Methods Note/ Dynamic of Sea Water Interface using Hydrochemical Facies Evolution Diagram by Elena Gim´ enez-Forcada Abstract This paper analyzes the results of a theoretical simulation of sea water intrusion and its dynamics. The assignment of hydrochemical facies identifies whether the aquifer is in the phase of sea water intrusion or freshening, indicating the status of the aquifer in terms of the advance or regression of the saline front. A new multi-rectangular diagram is proposed that aids interpretation of these important processes through the representation and evolution of hydrochemical facies (hydrochemical facies evolution diagram, HFE-D). As an example, the HEF-D is applied to an alluvial aquifer in the Vinaroz-Pe˜ n´ ıscola Plain (Spain), where Ca-Cl facies characterize the sea water intrusion phase, while Na-MixHCO 3 /MixSO 4 facies characterize a freshening stage. Introduction In coastal aquifers it is important to realize that sea water intrusion is a dynamic, not a static, process that depends on the periodic changes in the recharge-discharge balance of the aquifer. Any influence, direct or indirect, on the aquifer’s water balance affects the position and movement of the sea water interface and chemistry of the groundwater. The relationship between the dynamic movement of the sea water interface and the hydrochemistry of ground- water suggests that a study of the chemical variations will allow the phase of sea water intrusion of the aquifer to be identified as well as the evolution and dynamics of the saline front (phases of sea water intrusion or freshening) within the aquifer. The aims of this paper are to identify the hydro- chemical facies that characterize each stage of intrusion and freshening, and to introduce a new type of diagram Instituto Geol´ ogico y Minero de Espa˜ na (IGME) - Geological Survey of Spain, Department of Research in Geological Resources, R´ ıos Rosas 23, 28003 Madrid, Spain; +34 91 349 5751; fax: +34 91 349 5955; e.gimenez@igme.es Received April 2009, accepted October 2009. Copyright  2009 The Author(s) Journal compilation  2009 National Ground Water Association. doi: 10.1111/j.1745-6584.2009.00649.x that aids identification of the processes occurring and the evolution of the hydrochemical facies, which taking into account the dynamic character of sea water intrusion. Cation Exchange Reactions and Dynamics of Sea Water Intrusion Ion exchange processes are useful in the study of the dynamic of sea water intrusion occurring in coastal alluvial aquifers. The following cation exchange reaction takes place (Appelo and Postma 2006): Na + + 1 / 2 Ca-X 2 ↔ Na-X + 1 / 2 Ca 2+ (1) During sea water intrusion, reverse cation exchange occurs (Na\Ca) (→), while direct cation exchange (Ca\Na) moves in the opposite direction (←). The Ca-Cl facies is recognized as the clearest feature of the advance inland of the saline wedge, while the Na-HCO 3 facies characterizes the freshening phase. Appelo and Willemsen (1987), Appelo et al. (1990), Beekman (1991), Boluda (1994), and Lambrakis and Kallergis (2001), among others, have studied this type of situation using laboratory simulations, based on the principles of ion chromatography and/or geochemical modeling. Appelo and Willemsen (1987) simulated the intrusion process using a geochemical model based on experimental chromatographic studies, using dilute sea 212 Vol. 48, No. 2 – GROUND WATER – March-April 2010 (pages 212 – 216) NGWA.org