Effect of Tributary Inflows on the Distribution of Trace Metals in Fine-Grained Bed Sediments and Benthic Insects of the Clark Fork River, Montana

Abstract

The effect of tributary inflows on metal concentrations in <63-μm sediments and benthic insects was examined on two scales (380 km and <2 km) in a river impacted by mining. A dilution−mixing model effectively described large-scale dispersion of Cd, Cu, and Pb in the sediments of the river. Input of metal from contaminated flood plains may introduce additional contamination in the middle reaches of the river. Intensive sampling around the confluences of two tributaries showed that there were significant, localized decreases in some metal concentrations immediately downstream of the inflows. Sediment metal concentrations 1 km below the inflows returned to values within the range predicted by the dilution−mixing model. Metal concentrations in benthic insects exhibited spatial patterns similar to those of the sediments, indicating that biological exposures to metals are at least partially dependent on the physical processes controlling the dispersion of sediment-bound metals. Tributary inflows introduce variability in metal contamination on different spatial scales that must be considered when assessing ecological risks in contaminated rivers. In addition to large-scale dilution of contaminants, smaller areas of reduced metal exposure occur near tributary inflows. These may shelter metal-sensitive taxa from severe metal contamination in the mainstem.