Dynamics of trace elements during seawater intrusion in a sedimentary aquifer of western Mexico

This study shows the dynamics of several trace elements in groundwater of a coastal sedimentary aquifer of western Mexico affected by seawater intrusion. The results of the chemical analyses performed in groundwater indicated that the elements Ba, Sr, Li, Fe, Co, V, Se, and Re are mobilized from the aquifer sediments to the aqueous phase during seawater intrusion, whereas Rb is removed from solution during salinization. Overall, the mobilization/removal of these elements may be due to processes such as cation exchange, the increase of the ionic strength due to increasing salinity and the weathering of biogenic carbonates in an oxidizing environment. Other elements such as Mo, Ni, Cr, Ta, and W displayed low or no mobilization during the seawater intrusion. This may indicate that dissolution of biogenic carbonates is not their main sources and that these elements are not affected by the ionic strength caused by groundwater salinization. Finally, other elements such as As, U, Ge, Sb, Cu, and Mn displayed an undefined tendency, showing concentrations above and around the theoretical seawater mixing line. These elements may be likely affected by other processes rather than seawater intrusion, which may explain their undefined tendency during groundwater salinization. In general, geochemical processes such as carbonate/sulfate complexation (in the case of U), organic matter complexation (in the case of Cu), metalloid co-occurrence (As, Ge, and Sb), and redox processes (in the case of As and Mn) may play a key role in determining the concentration levels of these elements in this coastal groundwater system affected by seawater-freshwater mixing.