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

近年来，米尔达姆的清除量有所增加，美国大西洋中部的清除率最高。拆除水坝的主要动机是改善鱼类通道和栖息地，提高娱乐用户的安全，并减少财务责任。然而，很少有研究研究大坝拆除如何影响水质。水坝往往会阻碍和减缓溪流的水流，并提高溪流和附近土壤的水位。这提供了一个意想不到的好处，增强了河边土壤和森林对氮的自然过滤服务，氮是我们国家水道的主要污染物。因此，拆除水坝可能会破坏这种宝贵的过滤服务，并增加清洁水道的成本。这项研究将调查的关键过程中，磨坊水坝提高水质的溪流和河边森林。这项研究的结果将传达给负责水质管理和大坝拆除的流域管理人员和自然资源机构。这些知识将帮助他们在大坝拆除方面做出更好、更明智的决定。这项研究还将为来自特拉华州大学的两名研究生和一名本科生提供宝贵的教育经验。一位当地的环境历史学家将与本科生一起工作，通过研究历史照片，文件和财产信息，记录米尔水坝是如何影响1700- 1900年代的河流状况的。米尔水坝降低了河流和地下水的流速，增加了水坝上游的地下水位。河岸带的高地下水位为反硝化创造了热点，并增加了脱氮的机会。同样，增加停留时间的流和沉积物和有机物上游的大坝增强损失的N通过instantly反硝化作用。低水头磨坊坝在河岸土壤中创造了一个水文过渡/发散区，这与为溪流提出的河流不连续体概念相符。这些假设将在特拉华州白色粘土溪的三个现有的连续低水头磨机坝址进行测试。地下水威尔斯井和相关的化学取样将表征地下水混合状况。流分析和测量将有助于表征流中过程和变化。反硝化，硝化和矿化孵化流沉积物和河岸土壤将量化N处理和消费。这些数据将被整合，以开发一个新的，耦合的，概念模型流和河岸过程和混合制度上游的milldams。这将是第一项研究，以调查如何现有的milldams影响河岸地下水水文和水质。这项研究还将提供重要的见解水文和地球化学条件与停滞或汇集沃茨或“lentic”河流制度。这些条件在殖民地和后殖民地时期的河流中确实存在，但在现代的、不连通的和城市化的河流网络中也存在。如果这项工作表明，河岸带和溪流附近的milldams确实是热点和水槽的N去除，大坝拆除的栖息地，安全和自然流动制度的好处将不得不权衡对潜在的负面影响，失去宝贵的水质生态系统服务N去除。这项研究的结果将在科学会议上发表，发表在权威期刊上，并将通过科学咖啡馆和社区charrettes传达给更广泛的社区和利益相关者。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。

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.