A private source serving Kurokawa Onsen Ryokan Wakaba. Hot at 70.1 °C (one of Japan's hotter wellheads) and weakly acidic at pH 3.7, yet the dissolved-solids load is light enough at 0.67 g/kg that the cert classes it as a simple thermal spring. Sulfate dominates the anion side at roughly 64 mval percent and a faint hydrogen sulfide note rides on the surface. A generous 181.9 mg/kg of metasilicic acid leaves the kind of silica film that makes skin feel noticeably smoother afterwards.
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⁺117mgP34 national69%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺18.6mgP38 national13%
- Potassiumminor; matters for drinking limitsK⁺23.0mgP67 national8%
- Magnesiummild; a laxative if you drink itMg²⁺4.1mgP44 national5%
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.20mgP59 national3%
- LithiumLi⁺0.60mgP61 national1%
- AluminumAl³⁺0.60mgP75 national0.9%
- AmmoniumNH₄⁺0.60mgP53 national0.5%
- Iron (II)Fe²⁺0.60mgP56 national0.3%
- ManganeseMn²⁺0.70mgP72 national0.3%
- BariumBa²⁺NDmg
- CopperCu²⁺NDmg
- StrontiumSr²⁺NDmg
- ZincZn²⁺NDmg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻228mgP68 national64%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻91.4mgP38 national35%
- FluorideF⁻0.80mgP44 national0.6%
- BromideBr⁻0.40mgP39 national0.1%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitrateNO₃⁻NDmg
- HydroxideOH⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃182mgP81 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.0mgP42 national
- Metaarsenious acidHAsO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP61 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- ChromiumCrNDmg
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.