Assessing Soil Quality and Heavy Metal Contamination on Scheelite Mining Sites in a Tropical Semi-arid Setting

The study area lies in a semi-arid setting of Brazil that comprises some of the largest scheelite mines in the country, but information regarding the mining impacts on the soil quality are relatively scarce. Here, we studied the changes in the soil physical and chemical characteristics caused by the scheelite mining activities in northeast Brazil, including the impacts on heavy metal concentrations in soils. Soil quality was evaluated in three sites: non-mining (reference), mining, and post-mining. The soil samples were physically and chemically characterized and had their content of the heavy metals Cd, Cr, Cu, Ni, Pb, and Zn determined. The results showed that mining activities significantly impacted soil quality. The main environmental degradation caused by scheelite mining was soil compaction, alkalization, and decreasing of soil organic matter, nitrogen, and phosphorus contents. Besides, the soil concentrations of Cd, Cr, Cu, Ni, Pb, and Zn were higher than the soil guideline values (SGVs) for metals proposed by the Environmental Protection Agency of Brazil. The soil characteristics varied widely in the three conditions of the study (non-mining, mining, and post-mining); therefore, our data provide a base for soil quality assessments of areas impacted by scheelite mining. Due to the improvement in soil physical and chemical characteristics promoted by the spontaneous vegetation cover of old tailings piles, the post-mining sites were statistically grouped with the non-mining areas. Such a result highlights the crucial role that vegetation plays in the recovery of mined sites.