Ecological risk assessment and soil quality in areas impacted by iron mining tailings deposition

Abstract

The failure of the Fundão tailings dam in 2015 in Mariana (Minas Gerais, Brazil) released millions of cubic meters of iron mining tailings that dispersed along the Doce River basin, affecting terrestrial ecosystems. Despite the extensive documentation of the initial impacts, uncertainties remain regarding how the deposition of these tailings’ influences, in the medium term, soil organisms and the biological processes responsible for maintaining the ecological functioning of affected soils. In this context, this thesis investigated how the deposition of mining tailings influences soil chemical, physical, and biological attributes and how these alterations are reflected in the response of soil organisms to environmental stressors and in ecological risk assessment in soils with tailings deposition eight years after the dam failure. To address this objective, the thesis was structured into three complementary studies. In the first study, the toxicity of tailings alone and the response of standardized soil organisms to exposure to tailings combined with a pesticide were evaluated in soils with tailings deposition and in reference soils. The test organisms used were Folsomia candida, Enchytraeus crypticus, and Eisenia andrei. The results indicated that the tailings alone produced low ecotoxicological effects on the evaluated species. However, when the pesticide was applied, greater sensitivity of the species was observed in soils with tailings deposition compared with reference soils, indicating that biological responses to contaminants may vary depending on soil characteristics. In the second study, soil biological processes and the structure of soil organism communities associated with the maintenance of ecological functioning were investigated. Three lines of evidence were integrated: physical, chemical, and biological. Soils with tailings deposition showed higher pH, greater bulk density, and enrichment in Fe and Mn. From a biological perspective, reductions exceeding 50% were observed in microbial biomass, basal respiration, and enzymatic activities. The mesofauna community showed structural simplification, characterized by reduced diversity and the predominance of Hymenoptera. In the third study, an integrated Ecological Risk Assessment (ERA) approach based on the Dutch Triad model was applied, traditionally structured around chemical, ecotoxicological, and ecological lines of evidence. As a methodological innovation, a fourth line of evidence based on soil physical attributes was incorporated. Although the integrated risk derived from the chemical assessment was classified as low due to the low concentrations of potentially toxic elements, the ecological and physical lines of evidence indicated relevant alterations in soil properties, including compaction, reduced porosity, and low microbial biomass. Soil Quality Index (SQI) values corroborated these findings and showed that alterations in physical attributes may help explain the effects observed in other system components. Overall, the results demonstrate that even eight years after the dam failure, soils with tailings deposition still exhibit persistent alterations in physical and biological attributes associated with soil ecological functioning. These findings indicate that assessments based exclusively on chemical parameters may underestimate persistent ecological alterations in areas affected by mining tailings. Incorporating soil physical attributes into integrated environmental assessment approaches contributes to a more robust interpretation of the processes regulating ecological functioning and recovery in these systems. Keywords: soil ecosystems; mining tailings; ecological risk; soil quality; soil fauna; microbial activity