By MARK STEVENSON, Associated Press

MEXICO CITY (AP) — A civil defense official says a private mine in northern Mexico did not immediately report a massive acid spill, allowing it to flow into a river that supplies water to tens of thousands of people.

Carlos Arias, director of civil defense for the northern state of Sonora, said the spill at a copper mine near the U.S. border was caused by defects in newly constructed leaching or holding ponds.

Such ponds hold the overflow of acids used to leach metal out of crushed rock. But Arias said a pipe either blew out or became unseated on Aug. 7, allowing about 10 million gallons (40,000 cubic meters) of mining acids to flow downstream into a river.

"Definitely ... it was an error" in the design or construction of the retaining pond, Arias said.

He said the sulfuric acid spill was detected by residents downstream the next day, and that the mine operators hadn't notified state authorities.

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.

The Grupo Mexico company operates the Buenavista mine in Cananea, Sonora. The company did not respond to requests for comment on Tuesday.

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. Rodriguez said mine operators should have been able to detect the leak before such a large quantity leaked into the river.

Authorities are trying to de-acidify the water in the Bacanuchi and Sonora rivers by adding calcium, Arias said. "What you can't get rid of are the heavy metals," he noted.