A sodium chloride spring, mildly alkaline and soft on the skin, carrying a generous 208.4 mg/kg of metasilicic acid: enough silica to leave a thin film and noticeably smoother skin, which puts it squarely in Japan's beauty-water family. At 99.3 °C the source is among the hottest in the country, and its moderate, chloride-led mineralization of about 2.4 g/kg gives the salty, heat-retaining body typical of the type. The park's signboard tells the origin story: during drilling in November 1926 a jet of 100 °C water shot some 50 metres into the air, and the self-flowing geyser, a rarity in Japan, still erupts with almost unchanged force from a well 36.4 m deep. The town manages the flow centrally and pipes it out to facilities including the Odoriko Onsen Kaikan, while the thick white mineral deposits that clog the vent are chiselled out by hand twice a week.
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⁺674mgP79 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺91.2mgP71 national
- Potassiumminor; matters for drinking limitsK⁺68.2mgP88 national
- StrontiumSr²⁺0.80mgP73 national
- LithiumLi⁺0.60mgP61 national
- Magnesiummild; a laxative if you drink itMg²⁺0.20mgP15 national
- AmmoniumNH₄⁺0.10mgP23 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,111mgP79 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻163mgP64 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻22.7mgP72 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻8.5mgP5 national
- Bromidetrace, sea-originBr⁻2.4mgP72 national
- Fluoridetrace; drinking limits applyF⁻1.5mgP55 national
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.20mgP39 national
- HydroxideOH⁻0.04mgP35 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃208mgP89 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂6.0mgP42 national
- Metaarsenious acidHAsO₂0.10mgP59 national
- 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.