Yamanakako Hirano Onsen's Ishiwari-no-Yu is a simple thermal spring, but a strongly alkaline one — pH 10.6 at the wellhead gives it a pronounced slippery feel on the skin and places it squarely in Japan's beauty-water family. Dissolved solids are light at 157 mg/kg, so the water is gentle rather than assertive, though at the source it carries a faint yellow-brown tint with slight turbidity and a light sulfurous note. The attached indications panel lists autonomic nervous system instability, insomnia and depressed mood among the spring-type indications, framing this as water for winding down. The source emerges at only 26.8 °C, so the bath is reheated; the public posting adds that the water is also diluted, recirculated through a filter and chlorine-disinfected before it reaches the tub.
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
Take care
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor, or marked physical debilitation such as severe anemia
- Severe heart or lung disease causing breathlessness on slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding
- Any visible bleeding
- Acute exacerbation of a chronic illness
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⁺42.1mgP10 national91%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.3mgP8 national5%
- Aluminumshows up in acidic volcanic waterAl³⁺0.40mgP73 national2%
- PotassiumK⁺0.40mgP4 national0.5%
- Iron (II)Fe²⁺0.30mgP43 national0.5%
- ZincZn²⁺0.20mgP84 national0.5%
- Iron (III)Fe³⁺0.10mgP57 national0.5%
- BariumBa²⁺0.0000mg0.0%
- HydrogenH⁺0.0000mg0.0%
- LithiumLi⁺0.0000mg0.0%
- MagnesiumMg²⁺0.0000mg0.0%
- ManganeseMn²⁺0.0000mg0.0%
- AmmoniumNH₄⁺0.0000mg0.0%
- StrontiumSr²⁺0.0000mg0.0%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻28.6mgP83 national36%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻31.1mgP34 national25%
- HSiO3HSiO₃⁻42.5mgP87 national21%
- HydroxideOH⁻6.7mgP99 national15%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1.3mgP2 national2%
- Fluoridetrace; drinking limits applyF⁻0.80mgP42 national2%
- Hydrogen sulfideHS⁻0.30mgP45 national0.4%
- BO2BO₂⁻0.20mgP40 national0.0%
- AsO2AsO₂⁻0.0000mg0.0%
- BromideBr⁻0.0000mg0.0%
- BicarbonateHCO₃⁻0.0000mg0.0%
- Hydrogen phosphateHPO₄²⁻0.0000mg0.0%
- HSO4HSO40.0000mg0.0%
- IodideI⁻0.0000mg0.0%
- SS²⁻0.0000mg0.0%
- ThiosulfateS₂O₃²⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃0.0000mg
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.0000mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Muscle stiffness associated with motor paralysis
- Cold sensitivity
- Peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Stress-related symptoms (sleep disorders, depressed mood)
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.