Herbicides distribution in sediments of the Argentina rolling pampas landscape

Soil losses due to water erosion exceed the tolerance in the edafoclimatic conditions of the rolling pampas. Erosion sediments transport pesticides outside their own limits. Increased knowledge about its polluting potential would allow agronomic practices to be redirected towards sustainability. The objectives of this work were to: a) analyze herbicide distribution patterns frequently used in agricultural production and b) evaluate some herbicide and soil properties to explain their landscape distribution pattern. In an area under exclusively agricultural production of the upper basin of Pergamino stream, rain simulations were carried out in different landscape positions (upland, mid slope, and lowland). In the upland and mid slope (well-drained Mollisols) agriculture is practiced with soybean monoculture tendency under no tillage; in the lowland (Mollisols and alkaline and saline Alfisols), cattle breeding and rearing is carried-out on improved grasslands. Sediments were obtained using a rain simulator for one hour at high intensity (60 mm h^-1) at 23 sampling points. In the sediments, 2.4-D, acetochlor, atrazine and metabolites, flurochloridone, glyphosate and AMPA, and s-metolachlor concentrations were determined. In addition, the following variables: basic infiltration, runoff coefficient (%), slope, amount of sediments, texture, soil organic carbon (SOC), pH, electrical conductivity and exchangeable sodium at 0-5 cm were obtained. Non-parametric tests of herbicide concentrations between landscape positions and correlations with the analyzed variables were performed. The production systems practiced in the landscape different positions, even with low grade slopes, against heavy rains, favor surface runoff (between 45 and 64%) and generate significant sediment losses. No differences were found in the amount of sediment between landscape positions. There was also no relationship between sediment quantity and herbicide concentration. The herbicides applied in agriculture were moved to the lower parts of the landscape, where they are not applied. Three patterns of distribution of concentrations were found that corresponded to some herbicides and soils properties. The average concentrations of 2.4-D, acetochlor and s-metolachlor were higher in the lowland than in the upland and mid slope. The low/moderate adsorption coefficients, the moderate/high solubilities and their relationship with higher sand content and SOC led to their accumulation in the lowland. On the contrary, the average concentrations of glyphosate and AMPA were higher in the upland and mid slope positions, as a consequence of their high adsorption coefficient in soils with higher clay and silt content. Finally, the average concentrations of atrazine-OH and flurochloridone did not differ between landscape positions. Its moderate adsorption to the soil, low solubility and lack of relationship with soil properties caused a relatively homogeneous distribution in the landscape. It is necessary to implement crop rotations that improve soil surface properties to increase its retention and degradation and, therefore, decrease the runoff, the herbicides load in runoff and the associated environmental risks.