A blended sodium chloride-bicarbonate spring drawing on the Miyuki main well and two further wells. At 11.1 g/kg of dissolved solids it is richly mineralized and hypertonic, pairing the heat-retaining feel of chloride water with a bicarbonate softness layered in. Weakly alkaline and gentle on the skin, it carries 89.4 mg/kg of metasilicic acid that leaves a thin film for noticeably smoother skin, along with a faint hydrogen sulfide note and a salty tang.
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
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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⁺3,178mgP94 national83%
- Magnesiummild; a laxative if you drink itMg²⁺199mgP96 national10%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺155mgP81 national5%
- Potassiumminor; matters for drinking limitsK⁺140mgP94 national2%
- ManganeseMn²⁺0.70mgP72 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4,515mgP92 national73%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻2,313mgP97 national22%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻467mgP86 national6%
- FluorideF⁻4.3mgP74 national0.1%
- ThiosulfateS₂O₃²⁻2.2mgP87 national0.0%
- Hydrogen sulfideHS⁻0.20mgP40 national0.0%
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- HSiO3HSiO₃⁻< —mg
- HydroxideOH⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃89.4mgP51 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂51.2mgP89 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂12.7mgP44 national
- 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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.