Authorities and environmentalists in Zambia fear the long-term impact of an acid spill at a Chinese-owned mine that contaminated a major river and could potentially affect millions of people after signs of pollution were detected at least 100 kilometers (60 miles) downstream.

The spill happened on Feb. 18 when a tailings dam that holds acidic waste from a copper mine in the north of the country collapsed, according to investigators from the Engineering Institution of Zambia.