An alkaline simple thermal spring (pH 9.03) led by bicarbonate. The mineralization is light at 346 mg/kg, but the alkalinity gives the slippery, soft-on-skin feel of Japan's beauty-water family. A fair 78.9 mg/kg of metasilicic acid leaves a thin silica film that makes skin feel smoother afterwards.
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⁺62.9mgP19 national89%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3.6mgP14 national6%
- Potassiumminor; matters for drinking limitsK⁺4.8mgP33 national4%
- MagnesiumMg²⁺0.40mgP18 national1.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻163mgP52 national75%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻16.2mgP61 national15%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻9.9mgP21 national6%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻5.3mgP8 national4%
- HydroxideOH⁻0.10mgP50 national0.3%
- Bromidetrace, sea-originBr⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃78.9mgP47 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.60mgP10 national
- Metaarsenious acidHAsO₂< —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.