下呂温泉(幸田ポンプ所)Gero Onsen (Koda Pump Station)
A simple alkaline thermal spring drawn from the Koda Pump Station of Gero Onsen. At pH 9.4 it sits firmly in the alkaline range that gives the slippery, skin-smoothing feel of Japan's beauty-water family. The water is lightly mineralized and gentle on the body, yet carries a generous 66.7 mg/kg of metasilicic acid that leaves a thin silica film for noticeably smoother skin.
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⁺96.5mgP28 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺2.5mgP10 national
- Potassiumminor; matters for drinking limitsK⁺1.0mgP14 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.09mgP37 national
- Magnesiummild; a laxative if you drink itMg²⁺< —mg
- Fluoridetrace; drinking limits applyF⁻10.0mgP90 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻8.1mgP11 national
- HydroxideOH⁻0.30mgP74 national
- HSiO3HSiO₃⁻—mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃66.7mgP42 national
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.