A large-scale laboratory experiment of seawater intrusion in heterogeneous aquifers affected by drought periods

Most density-dependent flow and transport models assume homogeneity of natural aquifers, a strong simplification with respect to reality, while subsurface formations are known to have spatially variable properties (e.g. Freeze, 1975). Previous studies of saltwater intrusion in heterogeneous aquifers have considered mainly macro-scale geological structures, but the effects of local heterogeneities on density-dependent flow and transport are known to be highly affected by spatially correlated distributions of hydraulic conductivities (e.g., Dagan and Zeitoun, 1998, Prasad and Simmons, 2003, Li et al., 2022). Moreover, investigations have been performed mainly via numerical modeling and, to the best of our knowledge, only in one case numerical results have been compared with physical evidence from laboratory reproduction of a heterogeneous media (Koch and Starke, 2001).

The present work describes the design and the realization activities developed to reproduce a controlled heterogeneous porous media in a laboratory flume, aimed at defining the influence of the hydraulic conductivity spatial variability on the density-dependent transport in coastal phreatic aquifers.

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 of the sandbox, with volumes of approximately 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. In both tanks the level is maintained constant via two spillways, whose height can be adjusted. The discharge through the downstream spillway can be measured.

The flume has been used in previous works (Bouzaglou et al., 2018, Crestani et al., 2022), but here 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 (Figure 1).

Figure 1- Sandbox 3D view from upstream (left side) to downstream (right side)

After a preliminary analysis carried out by constant head permeameter tests on each glass beads nominal size range, the hydraulic characterization of the whole heterogeneous formation has been developed considering the filtration process that affects different thicknesses of the aquifer (10, 20, 30, 40 cm) forced by three upstream-downstream head differences (2, 4 and 6 cm).

During the saltwater intrusion experiment a water level difference upstream - downstream of 2 cm has been maintained for 8 days, introducing two separate drought periods (about 8 and 10 hours) at the end of the second and of the third days respectively.

The findings from the heterogeneous media characterization and the seawater advance-retreat phenomenon are discussed in comparison with the results of a numerical model.

This study has been co-funded by the Interreg Italy–Croatia CBC Programme 2014–2020 (Priority Axes: Safety and Resilience) through the ERDF as a part of the projects MoST (AID: 10047742) and SeCure (AID: 10419304).