Nitrogen Dynamics in Riverine Wetlands of the Leveed Middle Mississippi River Floodplain

Author

• Joseph M. Krienert, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2026

Degree Name

Doctor of Philosophy

Department

Geology

First Advisor

Remo, Jonathan

Abstract

Excess dissolved inorganic nitrogen (DIN) in the Mississippi River basin degrades aquatic ecosystems and drinking‑water quality. Riverine wetlands can mitigate these impacts by providing hydrologic and geochemical conditions that support nitrification and denitrification, converting DIN to inert nitrogen gas. This service may be especially important along high‑order rivers, yet its magnitude and controls remain unclear. This research evaluates DIN processing in two riverine wetlands along the 8th‑order middle Mississippi River (MMR) floodplain, one connected to the River and one disconnected by levees.This valley was shaped over millennia by lowland river processes and glacial outwash, filling the confined bedrock valley with alluvium. These sediments form the physical template of flow paths, redox gradients, and reaction zones where DIN processing can occur. Historically, ~80% of the floodplain inundated during moderate to large floods, supporting extensive wetlands. Construction of levees in the 1940s reduced inundation to < 40%, leaving wetlands that are connected to (Wilkinson), and disconnected from (Station) the MMR. This study integrates two years of monthly sampling of these wetlands rain, snow, river water, surface water, and groundwater from a three‑unit aquifer system: a < 10 m low‑permeability silt‑and‑clay layer, a >50 m higher‑permeability sand aquifer, and karstic, faulted bedrock underlying up to 100 m of alluvium. Hydrologic exchange among these reservoirs was quantified using hydraulic gradients, numerical modeling (hydrodynamic, satellite‑derived inundation mapping, and groundwater flow), and stable isotope ratios of water. Results show that Wilkinson supports two‑way exchange with the MMR, enabling low level recharge during floods and baseflow during draining. The Station showed trace recharge and persistent low level baseflow. These dynamics sustain three mixing zones: (1) hyporheic mixing of Wilkinson surface water with shallow and sand‑aquifer groundwater, which was the most favorable hydrologic conditions for DIN processing; (2) perirheic mixing between wetlands along levees; and (3) local mixing at the Station driven by upward flow from bedrock and the sand aquifer. Comparisons of transport, mixing, and DIN reactivity show that Wilkinson’s hyporheic and perirheic zones supported the nitrification–denitrification sequence, with nitrate and nitrite concentrations declining during winter flooding and summer draining. The sand aquifer provided the most consistent denitrification conditions, yet recharge to it was limited. The Station also lacked the groundwater recharge needed for denitrification processing and instead showed trends of increasing nitrate and nitrite concentrations exported for all seasons. Microbial, carbon, and isotopic analyses corroborate these patterns. Wilkinson hosted diverse low-abundance denitrifiers and isotopic tracers of low‑level denitrification, whereas the Station showed high abundance of nitrifiers and tracers of nitrification, potentially supported by the unexpected trace nitrate and high level nitrite in bedrock and sand aquifer groundwater baseflow. Those bedrock sourced waters were also the only consistent synthetic DIN, likely from agriculture in the uplands that transports through the valleys karstic and faulted strata. Overall, the Station wetland showed limited capacity for DIN removal and may instead export DIN through groundwater‑fed drainage. In contrast, Wilkinson supports hydrologic and biogeochemical conditions that facilitate DIN removal and help improve MMR water quality.