A sodium-dominant bicarbonate spring in the classic soda-water (juso-sen) style — sodium accounts for close to 88 percent of the cations and bicarbonate for 93 percent of the anions. It runs neutral and soft on the skin, which places it squarely in Japan's beauty-water family, and at 2,457 mg/kg of dissolved solids it carries enough mineral body to feel substantial rather than thin. Straight from the wellhead the water is colorless and clear but carries a faint metallic tang and a trace of sulfur on the nose; by the time the sample reached the lab a day later, the smell had gone.
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
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
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⁺572mgP79 national88%
- Potassiumminor; matters for drinking limitsK⁺95.4mgP92 national9%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺11.5mgP27 national2%
- Magnesiummild; a laxative if you drink itMg²⁺3.5mgP42 national1%
- LithiumLi⁺0.60mgP59 national0.3%
- StrontiumSr²⁺0.50mgP68 national0.0%
- BariumBa²⁺0.40mgP80 national0.0%
- Iron (II)Fe²⁺0.20mgP38 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- CopperCu²⁺NDmg
- Iron (III)Fe³⁺NDmg
- Manganesetrace; companion to iron springsMn²⁺NDmg
- AmmoniumNH₄⁺NDmg
- ZincZn²⁺NDmg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻1,643mgP94 national93%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻66.8mgP30 national7%
- FluorideF⁻1.0mgP46 national0.2%
- SulfateSO₄²⁻1.3mgP8 national0.1%
- Bromidetrace, sea-originBr⁻NDmg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻NDmg
- Hydrogen phosphateHPO₄²⁻NDmg
- Hydrogen sulfidesulfur character — antibacterialHS⁻NDmg
- HSO4HSO4NDmg
- Iodideyellows in air; helps blood lipidsI⁻NDmg
- NitrateNO₃⁻NDmg
- Thiosulfateoxidized sulfurS₂O₃²⁻NDmg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃49.4mgP28 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.9mgP55 national
- Metaarsenious acidHAsO₂0.80mgP87 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂144mgP84 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- ArsenicAs0.58mgP91 national
- CadmiumCd< —mg
- ChromiumCr< —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.
- 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.