Abstract:
Coastal aquifers are dynamic mixing zones where fresh groundwater and seawater interact while undergoing hydro-biogeochemical transformations that modify groundwater composition before discharge to the coastal ocean. This study investigates how freshwater–seawater mixing and reactive processes jointly shape groundwater chemistry in the multilayer alluvial aquifer of the Argentona stream, northeastern Spain. The aquifer is characterized by permeable sandy and gravelly units separated by finer-grained layers that restrict vertical connectivity and create contrasting hydro-biogeochemical environments with depth.

Groundwater was sampled during winter and summer across a multilevel monitoring network, and 21 hydrochemical and isotopic variables were integrated to characterize mixing, redox conditions, nutrient cycling, cation exchange, mineral interactions, and organic-matter transformations. We applied the AMAR/rEMMA-MIX framework to identify freshwater and seawater end-members, estimate mixing proportions using reaction-invariant components, and quantify apparent reaction amounts under both global and domain-specific reaction networks.

By linking groundwater mixing with independently constrained hydro-biogeochemical domains, this study explores how aquifer heterogeneity influences the spatial organization and apparent magnitude of reactions that modify groundwater before discharge to the coastal ocean.