(UPDATE) It turned out to be apple juice. Someone called the agency in Iwate who did the testing and asked. Why didn't they just say it upfront, that it was apple juice? That agency will probably be scolded by the Iwate prefectural government for sharing the information.
People have been freaking out on their own speculation that radioactive cesium must have been from the water itself.
No details other than it was "soft drink" bottled in Ichinoseki City in Iwate Prefecture, whose contamination had been initially denied by the city officials in the early days of the nuclear accident. It could be juice, it could be plain water. I can't find information on the Iwaki Prefecture's website.
One bottle tested had 4.1 Bq/kg of radioactive cesium, the other had 2.2 Bq/kg. The national safety standard for radioactive cesium is 10 Bq/kg.
In pre-Fukushima Japan, the level of radioactive cesium (Cs-137) in tap water in Iwate Prefecture was ND (not detected), according to the database by Japan Chemical Analysis Center.
Ministry of Health and Welfare's data on January 15, 2013 says these bottles of soft drink were sampled from those sold in retail stores: