A well drilled beneath Togoshi in Shinagawa, Tokyo. The certificate assigns no therapeutic spring-quality name: the water qualifies as an onsen under Article 2 of the Hot Spring Law specifically on its metasilicic acid content. It is lightly mineralized at 352 mg/kg, neutral at pH 7.3, colorless, clear and all but odorless, with bicarbonate and calcium leading the ion table. Metasilicic acid at 76.9 mg/kg puts it in Japan's beauty-water family, enough silica to leave skin feeling noticeably smoother.
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.
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺38.1mgP51 national49%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺19.7mgP4 national22%
- Magnesiummild; a laxative if you drink itMg²⁺10.3mgP62 national22%
- Potassiumminor; matters for drinking limitsK⁺6.2mgP37 national4%
- Aluminumshows up in acidic volcanic waterAl³⁺0.40mgP74 national1%
- AmmoniumNH₄⁺0.40mgP43 national0.5%
- ManganeseMn²⁺0.30mgP54 national0.3%
- FeTotalFe0.07mgP37 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻120mgP47 national52%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻31.6mgP21 national24%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻39.1mgP38 national21%
- NitrateNO₃⁻5.2mgP97 national2%
- NitriteNO₂⁻0.40mgP71 national0.3%
- FluorideF⁻0.10mgP8 national0.3%
- Bromidetrace, sea-originBr⁻NDmg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃76.9mgP46 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂3.0mgP24 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂2.2mgP22 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.