Abstraction, desalination and recharge method to control seawater intrusion into unconfined coastal aquifers

Fresh-saline water dynamics in coastal aquifers: Sand tank experiments with MAR-wells injecting at intermittent regimes

Seawater intrusion occurs commonly in coastal aquifers around the world, threatening the availability and usability of fresh groundwater resources for vegetation and human uses. The rapid growth of the world population and urbanization requires sound strategies for protection and management of the freshwater resources, especially coastal groundwater. This goal can only be achieved through proper understanding of the processes that underlie seawater intrusion in coastal aquifers. Major insights have been gained over the past several decades, in particular, roles of the density-driven flow in driving and maintaining the invasive flow of saltwater. However, most studies have linked the seawater intrusion process merely to the level of salt content through the free convection induced by the salinity contrast between groundwater and seawater but ignored their temperature difference. In reality, the thermal contrast between coastal groundwater and marine seawater may range up to 15°C in absolute value with either warmer or colder seawater. Such thermal contrast can alter seawater circulation through the coastal aquifer, which in turn affects the biogeochemical reactions of land-sourced pollutants in the aquifer prior to discharge to the marine ecosystem.This research aimed to investigate the combined effects of salinity and temperature contrasts on the interactions between freshwater and seawater in unconfined coastal aquifers. Using laboratory experiments and numerical simulations, this research explored the coastal groundwater dynamics under various boundary settings with regards to thermal variations, tidal forcing and seasonal changes of seawater temperature. Findings from this research provided insights into the importance of temperature variations on various key processes in coastal aquifers under the condition of either static sea level or tidal oscillation.The effects of temperature contrast were first investigated for the seaward boundary of static level using physical experiments and numerical models in combination with tracer tracking. With the static sea level, the thermal contrast induced long-term impacts on the aquifer and altered background flow patterns and transport activities. The position of the saltwater wedge toe was modified significantly by the presence of temperature gradient either landward or seaward. Colder seawater enhanced the advancement of saltwater while warmer seawater hindered it. More importantly, the seawater circulation pattern changed dramatically in the latter case. A second circulation cell was discovered for the first time near the seaward boundary. The regular landward circulation cell was pushed to the vicinity of the interface where considerably larger velocity was observed. In-depth sensitivity analysis revealed the important role of spatial correlation between temperature-induced and salinity-induced density gradients, especially at the base of the aquifer, in driving the formation of the new cell.Both laboratory experiments and numerical simulation were carried out to investigate the thermal effects under the condition of tidal oscillation. The responses of the saltwater wedge were found to be similar to those under the static sea level, in particular, the retreat and advance of the wedge with warmer and colder seawater, respectively. The mixing zone widened as a result of the tidal fluctuation. Meanwhile, the upper saline plume and the freshwater discharge zone expanded in the warmer seawater case and contracted with colder seawater. The increased seawater temperature also intensified water exchange across aquifer-ocean interface, seawater circulation and the submarine groundwater discharge. Furthermore, tidally induced seawater circulation intensified with increased contribution to the submarine groundwater discharge compared with density-driven seawater circulation. All these characteristics were persistent over a range of tidal amplitudes. These results shed light on the importance of the thermal effects and have important implications for the assessment of the biogeochemical processes in coastal aquifers.The seasonal variations of the temperature contrast were then examined based on numerical simulations. The results showed clearly seasonality of the aquifer – ocean exchange and seawater circulation induced by the seasonal variation of seawater temperature in both cases with the static sea level and tidal conditions. Compared with the cases of the isothermal condition, all fluxes increased during colder months and decreased during warmer months. The periodic oscillation of the thermally induced density gradient resulted in a continuously changing mode of saltwater flow in the saltwater wedge. The flow path and transit time of circulating seawater shortened considerably in comparison with that in the isothermal case. This finding is particularly important for the evaluation of transport of land-sourced contaminants to the marine environment.The insights into the thermal effects on coastal unconfined aquifers gained from laboratory experiments and numerical simulations were applied to calculate a thermal impact factor and a thermal sensitivity index for aquifers along global coastlines based on local conditions of freshwater temperature and temperature contrast. The results suggested that the temperature effect is significant and would either amplify or reduce the impact of sea level rise on the vulnerability of coastal aquifers over a large proportion of the global coastlines.

Groundwater systems are considered major freshwater sources for many coastal aquifers worldwide. Seawater intrusion (SWI) inland into freshwater coastal aquifers is a common environmental problem that causes deterioration of the groundwater quality. This research investigates the effectiveness of using an injection through a well to mitigate the SWI in sloping beds of unconfined coastal aquifers. The interface was simulated using SEAWAT code. The repulsion ratios due to the length of the SWI wedge (RL) and the area of the saltwater wedge (RA) were computed. A sensitivity analysis was conducted to recognize the change in the confining layer bed slope (horizontal, positive, and negative) and hydraulic parameters of the value of the SWI repulsion ratio. Injection at the toe itself achieved higher repulsion ratios. RL and RA declined if the injection point was located remotely and higher than the toe of the seawater wedge. Installation at the toe achieved a higher RL in positive sloping followed by horizontal and negative slopes. Moreover, the highest value of RA could be reached by injecting at the toe itself with a horizontal bed aquifer, followed by negative and positive slopes. The recharge well is confirmed as one of the most effective applications for the mitigation of SWI in sloping bed aquifers. The Akrotiri case study shows that the proposed recharging water method has a significant impact on controlling SWI and declines in both SWI wedge length and area.

Increased groundwater extraction, coupled with potential declines in recharge rates, is anticipated to negatively impact the sustainability of groundwater resources (Mehdizadeh, 2019). Coastal aquifers are particularly vulnerable, as these changes can result in a significant risk of saltwater intrusion (SI). While the fundamental mechanisms of SI are well-understood, tracking the encroachment of saline water into coastal aquifers and its risk to water extraction sources remains a complicated and expensive task. Studies have shown that self-potential (SP) could serve as an effective tool for remotely monitoring the movement of saline-freshwater interfaces due to SI (Graham, 2018).Self-potential voltages, which originate from subsurface pressure and concentration gradients, occur when these gradients lead to ion separation. This separation results in an electrical potential and a subsequent electron flow to preserve electrical neutrality. These potentials, usually in the millivolt range, can be observed and recorded in the field using electrodes. SP primarily consists of two types: electro-kinetic potentials (VEK), arising from differing flow velocities, and exclusion-diffusion potentials (VED), stemming from ion concentration gradients with varying mobilities. A previous study recorded tidal signatures in SP within a Chalk borehole located less than 2 kilometres from the English Channel (MacAllister et al., 2016). More recent research has detected similar signatures in a sand aquifer on the north coast of Northern Ireland and in a gravel aquifer on the south coast of England.In each instance, the tidal signature most prominently reflected the M2 (Principal lunar-semidiurnal) component, although in some cases other, less prominent, elements were also identified. Analysis of water level, electrical conductivity and temperature data suggests that these signatures were not due to electrokinetic potentials from tidally induced flows in and around the borehole. Rather, a more likely explanation is that nearby saline-freshwater interfaces, inducing exclusion-diffusion potentials, are responsible. Whilst further investigation is necessary to quantify, model, and fully comprehend these signals, the detection of tidal signatures in an increasing and diverse number of aquifers suggests that self-potential might be a viable technique for monitoring and providing an early warning of saline intrusion.BibliographyGraham, M. T., MacAllister, DJ., Vinogradov, J., Jackson, M. D., and A. P., Butler, (2018). Self‐potential as a predictor of seawater intrusion in coastal groundwater boreholes. Water Resources Research, 54, 6055– 6071.MacAllister, DJ., Jackson, M. D., Butler, A. P., and Vinogradov, J. (2016), Tidal influence on self‐potential measurements, J. Geophys. Res. Solid Earth, 121, 8432– 8452.Mehdizadeh, S., Badaruddin, S. and S. Khatibi, (2019). Abstraction, desalination and recharge method to control seawater intrusion into unconfined coastal aquifers. Global Journal of Environmental Science and Management, 5, 107-118.

Abstract. In coastal zones with saline groundwater, fresh groundwater lenses may form due to infiltration of rain water. The thickness of both the lens and the mixing zone, determines fresh water availability for plant growth. Due to recharge variation, the thickness of the lens and the mixing zone are not constant, which may adversely affect agricultural and natural vegetation if saline water reaches the root zone during the growing season. In this paper, we study the response of thin lenses and their mixing zone to variation of recharge. The recharge is varied using sinusoids with a range of amplitudes and frequencies. We vary lens characteristics by varying the Rayleigh number and Mass flux ratio of saline and fresh water, as these dominantly influence the thickness of thin lenses and their mixing zone. Numerical results show a linear relation between the normalised lens volume and the main lens and recharge characteristics, enabling an empirical approximation of the variation of lens thickness. Increase of the recharge amplitude causes increase and the increase of recharge frequency causes a decrease in the variation of lens thickness. The average lens thickness is not significantly influenced by these variations in recharge, contrary to the mixing zone thickness. The mixing zone thickness is compared to that of a Fickian mixing regime. A simple relation between the travelled distance of the centre of the mixing zone position due to variations in recharge and the mixing zone thickness is shown to be valid for both a sinusoidal recharge variation and actual records of daily recharge data. Starting from a step response function, convolution can be used to determine the effect of variable recharge in time. For a sinusoidal curve, we can determine delay of lens movement compared to the recharge curve as well as the lens amplitude, derived from the convolution integral. Together the proposed equations provide us with a first order approximation of lens characteristics using basic lens and recharge parameters without the use of numerical models. This enables the assessment of the vulnerability of any thin fresh water lens on saline, upward seeping groundwater to salinity stress in the root zone.

This study, building on the comprehensive discharge potential theory presented by Strack and Ausk (2015) https://doi.org/10.1002/2015WR016887, discovered that the effects of layer arrangements on steady state seawater intrusion and groundwater discharge in stratified confined coastal aquifers can be approximated via transmissivity centroid elevation (TCE), defined as the summation of the product of the transmissivity and elevation (above the aquifer base) of each layer divided by the total aquifer transmissivity. Specifically, a higher TCE in aquifers with high transmissivity layers at high elevations results in a more landward interface‐toe position in both flux‐ and head‐controlled coastal aquifer systems, as well as a higher freshwater discharge rate in head‐controlled systems. Furthermore, the toe position in head‐controlled stratified systems is only a function of the TCE and independent of the total transmissivity. Therefore, we can estimate the upper limit of seawater intrusion, that is, the furthest inland toe position, in stratified aquifers by letting the TCE equal to the elevation of the aquifer top. Another important implication of our results is that the interface toe position in coastal aquifers containing a preferential flow layer is controlled by the elevation of the preferential flow layer. Additionally, effective parameters and homogenization of the interface flow in stratified aquifers were developed to approximate the toe position and discharge rate. The results developed in this study provide significant advances in understanding the effect of aquifer stratification on groundwater flow and seawater intrusion in coastal aquifers.

Effects of multi-constituent tides on a subterranean estuary

Unsaturated flow influences both the seawater extent under steady‐state conditions and the propagation of tides in coastal aquifers. However, its effects on salt distributions in tidally influenced coastal aquifers are little investigated. The present study used numerical simulations and data from laboratory experiments to analyze the effects of unsaturated flow on density‐dependent solute transport in coastal unconfined aquifers. The effects of the inland boundary condition (i.e., constant‐head or constant‐flux) were tested. Compared to a stable sea level, the results show that unsaturated flow has a more pronounced influence on salt distributions in coastal unconfined aquifers when tides are considered, regardless of the type of inland boundary condition. Neglect of unsaturated flow effects leads to expansion of the upper saline plume (USP), shrinkage of the saltwater wedge (seaward movement of saltwater wedge), and overestimation of water and salt exchange across the aquifer‐ocean interface. This is caused by a lower head in the nearshore area during high‐tide periods with the unsaturated zone effects removed. Thus, without the unsaturated zone, stronger head gradients within the nearshore aquifer occur at high tide, leading to stronger tidally driven seawater infiltration and hence a larger USP. Counterintuitively, ignoring unsaturated flow effects leads to greater average inland head over a tidal period, which shifts the saltwater wedge seaward. It is concluded that unsaturated zone effects should not be neglected for modeling tide‐affected seawater intrusion, especially if quantification of near‐shore conditions is important.

Despite its purported importance, previous studies of the influence of sea-level rise on coastal aquifers have focused on specific sites, and a generalized systematic analysis of the general case of the sea water intrusion response to sea-level rise has not been reported. In this study, a simple conceptual framework is used to provide a first-order assessment of sea water intrusion changes in coastal unconfined aquifers in response to sea-level rise. Two conceptual models are tested: (1) flux-controlled systems, in which ground water discharge to the sea is persistent despite changes in sea level, and (2) head-controlled systems, whereby ground water abstractions or surface features maintain the head condition in the aquifer despite sea-level changes. The conceptualization assumes steady-state conditions, a sharp interface sea water-fresh water transition zone, homogeneous and isotropic aquifer properties, and constant recharge. In the case of constant flux conditions, the upper limit for sea water intrusion due to sea-level rise (up to 1.5 m is tested) is no greater than 50 m for typical values of recharge, hydraulic conductivity, and aquifer depth. This is in striking contrast to the constant head cases, in which the magnitude of salt water toe migration is on the order of hundreds of meters to several kilometers for the same sea-level rise. This study has highlighted the importance of inland boundary conditions on the sea-level rise impact. It identifies combinations of hydrogeologic parameters that control whether large or small salt water toe migration will occur for any given change in a hydrogeologic variable.

Assessment of mechanical dispersion effects on mixing zone under extreme saltwater intrusion

Saltwater intrusion into coastal aquifers has become a prominent environmental concern worldwide. As such, there is a need to prepare and implement proper remediation techniques with careful planning of freshwater withdrawal systems for controlling saltwater intrusion in coastal marine and estuarine environments. This paper investigates the performance of groundwater circulation well (GCW) in controlling saltwater intrusion problems in unconfined coastal aquifers. The GCWs have been established as a promising in-situ remedial technique of contaminated groundwater. The GCW system creates vertical circulation flow by extracting groundwater from an aquifer through a screen in a single well and injecting back into the aquifer through another screen. The circulation flow induced by GCW force water in a circular pattern between abstraction and recharge screens and can be as a hydraulic barrier for controlling saltwater intrusion problem in coastal aquifers. In this study, an effort has been made to investigate the behavior of saltwater intrusion dynamics under a GCW. An experiment has been conducted in a laboratory-scale flow tank model under constant water head boundary conditions, and the variable-density flow and transport model FEMWATER is used to simulate the flow and transport processes for the experimental setup. The evaluation of the results indicates that there is no further movement of saltwater intrusion wedge towards the inland side upon implementation of GCW, and the GCW acts as a hydraulic barrier in controlling saltwater intrusion in coastal aquifers. The present study reveals the GCWs system can effectively mitigate the saltwater intrusion problem in coastal regions and could be considered as one of the most efficient management strategies for controlling the problem.

Saltwater intrusion (SWI) is a widespread environmental problem that poses a threat to coastal aquifers. To address this issue, this research employs both numerical and experimental methods to study saltwater intrusion under the impact of sea level rise and varying freshwater boundary conditions in two homogeneous aquifers. The study compares transient numerical groundwater heads and salt concentrations to experimental results under receding-front and advancing front conditions. In the low permeability aquifer, the root mean square error is 0.33 cm and the R2 is greater than 0.9817. Similarly, in the high permeability aquifer, the root mean square error is 0.92 cm and the R2 is greater than 0.9335. The study also compares the results of ten experimental tests for steady-state saltwater intrusion wedge and toe length with seven different analytical solutions. The experimental results are then compared to these analytical solutions to find the most suitable equation. The Rumer and Harleman equation shows good agreement with experimental saltwater intrusion wedge, while the Anderson equation is a good fit for saltwater intrusion toe length. Overall, this research provides valuable insights into saltwater intrusion in coastal aquifers, and the findings can be used to inform policies and management strategies to mitigate the negative impacts of saltwater intrusion. The investigation shed light on how inland water head and Sea Level Rise (SLR) affect SWI behavior.

Watertable fluctuations and seawater intrusion are characteristic features of coastal unconfined aquifers. The dynamic effective porosity due to watertable fluctuations is analyzed and then a modified (empirical) expression is proposed for the dynamic effective porosity based on a dimensionless parameter related to the watertable fluctuation frequency. After validation with both experimental data and numerical simulations, the new expression is implemented in existing Boussinesq equations and a numerical model, allowing for examination of the effects of the dynamic effective porosity on watertable fluctuations and seawater intrusion in coastal unconfined aquifers, respectively. Results show that the Boussinesq equation accounting for the vertical flow in the saturated zone and dynamic effective porosity can accurately predict experimental dispersion relations (that all existing theories fail to predict), highlighting the importance of the dynamic effective porosity in modeling watertable fluctuations in coastal unconfined aquifers. This in turn confirms the utility of the real-valued expression of the dynamic effective porosity. An outcome is that the phase lag between the total moisture (above the watertable) and watertable height measured in laboratory experiments using vertical soil columns (1D systems) can be ignored when predicting watertable fluctuations in coastal unconfined aquifers (2D systems). A dynamic effective porosity that is, by comparison, smaller than the soil porosity leads to a reduction in vertical water exchange between the saturated and vadose zones and hence watertable waves can propagate further landward. The dynamic effective porosity further plays a critical role in simulations of seawater intrusion, since it predicts a more landward seawater-freshwater interface and a higher position of the upper saline plume.

Watertable fluctuations and seawater intrusion are characteristic features of coastal unconfined aquifers. The dynamic effective porosity due to watertable fluctuations is analyzed and then a modified (empirical) expression is proposed for the dynamic effective porosity based on a dimensionless parameter related to the watertable fluctuation frequency. After validation with both experimental data and numerical simulations, the new expression is implemented in existing Boussinesq equations and a numerical model, allowing for examination of the effects of the dynamic effective porosity on watertable fluctuations and seawater intrusion in coastal unconfined aquifers, respectively. Results show that the Boussinesq equation accounting for the vertical flow in the saturated zone and dynamic effective porosity can accurately predict experimental dispersion relations (that all existing theories fail to predict), highlighting the importance of the dynamic effective porosity in modeling watertable fluctuations in coastal unconfined aquifers. This in turn confirms the utility of the real-valued expression of the dynamic effective porosity. An outcome is that the phase lag between the total moisture (above the watertable) and watertable height measured in laboratory experiments using vertical soil columns (1D systems) can be ignored when predicting watertable fluctuations in coastal unconfined aquifers (2D systems). A dynamic effective porosity that is, by comparison, smaller than the soil porosity leads to a reduction in vertical water exchange between the saturated and vadose zones and hence watertable waves can propagate further landward. The dynamic effective porosity further plays a critical role in simulations of seawater intrusion, since it predicts a more landward seawater-freshwater interface and a higher position of the upper saline plume.

Watertable fluctuations and seawater intrusion are characteristic features of coastal unconfined aquifers. The dynamic effective porosity due to watertable fluctuations is analyzed and then a modified (empirical) expression is proposed for the dynamic effective porosity based on a dimensionless parameter related to the watertable fluctuation frequency. After validation with both experimental data and numerical simulations, the new expression is implemented in existing Boussinesq equations and a numerical model, allowing for examination of the effects of the dynamic effective porosity on watertable fluctuations and seawater intrusion in coastal unconfined aquifers, respectively. Results show that the Boussinesq equation accounting for the vertical flow in the saturated zone and dynamic effective porosity can accurately predict experimental dispersion relations (that all existing theories fail to predict), highlighting the importance of the dynamic effective porosity in modeling watertable fluctuations in coastal unconfined aquifers. This in turn confirms the utility of the real-valued expression of the dynamic effective porosity. An outcome is that the phase lag between the total moisture (above the watertable) and watertable height measured in laboratory experiments using vertical soil columns (1D systems) can be ignored when predicting watertable fluctuations in coastal unconfined aquifers (2D systems). A dynamic effective porosity that is, by comparison, smaller than the soil porosity leads to a reduction in vertical water exchange between the saturated and vadose zones and hence watertable waves can propagate further landward. The dynamic effective porosity further plays a critical role in simulations of seawater intrusion, since it predicts a more landward seawater-freshwater interface and a higher position of the upper saline plume.