Stochastic and Robust Multi-Objective Optimal Management of Pumping from Coastal Aquifers Under Parameter Uncertainty

Combined simulation-optimization approaches have been used as tools to derive optimal groundwater management strategies to maintain or improve water quality in contaminated or other aquifers. Surrogate models based on neural networks, regression models, support vector machies etc., are used as substitutes for the numerical simulation model in order to reduce the computational burden on the simulation-optimization approach. However, the groundwater flow and transport system itself being characterized by uncertain parameters, using a deterministic surrogate model to substitute it is a gross and unrealistic approximation of the system. Till date, few studies have considered stochastic surrogate modeling to develop groundwater management methodologies. In this study, we utilize genetic programming (GP) based ensemble surrogate models to characterize coastal aquifer water quality responses to pumping, under parameter uncertainty. These surrogates are then coupled with multiple realization optimization for the stochastic and robust optimization of groundwater management in coastal aquifers. The key novelty in the proposed approach is the capability to capture the uncertainty in the physical system, to a certain extent, in the ensemble of surrogate models and using it to constrain the optimization search to derive robust optimal solutions. Uncertainties in hydraulic conductivity and the annual aquifer recharge are incorporated in this study. The results obtained indicate that the methodology is capable of developing reliable and robust strategies for groundwater management.     

Optimal Management of Coastal Aquifers Using Linked Simulation Optimization Approach

Saltwater intrusion management models can be used to derive optimal and efficient management strategies for controlling saltwater intrusion in coastal aquifers. To obtain physically meaningful optimal management strategies, the physical processes involved need to be simulated while deriving the management strategies. The flow and transport processes involved in coastal aquifers are difficult to simulate especially when the density-dependent flow and transport processes need to be modeled. Incorporation of this simulation model within an optimization-based management model is very complex and difficult. However, as an alternative, it is possible to link a simulation model externally with an optimization-based management model. The GA-based optimization approach is especially suitable for externally linking the numerical simulation model within the optimization model. Further efficiency in computational procedure can be achieved for such a linked model, if the simulation process can be simplified by approximation, as very large number of iterations between the optimization and simulation model is generally necessary to evolve an optimal management strategy. A possible approach for approximating the simulation model is to use a trained Artificial Neural Network (ANN) as the approximate simulator. Therefore, an ANN model is trained as an approximator of the three dimensional density-dependent flow and transport processes in a coastal aquifer. A linked simulation – optimization model is then developed to link the trained ANN with the GA-based optimization model for solving saltwater management problems. The performance of the developed optimization model is evaluated using an illustrative study area. The evaluation results show the potential applicability of the developed methodology using a GA- and ANN-based linked optimization – simulation model for optimal management of coastal aquifer.     

Inclined Physical Subsurface Barriers for Saltwater Intrusion Management in Coastal Aquifers

Saltwater intrusion (SWI) has a negative environmental impact on groundwater quality in coastal areas. Therefore, effective management strategies are required to preserve fresh groundwater resources. Historically, vertical barriers have been exclusively considered in both numerical studies and practical applications. The novelty of this study consists in investigating the SWI mitigation effectiveness of inclined physical subsurface barriers (PSBs), and specifically cutoff walls (CWs) and subsurface dams (SDs). An initial benchmark analysis of the Henry problem was performed. Following verification, the proposed model was applied to a real case study - the Biscayne aquifer (Southeastern Florida, USA). The model simulations run for different scenarios considering the vertical placement of the PSB, an inclined placement of the PSB according to different slopes (1/4, 1/2 and 1/1, at sea- and landside) and the combination of the best scenario. The results showed that CWs are more effective in limiting SWI in comparison with SDs. The most positive impact in both cases was achieved for a slope of 1/4, indicating that a moderate vertical inclination of the PSB better preserve coastal groundwater resources. The model presented in this work can be a valuable tool for policy makers in predicting the coastal aquifer response. However, a comprehensive cost–benefit analysis is required to further account for the feasibility and the economic costs related to the construction of inclined PSBs.

     

An Artificial Intelligence Approach for the Stochastic Management of Coastal Aquifers

Aquifer recharge rates and patterns are often uncertain, especially in arid areas due to sporadic and erratic rainfall. Therefore, determining the optimal groundwater abstraction using classical approaches such as Monte Carlo Simulation (MCS) requires a large number of groundwater simulations and exorbitant computational efforts. The problem becomes even more complex and time consuming for regional coastal aquifers whose domains must be discretized using high-resolution meshes. In fact, even fast evolutionary multi-objective optimization techniques generally require a large number of simulations to determine the Pareto-front among the objectives. This study explores the performance of a Decision Tree (DT) approach for the generation of the Pareto optimal solutions of groundwater extraction. This paper applies the DTs for the optimal management of the Al-Khoud coastal aquifer in Oman. The learning process of the developed DT-based model uses the output of a numerical simulation model to assess the aquifer response based on different abstraction policies. The trained DT network then utilizes the NSGA-II to determine the Pareto-optimal solutions. The simulation show that the general flux pattern in the study area is toward the sea and the hydraulic head following a similar pattern in both best and worst recharging scenarios downstream of the studied recharging dam. Statistical tests showed a good correlation between the DT-based and simulation-based results and demonstrate the capability of the DT approach to obtain high-quality solutions by incorporating a large number of recharge scenarios. Moreover, the required runtime of the DT-based approach is extremely low (5 min) compared to that of the simulation-based method (several days). This means that including additional Monte-Carlo simulations can be readily done in few minutes using the obtained DTs, instead of the long computational time needed by the simulation-based approach.     

潮汐引起滨海承压含水层地下水位变化的数值模拟

海潮波动可以引起海岸带地下水位发生波动。建立基于有限差分法的滨海地区一维承压含水层地下水运动数值模型。通过将潮汐波动概化为正弦波,模拟滨海地区地下水位随潮汐波动的变化。结果表明,受海潮影响的滨海含水层地下水位与海潮有相似的波动特征,但变幅减小,受海潮的影响程度与离海岸的距离有关,随着离海岸距离的增加地下水位的变幅及潮汐效率呈负指函数衰减,地下水位对海潮的滞后时间随距离呈线性增加。例如,当承压含水层导水系数为31．25m^2／h、储水系数为4．5×10^-4及海潮振幅为2．5m、周期为24．7h时,在距离海岸500m处,振幅为1．35268m,潮汐效率为0．54107,滞后时间为1．825h。在距海岸达4389m以远时可以认为地下水位不受海潮波动的影响。     

3D Variable Density Flow Simulation to Evaluate Pumping Schemes in Coastal Aquifers

A three dimensional model is presented for the simulation of seawater intrusion in coastal aquifers by considering the development of a transition zone and thus the variable density flow approach. The model is applied to a heterogeneous coastal aquifer to study the effects of the pumping rate, the salinity of freshwater inflow and the thickness of the aquifer, on the degradation of pumped water quality through wells in certain location. Even for an optimum pumping scheme solution based on a simple two-dimensional flow model, we simulate freshwater degradation in pumped water which depends on the salinity of freshwater inflow and aquifer thickness.     

Elitist Continuous Ant Colony Optimization Algorithm for Optimal Management of Coastal Aquifers

This paper presents an evolutionary based approach to achieve optimal management of a coastal aquifer to control saltwater intrusion. An improved Elitist Continuous Ant Colony Optimization (ECACO) algorithm is employed for optimal control variables setting of coastal aquifer management problem. The objectives of the optimal management are; maximizing the total water-pumping rate, while controlling the drawdown limits and protecting the wells from saltwater intrusion. Since present work is one of the first efforts towards the application of an ECACO algorithm, sharp interface solution for steady state problem is first exploited. The performance of the developed optimization model is evaluated through application examples available in the literature. The comparisons indicate the applicability of the ECACO algorithm. In the second approach, the numerical simulation is combined with ECACO algorithm. In this model, through some simple schemes, such as continuity equations in the porous media cells and existing hydraulic systems in the study area, further details can be investigated. The evaluation results show the potential applicability of the proposed numerical based model for optimal management of coastal aquifers.     

Optimal Locations of Groundwater Extractions in Coastal Aquifers

A regional water supply management model for coastal aquifers was developed. One of its outcomes is the definition of the optimized locations for groundwater withdrawal. Such a tool permits the analysis of alternative plans for groundwater extraction and the sustainable use of water resources in a coastal aquifer subject to saltwater intrusion. The principal components are the evolutionary optimization and the analytical/numerical simulation models. The optimization technique looks for the best well locations taking into consideration the economic results and the satisfaction of the societal water demand. However these two concerns are conditioned by trying to control the saltwater intrusion, i.e., preserving the environmental equilibrium. The simulation model uses the governing mathematical equations for groundwater movement to find the interface between freshwater and saltwater. Because of the non-linearity in the system and the possibility of a jumping interface, a security distance was defined. This is a controlling variable which can be set by the decision makers. The model was applied to a typical case with interesting results. For example, diagrams showing the relationship between the location of the wells and the security distance(s) are of importance to the managers. It was also crucial to have an understanding of the tradeoffs between groundwater withdrawals, positions of the wells from the coast line, and the security distance. The model was also applied to a real case in order to relate the extractions, distances and artificial recharge (not presented in this paper).     

Management of Saltwater Intrusion in Data-scarce Coastal Aquifers: Impacts of Seasonality,Water Deficit,and Land Use

Coastal aquifers are vulnerable to saltwater intrusion (SWI) due to several drivers particularly increased water demand and groundwater overexploitation associated with population growth, reduced groundwater recharge, and lately climate change. This study examines the status of SWI in four data scarce coastal aquifers located along the Eastern Mediterranean by assessing how water cycle seasonality, water deficits, and changes in land use and land cover (LULC) have contributed to increased salinity. A framework that combines field monitoring with hydro-geochemical techniques, as well as multivariate and inferential statistical analysis was used to identify the main SWI drivers at play at each aquifer. The overall assessment showed that all four pilot areas exhibited signs of salinization with different severities. The current state of the aquifers ranged from slightly saline (TDS < 1500 ppm) to highly saline (15,000 < TDS < 31,000 ppm). While the level of the SWI was significantly correlated to the dominant land uses at each site, the extent of the water deficit played a dominant role in explaining the occurrence and intensity of observed SWI rates. The findings suggest a synergistic effect between increased water deficits and urbanization and SWI. Site specific measures are discussed for mitigating the impacts of land use, water demand and deficit towards the sustainable management of the groundwater aquifers.

     

A Cost-Effective Method to Control Seawater Intrusion in Coastal Aquifers

Intrusion of seawater into coastal aquifers is considered one of the most important processes that degrade water-quality by raising the salinity to levels exceeding acceptable drinking standards. Therefore saltwater intrusion should be prevented or at least controlled to protect groundwater resources. This paper presents a cost-effective method to control seawater intrusion in coastal aquifers. This methodology ADR (Abstraction, Desalination and Recharge) includes; abstraction of saline water and recharge to the aquifer after desalination. A coupled transient density-dependent finite element model is developed for simulation of fluid flow and solute transport and used to simulate seawater intrusion. The simulation model has been integrated with an optimization model to examine three scenarios to control seawater intrusion including; abstraction, recharge and a combination system, ADR. The main objectives of the models are to determine the optimal depths, locations and abstraction/recharge rates for the wells to minimize the total costs for construction and operation as well as salt concentrations in the aquifer. A comparison between the combined system (ADR) and the individual abstraction or recharge system is made in terms of total cost and total salt concentration in the aquifer and the amount of repulsion of seawater achieved. The results show that the proposed ADR system performs significantly better than using abstraction or recharge wells alone as it gives the least cost and least salt concentration in the aquifer. ADR is considered an effective tool to control seawater intrusion and can be applied in areas where there is a risk of seawater intrusion.     

Development a Novel Integrated Distributed Multi-objective Simulation-optimization Model for Coastal Aquifers Management Using NSGA-II and GMS Models

Water Resources Management - Groundwater is one of the most valuable water resources in the world in terms of quantity and quality. Therefore, their protection as an important issue should be...     

Solving Complex Rainfall-Runoff Processes in Semi-Arid Regions Using Hybrid Heuristic Model

In the current research, a hybrid model was proposed to solve the complexity of rainfall-runoff models in semi-arid regions. The proposed hybrid model structure consists of linking two data mining models, namely, Group Method of Data Handling (GMDH) and Generalized Linear Model (GLM). The proposed hybrid model structure consists of two phases. The GMDH model was used in the first phase of the hybrid model to predict daily streamflow. The first phase consists of two sections. In the first section a predictive model is developed using the time series of the daily streamflow. In the second section the rainfall-runoff model was developed. The outputs of the first phase of the hybrid model are used as inputs to the second phase of the hybrid model. The second phase of the hybrid model was developed using the GLM model. The Gomel River in Iraq was selected as a case study. The daily rainfall data and daily streamflow data for the period from January 1, 2004 to December 19, 2016 were used to train and validate the model. The results proved the accuracy of the proposed hybrid model in estimating the daily streamflow of the study area, where the value of R² was 0.92 in the training period and 0.88 in the validation period of the model.

     

A Methodology to Analyse and Assess Pumping Management Strategies in Coastal Aquifers to Avoid Degradation Due to Seawater Intrusion Problems

Water Resources Management - In this paper we will focus on an interesting and complex problem, the analysis of coastal aquifer management alternatives in aquifers affected by seawater intrusion...     

Transient Investigation of the Critical Abstraction Rates in Coastal Aquifers: Numerical and Experimental Study

This research investigated the transient saltwater upconing in response to pumping from a well in a laboratory-scale coastal aquifer. Laboratory experiments were completed in a 2D flow tank for a homogeneous aquifer where the time evolution of the saltwater wedge was analysed during the upconing and the receding phase. The SEAWAT code was used for validation purposes and to thereafter examine the sensitivity of the critical pumping rate and the critical time (the time needed for the saltwater to reach the well) to the well design and hydrogeological parameters. Results showed that the critical pumping rate and the critical time were more sensitive to the variations of the well location than the well depth. The critical time increased with increasing the location and depth ratios following a relatively linear equation. For all the configurations tested, the lowest critical pumping rate was found for the lower hydraulic conductivity, which reflects the vulnerability of low permeability aquifers to salinization of pumping wells. In addition, higher saltwater densities led to smaller critical pumping rate and shorter critical time. The influence of the saltwater density on the critical time was more significant for wells located farther away from the initial position of the interface. Moreover, increasing the dispersivity induced negligible effects on the critical pumping rate, but reduced the critical time for a fixed pumping rate.     

Analytical Solutions and Simulation Approaches for Double Permeability Aquifers

In this paper we develop analytical solutions for 1D unsteady flow in an infinite double permeability aquifer. Two asymptotic cases are considered: the near-hydraulic-equilibrium-state and the far-hydraulic-equilibrium-state. It is demonstrated that for both cases the equation for the piezometric head can be expressed in leading order, as the classical solution of the simple permeability medium, to which correction terms are added. On the basis of these findings the accuracy of the simulations using conventional approaches (i.e. MODFLOW, MT3D codes) is discussed, while decision tools for the choice of the adequate double continuum model are also provided.     

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

Water Resources 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...     

Pumping Optimization of Coastal Aquifers Assisted by Adaptive Metamodelling Methods and Radial Basis Functions

The application of metamodelling frameworks is a popular approach to handle the computational cost arising from complex computer simulations and global optimization algorithms in simulation-optimization routines. In this paper, Radial Basis Functions (RBF) are used as metamodels for the computationally expensive variable-density flow and salt transport numerical simulations, in a pumping optimization problem of coastal aquifers. While RBF metamodels have been fairly utilized in many engineering optimization problems, their use is very limited in coastal aquifer management. Two adaptive metamodelling frameworks are employed, that is, the adaptive-recursive approach and the metamodel-embedded evolution strategy. In both frameworks, cubic RBF models are used to approximate the constraint functions imposed on the coastal aquifer pumping optimization problem. The optimal pumping rates are first calculated based on the variable-density and salt transport numerical models of seawater intrusion. The resulting optimal solutions and the computational times are set as benchmark values in order to assess the performance of the metamodelling optimization strategies. Results indicate that the metamodel-embedded evolution framework outperformed in terms of computational efficiency the adaptive-recursive approach while it successfully located the region of the global optimum. Furthermore, with the metamodel-embedded evolution strategy the computational time of the variable-density-based optimization was reduced by 96 %.     

Simulation Modeling for Efficient Groundwater Management in Balasore Coastal Basin,India

The Balasore coastal groundwater basin in Orissa, India is under a serious threat of overdraft and seawater intrusion. The overexploitation resulted in abandoning many shallow tubewells in the basin. The main intent of this study is the development of a 2-D groundwater flow and transport model of the basin using the Visual MODFLOW package for analyzing the aquifer response to various pumping strategies. The simulation model was calibrated and validated satisfactorily. Using the validated model, the groundwater response to five pumping scenarios under existing cropping conditions was simulated. The results of the sensitivity analysis indicated that the Balasore aquifer system is more susceptible to the river seepage, recharge from rainfall and interflow than the horizontal and vertical hydraulic conductivities and specific storage. Finally, based on the modeling results, salient management strategies are suggested for the long-term sustainability of vital groundwater resources of the Balasore groundwater basin. The most promising management strategy for the Balasore basin could be: a reduction in the pumpage from the second aquifer by 50% in the downstream region and an increase in the pumpage to 150% from the first and second aquifer at potential locations.     

A Fast Semi Distributed Rainfall Runoff Model for Engineering Applications in Arid and Semi-Arid Regions

A new GIS based rainfall runoff model is developed for engineering applications, achieving a highly automated watershed analysis process starting from watershed delineation and up to the runoff hydrograph calculation. The model can be classified as a semi-distributed time area model that adopts an improved grid based approach for calculation of watershed response. The model deals with each grid cell in the digital elevation model as an independent hydrologic unit. Travel time through each grid cell is estimated using Manning’s formula and a stream power formula that relates the hydraulic radius at the cell to the characteristics of its upstream watershed area and excess rainfall depth. The watershed response at its outlet is estimated by routing the response of each grid cell using a flow path response function that is defined for that cell. The routed responses of all watershed cells are then convoluted to produce the outflow hydrograph. Model advantages include accuracy improvements due to the incorporation of grid-based routing calculations (both translation and attenuation), fully automated model structure, and fast ability to model many watersheds simultaneously. The combination of these advantages constitutes the novelty of the model that makes it very suitable for engineering design as well as for real-time applications. The model was tested using the data of the experimental watershed, Walnut Gulch, Arizona, USA, gauged by 88 rainfall stations and several discharge recording flumes. The results show that the model can accurately predict the runoff hydrograph where suitable input is available.     

A Standardized Index for Assessing Seawater Intrusion in Coastal Aquifers: The SITE Index

A large number of coastal aquifers worldwide are impacted by seawater intrusion. A major aim of European Directives 2000/60/EC and 2006/118/EC is to achieve good ecological status in groundwater bodies, including coastal aquifers. To this goal, information is needed about the current state of, and changes over time in, individual aquifers. This information can be obtained by applying methods that determine the status of aquifers in an uncomplicated manner. Methods for this type of assessment must comply with three essential criteria. First, calculation of the index must be straightforward and should be based on easy-to-obtain or commonly available data. Next, the index should be able to highlight important characteristics in understandable terms. Finally, the results should be objective and should be expressed in such a way that different time periods and different aquifers can be compared. In this paper we describe the development of a method to characterize seawater intrusion that meets these criteria and is based on four basic parameters: surface area, intensity, temporality, and evolution. Each parameter is determined by specific calculations derived from the groundwater chloride concentrations. Results are specified as a numerical index and an alphanumeric code. This index, known as SITE, has been applied to four Mediterranean coastal aquifers. The standardized results allowed us to discriminate between, and objectively compare the status of these groundwater bodies. Further, this index will make it possible to prioritize management actions and evaluate the effectiveness of these actions over time.