Geologic controls on hydrothermal groundwater mixing in Yellowstone National Park

Abstract

The hydrothermal system in Yellowstone National Park, arguably the most well-studied hydrothermal system in the world, offers a unique opportunity to study the complex interaction between deep, superheated, anoxic hydrothermal fluids and meteorically-sourced oxidizing waters in the near subsurface. Past research has established that hydrothermal waters originate from a hot source gas infused deep aquifer below the surface, and that mixing with modern meteoric water occurs to various degrees. However, many of mechanisms driving flow and mixing are still poorly understood. This study aims to geophysically image the conduits sustaining hot springs, and the below ground features that drive mixing of deeply-sourced hydrothermal water and meteorically-derived groundwater. We focus on the large-scale spatial relationships (10s of meters) between ascending hydrothermal fluids and geologic units, the sources of shallow hydrothermal water, and most importantly whether hydrologic mixing occurs prior to discharge at either the various hydrothermal features or “cold” springs. Our results reveal subsurface permeable zones in rhyolites that determine large-scale vertical flow paths of hydrothermal fluids. We compare the geophysical results and spatial relations of the hot springs to Cl− and SO42− concentrations in Yellowstone waters. From our interpretations, we propose an updated conceptual model showing the interactions between the shallow meteoric recharge system and deeper originated hydrothermal system that may inform interpretations of hydrothermal systems in Yellowstone and elsewhere.