Heatmap visualizations of arsenic, cadmium, and copper concentrations in streambed sediment in the Clark Fork River, Montana, 1996-2020

Long-term monitoring of stream-bed sediments reveals spatial and temporal trends in metal concentrations. Here we use concentration gradient “heat maps” as a visualization tool to report annual mean arsenic, cadmium and copper concentrations along a contamination gradient in the Clark Fork River (CFR) in Western Montana. The CFR has been heavily impacted by large-scale mining operations since the 19th century. Legacy mine waste and tailings have been deposited within the streambed, banks, and floodplains more than 200 kilometers downstream. Sieved sediment samples (<63µm) have been collected at 10 stations along a 200 kilometer contamination gradient annually since 1996. Ongoing remediation activities in the upper basin (60 Km) have reduced the tailings deposits along the flood plain and may account for the reduction of some metals in the upper stations. Arsenic concentrations ranged from 12-204 µg/g from 2003-2020. The highest concentrations were observed in 2003 at river Km 11 while the lowest concentrations in all years occurred in the most downstream reaches. Concentrations of cadmium (1996-2020) ranged from 1-11 µg/g with the highest in the most upstream reaches between 2000-2003. Temporal trends in copper (1996-2020) ranged from 125-2053 µg/g with the highest concentrations observed in the most upstream stations during the earlier years.