Determining nitrate and sulfate pollution sources and transformations in a coastal aquifer impacted by seawater intrusion—A multi-isotopic approach combined with self-organizing maps and a Bayesian mixing model

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 (δ 2 H H2O , δ 18 O H2O , δ 15 N NO3 , δ 18 O NO3 , δ 34 S SO4 , δ 18 O 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. • 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.