GEOPHYSICAL EVIDENCE OF HYDROTHERMAL GROUNDWATER MIXING IN SENTINEL MEADOWS, YELLOWSTONE NATIONAL PARK

Yellowstone National Park (YSNP) is arguably the world’s most extensively studied hydrothermal system. Sentinel Meadows in YSNP’s Lower Geyser Basin provides a natural laboratory to investigate complex interactions between hydrothermal fluids sourced from deep reservoirs and oxidizing meteoric waters flowing in the near surface. Past research on the interactions between deeply sourced waters and shallow groundwater has shown that hydrothermal waters originate from a hot, gas-infused source and that mixing occurs in the subsurface. However, the mechanisms driving fluid flow and mixing are not well understood. In Smeltz et al., 2022 (JVGR, 431, 107650), we use near-surface geophysical imaging to reveal conduits sustaining hot springs and the geologic features enabling the mixing of hydrothermal water with cooler groundwater sourced from runoff and infiltration. We evaluate the spatial relationships between ascending hot fluids and associated geologic units, the sources of hydrothermal water, and where hydrologic mixing occurs before surface discharge from both hot and cold water springs. Our near-surface geophysical imaging provides a unique conceptual model of the subsurface architecture and fluid flow beneath Sentinel meadows. We show that porous zones in the local rhyolites permit vertical and horizontal flow of hydrothermal fluids and demonstrate the influence of groundwater recharge on Yellowstone’s hydrothermal system. Importantly, our imaging of shallow groundwater and deep hydrothermal water interaction, coupled with existing geochemical data, also provides a new spatial understanding of the long-observed mixing relationships between Cl- and SO₂^4- concentrations in Yellowstone’s hydrothermal waters. Although our results are specific to Sentinel Meadows, they provide an important three-dimensional perspective on the influence of shallow water recharge in continental hydrothermal systems.