On 27 August, intense rainfall caused the overflow of two retention basins containing effluent from a copper and silver mine that had been operated between 1980 and 1997 and then abandoned. The structures involved were 120 m high and offered a total capacity of 110 million m³ of consolidated tailings. An inspection noted the state of weir overflow and ensuing erosion, as well as the discharge of effluent into MAPANUEPE Lake and the SAINT-THOMAS River downstream.

On 5 September, the Department of the Environment and Natural Resources (DENR) considered the prospect of a sudden rupture of both dams highly unlikely and, as such, issued an opinion that MAPANUEPE Lake was capable of withstanding the eventual overload caused by the release of a maximum water volume (estimated to equal 9 million m³).

On 11 September at 1 pm, a leak appeared at the level of the damaged weirs, resulting in a small volume discharge of sludge. 250 families from 3 neighbouring villages were evacuated out of precaution, followed by another 750 families on 12 September due to the incessant pounding rains. The operator called another mining company to request substantial backup pumping resources in order to remove water from the basin and thereafter undertook the necessary repair work.

High-intensity rainfall observed during the months of July and August were responsible for a rise in water volume inside the upper retention facility, to a point of exceding the weir discharge capacity. The mine had been abandoned for 3 years ahead of the initially scheduled date as a result of slope instabilities and annual flooding.