Seawater Intrusion Model Research Articles

Modeling and uncertainty analysis of seawater intrusion based on surrogate models.

When using a simulation model to study seawater intrusion (SI), uncertainty in the parameters directly affects the results. The impact of the rise in sea levels due to global warming on SI cannot be ignored. In this paper, the Monte Carlo method is used to analyze the uncertainty in modeling SI. To reduce the computational cost of the repeated invocation of the simulation model as well as time, a surrogate model is established using a radial basis function (RBF)-based neural network method. To enhance the accuracy of the substitution model, input samples are sampled using the Latin hypercube sampling (LHS) method. The results of uncertainty analysis had a high reference value and show the following: (1) The surrogate model created using the RBF method can significantly reduce computational cost and save at least 95% of the time needed for the repeated invocation of the simulation model while maintaining high accuracy. (2) Uncertainty in the parameters and the magnitude of the rise in sea levels have a significant impact on SI. The results of prediction were thus highly uncertain. In practice, it is necessary to quantify uncertainty to provide more intuitive predictions.

Physical and Numerical Modelling of Post-Pumping Seawater Intrusion

This paper explores post-pumping seawater intrusion (PP-SWI), which is the phenomenon of seawater intruding further inland than the location of a well, after pumping has ceased. Despite numerous papers on the topic of seawater intrusion and pumping, this is the first time that PP-SWI has been described in the literature, to our knowledge. This paper describes a laboratory-scale investigation of the phenomenon and we demonstrate that PP-SWI can be reproduced within physical experiments. We also show, using numerical modelling, that PP-SWI is caused by disequilibrium in the flow field following the cessation of pumping. Specifically, in our simulations, the cone of depression persisted after the cessation of pumping (first moving inland and then retreating toward the coastal boundary) which caused a lag in the reestablishment of fresh water flow toward the coast, after pumping had stopped. It was during this period of flow-field disequilibrium that PP-SWI occurred. We expect systems with larger postextraction disequilibrium to be most susceptible to PP-SWI and recommend future research to improve understanding of the relationship between hydrogeological parameters, extraction rates, well location, and incidence of PP-SWI. In those systems where PP-SWI is most likely, quantitative analysis of groundwater extraction and SWI will need to employ transient approaches to ensure that SWI is not underestimated.

Smoothed Particle Hydrodynamics modelling of fresh and salt water dynamics in porous media

Seawater intrusion modelling is often used to assist with groundwater management in coastal areas and islands. Although Smoothed Particle Hydrodynamics (SPH) schemes are able to simulate multi-fluid flows in porous media, they have not been widely tested against experimental observations yet. Additionally, numerical methods for groundwater flow problems need to be able to simulate pumping, which is an unexplored area in SPH. In this study, an Explicit Incompressible SPH (EISPH) solver for multi-fluid flow in porous media is used to simulate the dynamics of freshwater lenses in small islands, and is further developed to simulate groundwater pumping and associated seawater upconing. Three methods to implement a sink term that models water pumping from an aquifer are proposed and compared. The model is successfully tested against data from published laboratory-scale experiments and other numerical models. The results of EISPH are comparable to other models. The inclusion of a sink for water particles to simulate pumping did not affect the stability of the simulations, although one of the three methods led to results that better compared to experimental data. Hence, SPH modelling of groundwater flows in porous media can be successfully achieved using the methods developed here.

Assessment of vulnerability to pollution and seawater intrusion of groundwater in the anthropized reclamation area of Arborea (W. Sardinia)

The phenomenon of seawater intrusion is one of the major problems in Sardinian coastal aquifers (Italy). In particular, it has been detected in the reclamation area of Arborea plain (west Sardinia) where intensive agriculture and dairy farming are the mainstays of the local economy.In this research SINTACS and GALDIT vulnerability indexes and the numerical model for simulating groundwater flow have been applied for evaluating respectively the intrinsic vulnerability to pollution, seawater intrusion and groundwater flow for a typical Mediterranean phreatic sandy aquifer such as Arborea plain aquifer.This research may offer a valuable contribution to the team of existing tools in the field of seawater intrusion and groundwater quality modelling.

Modelling and optimisation of water management in sloping coastal aquifers with seepage, extraction and recharge

We consider the management of sloping, long and thin, coastal aquifers. We first develop a simple mathematical model, based on Darcy flow for porous media, which gives the water table height, and the flow velocity as a function of the underground seepage rate, the recharge rates and the extraction rates, neglecting sea water intrusion. We then validate the model with recent data from the Germasogeia aquifer which caters for most of the water demand in the area of Limassol, Cyprus. The data is provided over a three-year period by the Cyprus Water Development Department (WDD), the governmental department managing the aquifer. Three different models of the recharge sources have been considered and it was found that Gaussian sources give the closest agreement with data. Furthermore, based on our model, we subsequently develop an optimised recharge strategy and identify the optimal recharge rates for a desired extracted water volume while the water table height is maintained at the acceptable level. We study several scenarios of practical interest and we find that we can achieve considerable water savings, compared to the current empirical strategy followed by WDD. Additionally, we model the transport of pollutants in the aquifer in the case of accidental leakage, using an advection-diffusion equation and the concentration is determined in the aquifer for an ongoing contamination and two pulsed contaminations (pulse duration 1 day and 30 days, respectively). We find that in the case of an undetected and unhindered contamination (worst case scenario) the aquifer would get polluted in about three years. Also, we find that double recharge rates flush the pollutant out of the aquifer faster. Finally, to incorporate the possibility of sea water intrusion, which can render aquifers unusable, we develop a new, transient two-dimensional model of groundwater flow based on the Darcy-Brinkman equations, and determine the position of the water table and the seawater-freshwater interface for conditions of drought, moderate rainfall and flooding. The validation of the new seawater intrusion modelling approach has been carried out via comparison with a widely-accepted code.

Comparison of Sharp Interface to Variable Density Models in Pumping Optimisation of Coastal Aquifers

A number of models have been developed to simulate seawater intrusion in coastal aquifers, which differ in the accuracy level and computational demands, based on the approximation level of the application. In this paper, four seawater intrusion models are employed to calculate the optimal pumping rates in a coastal aquifer management problem. The first model considers both fluid flow and solute transport processes and assumes a variable-density transition zone between saltwater and freshwater. The implementation of the model in simulation-optimisation routines is impractical, due to the computational time required for the simulation. The second model neglects the dispersion mechanism and assumes a sharp interface between saltwater and freshwater. The sharp interface model is significantly faster than the variable density model, however, it may introduce errors in the estimation of the seawater intrusion extent. The remaining two models are modifications of the second model, which intent to correct the inaccuracies of the simplified sharp interface approximation. All four models are utilised to simulate an unconfined coastal aquifer with multiple pumping wells and an optimisation method is used to calculate the maximum allowed pumping rates. The optimisation results are then analysed, in order to examine if the three sharp interface models could provide feasible solutions in the area of the variable density optimum, which is considered as a benchmark solution.

Modeling and uncertainty analysis of seawater intrusion in coastal aquifers using a surrogate model: a case study in Longkou, China

Seawater intrusion is a complex problem involving groundwater with variable density. It has an important impact on the development and utilization of groundwater resources in coastal areas. In previous studies, numerical simulations of seawater intrusion were performed by assigning fixed values to model parameters, thereby not taking into account the influence of stochastic variability of hydrogeological parameters on model predictions. In this study, we developed a three-dimensional mathematical model of seawater intrusion by modeling vertical solute transport due to density changes resulting from seawater intrusion. We then applied a Monte Carlo–based method to analyze uncertainty in model simulations of seawater intrusion. Because the Monte Carlo method requires the simulation model to be repeatedly run, we used the Kriging method to build a surrogate model that significantly reduced the computational load compared to the simulation model while ensuring the desired accuracy. Then we used a local sensitivity analysis method to select the two parameters with the greatest influence on model output. We treated the two selected parameters as random variables, and results show that (1) the three-dimensional, variable-density seawater intrusion model can effectively simulate and predict the distribution of seawater intrusion in the study area; (2) the local sensitivity analysis can accurately identify the hydrogeological parameters that most influence model output; (3) the uncertainty analysis based on the surrogate model reduces computing time substantially and provides a realistic assessment of the effect of hydrogeological parameter variability on seawater intrusion numerical simulation results.

Numerical modeling of seawater intrusion in karstic aquifer, Northwestern Coast of Egypt

The growing population and increasing of the urbanization activities at the Egyptian coastal areas create a great demand on groundwater. El Negila which is located on the Northwestern coast of Egypt is of great environmental, economic, and social importance, so the sustainable management and preservation of the groundwater quantity and quality is imperative. Such task can be addressed through modeling tools that take into account both the human impacts and the effects of climate changes on the hydrogeological system. A variable-density flow model (SEAWAT) had been used to investigate the extent of the seawater intrusion the study area, and to introduce an effective management policy for sustainable development of the groundwater system. An attempt had been made for model calibration; based on adjusting the hydraulic head of the hydrogeological system with respect to the lineaments frequency in the study area. The calibrated numerical model showed the limited extent of the seawater intrusion in the study area under current recharge and abstraction conditions, in addition to that, the model was validated using hydrochemical and stable isotopic data. The aquifer response to different stressing pumping scenario has been tested and predicted up to the year 2022. Furthermore, the impact of rising the seawater level as a result of rapid climatic change was assessed, with pumping stress of the second scenario. It was concluded that the tableland area acts as recharge source and providing an opportunity of recharging the karstic aquifer in the study area especially through the lineaments. The extent of seawater intrusion in the study area for the karstic aquifer is limited, and would increase under the induced activities of pumping as well as seawater rise conditions. A range of withdrawal rate from the study area was recommended to avoid more invasions from seawater.

Global weak solutions for a degenerate parabolic system modelling seawater intrusion in confined aquifers

ABSTRACT This article is devoted to prove the existence of a non-negative solution for a degenerate parabolic strongly coupled system which arises from seawater intrusion model in confined aquifers. An approximate linear discretized in time scheme is then set up yielding to the existence result with no restriction on the space dimension. The availability of an entropy estimate turns out to be a central tool to obtain the existence result.

Pumping Optimization of Coastal Aquifers Using Seawater Intrusion Models of Variable-Fidelity and Evolutionary Algorithms

Variable-fidelity modelling has been utilized in several engineering optimization studies to construct surrogate models. However, similar approaches have received much less attention in coastal aquifer management problems. A variable-fidelity optimization framework was developed utilizing a lower-fidelity and computationally cheap model of seawater intrusion, based on the sharp interface assumption, and a simple correction process. The variable-fidelity method was compared to the direct optimization with the high-fidelity variable density and salt transport model and to conventional surrogate-based optimization. The surrogate-based approaches were embedded into the operations of an evolutionary algorithm to implement an efficient online update of the surrogate models and control the feasibility of the optimal solutions. Multiple independent optimization runs were performed to provide more insightful comparison outcomes. Although the variable-fidelity method found a better optimum than the conventional approach, the overall sample statistics showed that the surrogate-based optimization frameworks performed equally well and provided good approximations to the high-fidelity solution. Despite the potential for an improved exploration of the optimal search space by using the variable-fidelity method, the conventional approach had a 30% faster average convergence time.

Topical Collection: Coastal aquifers in the Middle East and North Africa region

The Middle East and North Africa (MENA) region suffers from low precipitation and high evaporation. Coastal areas of the MENA region are usually densely populated; hence, the coastal aquifers are easily over-exploited beyond their safe yield, and seawater intrusion and aquifer salinization have been caused by the groundwater-level declines. Four studies in MENA coastal aquifers, on seawater intrusion modeling and aquifer recharge and salinization using isotopes, have been brought together in a topical collection and are discussed in this essay. A brief overview is given of managed aquifer recharge as an effective method to combat groundwater-level decline, seawater intrusion and aquifer salinization in MENA counties.

Ensemble Kalman Filter Assimilation of ERT Data for Numerical Modeling of Seawater Intrusion in a Laboratory Experiment

Seawater intrusion in coastal aquifers is a worldwide problem exacerbated by aquifer overexploitation and climate changes. To limit the deterioration of water quality caused by saline intrusion, research studies are needed to identify and assess the performance of possible countermeasures, e.g., underground barriers. Within this context, numerical models are fundamental to fully understand the process and for evaluating the effectiveness of the proposed solutions to contain the saltwater wedge; on the other hand, they are typically affected by uncertainty on hydrogeological parameters, as well as initial and boundary conditions. Data assimilation methods such as the ensemble Kalman filter (EnKF) represent promising tools that can reduce such uncertainties. Here, we present an application of the EnKF to the numerical modeling of a laboratory experiment where seawater intrusion was reproduced in a specifically designed sandbox and continuously monitored with electrical resistivity tomography (ERT). Combining EnKF and the SUTRA model for the simulation of density-dependent flow and transport in porous media, we assimilated the collected ERT data by means of joint and sequential assimilation approaches. In the joint approach, raw ERT data (electrical resistances) are assimilated to update both salt concentration and soil parameters, without the need for an electrical inversion. In the sequential approach, we assimilated electrical conductivities computed from a previously performed electrical inversion. Within both approaches, we suggest dual-step update strategies to minimize the effects of spurious correlations in parameter estimation. The results show that, in both cases, ERT data assimilation can reduce the uncertainty not only on the system state in terms of salt concentration, but also on the most relevant soil parameters, i.e., saturated hydraulic conductivity and longitudinal dispersivity. However, the sequential approach is more prone to filter inbreeding due to the large number of observations assimilated compared to the ensemble size.

Numerical modeling and sensitivity analysis of seawater intrusion in a dual-permeability coastal karst aquifer with conduit networks

Abstract. Long-distance seawater intrusion has been widely observed through the subsurface conduit system in coastal karst aquifers as a source of groundwater contaminant. In this study, seawater intrusion in a dual-permeability karst aquifer with conduit networks is studied by the two-dimensional density-dependent flow and transport SEAWAT model. Local and global sensitivity analyses are used to evaluate the impacts of boundary conditions and hydrological characteristics on modeling seawater intrusion in a karst aquifer, including hydraulic conductivity, effective porosity, specific storage, and dispersivity of the conduit network and of the porous medium. The local sensitivity analysis evaluates the parameters' sensitivities for modeling seawater intrusion, specifically in the Woodville Karst Plain (WKP). A more comprehensive interpretation of parameter sensitivities, including the nonlinear relationship between simulations and parameters, and/or parameter interactions, is addressed in the global sensitivity analysis. The conduit parameters and boundary conditions are important to the simulations in the porous medium because of the dynamical exchanges between the two systems. The sensitivity study indicates that salinity and head simulations in the karst features, such as the conduit system and submarine springs, are critical for understanding seawater intrusion in a coastal karst aquifer. The evaluation of hydraulic conductivity sensitivity in the continuum SEAWAT model may be biased since the conduit flow velocity is not accurately calculated by Darcy's equation as a function of head difference and hydraulic conductivity. In addition, dispersivity is no longer an important parameter in an advection-dominated karst aquifer with a conduit system, compared to the sensitivity results in a porous medium aquifer. In the end, the extents of seawater intrusion are quantitatively evaluated and measured under different scenarios with the variabilities of important parameters identified from sensitivity results, including salinity at the submarine spring with rainfall recharge, sea level rise, and a longer simulation time under an extended low rainfall period.

Numerical analysis of a finite volume scheme for a seawater intrusion model with cross‐diffusion in an unconfined aquifer

We consider a degenerate parabolic system modeling the flow of fresh and saltwater in a porous medium in the context of seawater intrusion. We propose and analyze a finite volume scheme based on two‐point flux approximation with upwind mobilities. The scheme preserves at the discrete level the main features of the continuous problem, namely the nonnegativity of the solutions, the decay of the energy and the control of the entropy and its dissipation. Based on these nonlinear stability results, we show that the scheme converges toward a weak solution to the problem. Numerical results are provided to illustrate the behavior of the model and of the scheme.

Modeling of seawater intrusion in a coastal aquifer of Karaburun Peninsula, western Turkey

Seawater intrusion is a major problem to freshwater resources especially in coastal areas where fresh groundwater is surrounded and could be easily influenced by seawater. This study presents the development of a conceptual and numerical model for the coastal aquifer of Karareis region (Karaburun Peninsula) in the western part of Turkey. The study also presents the interpretation and the analysis of the time series data of groundwater levels recorded by data loggers. The SEAWAT model is used in this study to solve the density-dependent flow field and seawater intrusion in the coastal aquifer that is under excessive pumping particularly during summer months. The model was calibrated using the average values of a 1-year dataset and further verified by the average values of another year. Five potential scenarios were analyzed to understand the effects of pumping and climate change on groundwater levels and the extent of seawater intrusion in the next 10 years. The result of the analysis demonstrated high levels of electrical conductivity and chloride along the coastal part of the study area. As a result of the numerical model, seawater intrusion is simulated to move about 420 m toward the land in the next 10 years under “increased pumping” scenario, while a slight change in water level and TDS concentrations was observed in “climate change” scenario. Results also revealed that a reduction in the pumping rate from Karareis wells will be necessary to protect fresh groundwater from contamination by seawater.

Direct and Inverse Modeling of Seawater Intrusion: A Perspective

ABSTRACT Sea Water Intrusion (SWI) is one of the major hydrological problem in coastal aquifers. This global issue is aggravated by increasing demands for freshwater in coastal regions. In this paper, different approaches of modeling SWI and the parameters affecting the process of SWI are introduced. This is followed by the discussion on the numerical models to solve the complex, three dimensional (3D) groundwater and solute transport problems in coastal aquifer. The importance of considering aquifer characteristics while modeling the groundwater system for flow and solute transport is emphasized. A brief discussion on previous methodology, novelty and limitations on direct simulation of SWI are tabulated. The significant aspects to be considered while direct modeling of coastal aquifers are discussed and the recent focus of research in this area of interest are stated. In direct modeling, information on aquifer parameters are often unknown, therefore, an inverse approach is explained briefly. The previous studies relating to determistic inverse modeling for coupled groundwater flow and solute transport problems reported in literature are summarized. The insights about prior information, estimated parameter sensitivities, variances and correlations are reported. The paper identifies some of the existing gaps in the modeling of SWI based on the previous work and provides comprehensive understanding on direct and deterministic inverse SWI modeling.

Identifying key factors of the seawater intrusion model of Dagu river basin, Jiaozhou Bay

Seawater intrusion is a complex groundwater - seawater interaction process, and it is influenced by many factors from ground surface to underground, from groundwater to seawater. Generally, for seawater intrusion model, some model parameters and boundary conditions are always specified by model users’ personal experiences or literature's reference value. The defective model would damage the groundwater management for controlling and preventing seawater intrusion when making decisions are based on this model. In order to improve the reliability of seawater intrusion model, the influences of model inputs on output should be identified prior at optimizing model inputs.Dagu river basin, Jiaozhou Bay is one of the most serious areas of seawater intrusion in China, and it is chosen as the study area in this study. The seawater intrusion model of Dagu river basin is built based on a general program SEAWAT4. The key influence factors of model output are analyzed by two sensitivity analysis methods, i.e., stepwise regression and mutual entropy. The results demonstrated that the most important influence factors which have largest sensitivities to groundwater Cl- concentration are the precipitation rate and groundwater pumping in agriculture area. In addition, the hydraulic conductivity of zone 1 has a non-negligible influence on seawater intrusion process. Stepwise regression analysis is capable of identifying most important influence factor, and it can’t handle complicated nonlinear input-output relationship. Mutual entropy analysis is reliable for identifying the influence factors for complex seawater intrusion model.

Uncertainty-based simulation-optimization using Gaussian process emulation: Application to coastal groundwater management

Combined simulation-optimization (S/O) schemes have long been recognized as a valuable tool in coastal groundwater management (CGM). However, previous applications have mostly relied on deterministic seawater intrusion (SWI) simulations. This is a questionable simplification, knowing that SWI models are inevitably prone to epistemic and aleatory uncertainty, and hence a management strategy obtained through S/O without consideration of uncertainty may result in significantly different real-world outcomes than expected. However, two key issues have hindered the use of uncertainty-based S/O schemes in CGM, which are addressed in this paper. The first issue is how to solve the computational challenges resulting from the need to perform massive numbers of simulations. The second issue is how the management problem is formulated in presence of uncertainty. We propose the use of Gaussian process (GP) emulation as a valuable tool in solving the computational challenges of uncertainty-based S/O in CGM. We apply GP emulation to the case study of Kish Island (located in the Persian Gulf) using an uncertainty-based S/O algorithm which relies on continuous ant colony optimization and Monte Carlo simulation. In doing so, we show that GP emulation can provide an acceptable level of accuracy, with no bias and low statistical dispersion, while tremendously reducing the computational time. Moreover, five new formulations for uncertainty-based S/O are presented based on concepts such as energy distances, prediction intervals and probabilities of SWI occurrence. We analyze the proposed formulations with respect to their resulting optimized solutions, the sensitivity of the solutions to the intended reliability levels, and the variations resulting from repeated optimization runs.

Surrogate-based pumping optimization of coastal aquifers under limited computational budgets

AbstractThe computationally expensive variable density and salt transport numerical models hinder the implementation of simulation-optimization routines for coastal aquifer management. To reduce the computational cost, surrogate models have been utilized in pumping optimization of coastal aquifers. However, it has not been previously addressed whether surrogate modelling is effective given a limited number of numerical simulations with the seawater intrusion model. To that end, two surrogate-based optimization (SBO) frameworks are employed and compared against the direct optimization approach, under restricted computational budgets. The first, a surrogate-assisted algorithm, employs a strategy which aims at a fast local improvement of the surrogate model around optimal values. The other, balances global and local improvement of the surrogate model and is applied for the first time in coastal aquifer management. The performance of the algorithms is investigated for optimization problems of moderate and large dimensionalities. The statistical analysis indicates that for the specified computational budgets, the sample means of the SBO methods are statistically significantly better than those of the direct optimization. Additionally, the selection of cubic radial basis functions as surrogate models, enables the construction of very fast approximations for problems with up to 40 decision variables and 40 constraint functions.

Mathematical Analysis of a Seawater Intrusion Model Including Storativity

This paper is devoted to a seawater intrusion model in free aquifers, including rock and fluid compressibility effects. This original model is based on the superimposition of a phase-field model on sharp front tracking. It leads to the analysis of a two-dimensional strongly coupled system of PDEs of parabolic type describing the evolution of the depths of the two free surfaces. The main difficulty is dealing with the strong nonlinearity occurring in the time derivative due to the fluid storage coefficient. This point yields to the manipulation of very weak solutions---solutions of an alternative variational formulation focusing on the structural weakness of the problem.