Effects of a subsurface dam on groundwater flow and salt transport in stratified coastal aquifers: Experiments and simulations

Subsurface dams are now widely used to mitigate seawater intrusion in coastal aquifers. Previous studies have mostly focused on the optimal design of subsurface dams under various conditions, but such an attempt in the context of a three-layered (a low-permeability layer in between two high-permeability layers) aquifer is lacked. In reality, many coastal aquifer exhibit such a geological layering feature. This study, using laboratory experiments and numerical simulations, filled the research gap. The results show that, the addition of a subsurface dam to a three-layered coastal aquifer may significantly prolong the particle travel times in both the freshwater zone and the saltwater wedge. Also, the saltwater-freshwater mixing zone in a stratified aquifer can be narrowed by a subsurface dam. However, this trend is reversed when the subsurface dam is located rather landward. Moreover, a subsurface dam may hinder the formation of “freshwater fingers”, which would have formed in aquifers with highly contrasting permeability coefficients between the three layers. Further analysis reveals high-level sensitivity of saltwater-freshwater mixing zone to the height and location of the subsurface dam, and the permeability of the low-permeability layer. These findings promote deeper insights into the impact of subsurface dams on the underground hydrodynamics in complex nearshore groundwater systems. More importantly, conclusions drawn from the study may help to evaluate the environmental impact and optimize the design of subsurface dams to be constructed, ultimately assisting the management of coastal fresh groundwater resources.