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, 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
Take care
- 病気の活動期（特に熱のあるとき）
- Active tuberculosis
- 進行した悪性腫瘍又は重症の心臓病、腎不全、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
- 泉質別禁忌症：無し
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⁺1,684mgP90 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,226mgP96 national
- Potassiumminor; matters for drinking limitsK⁺69.3mgP89 national
- StrontiumSr²⁺3.6mgP87 national
- LithiumLi⁺0.80mgP70 national
- AmmoniumNH₄⁺0.80mgP61 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⁺0.02mgP17 national
- ZincZn²⁺0.0030mgP35 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻4,626mgP92 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻250mgP70 national
- Bromidetrace, sea-originBr⁻13.3mgP90 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻7.2mgP43 national
- Fluoridetrace; drinking limits applyF⁻1.3mgP52 national
- HydroxideOH⁻1.1mgP87 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃54.3mgP33 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂38.4mgP84 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂14.3mgP47 national
- ArsenicAs0.06mgP52 national
Measured at the source
On the certificate
- 筋肉又は関節の慢性的な痛みやこわばり（関節リウマチ、変形性関節症、腰痛症、神経痛、五十肩、打撲、捻挫などの慢性期）
- 冷え性、末梢循環障害
- 胃腸機能の低下（胃がもたれる、腸にガスがたまるなど）
- Mild hypertension
- 耐糖能異常（糖尿病）
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- 自律神経不安定症、ストレスによる諸症状（睡眠障害、うつ状態など）
- 病後回復期、疲労回復、健康増進
- きりきず、末梢神経痛、冷え性、うつ状態、皮膚乾燥症（泉質別適応症）
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.