Hydrogeochemistry and assessment of pollution indices for diagnosing drinking water quality in abandoned mining areas

Artisanal mining is responsible for extensive contamination of ecosystems, including humans and animals. Abandoned areas can present environmental problems related with potentially toxic metals and metalloids that are not bio(decomposed). This study focuses on the two-century-old Lavras do Sul Gold District, an Au-Ag-Cu mining area in southern Brazil abandoned 40 years ago. Currently, these areas are inhabited and used for agricultural and pasture activities. Based on chemical analysis of water for human consumption from homemade/private wells collected in two field campaigns, we evaluated the presence, distribution and quantity of potentially toxic elements. Furthermore, the determination of the pollution indices Heavy Metal Pollution Index (HPI) and Heavy Metal Evaluation Index (HEI), helped in the evaluation of potability. The proposed workflow includes hydrogeochemical analysis, mathematical modelling for element speciation, index calculation and spatial interpolation. The hydrogeochemical results indicate that source of anions and cations has influence of the agricultural activity and, to a lesser extent, geological activities. The elements in the highest concentrations are NO ₃ ^- , Cl- and Cd ²⁺ , Pb ²⁺ , As(V), Al ³⁺ and Fe ²⁺ , which made 70% and 100% of the water samples unsuitable for consumption, respectively from the first and second field campaigns. The analysis of the indices revealed significant discrepancies in potability diagnoses, with the HPI reaching a value 72.1 times higher than the HEI in the same sample. The index that best represented contamination by critical metals in the area was the HPI, as it favors small quantities of elements in defining the weights of these pollution indices.