Metal Distribution in Clark Fork River Sediments

Accurate sampling of pore waters and sediments in
gravel river beds is problematic because cobbles preclude
most coring techniques, while the angularity of sand
grains destroys the membranes of standard pore water
samplers. In the Clark Fork River, the recipient of over 100
years of mining activity, a modified single tube core-freezing device was used to profile bed sediment (∼65%
cobbles and 35% gravel substrate), and a novel pore water
sampler was developed to collect interstitial water. In
the sediment, the <63 μm fraction comprised <4 wt %
yet contained approximately an order of magnitude higher
metal concentrations compared to the 63 μm−2 mm
fraction (constituting ∼20% of the mass). However, on a
mass basis the sand fraction contained 60% of the metals,
compared to 40% in the clay/silt fraction in the gravels,
while in point bars the distribution was ∼50:50. The metals
occur predominantly in sulfides frequently armored with
an oxide rim and other sparingly soluble phases that may
explain the low pore water metal concentrations. These
data demonstrate that consideration of multiple particle size
cutoffs is necessary to accurately characterize fluvial
bed sediment metal conditions and that the form of the
metal is important in understanding metal solubility in the
benthos.