Tests at Japan’s Fukushima power plant have revealed a 90-fold increase in caesium-134 in groundwater at the site between July 5 and July 7.

Tepco, which runs the facility that was badly damaged by the 2011 earthquake and tsunami, said in a press release that the surge had been detected at an observation well between a damaged reactor and the sea.

The record spike in caesium-134 comes after recent discoveries of high levels of other radioactive elements such as tritium and strontium. It all suggests that contaminated water could be spreading through the plant towards the Pacific Ocean.

Masao Yoshida, the plant’s chief at the time of the tsunami, has died of oesophageal cancer at the age of 58. He led efforts to stabilise the crippled site in the wake of the disaster.

Tepco said Yoshida’s illness was not connected to his duties at Fukushima.