Hydrochemical and physical processes influencing salinization and freshening in Mediterranean low-lying coastal environments

Abstract

Ground- and surface water chemistry and stable isotope data from the coastal zone near Ravenna (Italy) have been examined to determine the geochemical conditions and processes that occur and their implications for fresh water availability in the various brackish/saline coastal environments. Fresh water in shallow coastal aquifers is often not enough to provide drinking water but it may sustain agriculture and natural environments such as pine forests, dunes and lagoons. The hydrochemistry and stable O and H isotopes of the ground and surface water near Ravenna were investigated in relation to these particular environments: rivers, drainage channels, lagoons, coastal dunes, paleo dunes, agricultural fields, and gravel pit lakes. Emphasis was put on the spatial distribution of ground water bodies with similar hydrochemistry. Most water samples are of the brackish to saline NaCl− type (NaCl concentration 300–20,000mg/L) with a negative Base Exchange Index (BEX) that is indicative of cation-exchange reactions caused by water rock interactions during the infiltration of saline water in a fresh water aquifer. Calcium-dominated fresh water is found only in the rivers, in the irrigation and drainage channels, and in a few ground water samples. The analysis of stable isotopes and SO42-/Cl- and δ18O/Cl− ratios shows that the major chemical and physical processes determining the salinity in the area are evaporation, mixing with saline water, cation exchange, oxidation of organic matter, migration and seepage of (hyper) saline water. The migration and upward seepage of (hyper) saline water driven by the hydraulic gradients induced by the drainage system causes the aquifer to become more saline over time. The areas where natural recharge occurs have always been small and this in combination with the Mediterranean climate results in very little natural recharge. Therefore, unless drastic changes to the hydraulic infrastructure are made, the coastal aquifer near Ravenna will become more saline and will threaten the ecology of current natural areas as well as the farmland.