Lake Kasumigaura in Ibaraki Prefecture is facing an environmental threat that has essentially turned it into a time bomb ticking away 60 km northeast of Tokyo.
Experts warn that Japan's second largest lake with a surface area of 220 sq. km is quietly but steadfastly accumulating radioactive cesium released from the crippled Fukushima No. 1 nuclear power plant.
It's no big surprise. The lake's catchment area is huge, covering 2,200 sq. km across 24 municipalities in Ibaraki, Chiba and Tochigi prefectures. It doesn't take a genius to understand that the radiation that fell across some of the Tohoku region, and beyond, in the wake of the March 2011 nuclear disaster found its way into the area's rivers and thus flowed into the lake. In addition to that, Lake Kasumigaura, which is the name given to three contiguous lakes (the largest is Lake Nishiura and the other two are called Kitaura and Sotonasakaura), is a closed lake with no outflow. That means incoming radioactive substances have nowhere else to go.
More disturbing than this, however, is that 20 months after the nuclear crisis, government agencies have shown no signs that they are trying to prevent the accumulation of cesium in the lake — which is not only rich with fishery resources but whose water is used for irrigation, industrial purposes, and even for consumption as drinking water for 960,000 people in Ibaraki Prefecture. Furthermore, no one knows how and by how much the problem has worsened over the months, except for one obvious thing: it hasn't gone away.
Hiroshi Iijima, director general of the nonprofit organization Asaza Fund in Ushiku, Ibaraki Prefecture, has tried to alert the public to the situation for months. "What's unique about Kasumigaura, as opposed to other lakes across the nation, is that it's fed by numerous small rivers and streams, not only the 56 rivers running directly into the lake but also hundreds of tributaries," Iijima told The Japan Times. "Also, the area is flat, meaning that the radioactive substances travel downstream very slowly; they will accumulate in the lake over a long period of time."
In the aftermath of the Fukushima disaster, the Environment Ministry and the Ibaraki Prefectural Government have been measuring cesium levels in mud and sludge once every three months at eight sample points in the lake and at 56 sample points at the bottom of the rivers flowing into it. According to the latest round of monitoring, which was the fourth of its kind, and carried out in September and October, no traceable amount of cesium was detected in the water itself. The mud samples from the lake and the rivers, meanwhile, were found to contain up to 5,200 becquerels/kg of cesium-134 and cesium-137, compared with a maximum of 500 Bq/kg detected a year ago, a maximum of 5,800 Bq/kg in February this year and a maximum of 4,800 Bq/kg in July. The sludge sampled from the bottom of the lake registered cesium contamination ranging from 97 Bq/kg to 520 Bq/kg. That is lower than the maximum 1,300 Bq/kg registered in February, but higher than the 340 Bq/kg detected in the first round of monitoring a year ago. The government safety limit for cesium-tainted food is 10 Bq/kg for water and 100 Bq/kg for most other foods. The mud samples from Kasumigaura have surpassed these figures, but mud is usually not ingested as food. Government regulations state that soil containing more than 8,000 Bq/kg of cesium is considered to emit levels of radiation that pose a danger to human health and therefore must be sealed away.
Based on those results, the Environment Ministry concluded in a report released Oct. 30 that the overall figures show that the contamination of rivers, the lake and water supply sites (in and around Kasumigaura) "has leveled off, or is in a downward trend."