Zoi Dokou

Saltwater intrusion estimation in a karstified coastal system using density-dependent modelling and comparison with the sharp-interface approach

Abstract Saltwater intrusion is a naturally occurring phenomenon that is exacerbated significantly by excessive groundwater exploitation in coastal aquifers. In order to determine the extent of saltwater intrusion in a karstified coastal aquifer in Crete, Greece, a three-dimensional, density-dependent groundwater flow and transport model was developed and compared to the more traditional sharp-interface approach. The karstified medium was modelled using a combination of the equivalent porous medium approach (for lower-order fractures) and a discrete fracture approach (for the main fractures/faults). The model takes into consideration the geomorphologic characteristics of the karstic system, such as the depth and orientation of the fault network, and the diffusion phenomena associated with the variable densities of freshwater and saltwater—parameters that create a complex system, inducing uncertainty in the model. The model results showed that the orientation of the fractures, the pumping activity and the fluid density effects drive the seawater intrusion front asymmetrically inland. Editor Z.W. Kundzewicz Citation Dokou, Z. and Karatzas, G.P., 2012. Saltwater intrusion estimation in a karstified coastal system using density-dependent modelling and comparison with the sharp-interface approach. Hydrological Sciences Journal, 57 (5), 985–999.

Groundwater-level forecasting under climate change scenarios using an artificial neural network trained with particle swarm optimization

Abstract Artificial neural networks (ANNs) have recently been used to predict the hydraulic head in well locations. In the present work, the particle swarm optimization (PSO) algorithm was used to train a feed-forward multi-layer ANN for the simulation of hydraulic head change at an observation well in the region of Agia, Chania, Greece. Three variants of the PSO algorithm were considered, the classic one with inertia weight improvement, PSO with time varying acceleration coefficients (PSO-TVAC) and global best PSO (GLBest-PSO). The best performance was achieved by GLBest-PSO when implemented using field data from the region of interest, providing improved training results compared to the back-propagation training algorithm. The trained ANN was subsequently used for mid-term prediction of the hydraulic head, as well as for the study of three climate change scenarios. Data time series were created using a stochastic weather generator, and the scenarios were examined for the period 2010–2020. Editor Z.W. Kundzewicz; Associate editor L. See Citation Tapoglou, E., Trichakis, I.C., Dokou, Z., Nikolos, I.K., and Karatzas, G.P., 2014. Groundwater-level forecasting under climate change scenarios using an artificial neural network trained with particle swarm optimization. Hydrological Sciences Journal, 59(6), 1225–1239. http://dx.doi.org/10.1080/02626667.2013.838005

Statistical analysis and ANN modeling for predicting hydrological extremes under climate change scenarios: The example of a small Mediterranean agro-watershed

The purpose of this study was to create a modeling management tool for the simulation of extreme flow events under current and future climatic conditions. This tool is a combination of different components and can be applied in complex hydrogeological river basins, where frequent flood and drought phenomena occur. The first component is the statistical analysis of the available hydro-meteorological data. Specifically, principal components analysis was performed in order to quantify the importance of the hydro-meteorological parameters that affect the generation of extreme events. The second component is a prediction-forecasting artificial neural network (ANN) model that simulates, accurately and efficiently, river flow on an hourly basis. This model is based on a methodology that attempts to resolve a very difficult problem related to the accurate estimation of extreme flows. For this purpose, the available measurements (5 years of hourly data) were divided in two subsets: one for the dry and one for the wet periods of the hydrological year. This way, two ANNs were created, trained, tested and validated for a complex Mediterranean river basin in Crete, Greece. As part of the second management component a statistical downscaling tool was used for the creation of meteorological data according to the higher and lower emission climate change scenarios A2 and B1. These data are used as input in the ANN for the forecasting of river flow for the next two decades. The final component is the application of a meteorological index on the measured and forecasted precipitation and flow data, in order to assess the severity and duration of extreme events.

Optimal management of saltwater intrusion in the coastal aquifer of Malia, Crete (Greece), using particle swarm optimization

Saltwater intrusion is a common phenomenon in coastal aquifers that can affect the quality of water intended for drinking and irrigation purposes. In order to provide sustainable management options for the coastal aquifer of Malia, located on the Greek island of Crete, a weighted multi-objective optimization methodology is employed. The methodology involves use of the particle swarm optimization algorithm combined with groundwater modelling. The sharp-interface approximation combined with the Ghyben-Herztberg equation is used to estimate the saltwater-intrusion front location. The prediction modelling results show that under the current pumping strategies (over-exploitation), the saltwater-intrusion front will continue to move inland, posing a serious threat to the groundwater quality. The management goal is to maximize groundwater withdrawal rates in the existing pumping wells while inhibiting the saltwater-intrusion front at locations closer to the coastal zone. This is achieved by requiring a minimum hydraulic-head value at pre-selected observation locations. In order to control the saltwater intrusion, a large number of pumping wells must be deactivated and alternative sources of water need to be considered.RésuméL’intrusion d’eau salée est un phénomène courant dans les aquifères côtiers qui peuvent affecter la qualité des eaux destinées à des fins de consommation d’eau potable et d’irrigation. Afin de fournir des options de gestion durable de l’aquifère côtier de Malia, situé en Crête, île grecque, une méthodologie d’optimisation multi-objectifs à pondération est utilisée. La méthodologie implique l’utilisation de l’algorithme d’optimisation par essaim de particules combiné à la modélisation des écoulements d’eaux souterraines. L’hypothèse d’une interface nette combinée avec l’équation de Ghyben-Herzberg est utilisée pour estimer la localisation du front d’intrusion d’eau salée. Les résultats de modélisation prédictive montrent que, pour les stratégies de pompages actuels (surexploitation), le front d’intrusion d’eau salée va continuer à progresser vers l’intérieur des terres, ce qui pose une grave menace pour la qualité des eaux souterraines. L’objectif de gestion est de maximiser les débits de pompages des eaux souterraines dans les puits existants tout en inhibant le front d’intrusion d’eau salée dans les secteurs les plus proches de la zone côtière. Ceci est réalisé en imposant des valeurs minimales de charge hydraulique dans des localisations présélectionnées pour l’observation. Afin de contrôler l’intrusion d’eau salée, un grand nombre de puits de pompage doit être désactivé et d’autres sources d’eau doivent être considérées.ResumenLa intrusión de agua salada es un fenómeno común en los acuíferos costeros que pueden afectar a la calidad de las aguas destinadas para fines potables y de riego. Con el objeto de proporcionar opciones de gestión sostenible en el acuífero costero de Malia, situado en la isla griega de Creta, se emplea una metodología de optimización multi-objetivo ponderado. La metodología consiste en el uso de un algoritmo de optimización por nubes de partículas combinado con el modelado de las aguas subterráneas. Se utiliza una aproximación de la interfaz marcada combinada con la ecuación Ghyben-Herztberg para estimar la ubicación frente de la intrusión de agua salada. Los resultados de los modelos de predicción indican que bajo las actuales estrategias de bombeo (sobreexplotación), el frente de la intrusión del agua salada continuará moviéndose hacia el interior, lo que representa una seria amenaza para la calidad del agua subterránea. El objetivo de gestión es maximizar las tasas de extracción de agua subterránea en los pozos de bombeo existentes al tiempo que se inhiba el avance de la parte frontal de la intrusión de agua salada en lugares cercanos a la zona costera. Esto se logra al requerir un valor mínimo de la carga hidráulica en los lugares de observación pre-seleccionados. Con el fin de controlar la intrusión de agua salada deben ser desactivados un gran número de pozos de bombeo y deben ser consideradas fuentes alternativas de agua.摘要海水入侵是沿海含水层一个常见的现象,可影响用于饮用和灌溉的水的水质。为了给位于希腊克里特岛的玛利亚沿海含水层提供可持续的管理选择,采用了加权多目标优化方法。该方法使用了粒子群优化算法和地下水模拟。利用明显界面近似法和Ghyben-Herztberg方程式估算了海水入侵前锋位置。预测模拟结果显示,在目前抽水状态(超采)下,海水入侵前锋将继续向内陆移动,对地下水质造成了严重威胁。管理目标就是在保持现有抽水井抽水量最大化状态下,抑制沿海带附近的海水入侵前锋。通过获取预先选择的观测点的水头值,这个目标可以达到。为了控制海水入侵,必须停止使用大量的抽水井,并且需要考虑替代水源。ResumoIntrusão de água salina é um fenômeno comum em aquíferos costeiros que podem afetar a qualidade da água utilizável em propósitos de irrigação e consumo. Para proporcionar opções sustentáveis de gerenciamento para o aquífero costeiro de Mália, localizado na ilha Grega de Creta, uma metodologia multi-objetiva ponderada foi empregada. A metodologia envolve a utilização do algoritmo da otimização por enxame de partículas combinado com a modelagem das águas subterrâneas. A aproximação da interface abrupta combinada com a equação de Ghyben-Herztberg foi utilizada para estimar a localização da frente de intrusão de água salina. Os resultados preditos da modelagem demonstram que sob as estratégias de bombeamento atuais (sobre-explotação), a frente de intrusão de água salina continuará a mover-se terra a dentro, tornando-se uma séria ameaça à qualidade das águas subterrâneas. O objetivo do gerenciamento é maximizar as taxas de retirada de água subterrânea nos poços de bombeamento existentes enquanto inibe a frente de intrusão de água salina próximas as zonas costeiras. Isso é obtido através da medição do valor de carga hidráulica mínimo em locais de observação pré-selecionados. Para controlar a intrusão de água salina, um grande número de poços de bombeamento deve ser desativado e fontes alternativas de água precisam ser consideradas.

Assessing groundwater quality in Greece based on spatial and temporal analysis.

The recent industrial growth together with the urban expansion and intensive agriculture in Greece has increased groundwater contamination in many regions of the country. In order to design successful remediation strategies and protect public health, it is very important to identify those areas that are most vulnerable to groundwater contamination. In this work, an extensive contamination database from monitoring wells that cover the entire Greek territory during the last decade (2000–2008) was used in order to study the temporal and spatial distribution of groundwater contamination for the most common and serious anionic and cationic trace element pollutants (heavy metals). Spatial and temporal patterns and trends in the occurrence of groundwater contamination were also identified highlighting the regions where the higher groundwater contamination rates have been detected across the country. As a next step, representative contaminated aquifers in Greece, which were identified by the above analysis, were selected in order to analyze the specific contamination problem in more detail. To this end, geostatistical techniques (various types of kriging, co-kriging, and indicator kriging) were employed in order to map the contaminant values and the probability of exceeding critical thresholds (set as the parametric values of the contaminant of interest in each case). The resulting groundwater contamination maps could be used as a useful tool for water policy makers and water managers in order to assist the decision-making process.

Multi-objective optimization for free-phase LNAPL recovery using evolutionary computation algorithms

Abstract A nonlinear, multi-objective optimization methodology is presented that seeks to maximize free product recovery of light non-aqueous phase liquids (LNAPLs) while minimizing operation cost, by introducing the novel concept of optimal alternating pumping and resting periods. This process allows more oil to flow towards the extraction wells, ensuring maximum free product removal at the end of the remediation period with minimum groundwater extraction. The methodology presented here combines FEHM (Finite Element Heat and Mass transfer code), a multiphase groundwater model that simulates LNAPL transport, with three evolutionary algorithms: the genetic algorithm (GA), the differential evolution (DE) algorithm and the particle swarm optimization (PSO) algorithm. The proposed optimal free-phase recovery strategy was tested using data from a field site, located near Athens, Greece. The PSO and DE solutions were very similar, while that provided by the GA was inferior, although the computation time was roughly the same for all algorithms. One of the most efficient algorithms (PSO) was chosen to approximate the optimal Pareto front, a method that provides multiple options to decision makers. When the optimal strategy is implemented, although a significant amount of LNAPL free product is captured, a spreading of the LNAPL plume occurs. Editor Z.W. Kundzewicz; Associate editor L. See Citation Dokou, Z. and Karatzas, G.P., 2013. Multi-objective optimization for free-phase LNAPL recovery using evolutionary computation algorithms. Hydrological Sciences Journal, 58 (3), 671–685.

Utilizing Successive Linearization Optimization to Control the Saltwater Intrusion Phenomenon in Unconfined Coastal Aquifers in Crete, Greece

In the present work, a simulation-optimization method is employed in order to manage saltwater intrusion in two unconfined coastal aquifers in Crete, Greece. The optimization formulation seeks to maximize groundwater withdrawal rates while maintaining the saltwater intrusion front at the current location or inhibiting it at locations closer to the coast. A combination of a groundwater flow model (MODFLOW) with the Ghyben-Herzberg saltwater front approximation and a sequential implementation of the Simplex algorithm (GWM) are employed. The results show that under the current pumping strategies, the saltwater intrusion front will continue to move inland, posing a serious threat to the groundwater quality of these regions. Optimal groundwater withdrawal scenarios that take into consideration the water needs of the local communities and environmental concerns are presented and discussed. In both case studies, significant reductions in pumping are required in order for the saltwater intrusion front to retract closer to the shoreline.

Groundwater Modeling and Remediation Scenarios of a Hexavalent Chromium Plume Released from an Industrial Site

In recent years, high concentrations of hexavalent chromium [Cr(VI)] have been found in the groundwater system of the Asopos River Basin. This work focuses on a Cr(VI) plume detected in the industrial area of Inofyta, the most important contamination hot spot of the Asopos River Basin. A groundwater flow and Cr(VI) transport model was developed to assess the behaviour of the plume and investigate the location of a suspected nearby source zone. According to the model results, the suspected source zone location is highly probable and the Cr(VI) plume does not migrate significantly. Based on the above findings, it is believed that the remediation effort could potentially be very effective in the area, if the remediation plan is designed properly. In order to assess the remediation potential of (a) natural attenuation and (b) polyphenol-coated nZVI treatment, two model scenarios were created and their results were compared and discussed.

Experimental Investigation and Numerical Modeling of Enhanced DNAPL Solubilization in Saturated Porous Media

The accidental release of organic contaminants in the form of non-aqueous phase liquids (NAPLs) into the subsurface is a widespread and challenging environmental problem. Successful remediation of sites contaminated with NAPLs is essential for the protection of human health and the environment. One technology that has received significant attention is the injection of chemical additives (such as cosolvents) upgradient of the NAPL zone for the enhanced dissolution and mobilization of the NAPL mass. A key process influencing the effectiveness of NAPL mass recovery is the interphase mass transfer which is the transfer of components across the interface separating the different phases. In this work, we examine the impact of cosolvent content, flushing solution velocity, and injection pattern (continuous versus intermittent) on the interphase mass transfer rate. A series of flushing experiments were conducted using an intermediate-scale tank which allows for the impact of density variations on DNAPL mobility. The target DNAPL selected in this study was trichloroethylene while the flushing solutions consisted of ethanol–water mixtures with ethanol contents ranging from 0 to 50% by volume. The experimental results were also modeled using the UTCHEM multiphase flow simulator that was modified to model cosolvent flushing. Results show that the observed interphase mass transfer coefficient, expressed as a modified Sherwood number, was much lower than predicted based on published correlations developed under idealized conditions. Moreover, interphase mass transfer rate decreased with time, indicating that a single interphase mass transfer coefficient cannot accurately model the entire flushing solution. The data also suggest that the interphase mass transfer coefficient is dependent on cosolvent content.

Attenuation capacity of a coastal aquifer under managed recharge by reclaimed wastewater

Managed Aquifer Recharge (MAR) is becoming an increasingly attractive water management option, especially in semiarid areas. Nevertheless, field studies on the fate and transport of priority substances, heavy metals and pharmaceutical products within the recharged aquifer are rare. Based on the above, the objective of this project is to study the hydrological conditions of the coastal aquifer of Ezousa (Cyprus) and its ability to attenuate pollutants. The Ezousa riverbed is a locally important aquifer used for a MAR project where treated effluent from the Paphos Waste Water Treatment Plant is recharged into the aquifer through a number of artificial ponds along the riverbed. Additionally, groundwater is pumped for irrigation purposes from wells located nearby. The hydrological conditions of the area are unique due to the construction of the Kannaviou dam in 2005 that reduced natural recharge of the Ezousa aquifer significantly, inducing the saltwater intrusion phenomenon. A three-dimensional finite element model of the area was constructed using the FEFLOW software to simulate the groundwater flow conditions and transport of Phosphorous and cooper in the subsurface from the recharge process. The model was calibrated using hydraulic head and chemical data for the time period of 2002-2011. The groundwater model was coupled with a geochemical model PHREEQC attempting to evaluate nitrate and Copper processes. Inverse modeling calculation was used to determine sets of moles transfers of phases that are attributed to the water composition change in groundwater between the mixture of natural groundwater and reclaimed wastewater and the final water composition.