A sodium-led bicarbonate spring — the classic juso-sen style, neutral in pH and soft on the skin, with moderate mineralization of about 1,169 mg/kg dissolved solids. It sits squarely in Japan's beauty-water family: a generous 159.9 mg/kg of metasilicic acid, backed by 113.4 mg/kg of metaboric acid, leaves the kind of silica film that makes skin feel noticeably smoother afterwards. The certificate takes its sample at the open-air bath inlet rather than at the wellhead, so the figures describe the water as it actually reaches the pool.
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⁺208mgP54 national72%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺32.7mgP48 national13%
- Magnesiummild; a laxative if you drink itMg²⁺19.6mgP70 national13%
- Potassiumminor; matters for drinking limitsK⁺10.8mgP47 national2%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻573mgP80 national87%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻35.2mgP23 national9%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻14.8mgP28 national3%
- FluorideF⁻1.2mgP49 national0.6%
- NitrateNO₃⁻0.30mgP49 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃160mgP74 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂113mgP96 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂93.8mgP78 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.0020mgP9 national
- CopperCuNDmg
- 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.
- 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.