Mercury in sediment of the upper Clark Fork River, Western Montana, USA

As the clean-up of pollutants from large-scale mining progresses in the Clark Fork River (CFR) watershed, mercury (Hg) emerges as an important factor limiting the environmental health and human use of the CFR and its tributaries. Our study is the first to provide a comprehensive set of field data for total Hg concentration in sediment along the upper CFR and its tributaries, with special focus on Flint Creek, the tributary where the majority of Hg in the CFR originates and where targeted remediation activities have recently started. We use spatial modeling to infer the baseline Hg contamination status from a highly variable dataset and to quantify Hg contributions from individual tributaries. Concentrations substantially exceed Severe Effects Levels (2 mg/kg) in most of Flint Creek and in two sections of the CFR. Our stepwise exponential model for Hg concentration integrates mass-balance mixing at tributary confluences and longitudinal decay of Hg concentration with measurement uncertainty through a Gamma error distribution. Longitudinal losses cause Hg concentration to decrease by half every ∼134 km in most of the CFR. We demonstrate the suitability of the model for predicting outcomes of targeted elimination of Hg sources, which may aid in the development of effective remediation programs.