Hydrological characterisation of pesticide loads using hydrograph separation at different scales in a German catchment

Pesticide pollution of surface waters was studied in a 49.7 km(2) catchment in Germany. The loads were differentiated into point source (PSP) and non-point source pollution (NPSP). The contribution of runoff, interflow, groundwater, drainage and spray drift to river contamination was defined as NPSP. Pesticides discharged into the river via wastewater treatment plants and sewer overflows were considered as PSP. Hydrology and pesticide loads were studied on the scale of the entire catchment and in two sub-catchments not influenced by PSP. River discharges and concentrations of 19 pesticides were measured at four locations over 447 days. The total load detected in the entire catchment amounted to 3249 g active ingredient (a.i.). The stream flow data were separated into base and direct flow applying a non-linear reservoir separation algorithm. Single rainfall events leading to runoff and interflow dominated NPSP in the sub-catchments. For the sub-catchment Rossberg 87% and for Leidenhofen 71% of the pesticide load could be attributed to direct flow. This corresponded to 14 and 34% time of peak flow events for Rossberg and Leidenhofen, respectively. For the entire catchment only 15% of the NPSP was transported with direct flow with 49% attributed to base flow. The difference between the entire and the sub-catchment contribution was attributed to dilution, hysteresis and merging effects. Applying a digital recursive filter technique for hydrograph separation confirmed the results of the non-linear reservoir separation algorithm. The impact of the hydrograph separation technique on the hydrological characterisation of pesticide loads was insignificant. The results emphasise the dynamic nature of pesticide transport and its complex interaction between size and position of source areas along a stream flow network. The measured loads in the sub-catchments were scaled up to the entire catchment as an estimate for the total NPSP. The fraction of the area treated with a pesticide was found not to be an appropriate scaling factor. Research is required to develop adequate methods to scale detailed measurements of pesticide loads from small scales up to catchments with complex drainage systems. (C) 2003 Elsevier Science B.V. All rights reserved.