Metal distribution in Clark Fork River sediments

Accurate sampling of pore waters and sediments in gravel river beds is prob lematic because cobbles preclude most coring techniques, while the angulari ty of sand grains destroys the membranes of standard pore water samplers. I n 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 sedime nt (similar to 65% cobbles and 35% gravel substrate), and a novel pore wate r sampler was developed to collect interstitial water. In the sediment, the < 63 um fraction comprised <4 wt % yet contained approximately an order of magnitude higher metal concentrations compared to the 63 mum-2 mm fraction (constituting similar to 20% of the mass). However, on a mass basis the sa nd fraction contained 60% of the metals, compared to 40% in the clay/silt f raction in the gravels, while in point bars the distribution was similar to 50:50. The metals occur predominantly in sulfides frequently armored with an oxide rim and other sparingly soluble phases that may explain the low po re water metal concentrations, These data demonstrate that consideration of multiple particle size cutoffs is necessary to accurately characterize flu vial bed sediment metal conditions and that the form of the metal is import ant in understanding metal solubility in the benthos.