Transboundary mining pollution? Metal(oid) concentrations in water, sediments, and risk assessment for fish consumption in a lower Andean-to-Chaco sediment-choked basin.

The Pilcomayo is a transboundary Andean-to-Chaco sediment-choked basin, where mining in the upper basin has a history of over 500 years. The river margins are home to many communities, primarily indigenous, who rely on the main fish resource, sábalo Prochilodus lineatus. High levels of metal(oid)s in the upper basin's water and sediments have sparked diplomatic and local debates about the state of resources in the lower area, which could be impacted by downstream contaminant transport. This study evaluates the concentrations of As, Cd, Cr, Cu, Hg, Li, Ni, Pb, and Zn in water and sediment at seven sampling stations during dry and wet seasons, as well as in sábalo from two sites in the lower Pilcomayo Basin (Argentina and Bolivia). These locations were compared with reference sites in the Pilcomayo and Bermejo basins that were not impacted by mining. A decrease in As, Cu, Hg, Pb, and Zn levels in water and sediments towards the lower basin was observed. Enrichment of As, Cd, Pb (up to 2.3), and Hg (2.5 to 10) was found in sediments of the lower Pilcomayo compared to regional unimpacted background, suggesting that mining activities may have a significant impact. In fish muscle, Hg (< 0.77 mg/kg) was enriched 4 times compared to the Paraná River (Argentina). Metal concentrations in fish were all below WHO recommended guidelines. The estimated daily intake and target hazard quotient (< 1) suggest no significant risk to adults or children from chronic consumption of a fish-rich diet in local populations.