Grand Onsen is an alkaline simple thermal spring (pH 8.9) — sodium-led and light in mineral content, sitting squarely in Japan's beauty-water family thanks to its alkalinity and silica. Total dissolved solids are modest at about 415 mg/kg, yet a generous 60.8 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel noticeably smoother afterwards. The poster lists it for cold sensitivity, peripheral circulation disorders and fatigue recovery among the standard bathing indications. Because the source emerges at only 34.3 °C, the large bath is reheated, recirculated through a filtration unit, and chlorine-disinfected to meet the city's public-bath hygiene standards.
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, severe anemia, or other marked physical debilitation
- Severe heart or kidney disease causing breathlessness on slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding
- 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⁺120mgP34 national97%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.8mgP10 national3%
- PotassiumK⁺1.0mgP13 national0.6%
- LithiumLi⁺0.10mgP28 national0.2%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- AmmoniumNH₄⁺< —mg
- StrontiumSr²⁺< —mg
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻79.3mgP34 national42%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻109mgP45 national34%
- Fluoridetrace; drinking limits applyF⁻19.2mgP98 national19%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻6.3mgP38 national4%
- SulfateSO₄²⁻1.5mgP9 national0.6%
- Hydrogen sulfideHS⁻0.50mgP55 national0.4%
- ThiosulfateS₂O₃²⁻0.60mgP64 national0.2%
- HydroxideOH⁻0.10mgP49 national0.2%
- BromideBr⁻0.20mgP29 national0.0%
- Hydrogen phosphateHPO₄²⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- NitrateNO₃⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃60.8mgP37 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂12.6mgP61 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂0.20mgP13 national
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —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.