EVALUATING NITROGEN TRANSPORT IN A HYDROLOGICALLY CONNECTED WETLAND OF THE MISSISSIPPI RIVER

The extent of coastal hypoxia in the northern Gulf of Mexico periodically grows to be the largest in the world, and negatively impacts the regions ecology and economy. The size and magnitude of this oxygen depleted zone is partially dependent on the interseasonal variability of nitrogen supplied by the Mississippi Rivers (MR) discharge into the Gulf. This flux of nitrogen originates from point and non-point sources along the river, such as sewage treatment effluent and agriculture fertilizer runoff. The distribution of these sources is widespread throughout the 3.2M square kilometer MR drainage basin, which makes organizing effective mitigation strategies problematic. However, hydrologically connected floodplain wetlands along the MR are a potentially important sink which may ameliorate riverine nitrogen loads. The objective of this research is to better understand nitrogen cycling in hydrologically connected floodplains by evaluating Wilkinson Island (WI) wetland, located in the Middle MR Wildlife Refuge, Jackson County, Illinois.

Preliminary measurements of the regions surface waters in late spring of 2018 indicate irregular changes in nitrate concentration [~10 mg/L/km], that were oriented obliquely to the inferred direction of hydrologic flow. These dynamics suggest WI sustains a reactive environment with yet unknown sources, sinks, and fluxes between that affect nitrogen cycling. These dynamics will be evaluated in two phases, 1) measure the WI geochemical system with high resolution (space and time) sampling of groundwater and surface-water quality [including Nitrate, Nitrite, Ammonium, free-Nitrogen, δ ¹⁸Oxygen / δ¹⁵Nitrogen isotopes], and 2) discretization of these observations into an accurate numerical model of the WI hydrologic flow and transport system to determine key variables that impact nitrogen cycling. The results of this analysis will be used to evaluate other hydrologically connected landscapes along the Middle MR floodplain, for their ability to reduce nutrient loads transported into the Gulf of Mexico.