Assessment of groundwater influenced by seawater using geochemical modeling and statistical analysis: Basrah Province, Iraq

Seawater intrusion in heterogeneous coastal aquifers under flooding events

Around the globe, seawater intrusion in the coastal aquifer is a significant problem. Excessive groundwater extraction because of population growth, industrialization, tourism, and other anthropogenic activities and geogenic processes initiates and accelerates this problem. The contaminated groundwater impacts the health, economic activities, and social and cultural development of coastal regions. This work aims to explore the current status and a holistic comprehending review of geophysical studies applied to delineate the seawater intrusion in the high-quality coastal aquifers in India, as well as its origin and causes, mitigation strategies, and recent advancements in geophysical techniques to access the qualitative and quantitative properties of the complex aquifer system. In the future, it is recommended to do a detailed subsurface imaging of the entire coastal belt of India to decipher the lateral and vertical variation of the lithological conditions and seawater intrusion in space and time with improved/advanced geophysical techniques, which can lead toward sustainable development.

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.

We introduce a novel groundwater flow model designed for the rapid estimation of seawater intrusion (SWI) in coastal aquifers. Drawing on Girinskii's potential theory (1947) for 2D aquifer flow, the model extends Strack's (1976) adaptation to coastal aquifers under transient state conditions. Key features include applicability to heterogeneous unconfined aquifer systems under time-dependent pumping and spatially distributed groundwater recharge. Assumptions include a sharp seawater-freshwater interface, prevailing horizontal flow, and neglect of flow rates within the saltwater wedge. Built upon a finite-element 2D saturated groundwater simulator, the model derives a solution for the potential using a non-linear formulation of the classic flow equation and can accommodate prescribed-head or prescribed-flux boundary conditions at inland boundaries, as well as time-varying head conditions at the shore boundaries, which is crucial for addressing long-term sea-level rise (SLR). Noteworthy modifications enable the model to estimate land subsidence through a 1D vertical consolidation model directly in terms of potential, providing a holistic view of aquifer dynamics. The model is successfully tested against analytically based solutions and can facilitate swift calculations of aquifer vulnerability indicators, such as SWI toe location, increase in dissolved salt mass, and apparent land subsidence due to compounded effects of SLR, groundwater pumping, and long-term variable recharge conditions. Our presentation will showcase the model's features, preliminary results focusing on the effects of aquifer heterogeneities on the intensity of SWI and land subsidence patterns, model strengths and limitations. Emphasis will be placed on the model computational efficiency, enabling rapid estimation of crucial SWI indicators. Furthermore, the discussion will outline future steps, highlighting the need to balance the simplicity of working hypotheses with the computational demands of more intricate variable density models in assessing complex coastal aquifer dynamics.

Sea-level rise impacts on seawater intrusion in coastal aquifers: Review and integration

Lee, W.D.; Yoo, Y.J.; Jeong, Y.M.; Jeong, Y.H., and Hur, D.S., 2018. Experimental Investigation of the Effects of Revetments on Seawater Intrusion in Coastal Aquifers. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 441–445. Coconut Creek (Florida), ISSN 0749-0208.Sea level rise due to global warming and the decline in groundwater levels due to the misuse of groundwater resources can decrease the seawater and groundwater pressure gradient, which can, in turn, increase seawater intrusion into coastal aquifers. Many researchers have conducted theoretical, experimental, and numerical analyses on the effects of changes in seawater and groundwater levels on coastal aquifers. In this study, modeling experiments were conducted in a sandy tank to analyze revetment and underground obstacles that influence seawater intrusion in coastal aquifers. We focused on areas that have not been examined by current research. We confirmed that as the gradient of the seawater level to groundwater level increased, seawater intrusion weakened, because the difference between the seawater and groundwater levels increased as the salinity difference between the seawater and freshwater decreased. Following the installation of a revetment in the experimental coastal aquifer, the groundwater level behind it increased. Due to a decrease in the cross-sectional area, a special seawater–freshwater interface formed. We found that the seawater–groundwater pressure gradient increased due to the revetment, the flow to the sea increased, and the seawater–freshwater boundary moved in the direction of the sea in the experimental tank. This study suggests that if revetment and similar underground obstacles are arranged appropriately, the groundwater level and groundwater flow can be changed, and seawater intrusion in coastal aquifers can be reduced. In the future, results from this experiment could be used by numerical models as verification data for the seawater–freshwater interface and the seawater intrusion distance.

Seawater intrusion in coastal aquifers is generally three dimensional (3-D) in nature. In the literature, there is a general lack of reported results on 3-D simulations. This paper presents some typical example simulations of 3-D seawater intrusion process for a specified hypothetical study area. The simulation results presented in this paper are based on the density-dependent miscible flow and transport modelling approach for simulation of seawater intrusion in coastal aquifers. A nonlinear optimization-based simulation methodology was used in this study. Various steady state simulations are performed for a specified study area. Response evaluations consider the effects of vertical recharge on seawater intrusion, effects of boundary conditions, and effects of spatially varying pumping from the aquifer. The 3-D simulations demonstrate the viability of using a planned strategy of spatially varying withdrawals from the aquifer to manage seawater intrusion. It is demonstrated that series of pumps near the ocean-face boundary induce a hydraulic head distribution that can be effectively used for controlling seawater intrusion.

The Heuningnes Catchment in the Republic of South Africa was used as a case study in this research to describe the application of saltwater fraction/quantification and hydrogeochemistry methods to evaluate the extent of saline intrusion in the coastal aquifers. The argument of the research is that the presence of seawater incursion may be conclusively determined by combining the examination of the major ions, seawater fraction, stable isotopes of water, bromide, and geochemical modeling. Using stable isotopes of oxygen (18O) and deuterium (2H), major ions chemistry, seawater composition, and geochemical modeling, the genesis of salinity and mixing of different water masses were examined. Twenty-nine (29) samples of groundwater were examined. All samples showed water facies of the Na-Cl type, indicating a seawater-related origin. The significance of mixing in coastal aquifers under natural conditions was shown by the hydrogeochemical characteristics of key ions derived from ionic ratios, which demonstrated substantial adherence to mixing lines among endmembers for freshwater as well as saltwater (seawater). The quantification of seawater contribution in groundwater percentages varied from 0.01 to 43%, with three samples having concentrations of seawater above 50%. It was clear from the hydrogeochemical analysis and determination of the proportion of saltwater that the seawater intrusion impacted the coastal fresh groundwater. In addition, the chloride concentration in the groundwater ranged from 81.5 to 26,557.5 mg/L, with the corresponding δ18O values ranging from −5.5‰ to −0.9‰, which suggested that freshwater and saltwater were mixing. The Br−/Cl− ratios showed that evaporation had played a part in elevating groundwater salinity as well. Since saturation indices were below zero, the mineral dissolution could also contribute to the salinization of groundwater. Further proof of seawater incursion in the investigated catchment was supplied by geochemical modeling and bromide. Even though such tools were not verified in multiple coastal aquifers for widespread generalization, the study offered a scientifically significant understanding of the application of such tools on seawater intrusion in coastal aquifers and has useful recommendations for the aquifer setting of similar environments.

Lee, H.; Kim, S.; Jun, K.W.; Park, H.K., and Park, J.S., 2016. The Effects of Groundwater Pumping and Infiltration on Seawater Intrusion in Coastal Aquifer. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 652–656. Coconut Creek (Florida), ISSN 0749-0208. The purpose of this study is to identify the characteristics of seawater intrusion; an examination was carried out of the impacts of groundwater pumping and infiltration by precipitation on the seawater intrusion in coastal aquifers. To simulate seawater intrusion, three layers (layers 1, 2 and 3) were identified based on a drilling log. The infiltration rate of 30 years of rainfall data was calculated as 26.43%. The change of salt concentration was simulated using the FEMWATER model by considering the infiltration rate and assuming the pumping rate as 185m3/day for 300 days. The proporti...

SUMMARYSeawater intrusion is one of the most serious environmental problems in many coastal regions all over the world. Mixing a small quantity of seawater with groundwater makes it unsuitable for use and can result in abandonment of aquifers. Therefore, seawater intrusion should be prevented or at least controlled to protect groundwater resources. This paper presents development and application of a simulation‐optimization model to control seawater intrusion in coastal aquifers using different management scenarios; abstraction of brackish water, recharge of freshwater, and combination of abstraction and recharge. The model is based on the integration of a genetic algorithm optimisation technique and a coupled transient density‐dependent finite element model. The objectives of the management scenarios include determination of the optimal depth, location and abstraction/recharge rates for the wells to minimize the total costs for construction and operation as well as salt concentrations in the aquifer. The developed model is applied to analyze the control of seawater intrusion in a hypothetical confined coastal aquifer. The efficiencies of the three management scenarios are examined and compared. The results show that combination of abstraction and recharge wells is significantly better than using abstraction wells or recharge wells alone as it gives the least cost and least salt concentration in the aquifer. The results from this study would be useful in designing the system of abstraction/recharge wells to control seawater intrusion in coastal aquifers and can be applied in areas where there is a risk of seawater intrusion. Copyright © 2011 John Wiley & Sons, Ltd.

To investigate the relative importance of projected sea-level rise, climate change effects on recharge, and groundwater extraction on seawater intrusion in important coastal aquifers in Atlantic Canada, a three-dimensional numerical model of density-dependent groundwater flow coupled with solute transport was developed for the Richibucto region of New Brunswick. The model was used, with an efficient 2k factorial design approach, to perform simulations for the period 2011–2100. The results of the factorial analyses indicate that the relative importance of the three factors investigated varies depending on the model location considered. The effect of declining recharge is most significant at shallow to intermediate depths along the freshwater–seawater transition zone, while the effect of increasing pumping rates dominates at a location relatively close to the well field. The effect of sea-level rise is shown to be significant only at the much deeper inland toe of the transition zone. The spatial variation in importance is related to how different model boundary conditions influence freshwater flow at the different locations within the model domain. This investigation indicates that sea-level rise has the least significant effect (of the three factors considered) on future seawater intrusion in sandstone aquifers in the Richibucto region.

Hydrogeochemical processes that accompany seawater intrusion in coastal aquifers can alter the resulting water quality and are important ingredients in coastal aquifer management. The presence of dissolution–precipitation reactions and ion exchange in the mixing zone of the Biscayne aquifer (FL, USA) are suggested based on changes in major ion concentrations and mineral saturation indices (SI). Major ion concentrations from 11 groundwater samples are compared with theoretical mixing between freshwater and seawater. PHREEQC code was used to calculate saturation indices of the samples with respect to common phases in the Biscayne aquifer. High Ca2+ and HCO3 − content of the samples is typical of waters in contact with carbonate aquifers. Water quality of the samples is mainly attributed to mixing and precipitation–dissolution reactions with calcite and dolomite. The samples were saturated with calcite (SI ~ 0) and undersaturated for dolomite (SI < 0), while a few samples showed dolomite saturation. Because gypsum and halite SI could be predicted by theoretical mixing, reactions with those minerals, if present, are thought to be insignificant. In the active intrusion areas, cation exchange also appears to modify water quality leading to excess Ca2+, but depleted Na+, Mg2+ and K+ concentrations. On the other hand, samples from previous intrusion areas plotted very close to the theoretical mixing line and approached equilibrium with the seawater.

Jeong, Y.H.; Lee, S.Y.; Lee, J.; Lee, T.K., and Lee, W.D., 2021. Characteristics of seawater intrusion in coastal aquifers due to seawater flooding: A numerical study. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 96–100. Coconut Creek (Florida), ISSN 0749-0208. In typical coastal aquifers, dense brine infiltrates under freshwater in a wedge form. However, during seawater flooding due to storm surges or tsunamis, dense brine infiltrates from the surface into the ground. In this study, to investigate the characteristics of seawater intrusion in coastal aquifers. we simulated seawater intrusion by seawater flooding using the Navier–Stokes solver based on the porous body model. The numerical analysis results showed that when the hydraulic gradient of the coastal aquifers was small, salt diffusion by seawater flooding occurred throughout the groundwater, and the infiltrated seawater remained in the freshwater layer for a long time. However, when the hydraulic gradient of the coastal aquifers was large, the salt damage by seawater flooding was relatively small, and the amount of infiltrated seawater that escaped into the sea through the groundwater flow increased. In other words, a larger hydraulic gradient of coastal aquifers leads to a lower salt diffusion in the groundwater by seawater flooding and a faster recovery. The results of this study will be useful for the preservation of coastal aquifers in the event of tsunamis and flooding.

A correction for Dupuit–Forchheimer interface flow models of seawater intrusion in unconfined coastal aquifers

The Quaternary aquifer in the western Nile Delta is threatened by seawater intrusion. Few studies have integrated diverse techniques for the assessment of seawater intrusion in this aquifer. The present study aims to determine the geochemical processes and impact of seawater intrusion on this aquifer. To accomplish this investigation, the integration of hydrogeochemical, statistical, multivariate statistical, and graphical tools were implemented on 75 groundwater samples and 5 soil samples. The physicochemical variables were analyzed using hierarchical cluster analysis (HCA), saturation index (SI), ionic ratios, ionic relationships, the seawater intrusion index (SWI) and the correlations among 16 hydrochemical parameters, to identify the influencing processes of groundwater quality in the study area. According to the statistical study, the groundwater is divided into four groups. Those are distributed, from north to south: Group1 (G1), Group2 (G2), Group4 (G4), and Group3 (G3). The samples of G1 and G2 are distinguished by Na–Cl chemical type. While G4 has two main ion associations, HCO3–Ca–Mg and Cl–SO4–Na, G3 is characterized by HCO3–Cl–SO4–Ca–Na type. The processes that affect the chemistry of the groundwater are the seawater intrusion, ion exchange, silicate and Ca-rich mineral weathering, and mineral deposition. G1 and G2 groups are primarily influenced by seawater incursion, evaporation, and the ion exchange mechanism. In addition, the weathering of silicate minerals has a substantial effect on G3 and G4 groups, resulting in the creation of carbonate minerals.