新井第2号泉・動力揚湯源泉、新井第5号泉・唐竹沢源泉 混合泉Arai Spring No.2 / Arai Spring No.5 (Karatakezawa) Blended Source
A hot (64 degC) blended source combining Arai Spring No.2 with the Karatakezawa Spring (Arai No.5). The chemistry is a sodium bicarbonate-chloride water — bicarbonate-led with a salty layer underneath — weakly alkaline and soft on the skin, of the classic beauty-water family. Metasilicic acid runs to a generous 237 mg/kg, leaving the kind of fine silica film that makes skin feel noticeably smoother afterwards.
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
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
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, netto 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
Take care
From a certificate analyzed in 2023.
- Active stage of any illness (especially with fever)
- Active tuberculosis, serious illness such as an advanced malignant tumor or severe anemia
- Severe heart or lung disease causing shortness of breath on slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding, visible bleeding
- Acute exacerbation of a chronic illness
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⁺267mgP60 national83%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺28.5mgP44 national10%
- Potassiumminor; matters for drinking limitsK⁺19.3mgP62 national3%
- Magnesiummild; a laxative if you drink itMg²⁺5.5mgP53 national3%
- StrontiumSr²⁺0.20mgP41 national0.1%
- Manganesetrace; companion to iron springsMn²⁺0.30mgP54 national
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- CopperCu²⁺< —mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< —mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺< —mg
- AmmoniumNH₄⁺< —mg
- ZincZn²⁺< —mg
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻548mgP78 national61%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻113mgP44 national22%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻119mgP57 national17%
- FluorideF⁻0.20mgP16 national0.1%
- Iodideyellows in air; helps blood lipidsI⁻1.2mgP83 national
- Bromidetrace, sea-originBr⁻< —mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻< —mg
- Hydrogen phosphateHPO₄²⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- NitrateNO₃⁻< —mg
- HydroxideOH⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃237mgP93 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂13.7mgP64 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂13.2mgP45 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs0.22mgP81 national
- CadmiumCd< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2023.
- Peripheral circulation disorders
- Chronic muscle or joint pain or stiffness (the chronic phase of rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains and the like)
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.