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

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.