Joint estimation of groundwater salinity and hydrogeological parameters using variable-density groundwater flow, salt transport modelling and airborne electromagnetic surveys

Freshwater aquifers in low elevation coastal zones are known to be threatened by saltwater intrusion (SWI). As these areas host a significant share of the world's population, an excellent understanding of this phenomenon is required to effectively manage the availability of freshwater. SWI is a dynamic process, therefore saline groundwater distributions can change quickly over time – particularly in stressed areas with anthropogenic drivers. To model these changes, regional 3D variable-density groundwater (3D-VDG) flow and coupled salt transport models are often used to estimate the current (and future distributions) of saline groundwater. Unfortunately, parameterising 3D-VDG models is a challenging task with many uncertainties. Generally, uncertainty is reduced through the addition of observational data – such as Airborne Electromagnetic (AEM) surveys or ground-based information – that offer information about parameters such as salinity and hydraulic head. Recent research has shown the ability of AEM surveys to provide accurate 3D groundwater salinity models across regional scales, as well as highlighting the potential for good survey repeatability. To this end we investigated the novel approach of using repeat AEM surveys (flown over the same area at different points in time) and 3D-VDG models to jointly improve the parameterisation of 3D-VDG models - while simultaneously providing a detailed 3D map of groundwater salinity distributions. Using detailed 3D synthetic models, the results of this study quantitatively highlight the usefulness of this approach, while offering practical information on implementation and further research