Hydrogeochemical Process Controls on Groundwater Quality in the Gran La Plata Multilayer Aquifer System, Argentina

ABSTRACT

Multilayer aquifer systems beneath the urbanising plains of Latin America combine intensive abstraction with a strong geological inheritance, and at their coastal margins the origin of groundwater salinity is a recurrent, often unresolved question. The Gran La Plata multilayer system, Argentina, is characterised here from 111 water samples along an inland‐to‐coastal transect, through a process‐based reading of the major‐ion signatures. Cation exchange is the dominant control (negative Chloro‐Alkaline Indices in 93.2% of the 103 groundwater samples), coupled with carbonate dissolution, yielding a sodium–bicarbonate composition more advanced in the semi‐confined Puelche than in the phreatic Pampeano. Within each unit, the few coastal samples ( n  = 4 in the Pampeano, n  = 7 in the Puelche) are far more saline than inland ones (median EC roughly six times higher in the Puelche) while the exchange signature shows no matching shift, remaining a property of the unit, so salinity tracks geomorphic position, not flow domain. The coastal composition, contrasted against published isotopic and hydrodynamic evidence for the system, is most consistent with a relict palaeo‐marine origin rather than active seawater intrusion from the present‐day fresh estuary. These processes leave a predictable imprint on quality metrics: the sodium adsorption ratio rates nearly all samples suitable for irrigation, yet the sodium‐dominance and carbonate‐balance indices reveal a latent hazard, and an adjusted sodium ratio confirms its direction. The weighted drinking‐water quality index, in turn, tracks geogenic mineralisation rather than direct health risk. Correctly diagnosing the dominant process is therefore a prerequisite for interpreting any quality metric. This process‐first framework is transferable across the urbanising multilayer loessic aquifers of the Chaco‐Pampean plain.