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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 HCO3– and SO 4 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 NO 3 − 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 NO 3 − 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. Unlabelled Image • Ca, Mg, Sr and Ba are exchanged from the sediment by Na, K, Rb during salinization. • Unexpectedly, Li is released into solution during saline intrusion by cation exchange. • B, I, HCO 3 – and SO 4 2− are mobilized into solution by chemical weathering. • Fe, Co, V, Se and Re are mobilized whereas Mi, Ni, Cr, Ta and W are not mobilized. • U, As, Ge, Sb, Cu and Mn show undefined tendency during seawater intrusion. [ABSTRACT FROM AUTHOR]

Over the past few decades, the La Paz aquifer system in Baja California Sur, Mexico, has been under severe pressure due to overexploitation for urban water supply and agriculture; this has caused seawater intrusion and deterioration in groundwater quality. Previous studies on the La Paz aquifer have focused mainly on seawater intrusion, resulting in limited information on nitrate and sulfate pollution. Therefore, pollution sources have not yet been identified sufficiently. In this study, an approach combining hydrochemical tools, multi-isotopes (δ2H H2O , δ18O H2O , δ15N NO3 , δ18O NO3 , δ34S SO4 , δ18O SO4), and a Bayesian isotope mixing model was used to estimate the contribution of different nitrate and sulfate sources to groundwater. Results from the MixSIAR model revealed that seawater intrusion and soil-derived sulfates were the predominant sources of groundwater sulfate, with contributions of ~43.0% (UI 90 = 0.29) and ~42.0% (UI 90 = 0.38), respectively. Similarly, soil organic nitrogen (~81.5%, UI 90 = 0.41) and urban sewage (~12.1%, UI 90 = 0.25) were the primary contributors of nitrate pollution in groundwater. The dominant biogeochemical transformation for NO 3 - was nitrification. Denitrification and sulfate reduction were discarded due to the aerobic conditions in the study area. These results indicate that dual-isotope sulfate analysis combined with MixSIAR models is a powerful tool for estimating the contributions of sulfate sources (including seawater-derived sulfate) in the groundwater of coastal aquifer systems affected by seawater intrusion. [Display omitted] • Multiple isotopes and Bayesian mixing model traced nitrate and sulfate sources. • First-time use of Bayesian mixing model to assess sulfate from seawater intrusion. • Fertilizer - ẟ15 N is affected by the mineralization–immobilization–turnover process. • Groundwater overuse promotes nitrification, seawater intrusion and sulfate increase. [ABSTRACT FROM AUTHOR]