Groundwater flowpaths, chemical weathering rates, and climate change patterns as determined using multiple environmental tracers

DOCTORATE DISSERTATION: The study of interactions within Earth s critical zone, or surface environment, provides an understanding of the integral links between the hydrosphere, lithosphere, atmosphere, and biosphere, Central to the study of the critical zone are chemical weathering, surface, and near-surface hydrology. Geochemical tracers provide insight into the interactions between processes in the critical zone. IN the series of studies described herein, chlorofluorocarbons (CFCs), tritium, major ion chemistry, as well as oxygen, hydrogen, and helium isotopes were used to understand groundwater residence times, groundwater movement, and the origin of solutes in the near surface zone. The two main field sites, Sagehen basin and the Mission Tunnel, differ greatly in their geologic settings and provide an interesting contrast and broad test for the applicability of using environmental tracers in near surface systems. Sagehen basin is located in the Sierra Nevada of California. The chemical and isotope composition of groundwater springs in Sagehen basin were used to decipher an archive of internal and external changes. A positive correlation was found between groundwater residence times and the chemistry of the waters. This correlation is strong evidence for the chemical evolution of groundwater and suggests internal changes in the system are recorded by shallow groundwater and is accessible with springs. The oxygen isotope content of the spring waters also correlates with groundwater residence times. This correlation is related to air temperature and atmospheric circulation pattern changes, suggesting groundwater systems also archive external changes. The Mission Tunnel of Santa Barbara, California, provides a very different environment for studying groundwater flowpaths and solute chemistry. By using multiple geochemical tracers, different age components of the groundwater system were separated. Due to the highly fractured nature of the aquifers, no correlation was found between the minimum flowpath lengths and groundwater age components. Ion chemistry of the goundwaters suggest that flow is primarily in the vertical direction. In addition, time series analysis of long-term records of precipitation and groundwater seepage into the tunnel suggest that there is a 3 month lag between precipitation and seepage events and that antecedent conditions of the groundwater system strongly control the response to precipitation loading.