Hot Spring Water Quality Guide
A summary of Japan's eight main hot-spring water types, based on the classification criteria in the Ministry of the Environment's official hot-spring analysis guidelines and the published indications for each type.
Onsen (温泉) vs. Kōsen (鉱泉): What's the Difference?
- “Onsen” is the legal term defined by Japan's Hot Spring Act (Onsen-hō, 1948): a spring qualifies if its source temperature is 25°C or higher, or if it contains at least one of a set of specified substances (such as metaboric acid, radon, or sulfur) above a minimum threshold — regardless of temperature.
- “Kōsen” (mineral spring) is an older, more general term for groundwater containing dissolved minerals. It predates the Hot Spring Act, and many low-temperature facilities still use “kōsen” in their name today.
- In practice, many places named “kōsen” actually meet the legal onsen threshold and are classified as onsen under the law. The naming difference is mostly a matter of history and custom, not a strict legal boundary.
- This site preserves each spring's name exactly as it appears in its source Wikipedia article, including any “kōsen” naming, without altering it.
Simple Thermal Spring
- Definition
- Total dissolved solids (excluding dissolved gas) under 1,000 mg per kg, with a source temperature of 25°C or higher. It's called “simple” because no single mineral dominates — not because the water is weak. A simple spring with pH 8.5 or higher is called an alkaline simple spring.
- Characteristics
- Colorless, odorless, and gentle on the skin. Alkaline simple springs in particular are known for a smooth, silky feel.
- Geological Background
- Because no one mineral dominates, it isn't tied to a particular geology and occurs widely across mountain valleys and basins nationwide — it's Japan's single most common water type.
- Bathing Indications
- Autonomic nervous instability, insomnia, low mood.
- Drinking Indications
- None established
Chloride Spring
- Definition
- Total dissolved solids of 1,000 mg/kg or more, with chloride ion (Cl⁻) as the dominant anion — a mineral profile close to seawater.
- Characteristics
- Tastes salty. A thin film of salt forms on the skin during bathing and slows sweat evaporation, so the body stays warm longer after getting out — hence its nickname “the warming spring.”
- Geological Background
- Common along the coast, but also found inland as “fossil seawater” trapped in strata that were once seabed (Matsunoyama Onsen in Niigata is thought to draw on seawater roughly 8–12 million years old). In volcanic areas, magmatic chlorine can also contribute.
- Bathing Indications
- Cuts, peripheral circulation disorders, sensitivity to cold, low mood, dry skin.
- Drinking Indications
- Atrophic gastritis, constipation.
Bicarbonate Spring
- Definition
- Total dissolved solids of 1,000 mg/kg or more, with bicarbonate ion (HCO3⁻) as the dominant anion. When sodium is the dominant cation it's colloquially called a “soda spring.”
- Characteristics
- Known as the “beauty-skin spring” for the smooth, silky feel it leaves after bathing — a soap-like, refreshing touch.
- Geological Background
- Often forms where groundwater reacts with limestone or other carbonate-bearing strata, or with volcanic-derived carbon dioxide; commonly found in mountain and basin areas.
- Bathing Indications
- Cuts, peripheral circulation disorders, sensitivity to cold, dry skin.
- Drinking Indications
- Gastric and duodenal ulcers, reflux esophagitis, impaired glucose tolerance, hyperuricemia.
Sulfate Spring
- Definition
- Total dissolved solids of 1,000 mg/kg or more, with sulfate ion (SO4²⁻) as the dominant anion.
- Characteristics
- Has a bitter taste. Long used for therapeutic bathing, it's traditionally nicknamed the “wound spring” or the “stroke-recovery spring.”
- Geological Background
- Forms through the oxidation of sulfur associated with volcanic activity, or contact with gypsum (calcium sulfate)-bearing strata; common around volcanoes and in sedimentary areas.
- Bathing Indications
- Same as chloride springs — cuts, peripheral circulation disorders, sensitivity to cold, low mood, dry skin.
- Drinking Indications
- Biliary dysfunction, high cholesterol, constipation.
Sulfur Spring
- Definition
- Total sulfur (the sum of hydrosulfide ion, thiosulfate ion, and free hydrogen sulfide) of 2 mg/kg or more. Divided into a “hydrogen sulfide type,” dominated by free H2S, and a “sulfur type” otherwise.
- Characteristics
- Its rotten-egg smell (hydrogen sulfide) is the classic “onsen smell.” Often cloudy or milky white, and known to tarnish silver black. Has strong antibacterial properties.
- Geological Background
- Forms when volcanic gases (hydrogen sulfide and sulfur dioxide) released from magma dissolve into groundwater, so it's almost entirely confined to volcanic zones and volcanic fronts — the most distinctly “volcanic” of the eight water types.
- Bathing Indications
- Atopic dermatitis, psoriasis vulgaris, chronic eczema, purulent skin infections.
- Drinking Indications
- Impaired glucose tolerance (diabetes), high cholesterol.
Caution: Its strong effect means people with sensitive skin, or older adults with dry skin, should bathe with care.
Carbon Dioxide Spring
- Definition
- Contains 1,000 mg/kg or more of free carbon dioxide (CO2). Also called a “carbonated spring.”
- Characteristics
- Fine carbonic bubbles cling to the whole body while bathing, earning it the nickname “bubble spring.” Drunk, it has a cider-like fizzy freshness.
- Geological Background
- Forms as magmatic carbon dioxide dissolves into groundwater. Because it often emerges at low temperatures (as at Nagayu Onsen in Oita), it tends to occur at the margins of areas with residual volcanic activity.
- Bathing Indications
- Cuts, peripheral circulation disorders, sensitivity to cold, autonomic nervous instability. Even at low temperature, the carbon dioxide dilates peripheral blood vessels, earning it the nickname “spring for the heart.”
- Drinking Indications
- Reduced digestive function.
Iron Spring
- Definition
- Contains 20 mg/kg or more of total iron ions (Fe²⁺ and Fe³⁺ combined).
- Characteristics
- Often colorless when it first emerges, but turns reddish-brown to yellowish-brown as the iron oxidizes on contact with air. Has an astringent taste.
- Geological Background
- Derives from groundwater that has passed through iron-bearing rock or strata, and is often found near old mining areas in mountainous regions. Can occur in both volcanic and non-volcanic geology.
- Bathing Indications
- None established
- Drinking Indications
- Iron-deficiency anemia.
Radium Spring
- Definition
- Contains 30×10⁻¹⁰ curies of radon (Rn) per kg or more (roughly 111 becquerels, or 8.25 Mache units).
- Characteristics
- Colorless and odorless, nearly indistinguishable from other water types by the senses alone — its defining trait is the trace radioactive gas radon it carries.
- Geological Background
- Forms as radon gas, released while uranium-bearing rock such as granite weathers and breaks down, dissolves into groundwater — so it's common in granite mountain areas (as at Misasa Onsen in Tottori and Masutomi Onsen in Yamanashi).
- Bathing Indications
- Hyperuricemia, rheumatoid arthritis, ankylosing spondylitis.
- Drinking Indications
- None established
Classification criteria (definitions) follow the Ministry of the Environment's official hot-spring analysis guidelines (2014 revision). Indications for each water type are based on the Ministry's 2014 notice on contraindications and bathing/drinking precautions, confirmed via a summary article from Toho Laboratories. Sensory characteristics reference the Japan Spa Association; geological background references individual onsen towns' official sites and general earth-science knowledge. This is not medical advice — consult a doctor if you have a pre-existing condition.
環境省 鉱泉分析法指針(平成26年改訂)「鉱泉の定義と分類」 ・ 泉質別適応症の解説 - 東邦微生物病研究所 ・ 温泉の泉質のいろいろ - 日本温泉協会 ・ 塩化物泉と化石海水 - 大鹿村中央構造線博物館 ・ 硫黄泉 - Wikipedia ・ 含鉄泉 - Wikipedia