What Minerals Are in Hot Springs?
Mostly dissolved salts that the water picked up on its way through the rock: sodium, calcium, magnesium, sulfate, chloride and bicarbonate, with silica and a little iron in many springs. Government scientists have tested the water at 736 of the hot springs on this site, and the map below colors each one by its pH, its dissolved minerals or its hardness.
- Acidic, under 5.5 15
- Slightly acidic 77
- Neutral, 6.8 to 7.2 96
- Slightly alkaline 306
- Alkaline, over 8.5 160
- Under 500 mg/L 159
- 500 to 2,000 105
- 2,000 to 10,000 57
- Over 10,000 13
- Soft, under 60 246
- Moderately hard 60
- Hard, 120 to 180 56
- Very hard, over 180 269
Tap a pin for the spring's readings and the year its water was sampled. Springs that were never tested are left off.
Why the Water Changes From Place to Place
Idaho is where the blue piles up. A huge body of granite called the Idaho Batholith sits under much of the state. Hot water working through granite dissolves very little, mostly some silica and sodium, and it comes out with a pH of 9 or more. Soakers call water like this silky.
Yellowstone runs the opposite way. Volcanic gas bubbles up through the pools there and turns into sulfuric acid, so Wyoming has more acidic readings than any other state. Most of those pools are far too hot and too sour to get into.
The darkest purple gathers in northern Utah. Springs there rise through salty lake beds left behind by Lake Bonneville, the ancient lake that shrank into the Great Salt Lake. Crystal (Madsens) Hot Springs tested at 40,700 mg/L, which beats seawater.
Switch to hardness and the map splits in two. Hardness counts the calcium and magnesium in the water, the same measure behind hard and soft tap water. Most springs in Idaho, Oregon and California come up soft. In Utah, Arizona and Colorado the majority test very hard, and that dissolved calcium is what builds the stone terraces and crusty rims around many of those pools.
Most of these samples are old. The first was taken in 1915 and the newest in 2023, and the bulk of them date from before 1990. The chemistry of deep spring water changes slowly, so an old reading tends to hold up. A spring that has since been piped into a resort pool can test differently from its source.
The Springs at Each Extreme
Alkaline water between 95 and 110°F, the range most people like for a long soak.
- 1 Dagger Creek Hot Spring Idaho pH 9.9 110°F water · sampled 1980
- 2 Palm Springs (Agua Caliente Spring) California pH 9.8 104°F water · sampled 2005
- 3 Running Warm Springs Idaho pH 9.7 106°F water · sampled 1982
- 4 (Upper) Frisco Hot Spring New Mexico pH 9.7 98°F water · sampled 1958
- 5 Pool Creek Hot Spring Idaho pH 9.6 108°F water · sampled 1982
- 6 Preis Hot Spring Idaho pH 9.6 106°F water · sampled 1981
- 7 Easley Hot Springs Idaho pH 9.6 100°F water · sampled 1988
- 8 Cape Horn Warm Springs Idaho pH 9.6 95°F water · sampled 1980
- 9 Badley Warm Spring Idaho pH 9.6 100°F water · sampled 1979
- 10 Hot Spring Oregon pH 9.6 106°F water · sampled 1992
The highest pH readings on record.
- 1 Boulder Creek Spring Idaho pH 9.9 82°F water · sampled 1980
- 2 Dagger Creek Hot Spring Idaho pH 9.9 110°F water · sampled 1980
- 3 Palm Springs (Agua Caliente Spring) California pH 9.8 104°F water · sampled 2005
- 4 Arling Hot Spring Idaho pH 9.8 90°F water · sampled 1972
- 5 Billy Hot Spring Idaho pH 9.7 sampled 1980
- 6 Running Warm Springs Idaho pH 9.7 106°F water · sampled 1982
- 7 Trail Creek Hot Spring Idaho pH 9.7 122°F water · sampled 1980
- 8 Unnamed spring California pH 9.7 sampled 1986
- 9 (Upper) Frisco Hot Spring New Mexico pH 9.7 98°F water · sampled 1958
- 10 Dutch Frank Hot Spring Idaho pH 9.6 149°F water · sampled 1981
The lowest pH readings. Nearly all of these are steaming vents and mud pots you can only look at.
- 1 Coso Hot Springs California pH 1.5 207°F water · sampled 1964
- 2 Sulphur Springs New Mexico pH 1.9 189°F water · sampled 1949
- 3 Gas Vent Wyoming pH 3.0 sampled 2006
- 4 Echinus Geyser Wyoming pH 3.4 185°F water · sampled 2007
- 5 Cistern Spring Wyoming pH 3.6 199°F water · sampled 2007
- 6 Unnamed spring Wyoming pH 4.5 201°F water · sampled 2007
- 7 Bijah Spring Wyoming pH 5.4 198°F water · sampled 2006
- 8 Apollinaris Spring Wyoming pH 5.4 sampled 2006
- 9 Artists Paintpots Wyoming pH 5.4 199°F water · sampled 2006
- 10 Gibbon Hill Geyser Wyoming pH 5.6 199°F water · sampled 2006
Total dissolved solids: everything dissolved in a liter of the water, by weight. Seawater comes in around 35,000 mg/L.
- 1 Crystal (Madsens) Hot Springs Utah 40,700 mg/L 129°F water · sampled 2022
- 2 Stinking Hot Springs Utah 33,800 mg/L 118°F water · sampled 1989
- 3 Mineral Spring (Lower Klawasi Cone) Alaska 27,500 mg/L sampled 1973
- 4 Grantsville Warm Springs Utah 26,700 mg/L 77°F water · sampled 1978
- 5 Unnamed spring Utah 24,900 mg/L 77°F water · sampled 1963
- 6 Iron Mountain Hot Springs Colorado 22,000 mg/L sampled 1965
- 7 Unnamed spring New Mexico 17,800 mg/L 80°F water · sampled 1941
- 8 Becks Hot Spring Utah 15,500 mg/L 133°F water · sampled 1967
- 9 Hot Spring Arizona 13,900 mg/L 138°F water · sampled 1978
- 10 Soda Lake Nevada 13,800 mg/L 86°F water · sampled 2018
The springs with the least dissolved in them. Many draw on snowmelt that spends little time underground.
- 1 Myers Warm Spring California 40 mg/L 75°F water · sampled 2007
- 2 Bumpass Hell California 42 mg/L 199°F water · sampled 1981
- 3 Devils Kitchen California 49 mg/L 203°F water · sampled 1981
- 4 Bigelow Hot Springs Oregon 49 mg/L 142°F water · sampled 2004
- 5 Mill Creek Springs California 88 mg/L 150°F water · sampled 1981
- 6 Sand Spring Massachusetts 114 mg/L 75°F water · sampled 1969
- 7 Thorn Spring West Virginia 118 mg/L 72°F water · sampled 1935
- 8 Chattanooga Hot Spring Colorado 126 mg/L sampled 1995
- 9 Eldorado Springs Colorado 130 mg/L 79°F water · sampled 1976
- 10 Reveille Mill Spring Nevada 143 mg/L 84°F water · sampled 1967
Sulfate readings, highest first. Sulfate has no smell. The rotten-egg smell at some springs comes from hydrogen sulfide gas, which these samples don't measure.
- 1 Unnamed spring New Mexico 8,900 mg/L 73°F water · sampled 1975
- 2 Spring New Mexico 6,200 mg/L 76°F water · sampled 1975
- 3 Laguna Pueblo Springs New Mexico 5,600 mg/L 80°F water · sampled 1975
- 4 Unnamed spring New Mexico 5,440 mg/L 80°F water · sampled 1941
- 5 Unnamed spring New Mexico 4,260 mg/L 68°F water · sampled 1963
- 6 Paoha Island Springs California 3,300 mg/L 181°F water · sampled 2018
- 7 Soda Lake Nevada 3,200 mg/L 86°F water · sampled 2018
- 8 San Ysidro Hot Springs New Mexico 2,600 mg/L 85°F water · sampled 1974
- 9 Brockman Creek Hot Spring Idaho 2,500 mg/L 95°F water · sampled 1976
- 10 Iron Mountain Hot Springs Colorado 2,210 mg/L sampled 1965
Iron leaves rust-orange stains on rock and pool bottoms. The EPA flags drinking water once it passes 300 µg/L.
- 1 Sulphur Springs New Mexico 92,000 µg/L 189°F water · sampled 1949
- 2 Coso Hot Springs California 28,000 µg/L 207°F water · sampled 1964
- 3 Surprise Lake Hot Spring Alaska 13,000 µg/L sampled 1976
- 4 San Ysidro Warm Springs New Mexico 11,000 µg/L 68°F water · sampled 1982
- 5 Gas Vent Wyoming 8,860 µg/L sampled 2006
- 6 Chocolate Pots Wyoming 5,050 µg/L 124°F water · sampled 2007
- 7 Pinkerton Hot Springs Colorado 4,500 µg/L sampled 1976
- 8 White Sulphur Springs (Hooniah H S) Alaska 3,100 µg/L sampled 1915
- 9 Stinking Hot Springs Utah 3,000 µg/L 118°F water · sampled 1989
- 10 Horner Hot Springs Alaska 2,700 µg/L 117°F water · sampled 1915
pH of Each State's Springs
The median pH of the tested springs in each state. The dashed line marks neutral, pH 7. Bars reaching right lean alkaline and bars reaching left lean acidic.
Hot Spring Minerals, Answered
What minerals are in hot springs?
Mostly dissolved salts. Sodium, calcium, magnesium, sulfate, chloride and bicarbonate make up the bulk, and many springs add silica and a little iron. Which ones lead depends on the rock the water moved through. The typical tested spring here carries 532 mg/L of dissolved minerals at a pH of 7.7.
Why do some hot springs smell like sulfur?
That rotten-egg smell is hydrogen sulfide, a gas that escapes as the water reaches the air. The samples on this page measure sulfate instead, a dissolved form of sulfur with no smell, so a spring can be high in sulfate and still smell clean. Laguna Pueblo Springs in New Mexico, near the top of the list, tested at 5,600 mg/L.
What is a mineral spring?
A spring whose water carries a noticeable load of dissolved minerals. No single rule defines one. Bottled water sold as mineral water in the US needs at least 250 mg/L of dissolved solids, and the USGS calls water saline once it passes 1,000 mg/L. 114 of the springs tested here are over that saline mark.
Is hot spring water hard or soft?
It depends on the region. Of the springs with calcium and magnesium readings, 39% test soft and 43% test very hard. Idaho, Oregon and California lean soft, while Utah, Arizona and Colorado mostly run very hard.
What makes hot spring water feel silky?
A high pH. Alkaline water above about 8.5 feels slippery because it reacts with the natural oils on your skin, much like soap does. 32 springs here pair that with water between 95 and 110°F, and the Silky soaks tab lists the top ones.
Mineral by Mineral
Water chemistry: USGS and partner agency samples from the Water Quality Portal, matched to each spring by location, sampled 1915 to 2023. Neighboring vents sometimes share one sample, and the rankings list each sample once. Download the data (CSV), free to reuse under CC BY 4.0 with credit to Soaking Springs.