"The entire ecosystem of the Marcal river has been destroyed, because the very high alkaline levels have killed everything," Tibor Dobson, head of the regional disaster relief services, told the Hungarian news agency MTI.
"All the fish are dead and we haven't been able to save the vegetation either," he said.
"We've tried to lower the alkaline levels at several points on the Marcal with acid and gypsum. But it's been in vain," he said, saying the aim was to bring the alkaline levels below a pH level of 9 in the Raba and Danube rivers so as to save their ecosystems.
On a scale of 1-14, pH values of 1-6 are acid, between 6 and 8 are neutral, and readings of 8-14 are alkaline.The pollution comes from a toxic mud spill that occurred on Monday in Ajka, 165 km west of Budapest, when the containing walls of a residue reservoir at an alumina plant burst, sending 1.1 million cubic metres of toxic red sludge into surrounding villages.
Four people died in one of the villages, Kolontar, from where the tiny Torna stream flows into the Marcal.
The Marcal is a tributary to the Raba, which in turn flows into the Danube, Europe's second longest river.The pollution reached the main branch of the Danube at midday (1000 GMT). But so far, there have been no sightings of dead fish in either the Raba or the Danube.