Loss of pesticides from agricultural fields in SE Norway - runoff through surface and drainage water

Loss of two pesticides with different mobility characteristics, bentazone (Koc similar to 34) and propiconazole ( Koc similar to 1800), were studied at three agricultural fields (Askim, Bjornebekk and Syverud) in SE Norway. A conservative tracer (Br) was used to follow the flow of water. The loss of pesticides varied among the fields, depending on hydrological characteristics and soil properties. The loss of pesticides was higher from two artificially levelled silty clay loam soils with poor aggregate stability ( Askim and Bjornebekk) compared to a loam/silt loam soil with increased content of organic carbon and better aggregate stability ( Syverud). The total accumulated loss was< 0.5% from all fields. The highest pesticide concentrations were measured at the first runoff episode after application for both the mobile ( bentazone) and less mobile pesticide ( propiconazole) in the surface runoff. In the drainage water, the peak for the less mobile pesticide coincided with the Br tracer, while the peak for mobile pesticide appeared earlier than the Br tracer. Rapid movement of water, particles and pesticides through soils indicate flow through macropores. Larger proportions ( in percent of total applied) of both the mobile and the strongly sorbed pesticides were lost through the drainage as compared to the loss through surface runoff at Askim. Here, it is suggested that macropore flow contribute to the increased loss of pesticides through the drainage. At Syverud, high infiltration capacity reduces the amount of water available for surface runoff, and somewhat higher loss of the mobile pesticide was registered in the drainage compared to the surface runoff. For the strongly sorbed pesticide, however, propiconazole was neither detected in surface nor in drainage water at Syverud. Generally, there was a higher percentage loss of the mobile compared to the strongly sorbed pesticide in both surface and drainage water, which is in agreement with the pesticides mobility characteristics in soil. An exception was, however, the erodible soil Bjornebekk, where a higher fraction of propiconazole was lost in the surface runoff compared to bentazone. Large amounts of sediment transport from the Bjornebekk field probably contributed to enhanced transport of the strongly sorbed pesticide.