Radioactive Springs: Traditional Radon Waters & Their Indications

Nekonaki Onsen rises beside the Kitasu River in Ishikawa, central Fukushima. Legend says that when the Heian-era poet Izumi Shikibu left for the capital, her cat cried for her until it fell ill, then recovered by bathing here, giving the spring its name, "crying cat." The Ishikawa area is dotted with radioactive springs, and Izutsuya Onsen No. 2, pumped from the ground at 18.4°C, is a cold mineral spring. Its total mineral content is a light 128.2 mg/kg, yet carbonate ions reach 36.1 mg, giving it a strongly alkaline pH of 10.1.

Rising at Kinoe on Osakikamijima, an island reached only by ferry, this spring once served a public lodge loved by islanders and sea travelers. Saltier than seawater and rich in calcium with a trace of radon, it is a powerful water for those who want to warm themselves slowly while gazing over the scattered islands.

Deyu Onsen, said to have been opened by Kobo Daishi some 1,200 years ago, is regarded as Niigata's oldest spa and forms the Gozu hot spring area with Murasugi and Imaita. Seiko-kan has over 300 years of history, and its three-story wooden building from 1928 is a Registered Tangible Cultural Property. The low-temperature spring of about 30°C flows naturally in modest volume, so the inn heats it with a boiler and feeds it in from the tub floor, free-flowing without recirculation or chlorine, and covers the tubs with lids to keep the precious water warm.

This Radon-sen No. 1 source plays a key role as one of the "Ginsen" (Silver Springs) that color the history of Arima Onsen, one of Japan's three ancient hot springs. In contrast to the reddish-brown "Kinsen" containing iron, Ginsen is characterized by its clear, smooth feel. The water from this source is said to gently stimulate cells and vitalize the body as it's absorbed through breathing.

About 30 minutes by car inland from Kansai International Airport, Matsuba Onsen in Kotsumi, Kaizuka, lies along the Kogi River valley and draws radon-bearing water from the granite beneath the grounds. The source is a cool 26.8°C and is pumped at 83.3 liters per minute. Sodium, chloride and bicarbonate lead its chemistry, yet total dissolved solids are a modest 0.729 g/kg, and its 236.8 Bq/kg of radon classifies it as a simple weak radioactive spring, known locally for warming the body and keeping it warm.

Murasugi Onsen is said to have been found in 1335, when Araki Masataka, a retainer of the Ashikaga clan fleeing war, dreamed of the Healing Buddha seven nights in a row, giving the spa some 700 years of history. Yakushinoyu No. 3 Well carries that name, gushing from a drilled well in Murasugi and supplying the inns of the spa town. Known since the Taisho era for its rich radon, the clear, mild water seems ordinary, yet soaking and breathing its steam lets a gentle warmth spread from within, suiting anyone seeking an unhurried toji stay among quiet cedar groves.

Murasugi Onsen is said to have been found in 1335, when Araki Masataka, a retainer of the Ashikaga clan fleeing war, dreamed of the Healing Buddha seven nights in a row, giving the spa some 700 years of history. Yakushinoyu No. 3 Well carries that name, gushing from a drilled well in Murasugi and supplying the inns of the spa town. Known since the Taisho era for its rich radon, the clear, mild water seems ordinary, yet soaking and breathing its steam lets a gentle warmth spread from within, suiting anyone seeking an unhurried toji stay among quiet cedar groves.

Tochiomata Onsen, in the mountains of Uonuma, is the oldest healing retreat of the Yunotani hot spring area; said to have been found some 1,200 years ago, it is known for the saying "bathe for a week and you won't catch a cold for a year." Tochiomata No. 1 rises naturally from granite ground at around 35°C and flows, unheated and undiluted, into the three shared "sacred spring" baths, Shita-no-Yu, Ue-no-Yu and Oku-no-Yu. Shita-no-Yu sits directly above the source, and the old custom of soaking quietly for one to two hours lives on.

Rising in a district long known by the place name "Yuden", this spring was developed in 1952. It emerges at 20.7°C as a cold mineral spring whose defining trait is a trace of dissolved radon. With only 0.185 g/kg of dissolved solids, the water is light and unassuming, like a quiet companion for unhurried, restorative bathing.

Misasa Onsen is said to have been discovered some 850 years ago when a samurai who spared a white wolf was guided to the spring in a dream, and its name is linked to the belief that "by the third morning, ailments are healed." It is known for radium springs with some of the world's highest radon levels. Rakusen wells up naturally from the ground along the Mitoku River, and hot water rises constantly from the spring pocket at the bottom of the tub.

Misasa Onsen is said to have been discovered some 850 years ago when a samurai who spared a white wolf was guided to the spring in a dream, and its name is linked to the belief that "by the third morning, ailments are healed." It is known for radium springs with some of the world's highest radon levels. Gankutsu-no-Yu uses the natural rocks along the Mitoku River, and water rises on its own from the floor of each of its three tubs, sending small bubbles up around your feet.

Said to have flowed since the Nanboku-cho period, this Amami spring is remembered in the tale of a "paradise bath" that soothed pilgrims bound for Mount Koya. Light in dissolved minerals, it is a radioactive spring carrying radon and a touch of free carbon dioxide. It suits travelers who trace the old Koya Kaido road and want a quiet soak by a clear stream.

Okudogo No. 4-1 Gensen is drawn from the gorge upstream along the Ishite River from Dogo's historic spa town. Its lukewarm 38.7°C water is alkaline at pH 9.4 and pairs hydrosulfide ions with radon, an uncommon combination. A faint sulfur scent and slippery feel suit bathers who like long, unhurried soaks to the sound of the stream.