Experimental Study of Seawater Intrusion in Stratified Layers with Sloping Ocean–Aquifer Boundary

Abstract

Seawater intrusion (SWI) is a concern for coastline-saturated aquifers because it increases salinity, which contaminates and degrades groundwater quality. Rising sea levels and other natural processes, as well as anthropogenic activities such as overpumping, are the causes of intrusion. Because these experiments are the most reliable methods for study, this study focused on the experimental investigation of SWI in a hypothetical stratified aquifer with parallel and perpendicular stratification and a sloping beach boundary. Stratified media were formed by stacking layers in parallel and perpendicular layers in different combinations. Sensitivity analysis was performed with 10-, 20-, and 30-cm gaps for interactions with 4-layer formations with height to toe length expanding from 0.12 to 2.33. High-resolution images were captured at specified intervals and demarcated for a clear visualization of intrusion. A homogenous base case was used to evaluate the feasibility of the proposed method and the results were compared with those of a sandbox apparatus and found to be consistent in terms of the shape of the wedge and the toe length. The height of the intrusion ranges from 28.3 to 32.2 cm. The intrusion was fast and accurate due to the higher permeability of the material. It was concluded that the toe length–height ratio varies from 0.43 to 7.0. For materials with lower permeabilities, this ratio is small because the intrusion is slow but steady in both directions; however, for materials with higher permeabilities, this ratio corresponds to a higher value. The intrusion is restricted to the individual layer until it encounters an impervious layer and follows a proportional pattern with an increasing area of intrusion as the opening area increases. This study focused on determining the intrusion behavior in coastal aquifers with monotonically increasing permeability under a layered stratification condition and subjected to the application of inclined boundaries. The study revealed that the use of a glass box apparatus with an inclined ocean–aquifer boundary is the best practical approach for studying intrusion. The occurrence of intrusion due to density differences and heterogeneity represents a case of passive seawater intrusion. The heterogeneity of the aquifer varies at the regional scale; thus, the results obtained from this study may be applied to real-world scenarios by implementing upscaling, temporal dynamics, heterogeneity, and desired boundary conditions. To quantify the intrusion in parallel stratification, a ratio of toe length to height of the intrusion is devised, which is the best approach to follow. The intrusion in the parallel configuration directly depends on the face of the inclined formation subjected to seawater, which forms a parabolic profile, while for the perpendicular configuration, the potential to intrude decreases with each layer as the intrusion is subjected to one layer at a time, initially leading to a parabolic profile that changes to linear in subsequent layers.