The Dynamics of Sea Tide-Induced Fluctuations of Groundwater Level and Freshwater-Saltwater Interface in Coastal Aquifers: Laboratory Experiments and Numerical Modeling

Abstract

Laboratory experiments were conducted in a vertical, two-dimensional, rectangular flow tank, simulating the response of a phreatic coastal aquifer to a sea tide. Imposed sinusoidal fluctuations of the saltwater level at one side of the flow tank caused three types of fluctuations: (a) hydraulic head throughout the aquifer, (b) saturation degree within the capillary fringe, and (c) salt concentration surrounding the freshwater-saltwater interface (FSI), all recorded by head, saturation, and salinity sensors, respectively. Significant time lags were observed both in the saturation degree within the unsaturated zone and in the salinity within the FSI. All measured values, recorded by the three types of sensors, were simulated and reproduced using a numerical model. The calibrated model was used for mapping the time lags throughout the aquifer. It was found that the time lag of saturation fluctuations within the unsaturated zone increased upward from the groundwater level as the unsaturated hydraulic conductivity decreased. Similarly, the time lag of salinity fluctuations within the FSI increased downward, with distance from the groundwater level. We interpret the low hydraulic conductivity at the capillary zone as the source of attenuation of both saturation and salinity, because both are controlled by the vertical advection of the whole freshwater body. This advection is significantly slower compared to the dynamics of pressure diffusion. The uniqueness of this study is that it provides quantitative data on the attenuation at the capillary zone and its effect on the salinity time lag in coastal aquifer systems.

Haline Convection within a Fresh-Saline Water Interface in a Stratified Coastal Aquifer Induced by Tide

“…Results show the time lag in stratified systems is generally higher than in homogeneous ones by a factor of two. In a homogeneous porous medium, the time lag increases with distance from the drainage point, in the vertical direction as well as in the horizontal one, as shown in previous studies [6]. Results also show (Figure 6) that, in the stratified system, the time lag is significantly higher at the aquitards than those at the aquifers.…”

“…The monitoring setup includes several measurement systems [6,20,21]. More than 150 sensors are placed at the backside of the flow tank for in situ voltage values (Figure 1b).…”

“…They induce groundwater level fluctuations with a periodicity similar to tidal periodicity [1,2], as well as fluctuations of the FSI's location at the same periodicity [3][4][5]. Using a laboratory physical model, together with a matching numerical model, Levanon et al [6] found a significant time lag between tide fluctuations and GWL and FSI responses. The time lag of the salinity values of groundwater within the FSI increases with depth and with distance from the sea floor.…”

The impact of aquifer stratification on saltwater intrusion characteristics. Comprehensive laboratory and numerical study

“…The majority of laboratory studies have recreated SWI in homogeneous synthetic aquifers (Abdelgawad et al, 2018;Abdoulhalik & Ahmed, 2018a, 2018bQ. Chang et al, 2019;Guo et al, 2019;Kuan et al, 2019;Lee et al, 2019;Levanon et al, 2019;Memari et al, 2020;Na et al, 2019;Noorabadi et al, 2017;Shen et al, 2020;Stoeckl et al, 2019;Yu et al, 2019). Nevertheless, sandbox setups have been successfully employed to study freshwater-saltwater interface in aquifers with structuredsedimentary heterogeneity (Houben et al, 2018), in freshwater lenses, located in heterogeneous island aquifers (Dose et al, 2014;Stoeckl et al, 2015), as well as inside fractured porous media (Etsias et al, 2021).…”

“…Sandbox laboratory setups have been extensively utilized to study the fundamental mechanisms of saltwater intrusion in coastal aquifers (Abdoulhalik et al, 2017; Armanuos et al, 2019; Goswami & Clement, 2007; Konz et al, 2008; Kuan et al, 2012; Liu et al, 2017; Robinson et al, 2016; Stoeckl & Houben, 2012; Takahashi et al, 2018; Zhang et al, 2002). The majority of laboratory studies have recreated SWI in homogeneous synthetic aquifers (Abdelgawad et al, 2018; Abdoulhalik & Ahmed, 2018a, 2018b; Q. Chang et al, 2019; Guo et al, 2019; Kuan et al, 2019; Lee et al, 2019; Levanon et al, 2019; Memari et al, 2020; Na et al, 2019; Noorabadi et al, 2017; Shen et al, 2020; Stoeckl et al, 2019; Yu et al, 2019). Nevertheless, sandbox setups have been successfully employed to study freshwater—saltwater interface in aquifers with structured—sedimentary heterogeneity (Houben et al, 2018), in freshwater lenses, located in heterogeneous island aquifers (Dose et al, 2014; Stoeckl et al, 2015), as well as inside fractured porous media (Etsias et al, 2021).…”