The River Danube has been apparently absorbing the toxic red sludge with little immediate harm, officials reported - even though the amount of caustic slurry spewed over western Hungary was nearly as great as the Gulf of Mexico oil spill.
Revising even higher earlier estimates, government officials said the reservoir break on Monday dumped 600,000 to 700,000 cubic metres (158 million to 184 million gallons) of sludge onto three villages - just barely less in a few hours than the 200 million gallons the blown-out BP oil well gushed into the Gulf over the course of several months starting in April.
"The consequences do not seem to be that dramatic," said Philip Weller, who heads the International Commission for the Protection of the Danube said by telephone from Vienna, when asked about harm to the waterways ecosystem up to now.
The risk of pervasive and lasting environmental harm remained nonetheless, with laboratory analyses organised by Greenpeace showing high concentrations of toxic substances in samples taken from the sludge.
Greenpeace told reporters in Vienna that the samples taken a day after the spill showed "surprisingly high" levels - 110 milligrams of arsenic and 1.3 milligrams of mercury per kilogram (2.2 pounds) of dry matter.
The results, which also show 660 milligrams of chrome per kilogram, are based on analyses carried out in laboratories in Vienna and in the Hungarian capital, Budapest.
Very roughly speaking, that translates into 50 tons of arsenic, 300 tons of chrome and 500 kilograms, or a half ton, of mercury set free by the spill, Greenpeace officials told reporters in Vienna on Thursday.
Greenpeace officials said the detected arsenic concentration is twice the amount normally found in so-called red mud. Analysis of water in a canal near the spill also found arsenic levels 25 times the limit for drinking water.
Hungary's state secretary for the environment, Zoltan Illes, said the henna-coloured sludge covering a 16-square-mile (41-square-kilometre) swathe of countryside has "a high content of heavy metals," some of which can cause cancer. He warned of possible environmental hazards, particularly to groundwater systems.
Officials with Hungary's national disaster relief service, meanwhile, said that a fifth person - an 81-year-old man - has died from unspecified injuries sustained in the flooding.