Terrestrial solute fingering flow behind subsurface physical barriers during seawater intrusion

Blocking seawater intrusion in coastal aquifers through impervious subsurface physical barriers is a commonly used method, which creates a relatively closed hydrodynamic environment behind the barrier for the transport of land-sourced contaminants. This study investigated the phenomenon of terrestrial solute fingering flow behind subsurface physical barriers for the first time under field-scale hydrological and hydrogeological conditions. A prediction model for the onset of flow instability and the size of subsequent solute fingers was proposed based on the identified effects of these conditions. Once a modified Rayleigh number exceeded 4000, solute fingering flow behind subsurface physical barriers would occur, and the size of individual fingers followed a sigmoidal function growth with increasing Rayleigh number. Solute fingering flow resulted in a series of morphological changes in individual fingers from finger-like to foot-like with decreased salinity. The movement of solute fingers altered the trajectory and residence time of tracer particles, which led to an early particle arrival for those initially located behind. Compared to a no-finger situation, solute fingering flow accelerated the movement of tracer particles out of the suspended barriers with a reduced residence time by 45%. Shortened residence times and changed transport patterns as a result of solute fingering flow may have significant implications for the fate and transport of terrestrial contaminants. Thus, caution should be exercised in the interpretation of field measurements on groundwater contamination in coastal aquifers if solute fingering flow behind subsurface physical barriers is expected, e.g., from the present prediction model.