湯SimpleSodium Bicarbonate spring
A gentle simple thermal spring, weakly alkaline at pH 7.9 at the wellhead (8.2 in the lab), soft on the skin and squarely in Japan's beauty-water family. It is lightly mineralized at 303 mg/kg of dissolved solids, with just a faint note of hydrogen sulfide perceptible at the source.
Good for
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
Cationsshare of charge (mval%)
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺62.0mgP18 national93%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.3mgP9 national4%
- Potassiumminor; matters for drinking limitsK⁺1.5mgP18 national1%
- Magnesiummild; a laxative if you drink itMg²⁺0.50mgP20 national1%
- AmmoniumNH₄⁺0.30mgP40 national0.6%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- LithiumLi⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- StrontiumSr²⁺< —mg
- ZincZn²⁺< —mg
Anionsshare of charge (mval%)
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻168mgP54 national93%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻0.40mgP4 national8%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5.4mgP8 national5%
- Hydrogen phosphateHPO₄²⁻0.10mgP34 national0.9%
- FluorideF⁻0.10mgP9 national0.2%
- Bromidetrace, sea-originBr⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
Non-dissociatedmeasured by mass
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃60.7mgP37 national
- Metaarsenious acidHAsO₂< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂< —mg
Dissolved gasesmeasured by mass
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂3.6mgP25 national
Trace metalsmeasured by mass
- ArsenicAs0.0040mgP11 national
- CadmiumCd< —mg
- ChromiumCr< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
pH
7.90
Weakly alkaline
Source temperature
41.6°C
Warm
Tonicity
0.30g/kg
Hypotonic
Flow rate
550L/min
High
Free CO₂
3.60mg/kg
Low
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.