Radioactive
river mud threatens lakes, Tokyo Bay
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
5
July, 2012, 2012
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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