Salinisation and freshening of a coastal aquifer system in the southern Marais Poitevin (France): a quantitative hydrogeochemical reassessment

This study presents a quantitative hydrogeochemical reassessment of salinisation and freshening processes in the coastal aquifer system of the southern Marais Poitevin (France). It reanalyses hydrochemical data collected in February 2007 and previously published in 2023, without incorporating new measurements. The methodological approach combines PHREEQC geochemical modelling, freshwater–seawater mixing analysis, quantification of ion exchange, stoichiometric reaction budgets, Monte Carlo simulations and compositional statistical methods. The results confirm the marine origin of groundwater salinity but lead to a revision of several previously proposed hydrogeochemical mechanisms. In the confined Upper Oxfordian aquifer, sodium deficits associated with calcium and magnesium enrichments are consistent with reverse cation exchange during salinisation rather than predominantly carbonate dissolution. In certain parts of the Quaternary aquifer, hydrochemical signatures instead suggest freshening accompanied by cation exchange. The study also identifies substantial sulfate deficits and strontium and fluoride enrichments, while explicitly addressing uncertainties and limitations in identifying the underlying geochemical reactions. The scientific contribution of this study lies in the quantitative revision of the conceptual model of hydrochemical evolution in these aquifers using previously published data, through a more comprehensive and reproducible analytical approach.