Geysers are highly dynamic systems controlled by poorly understood mechanisms in the subsurface. The purpose of this work is to better understand the role of non-condensable gas in geysers. Two case studies were conducted, the first to determine how non-condensable gas may influence the eruption mechanism in hot water systems, and the second to elucidate gas dynamics in a cold water geyser driven by non-condensable gas.
The first case study was carried out in five hot water geysers and springs and one cool water spring within the hydrothermal system of Yellowstone National Park. Non-condensable gases, primarily magmatic CO2, are present in Yellowstone waters, but to our knowledge no field-based research has directly addressed how such gases may affect the eruption process. In order to understand the evolution of geyser waters and dissolved gases on an eruption interval time-scale, we collected continuous in situ water quality data during eruption intervals and took time series water samples for major element chemistry, isotope, and dissolved gas analysis. Total dissolved gas pressure (PTDG) and temperature measurements were used to isolate water vapor pressure and non-condensable gas pressure. Using these data, we estimated CO2 concentrations in deep groundwater below Spouter Geyser. Although dissolved gas analysis shows that CO2 concentration is a minor component (< 1% of PTDG) in surface waters of Spouter Geyser, subsurface concentrations may reach levels greater than solubility preceding an eruption at corresponding pressures and temperatures. We propose that ebullition of CO2 may induce boiling to drive an eruption in hot water systems.
The second case study was conducted at Crystal Geyser, a borehole drilled into a natural CO2 reservoir in east-central Utah. Studying this feature is useful for understanding modes of leakage from future anthropogenic carbon storage sites, but comprehensive gas monitoring at this site is currently lacking. We measured PTDG along with water pressure, temperature, and electrical conductivity at three depths every minute throughout the eruption cycle, and collected water samples for the same analyses as above. We found that the geyser cycle consists of three phases, 1) Recovery Phase, 2) Minor Eruptions Phase, and 3) Major Eruption Phase. Waters contain 100% CO2, and thus PTDG can show variations in CO2 release. PTDG is a necessary tool for future Crystal Geyser studies and is recommended for use in carbon storage projects as well.