8 constituents. 500+ mapped springs. One USGS-sourced dataset.
Soak Trail profiles eight constituents of hot-spring water — pH, total dissolved solids, and six core minerals — and ranks U.S. hot springs by each one's measured concentration. A "concentration" is the amount of that constituent dissolved in the water, reported in mg/L (pH is unitless); each ranking lists only springs with a verified USGS water-quality sample matched to that spring.
A complete guide to the science behind hot spring water — pH, mineral content, geothermal chemistry, and what it means for your soak.
Read the guideAcidity vs. alkalinity, from sour to slippery.
pH measures how acidic or alkaline water is on a 0-14 scale, where 7 is neutral. Below 7 is acidic, above 7 is alkaline.
View profile & rankings →A single number for how much mineral load the water carries.
Total dissolved solids (TDS) is the sum of everything dissolved in the water, mainly minerals, salts, and metals. It is the simplest proxy for how "mineral-rich" a spring is.
View profile & rankings →The main driver of water "hardness."
Calcium is the most abundant cation in many hot springs and the main contributor to water hardness. It enters groundwater as it dissolves limestone and other carbonate rocks.
View profile & rankings →The most-studied mineral in balneology.
Magnesium is the mineral most studied in balneology and the dominant cation in Dead Sea brine and many mineral springs. Magnesium sulfate (Epsom salt) baths are a popular home remedy for sore muscles.
View profile & rankings →Pairs with chloride to make a spring "saline."
Sodium is an alkali metal and the partner of chloride in saline springs. In hot springs it also appears with bicarbonate. It is one of the most common dissolved ions in geothermal water.
View profile & rankings →The signature of traditional "sulfur springs."
Sulfate is a sulfur-oxygen ion. When sulfate is reduced to hydrogen sulfide underground, it gives some springs the familiar "rotten egg" smell. Sulfur springs have been used for skin and joint conditions for centuries.
View profile & rankings →The anion that makes a spring salty.
Chloride is the dominant anion in saline springs and the partner of sodium. High-chloride springs are sometimes called "salt springs."
View profile & rankings →The rust-colored mineral that stains the rocks.
Iron is a dissolved metal that gives some springs a rust or orange tint and a metallic taste. Iron-rich springs stain surrounding rocks orange as the iron oxidizes on contact with air.
View profile & rankings →