A New Multi-tracer Approach for Dating Groundwater on 10,000-year Timescales Applied to a Southern Californian Aquifer

地下水是美国和世界各地饮用水和灌溉用水的重要来源。地壳上层一公里的大部分地下水被认为是“化石”——这意味着它在大约一万二千年前的全新世时期之前首次进入（即重新进入）地下。随着对地下水需求的增加，以及新钻的井进入更深的含水层，我们对地下水的依赖可能会增加。该研究项目的重点是应用独立的、最先进的地球化学示踪剂来了解以化石地下水为主的含水层系统的停留时间（即水在地下停留的时间）。通过从新的示踪剂中获得见解，提出的工作目标是改进传统年龄示踪剂的应用，测试概念模型，并最终提高准确确定全球含水层中化石地下水停留时间的能力，这将有助于可持续的地下水管理。该项目包括培训一名博士后学者，他将在伍兹霍尔海洋研究所和阿贡国家实验室的实验室进行新颖的高精度示踪剂测量，以及一名高中生，他将把这些相同的技术应用于地下水池塘，以了解混合和循环时间尺度。准确确定地下水停留时间对了解地下水流动和混合具有重要意义，对地下水的可持续管理具有重要意义。然而，在表征地下水化石特征的一万年时间尺度上，最常用的地下水“测年”地球化学工具（例如放射性碳和氦）容易产生很大的系统误差。本研究项目结合了新的高精度81Kr测量（精度为1%，通过阿贡国家实验室的原子阱痕量分析）和放射性40Ar（精度为0.01‰，通过伍兹霍尔海洋研究所的动态惰性气体质谱），以及传统的测年工具（14C和4He），以限制圣地亚哥含水层系统（加利福尼亚州）数十个科学监测井的地下水停留时间分布。主要目标是定量了解氦和放射性碳概念定年模型（例如，通过评估关于化石碳添加和氦积累速率的假设），采用新的约束来确定最佳停留时间分布函数，并开发这些新的示踪剂用于世界范围内其他化石地下水系统。该项目将包括多个实地活动，培训一名博士后学者，他将在该项目中担任主要领导角色，并为一名高中生提供动手的带薪暑期实习机会。该项目由水文科学和地球生物学及低温地球化学项目共同资助。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。

Groundwater is a crucial source of drinking and irrigation water in the United States and around the globe. Much of the groundwater in the upper kilometer of Earth’s crust is considered to be “fossil” – meaning it first entered (i.e., recharged into) the subsurface before the Holocene period around twelve thousand years ago. As demand for groundwater increases, and newly drilled wells tap into deeper aquifers, our reliance on fossil groundwater will likely grow. This research project is focused on applying independent, state-of-the-art geochemical tracers to understand residence times (i.e., the time water spends in the subsurface) of an aquifer system dominated by fossil groundwater. By gaining insight from new tracers, the objective of the proposed work is to refine applications of traditional age tracers, test conceptual models, and ultimately improve the ability to accurately determine residence times of fossil groundwater in aquifers worldwide, which will aid in sustainable groundwater management. The project involves training of a postdoctoral scholar, who will make novel high-precision tracer measurements in labs at Woods Hole Oceanographic Institution and Argonne National Lab, and a high school student, who will apply these same techniques to a groundwater-fed pond to learn about mixing and circulation timescales.Accurately determining groundwater residence time is important for understanding groundwater flow and mixing, with key implications for sustainable groundwater management. However, on the 10,000-year timescales that characterize fossil groundwater, the geochemical tools most commonly used to “date” groundwater (e.g., radiocarbon and helium) are prone to large sources of systematic error. This research project combines new high-precision measurements of 81Kr (1% precision; via Atom Trap Trace Analysis at Argonne National Lab) with radiogenic 40Ar (0.01‰ precision; via dynamic noble gas mass spectrometry at Woods Hole Oceanographic Institution) with traditional dating tools (14C and 4He) to constrain groundwater residence time distributions in dozens of scientific monitoring wells in the San Diego aquifer system (California). The key objectives are to gain quantitative insight into helium and radiocarbon conceptual dating models (e.g., by evaluating assumptions about fossil carbon addition and helium accumulation rates), employ new constraints to determine optimal residence time distribution functions, and develop these new tracers for application in other fossil groundwater systems worldwide. This project will include multiple field campaigns, training of a postdoctoral scholar who will take on a major leadership role in the project, and support for a hands-on, paid summer internship opportunity for a high school student.This project is co-funded by the Hydrologic Sciences and Geobiology & Low-Temperature Geochemistry programs.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.