Matrix diffusion controls mountain hillslope groundwater ages
Simulated 4Heterr and 3H concentrations using the EcoSLIM model, showing the mean age (τ) of the simulations is positively correlated with the magnitude of matrix diffusion. Image courtesy of Thiros et al., 2025.
The Science
Researchers investigated groundwater age distributions in a mountainous catchment in Colorado, USA, using a combination of numerical modeling and environmental tracers. They found that matrix diffusion plays a crucial role in controlling groundwater age distributions, and that advection-dominated models alone cannot capture the observed co-occurrence of young and old-aged water. The study highlights the importance of considering matrix diffusion when interpreting environmental tracers in bedrock groundwater systems. The results show that a dynamic bedrock groundwater reservoir is susceptible to considerable storage losses during low-snow periods, amplifying the need to assimilate deeper bedrock groundwater into watershed hydro-biogeochemical predictions.
The Impact
By understanding the role of matrix diffusion in controlling groundwater age distributions, policymakers and scientists can better manage water resources and predict the impacts on future perturbations. This study also informs the development of more accurate numerical models, which can be used to simulate groundwater flow and inform decision-making.
Summary
The study employed a combination of numerical modeling and environmental tracers, including 3H and 4He concentrations, to investigate groundwater age distributions in a lower montane hillslope in the East River Watershed, Colorado, USA. The researchers used a convolution-based approach to propagate fracture-matrix diffusion processes to the EcoSLIM advection-dominated age distributions. A Monte Carlo analysis was conducted to consider uncertain matrix and fracture parameters, which revealed that matrix diffusion is necessary to jointly predict 3H and 4He observations at two wells. The results show that the model scenario that best matches the 3H, 4He, and water level observations has a dynamic bedrock groundwater reservoir that is susceptible to considerable storage losses during low-snow periods. This highlights the need to assimilate deeper bedrock groundwater into watershed hydro-biogeochemical predictions, and demonstrates the importance of considering matrix diffusion when interpreting environmental tracers in bedrock groundwater systems. The study’s findings have important implications for predicting the impacts of climate change on water resources in mountainous regions.
Contact
Eoin L. Brodie, Watershed Function SFA LRM
Lawrence Berkeley National Laboratory
Publications
Thiros, N. et al., Matrix Diffusion Controls Mountain Hillslope Groundwater Ages and Inferred Storage Dynamics, Groundwater, 63: 306-318, (2025). [DOI: 10.1111/gwat.13475]