Tobiya-bara Spring (Morden no Yu) is a lightly mineralized simple thermal water (about 0.9 g/kg dissolved solids), gentle on the body. At pH 8.5 it is alkaline with the soft, slightly slippery feel of Japan's beauty-water family. The wellhead runs hot at 55.7 °C; for bathing it is reheated to 40-42 °C, recirculated, and disinfected.
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
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⁺251mgP57 national99%
- CalciumCa²⁺1.5mgP5 national0.7%
- PotassiumK⁺0.52mgP6 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻538mgP78 national82%
- Fluoridetrace; drinking limits applyF⁻10.1mgP92 national5%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻11.5mgP14 national3%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻23.5mgP72 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.20mgP40 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃35.9mgP18 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.27mgP7 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.8mgP24 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
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.
- 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.