Water profile — Mukawa No Yu

むかわの湯Mukawa no Yu

塩ChlorideSodium Chloride-Bicarbonate spring · +Bicarbonate

A sodium chloride spring with bicarbonate softness layered in — the salty, heat-retaining type that keeps you warm long after you leave the bath. The water is weakly alkaline and gentle on the skin, emerging pale yellow and clear with a faint salty tang on the lips. Moderately mineralized, it has noticeable body without being harsh, and the bicarbonate side lends a smooth, skin-friendly finish.

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

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

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⁺530mgP77 national89%
Calciumsettling and calming; can leave a white mineral bloomCa²⁺32.2mgP47 national6%
Magnesiummild; a laxative if you drink itMg²⁺9.0mgP60 national3%
PotassiumK⁺7.6mgP40 national0.7%
AmmoniumNH₄⁺2.4mgP83 national0.5%
LithiumLi⁺0.90mgP72 national0.5%
StrontiumSr²⁺3.9mgP88 national0.3%
Iron (III)Fe³⁺0.70mgP82 national0.1%
BariumBa²⁺1.3mgP92 national0.1%
Iron (II)Fe²⁺0.10mgP30 national0.0%
AluminumAl³⁺0.0000mg0.0%
HydrogenH⁺0.0000mg0.0%
ManganeseMn²⁺0.0000mg0.0%
ZincZn²⁺0.0000mg0.0%

Anionsshare of charge (mval%)

Chloridetable-salt anion — coats the skin and holds heat inCl⁻700mgP77 national73%
Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻452mgP74 national27%
BromideBr⁻1.8mgP70 national0.1%
FluorideF⁻0.40mgP31 national0.1%
Hydrogen phosphateHPO₄²⁻0.30mgP60 national0.0%
Hydrogen sulfideHS⁻0.30mgP47 national0.0%
IodideI⁻0.50mgP69 national0.0%
AsO2AsO₂⁻0.0000mg0.0%
BO2BO₂⁻0.0000mg0.0%
CarbonateCO₃²⁻0.0000mg0.0%
HSiO3HSiO₃⁻0.0000mg0.0%
HSO4HSO40.0000mg0.0%
HydroxideOH⁻0.0000mg0.0%
SS²⁻0.0000mg0.0%
ThiosulfateS₂O₃²⁻0.0000mg0.0%
SulfateSO₄²⁻0.0000mg0.0%

Non-dissociatedmeasured by mass

Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃35.0mgP17 national
Metaboric acidmetaboric acid — mildly antibacterialHBO₂14.3mgP68 national
H2SO4FreeH2SO4Free0.0000mg
Phosphoric acidH₃PO₄0.0000mg
Metaarsenious acidHAsO₂0.0000mg

Dissolved gasesmeasured by mass

Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂37.2mgP64 national
Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.10mgP61 national

Trace metalsmeasured by mass

ArsenicAs< —mg
CadmiumCd< —mg
CopperCu< —mg
Mercurytoxic trace — part of the drinking-safety checkHg< —mg
LeadPb< —mg

Measured at the source

7.90

Weakly alkaline

Source temperature

39.8°C

Warm

Tonicity

1.79g/kg

Hypotonic

Flow rate

147L/min

Typical

Free CO₂

37mg/kg

Low

Free H₂S

0.10mg/kg

Low

Evidence

むかわ鉱泉Mukawa Mineral Spring

Sodium-Calcium-Magnesium Bicarbonate spring

An unusual spring with no official quality name: the certificate itself notes that the water qualifies as an onsen under the metasilicic acid clause of the Onsen Law's Article 2 appendix, while falling short of therapeutic-spring status. It is a cold mineral spring (below 25 °C), colorless and clear, with a bitter taste and a faint metallic iron note. The very mineral that earns it onsen status also defines its character — the water is rich in metasilicic acid, the skin-smoothing component of Japan's beauty-water family, leaving a thin moisturizing silica film on the skin.