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: 0738912
• 负责人: Simon Poulson
• 金额: --
• 依托单位: Board of Regents, NSHE, obo University of Nevada, Reno
• 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=0738912&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 £_18O-DO (the stable isotopic composition of DO) with reaction progress for different biotic and abiotic processes. The fractionation of £_18O-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 £_18O-DO in natural groundwater settings. However, it is also vital to demonstrate to the scientific community that large variations in £_18O-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 £_18O-DO and ?Ô13C-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 £_18O-DO, ?Ô13C-DIC and ?Ô13C-DOC as progress variables will produce a better understanding of processes that affect these underground reservoirs.

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洪泛区含水层是自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。浅层含水层系统中溶解氧浓度对水质和微生物有很大影响。 许多研究表明，DO的浓度通常沿沿着地下水流路径耗尽;然而，生物的或非生物的）通常是模糊的。 使用稳定同位素的分子氧有很大的潜力，区分各种机制，造成溶解氧耗尽在地下，是本研究的关键。 由于这一研究领域是相对较新的，有必要更好地从根本上了解的机制，有助于O2消耗在地下，这样的方法获得广泛的实用性之前。 特别是对于不同的生物和非生物过程，建立δ 18 O-DO（DO的稳定同位素组成）变化与反应进程之间的函数关系是必要的。 微生物呼吸过程中ε_（18）O-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而知道。 然而，几乎没有关于伴随无机反应的18 O分馏的公开数据，例如溶解的Fe（II）或溶解的硫化物（H2S，HS #8254;）的氧化。 目前的建议有一个主要的实验组成部分，其目的是确定这些无机DO消耗反应的动力学和平衡分馏因子在不同条件下的pH值，温度和还原剂浓度。实验确定的平衡和动力学同位素分馏因子是解释天然地下水环境中18 O-DO变化所需的“质量作用约束”。 然而，同样重要的是向科学界证明，地下水的ε_18 O-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。 这是本提案外地部分的动力。 两个拟定的现场位置分别是蒙大拿州卡利斯佩尔附近平头河洪泛区中叉沿线沿着的Nyack含水层和蒙大拿州巴特的银弓溪含水层。 奈亚克含水层系统的水文地质学、地球化学和微生物学已经被一组不同的科学家很好地描述了，他们的主要资金来自NSF生物复杂性计划。 有一组维护良好的地下水监测威尔斯井，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，DO将成为逐步丰富的18 O在地下停留时间的增加，作为微生物的过程优先消耗16 O。 由于微生物消耗的DO耦合到释放同位素轻生物CO2，我们还预测了反关系£_18 O-DO和？溶解无机碳同位素组成（Isotopic Composition of Dissolved Inorganic Carbon） Butte油田是一个受到良好监测的含水层，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）。 该项目的成果之一将是开发一套新的工具，可用于评估作用于地下水资源的各种化学和生物过程。 此外，解释地下水数据，包括以下£_18 O-DO，？β 13 C-DIC和？13 C-DOC作为进展变量将产生更好的理解的过程，影响这些地下水库。