Coastal Aquifer Management Research Articles

The Bajo Guadalhorce coastal aquifers (Málaga, Spain): five decades of human induced evolution

This work makes a synthesis of the hydrogeological knowledge acquired from different works from the decade of 1970 in the Bajo Guadalhorce detrital, coastal aquifers. Hydrodynamic, hydrochemical and geophysical studies were done in the frame of the aforementioned works to determine the hydrogeological properties of these aquifers. Besides, the drilling of research piezometers was carried out in the framework of some of these studies. This information allowed to study the spatial and temporal behavior of groundwater table and some chemical components of the groundwater of these aquifers, as well as to estimate some parameters as hydraulic conductivity or transmissivity. Overexploitation of aquifers was also studied, because marine intrusion situations were detected over the years. However, some studies carried out in the decade of 2010 have remarked a noticeable difference in the hydrodynamic and hydrochemical state of aquifers, due to intense changes in land use during the end of the 20th century. These changes have also influenced over some groundwater dependent ecosystems, as the Guadalhorce Delta Wetlands, located in the Guadalhorce River mouth. So, this article is an example of the remarkable influence that human activities can have on groundwater due to irrigated agriculture, a rising tourism and the building of big hydraulic infrastructures, which demonstrate the necessity of an adequate management of Mediterranean coastal aquifers.

A multidisciplinary approach to characterizing coastal alluvial aquifers to improve understanding of seawater intrusion and submarine groundwater discharge

Coastal aquifers are affected by seawater intrusion (SWI), which causes their salinization, and yield submarine groundwater discharge (SGD), which feeds marine ecosystems. Characterizing groundwater dynamics in coastal aquifers is fundamental for understanding both processes and their interaction. In order to gain insights into SWI and SGD, we developed a 100 m-scale experimental field site located in a coastal alluvial aquifer at the mouth of an ephemeral stream on the Maresme coastline (Barcelona, Spain). Given the complexity of coastal aquifers and the dynamism of the processes occurring therein, understanding of the coupled processes can be achieved by combining methods and approaches across different hydrogeological disciplines. In this study, we conduct a detailed aquifer characterization based on the four pillars of hydrogeology: geology (lithological description and core samples analyses), geophysics (downhole and cross-hole measurements), hydraulics (pumping and tidal response tests) and hydrochemistry (major and minor elements, together with stable and Ra isotopes). Each discipline contributed to the characterization of the aquifer: (1) geological characterization revealed that the aquifer consists of fluvial sediments, organized in fining upwards sequences with alternating layers of gravel, sand and silt; (2) geophysics helped in identifying silt layers and their continuity, which play a segmenting role in the aquifer hydrodynamics; (3) hydraulics tests, specifically tidal response tests, evidenced that tidal loading, rather than hydraulic connection to the sea, drives the tidal response; and (4) hydrochemistry revealed a surprising high reactivity, as most ions reflect some reaction, beyond the expected cation exchange. The summary is that the aquifer, which initially looked like a homogeneous unconfined aquifer 22 m thick, effectively behaves as a multi-aquifer and reactive system with freshwater discharging beneath saltwater at several depths. The fact that thin silt layers caused such a significant impact opens new paths beyond this study both for coastal aquifer management (the possibility of transient pumping for freshwater resources) and marine ecology (expect diffuse groundwater discharge).

Appraisal of urban growth impacts on seawater intrusion vulnerability using GIS-based modified GALDIT-U model: a case study of Kozhikode coastal stretch, Kerala, South India

We investigate the potential vulnerability to seawater intrusion into the aquifers along the Kozhikode coastal stretch using the modified GALDIT (GALDIT-U) model from an urbanization perspective. Urban growth impact (expansion of the impervious surface) was added as an additional input parameter in the GALDIT model, and the analyses were performed using GIS. The results indicated that the area of 16.84 km2, found within the proximity of urban settlements and eroded shorelines along with the Kozhikode, Beypore, Kadalundi, and Faroke, falls under very high vulnerability to seawater intrusion with vulnerability index values ranging 24.82 to 29.82. In the southern sector, the area of 16.84 km2 between Elathur and Beypore also falls within very high vulnerability zones. Similarly, the area of 37.01 km2 in Quilandi, Faroke, Puthiyankadi, Panniyankara, and the east sector of Kozhikode municipality is seen as high vulnerability zones with the vulnerability index values ranging 19.84 to 24.82. Seawater intrusion estimated under moderate vulnerability zones is found to occupy 47.85 km2 (vulnerability index values 14.88 to 19.84). The low and very low vulnerability zones cover 65.07 and 48.07 km2, with the vulnerability index values ranging 9.92 to 14.88 and 4.96 to 9.92, respectively. When cross-validated using major hydrochemical parameters, the results indicate a strong correlation between the vulnerability index classes and groundwater physicochemical parameters. The sensitivity analysis carried out indicated that the distance from the shoreline (D), impacts of urban growth (U), and depth to groundwater table above sea level (L) to be highly influencing parameters for seawater intrusion vulnerability in the coastal stretches. The modified GALDIT-U model shows reliable accuracy for estimating and demarcating seawater intrusion vulnerability zones and could act as an efficient tool in the sustainable management of coastal aquifers in an urban environment.

Simulation of horizontal injection wells in Managed Aquifer Recharge facilities using the conduit flow process (CFP) code for MODFLOW-2005

Groundwater management of coastal aquifers is a scientific and engineering challenge, as coastal aquifer systems are hydrogeologic features that are hydraulically connected with the sea while at the same time are stressed with extensive abstraction practices for irrigation. To remedy this, Managed Aquifer Recharge -MAR- is proved to be a sound and effective technology that is able to fill the gap between groundwater supply and demand. Essential part of this procedure is reliable groundwater modeling that simulates effectively all involved hydrologic processes in order to be able to predict the response of the system under different natural or artificially induced conditions. This paper describes the application of a modeling tool, for the simulation of the relative hydrologic processes between the open filter pipe of the injection well and the surrounding aquifer material with the finite difference method. More specifically, a MAR pilot setup with a horizontal direction injection well was simulated with the Conduit Flow Process (CFP) of MODFLOW-2005 code. To investigate further the applicability of CFP code, the HDDW was conceptualized with the simpler MODFLOW WEL package and the results of the two models were compared. For the two models, sensitivity analysis was conducted using UCODE_2014. Both models were calibrated successfully as the simulated values showed an acceptable fit to the observed data. The main difference between the two packages was observed in the rise of groundwater level along the horizontal well. To this end, it was indicated that CFP offered a more realistic representation of the horizontal well pipe.

Laboratory and Numerical Study of Saltwater Upconing in Fractured Coastal Aquifers

This study investigated the saltwater upconing mechanism in fractured coastal aquifers. Head-induced saline intrusion was initiated into three narrow sandbox aquifers containing individual horizontal discontinuities placed on different positions. Subsequently, using a peristaltic pump, freshwater was abstracted from the aquifers’ center, triggering saltwater upconing. Progressively larger pumping rates were applied until critical conditions, resulting in the wells’ salinization, were achieved. Advanced image analysis algorithms were utilized to recreate the saltwater concentration fields and quantify the extent of the saline wedges with a high accuracy. A numerical model was successfully employed to simulate the laboratory results and conduct a comprehensive sensitivity analysis, further expanding the findings of this investigation. The impact of the fractures’ length, permeability and position on the upconing mechanism was identified. It was established that the presence of high permeability discontinuities significantly affected aquifer hydrodynamics. The conclusions of this study could constitute a contribution towards the successful management of real-world fractured coastal aquifers.

Review of drivers and threats to coastal groundwater quality in China

With rapid socio-economic development, China's coastal areas are among the fastest growing and most economically dynamic regions in the world. Under the influence of climate change and human activities, protecting the quality of coastal groundwater has emerged as one of the key environmental and resource management issues for these areas. This paper reviews (for the first time) groundwater quality data for the coastal basins of China, where over 600 million people live, focussing on key inorganic indicators/pollutants; groundwater salinity, nitrate, fluoride, and arsenic. These pollutants present major water quality issues and are also valuable as indicators of wider processes and influences impacting coastal groundwater quality – e.g. saltwater intrusion, agricultural pollution and release of geo-genic contaminants. We discuss the major drivers causing water quality problems in different regions and assess future trajectories and challenges for controlling changes in coastal groundwater quality in China. Multiple processes, including modern and palaeo seawater/brine migration, groundwater pumping for agricultural irrigation, pollution from agrochemical application, rapid development of aquaculture, urban growth, and water transfer projects, may all be responsible (to different degrees) for changes observed in coastal groundwater quality, and associated long-term health and ecological effects. We discuss implications for sustainable coastal aquifer management in China, arguing that groundwater monitoring and contamination control measures require urgent improvement. The evolution and treatment of coastal groundwater quality problems in China will serve as an important warning and example for other countries facing similar pressures, due to climate change, coastal development, and intensification of anthropogenic activity in coming decades.

A New Normalized Groundwater Age-Based Index for Quantitative Evaluation of the Vulnerability to Seawater Intrusion in Coastal Aquifers: Implications for Management and Risk Assessments

The vulnerability of coastal aquifers to seawater intrusion has been largely relying on data-driven indexing approaches despite their shortcomings to depict the complex processes of groundwater flow and mass transport under variable velocity conditions. This paper introduces a modelling-based alternative technique relying on a normalized saltwater age vulnerability index post-processed from results of a variable density flow simulation. This distributed index is obtained from the steady-state distribution of the salinity and a restriction of the mean groundwater age to a mean saltwater age distribution. This approach provides a novel way to shift from the concentration space into a vulnerability assessment space to evaluate the threats to coastal aquifers. The method requires only a sequential numerical solution of two steady state sets of equations. Several variants of the hypothetical Henry problem and a case study in Lebanon are selected for demonstration. Results highlight this approach ability to rank, compare, and validate different scenarios for coastal water resources management. A novel concept of zero-vulnerability line/surface delineating the coastal area threatened by seawater intrusion has shown to be relevant for optimal management of coastal aquifers and risk assessments. Hence, this work provides a new tool to sustainably manage and protect coastal groundwater resources.

Performance evaluation of Auto-Regressive Integrated Moving Average models for forecasting saltwater intrusion into Mekong river estuaries of Vietnam

The Mekong Delta is the most severely affected area by saltwater intrusion in Vietnam. Recent studies have focused on predicting this disaster with weekly and decade lead times without many seasonal forecasts, which is important for planning crop selection, crop structure, and sowing time. This study aims to forecast the spatial distribution of saltwater intrusion into the Mekong river estuaries of Vietnam during the dry season of 2021 by integrating Auto-Regressive Integrated Moving Average with Geographic Information System. ARIMA models were trained with a single input of water salinity measurements from 2012 to 2020. Compared to the weekly salinity observations in 2021, these models predicted very well in the My Tho and Ham Luong rivers but unsatisfactory performance in the Co Chien river. The deepest saltwater intrusion will occur between March 19th and April 16th of 2021, when the 4‰ saline front will move the farthest distance of 41,41 and 44 kilometers inland from the sea through My Tho, Ham Luong Co Chien rivers, respectively. The entire river system will be exposed to moderate risk of saltwater intrusion. Freshwater zones decreased significantly to 0.73% of the whole area of Ben Tre province. These findings could provide a valuable scientific foundation for the appropriate management of coastal aquifers to control or reduce saltwater intrusion.

A Probabilistic Multiperiod Simulation–Optimization Approach for Dynamic Coastal Aquifer Management

A Probabilistic Multiperiod Simulation–Optimization Approach for Dynamic Coastal Aquifer Management

Effect of unconventional water resources interventions on the management of Gaza coastal aquifer in Palestine

Abstract The non-conventional water resources of seawater desalination, wastewater treatment, and stormwater harvesting are promising water resources to enhance the water supply and to cope with the groundwater depletion of the Gaza coastal aquifer (GCA). In total, the current daily operation of the short-term low-volume (STLV) seawater desalination plants produces 36,000 m3 and, on the large-scale perspective, the seawater desalination capacity is planned to increase from 150,000 to 300,000 m3 per day by the years 2025 and 2035, respectively. The wastewater treatment and reuse activities are processed through three wastewater treatment plants with a total daily capacity of 130,000 m3 which is proposed to be increased to a capacity of 235,000 m3 by the beginning of 2025. Stormwater collecting and harvesting supply the water sector by about 550–820 cubic meters per day. The proposed stochastic and artificial intelligence model that was developed in this study to simulate the interactive conditions between the groundwater and the water intervention plan showed proper performance in terms of (r) = 0.95–0.99 and the root mean square error (RMSE) = 0.09–0.21. The model outputs reveal that the annual groundwater abstractions will reach 192 million cubic meters by 2040 with an annual increasing rate of +3%. By applying the model, the optimum utilization of the unconventional water resources contributes positively to the recovery of the GCA, which is experiencing a decline hot spot in the water level that reaches to −19 m below mean sea level (MSL) and is expected to drop to −28 m MSL by 2040. The impact of unconventional water resources interventions was investigated by simulating the water table trend using stochastic models and artificial neural networks (ANNs) through three scenarios. The first scenario, which addresses the non-intervention status, indicates that the groundwater table will decline by −1.5% in the northern governorates and by −51% in the southern governorates of the Gaza Strip within 2020–2040. The second scenario demonstrates the impact of the existing water interventions that revealed an interim recovery in the groundwater balance until 2025 in which the water consumption tended to increase rapidly. The third scenario illustrates the impact of applying the full water management intervention plan in which the depression cone in the groundwater level will be restored by about +10 m.

Management of Coastal Aquifers with special reference to seawater intrusion and its remediation

Coastal areas are home to a significant portion of the world's population, which is mostly concentrated in major cities. In India alone 28.62 million which is almost 25 percent of people living in the state live near the coastal areas. Population and economic development continue to bring pressure on already-strained water supplies, putting coastal aquifers at risk of seawater intrusion. Groundwater salinization has occurred in coastal areas all over the world as a result of intensive groundwater abstraction. Rising sea levels and storm surges as a result of climate change, as well as land subsidence, are expected to escalate the problem in the coming decades. The handling of coastal aquifers includes identifying an appropriate body of salt water and calculating the amount of freshwater discharging in the sea which is needed to maintain the seawater-freshwater interface. Although data are the most important precondition for management, good governance is also essential. Groundwater supplies face some general governance problems due to invisibility of ground water management is a pressing problem due to the increased dependency on it. The paper discusses common governance problems and the remedial measures that can be taken against seawater intrusion in coastal aquifers with great detail along with 4 case studies discussing water management challenges in coastal zones and improved methods of aquifer recharge. Remedial measures using conventional and temporary methods as well as modern methods like hydraulic barriers are also discussed and elaborated.

Surrogate-Based Stochastic Multiobjective Optimization for Coastal Aquifer Management under Parameter Uncertainty

Linked simulation-optimization (S/O) approaches have been extensively used as tools in coastal aquifer management. However, parameter uncertainties in seawater intrusion (SI) simulation models often undermine the reliability of the derived solutions. In this study, a stochastic S/O framework is presented and applied to a real-world case of the Longkou coastal aquifer in China. The three conflicting objectives of maximizing the total pumping rate, minimizing the total injection rate, and minimizing the solute mass increase are considered in the optimization model. The uncertain parameters are contained in both the constraints and the objective functions. A multiple realization approach is utilized to address the uncertainty in the model parameters, and a new multiobjective evolutionary algorithm (EN-NSGA2) is proposed to solve the optimization model. EN-NSGA2 overcomes some inherent limitations in the traditional nondominated sorting genetic algorithm-II (NSGA-II) by introducing information entropy theory. The comparison results indicate that EN-NSGA2 can effectively ameliorate the diversity in Pareto-optimal solutions. For the computational challenge in the stochastic S/O process, a surrogate model based on the multigene genetic programming (MGGP) method is developed to substitute for the numerical simulation model. The results show that the MGGP surrogate model can tremendously reduce the computational burden while ensuring an acceptable level of accuracy.

A conjunctive management framework for the optimal design of pumping and injection strategies to mitigate seawater intrusion

Coastal aquifer management (CAM) considering conjunctive optimization of pumping and injection system for seawater intrusion (SI) mitigation poses significant decision-making challenges. CAM needs to pose multiple objectives and massive decision variables to explore tradeoff strategies between the conflicting resources, economic, and environmental requirements. Here, we investigate a joint artificial injection scheme for ameliorating SI by establishing an evolutionary multi-objective decision-making framework that combines simulation-optimization (S–O) modelling with a cost-benefit analysis, and demonstrate the framework on a large-scale CAM case in Baldwin County, Alabama. First, a SI numerical model, using SEAWAT, was configured to predict the vulnerable region as an SI encroachment area with the scenarios of minimum and maximum pumping capacity. As a result, a smaller number of candidate sites were selected in the SI encroachment area for implementing groundwater injection to avoid the computationally infeasible SI optimization with an inordinate number of injection related decision variables. Second, the effective S–O methodology of niched Pareto tabu search combined with a genetic algorithm (NPTSGA), which considers the moving-well option, was applied to discover optimal pumping/injection (P/I) strategies (including P/I rates and injection well locations) between three conflicting management objectives under complicated SI constraints. Third, for practical operation of the P/I schemes, a cost-benefit analysis provides judgment criteria to allow decision-makers to implement more sustainable P/I strategies to capture the different realistic preferences. The implementation of three extreme optimization solutions for the case study indicates that, compared to the initial unoptimized scheme, a maximum increase of a factor of 3 in groundwater extraction rates, a maximum reduction of 17% in extent of SI, and a maximum 82.3 million US dollars in comprehensive benefits are specifically achieved by conjunctive P/I optimization. The robustness in the decision alternatives attributed to the uncertainty in physical parameters of hydraulic conductivity was discovered through global sensitivity analysis. The proposed framework provides a decision support system for multi-objective CAM with combined pumping control and engineering measures for SI mitigation.

The Issue of Groundwater Salinization in Coastal Areas of the Mediterranean Region: A Review

The Mediterranean area is undergoing intensive demographic, social, cultural, economic, and environmental changes. This generates multiple environmental pressures such as increased demand for water resources, generation of pollution related to wastewater discharge, and land consumption. In the Mediterranean area, recent climate change studies forecast large impacts on the hydrologic cycle. Thus, in the next years, surface and ground-water resources will be gradually more stressed, especially in coastal areas. In this review paper, the historical and geographical distribution of peer-review studies and the main mechanisms that promote aquifer salinization in the Mediterranean area are critically discussed, providing the state of the art on topics such as actual saltwater wedge characterization, paleo-salinities in coastal areas, water-rock interactions, geophysical techniques aimed at delineating the areal and vertical extent of saltwater intrusion, management of groundwater overexploitation using numerical models and GIS mapping techniques for aquifer vulnerability to salinization. Each of the above-mentioned approaches has potential advantages and drawbacks; thus, the best tactic to tackle coastal aquifer management is to employ a combination of approaches. Finally, the number of studies focusing on predictions of climate change effects on coastal aquifers are growing but are still very limited and surely need further research.

Nitrate-Enrichment Structures Phytoplankton Communities in the Shallow Eastern Mediterranean Coastal Waters

Submarine groundwater discharge (SGD) has been shown to be an important source of nutrients in coastal environments, especially nitrogen and silica, and thereby relive nutrient limitation to phytoplankton. Here, we followed autotrophic microbial biomass, activity, and community composition at a site strongly influenced by SGD and a nearby nutrients-poor reference site at the oligotrophic Israeli shallow rocky coast [southeastern Mediterranean Sea (SEMS)] between 2011 and 2019. The surface water at the SGD-affected area had significantly higher NO3 + NO2 (∼10-fold) and Si(OH)4 (∼2-fold) levels compared to the reference site, while no significant differences were observed for PO4 or NH4. This resulted in a significant increase in algae biomass (∼3.5-fold), which was attributed to elevated Synechococcus (∼3.5-fold) and picoeukaryotes (∼2-fold) at the SGD-affected site, and in elevated primary production rates (∼2.5-fold). Contrary to most SGD-affected coastal areas, diatoms biomass remained unchanged between sites, despite the elevated N and Si, suggesting the dominance of picophytoplankton over microphytoplankton at the SEMS. DNA sequencing of the 16S and 18S rDNA supported these findings. These results highlight the influence of SGD on shallow-water microbial populations. Our observations are consistent with recent studies showing that phytoplankton along the Israeli coast are likely nitrogen + silica limited, and may have important ecological and regulatory implications for environmental policy and management of coastal aquifers.

Stochastic time-series models for drought assessment in the Gaza Strip (Palestine)

Abstract The Eastern Mediterranean region of the Middle East and North Africa (MENA) is experiencing patterns of major drought due to the effects of rising temperatures and falling precipitation levels. The multiscale drought evaluation Standardized Precipitation Evapotranspiration Index (SPEI) reveals evolving and severe drought from North Africa and the Sinai desert toward the Middle East. While there has been a period without drought between 1970 and 1990, the severity and frequency of drought increased considerably after 1990. Current drought conditions in the Eastern Mediterranean region of MENA are moderate to severe with a 60–100% likelihood of occurrence, according to time parameters. The Gaza Strip is especially vulnerable to the consequences of increasing drought because it is situated in the vicinity of the Sinai Desert; therefore, a downscaled study of drought in the region is essential to implement mitigation measures for the sustainable management and planning of coastal aquifer and agricultural activities in the Gaza Strip. Considerable availability of precipitation time series from various meteorological stations helped provide a local drought study for the Gaza Strip, in accordance with the Standardized Precipitation Index (SPI). The stochastic time-series model of (4,0,1) (5,1,1)12 shows a robust simulator for modeling and forecasting the future trend of precipitation at the nine meteorological stations. In terms of correlation accuracy, the model achieves a correlation (r) of approximately 93–97% in the calibration range and a correlation (r) of about 92–99% in the validation range. In terms of measuring the difference between the values, the root mean squared error (RMSE) of the model results shows that the RMSE was between 7–21 in the calibration range and 11–21 in the validation range. The model reveals a slightly stable trend in precipitation patterns at the northern meteorological stations of Beit Hanon, Beit Lahia, Shati, and Remal. However, declining precipitation tendency was recorded at the southern meteorological stations of Mughraka, Nussirat, Beir Al-Balah, Khanyounis, and Rafah. The SPI-based drought assessment implies that the precipitation annual threshold levels at SPI = 0 drop territorially from 474 mm in the north to about 250 mm in the south of the Gaza Strip. In this study, a representative 12-month local scale SPI12 at an annual precipitation threshold level of 370 mm was formulated to address the drought conditions in the Gaza Strip. Standing on the outputs of the local SPI12 scale might signify that the region of the Gaza Strip risks drought status with an incidence likelihood varying from 8% in the north to 100% in the south. Regular drought is prevalent in the northern governorates, but the hazards of extreme and severe drought are high in the southern areas with an incidence risk of about 83%. Sequentially, southern governorates of Rafah and Khanyounis experience chronic annual drought, while the return period of drought is reported to be every 9–12 years in the northern governorates of the Gaza Strip. The rain-fed years of 1998 and 2010 reported the worst periods of drought, while the period of 2016 showed a good droughtless water balance. Overall, the no-drought status might define the prospective conditions in the governorates of North Gaza, Gaza, and central Gaza over the next 20 years, while Rafah and Khanyounis are anticipated to be under normal to severe drought conditions.

A surrogate-based simulation–optimization approach for coastal aquifer management

Abstract This study proposed a pumping-injection (P-I) groundwater management strategy based on a simulation–optimization (S-O) framework to mitigate seawater intrusion (SI). The methodology was applied to a real case in Longkou, China. A three-dimensional variable-density groundwater simulation model was established to simulate and predict the SI process. In the S-O framework, while solving the optimization model, it is required to call the simulation model thousands of times, which leads to enormous computational load. In this case, the Kriging and support vector regression (SVR) surrogate models were established for the simulation model respectively. Furthermore, the ensemble surrogate modeling technique was applied to construct the Kriging-SVR ensemble surrogate model. The most accurate surrogate model was selected as the substitute for the simulation model, saving considerable computing costs. The results show that the ensemble surrogate model performs better than the stand-alone surrogate models in accuracy, indicating that combining stand-alone surrogate models is a potential modeling method for the surrogate model of the variable-density groundwater simulation model. By solving the optimization model, the optimal pumping and injection schemes under different scenarios were obtained. The optimization results demonstrate that the proposed methodology is effective and stable in coastal groundwater management.

Modelling and management of saltwater intrusion in a coastal aquifer system: A regional-scale study

Increasing saltwater intrusion in the Barguna aquifer due to unplanned groundwater abstraction is a concern for the people of Barguna district, Bangladesh. A sustainable groundwater abstraction strategy capable of confining saltwater concentrations in the aquifer within acceptable limits is required for the Barguna aquifer system. In this study, a regional-scale integrated simulation-optimization (S–O) approach was used to prescribe an optimal groundwater abstraction strategy for the aquifer. The flow and transport processes were simulated by a calibrated and validated numerical simulation model using the available hydrogeological data. The simulation model was then used within the integrated S–O based management model to evolve ideal groundwater abstraction patterns to restraint saltwater intrusion in the study area. Computational efficiency of the management model was achieved by using a Multivariate Adaptive Regression Spline (MARS) based meta-model emulating the density dependent salinity transport processes of the study area. The developed MARS-based emulators were then externally linked with a Controlled Elitist Multiple Objective Genetic Algorithm (CEMOGA) as a replacement of the numerical model. The study area comprised of a set of production wells for abstracting groundwater for beneficial purposes. Barrier extraction wells placed along the coastline were also considered as a management option to control saltwater intrusion hydraulically. Grey Relational Analysis was utilized as a quick decision-making tool for selecting a relatively better solution from a large number of non-dominated solutions from the Pareto front. The proposed MARS-CEMOGA based saltwater intrusion management methodology was capable of obtaining accurate solutions for optimal groundwater abstractions from a combination of production and barrier extraction bores in a real-life coastal aquifer system. This study demonstrates the efficiency of the MARS meta-model, together with an integrated S–O approach, to solve real-life complex coastal aquifer management problems.

Application of the group method of data handling and variable importance analysis for prediction and modelling of saltwater intrusion processes in coastal aquifers

Data-driven mathematical models are powerful prediction tools, which are utilized to approximate solution responses obtained using numerical saltwater intrusion simulation models. Employing data-driven prediction models as a replacement of the complex groundwater flow and transport models enables prediction of future scenarios. Most important, it also helps save computational time, effort and requirements when developing optimal coastal aquifer management methodologies using complex and large-scale coupled simulation-optimization models. In this study, a new data-driven mathematical model, namely group method of data handling (GMDH)-based prediction models, is developed and utilized to predict salinity concentration in a coastal aquifer by mimicking the responses of a variable-density flow and solute transport numerical simulation model. For comparison and evaluation purpose, the prediction performances of GMDH models were compared with well-established support vector machine regression and genetic programming based models. In addition, one important characteristic of the GMDH models is explored and evaluated, i.e. the ability to identify a set of most influential input predictor variables (pumping rates) that had the most significant impact on the outcomes (salinity concentration at monitoring locations). To confirm variable importance, 3 tests are conducted in which new GMDH models are constructed using subsets of the original datasets. In TEST 1, new GMDH models are constructed using a set of most influential variables (consisting of pumping rates at selected locations) only. In TEST 2, a subset of 20 variables (10 most and least influential variables) is used to develop new GMDH models. In TEST 3, a subset of the least influential variables is used to develop GMDH models. The performance evaluation results demonstrate that GMDH models developed using the entire dataset had reasonable prediction accuracy and efficiency. The comparison performance evaluation results for the three test scenarios highlighted the importance of the appropriate selection of relevant input pumping rates when developing accurate prediction models. The results suggested that incorporating the least influential variables deteriorate the accuracy of the prediction models; thus, considering the most influential pumping rates it is possible to develop more accurate and efficient salinity prediction models. Overall, the evaluation results from this study establish that the GMDH models and the inherent input variable ranking capability can be utilized as accurate and efficient coastal saltwater intrusion prediction models. Hence, GMDH models are viable saltwater intrusion modelling tools, which can be employed in future regional-scale saltwater intrusion prediction and management investigations.

Global sensitivity analysis on a numerical model of seawater intrusion and its implications for coastal aquifer management: a case study in Dagu River Basin, Jiaozhou Bay, China

Seawater intrusion (SWI) has triggered an accelerating process of freshwater contamination and significantly affected the soil fertility and local groundwater supply in the coastal area of Jiaozhou Bay, Shandong Province, China. This study establishes a three-dimensional numerical model based on SEAWAT code to simulate transient regional SWI to coastal aquifers in Dagu River Basin (DRB) adjacent to Jiaozhou Bay. The hydrogeological parameters in the SEAWAT model are calibrated and validated with the observed data of groundwater level and chloride (Cl−) concentration from 1 January 2010 to 1 June 2018. Also, global sensitivity analysis is used to evaluate the impacts of hydrogeological parameters (aquifer hydraulic conductivity and specific yield), along with sources/sinks consisting of recharge from precipitation infiltration and groundwater abstraction, on the SWI model. The sensitivity analysis results indicate that the progression of SWI is sensitive to the groundwater recharge from precipitation infiltration and the groundwater abstraction in the study area, while the hydraulic conductivity is of secondary importance. Furthermore, the baseline SWI model is applied to predict the extent of SWI under different scenarios considering the possible future precipitation and groundwater abstraction. It is shown that increased recharge and reduced groundwater abstraction could effectively lessen the extent of future SWI. As a case study, the research efforts on the regional SWI model are of critical importance for investigating the occurrence of SWI, identifying the factors most influential on the SWI process, and providing useful predictive information for SWI management in the DRB aquifer of Jiaozhou Bay.