湯SimpleSodium Bicarbonate spring
A simple thermal spring with sodium and bicarbonate as the leading ions. Alkaline at pH 8.2, with the soft, slightly slippery feel that classifies it as a gentle thermal water. Lightly mineralized at 752 mg/kg total solutes — easy on the body, without sharp character.
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⁺173mgP45 national91%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺5.5mgP18 national3%
- Potassiumminor; matters for drinking limitsK⁺7.1mgP39 national2%
- AmmoniumNH₄⁺2.0mgP81 national1%
- Magnesiummild; a laxative if you drink itMg²⁺1.3mgP28 national1%
- LithiumLi⁺0.30mgP44 national0.5%
- Iron (III)Fe³⁺0.30mgP72 national0.2%
- StrontiumSr²⁺0.40mgP62 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- BariumBa²⁺NDmg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺NDmg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺NDmg
- Manganesetrace; companion to iron springsMn²⁺NDmg
- ZincZn²⁺NDmg
Anionsshare of charge (mval%)
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻508mgP76 national97%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻8.9mgP12 national3%
- FluorideF⁻0.60mgP38 national0.3%
- Hydrogen phosphateHPO₄²⁻0.10mgP34 national
- AsO2AsO₂⁻NDmg
- Bromidetrace, sea-originBr⁻NDmg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- HSO4HSO4NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻NDmg
Non-dissociatedmeasured by mass
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃39.5mgP21 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂5.2mgP38 national
- H2SO4FreeH2SO4FreeNDmg
- Phosphoric acidH₃PO₄NDmg
- Metaarsenious acidHAsO₂NDmg
Dissolved gasesmeasured by mass
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂28.5mgP59 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
Trace metalsmeasured by mass
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
pH
8.20
Alkaline
Source temperature
41.0°C
Warm
Tonicity
0.75g/kg
Hypotonic
Free CO₂
29mg/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.