湯Simple
Hanasaku Onsen's Koshizawa-no-Yu is an alkaline simple thermal source (pH 8.50) — gentle in composition with the slippery, beauty-water feel that pH-8.5+ springs carry. The source emerges at a modest 29.2 °C and is reheated for bathing use, arriving in the bath at around 40.5 °C.
Good for
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
On the certificate:ConvalescenceFatigue recoveryGeneral health improvement
Take care
Contraindications
- 急性疾患(特に熱のある場合)
- Active tuberculosis
- Malignant tumor
- 重い心臓病
- 呼吸不全
- Renal failure
- Bleeding disorders
- Severe anemia
- その他一般に病勢進行中の疾患
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.
Cationsshare of charge (mval%)
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺41.1mgP10 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺4.7mgP17 national
- Potassiumminor; matters for drinking limitsK⁺0.38mgP3 national
- Magnesiummild; a laxative if you drink itMg²⁺0.29mgP15 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.03mgP15 national
- Manganesetrace; companion to iron springsMn²⁺0.01mgP17 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
Anionsshare of charge (mval%)
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻56.5mgP30 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻13.8mgP26 national
- Carbonatestrongly alkaline; slippery feelCO₃²⁻12.0mgP56 national
- Fluoridetrace; drinking limits applyF⁻6.2mgP82 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻3.7mgP6 national
Measured at the source
Source temperature
29.2°C
Warm
On the certificate
- 神経痛
- Muscle pain
- Joint pain
- Frozen shoulder
- Motor paralysis
- Joint stiffness
- Bruises
- Sprains
- Chronic digestive disorders
- Hemorrhoids
- Cold sensitivity
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.
- 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.