TEPCO said it first detected the leak in the first week of April 2013, when radiation levels spiked in samples taken from water outside the tank, indicating a likely leak. TEPCO is in the process of moving the toxic water to another nearby storage tank.
Fish caught near the plant have had high levels of radiation since the meltdown, during which contaminated water from the facility spilled in to the sea several times, according to reports.
Radioactive water associated with the meltdown at Fukushima is believed to have been leaking continuously into the sea via an underground water system as well.
TEPCO has also recently reported the loss of power to Fukushima's spent fuel ponds. The outages were suspected to have been caused by a shorted out control board.
Plant operators have been working to contain damaged to Fukushima plant since the March 11, 2011 earthquake and tsunami caused a trio of meltdowns at the plant.