共同泉Kyodosen (communal spring)
An alkaline simple thermal spring drawn from the communal Kyodosen source at Shimobe Onsen. With a fairly high pH it sits in the alkaline range, giving the slippery, soft-on-the-skin feel of Japan's beauty-water family. Colorless, tasteless and odorless, it is lightly mineralized (about 372 mg/kg of dissolved solids) and gentle on the body, with a quiet edge from its modest sulfate and chloride content.
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⁺64.2mgP19 national53%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺48.5mgP57 national46%
- MagnesiumMg²⁺0.50mgP20 national0.8%
- PotassiumK⁺0.60mgP7 national0.4%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻133mgP59 national55%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻60.9mgP31 national34%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻22.3mgP14 national7%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻5.4mgP36 national4%
- FluorideF⁻0.30mgP24 national0.4%
- HydroxideOH⁻0.10mgP50 national0.2%
- Bromidetrace, sea-originBr⁻0.20mgP30 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂35.3mgP82 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃32.9mgP15 national
- CopperCu0.0090mgP58 national
- LeadPb0.0030mgP27 national
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —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.