Geochemical evolution and timescale of seawater intrusion into the coastal aquifer of Israel

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 14C and 3H. 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 13C DIC and 14C 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 3H 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 14C DIC mass balance that explains the relatively low 14C DIC activities in the SDS as influenced by diagenesis and not by simple radioactive decay.