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Zoi Dokou

California State University, Sacramento, Civil Engineering, Faculty Member

Technical University of Crete, Environmental Engineering, Post-Doc

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I am currently an Assistant Professor at the Civil Engineering Department, California State University, Sacramento. My primary research interests revolve around the area of hydrogeology with focus on groundwater flow and contaminant transport and remediation. I specialize in employing, as well as developing, numerical, statistical and surrogate approaches to integrate complex system models with laboratory and field scale observations and optimization techniques. I am currently engaged in the development of scientific information, in the form of seasonal forecasting, and studying its adoption by local communities in the Blue Nile region, Ethiopia.

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Papers by Zoi Dokou

Saltwater intrusion in an irrigated agricultural area: combining density-dependent modeling and geophysical methods

by Pantelis M Soupios and Zoi Dokou

Saltwater intrusion is one of the most important water quality problems in coastal aquifers, espe... more Saltwater intrusion is one of the most important water quality problems in coastal aquifers, especially in areas with increased water demands. Geophysical techniques can provide a non-invasive and cost-effective approach for determining the geometrical characteristics of an aquifer and for guiding the saltwater intrusion modeling process and in turn reducing the model's inherent uncertainty. In this work, the above concept was applied in the Tympaki basin in Heraklion, Greece. The transient electromagnetic method was used to obtain an accurate 3-D geomodel (bedrock geometry and fault detection) of the basin. This, in turn, was used to guide the construction of a density-dependent groundwater flow and transport simulation model. The results show significant advancement of the saltwater intrusion front in the northern part of the study area, while the phenomenon is less pronounced in the central and southern parts. This is mainly attributed to the combined effect of the fault in the northern part of the basin, the uplifted Neogene deposits in the central part and the freshwater inflow from the Festos corridor in the southern part.

Groundwater level forecasting using artificial neural networks

by Paulin Coulibaly and Zoi Dokou

Journal of Hydrology, 2005

The purpose of this study is to examine the use of particle swarm optimization algorithm in order... more

Optimal Search Strategy for the Definition of a DNAPL Source

s u m m a r y Contamination created by dense non-aqueous phase liquids (DNAPLs) is a serious thre... more s u m m a r y Contamination created by dense non-aqueous phase liquids (DNAPLs) is a serious threat to the quality of the groundwater supply; thus, delineating and removing the DNAPL source is an essential step in a successful remediation strategy. The goal of this work is to create an optimal search strategy that identifies a DNAPL source using the least number of water quality samples.

Reviewing the basic design parameters affecting the disinfection of sand filters

Assessing groundwater quality in Greece based on spatial and temporal analysis

by Zoi Dokou, Nektarios Kourgialas, and George P Karatzas

Environmental Monitoring and Assessment, 2015

The recent industrial growth together with the urban expansion and intensive agriculture in Greec... more 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.

Large scale groundwater flow and hexavalent chromium transport modeling under current and future climatic conditions: the case of Asopos River Basin

by Zoi Dokou and George P Karatzas

Environmental Science and Pollution Research, 2015

In recent years, high concentrations of hexavalent chromium, Cr(VI), have been observed in the gr... more In recent years, high concentrations of hexavalent chromium, Cr(VI), have been observed in the groundwater system of the Asopos River Basin, raising public concern regarding the quality of drinking and irrigation water. The work described herein focuses on the development of a groundwater flow and Cr(VI) transport model using hydrologic, geologic, and water quality data collected from various sources. An important dataset for this goal comprised an extensive time series of Cr(VI) concentrations at various locations that provided an indication of areas of high concentration and also served as model calibration locations. Two main sources of Cr(VI) contamination were considered in the area: anthropogenic contamination originating from Cr-rich industrial wastes buried or injected into the aquifer and geogenic contamination from the leaching process of ophiolitic rocks. The aquifer&amp;amp;amp;amp;#39;s response under climatic change scenario A2 was also investigated for the next two decades. Under this scenario, it is expected that rainfall, and thus infiltration, will decrease by 7.7 % during the winter and 15 % during the summer periods. The results for two sub-scenarios (linear and variable precipitation reduction) that were implemented based on A2 show that the impact on the study aquifer is moderate, resulting in a mean level decrease less than 1 m in both cases. The drier climatic conditions resulted in higher Cr(VI) concentrations, especially around the industrial areas.

Modeling Ethanol-Enhanced Dnapl Remediation in a Two- Dimensional Experimental Setting

by Zoi Dokou, Nadim Copty, Derya AYDIN, and George P Karatzas

The accidental release of organic contaminants in the form of non-aqueous phase liquids (NAPLs) i... more The accidental release of organic contaminants in the form of non-aqueous phase liquids (NAPLs) is a widespread and challenging environmental problem. Successful remediation of sites contaminated with NAPLs is essential for the protection of human health. An effective remediation method is the injection of chemical additives (cosolvents) together with water upgradient of the NAPL-contaminated area (flushing). The additives alter the physio-chemical properties of NAPLs, such as interfacial tension, and enhance their solubilities. Solubilized contaminants are carried downgradient and collected through recovery wells. The goal of the work presented here was the development of a numerical model that simulates accurately the experimental results of a DNAPL (TCE) flushing process using ethanol as a cosolvent. The effect of different ethanol contents (0%, 20% and 50%) on the remediation process were tested and evaluated. Experiments were conducted using a two-dimensional sand box, to allow for the impact of density variations on DNAPL mobility. The experimental results were used to calibrate a multiphase flow model based on a modified version of the UTCHEM simulator that includes an interfacial tension calculation. This IFT method enables UTCHEM to accurately simulate the process of ethanol concentration-dependent interfacial tension lowering.

Modeling Groundwater and Nitrate Transport in the Upper Aquifer of the Coastal Area of the Corinthian Gulf

by Zoi Dokou and George P Karatzas

Cosolvent Flushing for Remediation of Groundwater Contaminated with DNAPL

by Zoi Dokou, Nadim Copty, Derya AYDIN, and George P Karatzas

The contamination of the subsurface by organic compounds in the form of non-aqueous phase liquids... more The contamination of the subsurface by organic compounds in the form of non-aqueous phase liquids (NAPLs) is among the most challenging environmental problems. Field investigations have shown that due to their low solubility and high interfacial tension, many NAPLs can persist in porous media for decades and even centuries. Among the more promising technologies for NAPL source zone remediation is in-situ cosolvent flushing which involves the injection of chemical agents for the enhanced solubilization and/or mobilization of the NAPL. In this study, we examined the significance of the cosolvent (ethanol) contact time on the DNAPL (Trichloroethylene-TCE) recovery from contaminated groundwater. Batch tests were first conducted to evaluate the operational parameters such as solubility, interfacial tension, and miscibility as a function of cosolvent content. Flushing experiments were performed using a two-dimensional tank (in the vertical plane with L= 80 cm, H=40 cm, and W=5 cm). The ef...

Stochastic optimization and numerical simulation for pumping management of the Hersonissos fresh water coastal aquifer in Crete

by Zoi Dokou, Yiannis Saridakis, and George P Karatzas

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

by Zoi Dokou and George P Karatzas

Hydrogeology Journal, 2015

Estimation of seawater intrusion front in a coastal karstified system using a density-dependent flow approach

by Zoi Dokou, Xrysoula Zax, and George P Karatzas

Proceedings of the 6th International Symposium on Enviornmental Hydraulics, Athens, Greece, 23-25 June 2010, 2010

The simulation of the contaminant transport to determine the penetration of seawater front is ext... more The simulation of the contaminant transport to determine the penetration of seawater front is extremely complex due to the uncertainty related to the depth and orientation of the fault network and also due to the diffusion phenomenon associated with the variable density between fresh and salt water. Furthermore, the pumping activity which takes place in the aquifer is variable upon demand and affects significantly the hydraulic characteristics of the aquifer during the simulation period. In this paper, using the FEFLOW numerical simulator a subsurface flow and contaminant transport model that estimates the penetration of seawater intrusion front for a karstified coastal aquifer in Crete, Greece was developed. The model takes into consideration the geomorphologic characteristics of the karstic system and the density-dependent groundwater flow. The model results showed that the connectivity of the fractures and the fluid density effects are important factors that drive the seawater intrusion front asymmetrically inland.

8. Dokou Z., and G.P. Karatzas, "Estimation of Saltwater Intrusion Front in a Coastal Karstified System using Density-Dependent Flow and Comparison with the Traditional Ghyben-Herzberg Approach", Hydrological Sciences, DOI: 10.1080/02626667.2012.690070, Vol. 57(5), 985-999, 2012

Saltwater intrusion is a naturally occurring phenomenon that is exacerbated significantly by exce... more 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 act...

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

by George P Karatzas and Zoi Dokou

Journal of environmental management, 2015

The purpose of this study was to create a modeling management tool for the simulation of extreme ... more 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 t...

Olive Oil Mill Waste Transport in Unsaturated and Saturated Laboratory Conditions using Geophysical Techniques and the FEFLOW Model

by Zoi Dokou, Apostolos Sarris, Pantelis M Soupios, Nektarios Kourgialas, and George P Karatzas

Groundwater Level Prediction under various Climate Change Scenarios using an Artificial Neural Network Trained with Particle Swarm Optimization Algorithm

by Zoi Dokou and George P Karatzas

Hydrological Sciences Journal/Journal des Sciences Hydrologiques

Artificial Neural Networks (ANNs) have been lately used to predict the hydraulic head in well loc... more Artificial Neural Networks (ANNs) have been lately used to predict the hydraulic head in well locations. In the present work, Particle Swarm Optimization (PSO) algorithm is used to train a feed-forward multi-layer ANN for the simulation of hydraulic head change at an observation well at the region of Agia, Chania, Greece. Three variants of the particle swarm optimization algorithm are considered, the classic one with the inertia weight improvement, PSO-TVAC and 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 midterm 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.

Mapping of Chromium in the Greater Area of Asopos Basin

by Nicolas Kalogerakis, George P Karatzas, and Zoi Dokou

The greater area of the Asopos river, situated at the Region of Sterea Ellada, north of Athens is... more The greater area of the Asopos river, situated at the Region of Sterea Ellada, north of Athens is a unique case of combined anthropogenic and geogenic origin of chromium contamination. The high concentrations of chromium and hexavalent chromium found at the groundwater system of Asopos have resulted in an increased public concern, since part of the groundwater is used for water abstraction for human consumption and most of it for irrigation purposes. The objective of this work was to organize all available data pertinent to the groundwater flow and chromium transport modeling of the area into maps, using geographical information systems, geotechnical graphics software and geostatistical techniques. The data was collected from various sources such as field campaigns, existing reports, data logs etc. One of the most important parameters that affect chromium transport is the geology of the area, thus a digitized geological map of the main formations encountered in the upper layers of the subsurface of the region was generated. In addition, geological boring logs for a large number of wells were created (using a geotechnical graphics software) and then combined in order to create vertical cross-sections, at various locations in the greater area, that define the geological characteristics of the deeper layers. The combination of the above defines the 3-D representation of the geological stratification of the physical system. Another parameter of interest in the area is the groundwater level and flow direction. In order to create a hydraulic head map of the area, the locations of more than 1000 shallow and deep wells that have been documented in the greater area of the Asopos river basin were defined using GIS software. A kriging technique was then applied to the available data in order to produce hydraulic head contours and consequently define the general groundwater flow direction. The same technique was applied for the generation of chromium concentration contours that provide an indication of the most problematic locations as well as a general chromium background level in the greater area.

MAPPING OF CHROMIUM IN THE GREATER AREA OF ASOPOS RIVER BASIN

by Nicolas Kalogerakis, George P Karatzas, and Zoi Dokou

Spatial and Temporal Distribution of Groundwater Contamination in Greece Based on Historical Data

by Zoi Dokou, Nektarios Kourgialas, and George P Karatzas

ABSTRACT The recent urban expansion together with the high population density and industrial deve... more ABSTRACT The recent urban expansion together with the high population density and industrial development has increased groundwater contamination considerably in European countries. Greece is not an exception, facing serious groundwater contamination problems 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 was used in order to generate groundwater contamination maps for various contaminants. Based on these records, the temporal and spatial distribution of groundwater contamination was studied 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. Moreover, the seasonal distribution of the pollutants was investigated for the study area. The resulting groundwater contamination maps could be used in turn as a useful tool for water policy makers and water managers in order to assist the decision making process.

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

by Zoi Dokou and George P Karatzas

Proceedings of SPIE - The International Society for Optical Engineering, 2013

Managed Aquifer Recharge (MAR) is becoming an increasingly attractive water management option, es... more Managed Aquifer Recharge (MAR) is becoming an increasingly attractive water management option, especially in semi-arid 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.

Saltwater intrusion in an irrigated agricultural area: combining density-dependent modeling and geophysical methods

by Pantelis M Soupios and Zoi Dokou

Saltwater intrusion is one of the most important water quality problems in coastal aquifers, espe... more Saltwater intrusion is one of the most important water quality problems in coastal aquifers, especially in areas with increased water demands. Geophysical techniques can provide a non-invasive and cost-effective approach for determining the geometrical characteristics of an aquifer and for guiding the saltwater intrusion modeling process and in turn reducing the model's inherent uncertainty. In this work, the above concept was applied in the Tympaki basin in Heraklion, Greece. The transient electromagnetic method was used to obtain an accurate 3-D geomodel (bedrock geometry and fault detection) of the basin. This, in turn, was used to guide the construction of a density-dependent groundwater flow and transport simulation model. The results show significant advancement of the saltwater intrusion front in the northern part of the study area, while the phenomenon is less pronounced in the central and southern parts. This is mainly attributed to the combined effect of the fault in the northern part of the basin, the uplifted Neogene deposits in the central part and the freshwater inflow from the Festos corridor in the southern part.

Groundwater level forecasting using artificial neural networks

by Paulin Coulibaly and Zoi Dokou

Journal of Hydrology, 2005

The purpose of this study is to examine the use of particle swarm optimization algorithm in order... more

Optimal Search Strategy for the Definition of a DNAPL Source

s u m m a r y Contamination created by dense non-aqueous phase liquids (DNAPLs) is a serious thre... more s u m m a r y Contamination created by dense non-aqueous phase liquids (DNAPLs) is a serious threat to the quality of the groundwater supply; thus, delineating and removing the DNAPL source is an essential step in a successful remediation strategy. The goal of this work is to create an optimal search strategy that identifies a DNAPL source using the least number of water quality samples.

Reviewing the basic design parameters affecting the disinfection of sand filters

Assessing groundwater quality in Greece based on spatial and temporal analysis

by Zoi Dokou, Nektarios Kourgialas, and George P Karatzas

Environmental Monitoring and Assessment, 2015

The recent industrial growth together with the urban expansion and intensive agriculture in Greec... more 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.

Large scale groundwater flow and hexavalent chromium transport modeling under current and future climatic conditions: the case of Asopos River Basin

by Zoi Dokou and George P Karatzas

Environmental Science and Pollution Research, 2015

In recent years, high concentrations of hexavalent chromium, Cr(VI), have been observed in the gr... more In recent years, high concentrations of hexavalent chromium, Cr(VI), have been observed in the groundwater system of the Asopos River Basin, raising public concern regarding the quality of drinking and irrigation water. The work described herein focuses on the development of a groundwater flow and Cr(VI) transport model using hydrologic, geologic, and water quality data collected from various sources. An important dataset for this goal comprised an extensive time series of Cr(VI) concentrations at various locations that provided an indication of areas of high concentration and also served as model calibration locations. Two main sources of Cr(VI) contamination were considered in the area: anthropogenic contamination originating from Cr-rich industrial wastes buried or injected into the aquifer and geogenic contamination from the leaching process of ophiolitic rocks. The aquifer&amp;amp;amp;amp;#39;s response under climatic change scenario A2 was also investigated for the next two decades. Under this scenario, it is expected that rainfall, and thus infiltration, will decrease by 7.7 % during the winter and 15 % during the summer periods. The results for two sub-scenarios (linear and variable precipitation reduction) that were implemented based on A2 show that the impact on the study aquifer is moderate, resulting in a mean level decrease less than 1 m in both cases. The drier climatic conditions resulted in higher Cr(VI) concentrations, especially around the industrial areas.

Modeling Ethanol-Enhanced Dnapl Remediation in a Two- Dimensional Experimental Setting

by Zoi Dokou, Nadim Copty, Derya AYDIN, and George P Karatzas

The accidental release of organic contaminants in the form of non-aqueous phase liquids (NAPLs) i... more The accidental release of organic contaminants in the form of non-aqueous phase liquids (NAPLs) is a widespread and challenging environmental problem. Successful remediation of sites contaminated with NAPLs is essential for the protection of human health. An effective remediation method is the injection of chemical additives (cosolvents) together with water upgradient of the NAPL-contaminated area (flushing). The additives alter the physio-chemical properties of NAPLs, such as interfacial tension, and enhance their solubilities. Solubilized contaminants are carried downgradient and collected through recovery wells. The goal of the work presented here was the development of a numerical model that simulates accurately the experimental results of a DNAPL (TCE) flushing process using ethanol as a cosolvent. The effect of different ethanol contents (0%, 20% and 50%) on the remediation process were tested and evaluated. Experiments were conducted using a two-dimensional sand box, to allow for the impact of density variations on DNAPL mobility. The experimental results were used to calibrate a multiphase flow model based on a modified version of the UTCHEM simulator that includes an interfacial tension calculation. This IFT method enables UTCHEM to accurately simulate the process of ethanol concentration-dependent interfacial tension lowering.

Modeling Groundwater and Nitrate Transport in the Upper Aquifer of the Coastal Area of the Corinthian Gulf

by Zoi Dokou and George P Karatzas

Cosolvent Flushing for Remediation of Groundwater Contaminated with DNAPL

by Zoi Dokou, Nadim Copty, Derya AYDIN, and George P Karatzas

The contamination of the subsurface by organic compounds in the form of non-aqueous phase liquids... more The contamination of the subsurface by organic compounds in the form of non-aqueous phase liquids (NAPLs) is among the most challenging environmental problems. Field investigations have shown that due to their low solubility and high interfacial tension, many NAPLs can persist in porous media for decades and even centuries. Among the more promising technologies for NAPL source zone remediation is in-situ cosolvent flushing which involves the injection of chemical agents for the enhanced solubilization and/or mobilization of the NAPL. In this study, we examined the significance of the cosolvent (ethanol) contact time on the DNAPL (Trichloroethylene-TCE) recovery from contaminated groundwater. Batch tests were first conducted to evaluate the operational parameters such as solubility, interfacial tension, and miscibility as a function of cosolvent content. Flushing experiments were performed using a two-dimensional tank (in the vertical plane with L= 80 cm, H=40 cm, and W=5 cm). The ef...

Stochastic optimization and numerical simulation for pumping management of the Hersonissos fresh water coastal aquifer in Crete

by Zoi Dokou, Yiannis Saridakis, and George P Karatzas

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

by Zoi Dokou and George P Karatzas

Hydrogeology Journal, 2015

Estimation of seawater intrusion front in a coastal karstified system using a density-dependent flow approach

by Zoi Dokou, Xrysoula Zax, and George P Karatzas

Proceedings of the 6th International Symposium on Enviornmental Hydraulics, Athens, Greece, 23-25 June 2010, 2010

The simulation of the contaminant transport to determine the penetration of seawater front is ext... more The simulation of the contaminant transport to determine the penetration of seawater front is extremely complex due to the uncertainty related to the depth and orientation of the fault network and also due to the diffusion phenomenon associated with the variable density between fresh and salt water. Furthermore, the pumping activity which takes place in the aquifer is variable upon demand and affects significantly the hydraulic characteristics of the aquifer during the simulation period. In this paper, using the FEFLOW numerical simulator a subsurface flow and contaminant transport model that estimates the penetration of seawater intrusion front for a karstified coastal aquifer in Crete, Greece was developed. The model takes into consideration the geomorphologic characteristics of the karstic system and the density-dependent groundwater flow. The model results showed that the connectivity of the fractures and the fluid density effects are important factors that drive the seawater intrusion front asymmetrically inland.

8. Dokou Z., and G.P. Karatzas, "Estimation of Saltwater Intrusion Front in a Coastal Karstified System using Density-Dependent Flow and Comparison with the Traditional Ghyben-Herzberg Approach", Hydrological Sciences, DOI: 10.1080/02626667.2012.690070, Vol. 57(5), 985-999, 2012

Saltwater intrusion is a naturally occurring phenomenon that is exacerbated significantly by exce... more 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 act...

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

by George P Karatzas and Zoi Dokou

Journal of environmental management, 2015

The purpose of this study was to create a modeling management tool for the simulation of extreme ... more 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 t...

Olive Oil Mill Waste Transport in Unsaturated and Saturated Laboratory Conditions using Geophysical Techniques and the FEFLOW Model

by Zoi Dokou, Apostolos Sarris, Pantelis M Soupios, Nektarios Kourgialas, and George P Karatzas

Groundwater Level Prediction under various Climate Change Scenarios using an Artificial Neural Network Trained with Particle Swarm Optimization Algorithm

by Zoi Dokou and George P Karatzas

Hydrological Sciences Journal/Journal des Sciences Hydrologiques

Artificial Neural Networks (ANNs) have been lately used to predict the hydraulic head in well loc... more Artificial Neural Networks (ANNs) have been lately used to predict the hydraulic head in well locations. In the present work, Particle Swarm Optimization (PSO) algorithm is used to train a feed-forward multi-layer ANN for the simulation of hydraulic head change at an observation well at the region of Agia, Chania, Greece. Three variants of the particle swarm optimization algorithm are considered, the classic one with the inertia weight improvement, PSO-TVAC and 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 midterm 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.

Mapping of Chromium in the Greater Area of Asopos Basin

by Nicolas Kalogerakis, George P Karatzas, and Zoi Dokou

The greater area of the Asopos river, situated at the Region of Sterea Ellada, north of Athens is... more The greater area of the Asopos river, situated at the Region of Sterea Ellada, north of Athens is a unique case of combined anthropogenic and geogenic origin of chromium contamination. The high concentrations of chromium and hexavalent chromium found at the groundwater system of Asopos have resulted in an increased public concern, since part of the groundwater is used for water abstraction for human consumption and most of it for irrigation purposes. The objective of this work was to organize all available data pertinent to the groundwater flow and chromium transport modeling of the area into maps, using geographical information systems, geotechnical graphics software and geostatistical techniques. The data was collected from various sources such as field campaigns, existing reports, data logs etc. One of the most important parameters that affect chromium transport is the geology of the area, thus a digitized geological map of the main formations encountered in the upper layers of the subsurface of the region was generated. In addition, geological boring logs for a large number of wells were created (using a geotechnical graphics software) and then combined in order to create vertical cross-sections, at various locations in the greater area, that define the geological characteristics of the deeper layers. The combination of the above defines the 3-D representation of the geological stratification of the physical system. Another parameter of interest in the area is the groundwater level and flow direction. In order to create a hydraulic head map of the area, the locations of more than 1000 shallow and deep wells that have been documented in the greater area of the Asopos river basin were defined using GIS software. A kriging technique was then applied to the available data in order to produce hydraulic head contours and consequently define the general groundwater flow direction. The same technique was applied for the generation of chromium concentration contours that provide an indication of the most problematic locations as well as a general chromium background level in the greater area.

MAPPING OF CHROMIUM IN THE GREATER AREA OF ASOPOS RIVER BASIN

by Nicolas Kalogerakis, George P Karatzas, and Zoi Dokou

Spatial and Temporal Distribution of Groundwater Contamination in Greece Based on Historical Data

by Zoi Dokou, Nektarios Kourgialas, and George P Karatzas

ABSTRACT The recent urban expansion together with the high population density and industrial deve... more ABSTRACT The recent urban expansion together with the high population density and industrial development has increased groundwater contamination considerably in European countries. Greece is not an exception, facing serious groundwater contamination problems 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 was used in order to generate groundwater contamination maps for various contaminants. Based on these records, the temporal and spatial distribution of groundwater contamination was studied 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. Moreover, the seasonal distribution of the pollutants was investigated for the study area. The resulting groundwater contamination maps could be used in turn as a useful tool for water policy makers and water managers in order to assist the decision making process.

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

by Zoi Dokou and George P Karatzas

Proceedings of SPIE - The International Society for Optical Engineering, 2013

Managed Aquifer Recharge (MAR) is becoming an increasingly attractive water management option, es... more Managed Aquifer Recharge (MAR) is becoming an increasingly attractive water management option, especially in semi-arid 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.