Reading the water: spring types and the analysis sheet

When water counts, and when it is therapeutic

Under the Hot Springs Act, water from the ground is onsen if it is 25°C or warmer at the source or carries enough of any one of a list of substances. The Ministry of the Environment's analysis guidelines set a stricter bar, the ryoyosen or therapeutic spring: 25°C, or 1,000 mg/kg of dissolved minerals, or a set amount of carbon dioxide, iron, hydrogen ions, iodide, sulphur or radon. Only a ryoyosen is given a spring type. Takefue calls its own source one; our guide to private onsen deals with baths that are not onsen at all.

The spring types

The Ministry recognises ten types, named by their main ions, so 'sodium-calcium chloride' is a chloride spring in which sodium leads. Each below with what the Ministry's guide for bath operators or the Japan Spa Association says of it.

• Simple spring (tanjun onsen): under 1,000 mg/kg of minerals; gentle on the skin. Alkaline ones, pH 8.5 and over, feel smooth
• Chloride (enkabutsu-sen): salt left on the skin forms a film that holds in heat and moisture
• Hydrogen carbonate (tansan-suisoen-sen): lifts old keratin and sebum and leaves the skin smooth
• Sulphate (ryusan'en-sen): posted for cuts, poor circulation and dry skin, among others
• Carbon dioxide (nisanka-tanso-sen): bubbles cling to the skin; the gas widens the capillaries and warms the body
• Iron (gantetsu-sen): turns a cloudy reddish brown in the air and smells of rust
• Acidic (sansei-sen): strongly antibacterial; sensitive skin should rinse in plain water afterwards
• Iodine (gan'yoso-sen): brown like tincture of iodine, with a medicinal smell
• Sulphur (io-sen): the smell of eggs is hydrogen sulphide
• Radioactive (hoshano-sen): radon, with nothing to see or smell; the Ministry lists gout and rheumatoid arthritis among its indications

Kinnotake Sengokuhara

Wasure no Sato Gajoen

The three terms in brackets

After the type, the analysis sheet (onsen bunsekisho) classes the water by temperature at the source, pH and tonicity, the concentration against that of the body's cells. Akan Tsuruga Bessou Hinanoza's reads 'simple spring (neutral, hypotonic, high-temperature)'. The sheet also gives the date of analysis, which must be renewed at least every ten years.

• Temperature: cold mineral spring (reikosen) under 25°C; low-temperature 25–34°C; onsen 34–42°C; high-temperature 42°C and over
• pH: acidic under 3; weakly acidic 3–6; neutral 6–7.5; weakly alkaline 7.5–8.5; alkaline 8.5 and over
• Tonicity: hypotonic under 8 g/kg; isotonic 8–10 g/kg; hypertonic 10 g/kg and over

Free-flowing, and the lines that say otherwise

Kakenagashi means the water runs through the bath and away rather than round again; gensen kakenagashi, straight from the source. The phrase is the inn's own, but the law obliges every public bath to post whether water is added (kasui), heated (kaon), recirculated and filtered (junkan roka), disinfected or given additives, and why.

Good water needs little help. At Yamagata The Takinami bath attendants (yumori) set the valve so the 58°C spring sits at about 43°C in the room baths with no water added; Iki Retreat Kairi Murakami cools its 69.2°C source by the flow alone; most of Takefue's tubs have a red valve for more spring and a light-blue one for its own cold spring water. Shoraiso adds well water to a 93.6°C source and is still free-flowing.

Recirculation is often necessary and not a fault. Satoyama Jujo's 27°C spring is heated and recirculated with fresh water fed in continuously; Lake Shikotsu Tsuruga Bessou Ao no Za says its suite baths are recirculated, filtered and chlorinated; Nishimuraya Honkan's shared baths are heated and diluted.

Yamagata The Takinami

Shoraiso

Reading an inn's bath description

Look for the source and whether it is the inn's own, the type, the handling words, and which baths they apply to. At Hinanoza the indoor large baths are free-flowing and the room baths recirculated; at Moku no Sho the public baths are recirculated while the private bath and some room baths run straight from the source. Fufu Hakone uses milky, sulphurous Owakudani water in its shared baths and clear Gora water in the rooms.

None of the inns on our list records an iron spring. The rest, by the type each record gives: