Geologic misfortune: Diel and tracer studies reveal surface flows, groundwater inputs, solute sources, rare earth element, and metal mobility in an undisturbed mineralized mountain catchment impacted by acid rock drainage

In mineralized regions of the Colorado Rockies, acid rock drainage (ARD) presents an ongoing challenge to water resources. Many watersheds are also experiencing shifts in climate, with linkages to decreasing water quality. Here we examined the Upper Snake River (USR), an ARD-impaired catchment abutting the Continental Divide where one small highly acidic tributary has been proven to be the dominant source of acidity, metals, and rare earth elements (REEs). Long-term research indicates that enhanced weathering and dissolution of metals and REEs is linked to warming summers, drought, and declining baseflows. To better understand these trends, we conducted diel and tracer studies at the confluence of this tributary with the USR. These revealed steady concentrations of ARD solutes over a 24-hour period, conservative solute behavior and substantial groundwater input near the confluence. The increased loadings below the confluence suggest strong hyporheic zone connectivity and contributions from an adjacent fen. Our findings underscore the importance of resolving streamflow and solute dynamics to reveal sources, sinks, and/or redox conditions driving downstream fluxes. Studies of the Snake River Watershed continue to offer insight into the hydrological and geochemical controls on ARD, exemplifying the connectivity of mountain hydrology and water quality in a changing world.