A naturally flowing source feeding the Yamaguchi Ryokan motoyu at Tarutama Onsen in the southern Aso highlands. Classed as a simple thermal spring, weakly acidic and hypotonic with dissolved solids of 717 mg/kg, it nonetheless emerges hot at 45.3 °C. A standout 303 mg/kg of metasilicic acid puts it squarely in Japan's beauty-water family, leaving a fine silica film that makes skin feel noticeably smoother. The water comes up faintly yellow-brown and clear, carrying the distinct rotten-egg note of H2S and a light metallic edge — a simple spring with real character.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺39.2mgP52 national35%
- Magnesiummild; a laxative if you drink itMg²⁺20.5mgP70 national30%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺26.8mgP6 national21%
- Potassiumminor; matters for drinking limitsK⁺12.0mgP48 national6%
- AmmoniumNH₄⁺3.7mgP88 national4%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺3.9mgP78 national3%
- AluminumAl³⁺0.40mgP74 national0.8%
- ManganeseMn²⁺1.1mgP78 national0.7%
- StrontiumSr²⁺0.40mgP62 national0.1%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻187mgP56 national55%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻111mgP56 national41%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻6.6mgP9 national3%
- FluorideF⁻0.40mgP31 national0.4%
- Hydrogen sulfideHS⁻0.20mgP40 national0.1%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃303mgP97 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.7mgP16 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂108mgP80 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S1.5mgP80 national
- Radonfaint radon — studied for joint pain (hormesis)Rn7.6mgP45 national
- ArsenicAs< —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.
- 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.