A sodium chloride cold-mineral spring at 13 °C, transported by tanker truck and blended 2:1 from two Wakayama wells (the Marina City source and a nearby Kegami-district well). Neutral pH and 5,608 mg/kg of dissolved solids place it firmly in the moderately mineralized chloride family — heat-retaining, with a salty edge on the lips. At the wellhead the water reads yellow-brown and slightly turbid; the bath is heated for use.
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
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
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⁺1,588mgP90 national76%
- Magnesiummild; a laxative if you drink itMg²⁺165mgP95 national15%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺135mgP78 national7%
- PotassiumK⁺31.7mgP76 national0.9%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- FeTotalFe< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- Manganesetrace; companion to iron springsMn²⁺—mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻2,583mgP89 national80%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻694mgP83 national13%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻311mgP74 national7%
- FluorideF⁻1.2mgP50 national0.1%
- BO2BO₂⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂48.2mgP87 national
- Metasilicic acidH₂SiO₃29.9mgP13 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂109mgP81 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —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.
- 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.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.