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

溶解氧的稳定同位素作为地下水化学和生物过程的示踪剂。

• 批准号: 0739054
• 负责人: Stephen Parker
• 金额: --
• 依托单位: Montana Technological University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-05-01 至 2011-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0739054&HistoricalAwards=false
• 关键词: Stable; Isotopes; Dissolved; Oxygen; Tracers

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.

作为地下水中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、工业和家庭用途的资产。保护我们的地下水储备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学受溶解氧（DO）浓度的影响很大。许多研究表明，DO的浓度往往沿着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）往往是模糊的。利用分子氧的稳定同位素来区分引起地下DO耗竭的各种机制具有很大的潜力，是本研究的关键。由于这一研究领域相对较新，在这种方法获得广泛应用之前，需要对地下氧气消耗的机制有更好的基本理解。特别是，在不同的生物和非生物过程中，建立ä18O-DO （DO的稳定同位素组成）的变化与反应过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的分馏是很大的，并且已经通过许多已发表的实验室研究知道。然而，几乎没有关于18O伴随无机反应(如溶解的Fe（II）或溶解的硫化物（H2S, HS⎯）氧化的分馏的公开数据。目前的提议有一个主要的实验组成部分，其目的是确定在不同的pH、温度和还原剂浓度条件下这些无机do消耗反应的动力学和平衡分馏因素。实验确定的平衡和动力学同位素分馏因子是？质量作用约束？这需要解释ä18O-DO在天然地下水环境中的变化。然而，向科学界证明自然界中确实存在地下水ä18O-DO的巨大变化，并且可以对这些同位素梯度作出合理的解释，这也是至关重要的。这是当前提案外地部分的推动力。两个被提议的野外地点分别是位于MT. Kalispell附近Flathead河漫滩中叉的Nyack含水层和MT. Butte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已经由不同的科学家小组在NSF生物复杂性计划的主要资助下进行了很好的表征。存在一套维护良好的地下水监测井，系统的氧动力学已得到很好的表征。我们的主要假设之一是，随着在地下停留时间的增加，DO在18O中会逐渐富集，因为微生物过程优先消耗16O。由于微生物对DO的消耗与同位素轻的生物源CO2的释放是耦合的，我们还预测了ä18O-DO和ä13C-DIC（溶解无机碳的同位素组成）之间的反比关系。Butte油田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。影响该系统中DO消耗的过程预计会受到非生物机制的强烈影响（例如，还原金属和硫化物的氧化）。该项目的一个成果将是开发一套新的工具，可用于评估对地下水资源起作用的各种化学和生物过程。此外，对地下水数据的解释包括将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。