Sulphurous, bicarbonate, iron-rich... behind these technical names lie very different waters. Here's how to tell them apart.
When visiting several thermal springs over the course of one's travels, it quickly becomes clear that none of them feel, smell, or behave quite the same way. This diversity comes from the rock the water passes through deep underground before rising back to the surface: depending on the minerals it dissolves along the way, it takes on a particular chemical composition, which shapes its color, smell, texture on the skin, and the sensation it gives during a soak. Here is a short guide to help you navigate the main families of thermal waters, without claiming any particular property — just to understand what you feel.
Instantly recognizable by its characteristic rotten-egg smell, sulphurous water owes this distinctive scent to the sulphur compounds it contains. It often takes on a slightly milky or greenish tint and leaves a soft, almost silky sensation on the skin once one gets used to it. It is historically one of the waters most associated with the thermal imagination, particularly in Europe, where many resorts developed around sulphurous springs.
Similar in composition to seawater, sodium chloride water is rich in dissolved salts, giving it a pronounced salty taste and a density that naturally aids buoyancy. The sensation is often compared to bathing in a warm sea, with that feeling of the body being effortlessly held up. It can be more or less concentrated depending on the source, with some reaching very high salinity levels.
Characterized by the presence of bicarbonates, this water is often distinguished by a slight natural fizz or a fine, delicate feel on the skin. It is frequently colorless and without a strong odor, making it a more discreet water than the sulphurous type, but just as widespread around the world. The sensation during a soak is generally described as light and enveloping.
When a water is naturally charged with carbon dioxide, it becomes carbo-gaseous: fine bubbles form spontaneously on the skin upon contact with the water, a rather striking phenomenon to watch and feel. This natural effervescence gives a sparkling, invigorating sensation that is immediately recognizable and sets this type of spring apart from calmer pools.
Rich in calcium, this water often comes from limestone terrain it has passed through at depth. It can leave a light whitish deposit on the walls of pools or surrounding rocks, a visible sign of its mineralization. To the touch, it is generally described as soft, without the marked character of other, more distinctive families.
Ferruginous water owes its name to its iron content, which often gives it a characteristic reddish or orange tint, visible especially on the rocks and ground around the source, sometimes stained with ochre deposits. Its taste, when drunk at a pump room, is generally described as metallic and unmistakable.
Less familiar to the general public, silica water contains a notable proportion of dissolved silica. It is often associated with a particular softness on the skin, almost silky, somewhat comparable to what is found in certain Icelandic springs known for their distinctive milky-blue water, a tint that is precisely linked to suspended silica.
Some thermal springs naturally contain radon, a gas present in trace amounts in certain granite or volcanic rocks. These are referred to as radioactive or radon waters, a term that may sound surprising but simply describes the presence of this natural gas at very low concentration in the water or ambient air of certain resorts. Such springs have always existed in this form and attract particular curiosity, without this fundamentally changing the immediate feel of the bath compared to other waters in the same region.
Beyond these technical classifications, the best way to appreciate these differences is, of course, to experience them firsthand, spring after spring. Each family of water tells a different geological story, that of the underground layers it passed through over sometimes many years before rising back to the surface, and it is this diversity that gives a thermal journey around the world all its charm.