The role of fluvial sediments in modifying phosphorus export from the Upper Illinois River Watershed, Arkansas

Anthropogenic phosphorus (P) loading to freshwaters is a key concern facing Northwest Arkansas and the Upper Illinois River Watershed (UIRW). the focus of this study. Inputs of P to surface waters are cited as a key contributor to accelerated eutrophication. Subsequent reductions in overall water quality due to diverse inputs from both urban and agricultural sources have been researched, and measures have been implemented to reduce P loads. While strategies have been put in place, stream sediments are a critical, while less understood, link in the fate and transport of P from the landscape to overlying waterbodies and represent a potential source of a legacy effect in which prior management strategies continue to influence stream dissolved reactive P (DRP) concentrations. Thus, a comprehensive study of stream sediments within the region is necessary to understand whether or not bed sediment is still potentially a key regulator of DRP in the region. A three-fold approach was implemented; classification of physical and chemical properties of bed sediments, evaluation of extracting solutions for use in P-sorption isotherm experiments to replicate stream water, and P uptake and release in a simulated stream channel utilizing a high P poultry litter source to evaluate typical agricultural practices of the region. Five sites were selected in the UIRW representing agricultural, forest, and urban sites. Sediment was collected and analyzed for a myriad of factors for each experiment. Results showed that streams in the UIRW are comprised of a mix of substrate sizes, a large portion of which are > 20-mm where P reactions are mainly biologically driven. Fine sediments which were approximately 15 % of bed sediment influenced chemically driven P reactions. Within the region DI water is most comparable to stream water and thus the most appropriate equilibrating solution to characterize P sorption and desorption properties of sediments. Based on fluvarium experiments, sediments in the watershed have a high affinity to sorb P (86 --- 96%), with varying amounts of P subsequently desorbed (1 --- 7 %). Across studies, sites showed a strong relationship to modified P saturation ratio (PSRmod). This implies that prior land use management of the watershed has affected sediment properties, which continue to influence DRP levels of streams. Thus, a lack of decrease in P loads in streams of the UIRW is not indicative of a lack of effectiveness of recently implemented management strategies, rather it is likely a lack of sufficient time for reduction in both soil P levels, through adoption of agricultural and urban best management practices (BMPs), and stream sediment bound P acting as continued regulators of stream DRP levels.