Managing saltwater intrusion using conjugate sharp interface and density dependent models linked with pumping optimization

Coastal areas are densely populated due to socioeconomic benefits and in turn also have a greater demand for fresh water. This ever-increasing demand for fresh water can be met by coastal aquifers, which act as large reservoirs of freshwater. Excessive and unmanaged pumping from coastal aquifer allows the salt water to flow inward encroaching on the voids created by the pumping of freshwater. This phenomenon is called saltwater intrusion. To stop the saltwater intrusion, an optimal pumping strategy needs to be adopted. Simulation models are generally linked with an optimization algorithm to develop an optimal pumping strategy for management of saltwater intrusion. Sharp interface based simulation models are often used which are computationally inexpensive but lacks in prediction accuracy, as it does not incorporate the effects of dispersion and diffusion. Density dependent simulation models include the effect of dispersion and diffusion, but have a very high computational budget in evaluating an optimal pumping strategy. To overcome above-mentioned limitation a new methodology is developed, where a density dependent model is used in conjunction with a sharp interface model to derive an optimal density ratio, such that interface obtained using this density ratio implicitly accommodated the effect of dispersion and diffusion in a sharp interface model. The performance of the developed methodology is evaluated for three hypothetical scenarios of saltwater intrusion. The performance evaluation results show the applicability of the methodology for management of saltwater intrusion while maximizing fresh water pumping in coastal aquifers.KeywordsGroundwater modellingCoastal aquifer managementPumping optimizationSaltwater intrusionDensity dependent modelSharp interface model

The main management challenge in coastal aquifers is to prevent saltwater intrusion, ensuring ample freshwater supply. Saltwater intrusion happens due to unregulated pumping from production wells. Therefore, it is essential to have an effective management policy, which ensures the requisite amount of freshwater to be withdrawn from coastal aquifers without causing saltwater intrusion. A methodology for optimizing production well locations and maximizing pumping from production wells is presented to achieve these conflicting objectives. The location of production wells directly affects the amount of freshwater pumped out of the coastal aquifer. Simultaneous optimization of production well locations and pumping from the same is achieved by linking mathematical simulation models with the optimization algorithm. A new methodology using coupled sharp-interface and density-dependent simulation models is developed to find optimal well locations and optimize the amount of freshwater pumped from the coastal aquifer. The performance of the developed methodology is evaluated for saltwater intrusion in the coastal city of Puri, India. The performance evaluation results show the developed methodology's applicability for managing saltwater intrusion while maximizing freshwater pumping in coastal aquifers under constraints of well location.

In pumping optimization of coastal aquifers, the evaluation of the objective function and constraints using density-dependent models is overwhelmed by complex and time-consuming numerical simulations. To address those cases where the available density-dependent model runs are very limited, due to excessive computational burden, an efficient optimization strategy is developed. The proposed methodology uses an efficient sharp interface model jointly with a complex density-dependent model in an evolutionary optimization algorithm. While most evaluations are based on the sharp interface model, the density-dependent model is selectively called to evaluate promising solutions and to improve the predictions of the sharp interface model through the adaptive modification of the saltwater-freshwater density ratio. The method is tested for pumping optimization problems in confined and unconfined coastal aquifers with multiple pumping wells. The optimal solutions are compared to those obtained by density-dependent as well as by sharp interface optimization alone. Under a very restrictive computational budget, the best feasible solution is attained in less than 25 density-dependent model runs for two optimization problems of 10 and 20 decision variables. The results indicate that this optimization method leads to good feasible solutions and that an improved estimation of optimal pumping rates can be achieved within a limited computational budget. The method could also stand as an efficient preliminary exploration of the optimal search space, to provide good feasible starting points for the implementation of more comprehensive methods of coastal aquifer management.

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.

Pumping well management in coastal aquifers required to account for the saltwater intrusion problem. The prevention saltwater contamination of pumping wells should be considered along with the objective of maximum groundwater withdrawal. Saltwater intrusion constraint can be based on (1) sharp interface model (2) density-dependent transport model. Sharp interface models are preferable in the case of limited computation cost available and density-dependent transport models are preferable for accuracy. The correction factor introduced to account for the density-dependent dispersion by Pool and Carrera (Water Resour Res 47(5):W05506, 2011) vastly improves the sharp interface solution. In this present study, the application of the modified sharp interface solution based on the density-dependent correction factor for the pumping optimization is demonstrated for a regional scale aquifer in Nellore, Andhra Pradesh, India. The proposed optimization model sought to maximize the total pumping and minimize the landward toe intrusion from the sea.

Management strategies for the optimal and sustainable use of groundwater resources are developed based on prescriptive models that use mathematical tools for simulation and optimization together with field data. Because of the uncertainty inherent in the groundwater systems, it is essential to verify the compliance of the implemented strategies to those prescribed by using proper monitoring techniques during and after the implementation stages of the groundwater management project. In this work, an adaptive management approach for optimal management and monitoring of coastal aquifers is proposed. A simulation-optimization approach is used to derive optimal pumping strategy for the management of saltwater intrusion in coastal aquifers. Then, an optimal monitoring network is designed to evaluate the compliance of the aquifer responses in the field with those predicted by the simulation-optimization model. The designed network can be used to monitor the compliance in the field in terms of the salinity concentration levels, which result from the implementation of the optimal pumping strategy. Uncertainty in the values of groundwater parameters and the uncertainty resulting from the deviation of the pumping strategies from the prescribed optimum values are characterized by considering different realizations of these values in the three-dimensional density dependent flow and transport simulation model. A new objective for monitoring is considered in this study. The objective function consists of maximizing the coefficient of variation of the salinity concentration at the monitored locations and minimizing the correlation coefficient between the concentrations at the monitored locations. Using this objective, the monitoring locations are chosen in regions where the uncertainty in the concentration values is highest, and those locations where the correlation between the concentrations of the monitored locations is lowest, so that the redundancy in monitoring data is the least. The concentration data collected at the optimal compliance monitoring locations can be used as feedback information to improve the initially developed optimal coastal aquifer management strategies. The sequential modification of the optimal pumping strategies in stages is illustrated using numerical experiments.

<p>In coastal areas, seawater intrusion is a main driver of groundwater salinization and numerical models are widely used to support sustainable groundwater management. Sharp interface models, in which mixing between freshwater and seawater is not explicitly simulated, have fast run times which enable the implementation of parameter estimation and uncertainty analysis. These are essential steps for decision-support modeling, however their implementation in sharp interface models has remained limited. Few guidelines exist regarding which observations to use, and what processing and weighting strategies to employ. We developed a data assimilation framework for a regional, sharp interface model designed for management purposes. We built a sharp interface model for an island aquifer using the SWI2 package for MODFLOW. We then extracted freshwater head observations from shallow wells, pumping wells and deep open wells, and observations of the seawater-freshwater interface from deep open wells, time-domain electromagnetic (TDEM) and electrical resistivity tomography (ERT) surveys. After quantification of measurement uncertainties, parameter estimation was conducted with PEST and a data worth analysis was carried out using a linear approach. Model residuals provided insight on the potential of different observation groups to constrain parameter estimation. The data worth analysis provided insight on these groups’ importance in reducing the uncertainty of model forecasts. Overall a satisfying fit was obtained between simulated and observed data, but observations from deep open wells were biased. While observations from deep open wells and geophysical surveys had a low signal-to-noise ratio, parameter estimation effectively reduced predictive uncertainty. Interface observations, especially from geophysical surveys, were essential to reduce the uncertainty of model forecasts. The use of different types of observations is discussed and recommendations are provided for future data collection strategies in coastal aquifers. This framework was developed in the Magdalen Islands (Quebec, Canada) and could be carried out more systematically for sharp interface seawater intrusion modeling.</p>

Optimal design of pumping networks in coastal aquifers using sharp interface models

Abstract. We investigate seawater intrusion in three prominent Mediterranean aquifers that are subject to intensive exploitation and modified hydrologic regimes by human activities: the Nile Delta, Israel Coastal and Cyprus Akrotiri aquifers. Using a generalized analytical sharp interface model, we review the salinization history and current status of these aquifers, and quantify their resilience/vulnerability to current and future seawater intrusion forcings. We identify two different critical limits of seawater intrusion under groundwater exploitation and/or climatic stress: a limit of well intrusion, at which intruded seawater reaches key locations of groundwater pumping, and a tipping point of complete seawater intrusion up to the prevailing groundwater divide of a coastal aquifer. Either limit can be reached, and ultimately crossed, under intensive aquifer exploitation and/or climate-driven change. We show that seawater intrusion vulnerability for different aquifer cases can be directly compared in terms of normalized intrusion performance curves. The site-specific assessments show that (a) the intruding seawater currently seriously threatens the Nile Delta aquifer, (b) in the Israel Coastal aquifer the sharp interface toe approaches the well location and (c) the Cyprus Akrotiri aquifer is currently somewhat less threatened by increased seawater intrusion.

Vertical leakage in sharp-interface seawater intrusion models of layered coastal aquifers

The demand for freshwater is very high in the coastal regions due to the high population density in coastal areas. To meet this demand for freshwater, the coastal aquifers are often heavily pumped without any regulation, resulting in saltwater intrusion. Therefore, the biggest challenge in the management of coastal aquifer is to meet the demand for freshwater by pumping the coastal aquifer without causing saltwater intrusion. In this study, a brief overview of various methods for identification, prediction, and management of saltwater intrusion is presented. Detection of saltwater intrusion is largely hindered due to insufficient spatiotemporal monitoring because of budgetary constraints. Application, merits, and demerits of the newer cost-effective techniques as well as conventional techniques for identifying saltwater intrusion are discussed in this chapter. The application of various prediction models and their computational difficulties is also presented in this study. Finally, advanced techniques for identification and sustainable management practice in saltwater intrusion are discussed. Though significant progress has been made in the recent past in the management of coastal aquifers, they still show gaps in addressing real-life scenarios. An attempt has been made to highlight the suitability of a developed methodology and their respective limitations.

Management of saltwater intrusion in coastal aquifers should be based on robust management strategies and monitoring of their impacts. A robust optimal management strategy is less sensitive to deviations from prescribed strategies at the field level. Development of robust management framework is an important issue that needs attention especially when it results in near optimal strategies even when deviations from prescribed strategies occur in the field implementation stage. Implementation of a strategy requires field scale monitoring to determine the impact in terms of compliance with management goals due to possible deviation from an optimal prescribed strategy. Design of such an optimal monitoring network for compliance also requires robust optimal design due to the uncertainties involved. Deviations from prescribed strategies in the field are often more sensitive to uncertainties in the implementation phase. A multiple objective management model for robust optimal management of saltwater intrusion in coastal aquifers is proposed. Both risk neutral and risk-based management model formulations are presented. A robust monitoring network design methodology is also proposed for compliance monitoring of proposed robust management strategies. Performances of the developed methodology are tested for an illustrative coastal aquifer study area, as presented by Dhar and Datta. Performance evaluations show potential applicability of the developed methodologies and some of the relative advantages.

A two-phase CFD model for autogenous pressurization and draining of a cryogenic storage tank is presented using both the Sharp Interface and Volume-Of-Fluid (VOF) approaches for capturing the front and the associated interfacial heat, mass and momentum transfer between the liquid and vapor regions. Both models are validated against data provided by the Cryogenic Propellant Storage and Transfer (CPST) Engineering Development Unit (EDU) experiment1. The results of the autogenous pressurization are presented first, focusing on the phase change and turbulence effects on the tank pressure and temperature predictions. Both the Sharp Interface (SI-CFD) and VOF (VOF-CFD) multiphase models predict tank pressure during pressurization within 3% of the measured values. The sensitivity of key physical and numerical parameters of the problem are tested using the Sharp Interface model. Effects of the accommodation coefficient (AC), the computational grid structure, and turbulence are studied. The second part of this paper is devoted to validating the SI-VOF model, with an enhanced capability of moving the liquid-vapor interface, against the draining segment of the EDU experiment. The VOF-CFD model was also used to simulate tank draining and its results were compared with the results of the Sharp Interface model with the moving interface. Both models predict tank pressure during draining within 3.5% of the measured values. Pressure decrease rate is underpredicted by both models during the first 100 seconds of draining but matches the experimental rate for the rest of the simulation.

The present study implements a stochastic optimization technique to optimally manage freshwater pumping from coastal aquifers. Our simulations utilize the well-known sharp interface model for saltwater intrusion in coastal aquifers together with its known analytical solution. The objective is to maximize the total volume of freshwater pumped by the wells from the aquifer while, at the same time, protecting the aquifer from saltwater intrusion. In the direction of dealing with this problem in real time, the ALOPEX stochastic optimization method is used, to optimize the pumping rates of the wells, coupled with a penalty-based strategy that keeps the saltwater front at a safe distance from the wells. Several numerical optimization results, that simulate a known real aquifer case, are presented. The results explore the computational performance of the chosen stochastic optimization method as well as its abilities to manage freshwater pumping in real aquifer environments.

The demand for fresh water is accelerating as the world population is increasing in alarming rate. In order to cope with the increased demands, the overexploitation of groundwater resources has become unavoidable in many parts of the world. It has been reported that at least 70 % of the world population is living in coastal areas. The main sources of fresh water for these people are the freshwater aquifer near the coastal region. The unplanned exploitation of freshwater from coastal aquifers, hydraulically connected with sea or ocean may cause saltwater intrusion into coastal aquifers. The saltwater intrusion in coastal aquifers contaminates the aquifers and makes the coastal aquifers unusable for further human utilization. The contamination of coastal aquifers may also cause serious consequences on environment, ecology, and the economy of the region. The remediation of contaminated aquifers is generally very expensive and time-consuming. In order to protect the vital resource, it is necessary to protect coastal aquifers from further contamination by saltwater intrusion. Saltwater intrusion can be controlled by suitable management policies. The main objective of a coastal aquifer management model is to evolve planned operational strategies to meet required demand of fresh water while maintaining the salinity of water within permissible limit. In order to evolve a physically meaningful strategy, the flow and transport processes need to be simulated within the optimization-based management model. Different methodologies like embedded optimization method, response matrix approach, linked simulation optimization method, etc., have been developed to incorporate the aquifer simulation models with the management model. These methods have their own advantages and disadvantages and none of the methods can be declared as the best method for solving coastal aquifer management models. As such, a suitable method should be selected based on the data availability and other related local advantages. The study presents a review on development of coastal aquifer management models.