A blended spring drawing on two sources — Kakuma Onsen and a deep well — combined in the facility's holding tank. The chemistry reads as a sodium-calcium chloride water, hot at the wellhead (47.5 °C) and moderately mineralized at roughly 8 g/kg of dissolved solids. Weakly alkaline pH gives a soft skin feel, and metasilicic acid at 54.3 mg/kg layers in a faint silica film that smooths the touch. Because the bath is fed from the mixing tank, it is diluted with water, reheated, recirculated, and disinfected before reaching the tub.
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, netto 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
Take care
From a certificate analyzed in 2014.
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor or severe heart disease, renal failure, gastrointestinal bleeding, visible bleeding
- Acute exacerbation of a chronic illness
- Spring-type contraindications: none
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⁺1,684mgP91 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,226mgP96 national
- Potassiumminor; matters for drinking limitsK⁺69.3mgP90 national
- StrontiumSr²⁺3.6mgP89 national
- LithiumLi⁺0.80mgP68 national
- AmmoniumNH₄⁺0.80mgP60 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.05mgP17 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.03mgP23 national
- Manganesetrace; companion to iron springsMn²⁺0.02mgP16 national
- ZincZn²⁺0.0030mgP34 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4,626mgP93 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻250mgP69 national
- Bromidetrace, sea-originBr⁻13.3mgP91 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻7.2mgP42 national
- Fluoridetrace; drinking limits applyF⁻1.3mgP52 national
- HydroxideOH⁻1.1mgP87 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃54.3mgP32 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂38.4mgP84 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂14.3mgP46 national
- ArsenicAs0.06mgP54 national
Measured at the source
On the certificate
From a certificate analyzed in 2014.
- Chronic muscle or joint pain or stiffness (the chronic phase of rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains and the like)
- Cold sensitivity, peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Autonomic nervous system instability, stress-related symptoms (sleep disorders, depressed mood)
- Convalescence, fatigue recovery, general health improvement
- Cuts, peripheral neuralgia, cold sensitivity, depressed mood, dry skin (spring-type indications)
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.