Sampling of herbicides in streams during flood events.

In stream water xenobiotics usually occur as pulses in connection with floods caused by surface run-off and tile drainage following precipitation events. In streams located in small agricultural catchments we monitored herbicide concentrations during flood events by applying an intensive sampling programme of ½ h intervals for 7 h. In contrast to grab sampling under non-flood conditions, clearly elevated concentrations were recorded during the floods, and pulses varying in occurrence, duration and concentration were recorded. Pulses of recently applied herbicides were the most prominent, but also agricultural herbicides used in previous seasons caused pulses in the streams. Asynchronism of chemographs may be related to the characteristics of the compounds as well as their transport pathways and transformation in compartments between the source and the point of sampling in the stream. Thus, the occurrence of chemographs is difficult to predict, which ought to be taken into account when designing a sampling strategy. Even though the chemographs of herbicides and their transformation products (glyphosate and aminomethylphosphonic acid (AMPA) as well as terbuthylazine and desethylterbuthylazine) seem to be synchronous, their occurrence may still be difficult to predict. It is evident that grab sampling under non-flood conditions yields insufficient information on the dynamics of occurrence of herbicides in stream water, both with respect to environmental effects and the calculation of the load to a recipient. In conclusion, the design of a sampling strategy regarding herbicides in stream waters should adequately consider the aim of the investigation.