SALINITY PROFILES OF TRANSITION ZONES BENEATH FRESHWATER LENSES IN STRIP ISLANDS

The configuration of the freshwater-seawater transition zone is traditionally treated in one dimension, the percent seawater increasing with depth according to an error function. The symmetry of the error function ensures that the 50% seawater line bisects the transition zone. As a result, the 50% seawater line represents the freshwater inventory, or the position of the interface in a lens with similar volume of fresh water but no transition zone. This is the interface which the Dupuit-Ghyben-Herzberg (DGH) theory predicts. The depth to the basement below the lens does not enter into DGH theory, because the seawater is assumed to be static.

To investigate whether the symmetry of the transition zone is disrupted by the presence of a shallow basement, we used SUTRA to perform a sensitivity analysis of parameters in models of a strip-island freshwater lens with varying depths to the no-flow boundary representing basement. We found that when we move up the basement in the model, the transition zone in the center of the island is translated upward and compressed, even in cases where the transition zone is entirely above the basement. The bottom part of the transition zone is compressed more than the top part, so that the 98% seawater line moves upward more than the 50% seawater line, which in turn moves upward more than the 2% seawater line. The top of the transition zone (with less than 50% seawater) is therefore thicker than the bottom of the transition zone (with more than 50% seawater) in the no-longer-symmetric transition zone. Nevertheless, the freshwater inventory line still coincides with the 50% seawater line along the axis of the lens.

Overall, then, the presence of an impermeable basement below the transition zone affects the freshwater inventory within the lens including the transition zone. DGH theory completely misses this effect.