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

近年来，密尔丹的清除有所增加，美国大西洋中部的清除率最高。拆除大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性，并减少经济责任。然而，很少有研究研究大坝拆除对水质的影响。水坝往往会堵塞和减缓水流，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，加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，拆除大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。本研究将探讨水坝改善溪流及河滨森林水质的关键过程。这项研究的结果将传达给负责水质管理和水坝拆除的流域管理者和自然资源机构。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。该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。当地一名环境历史学家将与本科生合作，通过研究历史照片、文件和财产信息，记录17世纪至20世纪期间水坝对河流状况的影响。水坝降低了河流和地下水的流速，并增加了水坝上游的地下水位。河岸地带的高地下水位为反硝化创造了热点，并增加了氮(N)去除的机会。同样，在河流中停留时间的增加以及大坝上游细沉积物和有机物的沉积增加了通过河流反硝化作用造成的N损失。低水头水坝在河岸土壤中创造了一个水文过渡/分化区，补充了溪流的河流间断概念。这些假设将在特拉华州怀特克莱溪的三个现有的、连续的低水头碾磨坝站点进行测试。地下水水井和相关的化学取样将描述地下水混合情况。流分析和测量将有助于描述流中的过程和变化。河流沉积物和河岸土壤的反硝化作用、硝化作用和矿化作用将量化氮的处理和消耗。这些数据将被综合起来，以开发一个新的、耦合的概念模型，用于河流和河岸过程以及水坝上游的混合制度。这将是第一个调查现有水坝如何影响河岸地下水水文和水质的研究。这项研究还将为与死水或池水或“死水”河流状况相关的水文和生物地球化学条件提供重要见解。这些条件当然存在于殖民和后殖民时期的水坝河流中，但也存在于今天的现代、断开的和城市化的河流网络中。如果这项研究表明，水坝附近的河岸地带和溪流确实是去除氮的热点和汇，那么拆除水坝对栖息地、安全和自然流动制度的好处将不得不与失去宝贵的水质生态系统服务以去除氮的潜在不利因素进行权衡。这项研究的结果将在科学会议上提出，发表在评审期刊上，还将通过科学咖啡馆和社区论坛向更广泛的社区和利益相关者传达。该奖项反映了美国国家科学基金会的法定使命，并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，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.