Abstract
Elemental Chemistry of Streambed Sediments of the St. Croix River Basin: A 25-Year Review
Rivers and their watersheds are dynamic systems that reflect both natural geologic processes and human influences, with streambed sediments serving as long-term integrators of trace-element inputs. The reassessment of the elemental chemistry of fine-fraction (<63 µm) streambed sediments in the St. Croix River Basin, a National Scenic Riverway spanning Minnesota and Wisconsin, was completed by replicating and updating the U.S. Geological Survey’s 2000 basin-wide survey. Sediments were collected from 30 locations in 2025, mirroring the original spatial distribution, and analyzed for major and trace elements including arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zinc. Comparison of 2000 and 2025 datasets reveal substantial declines in both major and trace-element concentrations over the 25-year interval. Median concentrations of lithophilic and chalcophilic metals decreased by up to an order of magnitude, with cadmium largely undetectable in 2025, indicating reduced anthropogenic inputs and improved sediment quality. Major-element declines, particularly in aluminum and iron, suggest altered sediment sourcing and reduced transport of fine mineral particles. Despite localized enrichments at individual sites, there is no consistent basin-wide pattern of contamination downstream of urban areas, and trace-element concentrations predominantly reflect natural geologic sources. These findings highlight the effectiveness of watershed management, regulatory controls, and improved land-use practices in mitigating trace-metal accumulation. The study underscores the value of long-term monitoring for understanding temporal changes in sediment chemistry, evaluating ecological risk, and informing conservation strategies in riverine systems.