Mitigation strategies against seawater intrusion in the context of climate change

In coastal regions, where about 40% of world population is settled, freshwater aquifers are affected by a saltwater intrusion. The presence of elevate salt concentrations, among the most common pollutants in groundwater aquifers, represents a world-wide spread problem for coastal areas, such as Mediterranean coasts, both East and West coasts in the U. S., Gulf of Mexico and Middle East coasts. The seriousness of the situation is enhanced by the high demand of water supply, especially in the drier periods, and by the mean sea-level rise due to climate change. So that seawater will encroach farther inland and will threaten the available fresh groundwater supply, affecting not only human livelihood, but also coastal ecosystems. The form and transformation of the seaward hydraulic gradient of the aquifer and a constant freshwater discharge into the sea are fundamental in order to control the rate of intrusion.

To maintain the seaward gradient in the system the aquifer may be artificially recharged by freshwater by increasing the inland piezometric heads. The purpose of this solution is to create a hydraulic barrier against the inland flow of saline water by injecting freshwater in the vicinity of the shoreline. For phreatic aquifers both injection wells and surface spreading of water, such as irrigation, may be applied. Surface reservoirs, lakes and canals can be used as recharge systems for unconfined aquifers through freshwater infiltration (Hussain et al., 2019).

To assess the effectiveness of this mitigation approach and the amount of volumes of freshwater required, physical experiments are developed in a laboratory canal developed to reproduce a controlled heterogeneous porous media.

The sandbox measures 500 cm long by 30 cm wide by 60 cm high, with 3 cm thick plexiglass walls. Two tanks are located upstream and downstream from the sandbox, with volumes of about 0.5 m³ and 2.0 m³, respectively. The upstream tank is filled with fresh-water and is continuously supplied by a small pump, providing fresh-water recharge. The downstream tank is filled with salt-water, previously prepared by adding salt to fresh-water till a proper density is reached, and it represents the sea. This canal has been used in previous works (Bouzaglou et al., 2018, Crestani et al., 2022), but in the present the homogeneous porous media has been substituted by three different nominal size ranges of glass beads, equal to 0.3-0.4, 0.4-0.8 and 1.0-1.3 mm respectively, organized in 250 cells, each of size 20x30x5 cm³ to reproduce a prescribed statistical anisotropic structure.

The evidences deduced from the physical experiments developed simulating the seawater intrusion-retreat phenomenon due to drought periods are discussed in comparison with the results of a numerical model.