CORRELATION AND SPECTRAL ANALYSES OF TURBIDITY AND CONDUCTIVITY TIME-SERIES OF THE CHALK AQUIFER IN HAUTE-NORMANDIE: RELATIONS WITH THE HYDROGEOLOGIC AND GEOMORPHOLOGIC CONTEXT

Groundwater from a chalk aquifer is the main water resource in Haute-Normandie (France). This karst aquifer has strong surface water / ground water connection resulting in frequent interruptions of water supply because of impaired water quality. The karst of this area is typically binary. Introduction of surface water in the aquifer is associated to a decrease in electrical conductivity and an increase in turbidity. This work aims to study the response of various sites to rain events and to compare the transport of both particulate and dissolved matter. Turbidity, temperature and electrical conductivity were measured in situ at 5 min intervals on 22 sites (well-bore and springs) geographically distributed throughout a 6000 km2 region. Signal processing and analyses were used for time-series analyses. Turbidity (particle transport) results from the input at sinkholes of turbid surface water induced by erosion on the plateaus. The decrease in conductivity results from the input of surface water (less mineralized waters). Being related to the arrival at outlets of surface waters, the decrease in conductivity represents the transport of dissolved matter. The autocorrelation functions computed for these parameters allow 1) to estimate the inertia of the corresponding hydrosystems, 2) to compare the transport of dissolved and particulate matter. The memory effects deduced from the correlograms were always shorter for turbidity compared to conductivity. In addition, the conductivity and turbidity correlograms showed similar trends for some sites but very different ones for the others. The greater inertia of systems with respect to conductivity traduces a larger volume of aquifer affected by dissolved matter transport (diffusion within the fissured matrix associated to karst-conduit transport) whereas particles are only transported through the conduits and affected by deposition. On the other hand, the different behaviours of the conductivity and turbidity correlograms for some other sites would imply more complex types of transport. These various responses could be related to the hydrogeologic context (hydraulic gradient, geographical distribution of geochemical variables) and the geomorphologic context (chalk lithology, thickness and spatial distribution of surficial layers), by means of GIS mapping.