Onsen water ( お湯 ) often has various elements in it - and this is what makes it so special. In onsens, the following types of water are commonly found, each with its own unique chemical composition and benefits:
Sulfur springs ( 硫黄泉 ) - milky white water with a distinct smell similar to rotten eggs, and yellowish deposits. Sulfurous water is known for its detoxifying properties, and helping to alleviate skin conditions like eczema, healing wounds ( 創傷 ), and soothes aching muscles and joints. Notable sulphur onsens are Kusatsu ( 草津 ), Bessho (別所温泉) and Unzen (雲仙温泉) onsens
Sulphate springs ( 硫酸塩泉 ) containing calcium, sodium or magnesium sulfates. Water is usually colorless but tastes bitter. Bathing in sulphate waters is good for treating cuts, burns and bruises and various skin problems, and drinking some is said to relieve constipation. Sulphate water can be found in Tamatsukuri (玉造温泉), Kawaguchiko (河口湖温泉) and Ashinomaki (芦ノ牧温泉) onsens.
Iron Springs ( 鉄泉 or 含鉄泉 ) are reddish-brown - due to iron. Actually the water comes from the ground colorless, but turns brown when it reacts with atmospheric oxygen. Iron springs are good for improving blood circulation, and are especially beneficial for anemia and menstrual disorders. Ikaho (伊香保温泉), Nagaragawa (長良川温泉), Noboribetsu (登別温泉) and Tsuchiyu (土湯温泉)
Sodium Chloride Springs (塩化物泉, Enkaibutsu-sen) have salty taste resembling seawater. Salt helps retain body heat, it is good for warming the body, and also beneficial for treating muscle and joint pain, cuts and burns. Some examples are Akiu 秋保温泉, Kinosaki 城崎温泉 and Ibusuki 指宿温泉
Hydrogen Carbonate Springs (炭酸水素塩泉, Tansansuisoshio-sen) are often colorless or slightly brownish. Bathin in hydrogen carbonate springs has a beautifying effect on the skin as it helps to softens the keratin. It can also help to improve digestion. Some examples are Yumura 湯村温泉, Naruko 鳴子温泉 and Ureshino 嬉野温泉
Acidic Springs (酸性泉, Sansei-sen) are often mildly cloudy, the water is strongly acidic. Such onsens are often found in highland areas. Acidic water has strong antibacterial effect and can be good for treating skin diseases and fungal infections, as well as gastrointestinal disorders. However, those with sensitive skin or weak health might consider to limit their exposure to these types of baths. You can find acidic water in Shin-Tamagawa (新玉川温泉), Dake (岳温泉) and Tateshina (蓼科温泉) onsens.
Carbonated onsens 二酸化炭素泉 are known for their beautifying effects. Essentially a water with dissolved CO2 gas, it feels soft and gentle and make the skin feel smooth. Carbonated water is good for treating a variety of skin ailments. It also keeps the body warm and helps to expand blood vessels and lower blood pressure. The bubbly water tends to be less hot than other hot spring types and is not so common compared to other types of onsens. Some examples are Arima (有馬温泉), Beppu (別府温泉郷) and Yunoko (湯の児温泉) onsens.
Radioactive - hot spring waters contain very small amounts of radium or radon which have beneficial effects on the body, such as lowering blood pressure and treating gout, circulatory disease, high blood pressure and rheumatism. Some examples are Yunoyama (湯の山温泉), Misasa (三朝温泉) and Tamana (玉名温泉) onsens
Simple Onsens (単純温泉, Tanjun Onsen) - as the name implies these are the ones without large concentrations of elements, however the water has to be at least 25 degrees Celsius warm. Naturally, the water is largely colorless, tasteless and odorless - but soft and gentle. This makes it good for children and people with sensitive skin. Nevertheless, bathing in simple onsens can help treating fatigue, nerve pain, insomnia and high blood pressure (高血圧症) among others ailments. Notable simple onsens are Kinugawa (鬼怒川温泉), Dogo (道後温泉) and Gero (下呂温泉)
There is more! #
This list is far from being complete, and even those types of onsens that were mentioned, represent a very rough classification. Each hot spring is unique - both in sense os chemical composition of its water, and in all other things that make an onsen. Just go and enjoy!
Ah, and another thing I forgot to mention - sometime one onsen would have several type of water in it! very convenient :D
Kanji quiz #
Try it at home #
You can actually buy bath salts that contain minerals from famous onsens. Do not expect crazy concentration of sulphur, expect added color and aromatization - but hey, you don’t go to the onsen every day. I personally add those to my bathwater once in a while, especially in winter. Something like https://amzn.to/3XOg8sF or https://amzn.to/3Ty8Uq2