新うなぎ湯3号泉・山道新1号泉・山道新2号泉・山道新3号泉・山道新4号泉・山道新5号泉・山道新6号泉混合泉Shin-Unagiyu Spring No.3 / Sando-Shin Springs No.1–6 mixed source
A sulfur-bearing sodium sulfate-bicarbonate spring blended from the inn's own wellfield in Naruko Onsen. The wellhead runs at 68.9 °C and the bath is served with no dilution, no reheating, no recirculation and no disinfection — fully untreated source flow. At the spout the water is clear, but the air carries a strong hydrogen sulfide note and a faint bitterness lingers if tasted; once the water settles, fine mineral scale (yuno-hana) drifts up. Metasilicic acid at 184 mg/kg paired with the sulfur fraction gives the slippery 'eel-water' (unagi-yu) feel the inn is named for, and salinity sits in the mid range — about 2,077 mg/kg of dissolved solids.
Good for
Skin conditions
Sulfur and acidic waters are antibacterial and keratolytic, studied for atopic dermatitis and psoriasis.
atopic skin, eczema
Circulation
Skin-absorbed CO₂ and hydrogen sulfide relax the vessel walls; carbon-dioxide springs can raise peripheral blood flow several-fold.
tired legs
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
Take care
Sensitive-skin caution
Strongly acidic or sulfur-rich water can sting broken, atopic-flare, or dry elderly skin. Ease in slowly and rinse off if it stings.
broken or dry skin
- 病気の活動期(特に熱があるとき)
- Active tuberculosis
- 進行した悪性腫瘍または高度の貧血など重い病気にかかっている時
- 少し動くと息苦しいような重い心臓または肺の病気
- むくみのあるような重い腎臓の病気
- 消化管出血
- 目に見える出血があるとき
- 慢性の病気の急性増悪期
- 皮膚または粘膜の過敏な人
- 高齢者の皮膚乾燥症
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⁺491mgP76 national81%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺42.8mgP55 national8%
- Magnesiummild; a laxative if you drink itMg²⁺23.8mgP72 national7%
- Potassiumminor; matters for drinking limitsK⁺31.3mgP76 national3%
- LithiumLi⁺0.50mgP55 national0.3%
- AmmoniumNH₄⁺—mg0.1%
- StrontiumSr²⁺0.50mgP68 national
- Manganesetrace; companion to iron springsMn²⁺0.40mgP59 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Iron (III)Fe³⁺< —mg
- ZincZn²⁺< —mg
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻625mgP90 national49%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻362mgP69 national22%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻204mgP54 national22%
- Hydrogen sulfidesulfur character — antibacterialHS⁻45.5mgP100 national5%
- Thiosulfateoxidized sulfurS₂O₃²⁻32.2mgP97 national2%
- FluorideF⁻0.0047mgP2 national0.1%
- Hydrogen phosphateHPO₄²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃184mgP82 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂33.2mgP81 national
- Metaarsenious acidHAsO₂0.75mgP88 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂44.5mgP68 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S6.5mgP88 national
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- 筋肉または関節の慢性的な痛みやこわばり
- 胃腸機能の低下
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- 軽い喘息または肺気腫
- Cold sensitivity
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.
- Tei C, Kihara T. Waon Therapy for Chronic Heart Failure. J. Cardiology 53(2):214–218, 2009.
- Coavoy-Sánchez SA et al. Hydrogen sulfide and dermatological diseases. Int. J. Dermatology, 2019.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Akiyama H et al. Antimicrobial effects of acidic hot-spring water on S. aureus. J. Dermatological Science 24(3):193–200, 2000.
- Toriyama T et al. Carbon dioxide foot bathing on critical limb ischemia. International Angiology 21(4):367–373, 2002.
- 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.
- Ministry of the Environment, Japan. 禁忌症及び入浴又は飲用上の注意事項 (2014 notice).official