Kameshimagawa Onsen is a sodium chloride-bicarbonate cold mineral spring drawn from beneath Shinkawa in central Tokyo. At 18.2 °C it is a cold-mineral source (below 25 °C) that is heated for bathing, and it arrives brown but clear, with a salty tang on the lips. Bicarbonate softness layers over the heat-retaining chloride base, and at pH 8.5 the water is alkaline with the slippery feel that places it in Japan's beauty-water family, helped along by 63.3 mg/kg of metasilicic acid. Moderately mineralized at 3,804 mg/kg of dissolved solids, it has noticeable body; the certificate's sample was taken from the source tank rather than at the wellhead.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, netto 熱の湯).
cold sensitivity
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
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
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⁺1,048mgP87 national78%
- Magnesiummild; a laxative if you drink itMg²⁺99.1mgP92 national14%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺68.8mgP64 national6%
- Potassiumminor; matters for drinking limitsK⁺49.4mgP85 national2%
- AmmoniumNH₄⁺6.4mgP93 national0.6%
- StrontiumSr²⁺0.80mgP77 national0.0%
- ManganeseMn²⁺0.40mgP61 national0.0%
- Iron (II)Fe²⁺0.20mgP38 national0.0%
- BariumBa²⁺0.07mgP33 national0.0%
- LithiumLi⁺0.01mgP11 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,595mgP86 national75%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻789mgP86 national22%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻53.5mgP42 national2%
- CarbonateCO₃²⁻15.6mgP57 national0.9%
- BromideBr⁻5.5mgP83 national0.1%
- NitrateNO₃⁻3.6mgP96 national0.1%
- Hydrogen phosphateHPO₄²⁻3.0mgP91 national0.1%
- NitriteNO₂⁻0.50mgP86 national0.0%
- FluorideF⁻0.10mgP8 national0.0%
- IodideI⁻0.10mgP34 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃63.3mgP38 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.9mgP17 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂NDmg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- 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.