Resolving time scales of groundwater flow and saline water displacement using noble gas radionuclide tracers

地下水是地球上最大的淡水资源之一，为全球人口提供了40%的饮用水。它储存在地下岩石的孔隙中，流动缓慢。随着来水化学成分的变化，孔隙水成分也随之变化，其变化速率取决于孔隙大小。较小的孔隙往往能将较老的水保留较长时间，但这一过程的速率通常较慢且难以量化，这使得污染地下水的修复变得困难。该项目将通过从加拿大米尔克河含水层收集地下水样本并分析化学和同位素组成（包括新型放射性同位素）来确定变化速度。研究生将接受各种水文方法的广泛训练。奥德赛授权学者（一项针对资源不足背景的荣誉本科生的计划）将接受培训，并获得最先进技术的研究经验。项目结果将对该含水层的可持续产量估算产生长期影响，该含水层是横跨蒙大拿北部和阿尔伯塔省南部地区最宝贵的淡水资源之一。溶质在小孔隙中的“动态储存”是平流的来源，长期影响地下水的质量。本项目将利用Milk River Aquifer作为天然实验室，利用81Kr和其他地球化学示踪剂确定平流的时间尺度。目标是通过建立一个综合地球化学模型来推断溶质位移、扩散和分散的速率。拟议的项目还将具有两项重要的技术意义：(i)确定输入的36Cl/Cl比率随气候变化的长期变化，以及（ii）确定使用39Ar/40Ar*比率作为古老（年龄远超过1000年）地下水的计时器的可行性。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 one of the largest freshwater resources on Earth, providing 40% of drinking water to the global population. It is stored in the pores of subsurface rocks and flows slowly. As the chemical composition of incoming water changes, the porewater composition also changes, the rate of which depends on the pore sizes. Smaller pores tend to retain the older water for a longer time, but the rate of this process is generally slow and difficult to quantify, which makes remediation of contaminated groundwater difficult. This project will determine the rate of change by collecting groundwater samples from the Milk River Aquifer in Canada and analyzing the chemical and isotopic compositions including novel radioisotopes. A graduate student will receive extensive training in various hydrological methods. An Odyssey & Empower scholar (a program for honors undergraduate students from under-resourced backgrounds) will be trained and acquire research experience in state-of-the art techniques. Project outcomes will have long-term impacts on estimations of sustainable yields from this aquifer, which serves as one of the most valuable freshwater resources in the region straddling northern Montana and southern Alberta.The ‘dynamic storage’ of solutes in small pores acts as a source to advective flow and impacts the quality of groundwater over the long term. Using the Milk River Aquifer as a natural laboratory, this project will apply 81Kr along with other geochemical tracers to determine the time scale of the advective flow. The goal is to deduce the rates of solute displacement, diffusion, and dispersion by establishing an integrated geochemical model. The proposed project will also have two important technical implications: (i) determining the secular variation of the input 36Cl/Cl ratios with varying climates and (ii) establishing the feasibility of using 39Ar/40Ar* ratios as a chronometer of old (much great than 1000 years in age) groundwater.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.