Matrix Diffusion Controls Mountain Hillslope Groundwater Ages and Inferred Storage Dynamics

Abstract Groundwater age distributions provide fundamental insights on coupled water and biogeochemical processes in mountain watersheds. Field‐based studies have found mixtures of young and old‐aged groundwater in mountain catchments underlain by bedrock; yet, the processes that dictate these groundwater age distributions are poorly understood. In this work, we use the coupled ParFlow‐CLM integrated hydrologic and EcoSLIM particle tracking models to simulate groundwater age distributions on a lower montane hillslope in the East River Watershed, Colorado (USA). We develop a convolution‐based approach to propagate fracture‐matrix diffusion processes to the EcoSLIM advection‐dominated age distributions. We compare observed 3 H and 4 He concentrations from two groundwater wells against model predictions that have varying advective transport times and matrix diffusion magnitudes. Based on a Monte Carlo analysis that considers uncertain matrix and fracture parameters, we find that matrix diffusion is needed to jointly predict 3 H and 4 He observations at both wells. The advection‐dominated age distributions lack adequate mixing of young and old‐aged water to capture the observed co‐occurrence of 3 H and 4 He. The model scenario that best matches the 3 H, 4 He, and water level observations when considering both advective flowpath and matrix diffusion mixing processes has a dynamic bedrock groundwater reservoir that is susceptible to considerable storage losses during low‐snow periods. This dynamic groundwater system amplifies the need to assimilate deeper bedrock groundwater into watershed hydro‐biogeochemical predictions. This work further highlights the importance of considering matrix diffusion when interpreting environmental tracers in bedrock groundwater systems.