Methodological Adaptations for Reliable Measurement of Radium and Radon Isotopes in Hydrothermal Fluids of Extreme Chemical Diversity in Yellowstone National Park, Wyoming, USA

Abstract:

To quantitatively model fluid residence times, water-rock-gas interactions, and fluid flow rates in the Yellowstone (YS) hydrothermal system we are measuring short-lived isotopes of Ra (228Ra, 226Ra, 224Ra, 223Ra) and Rn (222Rn, and 220Rn) in hydrothermal fluids and gases. While these isotopes have been used successfully in investigations of water residence times, mixing, and groundwater discharge in oceanic, coastal and estuarine environments, the well-established techniques for measuring Ra and Rn isotopes were developed for seawater and dilute groundwaters which have near neutral pH, moderate temperatures, and a limited range of chemical composition. Unfortunately, these techniques, as originally developed are not suitable for the extreme range of compositions found in YS waters, which have pH ranging from <1 – 10, Eh -.208 to .700 V, water temperatures from ambient to 93 degree C, and high dissolved CO₂ concentrations.

Here we report on our refinements of these Ra and Rn methods for the extreme conditions found in YS. Our methodologies are now enabling us to quantitatively isolate Ra from fluids that cover a large range of chemical compositions and conduct in-situ Rn isotope measurements that accommodate variable temperatures and high CO₂ (Lane-Smith and Sims, 2013, Acta Geophys. 61).

These Ra and Rn measurements are now allowing us to apply simple models to quantify hot spring water residence times and aquifer to surface travel times. (224Ra/223Ra) calculations provide estimates of water-rock reaction zone to discharge zone of 4 to 14 days for Yellowstone hot springs and (224Ra/228Ra) shallow aquifer to surface travel times from 2 to 7 days. Further development of more sophisticated models that take into account water-rock-gas reactions and water mixing (shallow groundwater, surface run-off, etc.) will allow us to estimate the timescales of these processes more accurately and thus provide a heretofore-unknown time component to the YS hydrothermal system.