104B Base Grants – Nutrient Removal Efficiency

The impact of nutrients exported from croplands on water quality can be minimized if, instead of being directly discharged directly into nearby streams, agricultural runoff and subsurface tile drainage can be channeled to temporary detention systems such as wetlands where bio-transformation of nutrients can occur. However, this approach could pose operational challenges. First, it could interfere with farming activities if wet soil conditions persist in adjacent crop fields. Thus, wetland operators need to strike a balance between field accessibility and increased system efficiency achievable with longer hydraulic residence time (HRT). Second, removal of NO3– and SRP – the pollutants most commonly present agricultural discharge often requires different redox conditions, suggesting that complete treatment can be accomplished in wetlands with multiple water-flow paths. Information is also lacking regarding the impact of treatment wetlands on air quality, specifically emission of greenhouse gases (GHG; N2O; CH4) into the atmosphere. Thus, the objectives of the proposed studies are to: (i) examine how HRT and water flow paths affect the performance of the constructed wetland, (ii) identify biogeochemical processes controlling nutrient transformation, and (iii) determine if constructed are potentially significant sources of N2O and CH4 emission in agricultural landscapes. The proposed study will be conducted at a constructed wetland (4-yr old) that comprises a surface and a subsurface flow cells. The wetland design also allows experimental adjustments of HRT and flow path (surface vs subsurface), thus making it possible to assess the impact of these parameters on system performance. The wetland is located downslope from a corn/soybean agricultural field that is actively monitored as part of a multi-agency edge-of-field effort. Significant synergies are expected between these two projects. Results of this study will have implications for the design, operations and acceptability of treatment wetlands in agricultural landscapes of the US Midwest.