A hot 54.9 °C simple thermal spring of the Kurokawa area. Weakly alkaline and soft on the skin, it sits on the gentle edge of Japan's beauty-water family. A generous 177 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards, with a faint rotten-egg note of H₂S drifting at the wellhead.
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⁺130mgP32 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺43.0mgP54 national20%
- Magnesiummild; a laxative if you drink itMg²⁺22.3mgP72 national17%
- Potassiumminor; matters for drinking limitsK⁺39.1mgP80 national9%
- LithiumLi⁺0.30mgP41 national0.4%
- ManganeseMn²⁺0.60mgP73 national0.2%
- StrontiumSr²⁺0.50mgP64 national0.1%
- Iron (II)Fe²⁺0.10mgP32 national0.0%
- BariumBa²⁺0.20mgP70 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- AmmoniumNH₄⁺< —mg
- ZincZn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻237mgP57 national36%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻124mgP44 national33%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻155mgP62 national30%
- FluorideF⁻0.40mgP32 national0.2%
- NitrateNO₃⁻0.40mgP59 national0.1%
- BromideBr⁻0.50mgP45 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃177mgP81 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂4.6mgP34 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂14.9mgP45 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.02mgP40 national
- ChromiumCr< —mg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.