Workers in Hungary are racing to build an emergency dam as cracks in a reservoir widened, threatening to unleash a second spill of toxic sludge on the village of Kolontar and nearby rivers.
A million cubic metres of the waste material leaked out of an aluminum plant reservoir into villages and waterways last Monday, killing seven people and injuring 123 others. Nearly a week into the disaster, one person was still missing.
Kolontar was evacuated on Saturday after cracks appeared in the northern wall of the reservoir, threatening a second spill of the toxic red sludge.
A by-product of alumina production, the thick, highly alkaline substance has a caustic effect on the skin. It contains heavy metals, such as lead, and is slightly radioactive. Inhaling its dust can cause lung cancer.
News agency MTI quoted environment state secretary Zoltan Illes as saying a 25-metre-long crack in the weakened wall of the sludge reservoir had widened slightly further by Sunday and the wall now looked beyond repair.
Mr Illes said the northern wall of the reservoir could collapse "within one day or a week" and crews at the scene were scrambling to complete a new dam to protect Kolontar and the nearby town of Devecser, home to 5,400 people.
He said authorities had amassed plaster and various types of acids along 100 kilometres of the river Marcal to neutralise the high alkaline content of any new spill before it reaches the Danube, a major European waterway.
The Interior Ministry said on its website that samples taken on Sunday showed alkaline levels in smaller rivers affected by Monday's spill, and in the Danube, had returned to normal.
Gyorgyi Tottos, a spokeswoman for disaster crews, said authorities hoped to complete the 600-metre emergency dam within days to stop 500,000 cubic metres of sludge still in the reservoir from escaping.
Late on Sunday, the Veszprem county disaster unit advised crews and residents in the area affected by Monday's spill to wear protective masks and glasses as dust levels in some places had exceeded the health limit as the sludge was drying out. It said the dust can irritate airways due to its alkalinity.