Floodplain reconnection in agricultural landscapes and tradeoffs in water quality

Floodplain reconnection in intensively managed agricultural landscapes has the potential to retain and possibly remove excess sediment, nutrients, and other pollutants. Reestablishing connectivity between the river and its floodplain can create optimal conditions for physical and biological retention to occur that positively influence water quality. While floodplain inundation creates anoxic conditions that promote nitrogen removal via denitrification and allows deposition of sediments and attached pollutants, it can also release phosphorus from sorption sites that are maintained under oxic conditions. Our work aims to understand drivers of these tradeoffs through an integrated modeling and monitoring approach. We are using the Wabash-Tippecanoe River (WTR) confluence as a model ecosystem that is representative of riverine floodplains in the Midwestern USA. We are using a combination of in situ monitoring of sediment and nutrient loading coupled with laboratory experiments to measure nutrient fluxes, timing of floodplain inputs, and environmental drivers of biogeochemistry. We are also using a novel groundwater-surface water model (ICPR, Interconnected Channel and Pond Routing) to inform event-scale drivers of biochemical processes and the widely applied HEC-RAS 2D to characterize river-floodplain connectivity over annual timescales. Hydrodynamic modeling and geomorphic metrics reveal complex flowpaths and residence times during peak flood pulses that persist for multiple weeks, which affects both redox and nutrient/sediment loading. Results from seasonal flooded sediment core mesocosms suggest that inundation periods of 4-8 days may enhance nitrogen removal while minimizing phosphorus release. Machine learning analysis show that denitrification rates are strongly influenced by internal flowpaths through the floodplain and vegetation, and to a lesser extent by substrate supply (nitrate and carbon). Ongoing experiments are assessing the influence of carbon quality via PARAFAC modeling on denitrification with depth and river connectivity. By integrating these approaches, we aim to better characterize drivers of water quality in these complex systems while providing much needed support for decision makers to prioritize restoration.