Year of Award

2024

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Other Degree Name/Area of Focus

Hydrogeology

Department or School/College

Geosciences

Committee Chair

W. Payton Gardner

Commitee Members

Dr. W. Payton Gardner, Dr. Joel T. Harper, Dr. Zachary Hoylman

Keywords

hydrogeology, environmental tracers, groundwater recharge, groundwater age

Subject Categories

Hydrology

Abstract

Mountain aquifer groundwater systems remain one of the least understood hydrologic systems in upland hydrology. In this study, we aim to characterize bedrock groundwater system dynamics and bedrock-saprolite interactions across a variety of landscape positions and lithology. We investigate groundwater hydraulics, recharge temperature and elevation, residence times, and bedrock recharge rates across two mountainous headwater catchments in west central Montana. We used a suite of environmental tracers (3H, CFCs, SF6, and 4He) and residence time distribution models to estimate groundwater mean residence times over a range of timescales from decades to millennia. Tracers were sampled from a stream, springs, and bedrock wells at four well sites located at various topographic positions within two adjacent catchment areas with differing lithologies in the Lubrecht Experimental Forest, MT. To constrain groundwater recharge temperature, elevation, and excess air, we sampled dissolved noble gases and calculated recharge parameter posterior probability distributions using Markov-chain Monte Carlo analyses. We used mean residence time posterior distributions to calculate annual bedrock groundwater recharge rates. Stable isotope samples were collected from wells and used to partition sources of groundwater recharge. The highest elevation well and spring of the Cap Wallace catchment area contain 3H, CFCs, and SF6. Exponential models estimate mean residence times of 108 and 30 years for this well and spring. Downslope wells contain 3H, CFCs, SF6, and elevated terrigenic 4He indicating a presence of modern (recharged after 1950) and pre-modern groundwater (recharged prior to 1950). Exponential models estimate mean residence times of 24, 214, and 1000 years for downslope wells of the Cap Wallace and North Fork Elk Creek catchment areas, respectively. Annual bedrock groundwater recharge rates range from 0 to 64 mm/yr, which is approximately 0.0 to 10.9% of annual precipitation of our study area. Mass balance mixing models applied to saprolite and bedrock wells show a >65% average snowmelt composition. Across gradients in topography and lithology, bedrock groundwater residence time, flux direction, and recharge do not appear to vary systematically with landscape position, although we have a limited sample size. This has implications for improving existing conceptual and numerical hydrologic models in mountainous catchments.

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© Copyright 2024 David L. Baude