Subsurface Nitrate Processing Beneath Drainageways: Are They Landscape Opportunities for Subsurface Drainage Remediation?

Highlights Drainageways contain fine-textured and nutrient rich alluvial soils conducive for denitrification. NO 3 -N concentrations in drainageway groundwater were 70% lower than observed in cropped fields. A new grass waterway design connects subsurface water from upland cropped fields to the drainageway deposits for NO 3 -N remediation. Abstract. Efforts to reduce nutrient export from agricultural crop production in the U.S. Midwest are leading to development of new conservation practices. In this study our objectives were to: (1) characterize subsurface soils and hydrogeology found in two main drainageway areas in eastern Iowa, (2) compare groundwater quality to upland agricultural fields, and (3) utilize a groundwater flow model to assess the capacity of drainageways to provide additional NO3-N processing in agricultural watersheds. Using data obtained from a network of 12 shallow wells installed across six different waterways, we found that the waterways contained fine-textured and nutrient rich alluvial soils derived from erosion and deposition of upland loess and till. Concentrations of NO3-N in waterway groundwater (3.1 mg/l) were 70% lower compared to groundwater beneath nearby cropped fields (10.5 mg/l). A shallow water table in the organic-rich drainageway soils provides the requisite organic carbon, anaerobic soil conditions, and nitrogen supply for denitrification to occur. Numerical modeling suggested that groundwater from the surrounding catchment discharges approximately 53 m3/day into the waterways and reduces NO3-N mass by 144.3 kg/yr, or 7.8 kg/ha. Results suggest that drainageways could be better exploited for additional NO3-N reductions from subsurface drainage if the flow could be diverted into these areas. Keywords: Denitrification, Grass waterway, Nitrate-nitrogen, Saturated buffer.