Growth of Coal Mining Operations in the Elk River Valley (Canada) Linked to Increasing Solute Transport of Se, NO(3)(–), and SO(4)(2–) into the Transboundary Koocanusa Reservoir (USA–Canada)

[Image: see text] Koocanusa Reservoir (KOC) is a waterbody that spans the United States (U.S.) and Canadian border. Increasing concentrations of total selenium (Se), nitrate + nitrite (NO(3)(–), nitrite is insignificant or not present), and sulfate (SO(4)(2–)) in KOC and downstream in the Kootenai River (Kootenay River in Canada) are tied to expanding coal mining operations in the Elk River Watershed, Canada. Using a paired watershed approach, trends in flow-normalized concentrations and loads were evaluated for Se, NO(3)(–), and SO(4)(2–) for the two largest tributaries, the Kootenay and Elk Rivers, Canada. Increases in concentration (SO(4)(2–) 120%, Se 581%, NO(3)(–) 784%) and load (SO(4)(2–) 129%, Se 443%, NO(3)(–) 697%) in the Elk River (1979–2022 for NO(3)(–), 1984–2022 for Se and SO(4)(2–)) are among the largest documented increases in the primary literature, while only a small magnitude increase in SO(4)(2–) (7.7% concentration) and decreases in Se (−10%) and NO(3)(–) (−8.5%) were observed in the Kootenay River. Between 2009 and 2019, the Elk River contributed, on average, 29% of the combined flow, 95% of the Se, 76% of the NO(3)(–), and 38% of the SO(4)(2–) entering the reservoir from these two major tributaries. The largest increase in solute concentrations occurred during baseflows, indicating a change in solute transport and delivery dynamics in the Elk River Watershed, which may be attributable to altered landscapes from coal mining operations including altered groundwater flow paths and increased chemical weathering in waste rock dumps. More recently there is evidence of surface water treatment operations providing some reduction in concentrations during low flow times of year; however, these appear to have a limited effect on annual loads entering KOC. These findings imply that current mine water treatment, which is focused on surface waters, may not sufficiently reduce the influence of mine-waste-derived solutes in the Elk River to allow constituent concentrations in KOC to meet U.S. water-quality standards.