Characterizing Deep Critical Zone Hydrologic Function in Mountainous Systems

Authors

William Payton Gardner1* (payton.gardner@umtonana.edu), Nicholas Thiros1,2, David Baude1, Kenneth H. Williams2

Institutions

1Department of Geosciences, University of Montana–Missoula, MT; 2Lawrence Berkeley National Laboratory, Berkeley, CA

Abstract

Researchers are working to characterize and model deep groundwater system dynamics across a gradient of landscape and sub-surface properties in montane to subalpine watersheds in west and central Montana and central Colorado. Multilevel groundwater wells were installed in a variety of landscape positions and subsurface geology across the different watersheds. Borehole geophysics, core logging and slug tests characterize subsurface structure and hydraulic properties at drilling locations. Continuous water-level and temperature logs provide information on seasonal hydraulic dynamics. Environmental tracers (i.e., chlorofluorocarbon, sulfur hexafluoride, tritium, helium-3, stable noble gas isotopes, stable isotopes of water) collected in wells and adjacent streams provide information on timing, location, and volume of groundwater circulation.

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 1000 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.