Ph.D Defense Seminar
Hydrology, Nutrient Removal, and Cost-Effectiveness of Small, Edge-of-Field Tile Drainage Treatment Wetlands
Agricultural Best Management Practices (BMPs) are being utilized to reduce the mass of nitrogen and phosphorus discharging into waterways. This research investigated the effectiveness and applicability of a small, edge-of-field constructed treatment wetland for reducing the discharge of nitrate and phosphorus from tile drainage in a 10.1 ha row crop field between 2013 and 2016.
The total area of the wetland was 0.22 ha and was seeded with wet prairie vegetation in 2013. Infiltration played a large role in the water budget of the wetland with 46-93% of the input volume infiltrating each year and infrequent surface flow through the outlet. Reductions of nitrate-N and total phosphorus over the course of four years were 61% (50-91%) and 10% (-74-49%) respectively. Infiltration greatly increased nitrate-N removal as it removed up to 98% of the nitrate-N which infiltrated into the subsurface flow of the wetland. Harvesting vegetation greatly improved phosphorus reductions as it could have removed 20-189% of the phosphorus load entering the wetland.
Reed canary grass (Phalaris arundinacea) began invading the wetland by the second year of the study. When the seeded plant communities and reed canary grass were isolated in a laboratory mesocosm experiment, the wet prairie, forb-dominant plant community removed more nitrate-N than the reed canary grass monocultures. Furthermore, the ratio of denitrifying bacteria to total bacteria was greatest in the wet prairie community soil compared to the soil of other plant communities. Therefore, the wet prairie, forb-dominant mix provided better conditions for nitrate removal.
The reduction results were used to model the best locations for treatment wetlands in the Elm Creek HUC12 watershed. It also compared the reduction potential and costs of small, edge-of-field treatment wetlands less than 0.4 ha compared to larger wetlands averaging 11.9 ha. The modeling component of this research estimated that the mass of nitrate-N removed from tile drainage discharge was comparable between the two wetland sizes. The cost for every kg of nitrate-N removed was also comparable between larger wetlands and smaller wetlands when the smaller wetlands had high infiltration rates.