2024 Abstracts

Estimating Groundwater Recharge Across Mountainous Catchments


W. Payton Gardner1* (payton.gardner@umontana.edu), David Baude1, Nicholas Thiros2, Kenneth Hurst Williams2


1University of Montana, Missoula, MT; 2Earth and Environmental Science, Berkeley National Laboratory, Berkeley, CA


This project reports on efforts to characterize and model deep groundwater system dynamics across a gradient of landscape and subsurface properties in montane to subalpine watersheds in west-central Montana and central Colorado. Multilevel groundwater wells have been installed in a variety of landscape positions and subsurface geology across the different watersheds. Borehole geophysics, core logging, and slug tests are used to characterize subsurface structure and hydraulic properties at drilling locations. Continuous water level and temperature logs are used to provide information on seasonal hydraulic dynamics. Environmental tracers (CFC, SF6, 3H-3He, stable noble gas isotopes, stable isotopes of water) collected in wells and adjacent streams are used to provide information on timing, location, and volume of groundwater circulation. Researchers include new results of 39Ar as a tracer capable of measuring intermediate groundwater ages. These datasets are incorporated into conceptual and numerical models of integrated surface and subsurface flow to provide insight into the role of groundwater in hillslope and watershed behavior. Researchers find that mountains host active groundwater systems with strong seasonal responses to changing infiltration and evapotranspiration. Measured groundwater ages in these deep bedrock systems are characterized by a mixture of young and old water, with mean ages of 100 to 1,000 years. Models of surface and subsurface flow are highly sensitive to subsurface hydraulic characteristics, and specific subsurface configurations are required to fit the observed water table dynamics and groundwater mean ages. The deep critical zone including the saprolite and bedrock groundwater systems are an important, but often underexplored control on watershed hydrogeochemical function in mountainous systems. This project is producing new insight into the form and function of these hidden systems, and the role of deep, old groundwater in watershed function.