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Rene L. Diaz · Hydrogeologist · 2026-07-23
The mineral analysis panel of a hot spring can look like a lab report. Calcium, bicarbonate, sulfate, fluoride, iron, hydrogen sulfide. For most visitors those names mean nothing. This guide translates them into terms that are useful for a traveler.
Hot springs are not simply hot water. They are geochemical solutions formed over years — sometimes centuries — as water travels underground, dissolving minerals from the rocks it passes through. The resulting composition depends on the local geology: a spring in a volcanic region has a completely different profile from one in a sedimentary basin.
Knowing the main minerals in a spring tells you three practical things: what kind of sensory experience to expect (smell, color, water texture), what cultural traditions have developed around that water, and whether there is any reason to consult a doctor before bathing.
The most common mineral in the world's hot springs. Bicarbonate gives thermal water that soft, slightly silky feel. Bicarbonate-rich waters are historically the ones used as "drinking cures" at the great European spa towns of Karlovy Vary, Vichy, and Baden-Baden. The water feels smooth on the skin and has no notable taste or smell.
"Sulfate-type" waters with a slight bitter taste. European balneary traditions historically associated them with the digestive system. They are most common at spas in the Pannonian Basin (Hungary, Austria, Czech Republic) and at the great spas of the Italian Riviera.
Responsible for the characteristic rotten-egg smell that some hot springs produce. Sulfurous springs like Terme di Saturnia (Italy) or Rotorua (New Zealand) owe that aroma to dissolved hydrogen sulfide, which escapes into the atmosphere on contact with air. Typical concentrations at visitable springs are very low — the smell is far more intense than the chemical analysis would suggest. From the traveler's perspective, the smell disappears within minutes of leaving the water.
The dominant components of seawater, these also appear in some hot springs — especially those of marine origin or those that pass through salt formations. High-sodium waters have a dense texture and slight buoyancy. The Blue Lagoon in Iceland is a well-known example of high-mineral water (though its buoyancy there owes more to dissolved silica than to sodium).
These tint water yellow, reddish, or brown, and can leave visible deposits on the spring bed. They are why some springs have intensely ochre-colored water. The Termas de Reyes in Jujuy, Argentina, and several iron-rich springs in northern Spain are examples of this profile.
High concentrations appear in young volcanic springs. Silica gives water a milky blue-white appearance — characteristic of the Icelandic Blue Lagoon and many Japanese onsen. It is not perceptible as taste or smell.
Some natural springs contain small concentrations of geologically derived radon. Regulated spa facilities take periodic measurements and ensure proper ventilation — certified establishments publish this data. It is not a relevant concern for the vast majority of visitable springs.
On each spring listing you'll see the Mineral Composition section with values in milligrams per liter (mg/L). A high bicarbonate reading (>500 mg/L) indicates an alkaline water with a smooth texture. A high sulfate reading (>200 mg/L) indicates sulfate-type water. Any value in the H₂S row explains the smell, even at concentrations of just 1–3 mg/L.
TDS (Total Dissolved Solids) is the summary figure: it adds up all dissolved minerals. Waters below 500 mg/L are weakly mineralized; between 500 and 1,500 mg/L, moderate; above 1,500 mg/L, strongly mineralized. The great European drinking-cure waters typically exceed 2,000 mg/L.
This guide describes the geochemistry of hot springs and the historical cultural traditions associated with each water type. It does not constitute medical advice. If you have a health condition and are considering thermal bathing as part of a treatment, consult a qualified health professional before visiting.
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