Ryusenji Onsen is an alkaline simple thermal spring at pH 9.25 — alkaline enough to give the slippery, soap-like feel that places it squarely in Japan's beauty-water family. Dissolved solids come to only 333.6 mg/kg, so the water is lightly mineralized and gentle on the body, led by sodium and chloride with small amounts of metasilicic acid and fluoride. The posted analysis records the source itself at 33.2 °C and the water at the public bathing point at 40 °C.
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
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⁺101mgP28 national92%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺7.3mgP22 national8%
- PotassiumK⁺0.60mgP7 national0.3%
- AmmoniumNH₄⁺0.20mgP29 national0.2%
- Iron (II)Fe²⁺0.10mgP29 national0.1%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻147mgP50 national76%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻18.0mgP62 national11%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻26.4mgP16 national8%
- Fluoridetrace; drinking limits applyF⁻5.1mgP78 national5%
- IodideI⁻0.80mgP77 national0.1%
- SulfateSO₄²⁻0.20mgP3 national0.1%
- BromideBr⁻0.30mgP37 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃23.1mgP8 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.6mgP26 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂NDmg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs< —mg
- CadmiumCd< —mg
- ChromiumCr< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
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.
- 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.