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
by
Judson W. Harvey (U.S. Geological Survey, Menlo Park, Calif.),
Christopher C. Fuller (U.S. Geological Survey, Menlo Park, Calif.), and
Brian J. Wagner (U.S. Geological Survey, Menlo Park, Calif.)
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.
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