Large-scale climate control on the occurrence of turbid events on interannual scales in a karstified, heavily exploited karst system in northwestern France

Abstract:

High-amplitude turbid episodes at water supplies can cause significant sanitary issues to populations. Owing to their hydrogeologic specificity, karst ground waters are particularly sensitive to such phenomena, involving either fast infiltration of turbid surface water or resuspension of intra-karstic sediments during flood events. In some regions, such as Upper Normandy (France), soil erosion and karst features in the chalk aquifer are at the origin of major turbid events which may result in interrupted water supply to the local populations. Thanks to a long daily turbidity time series corresponding to measurements at one major karst spring since the mid-80's, we could investigate the large-scale atmospheric circulation control on below- or above-average turbidity periods. The turbidity time-series actually display periods on pluriannual duration during which daily turbid events are more frequent and have higher amplitudes, which can not be seen on daily precipitation records. Comparison was made between annual precipitation amounts, chalk aquifer water table variations and turbidity throughout this approximately 25-year period, which showed interannual recharge periods associated to above-normal turbid conditions. We then studied the linkages between such variations and large-scale atmospheric circulation using a NOAA sea level pressure reanalysis product. A wavelet multiresolution analysis of all hydrological and climatic signals revealed common aperiodic oscillations on interannual scales and allowed identification of the large-scale, interannual-scale atmospheric pattern that was responsible for those above-normal turbid periods; this atmospheric pattern was not necessarily similar to that responsible to any individual short-term turbid event.