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Rescue workers in Hungary have found three more victims of the catastrophic flood of toxic sludge, raising the death toll to seven.

The body of an 81-year-old man who died from injuries sustained in the torrent and two other bodies were found Friday afternoon on the outskirts of the village of Devecser, a town in western Hungary inundated by the sludge.

The unidentified victims were most likely residents missing from the nearby town of Kolontar, said rescue agency spokesman Tibor Dobson. One more Kolontar resident is still missing.

Authorities said the concentration of toxic heavy metals where the spill entered the Danube had dropped to the level allowed in drinking water. The test results released by the National Catastrophe Management Directorate on Friday further allay concerns that the river would be significantly polluted, but environmentalists and local residents are worried about possible long-term effects.

Red mud: Toxic waste of aluminum refining

The torrent of sludge poured through several communities Monday after a reservoir at an alumina plant in Ajka burst, injuring hundreds. 

The slurry reached the Danube River on Thursday, prompting environmental officials in Hungary and neighbouring countries to step up their environmental monitoring.

On Friday, Prime Minister Viktor Orban told the MTI news agency that officials managed to take control of the situation in time to prevent major damage to the Danube.

Rivers and creeks surrounding the breached reservoir were badly damaged, but the consequences in the Danube "do not seem to be that dramatic," said Philip Weller, who heads the International Commission for the Protection of the Danube.

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Residents stand on a street Friday during the cleanup of the Hungarian village of Devecser. The village was one of several flooded by sludge from the burst reservoir of an alumina plant owned by the Hungarian Aluminum Production and Trade Company. ((Bernadett Szabo/Reuters))

Tibor Dobson, chief of Hungary's disaster relief services, told Reuters on Friday that pollution levels in the Danube were decreasing.

"The latest measurements show that the values for alkaline content are only slightly above normal," Budapest-based reporter Henk Hirs told CBC News.  

"That is only because upstream they are still pouring acids and plaster into the branches of the Danube to neutralize the effects of the alkaline but it seems to be working."

Meanwhile, government officials lowered their estimate of the size of Monday's catastrophic spill, saying Friday that the alumina plant dumped up to 700,000 cubic metres on surrounding villages. That's a decrease from the original estimate of one million cubic metres.

Emergency crews and environmental officials were still on the scene Friday, working to contain and neutralize the remaining sludge. Teams were also on hand to drain a second reservoir near the one that burst in the hopes of avoiding another disaster.

Police and other officials are still trying to determine what caused the reservoir to burst.

Environmental monitoring continues

As the cleanup continues, officials are still trying to determine exactly what risks the sludge poses — particularly in the towns surrounding the burst reservoir.

The Hungarian Academy of Sciences said while the material was a continued hazard, its heavy metal concentrations were not considered dangerous to the environment.

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Firefighters in protective gear discard belongings from a house damaged by flooding toxic mud in the village of Kolontar on Thursday. ((Bela Szandelszky/Associated Press))

"The academy can say whatever it wants," fumed Barbara Szalai Szita, who lives in Devecser, one of the hardest-hit villages. "All I know is that if I spend 30 minutes outside, I get a foul taste in my mouth and my tongue feels strange."

Hungary's environment minister, Zoltan Illes, said the sludge is covering 41-square-kilometre swath of countryside does have "a high content of heavy metals."  He warned of possible environmental hazards, particularly to groundwater systems.

Meanwhile, Greenpeace said Friday that there were "surprisingly high" levels of arsenic and mercury in the sludge. They based their findings on samples taken Tuesday from the town of Kolontar, one of the communities affected by the flood.

Greenpeace officials told reporters Thursday that the detected arsenic concentration is twice that normally found in so-called red mud. Analysis of water in a canal also found arsenic levels 25 times the limit for drinking water.

The owner of the plant, Magyar Aluminium Termelo es Kereskedelmi (the Hungarian Aluminum Production and Trade Company), has offered its condolences to families affected by the flood but maintains that it didn't do anything wrong, the BBC reported.

The company also said it is working alongside officials to help with the cleanup effort.

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With files from The Associated Press