The wall of a reservoir filled with caustic red sludge will inevitably collapse and unleash a new deluge that could flow about a half-mile to the north, a Hungarian official has said.
That would flood parts of the town already hit by the industrial waste on Monday but stop short of the next town to the north.
Environmental State Secretary Zoltan Illes said recently discovered cracks on the northern wall of the reservoir at the alumina plant have temporarily stopped widening because of favourable weather conditions but will continue to expand, especially at night.
Disaster agency spokesman Tibor Dobson said engineers didn't detect any new cracks overnight, and the older cracks were being repaired, but that it was too soon to consider lowering the current state of alert. Protective walls were being built around the reservoir's damaged area to hold back any further spills.
"I would describe the situation as hopeful, but nothing has really changed," Mr Dobson said.
"The wall to protect Kolontar is planned to be finished by tonight, but it will likely be several days before residents may be able to move back."
Nearly all of Kolontar's 800 residents were evacuated on Saturday, when Prime Minister Viktor Orban said the north wall of the massive storage pool was expected to "very likely" collapse after cracks were detected at several points along the dam.
The roughly 6,000 residents of neighbouring Devecser, just north of Kolontar, were told by police to pack a single bag and get ready to leave at a moment's notice.
"This hasn't changed," Mr Dobson said. "We are still on guard in case of any more spills."
Red sludge is a by-product of the refining of bauxite into alumina, the basic material for manufacturing aluminium. Treated sludge is often stored in ponds where the water eventually evaporates, leaving behind a largely safe red clay. Industry experts say the sludge in Hungary appears to have been insufficiently treated, if at all, meaning it remained highly caustic.