Quantifying Nitrate Dynamics of a Confluence Floodplain Wetland in a Disturbed Appalachian Watershed: High-Resolution Sensing and Modeling

Abstract

Abstract. Floodplain wetlands often form at confluences of main river channels and small, low-order tributary streams, yet their impact on nutrient removal and downstream loading is poorly understood. We coupled high-resolution water quality data with deterministic numerical modeling of hydrologic, hydraulic, and biochemical processes impacting nitrate fate and transport in a confluence floodplain wetland along the Ohio River from June 2017 to May 2018. The modeling results were used to quantify loading from a disturbed forested watershed (Fourpole Creek watershed) and the nitrate removal capacity of the wetland. Loading from the Fourpole Creek watershed to the wetland was on the same order of magnitude of other disturbed forested systems on an annual basis (3.6 kg N ha-1 year-1); however, the timing of peak nitrate loading and concentration from the tributary was associated with high flow conditions, contrasting with the timing of loadings typical of disturbed forested landscapes. Our results show that coupling the model with high-resolution data allowed us to estimate removal rates in the wetland, suggesting that 2.6% to 58.5% (optimally 12.7%) of nitrate was removed on an annual basis, despite the wetland comprising only 0.42% of the overall drainage area. We found that increasing inundation of the wetland confluence promoted enhanced residence times of stormflows and was the most important driver of nitrate removal rate and loading, with peak wetland inundation periods representing 26% of the removal load but occurring only 9% of the time. The findings of this study highlight the importance of using high-resolution data-model integration to quantify nutrient dynamics in complex landscapes, identifies the significance of connectivity of watershed nitrate loadings to floodplain wetland soils, and highlights the importance that these features will have in the future, given the enhanced potential for harmful algal blooms in the Ohio River. Keywords: Confluence floodplain wetlands, High-resolution water quality data, Nitrate fate and transport modeling, Nitrate loading, Wetland nitrate removal.