Stable Isotopes of Dissolved Oxygen as Tracers of Chemical and Biological Processes In Groundwater.

作为地下水化学和生物过程示踪剂的溶解氧稳定同位素是天然河流生态系统的重要组成部分，也是农业、工业和家庭使用的资产。保护我们的地下水储量是国家和全球经济可持续增长的先决条件。浅层含水层系统的水质和微生物受溶解氧浓度的影响很大。大量研究表明，DO的浓度通常会沿地下水流动路径耗尽；然而，消耗氧气的机制(例如，生物或非生物)往往是含糊不清的。氧分子稳定同位素的使用具有很大的潜力来区分导致亚表面DO耗竭的各种机制，这是本研究的关键。由于这一研究领域相对较新，在这种方法得到广泛应用之前，需要更好地从根本上了解地下氧气消耗的机制。特别是，在不同的生物和非生物过程中，建立DO的稳定同位素组成的变化与反应进程之间的函数关系是至关重要的。在微生物呼吸过程中，ä18O-DO的分馏程度很大，这一点已经通过许多已发表的实验室研究得到了证实。然而，几乎没有公开的数据关于18O伴随着无机反应的分馏，例如溶解的Fe(II)的氧化或溶解的硫化物(硫化氢，HS&amp；#9135；)。目前的方案有一个主要的实验部分，其目的是确定这些无机耗氧反应在不同pH、温度和还原剂浓度条件下的动力学和平衡分馏因子。这是解释天然地下水环境中ä18O-DO变化所必需的。然而，同样重要的是向科学界证明，地下水的18O-DO在自然界中确实存在很大的变化，并且可以对这些同位素梯度做出合理的解释。这就是当前提案的实地部分的动力。拟建的两个油田分别是位于密苏里州卡利斯佩尔附近的Flathead河泛滥平原中叉处的Nyack含水层和密苏里州巴特的Silver Bow Creek含水层。Nyack含水层系统的水文地质学、地球化学和微生物学一直由一群不同的科学家很好地描述，他们得到了NSF生物复杂性计划的主要资助。有一套维护良好的地下水监测井，系统的氧气动力学得到了很好的表征。我们的一个主要假设是，随着在地下停留时间的增加，DO将在18O中逐渐变得丰富，因为微生物过程优先消耗16O。由于微生物对DO的消耗与同位素轻质生物成因二氧化碳的释放相耦合，我们还预测了ä18O-DO和ä13C-DIC(溶解无机碳的同位素组成)之间的反向关系。巴特矿场是一个监测良好的含水层，该地区100多年来一直受到采矿和冶炼活动的严重影响。在这个系统中，影响DO耗竭的过程预计会受到非生物机制(例如，还原金属和硫化物的氧化)的强烈影响。该项目的一个成果将是开发一套新的工具，可以用来评估作用于地下水资源的各种化学和生物过程。此外，对地下水数据的解释，包括以ä18O-DO、ä13C-DIC和ä13C-DOC作为进度变量，将更好地理解影响这些地下水库的过程。

Stable isotopes of dissolved oxygen as tracers of chemical and biological processes in groundwaterFloodplain aquifers are a critical component of natural river ecosystems as well as being assets for agricultural, industrial and domestic uses. The preservation of our groundwater reserves is a prerequisite for sustainable growth of the national and global economy. The water quality and microbiology of shallow aquifer systems is greatly influenced by the concentration of dissolved oxygen gas (DO). Numerous studies have shown that concentrations of DO are often depleted along groundwater flow paths; however, the mechanisms (e.g., biotic or abiotic) that consume oxygen are often ambiguous. The use of stable isotopes of molecular O2 has great potential to discriminate between the various mechanisms causing DO depletion in the subsurface, and is the crux of this study. Since this field of study is relatively new, there is a need for a better fundamental understanding of the mechanisms that contribute to O2 consumption in the subsurface before such an approach gains widespread utility. In particular, it is essential to establish functional relationships between changes in ä18O-DO (the stable isotopic composition of DO) with reaction progress for different biotic and abiotic processes. The fractionation of ä18O-DO during microbial respiration is large, and is already known through numerous published laboratory studies. However, there are virtually no published data on the fractionation of 18O accompanying inorganic reactions such as oxidation of dissolved Fe(II) or dissolved sulfide (H2S, HS⎯). The current proposal has a major experimental component, the objective of which is to determine kinetic and equilibrium fractionation factors for these inorganic DO-consuming reactions under differing conditions of pH, temperature, and reductant concentration.Experimentally-determined equilibrium and kinetic isotopic fractionation factors are the ?mass action constraints? that are needed to interpret changes in ä18O-DO in natural groundwater settings. However, it is also vital to demonstrate to the scientific community that large variations in ä18O-DO of groundwater do indeed exist in nature, and that plausible explanations for these isotopic gradients can be formulated. This is the impetus for the field segment of the current proposal. The two proposed field sites are the Nyack aquifer along the Middle Fork of the Flathead River floodplain near Kalispell, MT and the Silver Bow Creek aquifer in Butte, MT. The hydrogeology, geochemistry, and microbiology of the Nyack aquifer system has been well characterized by a diverse group of scientists with major funding from the NSF Biocomplexity Program. A well-maintained set of groundwater monitoring wells exists and the oxygen dynamics of the system have been well characterized. One of our main hypotheses is that DO will become progressively enriched in 18O with increased residence time in the subsurface, as microbial processes preferentially consume 16O . Since microbial consumption of DO is coupled to release of isotopically-light biogenic CO2, we also predict an inverse relationship between ä18O-DO and ä13C-DIC (isotopic composition of dissolved inorganic carbon). The Butte field site is a well monitored aquifer in an area that has been heavily impacted by mining and smelting activities for over 100 years. The processes affecting DO depletion in this system are expected be strongly influenced by abiotic mechanisms (e.g., oxidation of reduced metals and sulfide).One outcome of this project will be the development of a new set of tools that can be used to assess the various chemical and biological processes acting on groundwater resources. Additionally, interpretation of groundwater data that includes following ä18O-DO, ä13C-DIC and ä13C-DOC as progress variables will produce a better understanding of processes that affect these underground reservoirs.