Onsen OniOnsen Oni

Search Onsen Oni

Search onsen, areas, prefectures and pages.

Yunoyama Onsen Green Hotel

Water profile

Yunoyama Onsen Green Hotel

Spring chemistry and what it does, with the research behind it.

新湯の山温泉2号源泉Shin-Yunoyama Onsen Source No. 2

湯SimpleSodium Bicarbonate-Chloride spring

A sodium-led simple alkaline thermal water carrying bicarbonate and a touch of chloride. Wellhead pH 8.5 gives the slippery, smooth feel that earns Japanese alkaline springs the 'beauty water' label. Lightly mineralized at about 0.70 g/kg dissolved solids, so the bath is soft on the body rather than emphatic.

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%)
  • Sodium200mgP50 national
  • Calcium4.6mgP16 national
  • Ammonium0.70mgP56 national
  • Potassium1.3mgP16 national
  • Magnesium0.27mgP15 national
  • FeTotal< —mg
Anionsshare of charge (mval%)
  • Bicarbonate330mgP66 national
  • Chloride103mgP41 national
  • Carbonate12.0mgP56 national
  • Fluoride5.0mgP79 national
Non-dissociatedmeasured by mass
  • Metasilicic acid31.0mgP14 national
  • Metaboric acid10.0mgP55 national
Trace metalsmeasured by mass
  • Cadmium< —mg
  • Copper< —mg
  • Radon—mg

Measured at the source

pH
8.50
Alkaline
Source temperature
39.4°C
Warm
Tonicity
0.70g/kg
Hypotonic
Flow rate
160L/min
Typical
Radon
29Bq/kg
Low

Evidence

  1. 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.
  2. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  3. Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
  4. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.

新湯の山温泉3号源泉Shin-Yunoyama Onsen Source No. 3

湯SimpleSodium Bicarbonate-Chloride spring

A hot (55 C at the wellhead) sodium-bicarbonate alkaline thermal water — clean, unadorned chemistry that belongs squarely in the Japanese 'beauty water' family. Wellhead pH 8.6 gives the slippery, soap-on-skin feel without any heaviness; the bath house keeps a small wellhead (gensen) tub running pure, no dilution and no reheating, so visitors can read the water on its own terms.

Good for

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

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%)
  • Sodium154mgP40 national
  • Calcium3.9mgP14 national
  • Potassium1.3mgP16 national
  • Magnesium0.40mgP18 national
  • FeTotal< —mg
Anionsshare of charge (mval%)
  • Bicarbonate251mgP61 national
  • Chloride81.2mgP36 national
  • Carbonate12.0mgP56 national
  • Fluoride4.3mgP75 national
Non-dissociatedmeasured by mass
  • Metasilicic acid34.7mgP17 national
  • Metaboric acid7.3mgP48 national
  • Metaarsenious acid0.02mgP36 national
Dissolved gasesmeasured by mass
  • Free carbon dioxide2.2mgP22 national
Trace metalsmeasured by mass
  • CadmiumNDmg
  • Radon—mg

Measured at the source

pH
8.60
Alkaline
Source temperature
55.0°C
Hot
Tonicity
0.55g/kg
Hypotonic
Flow rate
550L/min
High
Free CO₂
2.20mg/kg
Low
Radon
23Bq/kg
Low

Evidence

  1. 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.
  2. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  3. Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
  4. Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
  5. Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
  6. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.

新湯の山温泉4号源泉Shin-Yunoyama Onsen Source No. 4

湯SimpleSodium Bicarbonate spring

A low-temperature spring (30.7 C at the wellhead) of the sodium-bicarbonate simple-thermal family. Wellhead pH 8.4 (lab 8.32) puts it in the weakly alkaline band, gentle on the skin. The wellhead sensory record notes a faint iron taste even though total iron is sub-trace — a small metallic note when sipped at the source. Heated before bathing because the source itself is below soaking temperature.

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%)
  • Sodium206mgP52 national
  • Calcium1.8mgP7 national
  • Potassium1.2mgP15 national
  • Magnesium0.30mgP16 national
  • FeTotal< —mg
Anionsshare of charge (mval%)
  • Bicarbonate464mgP74 national
  • Carbonate12.0mgP56 national
  • Chloride13.4mgP15 national
  • Fluoride6.8mgP84 national
  • Sulfate0.10mgP2 national
  • Iodide< —mg
Non-dissociatedmeasured by mass
  • Metasilicic acid28.4mgP11 national
  • Metaboric acid12.0mgP61 national
Dissolved gasesmeasured by mass
  • Free carbon dioxide2.0mgP21 national
Trace metalsmeasured by mass
  • Cadmium< —mg
  • Radon—mg

Measured at the source

pH
8.40
Weakly alkaline
Source temperature
30.7°C
Low temp.
Tonicity
0.75g/kg
Hypotonic
Flow rate
83L/min
Typical
Free CO₂
2.00mg/kg
Low
Radon
12Bq/kg
Low

Evidence

  1. 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.
  2. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  3. Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
  4. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.

Educational, not medical advice. If you have a health condition, consult a physician before onsen therapy.