Why the analysis exists
Facilities using onsen water are required by the Hot Spring Act to post a "water analysis" prepared by a registered laboratory. Since the 2007 amendment, the analysis must be renewed every ten years, because a source's temperature and chemistry drift over time.
An analysis covers one source. A facility with several sources has several analyses; which tub uses which source is stated separately or explained by the facility.
Look 1: temperature and flow
Near the top: "Temperature 51.3 °C (air 12 °C)", "Flow 120 L/min (pumped)". Temperature is the source, not the tub. Flow is litres per minute, noted as natural or pumped. The greater the flow, the easier a free-flowing bath.
Look 2: pH and the senses
"pH 2.1" is acidic; "pH 9.2" alkaline. pH 7 is neutral; acidic springs are below 3, alkaline simple springs 8.5 or above.
The "sensory test" records what the analyst saw and smelled: "colourless and clear, hydrogen-sulfide odour, slightly bitter". This is the official record of colour and smell; where possible, the atlas's "water colour" rests on this line or on official descriptions.
Look 3: the component table and dissolved solids
The central table lists cations (sodium, calcium…) and anions (chloride, bicarbonate, sulfate…) in mg/kg, mval and mval%. Ions at 20 mval% or more enter the spring-type name: "sodium-calcium chloride-sulfate spring" means sodium and calcium, chloride and sulfate are each at 20% or above.
Below the table, "dissolved solids (excluding gases)" of 1,000 mg/kg (1 g/kg) or more makes a saline spring; less, a simple hot spring. "Total components" adds the gases.
Look 4: special components
Free carbon dioxide, total iron, total sulfur, iodide, radon and so on. Above their thresholds they add "…-bearing" or a class to the name. Radon appears in one of three units — "× 10⁻¹⁰ Ci/kg", "Mache" or "Bq/kg" — so check which (8.25 Mache ≈ 111 Bq/kg is the threshold).
Look 5: name, indications, date
Finally the official spring-type name, "indications for bathing", "contraindications", and the date and laboratory. Indications are an official statement under Ministry of the Environment guidelines, not a guarantee of treatment. Always read the contraindications (acute illness, early pregnancy and so on).
If the analysis is more than ten years old it may not have been renewed. The atlas shows the analysis date where known.
The analysis and the "notice" are separate
Beside the analysis is a second sheet, the "notice under the Hot Spring Act", stating whether water is added, heated, recirculated, treated with additives or disinfected, and why. The analysis is about the source; the notice about the tub. Read both and you know what happens between the two.
Glossary
- mval%
- Share of an ion by electrical equivalent; the 20% rule decides whether it enters the spring-type name.
- Registered laboratory
- A laboratory registered with the prefecture to analyse onsen water; named at the foot of the analysis.
Related onsen
三塚 · CC BY-SA 4.0
Misasa Onsen
三朝温泉
Tottori · 100 Onsen to Know Japan — No. 74
Soak it, drink it, breathe it. A spa built around taking radon into the body three ways, with a free riverside bath and free drinking fountains open to the whole town.
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.
663highland · CC BY-SA 4.0
Arima Onsen
有馬温泉
Hyogo · 100 Onsen to Know Japan — No. 62
Salt and iron thicker than seawater in the 'gold' spring, carbon dioxide and radon in the 'silver': two utterly different waters rise five minutes' walk apart.