Unconfined Coastal Aquifer Research Articles

Visual MODFLOW, solute transport modeling, and remote sensing techniques for adapting aquifer potentiality under reclamation and climate change impacts in coastal aquifer

Global environmental changes, such as climate change and reclamation alterations, significantly influence hydrological processes, leading to hydrologic nonstationarity and challenges in managing water availability and distribution. This study introduces a conceptual underpinning for the rational development and sustainability of groundwater resources. As one of the areas intended for the development projects within the Egyptian national plan for the reclamation of one and a half million acres; hundreds of pumping wells were constructed in the Moghra area to fulfill the reclamation demand. This study investigates the long-term impacts of exploiting the drilled pumping wells under climate change. The approach is to monitor the groundwater levels and the salinity values in the Moghra aquifer with various operational strategies and present proposed sustainable development scenarios. The impact of global warming and climate change is estimated for a prediction period of 30 years by using satellite data, time series geographical analysis, and statistical modeling. Using MODFLOW and Solute Transport (MT3DMS) modules of Visual MODFLOW USGS 2005 software, a three-dimensional (3D) finite-difference model is created to simulate groundwater flow and salinity distribution in the Moghra aquifer with the input of forecast downscaling (2020–2050) of main climatic parameters (PPT, ET, and Temp). The optimal adaptation-integrated scenario to cope with long-term groundwater withdrawal and climate change impacts is achieved when the Ministry of Irrigation and Water Resources (MWRI) recommends that the maximum drawdown shouldn’t be more significant than 1.0 m/ year. In this scenario, 1,500 pumping wells are distributed with an equal space of 500 m, a pumping rate of 1,200 m3/day and input the forecast of the most significant climatic parameters after 30 years. The output results of this scenario revealed a drawdown level of 42 m and a groundwater salinity value of 16,000 mg/l. Climate change has an evident impact on groundwater quantity and quality, particularly in the unconfined coastal aquifer, which is vulnerable to saltwater intrusion and pollution of drinking water resources. The relationship between climate change and the hydrologic cycle is crucial for predicting future water availability and addressing water-related issues.

Characteristics of Solute Transport Continuously Released from Coastal Unconfined Aquifers under the Tidal Action Based on Laboratory Experiment

Characteristics of Solute Transport Continuously Released from Coastal Unconfined Aquifers under the Tidal Action Based on Laboratory Experiment

Effects of groundwater pumping on pore water flow and salt transport in tide‐controlled unconfined coastal aquifers

AbstractUnconfined coastal aquifers are a main pathway for land‐sourced solutes to enter the oceans. The migration of these solutes in aquifers is highly affected by the groundwater flow and salt transport processes, which are, to a great extent, controlled by tides. While many studies have examined how tidal oscillations would influence the subsurface hydrodynamics in coastal aquifers, most of them ignored the potential impact of groundwater pumping, a common practice in coastal areas to satisfy the demand for freshwater. This study, by means of laboratory experiments and numerical simulations, explored the combined effects of tides and groundwater pumping on the pore water flow and salinity distributions in an unconfined coastal aquifer. The results show that, in a tide‐controlled aquifer, the addition of groundwater pumping would exacerbate the degree of seawater intrusion and lead to wider spreading and deeper penetration of the upper saline plume. Moreover, groundwater pumping would enhance the tide‐driven circulation in the upper saline plume and weaken the density‐driven circulation in the saltwater wedge, ultimately leading to the reduction in total submarine groundwater discharge. These findings may promote a deep insight into the complex coastal groundwater systems experiencing human activities, and provide guidance for better evaluating the environmental impact of groundwater pumping.

Dense Contaminants Mixing Into the Saltwater Wedge in Coastal Aquifers: Laboratory and Numerical Investigations

AbstractThe saltwater‐freshwater mixing zones in coastal aquifers can host complex physical exchange and biogeochemical transformations. The land‐sourced dense contaminant plumes could be transferred into the mixing zone of the saltwater wedge due to the density effect prior to discharge to the sea. However, the mixing process between dense contaminants and the saltwater wedge has not received much attention, largely due to the lack of physical evidence. This study used laboratory experiments and numerical simulations to investigate the transport and discharge behaviors of variable‐density contaminant plumes in tidally influenced unconfined coastal aquifers. Results demonstrate that the highly dense contaminants mix with the underlying saltwater and finally merge with the saltwater wedge. This process significantly extends the contaminant discharge durations, thereby reducing the peak value of contaminant efflux. The dense contaminants are elongated along the landward margin of the saltwater wedge, leading to a larger spreading area (Ms) than that of constant‐density contaminants. The sensitivity analysis indicates that the high density of contaminants acts as a trigger to induce the mixing of them and wedges. The higher hydraulic conductivity, lower dispersivities and reduced inland freshwater flux significantly increase the residence times (Rt) and discharge duration (Dt) by enhancing the mixing of dense contaminants with seawater. In contrast, both Rt and Dt values are not only non‐monotonic functions of tidal amplitudes but also less sensitive to tidal effects. Compared with the non‐tidal condition, however, the addition of tides significantly increases both Rt and Ms values of dense contaminant plumes. The results presented herein provide valuable insights into the mechanisms of dense contaminants mixing into saltwater wedges, which could guide practitioners in designing effective strategies to protect coastal environments from land‐sourced contaminants.

SUPHRE: A Reactive Transport Model With Unsaturated and Density‐Dependent Flow

AbstractAlthough unsaturated and density‐dependent flow affect solute fate in groundwater, they are rarely both included in reactive transport models. Using the operator‐splitting method, a new reactive transport model (SUPHRE) was developed by combining a variable‐saturation and variable‐density multiple‐component solute transport model (SUTRA‐MS) with a geochemical reaction module (PhreeqcRM). In contrast to existing reactive transport models, SUPHRE accounts for both unsaturated and density‐dependent flow. Model setup for SUPHRE is convenient, as only one setup file is required in addition to the usual input files of SUTRA‐MS and PhreeqcRM. By further implementing a time‐variant boundary condition into SUTRA‐MS, SUPHRE can simulate multi‐component reactive transport in tidally influenced coastal unconfined aquifers where unsaturated and density‐dependent flow prevail. Two examples were used to validate the new reactive transport model, including single‐species decay and sorption within a one‐dimensional soil column and a four‐species decay chain in a two‐dimensional aquifer. Following validation, SUPHRE was adopted to reveal unsaturated flow effects on oxygen consumption and nitrate reduction in tidally influenced coastal unconfined aquifers. Whether for simulating oxygen consumption or nitrate reduction, there were visible deviations between numerical results without and with unsaturated flow, highlighting that unsaturated flow can affect reactive solute transport and transformation.

Variable-density solute transport in unconfined coastal aquifers with a subsurface dam

Recently, the influence of subsurface dams on solute transport in coastal aquifers has become a hot research topic. Although many studies have been conducted, the combined effects of subsurface dam and tides on variable-density solute transport remain unclear, and this study aimed to fill this research gap. In the current study, a numerical model representing a 2-D cross-shore coastal aquifer was established. The model simulated cases with and without a subsurface dam, and sensitivity analysis cases with different height and location of the subsurface dam and solute concentration. The results show that a subsurface dam blocks a portion of the solute plume, which can only be discharged by dilution at the edges, thereby altering its discharge pattern and reducing their discharge rate. The addition of a subsurface dam may either prolong or shorten the residence time of solute,depending on the location rather than the height of the subsurface dam. In particular, a more landward subsurface dam would significantly increase the residence time; sensitivity analysis demonstrates that both the landward shift and the height increase of the subsurface dam contribute to a heightened ratio of dynamic mass distribution for the solute plume within the freshwater and saltwater zones of the aquifer, with maximum changes in mass distribution ratios of 87.22% and 300%, respectively. Also, these factors cause the solute to migrate both seaward and landward, respectively, across the primary outflow regions of the aquifer-ocean interface. Results from this study may provide theoretical guidance for the optimal design and environmental impact assessment of subsurface dams.

Nitrate fate in coastal unconfined aquifers influenced by preferential flows

This study examined the influence of preferential flow on pore water flows and marine nitrogen transport reaction in variable saturation and variable density coastal aquifers. The 2-D unconfined aquifer model established was based on the software COMSOL by coupling the dynamic and chemical processes together. The results showed that preferential flow affects groundwater flow and salinity distribution, leading to a more complicated mixing process. The preferential flow resulted in an increase in mixing zone area and the upper saline plume area of 10.33 and 2.62 m2, respectively, a decrease in saltwater wedge area of 7.22 m2, and an increase in nitrate (NO3-) removal efficiency from 7.9% to 8.97%. The NO3- removal efficiency increases progressively with the depth (h) and quantity (n) of preferential flows; however, it decreases after a certain quantity. Further quantitative analysis revealed an increase in the intensity of nitrification and dissolved oxygen inflow flux with preferential flow depth and quantity increase. This phenomenon usually occurs on coasts where biological caves are abundant. The results also offer significant implications for designing engineering measures to mitigate saltwater intrusion and are significant to prevent groundwater quality deterioration in coastal zones.

Investigating the occurrence of flow instabilities in coastal unconfined aquifers using sand tank experiments

Subterranean estuaries form the land-sea transition zone in coastal aquifers and are relevant for coastal freshwater management and coastal ecology. Tidal fluctuations typically lead to the formation of upper saline plumes, where denser saltwater circulates on top of the discharging, less dense freshwater. Extensive numerical simulations and only a few experimental trials previously demonstrated that these saline plumes may become unstable and exhibit salt fingering flow. This work used laboratory sand tank experiments to study the formation and evolution of tide induced flow instabilities in homogenous, unconfined coastal aquifers in a systematic manner, addressing the combined effects of relevant parameters such as sediment type, beach slope, fresh groundwater inflow, tidal amplitude, and tidal period variation. Some of these parameters have never been targeted in laboratory-based studies before and a comparable systematic and comprehensive approach has so far not been conducted. The resulting flow patterns were studied visually through the application of a colour tracer to the saltwater. The results show that the upper saline plume may be stable, unstable, or transitional, whereby the beach slope predominantly defines its state. When salt fingers form, they amplify the mixing zone between ground- and saltwater and force the freshwater to discharge at multiple locations along the beach. The processes were best visible in the coarsest sediment (1.02 × 10-2 m s−1). The finer grained the sediment was, the smaller the upper saline plume that formed. The present study further reveals the possibility of three-dimensional fingering flow, even though the physical model was intended to represent a two-dimensional cross-section. The application of a previously proposed non-dimensional stability diagram yielded reasonably accurate predictions of stability behaviour in coastal unconfined aquifers under laboratory settings. This further supports the plausibility of flow instabilities in subterranean estuaries within specific ranges of influencing hydrogeological factors.

Developing Functional Recharge Systems to Control Saltwater Intrusion via Integrating Physical, Numerical, and Decision-Making Models for Coastal Aquifer Sustainability

Controlling the hydraulic heads along a coastal aquifer may help to effectively manage saltwater intrusion, improve the conventional barrier’s countermeasure, and ensure the coastal aquifer’s long-term viability. This study proposed a framework that utilizes a decision-making model (DMM) by incorporating the results of two other models (physical and numerical) to determine proper countermeasure components. The physical model is developed to analyze the behavior of saltwater intrusion in unconfined coastal aquifers by conducting two experiments: one for the base case, and one for the traditional vertical barrier. MODFLOW is used to create a numerical model for the same aquifer, and experimental data are used to calibrate and validate it. Three countermeasure combinations, including vertical barrier, surface, and subsurface recharges, are numerically investigated using three model case categories. Category (a) model cases investigate the hydraulic head’s variation along the aquifer to determine the best recharge location. Under categories (b) and (c), the effects of surface and subsurface recharges are studied separately or in conjunction with a vertical barrier. As a pre-set of the DMM, evaluation and classification ratios are created from the physical and numerical models, respectively. The evaluation ratios are used to characterize the model case results, while the classification ratios are used to classify each model case as best or worst. An analytical hierarchy process (AHP) as a DMM is built using the hydraulic head, salt line, repulsion, wedge area, and recharge as selection criteria to select the overall best model case. According to the results, the optimum recharging location is in the length ratio (LR) from 0.45 to 0.55. Furthermore, the DMM supports case3b (vertical barrier + surface recharge) as the best model case to use, with a support percentage of 48%, implying that this case has a good numerical model classification with a maximum repulsion ratio (Rr) of 29.4%, and an acceptable wedge area ratio (WAR) of 1.25. The proposed framework could be used in various case studies under different conditions to assist decision-makers in evaluating and controlling saltwater intrusion in coastal aquifers.

Verification of experimental saltwater intrusion interface in unconfined coastal aquifers using numerical and analytical solutions

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.

Effect of mixed physical barrier on seawater intrusion and nitrate accumulation in coastal unconfined aquifers.

Physical barrier has been proven to be one of the most effective measures to prevent and control seawater intrusion (SWI) in coastal areas. Mixed physical barrier (MPB), a new type of physical barrier, has been shown to have higher efficiency in SWI control. As with conventional subsurface dam and cutoff wall, the construction of MPB may lead to the accumulation of nitrate contaminants in coastal aquifers. We investigated the SWI control capacity and nitrate accumulation in the MPB using a numerical model of variable density flow coupling with reactive transport, and performed sensitivity analysis on the subsurface dam height, cutoff wall depth and opening spacing in the MPB. The differences in SWI control and nitrate accumulation between MPB and conventional subsurface dam and cutoff wall were compared to assess the applicability of different physical barrier. The numerical results show that the construction of MPB will increase the nitrate concentration and contaminated area in the aquifer. The prevention and control efficiency of MPB against SWI is positively correlated with the depth of the cutoff wall, reaching the highest efficiency at the minimum effective dam height, and the retreat distance of the saltwater wedge is positively correlated with the opening spacing. We found a non-monotonic relationship between the change in subsurface dam height and the extent of nitrate accumulation, with total nitrate mass and contaminated area increasing and then decreasing as the height of the subsurface dam increased. The degree of nitrate accumulation increased linearly with increasing the height of the cutoff wall and the opening spacing. Under certain conditions, MPB is 46-53% and 16-57% more efficient in preventing and controlling SWI than conventional subsurface dam and cutoff wall, respectively. However, MPB caused 14-27% and 2-12% more nitrate accumulation than subsurface dam and cutoff wall, respectively. The findings of this study are of great value for the protection of coastal groundwater resources and will help decision makers to select appropriate engineering measures and designs to reduce the accumulation of nitrate pollutants while improving the efficiency of SWI control.

Mitigating seawater intrusion in coastal aquifers: Novel approach with treated wastewater injection and groundwater circulation

Seawater intrusion (SWI) is a natural phenomenon that negatively impacts the potability of groundwater and is expected to worsen with rising sea levels due to climate change. Artificial recharge of freshwater is a commonly-used remediation method to mitigate SWI and improve freshwater supply security in affected coastal aquifers. However, limited freshwater availability can limit the effectiveness of this approach. This study proposes a novel mitigation measure, called Inj-GCW, which combines the injection of reclaimed water with the use of groundwater circulation wells (GCW) to enhance the effectiveness of artificial recharge in controlling SWI. GCW is a dual-screened well with isolated screens that extract and inject groundwater into the aquifer. The performance of the proposed measure was quantitatively evaluated using an illustrative simplified unconfined coastal aquifer. Based on the findings, the design parameters were estimated for a field-scale case study of the Nile Delta aquifer (NDA), a large Mediterranean coastal aquifer in Egypt. The study adopts a future scenario that considers Sea-level rise due to climate change and projected population growth by 2100. Results demonstrate that introducing of GCWs into the saltwater wedge, along with injection, effectively retreated the saltwater wedge, due to enhanced velocity, seaward fluxes, and dilution of contamination. Inj-GCW measure led to an 8.9% reduction in SWI and a 5.2% decrease in aquifer salinity compared to the expected intrusion in 2100. Furthermore, the Inj-GCW measure resulted in a 2.2% higher repulsion rate and 0.3% reduction in total salt mass compared to injection alone. The Inj-GCW measure presents a promising solution to SWI challenges in the NDA and other coastal aquifers facing similar issues. The formation of a brackish water bubble at the injection well screen of the GCW and the generated vertical groundwater circulation cells acted as a hydraulic barrier and contributed to the proposed method effectiveness.

Estimating the Maximum Safe Pumping Rate in Sloping Unconfined Coastal Aquifers

AbstractUnconfined coastal aquifers with a sloping aquifer bed are ubiquitous. However, most analytical solutions previously developed to estimate pumping‐induced seawater intrusion considered a horizontal aquifer bed. In this study, we first developed a steady‐state analytical solution to quantify the maximum safe pumping rate in sloping unconfined coastal aquifers with a fixed‐flux inland boundary condition, using the single potential approach. The analytical solution corrected by an empirical factor is validated against the numerical simulation, which reproduces the corresponding numerical results well. It is found that coastal aquifers with a higher aquifer bed toward inland (i.e., a positive sloping angle) yield a higher maximum safe pumping rate than that of coastal aquifers with a horizontal aquifer base, since the elevated freshwater head prevents seawater intrusion and enhances groundwater pumping. Specifically, for the aquifer bases with the sloping angle of 0.01 and −0.01, the maximum safe pumping rate is increased by 42.6% and decreased by 48.4%, respectively, in comparison to the case of a horizontal aquifer base, suggesting that neglecting the aquifer base slop can result in a significant error in estimating the maximum safe pumping rate. Moreover, the sensitivity analysis reveals that the maximum safe pumping rate increases with a lower hydraulic conductivity and a larger dispersivity. Our results provide valuable insights into the effect of the aquifer base slope on the maximum safe pumping in coastal aquifers, and analytical solutions developed can serve as a powerful tool for quick assessment of pumping‐induced seawater intrusion in sloping unconfined coastal aquifers.

Watertable fluctuations in coastal unconfined aquifers with a sloping sea boundary: Vertical flow and dynamic effective porosity effects

Interactions between the tide and sloping sea boundary make watertable fluctuations in coastal unconfined aquifers complicated. Based on a perturbation method, we derived a new analytical solution to predict watertable fluctuations for coastal unconfined aquifers with a sloping sea boundary. Following validation with a numerical model, the analytical solution was used to explore the effects of the vertical flow (in the saturated zone) and dynamic effective porosity on watertable fluctuations. Results show that the new analytical solution accurately predicts watertable fluctuations for coastal unconfined aquifers with a sloping sea boundary. Compared with sand coastal unconfined aquifers, both vertical flow and dynamic effective porosity effects on watertable fluctuations are more pronounced for loam coastal unconfined aquifers. Vertical flow has a minor influence on the fluctuation amplitude while it significantly decreases the phase lag of the watertable fluctuation at a given location. In contrast to vertical flow, accounting for the dynamic effective porosity not only decreases the phase lag, but also significantly amplifies the fluctuation amplitude for a given location, which enables watertable wave propagation further inland. Increasing the beach slope weakens the effects of the vertical flow and dynamic effective porosity on watertable fluctuations. Furthermore, including either the vertical flow or dynamic effective porosity effects leads to a lower watertable overheight. These results highlight the importance of vertical flow and dynamic effective porosity effects in models of watertable fluctuations.

Effects of Unsaturated Flow on Salt Distributions in Tidally Influenced Coastal Unconfined Aquifers

AbstractUnsaturated 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.

Spatiotemporal estimation of fresh submarine groundwater discharge across the coastal shorelines of Oahu Island, Hawaii

Abstract The integrated hydrological model is a powerful tool that is used to assess the temporal distribution of fresh groundwater discharge especially in coastal areas. The coastal regions of Hawaii are examples of crucial natural resources for the Hawaiian economy and general ecological health. To fully comprehend the intricate interactions between coastal hydrology processes and ecosystems, it is necessary to evaluate the fresh submarine groundwater discharge (FSGD) at the Heeia shoreline using an integrated hydrological modeling technique. Under steady-state settings, the results showed that the present daily average of FSGD is around 0.43 m3/days across 1 m of the shoreline. However, we showed that the FSGD values were considerably impacted by climate change, groundwater head of the coastal aquifer, recharge rate, and sea level rise, particularly by the end of the 21st century. The post-development FSGD fluxes were 1.5–3.5 times greater than the freshwater transported by the Heeia stream, demonstrating the considerable contribution of the FSGD to the coastal zones of Heeia. The results also showed an exponential association between the FSGD and the groundwater level for the coastal unconfined aquifer.

Thermal effects of freshwater injection on flow and salinity distributions in tidally-affected coastal unconfined aquifers

Freshwater injection is a practical and efficient solution to mitigate seawater intrusion in coastal aquifers, but its thermal impacts on groundwater flow and salt transport are still poorly understood. This study investigates the dynamic characteristics of salinity distributions and seawater recirculation in coastal unconfined aquifers subjected to freshwater injection and tides. The results show that the seaward development of thermal plume modifies the transience of upper saline plume and saltwater wedge, significantly prolonging the aquifer response to injected freshwater. Three temporal variations of upper saline plume due to freshwater injection are driven by pressure waves of injected freshwater, thermal plume-induced changes of hydraulic gradients and saltwater wedge, and the hydraulic conductivity within the upper saline plume. Moreover, a delayed reduction in seawater recirculation in response to the thermal impacts of cold-water injection induces a significant and long-lasting overshoot of total water efflux. An increased tidal amplitude facilitates thermal effects on flow and salinity distributions. A deeper injection well extends the flow path of injected freshwater and thermally regulates discharge rates along the beach surface, thereby enhancing overshoot magnitude of total water efflux. These results improve the understanding of underlying processes related to freshwater injection in coastal aquifers and provide guidance for better managing coastal groundwater resources.

Effect of the cutoff wall on the fate of nitrate in coastal unconfined aquifers under tidal action

This paper investigates the effects of the cutoff wall on the fate of nitrate (NO3-), the NO3- removal rate, and the salinity distribution in a coastal aquifer under tidal action. A numerical study was performed based on a coupled model with variable-saturation and variable-density flow and a convection-diffusion-reaction equation for solute transport in a coastal unconfined aquifer. The results showed that the cutoff wall led to a larger upper salinity plume (USP) and that the saltwater wedge (SW) further retreated seaward. The recirculation pathways of saltwater and groundwater were largely modified by the wall. The cutoff wall within the tidal range could increase the NO3- mass of denitrification and the NO3- removal efficiency and decrease the length of the SW and the freshwater flux. This modification of the saltwater and groundwater recirculation pathways was enhanced with increasing wall depth. A deeper cutoff wall led to a further retreated SW, lower freshwater flux, and greater improvements in the NO3- mass of denitrification and the NO3- removal efficiency. In addition, the cutoff wall significantly decreased the terrestrial dissolved organic carbon (TDOC) discharge into the sea. Dissolved organic carbon source (SDOC) promoted a higher NO3- removal efficiency. This study provides us with a better understanding of coastal physical-biogeochemical processes and dynamic mechanisms, as well as a guide for designing engineering measures to mitigate NO3- contamination and thus enhance groundwater quality management.

Impacts of Subsurface Dam Construction on Downstream Groundwater Levels and Salinity in Coastal Aquifers.

Subsurface dam is a promising engineering technology for groundwater resources development. However, the possible impacts of these dams on the groundwater environment have been a major concern. Here, we used a three-dimensional (3D), variable-density, unsaturated-saturated groundwater flow model to explore how a groundwater-storage-type subsurface dam, built in the freshwater domain of an unconfined coastal aquifer, affected groundwater levels and salinity in the downstream area. Model results suggested that, after subsurface dam construction, groundwater levels in the downstream area showed intensified fluctuations in terms of phase advances, greater amplitudes, and higher frequencies following heavy rainfall events. Numerical simulations with variable subsurface dam scenarios indicated that the groundwater level fluctuations were further intensified with a higher crest elevation or a shorter distance from the coast. Moreover, during the recharging period of the subsurface reservoir, sea water in the downstream area intruded landward from its initial location, which can at least temporarily threaten water quality near the coast. A higher dam crest elevation prolonged the duration of sea water intrusion, while a dam positioned closer to the coast induced sea water intrusion with a greater horizontal extent. General implications are discussed with respect to improving assessment methodologies and engineering designs of subsurface dams.

Modeling of Hydrodynamic Dispersion in the Sidi Kirayr Locality’s Coastal Unconfined Aquifer West of Alexandria, Egypt

The finite difference method was used in this study to solve the two-dimensional hydrodynamic dispersion equation for a typical coastal unconfined aquifer. A mathematical model is prepared to solve this dispersion problem, which is then modeled and analyzed over time. The salinity profiles for the aquifer transition zone in the Sidi Kirayr locality are determined using the finite difference method. The computed salinity profiles are compared to those obtained from an actual well drilled in the area and used to determine electric conductivity. Profile correlation reveals a roughly parallel trend along the tested depth interval. Additionally, these results correlate well with the chemical analysis of water from the same well. The current work promotes the application of the solution methodology in comparable coastal regions in the absence of dispersion measures for the analyzed aquifer. This innovative approach can be used to forecast groundwater quality in comparable coastal regions. It can also help in selecting the ideal sites for man-made wells that pump groundwater.