Hot spring is known as a geothermal spring is a spring produced by the emergence of groundwater that has been strongly heated geothermally below the earth surface. Hot spring water often contains large amounts of dissolved minerals. The chemistry of hot springs ranges from acid sulphate springs with a pH as low as 0.8, to alkaline chloride springs saturated with silica, to bicarbonate springs saturated with carbon dioxide and carbonate minerals. Some springs also contain abundant dissolved iron. The minerals brought to the surface in hot springs often feed communities of extremophiles, microorganisms adapted to extreme conditions, and it is possible that life on Earth had its origin in hot springs (Farmer, 2000).
Generally, the chemical composition of a hot spring varies from alkaline to acid sulphate to bicarbonate to iron-rich, and each of them defines an end member of a range of possible chemistry of that hot spring (Drake, et al., 2014). According to White, et al. (1956), Alkaline chloride hot springs are derived from the hydrothermal fluids that form when groundwater containing dissolved chloride salts reacts with silicate rocks at high temperature. These springs are saturated with silica (SiO2) but their pH is nearly neutral. Since silica solubility depends on temperature, upon cooling, the silica is deposited as geyserite, a form of opal (opal-A: SiO2.nH2O) (White, Brannock, & Murata, 1956). On the other hand, acid sulphate hot springs are made when the hydrothermal fluids rich in hydrogen sulphide (H2S), is oxidized to form sulphuric acid, H2SO4 (Drake, et al., 2014).
The resulted PH in this formed fluid is lowered to an acidic value less than 0.8 (Cox, Shock, & Havig, 2011). The so formed acid reacts with the rock to change it to clay minerals and silica residue. In the case where carbon dioxide (CO2) and groundwater would react with carbonate rocks, this would lead to the formation of hydrothermal fluid of bicarbonate hot springs (Drake, et al., 2014). When these hydrothermal fluids reach the surface, CO2 is rapidly evaporated and carbonate minerals precipitate to form travertine, so that bicarbonate hot springs tend to form high-relief structures around their openings (Des Marais & Walter, 2019).The Iron rich hot springs are characterized by the presence of groups of microorganisms that produce the small groups of oxidized iron from the iron found in the hydrothermal fluids, and then feed the springs (Parenteau & Cady, 2010).
Recreational waters may contain chemicals of organic or inorganic source. Generally, chemicals get into hot spring water through rock dissolution where it passes, and soil in which water percolates (Zhang, et al., 2019). This is due to the fact that dissolution increases with temperature. Other than temperature, low pH of water also causes greater leaching of inorganic materials from rocks and soil, increasing the chance that naturally occurring inorganic substances will accumulate at higher concentrations than expected (Van der Sloot et al., 1997).
The main sources of sulfate ion in the underground waters are: i) Fumarolic gas of sulfur dioxide, ii) Rock forming minerals gypsum (CaSO4.2H2O), and 3) Rock forming minerals of pyrite (Seki et al, 2004). However, pyrite needs much oxygen to form sulfate ion ( FeS2 + 4O2 + 3H2O → Fe(OH)3 + 2SO42 − +6H+) (Jolie, 2010). Geothermal water tends to reduce the resistivity and increase the conductivity of its host rock caused by high concentrations of dissolved particles in the fluid and secondary alteration minerals that are formed when the thermal waters interact with the host rock. The type of alteration minerals being formed depends on the type of the host rock and the temperature and salinity of the fluid (Jolie, 2010). In the complex circulation processes, water-rock interactions play an important part in hydro-geochemical indicators of the waters in the geothermal systems which can be used to examine the properties of geothermal reservoirs and the mixing behavior of groundwater (Capasso et al., 2001; Han et al., 2010; Baioumy et al., 2015; Shi et al., 2017). Mixing processes between thermal and non-thermal water can be enhanced by faults in a geothermal area (Han et al., 2010).
The minerals in hot springs have a detoxifying capacity and remedy for skin ailments including acne, eczema and psoriasis because of their dissolved chemicals including Sodium bicarbonate, chloride and sulfur (Gupta and Nicol, 2004; Levin and Miller, 2011). Nyamyumba hot springs attract many people for swimming purpose. This research aimed at studying the chemical composition of Nyamyumba hot springs and analyze its safety for recreation purpose.