Geochemical Erosion in Fluvial Channels During Intense Rainfall Events in Southern Brazilian Watersheds

The high average load of suspended sediments in rivers in southern Brazil is related to soil degradation in agricultural areas due to erosive processes resulting from inadequate management. In this context, understanding the dimensions of transported particles and their biogeochemical dynamics is essential, as well as their transport capacity, load, concentration, deposition potential, and degradation within the fluvial system. This study analyzed hydrological characteristics, particle size, and chemical properties (TOC, NT, PT, POrg, Pd) in three river basins in Rio Grande do Sul (2012-2013): Arroio Lajeado Ferreira (1.19 km²), Rio Guaporé (1,980 km²), and Rio Conceição (804 km²), all located in southern Brazil. This research revealed that erosion during heavy rainfall in southern Brazil acts as a selective "geochemical erosion" process, preferentially transporting fine sediments (<63 µm) enriched with nutrients. The dynamics of phosphorus, during intense rainfall events can become predominantly organic (>50%), and quantifying significant loads of bioavailable phosphorus and organic carbon to rivers. The Conceição River Basin stood out with the greatest nutrient loss per area, demonstrating that agricultural practices exacerbate this impact. Suspended sediments (< 63 µm) showed high concentrations of organic carbon, nitrogen, and phosphorus (especially Pd, which is of environmental concern). The particle size D(25), D(50), D(75) did not vary significantly between basins or rainfall events. Phosphorus exhibited a dynamic behavior, at basal flow, it was predominantly inorganic, but during rainfall events, more than 50% could be organic.