Sediment that lies beneath the Mississippi River could, with better basic management practices, hold the potential to improve the river's natural ability to decrease its nitrate load, U.S. Geological Survey researchers say.
William Werkheiser, USGS associate director for water, says increasing nitrogen loads in part due to agricultural fertilization have been a concern for downstream waters, and have led to more in-depth study of how the rivers themselves already remove nitrogen from the water.
Part of the nitrogen removal process is completed by the thin sediment through which water flows underneath and to the side of the stream. This area, known as the "hyporheic zone," enhances chemical reactions by microbes that perform denitrification, a reaction that removes nitrogen from the water and converts it to nitrogen gas, explains Jesus Gomez-Velez, lead author of the study.
The research team determined that, throughout the Mississippi River network, vertical hyporheic exchange with sediments directly beneath streams and rivers has denitrification potential that far exceeds lateral hyporheic exchange with bank sediments.
"Rivers with more vertical exchange are more efficient at denitrification, as long as the contact time with sediment is matched with a reaction time of several hours," observed co-author Jud Harvey, the USGS team leader for the study.
The study findings suggest that managing rivers to help avoid the sealing of streambeds with fine sediments, which decreases hyporheic flow, would help exploit the valuable natural capability of rivers to improve their own water quality.
Other river management and restoration practices that protect permeable river bedforms could also boost efficiency, such as reducing fine sediment runoff to rivers, USGS says.
However, typical river channel restoration strategies that realign channels to increase meandering would not be as effective, because a comparatively small amount of water and river nitrate are processed through river banks compared with river beds.
Although not yet tested in the model, allowing natural flooding over river banks onto floodplains may also be an effective means of processing large amounts of river water to remove nitrogen before it reaches sensitive coastal waters, USGS says.