Groundwater salinization dynamics - an isotope approach

The evolution of groundwater salinization often is difficult identify, track and characterize. Additional factors such as hydro-climatic variability, pumping, pollution, and mixing with other groundwater end members in the borehole add complexity to the hydrochemical signal. Different isotope methods can provide insight into the inherent dynamics of groundwater salinization. The application of different isotope systems has been studied both theoretically using reactive transport models and in the field in different case studies. Tritium, SF₆ and ¹⁴C, in combination with ¹³C as a marker of geochemical interaction, provide a straightforward access to information on the hydrodynamics of the flow system. Combined with data on salinity and on the geochemical fingerprint of salinization, these residence time tracers provide a first insight into the expected dynamics of changes. Radium isotope ratios allow an even more detailed reconstruction as the sorption of radium depends on the salinity of ambient groundwater and affects the transport behaviour. The chromatographic effect induced by salinity dependent transport behaviour can be used to date the onset of salinization. These concepts have been validated both by applying coupled groundwater flow and reactive transport models with PhreeqC and Geochemical Workbench and practically in several test sites. The transport modeling indicated that especially the combination of conservative residence time tracers and non-conservative tracers yield information on the dynamics of groundwater salinization, especially at time scales of decades to centuries and more. The verification of these concepts in case studies in Saudi-Arabia, in the Dead Sea valley and in the coastal aquifer of Samos within the PRIMA project MEDSAL confirms the viability of isotope methods in groundwater salinization studies.