A mineral spring is a natural spring whose water is hard, that is, contains dissolved minerals. Salts, sulfur compounds and gases may dissolve in the water as it passes through rocks underground. In this respect mineral springs differ from sweet springs, which produce soft water without noticeable dissolved gases. Dissolved minerals can change the taste, colour and odour of the water and may give it specific physiological effects.
Mineral water obtained from springs, and salts precipitated from it such as Epsom salt, have long been important commercial products. Bottling and sale of mineral water form a global industry, and salts, travertine and iron oxides deposited at springs are used as medicinal materials, building stone or mineral raw materials.
Some mineral springs contain high concentrations of harmful dissolved minerals such as arsenic and should not be drunk. Sulfur springs smell of rotten eggs because they contain hydrogen sulfide, which is dangerous and sometimes fatal. It is a gas and usually enters the body by inhalation. The amount ingested in drinking water is much lower and unlikely to cause harm, but as of 2003 there had been little research on long-term low-level exposure.
Mineral spring water has sometimes been considered to have therapeutic value. Mineral spas are resorts developed around springs where guests drink or bathe in the water. Historic spas often had elaborate stone buildings, including artificial pools, retaining walls, colonnades and roofs, sometimes in the form of Greek temples, pavilions or pagodas, while others were entirely enclosed in spring houses.
For centuries, commercial promoters in Europe, North America and elsewhere classified mineral springs by the chemical composition of the water and the supposed medicinal benefits of each spring:
These categories are largely historical or commercial names and do not correspond to modern hydrochemical classification. Modern studies normally analyse mineral waters systematically by major-ion composition, total dissolved solids, pH, temperature and gas content.
Sedimentary rock, usually limestone or calcium carbonate, sometimes forms when minerals evaporate or precipitate rapidly at a spring outlet, especially at hot springs. At cold springs, rapid precipitation results from a decrease in acidity when gases escape. Such mineral deposits are also found in dry lake beds.
Spring deposits can form spectacular landforms, including terraces, stalactites, stalagmites and frozen waterfalls, such as those at Mammoth Hot Springs. A light, porous variety of calcite called travertine is widely used as building stone in Italy and elsewhere. Travertine may be white, tan or cream and commonly has fibrous or concentric textures.
Another spring deposit containing siliceous and calcareous minerals is tufa. Tufa resembles travertine but is softer and more porous. Iron-bearing springs may deposit iron compounds such as limonite, and some of these deposits are large enough to be mined as iron ore.
Modern uses of mineral springs include bottled mineral water, spa bathing, geothermal heating and tourism. The bottled-water industry must test sources for microorganisms, heavy metals, radionuclides and organic pollutants and comply with drinking-water or natural-mineral-water standards. Spa development must assess water temperature, flow rate, recharge area and contamination risk, and avoid over-extraction that reduces spring flow or degrades water quality.
Environmental risks include aquifer contamination, gas hazards and sediment clogging. Hydrogen sulfide from sulfur springs can accumulate to dangerous concentrations in enclosed spaces, arsenic-rich or saline water can contaminate drinking-water sources, and spring deposits can block pipes and drainage. Development therefore requires hydrogeological investigation, water-quality monitoring and safety management.