Seawater intrusion in the Guanahacabibes Peninsula (Pinar del Rio Province, western Cuba): effects on karst development and water isotope composition

The water resources in the Guanahacabibes Peninsula are distributed in two areas. The northeastern area is characterized by swamps, wetlands and lagoons, with a low contribution of seawater, whereas the area in the southwestern plain shows a considerable development of the karst structures that limits the existence of superficial waters but permits the ingression of the surrounding seawater. In this latter area, the groundwater showed a marked increase in salinity with the depth. In particular, groundwater with a seawater fraction of 0.1 had the lowest Ca-(Mg)-carbonates saturation indexes calculated by modeling the mixing between freshwater and seawater using different software, thermodynamic databases and equations for activity coefficients. Generally, seawater and groundwaters with an added seawater fraction above 0.60–0.65 showed similar oversaturated indexes in high-Mg calcites and pure Ca-carbonates (calcite and aragonite). Differently, in the groundwater that showed carbonates undersaturation (generally with a seawater fraction between 0.02 and 0.60), the saturation indexes in high-Mg calcites were 0.2 lower than pure Ca-carbonates. Locally, the bacterial reduction of the dissolved sulfate enhanced the dissolution of the limestone, contributing to the increased development of the karst structures and the seawater intrusion. Finally, the presence near the coastline of fresh Ca- and Na-bicarbonate waters was in accordance with the upward flow of the shallow freshwater during the formation of the saline wedge. However, the oxygen and hydrogen stable isotope composition of the waters showed a probable contribution to the area from a deep aquifer that is recharged in the highest reliefs of the province (Cordillera de Guaniguanico).