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The identification of nitrate (NO 3 - ) sources and biogeochemical transformations is critical for understanding the different nitrogen (N) pathways, and thus, for controlling diffuse pollution in groundwater affected by livestock and agricultural activities. This study combines chemical data, including environmental isotopes (δ 2 H H2O , δ 18 O H2O , δ 15 N NO3, and δ 18 O NO3 ), with land use/land cover data and a Bayesian isotope mixing model, with the aim of reducing the uncertainty when estimating the contributions of different pollution sources. Sampling was taken from 53 groundwater sites in Comarca Lagunera, northern Mexico, during 2018. The results revealed that the NO 3 - (as N) concentration ranged from 0.01 to 109 mg/L, with more than 32% of the sites exceeding the safe limit for drinking water quality established by the World Health Organization (10 mg/L). Moreover, according to the groundwater flow path, different biogeochemical transformations were observed throughout the study area: microbial nitrification was dominant in the groundwater recharge areas with elevated NO 3 - concentrations; in the transition zones a mixing of different transformations, such as nitrification, denitrification, and/or volatilization, were identified, associated to moderate NO 3 - concentrations; whereas in the discharge area the main process affecting NO 3 - concentrations was denitrification, resulting in low NO 3 - concentrations. The results of the MixSIAR isotope mixing model revealed that the application of manure from concentrated animal-feeding operations (∼48%) and urban sewage (∼43%) were the primary contributors of NO 3 - pollution, whereas synthetic fertilizers (∼5%), soil organic nitrogen (∼4%), and atmospheric deposition played a less important role. Finally, an estimation of an uncertainty index (UI90) of the isotope mixing results indicated that the uncertainties associated with atmospheric deposition and NO 3 - -fertilizers were the lowest (0.05 and 0.07, respectively), while those associated with manure and sewage were the highest (0.24 and 0.20, respectively)., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

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 Ca 2+ , Mg 2+ , Ba 2+ , Sr 2+ 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 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., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)