湯の里1号、湯の里2号、湯の里3号及び湯の里4号の混合泉Yunosato No.1-4 Blended Spring
A blend of four wells — Yunosato No.1 through No.4 — sampled from the mixing tank and piped to the main hall, wooden tubs and sand-rinse baths of the Genroku bathhouse. It is a sodium chloride spring, hypotonic and neutral, moderately mineralized at 4,345 mg/kg with a faint salt tang on the lips. The standout figure is metasilicic acid at 211.6 mg/kg — silica in that range leaves a thin moisturizing film and puts the water squarely in Japan's beauty-water family.
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, nettō 熱の湯).
cold sensitivity
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
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⁺1,246mgP88 national80%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺181mgP84 national13%
- Potassiumminor; matters for drinking limitsK⁺115mgP93 national4%
- Magnesiummild; a laxative if you drink itMg²⁺13.7mgP65 national2%
- LithiumLi⁺1.6mgP88 national0.3%
- ManganeseMn²⁺0.80mgP73 national0.0%
- AmmoniumNH₄⁺0.30mgP40 national0.0%
- StrontiumSr²⁺0.60mgP72 national0.0%
- Iron (II)Fe²⁺0.40mgP48 national0.0%
- ZincZn²⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,358mgP89 national95%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻93.9mgP53 national3%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻91.5mgP41 national2%
- BromideBr⁻7.0mgP85 national0.1%
- Hydrogen phosphateHPO₄²⁻0.10mgP34 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃212mgP88 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂22.4mgP76 national
- Metaarsenious acidHAsO₂0.20mgP70 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂26.4mgP58 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —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.
- 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.
- 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.