doi:10.1016/j.gca.2004.07.023 Geochemical evolution and timescale of seawater intrusion into the coastal aquifer of Israel ORIT SIVAN, 1,2, *YOSEPH YECHIELI, 2 BARAK HERUT, 3 and BOAZ LAZAR 1 1 Institute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel 2 Geological Survey of Israel, Jerusalem 95501, Israel 3 Israel Oceanographic and Limnological Research, National Institute of Oceanography, Haifa 31080, Israel (Received July 1, 2004; accepted in revised form July 23, 2004) Abstract—This study is an attempt to quantify the geochemical processes and the timescale of seawater intrusion into a coastal aquifer from changes in the major ionic composition of the water and the natural distribution of the cosmogenic isotopes 14 C and 3 H. For that purpose, we sampled saline and brackish groundwaters from the Israeli coastal aquifer. A multilayer sampler (MLS) was used to obtain very high resolution (10 cm) profiles across the fresh-saline water interface (FSI). The chemical and stable isotope data revealed three distinct water types (end members) that are located in different zones on the route to the coastal aquifer: (1) slightly modified Mediterranean seawater (SWS); (2) slightly diluted (with up to 20% fresh groundwater) saline groundwater (SDS); and (3) fresh groundwater (FGW). The SWS samples generally show an excess of total alkalinity and total dissolved inorganic carbon (DIC), and a depletion of 13 C DIC and 14 C DIC with respect to normal seawater indicating that anaerobic oxidation of organic matter is the first diagenetic reaction that affects seawater during its penetration into the bottom sediments. SDS waters appear when SWS is slightly diluted, gain Ca 2+ and Sr 2+ , and is depleted in K + , suggesting that the main processes that transform SWS into SDS are slight dilution with fresh groundwater and cation exchange. At the fresh-saline water interface, SDS generally shows conservative mixing with FGW. Inspection of chemical data from coastal aquifers around the world indicates that intensive ion exchange in slightly diluted saline groundwater is a globally important phenomenon of seawater intrusion. Most of our saline groundwater samples contain substantial amounts of 3 H suggesting that penetration of Mediterranean seawater and its inland travel to a distance of 50 –100 m onshore occurred 15–30 yr ago. This is supported by the 14 C DIC mass balance that explains the relatively low 14 C DIC activities in the SDS as influenced by diagenesis and not by simple radioactive decay. Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Sea level has risen since the end of the last glacial period (ca. 18 kya) by 120 m and stabilized at 5 kya (e.g., Fairbanks, 1989; Chappell et al., 1996). The rise caused worldwide intru- sion of seawater into coastal aquifers, elevating and shifting inland the fresh-saline water interface (FSI) and replacing fresh groundwater with seawater (e.g., Jones et al., 1999). Such replacement is generally accompanied by intensive transport and water/rock interaction processes that may considerably alter the chemical composition of the original seawater. The water/rock processes and the dynamics of seawater intrusion are affected by various characteristics of the aquifers such as lithology, tectonics, fresh groundwater flow, and anthropogenic activity (e.g., pumping). Most of the information on intruding seawater/rock interac- tion processes comes from studies analyzing the deviation of the chemical and isotopic composition of the groundwater at the FSI from a simple mixing line (e.g., Magaritz and Luzier, 1985; Price and Herman, 1991; Wicks et al., 1995; Appelo and Postma, 1996; Wicks and Herman, 1996; Barker et al., 1998; Panteleit et al., 2002). These studies suggest processes such as cation exchange, oxidation of organic matter in aerobic or anaerobic conditions, and CaCO 3 dissolution or precipitation. This study is an attempt to describe the geochemical evolu- tion of seawater intrusion into the coastal aquifer of Israel through detailed chemical (major ions) and isotopic character- istics of the porewater/groundwater and the sediments/rock, with specific emphasis on mass balance calculations of the dissolved inorganic carbonate (DIC) system ( 13 C DIC and 14 C DIC ). A fully perforated research well, 46 m deep, and 70 m from the shore, was dedicated to this study. The detailed chemical profiles enabled the identification and quantification of the major stages in the geochemical evolution of intruding seawater. The extent of seawater intrusion into coastal aquifers is commonly estimated by a salinity rise in observation wells (e.g., Melloul et al., 1999) or by hydrological models. Direct dating of the intrusion may be achieved using a suite of radio- active isotopes such as 14 C (t 1/2 of 5730 yr) of dissolved inorganic carbon ( 14 C DIC ) and 3 H (t 1/2 of 12.4 yr) of the water ( 3 H H 2 O ). 3 H H 2 O is a very good dating tool for “young” water since it is part of the H 2 O molecule, while 14 C DIC , which is suitable for dating older events, is part of the dissolved inor- ganic carbon in the water, and hence may “suffer” from several water/rock interaction processes that alter its apparent age. Seawater intrusion ages have been reported in several studies (Rao et al., 1987; De Breuck and De Moor, 1991; Hahn, 1991; Voss and Wood, 1994). The apparent 14 C ages reported in these studies for saline groundwaters in proximity to the shore were a few thousand years. However, they did not provide a com- * Author to whom correspondence should be addressed (sivan@ fas.harvard.edu). Geochimica et Cosmochimica Acta, Vol. 69, No. 3, pp. 579 –592, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 579