Using Compressed Air Injection to Control Seawater Intrusion in a Confined Coastal Aquifer

Seawater intrusion into groundwater is an important problem in many coastal regions. Freshwater injection has been widely used to avoid seawater contaminating freshwater systems, but is not an attractive solution where freshwater is limited. We investigated the effects of injecting compressed air on seawater movement in a confined coastal aquifer using a numerical simulation. We used TOUGH2/EOS7 software to analyze the effects of injecting compressed air in preventing seawater intruding into a hypothetical confined coastal aquifer (Henry’s problem), simulating steady-state initial conditions and comparing the results with the literature sources. We then performed a transient-state numerical simulation to quantify the seawater intrusion control efficiency achieved by injecting air. The results showed that injecting compressed air can mitigate seawater intrusion: Saltwater was ejected from the aquifer and the seawater circulation disappeared. The injected air flowed upward and spread laterally near the top of the aquifer because of the groundwater and air densities. Injecting air significantly increased the groundwater and gas pressures near the air injection zone and at the top of the aquifer. The air injection rate increased rapidly, then increased gradually. Freshwater injection was also simulated using settings similar to those used for air injection, and this showed that seawater intrusion is prevented more efficiently by freshwater injection than air injection. However, freshwater resources are valuable, whereas air is readily available, so injecting air to mitigate seawater intrusion has great potential. The modeling approach that we used will be used as a foundation for future work.     

Modeling and Analysis of Seawater Intrusion in the Coastal Aquifer of Eastern Cap-Bon, Tunisia

A numerical model that treats density-dependent variably saturated flow and miscible salt transport is used to investigate the occurrence of seawater intrusion in the Korba aquifer of the eastern coast of Cap-Bon in northern Tunisia. We examine the interplay between pumping regimes and recharge scenarios and its effect on the saline water distribution. More localized simulations are used to examine, in vertical cross sections, the effects of well location and soil type and the role of the vadose zone in possible remediation actions. The exploratory simulations suggest interesting interactions between the unsaturated zone and the saltwater–freshwater interface with possible implications for groundwater exploitation from shallow unconfined coastal aquifers, involving in one case feedback between seawater intrusion and the high pressure head gradients around the pumping-induced drawdown cone and in another case threshold-like interface displacement for tight soils such as clays. The data processing steps undertaken in this GIS and modeling study are described in some detail, and a critical assessment is given of the data availability and of the requirements for successful monitoring and modeling of seawater intrusion risks in heavily exploited coastal aquifers such as those found in the semi-arid regions of the Mediterranean basin. It is shown how, with the aid of GIS, reasonably reliable information can be assembled from maps, surveys, and other sources of geospatial and hydrogeological data, an approach that is necessary in the many regions of the world with acute water resource problems but with limited means for undertaking systematic data acquisition and environmental monitoring actions. Nonetheless the need for more concerted monitoring of relevant parameters and processes and of closer coordination between monitoring and modeling is stressed. An idea of the extent of over-exploitation of the Korba aquifer is obtained by examining the pumping and rainfall/infiltration data, and the simulation results support groundwater pumping as the mechanism for and seawater intrusion as the origin of the salt contamination observed in the soils and subsurface waters of the Korba plain.     

Integrating Hydrogeochemical and Geophysical Data for Testing a Finite Volume Based Numerical Model for Saltwater Intrusion

In this paper, hydrogeological and geophysical data are used to validate a numerical model developed to predict seawater intrusion into coastal aquifers. The cell-centered finite volume method is adopted here to solve the set of coupled partial differential equations describing the motion of saltwater and freshwater separated by a sharp interface. These equations are based on the Dupuit approximation and are obtained from integration of 3D flow equations for fresh and salt water zones over the vertical dimension. In order to have flexibility upon complex configurations domain, non structured grid meshing is utilized. To approximate the diffusion fluxes, Green-Gauss type reconstruction, based on diamond-cell and least squares interpolation, is performed. The model is first validated using academic test case studies with known closed form solutions. The mathematical model has been calibrated using hydrogeochemical and geophysical data. The geophysical method applied in this study has been a frequency domain electromagnetic method. In this method the apparent electrical conductivity is measured by induction using two separate hand-held transmitter and receiver coils. During the operation the transmitter coil is energized by a low frequency alternating current that radiates an electromagnetic field and the receiver coil detects the resulting field. Taking into account the relationship between the bulk conductivity of the subsoil and the conductivity of groundwater, EM soundings have been interpreted to provide complementary information to hydrogeochemical data to outline the fresh–saltwater interface. This methodology has been applied to the case of saltwater intrusion into the Llobregat delta aquifer, near Barcelona, Spain.     

The analysis of groundwater flows in unconfined aquifers with nonuniform hydraulic conductivity

Horizontal groundwater flows in unconfined aquifers with horizontal lower boundaries can be found exactly by the seepage analysis that allows the hydraulic conductivity to vary in the vertical direction. The exact analysis of flows when the lower boundary of the aquifer is not a horizontal plane, requires the soil-water pressure on this boundary to be known, and this is not generally the case except in the situation of a freshwater aquifer overlying saline water fed from the sea. For aquifers with spatial variations of hydraulic conductivity in both the vertical and horizontal directions, the seepage analysis can be modified to give groundwater flows in situations where the hydraulic conductivity can be represented by the product of independent functions of the three spatial coordinates. Different forms of three-dimensional variation are generated from suitably chosen functions. The use of such forms in calculations of equivalent uniform hydraulic conductivities of some groundwater flow regions demonstrates the dependence of equivalent hydraulic conductivity values on the flow boundary conditions. The exact groundwater flows calculated for particular groundwater situations by the seepage analysis provide results that are useful in validating numerical methods for solving groundwater problems in heterogeneous soils.     

A Two-Dimensional Areal Model for Density Dependent Flow Regime

A two-dimensional (2D) plane model of saltwater intrusion was developed, for the simulation of groundwater level and the average solute concentration in a 2D horizontal plane, together with the estimation of the saltwater depth. The proposed approach is of particular interest when assessing the effect of different regional pumping scenarios on groundwater level and its quality. The corresponding MEL2DSLT code was developed on the basis of the Modified Eulerian–Lagrangian (MEL) method to overcome difficulties arising from hyperbolic behavior of flow and transport equations, due to the advective nature of solute transport and heterogeneity of the soil characteristics (permeabilities and dispersivities). The code was verified against the 2D cross sectional model SUTRA and the three-dimensional (3D) model SWICHA. Simulation was conducted concerning the problem of saltwater intrusion in the Khan Yunis portion of the phreatic coastal aquifer of Gaza Strip. After calibrating the model for the aquifer parameters, we investigated its predictions resulting from various regional pumping scenarios using the actual pumping intensity from the year 1985 and extrapolating on the basis of 3.8% annual population growth. Results show a considerable depletion of groundwater level and intrusion of seawater due to excessive pumping.     

The Saltwater/Freshwater Regime in the Sedimentary Cover of the Gorleben Salt Dome

Hydrogeological investigations were conducted around the Gorleben salt dome, in the northeastern part of Lower Saxony in Germany, to study the aquifer system in the cover deposits consisting of unconsolidated Quaternary and Tertiary sediments up to 250m thick. An upper freshwater body is underlain here by saline water. The salinity of the water generally increases with depth. Saturated brines are found in the lower aquifer in a subglacial erosion channel above the salt dome. Hydrochemical analyses and geophysical and isotope studies provide information on development history and behaviour of the flow system. Highly saline groundwater formed by subrosion of salt collects in the lower aquifer above the salt dome. It generally flows north through the erosion channel in the direction of the regional flow field and collects in a basin-like aquifer structure north of the dome. The post-Pleistocene groundwater circulation is progressively replacing the Pleistocene brines with Holocene groundwater. Only a small amount of salt water is being transported into the near-surface aquifer.     

Numerical simulation of ground water flow via a new approach to the local radial point interpolation meshless method

A novel approach to local radial point interpolation meshless (LRPIM) method is introduced to investigate the influence of leakage on tidal response in a coastal leaky confined aquifer system, based on a local weighted residual method with the Heaviside step function as the weighting function over a local sub-domain. The present approach is a truly meshless method based only on a number of randomly located nodes. In this approach, neither global background integration mesh nor domain integration is needed. Radial basis functions (RBFs) interpolation is employed in shape function and its derivatives construction for evaluating the local weak form integrals. Due to satisfaction of kronecker delta property in RBF interpolation, no special treatment is needed to impose the essential boundary conditions. In order to obtain the optimum parameters, shape parameters of multiquadrics (MQ)-RBF are tuned and studied. The leakage has a significant impact on the tidal behaviour of the confined aquifer. The numerical results of this research indicate that both tidal amplitude of groundwater head in the aquifer and the distance over which the aquifer can be disturbed by the tide are considerably reduced by leakage. The novelty of the approach is the use of a local Heaviside weight function in the LRPIM which does not need local domain integration and only integrations on the boundary of the local domains are needed. Therefore, in this research a new local Heaviside weight function has been proposed. Numerical results are presented and compared with the results of analytical solution. It is observed that the obtained results agreed very well with the results of analytical solution. The numerical results show that the use of a local Heaviside weight function in the LRPIM is highly accurate, fast and robust. It is also noticed that this novel meshless approach using MQ radial basis is very stable.     

Investigation of Groundwater Contaminant Discharge into Tidally Influenced Surface-Water Bodies: Experimental Results

An investigation of the effect of tidally influenced water elevations on the concentration of groundwater contaminants discharging to a surface-water body is studied using a one-dimensional homogeneous sand column. A constant water level is imposed upstream, and the downstream water level is controlled by a wave generator that controls the hydraulic head to mimic a 12-h tidal fluctuation. The experimental results demonstrate that the tidal fluctuations in the downstream reservoir result in a decrease in average contaminant concentration at the point of discharge to the tidally influenced surface-water body. The further upstream an observation well is located, the smaller the amplitude of the concentration oscillation. Fourier analysis suggests that the dominant frequency of the pressure at different locations along the length of the column is identically two cycles per day and that the concentration data have a dominant frequency of two cycles per day, but also exhibit harmonics.     

Abrupt-Interface Solution for Carbon Dioxide Injection into Porous Media

We derive an approximate analytical solution, which describes the interface dynamics during the injection of supercritical carbon dioxide into homogeneous geologic media that are fully saturated with a host fluid. The host fluid can be either heavier (e.g., brine) or lighter (e.g., methane) than the injected carbon dioxide. Our solution relies on the Dupuit approximation and explicitly accounts for the buoyancy effects. The general approach is applicable to a variety of phenomena involving variable-density flows in porous media. In three dimensions under radial symmetry, the solution describes carbon dioxide injection; its two-dimensional counterpart can be used to model seawater intrusion into coastal aquifers. We conclude by comparing our solutions with existing analytical alternatives.     

Some nonlinear problems of groundwater flow through a porous medium

Self-similar solutions of the equation of groundwater gravity flow through a porous medium are given. The water flows from a basin into an underground permeable reservoir bounded from below by an impervious bed. A periodic problem of flow through a porous medium in which the water level in the basin periodically oscillates is considered. It is shown that in this case the water table elevation effect, i.e., water pumping into the aquifer, develops at a sufficient distance from the littoral zone. The invariant-group solutions of the problem of flow through a porous medium are investigated with allowance for evaporation from the groundwater surface.     

Three-dimensional hydrodynamic modelling of tidal flows interacting with aquaculture fish cages

Three-dimensional, non-linear, non-hydrostatic simulations of rotating tidal flows interacting with aquaculture cages (represented as drag elements) at the geophysical scale are performed using an adaptive, finite volume fluid code “Gerris”. Exploiting the Gerris grid structure, sub-metre scale resolution can be obtained even for the farm scale experiments, enabling examination of the impact of the cage on the imposed tidal flows. Passive tracers are used to try to quantify these cage impacts, representing either feed or faecal matter (with specified fall speeds), or other biogeochemical markers such as dissolved oxygen. Using a relatively simple drag formulation, we show that the model is able to reproduce laboratory observations. The farm scale simulations can also be “tuned” in a similar fashion, for example by comparison with observations of total drag force on such structures, or with field measurements of flow retardation by cages. Single and multi-scale cage experiments are then examined to explore the potential impacts of perturbed horizontal and vertical flows on material redistribution through and within the cages. Even with the relatively smooth forcing and drag formulation the experiments reveal a surprising level of complexity in terms of the perturbed flows and their impact on transporting and diffusing passive material.     

Mitigation of Seawater Intrusion by Pumping Brackish Water

This paper presents a technique to restore the balance between freshwater and saline water in coastal aquifers in order to mitigate seawater intrusion problems. Brackish water can be pumped from the dispersion zone and then used to develop green lands in the coastal areas or to irrigate certain types of crops. A two-dimensional finite element model (2D-FED), has been employed to verify this technique. The model is based on the dispersion zone approach with a variable density flow. Simulations were preformed in the vertical view and equiconcentration and equipotential lines were plotted for different locations of brackish water pumping. In all of the tested runs the width of dispersion zone has reduced significantly due to brackish water pumping. The quality of the pumped water differs according to the location of pumping. A study case on the Madras aquifer in India is presented.     

Wall pressure and its relation to turbulent structure of a boundary layer

Wall pressure fluctuations were measured in relation to the structure of coherent motions of a turbulent boundary layer. Their high and low frequency parts were found to be related with the large scale motions of the outer layer and the bursts in the wall region respectively. Based on the experimental results, a structure model of the coherent motions is presented.This paper was presented at the Ninth symposium on turbulence, University of Missouri-Rolla, October 1–3, 1984     

An unstructured-grid numerical model for interfacial multiphase fluids based on multi-moment finite volume formulation and THINC method

We present a practical numerical framework for incompressible interfacial multiphase flows on unstructured grids with arbitrary and hybrid elements. The numerical framework is constructed by combining VPM (volume-average/point-value multi-moment) and UMTHINC (unstructured multi-dimensional tangent of hyperbola interface capturing) schemes. To facilitate accurate and reliable simulations for interfacial multiphase flows on arbitrary and hybrid unstructured grids, we have made the following major new efforts in this work. (1) UMTHINC scheme on prismatic and pyramidal elements to facilitate computations on hybrid arbitrary unstructured grids; (2) Consistent numerical formulation for mass and momentum transports to simulate multiphase flows of large density ratio; (3) Combined FVM-FEM for accurate solution to diffusion equation; (4) Pressure-projection formulation in consistent with the balanced-force model. Integrating all these numerical techniques effectively enhances the accuracy and robustness in interface capturing and numerical solution of multiphase fluid dynamics, which results in a numerical framework of great significance for practical applications. Numerical verifications have been carried out through benchmark tests ranging from surface tension dominant flows of small scale to large scale flows with violently-changing interfaces. Numerical results demonstrate that the present framework is robust with adequate accuracy for simulating multiphase flows in complex geometries.     

The Interface Between Fresh and Salt Groundwater in Horizontal Aquifers: The Dupuit–Forchheimer Approximation Revisited

We analyze the motion of a sharp interface between fresh and salt groundwater in horizontal, confined aquifers of infinite extend. The analysis is based on earlier results of De Josselin de Jong (Proc Euromech 143:75–82, 1981). Parameterizing the height of the interface along the horizontal base of the aquifer and assuming the validity of the Dupuit–Forchheimer approximation in both the fresh and saltwater, he derived an approximate interface motion equation. This equation is a nonlinear doubly degenerate diffusion equation in terms of the height of the interface. In that paper, he also developed a stream function-based formulation for the dynamics of a two-fluid interface. By replacing the two fluids by one hypothetical fluid, with a distribution of vortices along the interface, the exact discharge field throughout the flow domain can be determined. Starting point for our analysis is the stream function formulation. We derive an exact integro-differential equation for the movement of the interface. We show that the pointwise differential terms are identical to the approximate Dupuit–Forchheimer interface motion equation as derived by De Josselin de Jong. We analyze (mathematical) properties of the additional integral term in the exact interface motion formulation to validate the approximate Dupuit–Forchheimer interface motion equation. We also consider the case of flat interfaces, and we study the behavior of the toe of the interface. In particular, we give a criterion for finite or infinite speed of propagation.     

Effects of velocity fluctuations on heat transfer enhancement

The three-dimensional velocity fluctuation effects on heat transfer enhancement were experimentally investigated using a wind tunnel system and cylinders placed upstream of the test section in the wind tunnel. The cylinders with different diameters were used as turbulators to generate vortical flow motions with three-dimensional velocity fluctuations. A heated plate, part of the tunnel wall, was placed far downstream of the cylinders such that it was subjected mainly to flows with velocity fluctuations but with negligible steady vortical motions. These studies included three-component velocity measurements to characterize the near-wall and cross-section velocity fields and to obtain the turbulent kinetic energy. The temperatures were measured by thermocouples on the heated plate to obtain the convection heat transfer coefficients and the Nusselt numbers. Results indicate that the heat transfer was enhanced by the velocity fluctuations, which is attributed to the modification of boundary layer velocity profiles without the modification of boundary layer thickness. The resulting normalized Nusselt number was approximately a parabolic function of a dimensionless parameter, the product of Reynolds number and normalized turbulent kinetic energy.     

Influence of porosity of the base on a plane standing wave of a heavy liquid

A study is made of the simple problem of the contact of two plane-parallel potential flows of incompressible fluid when one takes place in a layer of finite thickness and the other in a semiinfinite space of a porous medium. At the interface, which is taken to be a plane, the same conditions are used as earlier in problems of the contact of two wave flows of fluids with different densities and the contact of a wave motion in a layer of compressible fluid and wave motions in an elastic semi-infinite space. These conditions reduce to equalities of the pressures and projections of the velocity vectors onto the normal to the interface.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 160–163, July–August, 1984.     

Effects of Clay Dispersion on Aquifer Storage and Recovery in Coastal Aquifers

Cyclic injection, storage, and withdrawal of freshwater in brackish aquifers is a form of aquifer storage and recovery (ASR) that can beneficially supplement water supplies in coastal areas. A 1970s field experiment in Norfolk, Virginia, showed that clay dispersion in the unconsolidated sedimentary aquifer occurred because of cation exchange on clay minerals as freshwater displaced brackish formation water. Migration of interstitial clay particles clogged pores, reduced permeability, and decreased recovery efficiency, but a calcium preflush was found to reduce clay dispersion and lead to a higher recovery efficiency. Column experiments were performed in this study to quantify the relations between permeability changes and clay mineralogy, clay content, and initial water salinity. The results of these experiments indicate that dispersion of montmorillonite clay is a primary contributor to formation damage. The reduction in permeability by clay dispersion may be expressed as a linear function of chloride content. Incorporating these simple functions into a radial, cross-sectional, variable-density, ground-water flow and transport model yielded a satisfactory simulation of the Norfolk field test – and represented an improvement over the model that ignored changes in permeability. This type of model offers a useful planning and design tool for ASR operations in coastal clastic aquifer systems.     

Parallel finite element methods for large-scale computation of storm surges and tidal flows

Massively parallel finite element methods for large-scale computation of storm surges and tidal flows are discussed here. The finite element computations, carried out using unstructured grids, are based on a three-step explicit formulation and on an implicit space–time formulation. Parallel implementations of these unstructured grid-based formulations are carried out on the Fujitsu Highly Parallel Computer AP1000 and on the Thinking Machines CM-5. Simulations of the storm surge accompanying the Ise-Bay typhoon in 1959 and of the tidal flow in Tokyo Bay serve as numerical examples. The impact of parallelization on this type of simulation is also investigated. The present methods are shown to be useful and powerful tools for the analysis of storm surges and tidal flows. © 1997 John Wiley & Sons, Ltd.