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In fissured aquifers, hydrodynamic phenomena combined with a network of permeable fissures imparts varying degrees of vulnerability depending on the type of contaminant. The study presented here examines the impact on a karst aquifer of a river which receives effluents from a sewage treatment plant just upstream from the point of recharge. This recharge constitutes a source of bacteriological and chemical pollution. The aquifer is accessible by boreholes at discrete points, facilitating investigation of the evolution of three of the contaminants characteristic of treatment station effluents, NH4+ and two biological contaminants with different survival capacities. The study was undertaken both under normal climatic conditions and during a flood occurring after a long dry period. The rate of infiltration, of subsurface transport, and of dissemination of contaminants into a zone usually protected from contamination was recorded. The results confirm the vulnerability of fissured aquifers to contamination.

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