Tidal, freshwater-saltwater interactions: Three-dimensional sand-tank experiments

Laboratory-scale experiments are commonly used to investigate the dynamics of freshwater-saltwater interactions in coastal aquifers. This study presents a rare attempt at producing tidal, freshwater-saltwater interactions in a three-dimensional laboratory sand tank, including with freshwater injection, which creates complex flow effects. Comprehensive monitoring was undertaken to capture key processes, fluxes and salinity changes using three different techniques: image analysis, monitoring wells that provide novel time-series of water table and freshwater-saltwater interface fluctuations, and flux measurements at the sand tank boundaries. A recently devised approach (float-laser device) for tracking the freshwater-saltwater interface and the water table was adopted for the first time in sand tank experiments to demonstrate its application. Numerical modeling using SEAWAT was applied to further examine experimental conditions, allowing for a critical evaluation of each approach. The results show that the float-laser devices successfully tracked the tidal freshwater-saltwater interface, consistent with image analysis results, but with slightly smaller amplitudes of oscillation compared to those obtained from the numerical model. The inclusion of imagery from underneath the sand tank allowed for unique insights into the three-dimensionality of the interface position, albeit variability in the third dimension was mild. This study provides a leading example of sand tank experimentation, providing new insight into the transience of the freshwater-saltwater interface under the actions of tides and freshwater injection.