Float-laser device for measuring the water table and freshwater–saltwater interface in laboratory-scale experimental systems

Processes in coastal aquifers involving variable-density fluids are often investigated using laboratory sand tanks. However, experimental monitoring of the interface between liquids of different densities is challenging, typically requiring dye tracers and photographic analysis. These are susceptible to problems of tracer adsorption and image inhomogeneity, and experiments are often limited to two-dimensional cross-sections. In addition, the water-table position is an important factor, but its accurate measurement is often difficult. A device has been developed for sand-tank experiments to obtain high-frequency, high-precision measurements of both the freshwater–saltwater interface (inferred from density) and the water-table depth, using a float and a laser sensor in a monitoring well. The proposed device was tested using a two-column experiment, allowing for the estimation of measurement errors, which were generally less than 1 mm. The measurement device can be modified to allow for the monitoring of multiple salinity values, and is well suited to sand-box experiments where salinity dynamics are otherwise difficult to visualize.