The Kuchisakamoto No. 2 source is a municipally operated well in a mountain hamlet of Shizuoka City. A sodium bicarbonate spring of the classic jūsō-sen style, it emerges with a visible evolution of gas and a faint hydrogen-sulfide note. At pH 9.0 the water is alkaline, sitting in Japan's beauty-water family with a noticeably slippery feel on the skin. Moderate mineralization (1,188 mg/kg) plus metasilicic and metaboric acids round out a soft, smooth bath.
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⁺315mgP63 national97%
- Potassiumminor; matters for drinking limitsK⁺10.7mgP47 national2%
- MagnesiumMg²⁺0.10mgP11 national0.1%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺—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 smoothHCO₃⁻713mgP84 national83%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻69.1mgP32 national14%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻13.7mgP58 national3%
- SulfateSO₄²⁻—mg0.2%
- HydroxideOH⁻0.20mgP66 national0.1%
- Hydrogen sulfideHS⁻0.20mgP40 national0.0%
- IodideI⁻0.20mgP52 national0.0%
- Bromidetrace, sea-originBr⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂32.9mgP81 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃29.6mgP12 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —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.