Radioactive river mud threatens lakes, Tokyo Bay

July 05, 2012


Lakes across eastern Japan are being contaminated with radioactive cesium from the crippled Fukushima No. 1 nuclear power plant, and scientists are warning of a growing problem in Tokyo Bay.

Radioactive mud carried down rivers is slowly accumulating in the lakes, in some cases making fish and shellfish dangerous to eat.

In March, a maximum cesium concentration of 9,550 becquerels per kilogram was detected in mud on the bottom of the Bizengawa river, 1.65 kilometers from where it flows into Japan’s second-largest lake, Lake Kasumigaura in Ibaraki Prefecture.

A month later, the highest reading was 800 meters closer to the lake and had increased to 9,980 becquerels per kilogram.

Hiroshi Iijima, who heads the Asaza Fund nonprofit organization, which conducted the surveys, has asked the central and prefectural governments to put cesium-absorbing zeolites in the lake and set up a temporary dam to stop the mud flowing from the river.

Ibaraki Prefecture is known for producing the largest eel catch in Japan. In May, the central government suspended shipments of eels caught in Kasumigaura and other locations in Ibaraki Prefecture after cesium levels exceeding the government standard of 100 becquerels per kilogram for food were detected.

In fiscal 2011, cesium levels over 100 becquerels per kilogram were found in fish and shellfish caught in the lake in eight of 71 surveys. The frequency increased to 28 of 87 surveys in the current fiscal year.

The prefectural government on April 1 asked fishermen to refrain from shipping three other fish from two rivers and other locations due to high levels of cesium.

Since April, cesium levels over 100 becquerels per kilogram have also been found in fish and shellfish in Lake Numazawako, Lake Inawashiroko and Lake Akimotoko in Fukushima Prefecture, Hinuma marsh in Ibaraki Prefecture, Teganuma marsh in Chiba Prefecture and Lake Chuzenjiko in Tochigi Prefecture.

“Despite decontamination work, radioactivity could remain in lakes as long as cesium flows in,” an Environment Ministry official said. “While giving priority to decontamination efforts on land, we want to find out to what extent radioactive materials will move to fish through the rivers.”

Scientists say freshwater fish tend to retain ingested cesium longer than their saltwater counterparts. They do not discharge as much of the material due to low osmotic pressure between their bodies and surrounding waters.

The Environment Ministry found higher cesium concentrations in fish and water insects in lakes and rivers than sea life in a survey in Fukushima Prefecture from December to February.

The highest level was 2,600 becquerels per kilogram in a type of rhinogobius in Manogawa river north of the Fukushima No. 1 nuclear power plant. The concentrations in most saltwater fish were below 100 becquerels.

Meanwhile, Yosuke Yamashiki, an associate professor of environmental engineering at Kyoto University, estimates that cesium concentrations at the bottom of Tokyo Bay will peak in 2014 and then remain roughly stable through 2021.

His modeling predicts that levels will rise to 300-500 becquerels per kilogram of sand at the bottom of the bay near the mouths of Edogawa and Arakawa rivers in March 2014. Those rivers flow through areas with relatively high radiation levels.

Hotspots with cesium levels of 4,000 becquerels per kilogram are possible, Yamashiki said.

“Even if no impact of radiation has yet to be found on fish and shellfish, we cannot tell what will happen in the future,” he said. “We need to begin to prevent contamination immediately by reducing the amount of sand flowing into the bay.”

Sand containing cesium tends to accumulate in Tokyo Bay because it has a relatively narrow opening to the Pacific Ocean.

Yamashiki simulated sand and mud movements in Tokyo Bay and rivers flowing into the bay since March 2011 and used cesium concentrations in soil measured by the government to produce his estimates.

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Local residents enjoy fishing where the Bizengawa river flows into Lake Kasumigaura in Tsuchiura, Ibaraki Prefecture. (The Asahi Shimbun)

Local residents enjoy fishing where the Bizengawa river flows into Lake Kasumigaura in Tsuchiura, Ibaraki Prefecture. (The Asahi Shimbun)

  • Local residents enjoy fishing where the Bizengawa river flows into Lake Kasumigaura in Tsuchiura, Ibaraki Prefecture. (The Asahi Shimbun)
  • A veteran fisherman said in May that he could not go fishing in Lake Kasumigaura after high levels of cesium were found in fish. (Asahi Shimbun file photo)
  • The Asahi Shimbun

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