Collaborative Research: Low-head milldams as hotspots for denitrification and nitrogen consumption: Hydrologic and biogeochemical controls

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Milldam removals have increased in recent years with the highest removal rate in the mid-Atlantic US. The primary motivation for dam removals has been to improve fish passage and habitat, enhance safety for recreational users, and reduce financial liability. Few studies have however studied how dam removals could influence water quality. Dams tend to back-up and slow down stream water and raise water levels in the streams and the adjacent soils. This provides an unintended benefit of enhancing the natural filtering service of streamside soils and forests for nitrogen, a key pollutant of our nation's waterways. Thus, dam removals could undermine this valuable filtering service and increase the cost of cleaning our waterways. This study will investigate the key processes by which milldams enhance water quality in streams and streamside forests. Results from this study will be conveyed to watershed managers and natural resource agencies responsible for water quality management and dam removals. This knowledge will help them make better and more informed decisions on dam removals. The study will also provide valuable educational experience for two graduate and one undergraduate student from the University of Delaware. A local environmental historian will work with the undergraduate student and document how milldams influenced stream conditions back in the 1700-1900s through study of historic photographs, documents, and property information.Milldams reduce stream and groundwater flow velocities and increase groundwater levels upstream of the dams. High groundwater levels in riparian zones create hotspots for denitrification and an increased opportunity for nitrogen (N) removal. Similarly, increased residence time in streams and deposition of fine sediments and organic matter upstream of the dam enhances the loss of N via instream denitrification. Low-head milldams create a zone of hydrologic transition/divergence in riparian soils that compliments the riverine discontinuum concept proposed for streams. These hypotheses will be tested for three existing, sequential low-head mill dam sites on White Clay Creek in Delaware. Groundwater wells and associated chemical sampling will characterize the groundwater mixing regime. Stream assays and measurements will help characterize the in-stream processes and changes. Denitrification, nitrification and mineralization incubations on stream sediments and riparian soils will quantify N processing and consumption. These data will be integrated to develop a new, coupled, conceptual model for stream and riparian processes and mixing regimes upstream of milldams. This will be the first study to investigate how existing milldams affect riparian groundwater hydrology and water quality. This research will also provide important insights into hydrologic and biogeochemical conditions associated with stagnant or pooled waters or "lentic" river regimes. These conditions certainly existed for the dammed streams during the colonial and post-colonial era, but also exist today in modern, disconnected, and urbanized stream networks. If this work shows that riparian zones and streams near milldams are indeed hotspots and sinks for N removal, the benefits of dam removal for habitat, safety, and natural flow regimes will have to be weighed against the potential downside of losing valuable water quality ecosystem services for N removal. Results from this study will be presented at scientific meetings, published in refereed journals, and will also be conveyed to the broader community and stakeholders through Science Cafes and community charrettes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.