Behavior of dissolved alkali and alkali-earth elements in a coastal aquifer of Mexico affected by saltwater intrusion

This comprehensive study shows the behavior of major and trace alkali and alkali-earth elements in a coastal sedimentary aquifer of Mexico moderately impacted by saltwater intrusion and anthropogenic activities. For this purpose, the concentrations of major cations (Na⁺, K⁺, Ca²⁺ and Mg²⁺), major anions (Cl^-, SO₄^2-, HCO₃^-, NO₃^-) and several alkali and alkali-earth trace elements (Li, Rb, Ba and Sr) were analyzed in all the active groundwater wells of the Todos Santos aquifer, Baja California Sur, northwestern Mexico. The results indicates that the percentage of seawater intruded into the aquifer ranges from 0.2% to 2.7%, with an average of 0.9%. In the recharge areas, groundwater is Ca²⁺–HCO₃^- type. However, groundwater evolves from Ca²⁺–HCO₃^- type to Na⁺–Cl^- type when salinity is increased in the direction of the flow path, suggesting that the intrusion of saltwater is affecting the groundwater chemistry in wells close to the coastline. The excess of Ca²⁺ and Mg²⁺ over the corresponding anions SO₄^2- and HCO₃^- shows that both alkali-earth elements are being replaced by Na⁺ in the aquifer matrix. Overall, the excess of all alkali-earth elements over the freshwater-seawater mixing line suggests that this process is extensive to all alkali-earth elements. Overall, the alkali-earth elements Ca²⁺, Mg²⁺, Ba²⁺ and Sr²⁺ are mobilized from the aquifer matrix during seawater intrusion, whereas the alkali elements Na⁺, K⁺ and Rb⁺ are removed from solution. This phenomenon can be driven by a cationic exchange process, where alkali-earth element are exchanged by alkali elements in the aquifer matrix. Unlike the other alkali elements, Li is mobilized during saline intrusion, probably also by cationic exchange. The high diffuse NO₃^- concentrations in wells close to the Todos Santos downtown indicates that nitrates could be provided by anthropogenic activities, specifically by sewage infiltration. This work can be useful as reference for knowing the effect of salinization in the concentration of alkali and alkali-earth trace elements in groundwater of coastal aquifers under sea level rise scenarios driven by climate change.