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Added water: too hot, or not enough
When the source is very hot, cold water is mixed in to make bathing possible. Yumura's Arayu in Hyogo emerges at 98 °C, Obama at 105 °C — unusable as they are. The other reason is insufficient flow: water is added to fill the tub.
Adding water dilutes the minerals; a pH 1 acidic spring becomes a little milder. Notices usually give the reason, such as "water added for temperature control".
Asturio Cantabrio · CC BY-SA 4.0
Yumura Onsen
湯村温泉
Hyogo · 100 Onsen to Know Japan — No. 64
Water at 98°C gushes from the rocks by the river at 470 litres a minute. People boil eggs in it, then soak their feet in the same water once it has cooled.
Aspere · CC0
Kusatsu Onsen
草津温泉
Gunma · 100 Onsen to Know Japan — No. 25
Thirty-two thousand litres a minute of pH 2.1 acid water rise in the middle of town and cool as they run down wooden channels.
Heating: warming a cool source
A source of roughly 25–38 °C cannot be used in winter as it is, so it is heated. Heating in a boiler or heat exchanger does not change the minerals, but in a carbonated spring heat drives the gas out.
In cool-spring towns it is traditional to set a tub of unheated "lukewarm" water beside a heated "hot" one and alternate between them (Tochiomata, Shimobe, Masutomi).
浅野ます道 · CC BY 3.0
Shimobe Spa
下部温泉
Yamanashi · 100 Onsen to Know Japan — No. 45
Water at 30 °C rises from the rock beneath your feet in 'Shingen's hidden spring'. An hour in the cool bath, then a warm-up in the new hot source: a toji village with two kinds of water.
Masutomi Onsen
増富温泉
Yamanashi · 100 Onsen to Know Japan — No. 44
Thirty quiet minutes in water around 30 °C. A therapeutic village where one of Japan's most radon-rich springs is bathed in unheated, exactly as it comes.
Recirculation: reusing the water
Bath water is drawn out, passed through a filter that removes hair and oils, reheated and returned. It lets a large bathhouse run on limited water and is standard in city hotels and big facilities. After Legionella outbreaks in recirculating baths in the 1990s, disinfection standards were set.
"No recirculation" means free-flowing; "recirculated" means reused; "partly recirculated" sits between. It differs bath by bath, so check the open-air and indoor baths separately.
Disinfection: chlorine and its smell
Recirculating baths are required by local ordinance to be disinfected, usually with hypochlorite. Some free-flowing baths are disinfected too, depending on local policy. Chlorine reacts with the hydrogen sulfide in sulfur springs, removing the cloudiness and changing the smell; in acidic springs chlorine is less effective, so other methods may be used.
Disinfection is for hygiene and nothing to reject. But if you want to enjoy the water's character, choosing "no disinfection" or free-flowing baths makes the difference obvious.
Reading the notice — and how this atlas records it
The notice required by the Hot Spring Act lists spring type, temperature, added water, heating, recirculation/filtration, additives and disinfection, with reasons. Most facilities repeat it on their websites.
This atlas records "free-flowing / added water / heating / recirculation / disinfection" per bath as "yes / no / unconfirmed". The search filters "no added water", "no heating" and "no recirculation" include only confirmed "no" entries; unconfirmed ones are excluded and their number is shown above the results.
Glossary
- Legionella
- Bacteria that multiply in warm water and can cause pneumonia if inhaled. Disinfection of recirculating baths exists to prevent it.
- Lukewarm bath (nuruyu)
- A bath of about 35–38 °C, suited to long therapeutic soaks.
Related onsen
Asturio Cantabrio · CC BY-SA 4.0
Yumura Onsen
湯村温泉
Hyogo · 100 Onsen to Know Japan — No. 64
Water at 98°C gushes from the rocks by the river at 470 litres a minute. People boil eggs in it, then soak their feet in the same water once it has cooled.
浅野ます道 · CC BY 3.0
Shimobe Spa
下部温泉
Yamanashi · 100 Onsen to Know Japan — No. 45
Water at 30 °C rises from the rock beneath your feet in 'Shingen's hidden spring'. An hour in the cool bath, then a warm-up in the new hot source: a toji village with two kinds of water.
Purplepumpkins · CC BY-SA 3.0
Takayu Onsen
高湯温泉
Fukushima · 100 Onsen to Know Japan — No. 22
Nine sources, 2,589 litres a minute, and not a drop of tap water or a single boiler: the milky water is cooled only by gravity on its way down the hill.