Matrix Diffusion Controls Mountain Hillslope Groundwater Ages and Inferred Storage Dynamics

Area of Science:

• Hydrology
• Hydrogeology
• Biogeochemistry

Background:

• Groundwater age distributions offer key insights into mountain watershed processes.
• Previous studies show mixed young and old groundwater in bedrock catchments, but underlying mechanisms remain unclear.

Purpose of the Study:

• To simulate groundwater age distributions on a lower montane hillslope using integrated hydrologic and particle tracking models.
• To investigate the role of fracture-matrix diffusion in shaping groundwater age distributions.
• To compare model predictions with observed environmental tracer data (³H and ⁴He).

Main Methods:

• Utilized the coupled ParFlow-CLM integrated hydrologic and EcoSLIM particle tracking models.
• Developed a convolution-based approach to incorporate fracture-matrix diffusion into age distributions.
• Performed a Monte Carlo analysis to assess uncertain matrix and fracture parameters.

Main Results:

• Matrix diffusion is essential for jointly predicting observed tritium (³H) and helium-4 (⁴He) concentrations.
• Advection-dominated models alone cannot replicate the observed mixing of young and old groundwater.
• The best model fit included a dynamic bedrock groundwater reservoir with significant storage loss during dry periods.

Conclusions:

• Fracture-matrix diffusion is critical for understanding environmental tracer data in bedrock groundwater.
• Dynamic bedrock groundwater systems require deeper integration into watershed hydro-biogeochemical models.
• Accurate modeling of groundwater age necessitates accounting for both advection and matrix diffusion processes.