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Nitrogen sinks or sources? Denitrification and nitrogen removal potential in riparian legacy sediment terraces affected by milldams

350 210 Stroud Water Research Center

Peck, E.K., S. Inamdar, M. Sherman, J. Hripto, M. Peipoch, A.J. Gold, and K. Addy. 2022. Journal of Geophysical Research: Biogeosciences, early online access.

https://doi.org/10.1029/2022JG007004

Summary

Floodplains adjacent to rivers are important ecosystems that provide valuable services including nutrient removal, especially nitrogen, from stream water. Because nitrogen is a major polluter of coastal waters, river floodplains are increasingly being restored as part of watershed best management practices. For example, millions of dollars are being spent annually in the Chesapeake Bay to install 900 miles of riparian buffers and on other watershed practices to mitigate nutrient pollution. However, the impact of small, colonial-era milldams on floodplain nitrogen mitigation is poorly understood, despite >14,000 such structures still present across streams of the eastern US. We studied the impact of two small milldams (Roller mill on Chiques Creek, Lancaster, Pennsylvania and Cooch mill on Christiana River, Newark, Delaware) on the ability of floodplains to remove or store nitrogen. We found that the stagnant water that accumulates behind milldams restricts floodplains from effectively removing nitrogen and may actually cause the accumulation of nitrogen. Whether accumulated nitrogen is released back into streams is unknown but concerning. Removal of dams would likely improve many ecosystem services of both streams and floodplains, with minimal consequences for the nitrogen mitigation abilities of these ecosystems.