Mutagenic impact on fish of runoff events in agricultural areas in south-west France.

When heavy rainfall follows herbicide application, the intense surface runoff causes stream water contamination. Aquatic organisms are then briefly exposed to a complex mixture of contaminants. The aim of the present study is to investigate the genotoxic impact of such events on fish. A model fish, the Crucian carp (Carassius carassius) was exposed in controlled conditions, for 4 days, to water sampled daily in the Save River (France). The watershed of this stream is representative of agricultural areas in south-west France. Three hydrological conditions were compared: basal flow, winter flood, and spring flood. Chemical analysis of the water samples confirmed the higher contamination of the spring flood water, mainly explained by a peak of metolachlor. Genotoxicity was evaluated by micronucleus (MN) test and comet assay in peripheral erythrocytes. A significant increase in DNA breakdowns compared to controls was detected by the comet assay for all conditions. Exposure to spring flood water resulted in the highest damage induction. Moreover, induced chromosomal damage was only detected in this condition. In addition, fish were exposed, for 4 days, to an experimental mixture of 5 herbicides representative of the spring flood water contamination. Fish exhibited moderate DNA damage induction and no significant chromosomal damage. The mutagenicity induced by field-collected water is then suspected to be the result of numerous interactions between contaminants themselves and environmental factors, stressing the use of realistic exposure conditions. The results revealed a mutagenic impact of water contamination during the spring flood, emphasizing the need to consider these transient events in water quality monitoring programs.