Watanoyu Gensen
Analysis VerifiedThis rare, naturally gushing source emerges from beneath 'Gensenkaku,' nestled beside Kusatsu Onsen's iconic Yubatake (Kusatsu's central hot-water field). Despite its strong acidity, its waters offer a remarkably soft bathing sensation, akin to being enveloped in silk floss. The milky-white waters, renowned for having Kusatsu's finest 'yunohana' (hot spring minerals), gently embrace the skin, contributing to silky skin and providing a restorative warmth that lingers deep within the body. Known as a 'phantom spring' for its history of disappearing once during the Taisho era, it now offers a profound escape from daily life, inviting deep relaxation and rejuvenation.
The bar length shows how abundant each component is compared to thresholds that make it "distinctive" (therapeutic spring standards, beauty spring criteria, etc.).
Strong astringent action that tightens skin. A key component of acidic springs, suited to oily or acne-prone skin.
The source of the distinctive sulfur smell. Dilates blood vessels via skin and inhalation to strongly boost circulation.
Mild antibacterial action keeps the skin clean. Trace transdermal absorption supports tooth and bone health.
A natural moisturizer also used in skincare lotions. Forms a thin veil on the skin to reduce moisture loss after bathing.
- Sulfur9.9 mg/kg
- Feel
- Smells like boiled eggs; the water may turn milky white or blue-green.
- Said to
- Said to be strongly antibacterial and to promote circulation. Bathing indications include atopic dermatitis and chronic eczema.
- Note
- With sensitive skin, rinse with fresh water after bathing. Remove silver jewelry—it tarnishes.
- Acidic (pH)pH 2.1
- Feel
- May tingle on the skin; tastes sour.
- Said to
- Said to be strongly antibacterial. Bathing indications include atopic dermatitis and plaque psoriasis.
- Note
- Can irritate sensitive skin—rinse with fresh water after bathing. Keep it out of your eyes.
- Chloride (salt)299 mg/kg
- Feel
- Tastes salty; the warmth lasts after you get out.
- Said to
- Salt is said to form a film on the skin that keeps in heat and moisture. Bathing indications include cuts, poor circulation and dry skin.
- Note
- May sting cuts. Strong salty water can tire you—avoid long soaks.
- Sulfate201 mg/kg
- Feel
- Your skin feels moist; the water may taste slightly bitter.
- Said to
- Said to retain heat well and help heal cuts. Bathing indications include cuts and dry skin.
- Note
- It warms you well—do not stay in too long.
- Metasilicic acid225 mg/kg
- Feel
- Your skin may feel smoother.
- Said to
- 50 mg/kg or more is said to mark a “skin-beautifying spring”; it is also used in skincare.
- Note
- The “skin-beautifying” label is a common rule of thumb, not a legal standard.
- Source temperature50.2℃
- Feel
- The source is 42°C or hotter; the bath may be on the hot side.
- Said to
- Heat is said to boost circulation and warm the body.
- Note
- Older people and those with high blood pressure or heart conditions should avoid baths of 42°C or more. Keep soaks short.
Effects are general descriptions based on the Ministry of the Environment’s indications by spring type, not a guarantee.
Scores are calculated from mineral analysis data based on the Mineral Spring Analysis Guidelines (Ministry of Environment, Japan). They are unrelated to facility atmosphere or service quality.
▸Note
The temperature, pH and component values shown here are based on the hot spring analysis (source). Actual values in the bath may differ depending on heating, dilution, recycling and filtration. Please check current usage status at the facility.
With a very strong acidity of pH 2.03, those with sensitive skin should be cautious of irritation. It's best to start with short baths of 3 to 10 minutes and gradually increase the time. If you feel the water is too harsh, rinsing off with fresh water after your bath is recommended.
Known for its powerful sterilizing effects, this strongly acidic spring with a pH of 2.03 is expected to be effective for chronic skin diseases and cuts. With a high metasilicic acid content of 225.8 mg/kg, it is said to leave the skin feeling moisturized. Its high temperature of 50.2°C warms you to the core, making it ideal for improving circulation and recovering from fatigue.
Due to its high acidity, individuals with sensitive skin or elderly persons with dry skin should exercise caution. Bathing should be avoided by those with acute illnesses, active tuberculosis, severe heart disease, bleeding disorders, or severe anemia.
※ See the analysis data below for detailed contraindications
ANALYSIS DATAView all analysis fields ▼
Source Info
- Source name
- Watanoyu Gensen
- Spring type
- 酸性・含硫黄-アルミニウム-硫酸塩・塩化物温泉
- Source temperature
- 50.2℃
- pH
- 2.1
- Flow type
- 自然湧出
- Osmotic pressure
- 低張性
Analysis Document
- Analysis date
- 2014-11-05
- Analyst organization
- 一般社団法人 群馬県薬剤師会 環境衛生試験センター
- Registration No.
- 群馬第2号
Cations
- Hydrogen ion H⁺
- 8.9 mg/kg
- Sodium ion Na⁺
- 56.3 mg/kg
- Potassium ion K⁺
- 27.5 mg/kg
- Magnesium ion Mg²⁺
- 34.1 mg/kg
- Calcium ion Ca²⁺
- 76.7 mg/kg
- Aluminum ion Al³⁺
- 44.3 mg/kg
- Manganese ion Mn²⁺
- 1.0 mg/kg
- Iron(II) ion Fe²⁺
- 16.5 mg/kg
- Total cations
- 267 mg/kg
Anions
- Fluoride ion F⁻
- 9.6 mg/kg
- Chloride ion Cl⁻
- 299 mg/kg
- Sulfate ion SO₄²⁻
- 201 mg/kg
- Bromide ion Br⁻
- 1.4 mg/kg
- Iodide ion I⁻
- 2.3 mg/kg
- Hydrogen sulfate HSO₄⁻
- 669 mg/kg
- Total anions
- 1182 mg/kg
Undissociated Components
- Sulfuric acid H₂SO₄
- 4.5 mg/kg
- Metasilicic acid H₂SiO₃
- 225 mg/kg
- Metaboric acid HBO₂
- 7.2 mg/kg
- Total undissociated
- 237 mg/kg
Dissolved Gas
- Free carbon dioxide CO₂
- 167 mg/kg
- Free hydrogen sulfide H₂S
- 10.5 mg/kg
- Total dissolved gas
- 178 mg/kg
Totals
- Total dissolved substances
- 1890 mg/kg
- Component total
- 1860 mg/kg
- Evaporation residue
- 1480 mg/kg
Usage
- Heating
- No
- Dilution
- No
- Recycle / Filter
- No
- Additives
- No
- Disinfection
- No
Indications & Contraindications
- Bathing indications
- Atopic dermatitis, Plaque psoriasis, Chronic eczema, Epidermal suppuration, Impaired glucose tolerance (diabetes), Cuts and grazes, Peripheral circulatory failure, Poor circulation, Depression, Xeroderma
- General indications
- High blood pressure (mild), Rheumatoid arthritis, Autonomic instability, Insomnia, Chronic pain or stiffness in muscles and joints, Muscle stiffness due to motor paralysis, Gastrointestinal dysfunction
- Contraindications
- Acute illness (especially with fever), Active tuberculosis, Malignant tumors, Severe heart disease, Respiratory failure, Renal failure, Bleeding disorders, Severe anemia, Any actively progressing illness, Pregnancy (especially early and late stages), Severe chronic dermatitis (sulphur springs), Sensitive skin or mucous membranes, eye conditions (acidic springs)
Drinking
- Drinkability
- Not drinkable
