Effect of subsurface dams on saltwater intrusion and fresh groundwater discharge

Subsurface dams are widely used to prevent saltwater intrusion around the world. A subsurface dam blocks the groundwater movement both from and towards the sea. This blockage often leads to an accumulation of pollutants and salt on the inland and sea-side of the dam, respectively. While the latter is intended, the former effect is not desired and poses a huge problem in groundwater management. Herein, we propose the use of dams of minimum effective height to prevent saltwater intrusion, and the use of the fresh groundwater discharge to assess the environmental performance of the subsurface dam. Laboratory tests and numerical simulations were used to study the effects of dam height, distance from the saltwater boundary, and head difference on the subsurface dam's saltwater intrusion prevention efficiency and fresh groundwater discharge. We found that i) the fresh groundwater discharge reaches its peak at the minimum dam effective height, and ii) the minimum effective dam height is shorter than the height of SWI without the dam. This means, under the premise of effectively preventing SWI, we can reduce both construction costs and increase fresh groundwater discharge through constructing the dam with the minimum effective dam height. When the dam height was less than the minimum effective dam height, the subsurface dam had little effect in preventing saltwater intrusion. As the dam distance to the shoreline increased, the minimum effective dam height and the peak fresh groundwater discharge decreased simultaneously. A reduction of the dam height is conducive to saving engineering cost, while a reduction of the peak fresh groundwater discharge favors the accumulation of land-based pollutants and salt. Although an increased distance of the dam to the coast seems more economic during construction, this also implies a larger inland soil salinization and accumulation of pollutants. The site selection of subsurface dams is therefore an optimization task that should consider both the engineering cost and ecological environmental effects.