Résumé
Urban activities release lots of contaminants on impervious surfaces and into drainage networks. These contaminants are driven from their point of emission to the catchment outlet by the rainfall events: surface runoff leaches contaminants from the urban areas and transfers them downstream to the drainage network and then to the sea. Since regional precipitation regimes are expected to be modified in the near future by the global climatic change, we could ask the following question: must we expect an impact of rainfall changes on the contaminant inputs from the catchment to the sea? First elements of an answer are tackled here for thermotolerant coliforms (TTC) on the bay of Aigues-Mortes, a very attractive site for summer seaside tourism and winter retirement in southern France. The methodology was divided into 4 steps: (1) current TTC inputs from the catchment to the sea were calculated from in-situ measurements at the outlets of the three sub-catchments; (2) antecedent precipitation indexes were calculated from local observed daily precipitation (Mdtdo France data) for the measurement period and for the past twenty years; (3) statistical regression models were established to link measured daily contaminant inputs to observed antecedent precipitation indexes and were applied to produce annual contaminant inputs to the sea for the past twenty years; (4) precipitation predictions issued from Med-CORDEX regional climatic model were used to generate antecedent precipitation indexes for the future and to estimate TTC inputs to the sea for the next century. The results showed that whatever the scenario applied for predicting precipitations in the future (optimistic RCP4.5 and pessimistic RCP8.5), annual TTC inputs were very close. Moreover the two scenarios had very limited impact on annual TTC inputs, compared to the past and current ones. However, the study must be enhanced by considering local changes in human activities in the future, since the statistical regression models of daily TTC inputs showed significant differences between the three different sub-catchments of the bay.