An alkaline simple thermal spring blended at Gero Onsen's distribution pumping station, of the classic Japanese beauty-water family. At pH 9.1 the water is alkaline with a noticeably slippery, skin-smoothing feel, while its light mineralization (374 mg/kg dissolved solids) keeps it gentle on the body. The bath is diluted with water, reheated, recirculated, and disinfected with sodium hypochlorite before reaching the large bathing hall.
Good for
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
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Take care
- Acute phase of illness (especially when feverish)
- Active tuberculosis, advanced malignant tumors, or severe anemia and other states of marked physical debility
- Severe heart or lung dysfunction causing breathlessness on slight exertion
- Kidney failure with edema
- Gastrointestinal bleeding or any visible bleeding
- Acute exacerbation of a chronic disease
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⁺110mgP32 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.3mgP16 national4%
- PotassiumK⁺1.9mgP22 national1.0%
- Aluminumshows up in acidic volcanic waterAl³⁺0.09mgP37 national
- Magnesiummild; a laxative if you drink itMg²⁺0.0010mgP4 national
- FeTotalFe< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻105mgP42 national56%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻26.0mgP79 national17%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻37.5mgP22 national12%
- Fluoridetrace; drinking limits applyF⁻10.1mgP92 national10%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻12.7mgP25 national5%
- Hydrogen sulfideHS⁻0.70mgP63 national0.4%
- NitrateNO₃⁻0.0000mg0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃59.7mgP36 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.1mgP43 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂4.0mgP27 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
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 in their chronic phase)
- Weak gastrointestinal function (heaviness in the stomach, gas in the intestines)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Autonomic nervous instability and stress-related symptoms (sleep disturbance, depressive state)
- Recovery from fatigue and health promotion
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.
- 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.