Radioactive caesium-137, which was released into the sky by the nuclear disaster and then fell to Earth, has a strong affinity for clay minerals, and quickly attached itself to clay particles in the local soils. Since then, rain and snow melt have flushed some of this contaminated sediment down the region's rivers and into local reservoirs. The Ogaki dam, located in the middle of the eastern Fukushima prefecture, was highly contaminated with caesium-137, which has a half-life of 30 years. Today, scientists are looking at ways of containing that contamination.

With that in mind, Susumu Yamada from the Japan Atomic Energy Agency and colleagues have simulated the movement of sediment into and out of the Ogaki dam during typical flood events. The sand and silt sediments quickly settle onto the reservoir floor, but the clay particles remain floating for many weeks after being washed into the reservoir. As a result, the contaminated clay can easily be washed downstream, but the new simulations show that this can be prevented if the height of the reservoir is increased.

"We calculated that by increasing the reservoir water level by 30 m, the amount of clay ultimately discharged from the reservoir can be reduced by a factor of three," said Yamada, whose findings are published in Environmental Research Letters (ERL).

The simulations show that the higher water level increases the period that floodwater spends within the reservoir before it flows downstream. Raising water levels for three months gives the clay particles enough time to settle, locking them up in the sediment at the bottom of the dam and preventing them from travelling downstream and contaminating the newly resettled coastal regions.

Although Yamada and his colleagues have only performed simulations for the Ogaki dam, they believe that the technique could also be used to help prevent the spread of radioactive contamination at other dams in the region: there are more than 10 dams close to the power plant in the eastern Fukushima prefecture.

"A number of these dams are similar to the Ogaki dam reservoir in that there is relatively high contamination in the upper river basin compared with the lower regions near to the coast," he said. "Therefore it is conceivable that contamination migration could be limited in other areas by judicious operation of these dams."

Right now, the Japan Atomic Energy Agency is monitoring the contamination at the Ogaki dam, to better understand the migration patterns of the radioactive material and to validate the results from the simulation. It is still early days, but it seems that careful management of the dams around the Fukushima Daiichi plant could help to speed up the resettlement process downstream.

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