A gentle simple thermal spring: weakly alkaline, soft on the skin, and only lightly mineralized, so it sits easy on the body rather than announcing itself. Sodium leads the cations, while chloride, sulfate and bicarbonate share the anion side almost evenly and none of them in quantity. What stands out instead is metasilicic acid at 180 mg/kg — the silica that leaves a thin moisturizing film and puts this water squarely in Japan's beauty-water family. The sample was drawn at the storage tank rather than at the wellhead, and a companion certificate for the same source takes its sample at an on-site drinking fountain.
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⁺120mgP34 national75%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺14.5mgP31 national10%
- Potassiumminor; matters for drinking limitsK⁺17.2mgP57 national6%
- Magnesiummild; a laxative if you drink itMg²⁺4.7mgP49 national6%
- LithiumLi⁺1.1mgP76 national2%
- AluminumAl³⁺0.10mgP54 national0.1%
- StrontiumSr²⁺0.10mgP26 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻114mgP43 national44%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻95.6mgP54 national27%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻115mgP46 national26%
- NitrateNO₃⁻9.1mgP98 national2%
- FluorideF⁻0.20mgP17 national0.1%
- BromideBr⁻0.30mgP36 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃180mgP79 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.1mgP31 national
- Metaarsenious acidHAsO₂0.30mgP76 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂15.0mgP48 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.20mgP80 national
- CadmiumCd< —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.