A sodium-led chloride-bicarbonate spring served from a 39.5 °C self-flowing well. Weakly alkaline (pH 8.0 at the source, 8.17 in the lab) and softly mineralized at about 1,020 mg/kg, it sits squarely in the Japanese 'beauty-water' family — chloride lays down a heat-retaining film while the bicarbonate sub-type adds a smoother, almost soda-like finish. The cert notes a pale yellow tint with fine suspended matter at the wellhead.
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⁺244mgP57 national80%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺32.7mgP48 national12%
- Magnesiummild; a laxative if you drink itMg²⁺7.7mgP58 national5%
- Potassiumminor; matters for drinking limitsK⁺10.3mgP46 national2%
- AmmoniumNH₄⁺2.7mgP85 national1%
- BariumBa²⁺0.10mgP53 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.10mgP30 national
- StrontiumSr²⁺0.10mgP26 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻279mgP61 national58%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻339mgP67 national41%
- Hydrogen phosphateHPO₄²⁻3.6mgP93 national0.5%
- IodideI⁻1.7mgP86 national0.1%
- BromideBr⁻0.90mgP57 national0.1%
- FluorideF⁻0.0000mg0.0%
- AsO2AsO₂⁻0.20mgP87 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃96.5mgP54 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂13.2mgP46 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.