Devecser, Hungary (CNN) -- Rescue workers searched Wednesday for six elderly people missing at Kolontar, one of three villages in southwest Hungary that was hit Monday by a wave of toxic red sludge from an alumina plant reservoir that burst.
Wearing chemical protection suits, the workers used metal sticks to poke through muck three-feet deep (1 meter) for the presumed victims, reported MTI, Hungary's official news agency.
At least 116 people were injured, eight of them seriously, when the mishap occurred Monday afternoon, the agency said. Most of them were flown to hospitals in the capital, Budapest.
The reservoir has been repaired and the flow from the pool has halted.
But the material that flowed out of the reservoir continued to pose a threat. On Wednesday, more than 500 National Disaster Management Authority staffers and soldiers and employees of Hungarian Aluminum Production and Trade Company (MAL), the company that owns the alumina plant's reservoir, were trying to halt the advance of the sludge before it reaches the Danube River's tributaries, said Jeno Lasztovicza, head of the defense committee, according to MTI.
The sludge had already reached the Marcal River, which flows into the River Raba, which empties into the Danube. It was expected to show up in the Danube as soon as this weekend, said Imre Szakacs, head of Gyor-Moson-Sopron County's defense authority, MTI said.
Emergency workers were pouring plaster and fertilizers into the Marcal River in hopes that it would bind with the sludge and counter its alkalinity before it reached the Danube, the continent's second-longest river, some 70 km (43 miles) north, reported MTI.
But the material will have been neutralized by the time it reaches the Raba, Interior Minister Sandor Pinter told reporters.
Untreated, the sludge contains heavy metals, which cause burns and eye irritation, he said.
Four people -- two children ages 1 and 3, an elderly woman and a 35-year-old man whose SUV overturned in the sludge -- have been confirmed dead in the environmental disaster, which occurred 160 km (99 miles) west of Budapest, near the town of Ajka.
Residents were advised not to eat produce from gardens that were covered when the dam burst, releasing at least 1 million cubic meters of thick red mud.
It was not clear when residents evacuated from affected areas in the villages of Kolontar, Devecser and Somlovasarhely would be able to return home, nor were the long-term consequences clear.
A state of emergency has been declared in three counties, the State Secretariat of Governmental Communications said.
In a statement on its website, MAL said it "offers its honest condolences to the relatives of all of the victims who lost their lives in the catastrophe."
CNN's Nic Robertson contributed to this story.