Effects of a subsurface dam on unstable flow pattern in coastal aquifers

This study examines the impact of the subsurface dam on saltwater-freshwater mixing and groundwater exchange in unstable flow systems within tidally influenced coastal aquifers. The results indicate that after the construction of the subsurface dam, the salinity transitions from irregular fluctuations to periodic oscillations. Additionally, the period calculated through the Fourier transform can reflect the speed of salt finger movement. Although the subsurface dam functions as an impermeable vertical barrier, it accelerates the speed of salt fingers movement. Furthermore, with increasing dam height or when positioned closer to the landside, this acceleration effect becomes more pronounced. In unstable flow systems, flow velocity exhibits oscillations on both long and short time scales. Specifically, in zones where salt fingers are actively developing, the vertical flow velocity is primarily influenced by salt fingers (a long timescale), and the subsurface dam reduces this influence. In other zones, the horizontal and vertical flow velocities are mainly governed by tidal forces (a short timescale). It is noteworthy that at the aquifer boundary influenced by salt fingers, as groundwater flux increases, salinity tends to decrease, exhibiting an inverse spatial relationship. This suggests that zones with significant salinity variations are often regions of active groundwater discharge, which require special attention. Finally, this study recommends that when constructing a subsurface dam near the mean sea level, with a height greater than half of the aquifer thickness, it can more effectively reduce seawater intrusion into the aquifer. This design can significantly enhance protective efficacy and protect freshwater resources from seawater contamination.