"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."