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

近年来，米尔丹的清除量有所增加，其中美国大西洋中部地区的清除率最高。拆除大坝的主要动机一直是为了改善鱼类通道和栖息地，提高娱乐用户的安全，并减少经济负担。然而，很少有研究研究大坝拆除如何影响水质。水坝往往会使溪水回流和减慢速度，并提高溪流和邻近土壤的水位。这提供了一个意想不到的好处，即加强河边土壤和森林对氮的自然过滤服务，氮是我国水道的一种关键污染物。因此，拆除大坝可能会破坏这种宝贵的过滤服务，并增加我们清理水道的成本。这项研究将调查水坝改善溪流和溪边森林水质的关键过程。这项研究的结果将传达给分水岭管理者和负责水质管理和大坝拆除的自然资源机构。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。这项研究还将为特拉华大学的两名研究生和一名本科生提供宝贵的教育经验。当地的一位环境历史学家将与这位本科生合作，通过研究历史照片、文献和财产信息，记录米尔坝是如何影响1700-1900年代的河流状况的。米尔坝降低了水流和地下水的流速，并提高了大坝上游的地下水位。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，河流停留时间的增加以及大坝上游细小沉积物和有机物的沉积，都会增加河流反硝化过程中氮的损失。低水头水坝在河岸土壤中创造了一个水文过渡/分流地带，这与为河流提出的河流不连续体概念相辅相成。这些假设将在特拉华州白泥溪的三个现有的、连续的低水头磨坊大坝上进行测试。地下水井和相关的化学采样将成为地下水混合制度的特征。河流分析和测量将有助于表征河流中的过程和变化。水系沉积物和河岸土壤上的反硝化、硝化和矿化培养将量化N的处理和消耗。这些数据将被整合起来，以开发一个新的、耦合的、关于河流和河岸过程以及水坝上游混合制度的概念模型。这将是第一次研究现有的水坝如何影响河岸地下水、水文和水质。这项研究还将对与停滞或积水或“狭小”河流状况有关的水文和生物地球化学条件提供重要的见解。这些条件当然存在于殖民时代和后殖民时代的堤坝溪流中，但今天也存在于现代、断开连接和城市化的河流网络中。如果这项工作表明，河岸带和水坝附近的溪流确实是氮去除的热点和汇，则必须权衡清除水坝对栖息地、安全和自然水流制度的好处与失去有价值的水质生态系统服务的潜在负面影响。这项研究的结果将在科学会议上发表，在被推荐的期刊上发表，也将通过科学咖啡馆和社区宪章传达给更广泛的社区和利益相关者。这一奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.