A. Maqsoud, M. Bakalowicz (Canada), N. Crampon (France), and B. Bussière (Canada)
chalk, groundwater, karst, hydrological functioning
Spring discharge time series of two chalk aquifers of the Paris Basin (France), the Lys springs (Pas-de-Calais) and the Fontaines-sous-Praux springs (Seine-Maritime), were studied by spectral and autocorrelation analysis to define their behaviour during several hydrological cycles. The functioning of these two aquifers presents a marked variability between them and according to the hydro climatic conditions at which they are subjected. This functioning evolves from a typical karstic to a porous or fissured pole, sometimes mixed, or a confined functioning. Those different functioning can occur together during the same hydrological year by producing a composite response. But they can also occur separately in particular hydrological conditions such as a dry or a wet year. The karstic functioning of the chalk aquifers revealed by rapid response during (e.g. Lys springs) or after (e.g. the Fontaines-sous-Praux springs) the overdrawn years. It seems that very marked low-water levels are thus necessary to produce a functioning of the system reporting the existence of a karstic drainage structure. As for the composite responses seems to be a particular character of the chalk aquifers. Thise responses indicate the coexistence of the functioning of karstic drainage (producing a rapid response in 2 or 3 days) and a very slow one from the groundwater stored in the chalk porosity (50 days after). This episodic karstic functioning shows the existence of preferential fast drainage responsible for the typical vulnerability of the aquifer. It also determines particular conditions either for draining or recharging the reserves. These particular drainage conditions of certain chalk aquifers should be taken into account in the plans of exploitation and protection of their groundwater resources.
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