Anthropic-induced salinization in a dolomite coastal aquifer. Hydrogeochemical processes

Abstract

The dolomite coastal aquifer of Balanegra (SE Spain) is the only source of irrigation water in an area of flourishing intensive agriculture. This scenario has consequently led to a severe exploitation of the water resources. This aquifer extends from the edges of the Sierra de Gador to the sea, deepening progressively under the thick Neogene sedimentary sequence that confines it.Based on the content of major ions, the groundwater was classified into four groups (G1 to G4). G1 samples were characterized by their high content of dominating ions HCO3- and Mg2+. Samples classified as G2 showed a similar composition, but with a higher SO42- ion content. As for samples corresponding to groups G3 and G4, they were characterized by their chloride content. However, Na+ is the major cation in G3 and Mg2+ in G4. Based on the study of ionic relationships, ionic deltas and mineral saturation indices, it was possible to elucidate the main processes that condition the chemical composition of each of these groups of samples. The most important among said processes is the dissolution of dolomite or dolomitic limestone, the primary component of the aquifer rock. Compositional changes are due to SO42- concentration resulting from gypsum dissolution (G2). In G3, salinity is the result of dissolution of salts, or contamination through saline waters of the upper detrital aquifer. Seawater intrusion, a consequence of the overexploitation of the aquifer, increases the salinity in the G4 samples. In this intrusion process, waters of marine origin rich in Mg, interact with the carbonated rock, favouring a cation exchange in which Mg is fixed in dolomite or dolomitic limestone and Ca ion is released. This process is supported by the ionic delta values of these two cations. The marine intrusion in the area is facilitated through groundwater overexploitation, and also by the structuring of the aquifer. Marine intrusion exists where tectonics has allowed interconnection between the carbonate rock and the sea. In other areas on this coast, marine intrusion has not been possible because of the presence of impermeable Neogene materials that prevent this interconnection.