Destino de agrotóxicos através do ciclo hidrológico na sub-bacia do Arroio Encantado

Abstract

The water cycle controls the transport and fate of pesticides in the environment. When it reaches any water body, the fate of the pesticide come about according to the physical characteristics of the water body, physico-chemical properties of the compound itself and climate conditions. The adverse effects of pesticides on the environment tend to be more pronounced in agricultural river basins. This study sought to evaluate the presence of pesticides in surface and underground waters of the Arroio Encantado sub-basin, a small hydrographic basin of predominantly agricultural use, to understand the patterns of movement of pesticides in the environment and their interactions with the water cycle, in addition to assess the ecological risk to exposed biota. To this end, 24 pesticides were analyzed using liquid chromatography–mass spectrometry (LC-MS) in 14 groundwater and 11 surface water samples in summer and winter. The main ions usually present in natural waters were also analyzed, to use the hydrogeochemical methods of Piper and Stiff plots and the statistical method of hierarchical clustering analysis, in order to characterize different flow systems. These data were evaluated together with land use, climate and hydrogeology of the study area and with the physico-chemical properties of each compound analyzed. Of the 24 pesticides analyzed, 22 were detected in at least one sampling point, exposing the biota to low to very high ecological risk in at least some points in surface waters. In periods of low rainfall, physico-chemical properties of pesticides were important in determining their fate in the environment whereas in rainier periods land use becomes more important. Two flow systems were identified in the study area, one composed of surface and groundwater, with relatively low residence times and dependent on rainwater recharge. The other flow system is exclusively of groundwater, with longer residence times, less dependent on rainwater recharge and consequently less connected with the surface. The second flow system has significantly fewer pesticide detections, proving to be more protected from surface contamination.