Davide Gatto

Seawater intrusion resulting from pumping in coastal aquifers has been extensively studied to assess the performance of coastal pumping wells. Researchers have examined various aspects such as the construction of wells, whether they fully or partially penetrate the aquifer, and the associated processes like saltwater up-coning. Sand tank experiments have been conducted to investigate saltwater up-coning, comparing the results with analytical solutions and numerical models. Analytical solutions typically assume immiscible freshwater and seawater, while numerical models account for more complex processes such as dispersive mixing, transient phenomena, and heterogeneity. Although intermittent pumping of coastal groundwater is common, most studies on seawater intrusion assume constant or long-term pumping. However, intermittent pumping is likely to create different patterns of seawater intrusion and up-coning compared to constant pumping. Temporal fluctuations in hydraulic gradients during intermittent pumping can lead to more dispersive conditions, similar to the additional dispersiveness caused by tides. Previous studies have shown that temporal fluctuations induce transverse solute spreading. Notably, Pool et al. found that transient tidal oscillations significantly widen the mixing zone through three-dimensional stochastic modeling. Therefore, to understand the key controlling processes of intermittent pumping in coastal aquifers, both physical experimentation and numerical simulation are necessary. For pumping experiments in sand tanks, a three-dimensional setup is the optimal choice to capture the dynamics of the problem and minimize the effect of sand tank boundaries on seawater intrusion phenomena. However, obtaining observations of salt plume dynamics in three-dimensional experiments can be challenging. As a result, two-dimensional sand tanks have been widely used to observe variations in saltwater plumes, focusing on a cross-sectional profile where the pumping well represents a horizontal extraction feature parallel to the coast. The current study aims to explore the behavior of the freshwater-seawater interface under different intermittent pumping scenarios in a laboratory-scale coastal aquifer using sand tank experimentation. This study builds upon prior research that predominantly considered constant pumping scenarios. Previous two-dimensional sand tank experiments examined transient saltwater up-coning under varying pumping rates, saltwater densities, and well penetration depths. They discovered that saltwater plumes exhibited dispersion during up-coning but eventually resembled sharp interfaces after reaching the pumping well, indicating that freshwater extraction from coastal groundwater wells can occur despite saltwater up-coning. Additional research revealed that the shape of salt plumes observed in some experiments was influenced by dye tracer adsorption. Furthermore, experiments considering inclined freshwater-saltwater interfaces demonstrated that the critical pumping rate and critical time for saltwater invasion were more sensitive to well location (distance to the coastal boundary) rather than well depth, and higher dispersion resulted in faster up-coning. Prior experimental studies of saltwater up-coning mostly assumed homogeneous aquifer conditions. However, coastal aquifer systems are typically heterogeneous and often consist of multiple layers. Therefore, further research is warranted to investigate the processes associated with saltwater up-coning in heterogeneous coastal aquifers. The objective of the current study is to compare the performance of a well in a laboratory-scale coastal aquifer under constant and intermittent pumping conditions while avoiding the extraction of saltwater. Two approaches are considered: (a) comparing intermittent pumping with a lower mean pumping rate to constant pumping based on the volume of extracted water before well salinization, and (b) assessing the effects of constant and intermittent pumping on seawater intrusion by comparing well salinity and the volume of seawater in the aquifer when the total extracted water volume is the same. Additionally, both homogeneous and layered aquifers are evaluated in the study.

The aim of the research will be to expand the materials with which the phenomenon has already been studied and also the surrounding conditions.

Porous and fractured media will be used. Both cases will be studied both in the free surface and in the confined stratum mode.

We expect to better understand how the pollutants inside the aquifers, whether they are porous or fractured, move.