Spring B at Daimaru Besso in Futsukaichi Onsen is a sodium-led alkaline simple thermal spring. At pH 8.59 it is alkaline with a slippery, soft feel that places it in Japan's beauty-water family. The water runs colorless and clear at the source, carrying only a faint hydrogen sulfide note, and at 588 mg/kg dissolved solids it is lightly mineralized and gentle on the body. It also carries enough radon to fall in the radon-bearing 'hormesis' tradition.
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⁺175mgP45 national95%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.9mgP17 national3%
- PotassiumK⁺2.6mgP25 national0.9%
- LithiumLi⁺0.40mgP50 national0.6%
- StrontiumSr²⁺NDmg0.0%
- Magnesiummild; a laxative if you drink itMg²⁺0.06mgP6 national
- Aluminumshows up in acidic volcanic waterAl³⁺NDmg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻120mgP44 national45%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻156mgP52 national34%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻47.5mgP39 national13%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻13.8mgP58 national6%
- Fluoridetrace; drinking limits applyF⁻2.4mgP62 national2%
- ThiosulfateS₂O₃²⁻0.40mgP62 national0.1%
- BromideBr⁻NDmg0.0%
- NitrateNO₃⁻NDmg0.0%
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.08mgP21 national
- HydroxideOH⁻0.07mgP38 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃58.2mgP35 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.2mgP47 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂NDmg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂SNDmg
- Radonfaint radon — studied for joint pain (hormesis)Rn—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.