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

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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.