Pesticides may impact aquatic ecosystems when entering water bodies. Measures for mitigation against pesticide inputs include vegetated treatment systems (VTSs). Some of these systems have very short hydraulic retention time (<span classCombining double low line"inline-formula"i>lt/i>/span> 1 h) but nevertheless manage to effectively reduce peak concentrations of contaminants as a result of dispersion. We hypothesize that the effect of dispersion on contaminant mitigation in VTSs depends on the shape of the contaminant input signal chemograph, which in turn is related to factors affecting contaminant mobilization in the contributing catchment. In order to test this hypothesis, we grouped chemographs of six contaminants originating from a viticultural catchment during 10 discharge events into clusters according to chemograph shape. We then compared peak concentration reduction and mass removal in a downstream VTS, both among clusters and in terms of compound properties and discharge dynamics. We found that chemograph clusters reflected combined effects of contaminant source areas, transport pathways, and discharge dynamics. While mass loss was subject to major uncertainties, peak concentration reduction rate was clearly related to chemograph clusters and dispersion sensitivity. These findings suggest that mitigation of acute toxicity in a VTS is stronger for compounds with sharp-peaked chemographs, whose formation is related to the contributing catchment and can be analyzed by chemograph clustering.