Asaya's "Kodakara-no-yu" wellhead is a simple alkaline thermal spring (hypotonic, alkaline, normal-temperature class). Both the on-site pH 8.8 and the laboratory pH 8.77 sit well into alkaline territory, giving the characteristic slippery, skin-smoothing feel of Japan's "beauty-water" family. The water is lightly mineralized — 0.304 g/kg dissolved solids and 0.311 g/kg evaporation residue — which keeps the bath gentle, with little of the assertive mineral edge of stronger chemistries. Colorless, tasteless, and odorless at the spout, it presents as a soft, unobtrusive single-thermal source whose appeal lies in the alkaline silk-water feel rather than in pronounced ions or aroma.
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
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⁺—mg88%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻—mg39%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻—mg37%
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.
- 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.