Locating the Zone of Saline Intrusion in a Coastal Karst Aquifer Using Springflow Data by Bruno Arfib 1 , Ghislain de Marsily 2 , and Jacques Ganoulis 3 Abstract Coastal fresh water aquifers are an increasingly desirable resource. In a karstic aquifer, sea water intrusion occurs as a salt water wedge, like in porous media. However, preferential flow conduits may alter the spatial and temporal distribution of the salt water. This is typically the case when the outlet of the aquifer is a brackish spring. This paper shows that salinity and flow rate variations at a spring, where salinity is inversely proportional to dis- charge, can help to understand the hydrodynamic functioning of the aquifer and to locate the fresh water–sea water mixing zone deep inside the aquifer. The volume of water-filled conduit between the sea water intrusion zone and the spring outlet is calculated by the integral over time of the flow rate during the time lag between the flow rate increase and the salinity decrease as measured at the spring. In the example of the spring at Almyros of Heraklio (Crete, Greece), this time lag is variable, depending on the discharge, but the volume of water-filled con- duit appears to be constant, which shows that the processes of salt water intrusion and mixing in the conduit are constant throughout the year. The distance between the spring and the zone where sea water enters the conduit is estimated and provides an indication of the position where only fresh water is present in the conduit. Introduction Coastal aquifers all over the world are important because of their potential to provide fresh water resources to coastal communities. During the past decade, a large number of studies have investigated porous coastal aqui- fers offering many efficient models based simply on a few representatives field measurements. These models can be used to test exploitation scenarios and devise plans for sustainable development of the coastal resources (e.g., Bear et al. 1999). There are, however, few studies of karstic coastal aquifers, and they are often limited to the description of one particular aquifer (e.g., Stringfield and Legrand 1971; Calvache and Pulido-Bosch 1994; Howard and Mullings 1996; Bonacci and Roje-Bonacci 1997; Gilli 1999; Arfib et al. 2002; Blavoux et al. 2004; Pinault et al. 2004). Moreover, owing to the highly heterogeneous conditions of most karstic aquifers, many measurements are needed to describe their functioning as a whole. The conduits typical of karstic systems constitute elements that perturb the flow and transport conditions in the aqui- fer. There are large hydraulic head differences between the conduits and the surrounding limestone formations. Moreover, variability is introduced in both space (space differences linked to the position of the conduits) and time (temporal variation induced by the strong head var- iations caused by sudden floods, characteristic of the karst). Tidal fluctuations can also create head variations, but they were not observed in our case study. Conduits in the karsts perturb the distribution of fresh water and sea water in a coastal aquifer. These disturbances are ex- pressed by the brackish springs discharging a mixture of 1 Corresponding author: Laboratoire Chimie et Environnement FRE2704/Laboratoire de Ge ´ologie FRE2761, Universite ´ de Provence, case 29, 3 place V. Hugo, 13331 Marseille cedex 3, France; 00 33 491 10 65 67; fax 00 33 491 10 63 77; arfib@up.univ-mrs.fr 2 Laboratoire de Ge ´ologie Applique ´e, UMR-CNRS 7619 Sisyphe, Universite ´ Pierre et Marie Curie, case 105, 4 place Jussieu, 75252 Paris cedex 05, France; 00 33 144 27 51 26; fax 00 33 144 27 51 25; gdm@ccr.jussieu.fr 3 Hydraulics Laboratory, School of Technology, Aristotle Uni- versity of Thessaloniki, 54124 Thessaloniki, Greece; 00 30 2310 99 56 82; fax 00 30 2310 99 56 81; iganouli@civil.auth.gr Received April 2005, accepted June 2006. Copyright ª 2007 The Author(s) Journal compilation ª 2007 National Ground Water Association. doi: 10.1111/j.1745-6584.2006.00252.x 28 Vol. 45, No. 1—GROUND WATER—January–February 2007 (pages 28–35)