport in Pinal Creek, Arizona

Abstract

Solute transport was investigated in shallow ground water and surface water along a 500-meter (m) reach of the perennial stream in Pinal Creek basin. A bromide tracer injection in the stream was conducted to quantify ground-water inflow and streamwater/streambed-water exchange. Respective gains and losses in iron (Fe) and manganese (Mn) in solution occurred in ground water discharging laterally through bank seeps into the stream, and not in ground water discharging vertically from below the active channel, or in the active surface channel itself. Aqueous metal concentrations measured in shallow ground water varied significantly with position across the narrow channel at Pinal Creek. Concentrations of Fe were twentyfold to fiftyfold higher in shallow ground water discharging laterally through channel banks compared to ground water discharging vertically from beneath the stream. Mn was 25 percent lower in bank seepage compared to subchannel ground water. Cross-channel variability in metal concentrations in contaminated ground water could not be explained by differential dilution of ground water discharging laterally to the channel versus vertically through the streambed. Mass-balance calculations for the 500-m reach of stream and shallow aquifer verified that Fe and Mn dynamics in bank seepage were sufficient to affect downstream concentrations of those constituents. Microbial processes in channel banks are hypothesized to drive reductive dissolution of solid phase Fe in ground water discharging laterally through banks, but not in subchannel ground water. The mechanism of Mn loss in this channel reach appeared to be sorption of Mn²⁺ on Fe-oxyhydroxides, which form where ground water emerges from bank seeps.