Tateyama Sanroku Onsen is a sodium bicarbonate spring — the classic jūsō-sen style — and at pH 8.82 it is firmly alkaline, giving the slippery, soap-like feel that places it in Japan's beauty-water family. Dissolved solids of 1,248 mg/kg make it moderately mineralized, with sodium and bicarbonate doing almost all of the work and a notably generous 29.9 mg/kg of fluoride alongside. The posted summary lists the water for cuts, peripheral circulation disorders, dry skin and cold sensitivity, on top of the standard chronic-pain, gastrointestinal and fatigue indications. It reaches the bath reheated and through recirculating filtration with disinfection, so expect a managed bath rather than free-flow.
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
- Marked physical debilitation, such as advanced malignant tumor or severe anemia
- Severe heart or lung disease causing breathlessness on slight exertion
- Renal failure 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⁺364mgP67 national
- Potassiumminor; matters for drinking limitsK⁺1.6mgP18 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺0.88mgP2 national
- AmmoniumNH₄⁺0.53mgP49 national
- LithiumLi⁺0.47mgP50 national
- Magnesiummild; a laxative if you drink itMg²⁺0.13mgP11 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻694mgP83 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻45.4mgP26 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻30.0mgP82 national
- Fluoridetrace; drinking limits applyF⁻29.9mgP100 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻20.8mgP32 national
- BO2BO₂⁻5.5mgP84 national
- HSiO3HSiO₃⁻4.8mgP54 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃36.4mgP18 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂14.1mgP65 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.4mgP23 national
- ArsenicAs0.02mgP33 national
- 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
- 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)
- Cuts
- Peripheral circulation disorders
- Cold sensitivity
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.