Evaluating the potential role of denitrifying bioreactors in reducing watershed-scale nitrate loads: A case study comparing three Midwestern (USA) watersheds

Authors: Moorman, Thomas; Tomer, Mark; Smith, Douglas; Jaynes, Dan

Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/29/2014
Publication Date: 2/1/2015
Publication URL: http://handle.nal.usda.gov/10113/60179
Citation: Moorman, T.B., Tomer, M.D., Smith, D.R., Jaynes, D.B. 2015. Evaluating the potential role of denitrifying bioreactors in reducing watershed-scale nitrate loads: A case study comparing three Midwestern (USA) watersheds. Ecological Engineering. 75:441-448. doi: 10.1016/j.ecoleng.2014.11.062.

Interpretive Summary: The transport of nitrate (NO3-N) from agricultural lands to surface waters is a complex and recalcitrant problem. Subsurface drainage systems that are especially prevalent in the corn-growing regions of the Midwestern USA facilitate NO3-N transport. Several conservation practices, including fertilizer and manure management, cover crops, natural and installed wetlands, and wood-chip denitrification bioreactors are options that can mitigate NO3-N losses from agricultural lands. Using simple methods of estimation we examine methods for determining the cumulative volume of denitrification of bioreactors to treat various amounts of NO3-N in base flow, which is composed mainly of tile drainage, at the watershed scale. The use of load duration curves from three different watersheds shows that NO3-N transport is disproportionately skewed towards larger base flows. Approximately 50% of the annual NO3-N is transported in the largest 30% of daily base flows. Our analyses suggest that cumulative watershed bioreactor volumes sufficient to achieve an hydraulic residence time (HRT) of 0.5 days will reduce at least 20% of the total annual NO3-N loss in one watershed and 30% in the other two watersheds. The area required for wood-chip bioreactors would be 0.27% or less of the watershed area. These results illustrate methods for determining the potenial effectiveness of this conservation practice which informs conservationists, watershed managers, and drainage districts.

Technical Abstract: The transport of nitrate (NO3-N) from agricultural lands to surface waters is a complex and recalcitrant problem. Subsurface drainage systems that are especially prevalent in the corn-growing regions of the Midwestern USA facilitate NO3-N transport. Several conservation practices, including fertilizer and manure management, cover crops, natural and installed wetlands, and wood-chip denitrification bioreactors are options that can mitigate NO3-N losses from agricultural lands. Using simple methods of estimation we examine methods for determining the cumulative volume of denitrification of bioreactors to treat various amounts of NO3-N in base flow, a proxy for tile drainage, at the watershed scale. The use of load duration curves from three different watersheds shows that NO3-N transport is disproportionately skewed towards larger base flows. Approximately 50% of the annual NO3-N is transported in the largest 30% of daily base flows. Using previous estimates of NO3-N removal by wood-chip bioreactors we evaluated various combinations of bioreactor sizing to achieve various hydraulic residence time (HRT) given different waterhed base flows. These analyses suggest that cumulative watershed bioreactor volumes sufficient to achieve an HRT of 0.5 days will reduce at least 20% of the total annual NO3-N loss in one watershed and 30% in the other two watersheds. The area required for wood-chip bioreactors would be 0.27% or less of the watershed area.