Cosmogenic Tracers to Reveal the Sensitivity of Groundwater-Dependent Ecosystems to Changing Climate

Project Overview

Groundwater dependent ecosystems (GDE) are made up of various plant and animal populations that rely on groundwater in order to sustain their existence and provide essential habitats for species that are threatened and endangered. An increase in the number of warmer storms throughout California is predicted by climate models, which indicates an escalation of precipitation as rain rather than snow. Since snow is a necessary supply of recharge to groundwater and aquifers, these declines in precipitation as snow will have postponed adverse effects to GDEs, causing a decrease in their water source as well as increased temperatures to the habitats and species that they support (such as the endangered Coho Salmon). Essential to understanding the sensitivity of a GDE to climate change is to identify where the water originates, its source, and the length of time the water takes to get from its source to the GDE (known as groundwater residence time), both of which can be revealed by means of water isotope measurements. Short flow paths from the source to the GDE are the most vulnerable to climate change and have a "younger" isotope age.

Our study aimed to understand these flow paths using two innovative methods developed at LLNL: cosmogenic isotopes sulfur-35 (half-life 87 days) and sodium-22 (half-life 2.6 years) at the groundwater-fed Big Springs GDE, located in Shasta Valley, California. In addition, our study utilized this unique age data to calibrate a ModPath model with collaborators at UC Davis to predict the influence of climate change on groundwater flow to Big Springs. We sampled Big Springs once a month starting in March of 2020 through August and once more in October of 2022. In addition to sampling Big Springs, a snow sample was taken at 2,073 meters on Mt. Shasta to determine the isotopic source signature of Mt. Shasta. These samples were analyzed at LLNL (in B151) for all isotopes including oxygen-18 for the stable isotopes of water to determine the source, as well as tritium, sulfur-35 and sodium-22 to determine residence times. The processed sodium-22 samples were sent to Sanford Underground Research Facility (SURF) to be analyzed on a LLNL-owned detector, but due to unseen Covid-19 related circumstances, they were unable to be analyzed in time. Isotope data as well as sampling locations and expected source elevation was provided to UC Davis for them to create new ModPath simulations.

Isotope data revealed that the direct source of groundwater to Big Springs was from Mt. Shasta with no mixing from other regional sources based on the isotope signatures of the snow collected from Mt. Shasta and the water from Big Springs. In addition, the cosmogenic isotope data revealed that a large percentage of water feeding Big Springs comes from thee current year's snow events, recharging in less than one year. The ModPath results confirmed this further with the transient cumulative recharge elevation distribution showing similar trends with time as the age distribution. The elevation results suggest that most of the particles come from shorter flow paths as the 60th percentile is just below 1000m, while the remaining 40% of particles are fairly evenly distributed from recharge elevations of 1000–2000m. This information can prove critical for local, regional and state water policy makers, enabling them to test various groundwater sustainability planning scenarios for specific groundwater basins. As patterns of precipitation are projected to change as more rain than snowfall, these simulation results will show how climate change will alter patterns of snowfall, therefore leading to variations in groundwater availability and sustainability.

Mission Impact

This project aligns directly with the new Climate Resilience Mission Focus Area and with LLNL's Climate and Energy Resilience mission to improve our comprehension of the earth system through enhanced data collection and assessments of mitigation strategies. Additionally, our work aligns with one of the goals in the new NACSD strategic plan to position LLNL for a scientific focus area project on watershed functioning funded by DOE Office of Biological and Environmental Research. The research from this study also supports the Scientific Grand Challenges identified by DOE's Climate and Environmental Science Division to advance a robust, predictive understanding of the Integrated Water Cycle

Our research will become the paradigm for prospective studies with regard to climate change involving GDEs. The results from this study will enable the use of our unique isotopic tracers in various hydrologic applications, as well as inspire collaborations between academia and government agencies as well as within LLNL. We anticipate that results from our study will provide important and crucial data in a discipline where there is a deficiency in residence-time data and isotopic model calibration.

Publications, Presentations, and Patents

Deinhart, A. et al., 2021. "Simplified Method for the In Situ Collection and Laboratory Analysis of Cosmogenic Tracers (Sulfur-35 and Sodium-22) to Determine Residence Time Distributions and Water Ages." Analytical Chemistry 93: 10 4472-4478. (2021). https://doi.org/10.1021/acs.analchem.0c04490.