The dynamics of seawater intrusion into a coast is commonly estimated by the landward movement of the freshwater - seawater interface and by theoretical hydrological models. Radioactive isotopes were used for this purpose only in several preliminary works that did not consider the effect of various geochemical processes on the apparent age of the water. Here we present an attempt to estimate the time scale for seawater intrusion into a coastal aquifer and the operating geochemical processes by using the natural distribution cosmogenic isotopes and changes in the major ions composition of the waters. Saline and brackish groundwaters were sampled from observation and pumping wells in the Israeli coastal aquifer. In addition, detailed profiles across the fresh-saline groundwater interface (resolution of 10 cm) were provided using a Multi Layer Sampler (MLS) that was installed three times in this zone. All groundwater samples were analyzed for their chemical composition, stable carbon and oxygen isotopes, 14CDIC (14C in the dissolved inorganic carbon) and tritium activity. The coastal rock was analyzed for its chemical contents and stable and radioactive carbon isotope composition of the carbonate and of the organic matter. The substantial tritium and dissolved oxygen contents in the saline groundwaters suggest that Mediterranean Sea water penetrated inland and traveled toward the shore to a distance of at least 100-200 meter only very recently (less than 40 years). It seems that the wide range of 14CDIC activities in the saline area (40-90 pmc) was achieved from diagenetic and transport processes in this zone and not from aging. The high concentrations of alkalinity, DIC, Ca2+ and Sr2+ and the good correlation that was found between δ 13CDIC and 14CDIC in the saline groundwaters indicate that the main diagenetic processes in this zone were oxidation of dead organic carbon and cations exchange. Most of the groundwaters at the fresh-saline water interface represent conservative mixing between the saline groundwater and the top fresh groundwater of the aquifer. In some cases there is an additional ion exchange in the mixing zone.
|Journal||Geophysical Research Abstracts|
|State||Published - 2002|