Análise de risco ecológico por metais e metaloides em águas superficiais: estudo de caso da Bacia do Rio Doce

Abstract

The Doce river basin has been suffering anthropogenic pressure through activities related to agriculture and mineral extraction, mainly in the upstream portions of the Piracicaba river and Carmo river sub-basins, which historically receive waste, effluents and stormwater “runoff” arising from activities carried out in the basin. Concentrations of metals in waters above the safe level for aquatic biota can cause harm to these organisms and consequently affect the entire trophic chain and ecossystem. The general objective of this study is to carry out a spatio-temporal assessment of the ecological risk (ERA) related to the occurrence of metals and metalloids in surface waters in the Doce River basin. The study area comprises 64 points distributed in the upper and middle portions of the Doce river basin, within the limits of the state of Minas Gerais, which covers 86% of this basin. Water quality data were obtained through the Institutional Repository of Minas Gerais Institute of Water Management – IGAM. The ecological risk assessment was carried by calculating the Risk Quotient (RQ) for each observation, focusing on toxicity to the most sensitive aquatic organisms. Acute and chronic RQ were calculated by the ratio between observed concentrations of metals and metalloids in 64 points distributed throughout the Minas Gerais portion of the Doce River basin from 2009 to 2022, and predicted concentration values that do not cause an effect (PNEC). For Cu, Mn, Ni, Pb and Zn, the PNEC was adjusted considering bioavailability (PNECbio) through the use of modeling tools based on biotic ligand models (BLMs) according to the chemical characteristics of the water in the Doce River Basin in every sample. Calculated risks were evaluated in light of land use obtained by MapBiomas in the region of influence of each sampling station, delimited using Grass tools in QGIS. The ecological risk was compared to toxicity analyzes carried out by IGAM at 13 monitoring points. Results showed that the elements Hg, Cr, Cd and As are not very representative in the basin and were not detected in more than 60% of the sampling stations. RQ indicated that Fe, Al, and Mn presented higher extreme values and greater variability in the basin. Results suggest that the ERA is conservative and that, despite pointing out critical points and periods for monitoring, it does not replace ecotoxicological tests. The ecological risks obtained for metals increases in the rainy season in the Doce River Basin, thus being the priority period for ecotoxicological monitoring. In the areas affected by the collapse of the Fundão dam, the 2015 hydrological year was the most affected and, since then, the ecological risk pattern has been gradually approaching values detected before the collapse, showing, in general, no statistically significant differences between recent years and the pre-disruption period. No correlation was found between land use and risks associated with metals in the basin. This is the first ERA for the Doce River Basin and results contribute with a deeper knowledge on the risks imposed by metals and metaloids in the basin and may contribute with decision making related to watershed management.