Optical remote sensing of large-scale water pollution in Angola and DR Congo caused by the Catoca mine tailings spill

Billions of tons of hazardous mine waste are stored in thousands of tailings storage facilities around the world. These impoundments represent one of the most important environmental risk factors of industrial mining, since occasional tailings spills or dam failures cause devastating impacts on humans and ecosystems, specifically along river corridors. In this study, we developed a satellite remote sensing methodology to assess the impacts of tailings spills on water quality focusing on the controversial incident that occurred at the Catoca diamond mine in Angola in late July 2021. The spill allegedly caused important river pollution in neighbouring Democratic Republic of the Congo (DR Congo) and led to public health concerns including the loss of human lives – however the mining company denied any responsibility. We processed high resolution imagery acquired by ESA’s Sentinel-2 satellites using the Python package Acolite for atmospheric correction and turbidity retrieval, and applied a river skeletonizing algorithm to automatically extract turbidity values for the entire river system. This allowed tracking the propagation of the pollution front from the source at the Catoca mine through the Tshikapa- and the Kasaï River during more than one month and across 1400 km, until the pollution front finally dissipated after discharging into the Congo River. We further analyzed a 6-year time series of virtual stations in the Tshikapa River located up- and downstream of the effluent discharge to compare the impacts of the tailings spill to seasonal variabilities of water quality. Turbidity values caused by the spill largely exceeded the seasonal variability in the Tshikapa River in recent years. These findings confirm that the Catoca tailings spill has significantly affected water quality of the Tshikapa- and the Kasaï River with total suspended solids concentrations that were several 10-fold above drinking water standards in Lunda Norte Province, Angola, and Kasaï Province, DR Congo, making severe public health impacts for residents and fish kills highly probable. After investigating whether this methodology could be applied to other tailings dam failures that have occurred since the Sentinel-2 mission began in 2015, we recommend to apply it to four other incidents in Mexico, Myanmar, Peru and China, respectively. Overall, this Sentinel-2 workflow provides the opportunity to assess the large-scale impacts of pollution incidents in mining areas around the world in locations where hydrological- and water quality data are scarce and monitoring capacities are limited.