306-9 A New Multipurpose Grass Waterway that Uses the Natural Landscape for Nitrate Reduction

Abstract:

In the rolling glacial till plains of the U.S. Midwest beyond the extent of Wisconsin glaciation, tile drainage often occurs in grass waterways and there is potential to utilize organic-rich waterway sediments for nitrate reduction. In this project funded by the Iowa Nutrient Research Center, we developed and tested a new “multipurpose grass waterway” (MGW) practice that leaves surface conditions unchanged while redirecting subsurface tile flow into shallow waterway sediments for nitrate remediation. Using standard control boxes and a series of tile valve structures, the MGW allows tile water to seep into subsurface sediments and hydrate waterway sediments from the bottom up. Geologic characterization indicated that the waterway was filled with fine-textured and organic-rich alluvial soils derived from erosion and deposition of upland loess and till. Using continuous flow monitoring data, approximately 51,500 m³ of tile water seeped into the waterway sediments over a two-year period. The water table was raised in a stepped fashion upgradient in the waterway but it remained >0.5 m below ground surface and allowed the producer to drive across the waterway with heavy farm machinery. Based on hydraulic heads and nitrate concentration reductions measured in 16 monitoring wells, the MGW removed 27 kg of nitrate mass from tile water. Given the ubiquity of grass waterways in rolling hills of the Corn Belt, there is great potential for MGWs to reduce tile nitrate loss to surface water across the region.

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