Saltwater Intrusion Problem Research Articles

WSUD Techniques for Urban Development with Focus on Saltwater Intrusion Problem, A Case Study of Kedonganan Village - Bali

Water is one of the most important resources in human’s life, and groundwater known as the largest freshwater source in the world. Along the development of the cities and population growth, the demand for water supply is also increasing. With the increasing of the freshwater needs, groundwater is also widely used for human’s daily activities. However, the irresponsible use and exploration of groundwater also certainly generate many problems, one of those is saltwater intrusion. Many coastal areas in Indonesia are starting to face saltwater intrusion problem which cause groundwater is no longer be used properly by the community. Therefore, the problem of saltwater intrusion needs to be addressed. This research aims to see the WSUD techniques which can be applied to prevent saltwater intrusion in coastal areas with Kedonganan Village as a case study. The research methodology used is descriptive qualitative method. For the result of this research, those are some WSUD techniques which can be applied in urban development to prevent saltwater intrusion occurs: rainwater tank, downpipe diverters, biofilters, infiltration basins & trenches, soakwells, swales & buffer strips, and pervious paving which have focus on accelerate infiltration into the aquifer.

Uncertainty quantification in the Henry problem using the multilevel Monte Carlo method

We investigate the applicability of the well-known multilevel Monte Carlo (MLMC) method to the class of density-driven flow problems, in particular the problem of salinisation of coastal aquifers. As a test case, we solve the uncertain Henry saltwater intrusion problem. Unknown porosity, permeability and recharge parameters are modelled by using random fields. The classical deterministic Henry problem is non-linear and time-dependent, and can easily take several hours of computing time. Uncertain settings require the solution of multiple realisations of the deterministic problem, and the total computational cost increases drastically. Instead of computing of hundreds random realisations, typically the mean value and the variance are computed. The standard methods such as the Monte Carlo or surrogate-based methods are a good choice, but they compute all stochastic realisations on the same, often, very fine mesh. They also do not balance the stochastic and discretisation errors. These facts motivated us to apply the MLMC method. We demonstrate that by solving the Henry problem on multi-level spatial and temporal meshes, the MLMC method reduces the overall computational and storage costs. To reduce the computing cost further, parallelization is performed in both physical and stochastic spaces. To solve each deterministic scenario, we run the parallel multigrid solver ug4 in a black-box fashion.

Uncertainty quantification in coastal aquifers using the multilevel Monte Carlo method

AbstractWe are solving a problem of salinisation of coastal aquifers. As a test case example, we consider the Henry saltwater intrusion problem. Since porosity, permeability and recharge are unknown or only known at a few points, we model them using random fields and random variables. The Henry problem describes a two‐phase flow and is non‐linear and time‐dependent. The solution to be found is the expectation of the salt mass fraction, which is uncertain and time‐dependent. To estimate this expectation, we use the well‐known multilevel Monte Carlo (MLMC) method. The MLMC method takes just a few samples on computationally expensive (fine) meshes and more samples on cheap (coarse) meshes. Then, by building a telescoping sum, the MLMC method estimates the expected value at a much lower computational cost than the classical Monte Carlo method. The deterministic solver used here is the well‐known parallel and scalable ug4 solver.

Organisation of Fish Assemblage in the Mekong Delta as a Potential Indicator of Salt Water Intrusion

The problem of salt water intrusion into the Mekong Delta is a research topic involving various branches of science. Among the causes of this phenomenon are global sea level rise and regulation of the natural river discharge due to hydropower dam construction. The scale of the delta and its water dynamics complicate instrument-based observations. Meanwhile, communities of living organisms can serve as indicators of environmental heterogeneity. This study aimed to analyze the spatial variability of fish assemblage organisation in the Mekong Delta for use as an indicator of salt water intrusion. The composition of fish assemblages in different parts of the delta were determined by midwater trawl catches during January and April 2021. A total of ~15 thousand individual fish from 74 trawl hauls were captured and analyzed. The null hypothesis was that the fish assemblages in the Mekong Delta are represented by three taxonomic complexes that are localized in the upper course (freshwater), the lower course (brackish) and the middle course (zone of the contact of the fresh and brackish waters) of the delta. The taxonomic composition of the freshwater complex is the most poor and is represented by 9 families. Meanwhile, marginal (the contact zone) and brackish complexes include 26 and 23 families, respectively. Specimens of families Cobitidae, Eleotridae, Plotosidae and Siluridae that are only found in the zone of contact of fresh and brackish waters could be considered as indicator species of its position. The actual boundaries of the brackish and freshwater complexes were determined and the distance from them to the marine delta edge equals 34 and 78 km, respectively. Assessment of the salinity values that correspond to the complex boundaries is a task for future investigation. The position of the boundaries of freshwater and brackish fish taxonomic complexes has a biological basis and could be used as an indicator of salt water intrusion into the Mekong Delta.

An efficient discontinuous Galerkin - mixed finite element model for variable density flow in fractured porous media

Modeling variable density flow (VDF) in fractured porous media is computationally challenging. The challenges are mainly related to: (i) the high permeability contrast between the matrix and the fractures and, (ii) the nonlinearity induced by density variations. Due to their local mass conservation property, cell-centered methods, such as finite volumes (FV), mixed finite elements (MFE) or Discontinuous Galerkin (DG) methods are well suited for modeling mass transfer in highly heterogeneous domains. When applied for fractured media, these methods require small grid cells next to fractures because of the cross-flow equilibrium assumption (i.e. the pressure and concentration in the fracture and in the adjacent matrix grid-cells are assumed the same). To avoid this constraint, an efficient model is developed in this work using advanced cell-centered numerical methods for VDF in fractured porous media with cross-flow equilibrium assumed only across the fractures. The new model uses the hybrid MFE method for the flow discretization in the matrix and in the fracture continua. Mass transport in the matrix is modeled using a monotonic upwind MFE scheme. Advection-dominated transport in fractures is discretized with the DG method, which is well adapted for hyperbolic equations. The new model ensures continuity of pressure, concentration, fluid mass flux, advective and dispersive contaminant fluxes at matrix-fracture interfaces as well as at the intersection of several fractures. The time discretization is performed using high-order adaptive time integration techniques via the method of lines (MOL). The developed model is first validated by comparison against a 2D-2D model for a test problem dealing with linear flow and transport in a 2D domain involving a “+”-shaped barrier/fracture network. Then, the MFE-DG model is used for the simulation of the Henry saltwater intrusion problem and the results are validated by comparison against the semi-analytical solution in the case of unfractured and fractured aquifers. Finally, the simulation of a transport problem with variable viscosity in a fractured heterogeneous domain points out the superiority of the new model in terms of accuracy and efficiency when compared to a standard finite element model.

Mathematical analysis of the resolution of the saltwater intrusion problem in a coastal aquifer using the RBF collocation method

Mathematical analysis of the resolution of the saltwater intrusion problem in a coastal aquifer using the RBF collocation method

Validation of boundary layer solution for assessing mixing zone dynamics of saltwater wedge in a coastal aquifer

In saltwater intrusion problems of coastal regions, the dynamics of narrower mixing zones are influenced by the amount of dispersive entrainment and its associated marine discharges. Modelling low-fresh water recharge and high-Peclet-number scenarios gives inaccurate results with the available methods. In this study, the physical accuracy of the BL solution provided by Paster and Dagan (2007) and Paster (2010) for such conditions is validated for discharge at outflow face and width of mixing zone (WMZ) with experimentally created datasets and further extended with numerical analysis to quantify the influence of dispersive entrainment on marine discharge and saline circulation. By comparing the measured and estimated saltwater outflows, the average transverse dispersion for all dimensionless freshwater flux rates (a) was calculated, and it ranged from 0.005 cm to 0.062 cm. The study showed that, for buoyancy-dominated, high Peclet number flow circumstances, the boundary layer solutions can be effectively used to predict the marine discharges up to a = 0.5. The combined effects of water flux and dispersion revealed that buoyant force has a considerable impact on the rate of saline circulation as it suppresses the dispersion. The findings lend credence to simplicity of the boundary layer solution in reducing numerical model convergence issues for field problems caused by low recharge and low transverse dispersion flow situations.

Spatial and Temporal Changes of Groundwater Storage in the Quaternary Aquifer, UAE

To study the temporal and spatial variations of the groundwater quantity and quality in response to intensive groundwater exploitation from the Quaternary aquifer in UAE, a water budget model with a cell size of one km2 was developed. The available historical records of groundwater levels and salinity have been used to develop the water table and salinity maps of UAE for the years 1969, 2005, 2010, and 2015. The available water resources and soil information system was used to facilitate validity, cogency, and consistency of the groundwater analysis. The spatial analysis module of GIS was used to define the aquifer setting, saturated thickness, aquifer base elevation, effective porosity, and groundwater salinity at each grid cell. The obtained results indicated that the volume of fresh groundwater resources in the Quaternary aquifer in UAE has decreased from 238 km3 in 1969 to around 10 km3 in 2015. A major part of these depleted fresh groundwater resources was replaced by brackish water, and, therefore, the total groundwater storage in this aquifer has only decreased from 977 in 1969 to 922 km3 in 2015, respectively. If the same groundwater exploitation continues, the freshwater storage in the surficial aquifer might be totally depleted in agricultural areas. Most probably, the brackish groundwater resources will be exploited. In such areas, more attention should be devoted to the management of brackish water resources to avoid the exacerbation of the saltwater intrusion problem. Despite the fact that the obtained results indicate the negative impacts of the improper water resources management in a small part of the arid area, the learned lessons are valid for other arid countries, in particular, using the proper steady state boundary conditions for the initial conditions in modeling the available future management alternatives.

Analysis of the Relationship between Vegetable Virtual Water Trade and Saltwater Intrusion in Shouguang City, China

Shouguang City is located in north-central of Shandong, distributing the piedmont plain and coastal plain from the south to north. This area, as a vegetable base in northern China, not only meets local needs but also has a large number of vegetables exported to domestic and foreign countries in the form of virtual water. The vegetable production in this area mainly depends on groundwater exploitation, which causes saltwater intrusion problems. This study establishes the linear model of virtual water and vegetable production, irrigation water consumption, total population, per capita vegetable sales volume, and the purchase amount of vegetable trade. The output of vegetable virtual water from 1980 to 2012 is calculated. 3.79 billion kg of vegetables were exported in the form of virtual water with a volume of 145 million m3 in 2012, accounted for 78% of the average annual groundwater resources in the region. The vegetable trade development in Shouguang City from 1980 to 2002 can be divided into two stages: bare land vegetable cultivation in 1980–1990 and greenhouse vegetable cultivation in 1991–2002. The correlations among the annual virtual water output volume of vegetable trade, annual groundwater cumulative storage, groundwater depth at the center of depression area, and saltwater intrusion area are established and analyzed. In the first stage, for every 10 million m3 of vegetable virtual water was transferred, the salt water intrusion area in this area increased by 13.8 km2; in the second stage, the saltwater intrusion area increased correspondingly by 5 km2. The influence of saltwater intrusion on adjusting crop-planting structure is also analyzed to achieve the purpose of effectively alleviating the ecological and environmental problems in the area.

Effect of saltwater intrusion on activated sludge flocculation.

Saltwater addition negatively impacted the settling properties of the activated sludge in a sequencing batch reactor, which in turn caused the effluent quality and the dewatering properties to deteriorate. Experiments showed that increasing sodium ion concentrations above 1meq/L interfered with bio-flocculation by ion exchange of calcium ions displaced by sodium in mixed liquor flocs, causing increases in the sludge volume index, total suspended solids, and chemical oxygen demand concentrations of the final effluent and in the times of the time to filter test of the mixed liquor. Experiments were also conducted to reverse mixed liquor de-flocculation by adding calcium ions in wastewater with different sodium chloride concentrations and a positive relationship was found between calcium ion added and improved effluent quality. Calcium ion concentrations above 1.6meq/L proved to be beneficial for adhesion of microbial aggregates leading to the formation of stable flocs and improved sludge dewatering by reversing the ion exchange phenomenon observed with sodium concentrations above 1meq/L. Monovalent-to-divalent ratios did not provide predictive information on de-flocculation. Hence, individual ion concentrations in addition to monovalent-to-divalent cation ratios are important in the onset of de-flocculation effects while a system-specific concentration threshold was measured under which no adverse effects were observed. PRACTITIONER POINTS: The adverse effects of varying concentrations of sodium chloride on the biological process affecting bioflocculation and effluent quality. Impact of sodium on the health of the mixed liquor as well as on dewatering processes. Prediction of sea level rise due to climate change will impact utilities situated along the coast with saltwater intrusion problems. Management of dewatered flows containing saltwater from onsite construction Identify management practices to reduce the impact of saltwater on the wastewater treatment plant.

Parameters Identification in a Saltwater Intrusion Problem

This article is devoted to the identification, from observations or field measurements, of the hydraulic conductivity K for the saltwater intrusion problem in confined aquifers. The involved PDE model is a coupled system of nonlinear parabolic-elliptic equations completed by boundary and initial conditions. The main unknowns are the saltwater/freshwater interface depth and the freshwater hydraulic head. The inverse problem is formulated as an optimization problem where the cost function is a least square functional measuring the discrepancy between experimental data and those provided by the model. Considering the exact problem as a constraint for the optimization problem and introducing the Lagrangian associated with the cost function, we prove that the optimality system has at least one solution. Moreover, the first order necessary optimality conditions are established for this optimization problem.

Groundwater Circulation Well for Controlling Saltwater Intrusion in Coastal aquifers: Numerical study with Experimental Validation

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.

Transforming Vulnerability Indexing for Saltwater Intrusion into Risk Indexing through a Fuzzy Catastrophe Scheme

Mapping vulnerability to Saltwater Intrusion (SWI) in coastal aquifers is studied in this paper using the GALDIT framework but with a novelty of transforming the concept of vulnerability indexing to risk indexing. GALDIT is the acronym of 6 data layers, which are put consensually together to invoke a sense of vulnerability to the intrusion of saltwater against aquifers with freshwater. It is a scoring system of prescribed rates to account for local variations; and prescribed weights to account for relative importance of each data layer but these suffer from subjectivity. Another novelty of the paper is to use fuzzy logic to learn rate values and catastrophe theory to learn weight values and these together are implemented as a scheme and hence Fuzzy-Catastrophe Scheme (FCS). The GALDIT data layers are divided into two groups of Passive Vulnerability Indices (PVI) and Active Vulnerability Indices (AVI), where their sum is Total Vulnerability Index (TVI) and equivalent to GALDIT. Two additional data layers (Pumping and Water table decline) are also introduced to serve as Risk Actuation Index (RAI). The product of TVI and RAI yields Risk Indices. The paper applies these new concepts to a study area, subject to groundwater decline and a possible saltwater intrusion problem. The results provide a proof-of-concept for PVI, AVI, RAI and RI by studying their correlation with groundwater quality samples using the fraction of saltwater (fsea), Groundwater Quality Indices (GQI) and Piper diagram. Significant correlations between the appropriate values are found and these provide a new insight for the study area.

A Nonlinear Optimization Method Applied to the Hydraulic Conductivity Identification in Unconfined Aquifers

This article is concerned with the identification, from observations or field measurements, of the hydraulic conductivity for the saltwater intrusion problem in an unconfined aquifer. The involved model consists in a cross-diffusion system describing the evolutions of two interfaces: one between freshwater and saltwater and the other one between the saturated and unsaturated zones of the aquifer. The inverse problem is formulated as an optimization problem, where the cost function is a least square functional measuring the discrepancy between experimental interfaces depths and those provided by the model. Considering the exact problem as a constraint for the optimization problem and introducing the Lagrangian associated with the cost function, we prove that the optimality system has at least one solution. Moreover, we establish the first-order necessary optimality conditions. A numerical method is implemented to solve this identification problem. Some numerical results are presented to illustrate the ability of the method to determine the unknown parameters.

Groundwater Quality Analyses along Kenyan Coastal Region, Case Study of Kilifi County

The Kenyan coast is a semi-arid region frequently faced with severe water scarcity especially during prolonged drought and a rapid population growth. Groundwater provides 50% of the water demand and most sources are poorly drilled, over-abstracted and abandoned. The aquifers occur in sedimentary formations of fluvial and lacustrine origin. There is a need to have a groundwater monitoring system in place; therefore this study analyses selected physico- chemical and microbial parameters using both laboratory and field methods. Arc-GIS 9.3, SURFER were used to generate thematic maps for some tested parameters. Results indicate pH values within acceptable range of 6.5 – 8.5, though shallow wells in Malindi were acidic, this atrributed to the heavy industries prevalent in the area. Electric conductivity (EC) was above the set standard of 1500µS/m indicating high dissolved ions present especially in Magarini where salt mining is done. Total dissolved solids (TDS) of up to 500mg/L suitable for drinking water was only found during the wet season. Turbidity of less than 5 NTU was established in most areas. Chlorides and salinity levels exceeded 250mg/L pointing to the saltwater intrusion problem that is a challenge along Kenyan coastal aquifers Boreholes and shallow wells had a total hardness ranging from 50mg/L -150mg/L indicating the water was ranging from being slightly hard to hard water. Escheriria coli was present in almost all the boreholes and shallow wells indicating the need to treat water before use. Water Quality index (WQI) calculation indicate that groundwater is unsuitable for human consumption in dry season.

Axisymmetric Modeling Using MODFLOW-USG.

Axisymmetric groundwater models are used for simulating radially symmetric conditions. Groundwater simulators built specifically to model axisymmetric conditions are most commonly used for simulating aquifer tests. Although some numerical models capable of simulating flow and solute transport that are developed in the cartesian coordinate system framework offer flexibility to simulate axisymmetric conditions, most of the numerical groundwater models, such as the MODFLOW family of codes, are based on structured grids in which axisymmetric flows cannot be directly simulated. Researchers in the past have provided methods to manipulate aquifer properties to mimic axisymmetric conditions. This study presents a methodology that takes advantage of the unstructured grids of MODFLOW-USG to simulate axisymmetric models within the MODFLOW framework. To develop axisymmetric models, the intercell interface area arrays of MODFLOW-USG were calculated to accurately represent coaxial cylindrical model cells. Three examples are presented to demonstrate the application of MODFLOW-USG for axisymmetric modeling: a pumping well with delayed yield effects, a vadose zone flow model simulating an infiltration basin, and a density-dependent saltwater intrusion problem for a circular island. Results were verified against analytical solutions and published numerical codes.

Saltwater scavenging optimization under surrogate uncertainty for a multi-aquifer system

Surrogate-assisted simulation-optimization technique is an effective approach for identifying aquifer remediation strategies. Using surrogate models to replace high-fidelity groundwater simulation models is key to reduce computing time. Inevitably, uncertainties persist in surrogate model structure and parameters and may lead to poor accuracy in prediction. However, past studies often adopt one surrogate to capture the original model response and rarely investigate uncertainties and approximation accuracy of surrogate models. In this study, we present an ensemble surrogate modeling approach to address uncertainty in surrogate model predictions for saltwater intrusion mitigation designs using a hypothetical horizontal scavenger well. Firstly, polynomial response surface surrogate models are constructed to capture the relationship between chloride concentrations and saltwater extraction rates of a horizontal well. Secondly, a stochastic optimization model that includes multiple ensemble surrogates is developed to determine the optimal saltwater extraction schedule. Chance-constrained (CC) programming is used to account for model selection uncertainty in probabilistic nonlinear concentration constraints. Akaike information criterion correction (AICc) based Bayesian model averaging (BMA) method is adopted to determine required statistics of concentrations in the chance constraints. Finally, a set of saltwater intrusion mitigation strategies are obtained through an interior point optimization algorithm. The proposed methodology is applied to the saltwater intrusion problem in Baton Rouge, Louisiana. A high-fidelity flow and solute transport model is developed, but requires 16 h for one model run. The study shows that the developed surrogate models perform well and have strong predictive capabilities with respect to the solute transport model response, while considerably reducing computation time. Including model selection uncertainty through multimodel inference and model averaging provides more reliable remediation strategies compared with the single-surrogate assisted approach.

MULTIPLE OBJECTIVE MANAGEMENT STRATEGIES FOR COASTAL AQUIFERS UTILIZING NEW SURROGATE MODELS

Coastal aquifers are hydraulically connected to the sea and therefore susceptible to saltwater intrusion problems. This study proposes the utilization of a new surrogate model within coupled simulation-optimization (S/O) model for the management of coastal aquifers subjected to density-dependent saltwater intrusion processes. The simulation of the transient 3-dimensional density-dependent flow and transport model is based on the solution of an implemented numerical simulation model. Direct coupling of the numerical simulation model into the multi-objective genetic algorithm (MOGA) is computationally expensive. Hence, the solution of the numerical simulation model with random input variables are used to train and test the support vector machine regression (SVMR) surrogate models for approximately simulating the flow and transport processes. The performances of the new surrogate models are evaluated using various performance evaluation criteria. The resulting validated SVMR surrogate models are coupled to the MOGA and implemented for an illustrative coastal aquifer with an aim to develop efficient coastal aquifer management strategies. Based on the objective functions, execution of S/O model presented a set of optimal groundwater withdrawal rates from the simulated aquifer. It also ensured salinity levels at the designated monitoring wells are constrained within specified limits. The efficiency of the new SVMR surrogate models is also demonstrated. Evaluation results suggested that the projected S/O model is an effective way of developing feasible and reliable coastal aquifer management strategies. It also enhances the possibility of solving more realistic large-scale problems and developing regional-scale coastal aquifer management methodologies.

Determination of Saltwater Intrusion Zone Based on Groundwater Physical Properties on Eretan Coastal Area Indramayu Jawa Barat

The research area is a coastal area that has great groundwater potential. However, if the management is not appropriate then it can cause saltwater intrusion problem. This condition can be indicated by the value of EC, TDS, pH and temperature. The greater EC and TDS value indicated the higher saltwater intrusion. From 40 measurement points conducted in 18 wells obtained the lowest value of the TDS 520 mg/L at SG10 at 1.21 masl and the highest value 3580 mg/L at SG12 at 0.5 masl. As for the lowest value of EC 1050 μS/cm at SG10 at 1,21 masl and highest value 7050 μS/cm at SG12 at 0,5 masl. Based on the research there is a points of anomaly that have higher EC and TDS than the surrounding area. This anomaly shows that at these points there has been overpumping and classified into the highest saltwater intrusion zone that requiring monitoring.

Parallel characteristic mixed element method for saltwater intrusion problem

Numerical simulation of saltwater intrusion problem has attracted more and more attention in the past decades. Based on parallel subspace correction method proposed by Xu (1992), a new overlapping domain decomposition parallel algorithm is constructed to solve saltwater intrusion problem. In this algorithm, the splitting positive definite mixed element method is used to discrete head water equation, while the characteristic Galerkin finite element method is used to approximate concentration equation. The relation between the convergence rate and discretization parameters, including the overlapping degree of the subspaces, is considered. Furthermore, the convergence of this algorithm is analyzed and the corresponding error estimate is given. The convergence result suggests that only two iterations are needed to reach the given accuracy at each time level.