An alleged acid mine spill from a mine in northern Mexico went unreported for nearly 24 hours by the mine and the spill reached a river that supplies water to tens of thousands of people, The Associated Press reported.

Carlos Arias, director of civil defense for the northern state of Sonora, said the spill at Groupo Mexico’s Buenavista Mine in Cananea, Sonora was caused by defects in newly constructed leaching or holding ponds.

Arias said a pipe either blew out or became unseated on Aug. 7, allowing about 40,000 m3 (10 million gal) of mining acids to flow downstream into a river.

Arias said the sulfuric acid spill was detected by residents downstream the next day, and that the mine operators had not notified state authorities, according to The Associated Press.

Arias said tests have revealed the spill contains pollutants like arsenic above acceptable levels. Water supplies from the river have been cut off to about 20,000 people.

Arturo Rodriguez, the head of industrial inspection for the Attorney General for Environmental Protection, said the mine did advise his agency almost a day after the spill, just within the 24-hour time limit for filing such reports.

He said lax supervision at the mine, along with rains and construction defects, appeared to have caused the spill.