A sodium chloride spring — salty and heat-retaining. At the wellhead it reads pH 8.6, putting it on the alkaline side of Japan's beauty-water family with a soft feel on the skin. The water emerges gray and turbid, carries a slightly salty taste and a mineral-oil note, and evolves gas at the source — a distinctive well. Moderately mineralized at 2,483 mg/kg of dissolved solids.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺832mgP82 national85%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺87.1mgP68 national10%
- Aluminumshows up in acidic volcanic waterAl³⁺7.5mgP86 national2%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺15.5mgP92 national1%
- MagnesiumMg²⁺2.9mgP38 national0.6%
- PotassiumK⁺8.2mgP41 national0.5%
- AmmoniumNH₄⁺1.0mgP66 national0.1%
- ManganeseMn²⁺0.40mgP59 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,408mgP83 national96%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻59.2mgP31 national2%
- FluorideF⁻5.7mgP80 national0.7%
- CarbonateCO₃²⁻4.8mgP34 national0.4%
- SulfateSO₄²⁻4.6mgP15 national0.2%
- BromideBr⁻4.3mgP79 national0.1%
- IodideI⁻1.1mgP80 national0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃37.2mgP19 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.2mgP25 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.