Why the water of some fountains smells of sulfur: the scientific explanation

Sometimes it can happen, especially in a small town or in spa areasto approach a water fountain and feel the typical “rotten egg” smell. That smell, pungent and generally unpleasant, is not the result of poor maintenance, but of a real natural phenomenon: the presence of sulfur compounds, that is, sulphur-based, dissolved in water. In particular, the main person responsible is hydrogen sulfide (H_₂S)a volatile gas that is formed by geological and microbiological processes. The water that contains it, once it resurfaces on the surface, releases vapors with the characteristic thermal smell that we know well. Although this gas is harmful if inhaled in large quantities (after all, it is the dose that creates the poison) in this case it should not be considered a direct health hazardbut as a real one chemical signal coming from the underground and geological history of water.

The cause of the smell of sulfur in water: geological origin

Many fountains draw from deep aquifers or natural springs that pass through layers of rich rocks sulphur-containing minerals. When water flows through these layers, it can dissolve sulfur compounds such as sulfates or sulfides. In the presence of reducing conditions, i.e poor in oxygen atmospheric, sulfates are transformed into hydrogen sulfide (H₂S) in the same water.

Now, the H₂S has aexcellent solubilitywhich, basically, would lead it to remain as long as possible in the aqueous medium, remaining dissolved inside without leaking out, but this that’s not all. In fact, gas also has ahigh volatilitya characteristic common to gases that tend to “fly away” from the solvent, in this case the water from fountains. As soon as the liquid comes into contact with air, hydrogen sulfide jumps out of the water reaching our nostrils, giving us that typical rotten egg smell.

It is the same principle that we find in sulphurous thermal waters: springs known since ancient times for their therapeutic properties, but with an unappealing smell. Many well-known spa resorts in Italy are famous for the pungent smell that pervades the air due to the strong concentration of H_₂S coming from volcanic subsoil.

The microbiological origin and “sulphur-eating” bacteria

In addition to geological phenomena, odor can originate from the activity of a particular type of microorganism: i sulfate-reducing bacteria (SRB, sulfate-reducing bacteria). These microbes, in the total absence of oxygen (anaerobic conditions), use sulfates dissolved in water as an energy source, reducing them to our hydrogen sulfide. It’s a bit like bacteria “were breathing” sulfates instead of oxygen, thus producing the typical odor.

This phenomenon is frequent in deep aquifers and in little-used water networks, especially where the water stagnates and there is a lack of oxygen. It is no coincidence that tap water in houses that have been uninhabited for weeks can develop the same sulphurous odor: stagnation favors bacterial activity. Don’t panic, just stop let the water flow for a few minutes to eliminate the H_₂S accumulated.

Is it dangerous for your health?

At low concentrations, such as those in fountains or drinking water that present this phenomenon, hydrogen sulfide does not represent a serious health riskbut more of a smell and taste issue. Furthermore, the World Health Organization (WHO) reports that even at very low concentrations (0.05 mg/L) H_₂S is perceivable by the human sense of smell. However, high concentrations, typical of industrial areas or extreme natural phenomena, can be toxic. Hydrogen sulfide is in fact a poisonous gas in high dosesso much so that it is regulated in the workplace as a dangerous substance, but in the case of public fountains the quantity present never reaches harmful levels: at most, it discourages drinking or washing hands.