United Utilities said this was caused by harmless sediment which normally lies at the bottom of water pipes and which can get stirred up due to the changes in pressure when a main bursts.

BBC: People evacuated after pipe bursts in Huyton

Since cesium binds to the sediment on the ocean floor, the radiation has likely had the biggest effect on bottom-dwelling fish and other animals though the consequences still aren't clear, said Tatsuo Aono, a specialist at Japan's National Institute of Radiological Sciences.

WSJ: In Japan, Relief at Radiation's Low Toll