Metais e metaloides em águas superficiais da Bacia do Rio Paraguai: distribuição espacial, ocorrência e lacunas de monitoramento ambiental

Abstract

The Paraguay River Basin, covering approximately 1,095,000 km² across Brazil, Bolivia, Paraguay, and Argentina, contains one of the largest floodplain systems on the planet, supporting high biodiversity and economically important activities. Despite its environmental relevance, information on the occurrence of metals and metalloids in its surface waters remains spatially scattered, temporally discontinuous, and methodologically heterogeneous, hindering the development of an integrated transboundary assessment. This study aimed to compile, systematize, and critically analyze the scientific literature on the occurrence of these elements in the basin between 1995 and 2025. Following the application of inclusion and exclusion criteria, 19 peer-reviewed studies were selected, encompassing 41 water bodies and 158 sampling sites across the four countries. Concentrations were standardized to mg L⁻¹ and compared with limits established by Brazilian, Paraguayan, and Bolivian legislation. The spatial distribution of sampling sites was represented through georeferencing and spatial analysis using QGIS 3.34.4. Eleven elements were identified as recurrent in the basin: aluminum (Al), arsenic (As), cad mium (Cd), chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn). The highest concentrations were recorded in tributaries of the Bolivian portion of the basin, particularly in water bodies located above 2,000 m in altitude and associated with acid mine drainage in the Potosí region, where Fe reached up to 950 mg L⁻¹, Al 630 mg L⁻¹, and Zn 560 mg L⁻¹. The highest frequencies of limit exceedances were observed for Pb (n = 67), Cd (n = 49), and As (n = 47). Copper (Cu) showed different classifications depending on the legislation adopted. The spatial distribution revealed marked asymmetry among countries, with most studies concentrated in Brazil and Bolivia and limited sampling coverage in Paraguay and Argentina. These results demonstrate that metal and metalloid contamination in the Paraguay River Basin is recurrent and spatially heterogeneous, reinforcing the need for continuous, standardized, and cooperative monitoring among the countries sharing the basin.