Water storage and transfer in the epikarst of karstic systems during high flow periods

A monitoring of spring and rain waters in the South of France during two hydrological cycles is presented. Rain waters were sampled after each precipitation event at 3 rain-gauge stations. Four karstic springs located in the same area have also been daily (high discharge events) to monthly (low flow periods) sampled. This paper focuses on compositional. changes in the Cl- and Br- ions and the oxygen-18 (delta 18O) and hydrogen (delta D) isotopes during the high discharge events. The responses of the different karstic systems are quite homogeneous and reflect the hydrological state of the system. The waters discharged during the major autumn and winter high discharge events originate from the epikarstic reservoir and show characteristics chemical variations related to residence in the unsaturated zone close to the surface. Their residence time in the order of 1-3 months. Correlations between the composition of the spring-water and the rainwater during three successive high discharge events during the summer of 1998 indicate that the water for high discharge event n is derived from water from precipitation event n-1 via a piston-type mechanism with residence time of 2 weeks. These results are interpreted as an indication of the major role of the epikarst reservoir in the karst recharge functioning. The similar behaviour of the four springs, although located in different geological contexts allows to think that the epikarst role could be more important than previously thought. (c) 2005 Elsevier B.V. All rights reserved.