Thermal baths, mineral water

Close-up of a pool with boiling geothermal water at Hveravellir is actually in the heart of Iceland

Mineral and thermals waters constitute a special category of groundwater which has been used by humans for drinking, bathing, healing, therapy or religious rites for thousands of years. 

Mineral waters is referred to specific groundwaters used for curative purposes which are characterized by higher than average concentrations of dissolved solids or gases or higher temperature. Groundwater is considered as a part of the geological environment remaining in constant interaction with rocks. This water-rock interaction forms groundwater chemical composition and is respsonsible for concentration of specific ions and gas phases, as well as the content of total dissolved solids. 

Thermal waters are groundwaters with a temperature of atleast 20 degrees at the outflow. Thermal waters are primarily extracted for their heat content and secondarily for their mineral content. High temperatures and favorable aquifer permeability are the most important factors giving rise to thermal water systems from which heat can be obtained. The heat is extracted from liquid water or steam, which may be brought to the surface at natural emergences such as springs and geysers.