Noble gases dissolved in groundwater: Techniques for exploring groundwater dynamics and paleoclimate variability

The inert nature of the noble gases, coupled with their distinctive isotopic and solubility characteristics, makes them ideal tracers for studying groundwater chronology, paleoclimatology, mechanism(s) of recharge, and tectonic influences on volatile fluxes to the shallow crust. The first chapter of this dissertation describes the methodology for determining noble gas concentrations in fluid samples. In the subsequent chapters, the measurement of the noble gases (and the helium and neon isotope ratios) dissolved in groundwater samples, the resolution of the different gas components, and the interpretation of the results from four study sites are discussed.; The concentrations of the noble gases in groundwater may be described by Henry's Law as a function of the temperature and salinity of the water, and the partial pressure of each gas in the atmosphere. Generally, recharging groundwaters have low salinities, and atmospheric noble gas abundances have remained constant over the past 3 million years; therefore, the principal control on the amount of dissolved noble gases is the temperature of the recharging water. Chapter 2 examines paleoclimate information that is available from noble gas concentrations in southern Africa groundwaters with the aim to determine the climate variability in this region since the last glacial period.; Helium is particularly useful as a hydrological tracer owing to its low solubility in water, large subsurface production rate (of the 4He isotope), and large and diagnostic variations in its isotopic composition (3He/4He ratios). The low solubility of helium makes it particularly sensitive to addition or loss. Whether from rapid addition of excess-air during groundwater infiltration or water-table fluctuation, or the protracted decay of radionuclides and diffusion from deep crustal or mantle reservoirs, each process has a characteristic impact on groundwater 3He/4He ratios. The third, fourth, and fifth chapters of this dissertation focus on the helium systematics of four semi-arid groundwater basins, two in central Australia and two in California. These studies assess the utility of the helium-4 chronometer, determine how regional tectonics influences the helium components in deep aquifers, identify volatile transport across the mantle-crust boundary, and investigate what role faults and seismic activity play in facilitating helium transport through the crust.