THE HYDROGEOLOGY OF THERMAL SPRINGS—TECTONIC SETTINGS, ORIGINS, AND NECESSARY TOOLS TO DOCUMENT OUR UNDERSTANDING

Thermal springs have captured human interest for millennia, and understanding major processes and controls affecting origin, flow, and their thermal and chemical transport has implications for a broad range of human endeavors. Present on all continents, thermal springs occur in almost every conceivable tectonic setting. Some appear to emanate from deep crustal sources, some from magmatic sources, and some appear to be sourced from local ground-water recharge that has moved deeply, and exited quickly without time for thermal equilibration. Mineral deposits derived from these springs hold detailed records of temporal variations of these flow systems, and offer opportunities for glimpsing environments and geochemical conditions not commonly accessible.

The preponderance of thermal springs have stable isotopic signatures that suggest a meteoric source of the water, with the geothermal gradient serving as the source of the heat. Mixing of distinguishable and discrete recharge waters is common, as has been demonstrated in Hot Springs National Park. However, thermal springs from New Zealand, Yellowstone National Park, Lassen Volcanic National Park, Nevada, Iceland, and Italy offer examples of a range of waters from diverse sources and unique chemistries—from acidic to near-neutral to basic.

Geochemical, and particularly stable-isotopic and radionuclide analyses, serve as key elements in the tool set necessary to elucidate water source and relative age. Geothermometry, equilibrium geochemical modeling, and thermal modeling allow hypothesis testing and development of conceptual models necessary to understand and utilize these rare resources.