Modeling the applicability of edge-of-field treatment wetlands to reduce nitrate loads in the Elm Creek watershed in southern Minnesota, United States

Abstract

Constructed agricultural treatment wetlands are key tools for removing nitrate (NO₃^–) from surface waters. Due to limited funds for NO₃^– removal practices, investments need to be in the most cost-effective practices. Furthermore, NO₃^– removal practices that take less land out of production may be more appealing for farmers. Therefore, three types of wetlands were compared to determine NO₃^– removal effectiveness and cost effectiveness across a watershed. The three wetland types included large wetlands with drainage areas greater than 60 ha; small, edge-of-field wetlands with drainage areas covering fewer than 60 ha; and small, edge-of-field wetlands with a dual treatment system of surface treatment and subsurface treatment following infiltration. The Agricultural Conservation Planning Framework (ACPF) toolbox model was used to determine best placements for each wetland type in the Elm Creek watershed in southern Minnesota, United States. The Soil and Water Assessment Tool (SWAT) model was used to estimate the volume of tile discharge and nitrate-nitrogen (NO₃-N) concentration into each wetland over a 10-year period. A spreadsheet model was used to estimate the reductions of NO₃^– in each wetland over the same 10-year period. Small, edge-of-field wetlands with a saturated hydraulic conductivity (Ksat) of 0.17 m d^–1 were more effective at removing NO₃^– for the area removed from crop production in order to create the wetland (kg ha^–1 y^–1) and as cost effective (US$ kg^–1 NO₃-N removed) as the large wetlands. When the small wetlands had a low Ksat (8.64 × 10^–5 m d^–1), they were more effective for each area removed from crop production (p = 0.06) but not as cost effective (p = 0.003). This study suggests that constructing many small, edge-of-field treatment wetlands with high conductivity, dual-treatment systems to reach nutrient reduction goals would cost the same as constructing large wetlands but would remove fewer hectares of cropland from production. If landowners are interested in removing only a few hectares from crop production at a time, the wetland restoration would possibly still be beneficial. Minimum hectares should not be a limitation for restoration program enrollment as long as the wetland to watershed area ratio is still viable.