The Impact of Wetland Biogeochemistry and Hydraulic Loading on Tile Drainage Nutrient Removal Efficiency

Nutrient releases from agriculture represent one of the major sources of nitrogen (N) and phosphorus (P) loading to the environment. Excess N and P can lead to deleterious effects in receiving waters, including eutrophication, species disruption, and aquatic hypoxia. One promising technology for reducing such releases is the use of constructed wetlands to remove primarily N through stimulated denitrification. More widescale adoption of this technology will require a better understanding of what limits N removal efficiencies. To address this knowledge gap, I have studied nutrient dynamics in two constructed wetlands to capture and treat agricultural tile drainage installed in Bureau County, Illinois. I analyzed inlet and outlet NO3--N concentration and sediment organic matter (OM) concentrations during wetlands development and operations. To help understand the nitrate removal process in the system, I characterized the hydraulic loading and performed tracer studies to measure residence time. These data used to determine mixing and short-circuiting, a process that can create dead zones and that impacts N removal efficiencies. I also studied the denitrifying microbial community structure in the wetland. Both constructed wetlands N removal efficiency improved over time by location, over the years, and over seasons with the minimum accumulated mass removal of 825 kg NO-3-N per year for each wetland. Species diversity and concentrations increased as labile OM increased, and the denitrifying community was largely gone following the sediment drying period, concomitant with a significant decrease in labile OM.