Dynamics of sediment and associated pesticide transfers in cultivated southernmost Brazil since 1982

Since the 1980s, South America has emerged as one of the world’s leading agricultural producers, resulting in significant environmental pressures, including land and water degradation.The consequences of this agricultural development, both past and present, are still poorly documented in this region, particularly in the Pampa biome. Retrospective analysis using sediment archive can provide valuable insights for the characterisation of the long-term environmental degradation.In this study, we analysed a sediment core collected in the Salto Grande dam constructed in 1982 on the Uruguay river, draining a 266,000 km2 catchment. We established an age model and characterised the sediment properties, using gamma spectrometry, high-resolution geochemical content analysis (XRF), pesticides, magnetic susceptibility measurements over time (1982-2022). This multi-proxy analysis of a sediment archive from the Salto Grande reservoir enabled the first long-term reconstruction of land degradation and pesticide fluxes in the very large Uruguay river transnational basin (comprising Brazil, Argentina and Uruguay) since 1982.The results indicate that sediment fluxes have decreased significantly since 2000 and sediment provenance has shifted toward the southern part of the basin after this period. These changes coincide with the construction of dams in the upstream part of the catchment, the expansion of agriculture in the south and the widespread adoption of no-tillage practices. This change in farming practices induced an increase in pesticide fluxes, thereby posing potential ecological risks. In this context, trade deals such as those between the European Union and the European Free Trade Association and Mercosur, combined with the anticipated increase in the area dedicated to soybean and cellulose production, should be considered in light of the potential consequences in terms of agriculture expansion and related environmental threats.