A sodium chloride spring, pumped from a drilled well at Okita in Shirakawa village. At the wellhead the water carries a faint yellow-brown tint while staying clear, with a slight bitterness and a whisper of hydrogen sulfide behind it. Weakly alkaline and soft on the skin, it holds 3,132 mg/kg of dissolved solids — moderately mineralized, with noticeable body. Chloride water of this kind is valued for the way it holds heat, so the warmth tends to linger well after the bath.
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, netto 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⁺1,114mgP87 national94%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺44.3mgP55 national4%
- Magnesiummild; a laxative if you drink itMg²⁺10.9mgP63 national2%
- PotassiumK⁺7.0mgP39 national0.3%
- Iron (II)Fe²⁺1.9mgP72 national0.1%
- Manganesetrace; companion to iron springsMn²⁺0.04mgP19 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.03mgP25 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,488mgP85 national85%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻408mgP72 national14%
- FluorideF⁻8.6mgP87 national0.9%
- SulfateSO₄²⁻9.3mgP20 national0.4%
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃25.5mgP10 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂15.3mgP68 national
- Metaarsenious acidHAsO₂0.20mgP68 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂17.6mgP48 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.