A simple thermal spring pumped at Shimizu in Ibaraki City, neutral at pH 7.2 and lightly mineralized at 793.6 mg/kg of dissolved solids, so it bathes gently with little bite. It carries a fair amount of metasilicic acid (69.9 mg/kg), the kind of silica that leaves a thin film for noticeably smoother skin, and runs colorless, transparent and odorless.
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⁺174mgP45 national63%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺40.9mgP54 national17%
- Magnesiummild; a laxative if you drink itMg²⁺22.7mgP72 national16%
- Potassiumminor; matters for drinking limitsK⁺—mg2%
- StrontiumSr²⁺4.8mgP89 national0.9%
- ManganeseMn²⁺0.10mgP36 national0.3%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- BariumBa²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻340mgP67 national80%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻116mgP46 national16%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻18.5mgP30 national3%
- NitrateNO₃⁻2.7mgP93 national0.3%
- BromideBr⁻0.70mgP51 national0.1%
- Fluoridetrace; drinking limits applyF⁻< —mg
- Iodideyellows in air; helps blood lipidsI⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃69.9mgP44 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.9mgP17 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂5.5mgP30 national
- ArsenicAs< —mg
- CadmiumCd< —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.