Tons of water exposed to radiation have leaked during a transfer operation at the destroyed Fukushima nuclear plant, owner TEPCO announced Tuesday. The new leak marks the latest in mounting failures to contain the ever-increasing quantity of water contaminated by efforts to cool the plant’s melted reactors.

Following heavy rains, workers pumped trapped water into a temporary tank, a TEPCO spokesperson stated Tuesday. In the process, workers “found water was spilling from the manhole on top of the tank,” releasing an estimated 4 tons of water into the ground, according to the spokesperson.

“The water itself was rain water. But it was from the Fukushima Daiichi nuclear plant and could contain radioactive materials,” the spokesperson said. “The water seeped into the ground.”

The leak comes amid growing signs of structural flaws with the temporary tanks where the radioactive water is currently being held, including constant reports that leaks are releasing the water into the groundwater, and by extension, the sea. TEPCO says it has resorted to patching tank leaks with plastic tape.