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."

A closer look at the situation, though, shows the reality is a lot more complex. Katsuhiko Sato, an official at the ministry's water environment section, says that the ministry cannot explain why the levels of cesiums in Kasumigaura seem to have leveled off. "To be honest we don't know," Sato said. "The figures are somewhat inconsistent."

Sato even hinted that the ministry's sampling of soils might not have been sophisticated enough. "We don't know if we can get the same kinds of samples each time," he said. "Cesium is known to stick to solid substances such as sludge and mud. The finer the grains are, the more cesium it absorbs. We try to pick the same kind of samples each time, but by just looking at them. Grains of sand vary from 0.06 mm to 0.3 mm. (A detailed analysis of the samples) would take a very long time, and at present, we haven't been able to do that."

Sato said the ministry has no plans to increase the number of radiation monitoring points or the frequency of the tests, citing "a limited budget," but claimed that the current levels of contamination pose no health risk for the area's residents, because radiation in the lake and the rivers is shielded by water. "The government priority is to decontaminate (irradiated) land spaces," he added.

Radiation on the lake's bottom has hit the local fisheries industry hard, however. Five species of fish, including eel, American catfish and carp have been banned from the market as samplings in those animals showed levels of cesium surpassing the government-set exposure limit of 100 Bq/kg. And while drinking water sourced from Kasumigaura is technically safe now, dry solids that are produced in the water sedimentation process contains cesium, according to the Ibaraki Prefectural Government.

Iijima from Asaza Fund says the government monitoring of radiation levels is far from satisfactory, as it only surveys one location per river. For its part, in cooperation with other citizens' groups and a local food-delivery cooperative, his own group measured radiation levels at some 200 locations in March-April and again in October covering up to 20 locations in one river. The results have shown that, over the six-month period, radioactive substances are believed to have traveled downstream, as figures of cesium-134 and cesium-137 in upstream locations have gone down while those downstream have increased. The highest level of cesium contamination so far detected by the group's volunteers is along the Onogawa River, which snakes through the cities of Ibaraki Prefecture and runs into Kasumigaura's Lake Nishiura, where, close to the river banks in the city of Ushiku, 13,200 Bq/kg of cesium was detected in a sample of sludge in May.

"What we have found is that there are 'hot spots' in the rivers as well as on land," Iijima said. "Measures should be taken to stop cesium from moving into the lake, because once it's absorbed into the lake, it will be too hard to track and collect."

Atsunobu Hamada, former director of the government-affiliated Ibaraki Freshwater Fisheries Research Institute, meanwhile, argues that preventing some cesium from making its way into the lake is impossible.

Both Hamada and Iijima maintain that the inevitable solution would be to release Kasumigaura's cesium into the Pacific Ocean via the Tone River, which the lake feeds. But to do that would involve a change in the national government's water management policy. The 250-meter-wide Hitachigawa Water Gate was built in 1963 at the southern end of Kasumigaura to store water and keep seawater out, thereby stopping salted water from damaging the area's crops, preventing floods and securing enough water for the region's industrial complexes. This might have worked while demands for industrial water were high —due to the booming economy of yesteryear — but it's now out of date, Hamada says, adding that the water quality has greatly suffered over the years from the policy of closing the water gate into the lake.

"In our negotiations with the Ibaraki government, we have repeatedly asked that that the gate be opened (to keep cesium from being accumulated further)," said Hamada, who now serves as secretary general of the environmental nonprofit Kasumigaura Academy. "We pressed the prefectural government until it finally said it'll keep monitoring the cesium levels and base their future decisions on the results of the monitoring. But it will be too late if we wait until the results come out."

Iijima says that institutions and individuals with a stake in the future of the lake, not just government agencies but people from the private sector and citizens' groups like his, should band together to investigate and deal with the issue. A systematic and comprehensive monitoring of cesium movement along the rivers and across the lake would only be possible through such collaboration, he said.

Unfortunately, Iijima says there is little sign of that happening now — and the group's letters to the Ibaraki prefectural government demanding joint action on Kasumigaura have fallen on deaf ears.

"We have a potential disaster waiting to happen," he said. "This is a lake in the Tokyo metropolitan area and the second-largest lake in Japan, and we are sitting idly by, letting it get contaminated."