This is due to the contaminated water overflowing from the seaside of Reactor 1 ~ 4 to Fukushima plant port.
They also announced 2 Billion Bq of Cesium-137 and 1 Billion Bq of Tritium flow to the sea every single day as well.
Fukushima plant port is not separated from the Pacific. Discharged nuclide naturally spreads to the sea.
Source: Fukushima Diary