Hungary's toxic sludge spill, which has killed four people, reached the Danube river Thursday, threatening to contaminate the waterway's entire ecosystem, officials told AFP.
"The red mud pollution has reached the Danube -- its so-called Mosoni Branch, about 10 kilometers (six miles) from the main branch of the river -- this morning," said Tibor Dobson, the local head of the disaster relief services.
"At 09:27 am (0727 GMT), the pH level stood at 9.3. The experts are still measuring the pollution levels and the pH levels are descending."
Water authority official Jozsef Toth told AFP earlier that samples taken at the confluence of the Raba river and the Danube showed "alkalinity slightly above normal, with a pH value of 8.96-9.07," against a normal tally of 8.0.
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.
A wave of toxic mud was unleashed Monday from the reservoir of an alumina plant at Ajka, 160 kilometers (100 miles) west of Budapest.
The red mud travelled down the Raba river and reached the Danube waters at around 0630 GMT at Gyor.
The industrial accident triggered by the collapse of walls at the factory reservoir on Monday has been described as an ecological disaster and is now threatening the entire ecosystem of the Danube, Europe's second longest river which runs from Hungary through Croatia, Serbia, Bulgaria, Romania and Ukraine before flowing into the Black Sea.
Three adults and a child died when the tidal wave of mud smashed into surrounding villages on Monday. More than 120 people were injured, suffering from burns, and three people are still missing.
At two of the hardest hit villages, Devecser and Kolontar, water samples from the river Torna, which flows into the Raba, showed pH levels of around 10 on Thursday, but these were falling, Dobson said.
On Monday, after the accident happened, the reading was pH 13.5.
Dobson said neutralizing efforts, using acid and gypsum, were helping to bring the pH levels down.
Groundwater in the wells in the region showed neutral readings of pH 7.5.
Earlier, Prime Minister Viktor Orban visited Kolontar -- where all four victims died -- to see the extent of the destruction.
The mud -- a residue from aluminum production which is highly corrosive and contains toxic heavy metals such as lead, cadmium, arsenic and chromium -- destroyed all vegetation other than trees and seeped into hundreds of houses in villages.
Witnesses say the force of the wave was so strong that cars were sucked out from their garages and it tore out windows and doors, covering everything.
A bridge in Kolontar was swept away and the army had to build a temporary one to replace it.
Orban suggested reconstruction could be very problematic.
"Unfortunately, my impression is that any reconstruction, beyond the bridge, would be useless.
"It'll probably be necessary to clear a new area for the villages and pull down the destroyed part for good. It'll be impossible for people to live here," Orban said.