Copper biogeochemistry in an abandoned mine site from a tropical critical zone (Brazil)

Mining activities are one of the most harmful anthropic activities in natural systems, causing large-scale ecosystem degradation, and affecting different critical zone compartments. Recently, Brazil drew attention from the international community due to a large mining disaster involving mining wastes (Rio Doce watershed). Another harmful mining impact occurred at the Pedra Verde (PV) copper mine (in NE-Brazil), where tons of waste rocks were abandoned in an open pit over 30 years, which caused significant impacts on soils, plants, water, and aquatic macroinvertebrates. Copper concentrations at the Pedra Verde mining waste reached up to 350,000 mg kg^-1, which is mostly associated to carbonates (42%), oxides (47%), and sulphides (9%). Weathering of copper minerals trigger several biogeochemical reactions that govern the mechanistic release of copper from wastes into the environment. Sulphide oxidation and carbonate dissolution influenced soil pH and metal mobility, releasing Cu²⁺ ions into the soil, where a new range of biogeochemical reactions (e.g., sorption and desorption) controlled metal mobility. In the Pedra Verde mine soils, copper concentrations exceed 11,000 mg kg^-1 in some sites. The geochemical distribution of Cu in soils affected by mining wastes is similar to that found in the original wastes (carbonates, oxides, and sulfides), but in the vegetated areas, different biogeochemical processes controlled Cu dynamics. The rhizosphere of local plants directly affected Cu fractionation (e.g., carbonates and sulphides), which enhanced copper mobility. The nearby river receives the mine runoff, from where we found high metal contents in the sediments and water. This contamination causes a clear disturbance in the diversity of aquatic macroinvertebrates in the stream, favouring the predominance of tolerant species and almost eliminating sensible species, with a tendency to decrease the overall biodiversity. Finally, approximately 7.2 tons of Cu were derived from the mining wastes into the environment in the last 30 years. Considering the high copper content remaining in the waste, this site will continue to be a source of contamination, affecting the different compartments of the tropical critical zone in Brazil.