Iodine-Containing Springs: A Non-Volcanic Rarity & Its Drinking Indication

Hot springs in Shinshinotsu began with the No.1 well drilled in 1990, whose water had to be heated. In 2015 the No.3 well, reaching 1,600 m, was completed, delivering water above 50°C ready to use. Rising beneath the Ishikari Plain beside oxbow Lake Shinotsu, it is a yellowish-brown, intensely salty water with a faint medicinal note from iodine and bromine. Its dense salts wrap the skin and keep you warm long after bathing, a fine companion for harsh winters.

This spring rose in 1985 from a 1,500 m well in Naganuma, a farming town at the eastern edge of the Ishikari Plain beneath the Umaoi Hills, and was designated a national health-resort hot spring in 1988. It is a hypertonic strong saline water with over 20 g/kg of dissolved minerals, including 11.2 mg/kg of iodide and 356.2 mg/kg of metaboric acid, with a sea-like salty taste. It suits anyone wanting to warm up to the core after a day in the fields or on the park-golf course.

Toyotomi Onsen began in 1926, when hot water burst out with natural gas during oil exploration. A 2020 analysis of the mixed water from five wells recorded 34.2°C and pH 7.5, with sodium 4432 mg/kg, chloride 5952 mg/kg, bicarbonate 2136 mg/kg, iodide 16.1 mg/kg and metaboric acid 494.6 mg/kg, for 13.32 g/kg of dissolved solids. Pale yellow, cloudy and faintly oily in scent, it is a lukewarm yet deeply enveloping water with the steady, unpretentious character of a craftsman.

Yasuda Onsen rises in the farmland of Yasuda, Agano, on the right bank of the Agano River with the Gozu Mountains behind it. Pumped from underground at 19.9°C and 105 liters per minute, the water was found in its 2024 analysis to carry 23,161 mg of dissolved matter per kilogram—a "strong brine" with salinity approaching that of seawater. Sodium makes up about 85% of the cations and chloride more than 90% of the anions. It also contains 34.1 mg of iodide and 84.6 mg of bromide, hinting at ancient seawater trapped in the strata as fossil brine, along with a generous 413.7 mg of free carbon dioxide. Nearly colorless and clear, it tastes strongly salty with a faint metallic edge. Think of it as a taciturn Sea of Japan fisherman: plain at first sight, yet its dense, briny strength wraps the body firmly and leaves a warmth that lingers long after bathing.