From IAEA, excerpt :
" IAEA Briefing on Fukushima Nuclear Accident (5 April 2011, 20:25 UTC)
Japan Begins Discharge of Low Level Radioactive Water
Japanese authorities have confirmed to the IAEA that they began to discharge 11,500 tonnes of low level radioactive water from the Fukushima Daiichi nuclear power plant into the sea on 4 April. The operation is being conducted to create storage capacity for highly radioactive water that has pooled in parts of the reactor facility, hindering efforts to restore electrical power from the grid to the facility.
Japanese officials have reported that they plan to release 10,000 tonnes of water from a waste treatment facility and 1,500 tonnes from drainage pits around reactor Units 5 and 6. The operation is expected to last no more than five days."