Assessment of Hydrochemical Properties in a Transboundary Alluvial Aquifer Using Chemical, Isotopic and Correlation Analyses

Abstract

This study investigates the hydrochemical characteristics of the transboundary alluvial aquifer of the Sava River, shared between Eastern Slavonia (Croatia) and Bosnian Posavina (Bosnia and Herzegovina). Hydrochemical parameters major ions, stable isotopes (δ¹⁸O and δ²H), correlation analysis and Piper diagrams were used to identify the dominant geochemical processes, evaluate groundwater–surface water interactions, and assess hydraulic connectivity across the border. Results indicate that groundwater is predominantly of the Ca2+Mg2+–HCO₃- type, with its chemistry mainly controlled by carbonate mineral dissolution and water–rock interactions. Stable isotope data confirm a predominantly meteoric origin of groundwater and reveal differences in recharge conditions between shallow and deeper aquifer layers. Groundwater–river interactions are largely confined to shallow hydraulically connected zones, whereas deeper confined aquifers exhibit limited present-day exchange with the Sava River. Overall, the study demonstrates that the Sava alluvial aquifer is a complex multilayered transboundary groundwater system with varying degrees of hydraulic connectivity and groundwater evolution.