A gentle simple thermal spring drawn from Beppu's city-owned Horita storage tank. Weakly acidic and lightly mineralized at 483 mg/kg of dissolved solids, it is soft and undemanding on the body — though at the wellhead the water carries a faint rotten-egg note of hydrogen sulfide, which has faded to almost nothing by the time the lab repeats the sensory test ten hours later. What lifts it above ordinary simple water is metasilicic acid at 122 mg/kg: enough to place it in Japan's beauty-water family, leaving the kind of thin silica film that makes skin feel noticeably smoother. The sample was taken at the storage tank's overflow outlet rather than at the wellhead itself.
Good for
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
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⁺64.8mgP19 national59%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺18.3mgP37 national19%
- Magnesiummild; a laxative if you drink itMg²⁺10.5mgP62 national18%
- Potassiumminor; matters for drinking limitsK⁺6.7mgP39 national4%
- LithiumLi⁺0.20mgP38 national0.6%
- StrontiumSr²⁺0.20mgP42 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻119mgP47 national38%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻93.8mgP52 national38%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻44.7mgP25 national24%
- FluorideF⁻0.20mgP16 national0.2%
- H2PO4H₂PO₄⁻0.30mgP74 national0.0%
- ThiosulfateS₂O₃²⁻0.20mgP53 national0.0%
- BromideBr⁻0.10mgP14 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃122mgP62 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.7mgP16 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Metaarsenious acidHAsO₂0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂261mgP90 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.02mgP35 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.
- 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.