A dam in a southwestern Bolivian mining town that helped keep dangerous pollutants out of a major regional waterway has burst, sparking an environmental emergency, officials said.

"The lower part of the dam broke -- there was some sort of explosion -- which caused the water to rush into the Pilcomayo," said Jose Luis Rios, a local prosecutor, speaking to Bolivian radio about the accident, which he said took place on Thursday.

The private Santiago Apostol mine extracts lead, silver and zinc.

Rios said authorities are investigating what caused the dam at the mine to fail.

Officials said residue from the facility risks despoiling the Pilcomayo River, which runs through Bolivia to Argentina and Paraguay.

The Pilcomayo also flows into the Paraguay River, one of South America's most important waterways.