Water profile — Yokikan Hotel

岡165号Oka No. 165 Spring

湯Simple

A simple thermal spring (Oka No. 165) at the gentler end of the spectrum. Weakly alkaline at pH 8.04, it sits softly in Japan's beauty-water family — slippery on the skin without sharpness. Mineralization is light at 744 mg/kg and the source temperature is a mild 38.8 °C, lightly warmed for the indoor bath at Yokikan.

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.

Cationsshare of charge (mval%)

Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺145mgP39 national
Calciumsettling and calming; can leave a white mineral bloomCa²⁺80.9mgP67 national
Potassiumminor; matters for drinking limitsK⁺3.8mgP30 national
Magnesiummild; a laxative if you drink itMg²⁺2.4mgP36 national

Anionsshare of charge (mval%)

Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻203mgP66 national
Chloridetable-salt anion — coats the skin and holds heat inCl⁻201mgP54 national
Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻40.7mgP24 national
Iodideyellows in air; helps blood lipidsI⁻0.10mgP37 national

Non-dissociatedmeasured by mass

Metaboric acidmetaboric acid — mildly antibacterialHBO₂52.0mgP89 national

Dissolved gasesmeasured by mass

Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂11.0mgP42 national

Measured at the source

Source temperature

38.8°C

Warm

Tonicity

0.74g/kg

Hypotonic

Free CO₂

11mg/kg

Low

Evidence

岡160号Oka No. 160 Spring

塩Chloride

A hot calcium-sodium chloride spring (Oka No. 160) — the wellhead runs at 53.9 °C and is cooled to about 41–42 °C for bathing. Weakly alkaline at pH 7.60, it leads with calcium and adds a sodium backbone, giving the water a salty, heat-retaining body. Mineralization is a moderately rich 6.6 g/kg, and Yokikan feeds it to the rooftop open-air bath.

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