Collaborative Research: In Situ Examination of Hydrogen Peroxide Sources and Sinks in a Slightly Alkaline Stream

光化学生成的过氧化氢（H2O2）在阳光照射的淡水中普遍存在，并可能发挥许多重要的生物地球化学作用。特别有趣的是，H2O2可能是地表水中羟基自由基（OH）的最重要来源，通过Fe（II）还原H2O2。羟基自由基氧化是将难降解有机化合物（包括污染物和天然有机物）转化为CO2和微生物底物的有效机制。目前尚不清楚湖泊和海洋系统中H2O2的来源和汇是否是河流中H2O2的最重要控制因素。特别是H2O2的还原作用，除了非常酸性的流外，还没有被探索过。此外，一个理论规模的河流示踪剂注射实验的初步建模表明，与保守示踪剂结合使用的H2-18O2可能对解决河流中生物地球化学过程建模的一个主要问题（例如氮循环、污染物生物降解、吸附）非常有用。需要一种示踪剂，能够区分涉及地表水池中的瞬时储存的流动路径和通过潜流带（地下）的流动路径。然而，H2-18O2作为示踪剂的应用需要更好地了解对表层和地下水中H2-18O2分解速率的控制。拟议工作的总体目标是进行第一次过氧化氢循环的综合研究，该研究被选为美国中部农业源头溪流的广泛代表。测量直接放置在溪流沉积物中的天然H2O2的命运和注入的同位素标记的H2-18O2示踪剂将用于测试我们目前对H2O2来源和汇在阳光下溪流水和底层沉积物中的理解。我们的研究结果将表明，过氧化氢产生羟基自由基是否可能是河流中有机污染物和天然有机物间接光解的主要机制。天然有机物的光解作用具有重要意义，因为它产生生物可利用的碳底物和二氧化碳，并被认为是生态系统尺度碳循环的重要控制因素。我们的工作也可以为开发加速芬顿反应在农业水源等系统中的方法奠定基础，以降低阿特拉津等污染物的浓度。最后，如果过氧化氢被证明是区分河流中地表和地下储存的良好示踪剂，那么所提出的工作将有助于开发更好的生物地球化学过程模型，例如这些环境中的氮循环和污染物的生物降解。这项提议的工作是一位新研究员（Durelle Scott）和一位中层女科学家（Bettina Voelker）的共同努力，并将支持科罗拉多矿业学院和内布拉斯加州大学的一名研究生和一名本科生。

Photochemically produced hydrogen peroxide (H2O2) is ubiquitous in sunlit freshwater and may play a number of important biogeochemical roles. Of particular interest is that H2O2 may be the most important source of hydroxyl radical (OH) in surface waters via reduction of H2O2 by Fe(II). Oxidation by hydroxyl radical is an effective mechanism for converting refractory organic compounds, including contaminants and natural organic matter, to CO2 and microbial substrates. It is still unclear whether the sources and sinks of H2O2 identified in lakes and marine systems are the most important controls on H2O2 in streams. The role of reduction of H2O2, in particular, remains unexplored in all but very acidic streams. In addition, preliminary modeling of a theoretical reach-scale stream tracer injection experiment suggests that H2-18O2, used in conjunction with a conservative tracer, could be uniquely useful for addressing one of the main problems of modeling biogeochemical processes in streams (e.g. nitrogen cycling, contaminant biodegradation, sorption): the need for a tracer that can distinguish flowpaths involving transient storage in surface water pools from flowpaths through the hyporheic zone (subsurface). However, H2-18O2's application as a tracer requires a better understanding of the controls on H2-18O2 decomposition rates in surface and subsurface waters. The overall goal of the proposed work is to conduct the first comprehensive study of hydrogen peroxide cycling in a natural stream chosen to be broadly representative of agricultural headwater streams in the central U.S. Measurement of the fate of natural H2O2 and an injected isotope-labeled H2-18O2 tracer in mesocosms placed directly into stream sediments will be used to test our current understanding of H2O2 sources and sinks in sunlit stream water and the underlying sediment. Our results will show whether hydroxyl radical production from hydrogen peroxide could be a dominant mechanism of indirect photolysis of organic contaminants and natural organic matter in streams. Photolysis of natural organic matter is of significance because it produces bioavailable carbon substrates as well as carbon dioxide, and is thought to be an important control on ecosystem-scale carbon cycling. Our work could also lay the groundwork for developing methods to accelerate Fenton's reaction in systems such as agricultural headwaters, for the purpose of decreasing concentrations of contaminants such as atrazine. Finally, if hydrogen peroxide turns out to be a good tracer for distinguishing between surface and subsurface storage in streams, the proposed work will contribute to the development of better models of biogeochemical processes such as nitrogen cycling and biodegradation of contaminants in these environments. The proposed work is a joint effort between a new investigator (Durelle Scott) and a mid-level female scientist (Bettina Voelker), and will support one graduate student and one undergraduate researcher each at the Colorado School of Mines and at the University of Nebraska.