Dynamics of major and trace elements during seawater intrusion in a coastal sedimentary aquifer impacted by anthropogenic activities

This study analyzed the dynamics of major ions and trace elements along the groundwater flow path of the coastal sedimentary Todos Santos aquifer in Baja California Sur, Mexico, moderately impacted by anthropogenic activities. The results indicate that the elements Ca²⁺, Mg²⁺, Ba²⁺, Sr²⁺ and Li⁺ are mobilized from the aquifer matrix during seawater intrusion, whereas the alkali-elements Na⁺, K⁺ and Rb⁺ are removed from solution, possibly due to cationic exchange process. The anions HCO^3- and SO₄^2- and the elements I and B are mobilized due to carbonate mineral weathering, whereas dissolved silica and the halides Br^- and F^- behave conservatively during salinization. Groundwater NO₃^- is provided by sewage infiltration. Regarding trace elements behavior, we identify three groups: i) elements that are mobilized during saline intrusion (Fe, Co, V, Se, Re), ii) elements revealing low or no mobilization (Mo, Ni, Cr, Ta, W) and iii) elements that show an undefined tendency (U, As, Ge, Sb, Cu, Mn). The U and NO₃^- levels in groundwater should be considered carefully because several wells have concentrations close to the permissible levels. This study may be useful as reference for knowing the possible effect of salinization in coastal aquifers under sea level rise scenarios driven by climate change.