RAPID: Draining the landscape: Will the drop in stream and groundwater levels following dam removal increase the leaching/leakage of nitrogen (N) from riparian zones?

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A large number of dams are currently being removed across the U.S. and at an accelerating pace. Most of the dam removals are driven by the need to increase public safety, reduce owner liability, and improve fish/aquatic habitat. However, little attention is being paid to how dam removals could influence water quality. Generally, dams slow down the flow of water and simultaneously raise the level of water in the stream and in the stream-side areas known as riparian areas. Unintentionally, this provides a valuable benefit: wet soils rich in carbon act as important filters for nitrogen, a key pollutant in waterways. This natural filtering service reduces stream water nitrogen concentrations and improves water quality. Dam removals could undermine this valuable service and potentially increase nitrogen in streams. This study will quantify how dam removal alters the capacity of riparian soils in filtering nitrogen. Results will be conveyed to landowners and natural resource agencies which will position them to make informed decisions on dam removals and better manage our watersheds. The study will provide a valuable opportunity for education and professional development for undergraduate and graduate students. In addition to presentations at professional conferences and meetings, project results will be disseminated to broader, non-academic audiences through talks at community Science Cafes.Near-surface groundwater and wet soil conditions in riparian zones, particularly upstream of dams, enhance denitrification in soils - a key mechanism by which riparian zones filter and remove nitrogen (N) from soils and act as hot spots for nitrogen buffering. Dam removals can lower groundwater levels, expose and dry riparian soils, and result in loss of valuable denitrification ecosystem services. Furthermore, drying of soils could enhance mineralization and nitrification, processes that release soil N and increase water pollution. This could result in riparian zones switching from watershed N sinks to sources. This hypothesis (increasing soil N leakage following dam removal) will be tested at the Chickies Creek site in Pennsylvania where the Krady Mill dam was recently removed. Stream water and ground waters at two riparian transects will be monitored and sampled weekly to study changes in concentration of nitrogen species. Riparian soil samples along the transects and at various soil depths will be collected to measure changes in soil N concentrations and the rates of mineralization, nitrification and denitrification. Measured process rates along with areal extent of the drained soils will be used to estimate/predict the N leakage (kg per unit stream length) following dam removal. Considering increasing dam removals across the U.S., results from this study will provide novel insights critical for watershed management and assessing the benefits and drawbacks of dam removal.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.