Planning and implementing environmental policies for the sustainable management of water resource... more Planning and implementing environmental policies for the sustainable management of water resources is a challenging task. In order to improve the effectiveness of these policies it is essential to explore their social implications. The present article aims to investigate environmental policies focusing on domestic water conservation and their interconnection with social capital elements. In particular, by means of an empirical study conducted in an insular community of Greece, citizens' perceptions are explored concerning the restrictions imposed from different environmental policy instruments for water consumption and their perceived level of effectiveness. Furthermore, the influence of social capital parameters on these perceptions is investigated. Aggregated indicators of social capital are estimated with Confirmatory Factor Analysis measuring social and institutional trust, participation in social networks and compliance with social norms. Through the results of ordinal regression models it is evident that significant connections exist between elements of social capital and perceptions of citizens towards water consumption policies.
A geochemical analysis and modelling was carried out to investigate the As occurrence and release... more A geochemical analysis and modelling was carried out to investigate the As occurrence and release in groundwater from two different geological environments on Lesvos Island: (i) the volcanic area of Mandamados (ignimbrite of rhyolithic to rhyodacitic composition) and (ii) the metamorphic area of Tarti (schists and marbles) that comprises the geologic basement under ignimbrite. Seven sampling campaigns were conducted between October 2010 and October 2011, including 65 groundwater samples from 11 wells and springs. Chemical analyses showed As concentrations exceeding the 10-μg/L national drinking water limit in 46 % of the samples from Mandamados. Groundwater composition in Mandamados evolved from Ca-HCO 3 type, to mixed type and finally to Na-Cl type along the groundwater flow direction, indicating the contribution of ion exchange in groundwater chemical composition, while Ca-HCO 3 type waters were observed in the Tarti area. Arsenic speciation analysis showed that As(V) was the main species in all samples, indicating that As was released under oxidizing conditions. Statistical analysis suggested silicate weathering as the prime mechanism of As release in groundwater in both cases, while, in the Tarti area, carbonate dissolution may represent a secondary mechanism which could be related to the observed relatively low As concentrations in the region. In both areas, pH-related desorption of As, primarily from Fe mineral phases, was found to be the most important factor controlling the mobilisation of As, while the contribution of the redox control to As release in groundwater was generally found to be less significant.
Using composite constituents to simulate the average behavior and fate of groups of individual hy... more Using composite constituents to simulate the average behavior and fate of groups of individual hydrocarbons in the subsurface may provide the means for dealing with mixtures of tens or hundreds of individual hydrocarbons while reducing the computational cost. There are two additional sources of uncertainty associated with this technique, namely, the time dependence of the effective properties of a composite constituent due to temporal composition changes, and the approximation in determining their effective property values. In this work, an approach to limit these sources of uncertainty is described and certified with published experimental data, that involved an artificially mixed kerosene. An inverse analysis is used to estimate the effective property values of composite constituents, and a methodology is introduced for optimizing component grouping, based on the minimization of an objective function. Aqueous solubility is found to be the optimal grouping criterion for the multicomponent organic mixture considered here. Under the assumption of equilibrium partitioning among phases, the effective property values of the composite constituents are expressed in terms of the property values of the member components using a step-wise inverse analysis. Simple arithmetic averages are found to provide poor approximations to the effective group properties and alternative schemes are proposed, based on experimental data.
Proceedings of the 16th International Conference on Environmental Science and Technology, Nov 19, 2022
In the present study, a geochemical analysis was conducted to investigate the As occurrence and r... more In the present study, a geochemical analysis was conducted to investigate the As occurrence and release in groundwater from two different geological environments on Lesvos island: (i) the volcanic area of Mandamados (ignimbrite) and (ii) the metamorphic area of Tarti that comprises the geologic basement under ignimbrite. Seven sampling campaigns were conducted between October 2010 and October 2011 including 65 groundwater samples from 11 wells and springs. Chemical analyses showed As concentrations exceeding the 10 μg/L national drinking water limit in 46% of the samples from Mandamados. Groundwater composition in Mandamados evolved from Ca-HCO3 type, to mixed type and finally to Na-Cl type along the groundwater flow direction, indicating the contribution of ion exchange in groundwater chemical composition, while Ca-HCO3 type waters were observed in the Tarti area. Arsenic speciation analysis showed that As(V) was the main species in all samples, indicating that As was released under oxidizing conditions. Statistical analysis suggested silicate weathering as the prime mechanism of As release in groundwater in both cases, while, in the Tarti area, carbonate dissolution may represent a secondary mechanism which could be related to the observed relatively low As concentrations in the region. In both areas, pH related desorption of As, primarily from Fe mineral phases, was found to be the most important factor controlling the mobilisation of As, while the contribution of the redox control to As release in groundwater was generally found to be less significant.
In earth and environmental sciences applications, uncertainty analysis regarding the outputs of m... more In earth and environmental sciences applications, uncertainty analysis regarding the outputs of models whose parameters are spatially varying (or spatially distributed) is often performed in a Monte Carlo framework. In this context, alternative realizations of the spatial distribution of model inputs, typically conditioned to reproduce attribute values at locations where measurements are obtained, are generated via geostatistical simulation using simple random (SR) sampling. The environmental model under consideration is then evaluated using each of these realizations as a plausible input, in order to construct a distribution of plausible model outputs for uncertainty analysis purposes. In hydrogeological investigations, for example, conditional simulations of saturated hydraulic conductivity are used as input to physically-based simulators of flow and transport to evaluate the associated uncertainty in the spatial distribution of solute concentration. Realistic uncertainty analysis via SR sampling, however, requires a large number of simulated attribute realizations for the model inputs in order to yield a representative distribution of model outputs; this often hinders the application of uncertainty analysis due to the computational expense B Stelios Liodakis
... Petros Gaganis and Leslie Smith Department of Earth and Ocean Sciences, University of British... more ... Petros Gaganis and Leslie Smith Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada ... Insight gained in updating the prior information on the model parameters is used to assess the correctness of the model structure, which is defined ...
Abstract Geostatistical simulation using controlled or stratified sampling methods, namely Latin ... more Abstract Geostatistical simulation using controlled or stratified sampling methods, namely Latin hypercube and stratified likelihood sampling, are capable of generating representative realizations from (log)Gaussian random fields, i.e., spanning efficiently the range of values corresponding to the (log)Gaussian multivariate probability distribution. Although such realizations often serve as parameters for physical process simulators, existing controlled sampling methods do not account for model sensitivity; hence, they need not yield representative realizations of model outputs. To address this shortcoming, controlled sampling methods are embedded within a two-step simulation procedure. The first step involves stratified sampling at a set of control points where attribute values are expected to exert a large impact on model predictions and/or where uncertainty in such predictions is expected to be largest. In the second step, control point samples are used to generate attribute realizations over the entire study region using classical geostatistical simulation. The application of the proposed controlled, two-step, geostatistical simulation procedure is illustrated in a hydrogeological context via a synthetic case study involving physically-based simulation of flow and transport in a porous medium with known boundary and initial conditions over a simple geometrical domain.
... Petros Gaganis a , Peter Kjeldsen b and Vasilis N. Burganos a ,* a Institute of Chemical Engi... more ... Petros Gaganis a , Peter Kjeldsen b and Vasilis N. Burganos a ,* a Institute of Chemical Engineering and High Temperature Chemical ProcessesFoundation for Research and Technology, Hellas (ICE/HT-FORTH), PO Box 1414, GR 26504 Patras, Greece b Environment ...
Proceedings of the 16th International Conference on Environmental Science and Technology, Nov 19, 2022
A research conducted in water wells located in the rhyolithic volcanic area of Mandamados, Lesvos... more A research conducted in water wells located in the rhyolithic volcanic area of Mandamados, Lesvos Island, Greece, indicated that significant seasonal variation of arsenic concentration in groundwater exists mainly in wells near the coastal zone. However, there were differences among those coastal wells with regard to the processes and factors responsible for the observed seasonal variability of the element, although they are all located in a small homogeneous area. On the other hand, in wells located in higher relief regions, the concentration of As in groundwater followed a fairly constant pattern throughout the year, which is probably related to the faster groundwater flow in this region due to a higher hydraulic gradient. In general, seasonal variation of As in groundwater in the study area was found to be related to geology, recharge rate, topography-distance from coast, and well depth.
Two recently developed approaches to quantification of model (conceptual) error in a single groun... more Two recently developed approaches to quantification of model (conceptual) error in a single groundwater model, a per-datum calibration methodology and a Bayesian model error analysis, were applied to a problem of 90 Sr migration to water wells at Chernobyl, Ukraine. The intent of this composition is to demonstrate their utility to accounting for the uncertainty due to model error in estimating risks (or costs) in decision models. Bayesian model error analysis resulted in a more conservative estimate of the probability of the Pripyat Town well field contamination than did the per-datum calibration approach. This difference in risk estimates is a result of the conceptual differences between the two methods. Per-datum calibration relies primarily on information on model error contained in the measurements of the dependent variables to quantify its effect on model predictions. The Bayesian model error analysis assigns equal importance to prior information on the parameters and measurements of the dependent variable, thus allowing the incorporation of a more informative description of parameter distributions, as well as subjective judgement into a risk analysis. The suitability of either of the two methods, when applied to a specific problem, may be determined based on the nature and quantity of available data.
Environmental Monitoring and Assessment, Dec 23, 2017
A survey conducted in water wells located in the rhyolithic volcanic area of Mandamados, Lesvos I... more A survey conducted in water wells located in the rhyolithic volcanic area of Mandamados, Lesvos Island, Greece, indicated that significant seasonal variation of arsenic concentration in groundwater exists mainly in wells near the coastal zone. However, there were differences among those coastal wells with regard to the processes and factors responsible for the observed seasonal variability of the element, although they are all located in a small homogeneous area. These processes and factors include (a) a higher rate of silicate weathering and ion exchange during the dry period followed by the dilution by the recharge water during the wet period, (b) enhanced desorption promoted by higher pH in summer and subsequent dilution of As by rainwater infiltration during the wet period, and (c) reductive dissolution of Mn during the wet period and by desorption under high pH values during the dry period. On the other hand, in wells located in higherrelief regions, the concentration of As in groundwater followed a fairly constant pattern throughout the year, which is probably related to the faster flow of groundwater in this part of the area due to a higher hydraulic gradient. In general, seasonal variation of As in groundwater in the study area was found to be related to geology, recharge rate, topography-distance from coast, and well depth.
The effect of systematic model error on the model predictions varies in space and time, and diffe... more The effect of systematic model error on the model predictions varies in space and time, and differs for the flow and solute trans port components of a groundwater model. The classical single-objective formulation of the inverse problem by its nature cannot capture these characteristics of model error. We introduce an inverse approach that allows the spatial and temporal variability of model error to be evaluated in the parameter space. A set of solutions for model parameters are obtained by this new method that almost exactly satisfies the model equation at each observation point (per-datum calibration). This set of parameter estimates are then used to define a posterior parameter space that may be translated into a probabilistic description of model output to rep resent the level of confidence in model performance. It is shown that this approach can provide useful information regarding the strengths and limitations of a model as well as the performance of classical calibration procedures.
Despite the growing discussion on vulnerability and adaptation in urban areas, there is limited r... more Despite the growing discussion on vulnerability and adaptation in urban areas, there is limited research on how smaller towns and cities in Small Island Developing States are being affected by and responding to climate change impacts. This study uses fuzzy cognitive mapping (FCM), field visits and semi-structured interviews with 40 stakeholders across 6 different stakeholder groups in the Nadi River Basin, Fiji Islands to identify, analyse and deconstruct climate change vulnerability and adaptation options to manage increasing flood risks. The research evidence suggests that vulnerability to floods in the basin is on the rise due to a complex mesh of three intersecting factors. Firstly, non-climatic pressures such as development, drainage, social change, agriculture, tourism growth and deforestation combine, juxtapose and interact in a rather unique way with global climate variability (interdependent systems) to increase the stress on the river and coastal ecosystems. Secondly, the most vulnerable or at-risk populations like the farmers, squatter households and in particular women within the community have weak coping capacity due to a combination of demographic and social characteristics. Thirdly, vulnerability is on the rise due to climate factors as well as the flurry of unplanned development, redevelopment and degradation of catchment resources. The research findings have implications for adaptation policies. In particular, the basin stakeholders should integrate climate change within sectorial planning processes, actively engage the vulnerable groups, promote knowledge, awareness and social learning, and invest in adaptive management across all levels of decision-making. Structural policy changes to land-use planning and insurance financing schemes are also necessary to address growing risks. These have the potential to enhance local capacities of communities to adapt to climate-induced floods and improve ecosystem integrity for resilience building.
ABSTRACT A major difficulty that is typically encountered when real contamination problems are in... more ABSTRACT A major difficulty that is typically encountered when real contamination problems are investigated is the typically large number of constituents in the actual VOC mixture compared to the number of species that can be handled numerically. An approach to dealing with complex mixtures of individual hydrocarbons while reducing the computational cost is to define a small number of effective composite constituents to simulate the average behavior and fate of groups of individual hydrocarbons in the subsurface. However, there are two additional sources of uncertainty associated with this technique that may influence the reliability of model predictions, namely, the time dependence of the effective properties of a composite constituent due to temporal composition changes, and the approximation in determining their effective property values. An approach to limit these sources of uncertainty was developed recently by the authors based on experimental data from a lysimeter study that involved artificially mixed kerosene. The minimization of the time dependence of the effective thermodynamic properties of the composite constituents is achieved through a methodology for optimizing component grouping, based on the minimization of an objective function. Simple arithmetic averages are found to provide poor approximations to the effective group properties and alternative algorithms are proposed to express the effective property values of composite constituents in terms of property values of the member components. The applicability of this approach to large-scale contamination problems is demonstrated here using data from field studies. Compared to the case of simulating all organics as individual compounds, our approach offers the advantage of a greatly reduced computational cost, reduced data requirements, as well as a smaller impact of the uncertainty in the compound properties on modeling results because effective values are used for each composite constituent, which follow a pdf with reduced variance compared to that of the individual values.
ABSTRACT A computationally cost effective approach to modeling transport and fate of a mixture of... more ABSTRACT A computationally cost effective approach to modeling transport and fate of a mixture of a large number of volatile organic contaminants consists of simulating specific target contaminants as individual constituents while the influence of the mixture composition on the transport process is taken into account by defining a small number of composite constituents to represent the cumulative effect of groups of individual hydrocarbons. Compared to the case of individual compounds, there are two additional sources of uncertainty associated with this approach, namely, the identification of the effective thermodynamic property values of a composite constitutent, and their time dependence due to temporal composition variability. In this work, an approach to limit these sources of uncertainty is described and demonstrated with data from a lysimeter experiment, that involved an artificially mixed kerosene. The mixture component that represents the main threat of groundwater contamination is selected as a target contaminant while the rest of the hydrocarbons are grouped into four composite constitutents. The minimization of the time dependence of the effective thermodynamic properties of the composite constituents is achieved through a selection of an appropriate grouping criterion. A methodology that is based on the minimization of an objective function is introduced for selecting the optimal grouping criterion, which may be different for different hydrogeologic settings and compositions of the organic mixture. Given the assumption of equilibrium partitioning among phases, the appropriate effective property values of the composite constituents in relation to the property values of the member hydrocarbons are then identified through a step-wise inverse analysis.
Petroleum products involve mixtures of different volatile organic compounds (VOCs) that may repre... more Petroleum products involve mixtures of different volatile organic compounds (VOCs) that may represent a risk for groundwater after a spill to the unsaturated zone. However, the majority of the published approaches and codes for unsaturated zone modeling handle only either a single or a limited number of compounds in order to cope with the complexity of processes involved in VOC attenuation. This work aimed at simulating complex VOC scenarios using a grouping procedure, which results in the definition of a small number of effective pseudospecies with carefully specified effective properties. Grouping criteria are developed for a hypothetical scenario that involves a mixture of seven VOCs with properties covering a wide range of values. These criteria are assessed through a comparison of group-based calculations with rigorous calculations using the individual components. Groups of species with similar solubility resulted in concentration profiles that compared very satisfactorily with the rigorous results. The approach is validated with data from a large-scale lysimeter experiment. The simulation results compare satisfactorily with the experimental data for several days and for the vast majority of the mixture components. The agreement was significantly improved when temporally varying solubility values were used, justified by the difference in the compound volatility and the concomitant alteration of the VOC mixture composition with time.
High-resolution single source-solute transport experiments in glass-etched pore networks presente... more High-resolution single source-solute transport experiments in glass-etched pore networks presented in a previous publication are used for quantifying the hydrodynamic dispersion as a function of Peclet number (Pe). This type of artificial porous media is ideal for understanding the effects of pore-scale phenomena on solute dispersion thanks to its well characterized topology and pore geometry. The hydrodynamic dispersion coefficients are estimated by matching the spatial and temporal distributions of the solute concentration over various regions of the network with the numerical solution of the advection-dispersion equation and using a parameter space analysis to ensure well conditioning of the parameters. The source release function and assumptions on boundary conditions are found to affect significantly the reliability of the dispersion coefficient estimates. The estimated longitudinal dispersion coefficients are in close quantitative agreement with literature data for dispersion in porous media. The extracted transverse dispersivity (a T) values indicate an apparent decrease of a T with increasing flow velocity, which is at variance of constant dispersivity approximation adopted commonly in soil hydrology models and is attributed to incomplete diffusive mixing at the pore intersections. Calculated values of an effective mixing ratio at pore junctions are in agreement with published theoretical estimates and reveal a transition from partial mixing to streamline routing as Pe increases within the range investigated here.
Typical contaminants found in the unsaturated zone (diesel oil, unleaded gasoline, etc.) are mixt... more Typical contaminants found in the unsaturated zone (diesel oil, unleaded gasoline, etc.) are mixtures of several volatile organic compounds (e.g., toluene, xylene, etc.) having variable properties. A large number of models and codes dealing with the description of contaminant attenuation in the subsurface have appeared in the literature. In the present work, several representative codes capable of unsaturated zone modeling are presented. Only a limited number of these codes consider vapor phase diffusion, despite its significance in practical applications. These codes are discussed here in more detail. Three of these codes treat contaminants as mixtures of more than one compound with variable properties. However, only two of them include degradation, which is also an important attenuation mechanism. New approaches trying to overcome code limitations-in terms of compound number they can handle-are also discussed.
A field experiment was conducted in Denmark in order to evaluate the fate of 13 volatile organic ... more A field experiment was conducted in Denmark in order to evaluate the fate of 13 volatile organic compounds (VOCs) that were buried as an artificial fuel source in the unsaturated zone. Compound-specific isotope analysis showed distinct phases in the 13 C/ 12 C ratio evolution in VOC vapors within 3 m from the source over 114 days. At day 3 and to a lesser extent at day 6, the compounds were depleted in 13 C by up to-5.7‰ with increasing distance from the source compared to the initial source values. This trend can be explained by faster outward diffusion of the molecules with 12 C only compared to molecules with a 13 C. Then, the isotope profile leveled out, and several compounds started to become enriched in 13 C by up to 9.5‰ with increasing distance from the source, due to preferential removal of the molecules with 12 C only, through biodegradation. Finally, as the amount of a compound diminished in the source, a 13 C enrichment was also observed close to the source. The magnitude of isotope fractionation tended to be larger the smaller the mass of the molecule was. This study demonstrates that, in the unsaturated zone, carbon isotope ratios of hydrocarbons are affected by gas-phase diffusion in addition to biodegradation, which was confirmed using a numerical model. Gas-phase diffusion led to shifts in δ 13 C >1‰ during the initial days after the spill, and again during the final stages of source volatilization after >75% of a compound had been removed. In between, diffusion has less of an effect, and thus isotope data can be used as an indicator for hydrocarbon biodegradation.
Planning and implementing environmental policies for the sustainable management of water resource... more Planning and implementing environmental policies for the sustainable management of water resources is a challenging task. In order to improve the effectiveness of these policies it is essential to explore their social implications. The present article aims to investigate environmental policies focusing on domestic water conservation and their interconnection with social capital elements. In particular, by means of an empirical study conducted in an insular community of Greece, citizens' perceptions are explored concerning the restrictions imposed from different environmental policy instruments for water consumption and their perceived level of effectiveness. Furthermore, the influence of social capital parameters on these perceptions is investigated. Aggregated indicators of social capital are estimated with Confirmatory Factor Analysis measuring social and institutional trust, participation in social networks and compliance with social norms. Through the results of ordinal regression models it is evident that significant connections exist between elements of social capital and perceptions of citizens towards water consumption policies.
A geochemical analysis and modelling was carried out to investigate the As occurrence and release... more A geochemical analysis and modelling was carried out to investigate the As occurrence and release in groundwater from two different geological environments on Lesvos Island: (i) the volcanic area of Mandamados (ignimbrite of rhyolithic to rhyodacitic composition) and (ii) the metamorphic area of Tarti (schists and marbles) that comprises the geologic basement under ignimbrite. Seven sampling campaigns were conducted between October 2010 and October 2011, including 65 groundwater samples from 11 wells and springs. Chemical analyses showed As concentrations exceeding the 10-μg/L national drinking water limit in 46 % of the samples from Mandamados. Groundwater composition in Mandamados evolved from Ca-HCO 3 type, to mixed type and finally to Na-Cl type along the groundwater flow direction, indicating the contribution of ion exchange in groundwater chemical composition, while Ca-HCO 3 type waters were observed in the Tarti area. Arsenic speciation analysis showed that As(V) was the main species in all samples, indicating that As was released under oxidizing conditions. Statistical analysis suggested silicate weathering as the prime mechanism of As release in groundwater in both cases, while, in the Tarti area, carbonate dissolution may represent a secondary mechanism which could be related to the observed relatively low As concentrations in the region. In both areas, pH-related desorption of As, primarily from Fe mineral phases, was found to be the most important factor controlling the mobilisation of As, while the contribution of the redox control to As release in groundwater was generally found to be less significant.
Using composite constituents to simulate the average behavior and fate of groups of individual hy... more Using composite constituents to simulate the average behavior and fate of groups of individual hydrocarbons in the subsurface may provide the means for dealing with mixtures of tens or hundreds of individual hydrocarbons while reducing the computational cost. There are two additional sources of uncertainty associated with this technique, namely, the time dependence of the effective properties of a composite constituent due to temporal composition changes, and the approximation in determining their effective property values. In this work, an approach to limit these sources of uncertainty is described and certified with published experimental data, that involved an artificially mixed kerosene. An inverse analysis is used to estimate the effective property values of composite constituents, and a methodology is introduced for optimizing component grouping, based on the minimization of an objective function. Aqueous solubility is found to be the optimal grouping criterion for the multicomponent organic mixture considered here. Under the assumption of equilibrium partitioning among phases, the effective property values of the composite constituents are expressed in terms of the property values of the member components using a step-wise inverse analysis. Simple arithmetic averages are found to provide poor approximations to the effective group properties and alternative schemes are proposed, based on experimental data.
Proceedings of the 16th International Conference on Environmental Science and Technology, Nov 19, 2022
In the present study, a geochemical analysis was conducted to investigate the As occurrence and r... more In the present study, a geochemical analysis was conducted to investigate the As occurrence and release in groundwater from two different geological environments on Lesvos island: (i) the volcanic area of Mandamados (ignimbrite) and (ii) the metamorphic area of Tarti that comprises the geologic basement under ignimbrite. Seven sampling campaigns were conducted between October 2010 and October 2011 including 65 groundwater samples from 11 wells and springs. Chemical analyses showed As concentrations exceeding the 10 μg/L national drinking water limit in 46% of the samples from Mandamados. Groundwater composition in Mandamados evolved from Ca-HCO3 type, to mixed type and finally to Na-Cl type along the groundwater flow direction, indicating the contribution of ion exchange in groundwater chemical composition, while Ca-HCO3 type waters were observed in the Tarti area. Arsenic speciation analysis showed that As(V) was the main species in all samples, indicating that As was released under oxidizing conditions. Statistical analysis suggested silicate weathering as the prime mechanism of As release in groundwater in both cases, while, in the Tarti area, carbonate dissolution may represent a secondary mechanism which could be related to the observed relatively low As concentrations in the region. In both areas, pH related desorption of As, primarily from Fe mineral phases, was found to be the most important factor controlling the mobilisation of As, while the contribution of the redox control to As release in groundwater was generally found to be less significant.
In earth and environmental sciences applications, uncertainty analysis regarding the outputs of m... more In earth and environmental sciences applications, uncertainty analysis regarding the outputs of models whose parameters are spatially varying (or spatially distributed) is often performed in a Monte Carlo framework. In this context, alternative realizations of the spatial distribution of model inputs, typically conditioned to reproduce attribute values at locations where measurements are obtained, are generated via geostatistical simulation using simple random (SR) sampling. The environmental model under consideration is then evaluated using each of these realizations as a plausible input, in order to construct a distribution of plausible model outputs for uncertainty analysis purposes. In hydrogeological investigations, for example, conditional simulations of saturated hydraulic conductivity are used as input to physically-based simulators of flow and transport to evaluate the associated uncertainty in the spatial distribution of solute concentration. Realistic uncertainty analysis via SR sampling, however, requires a large number of simulated attribute realizations for the model inputs in order to yield a representative distribution of model outputs; this often hinders the application of uncertainty analysis due to the computational expense B Stelios Liodakis
... Petros Gaganis and Leslie Smith Department of Earth and Ocean Sciences, University of British... more ... Petros Gaganis and Leslie Smith Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada ... Insight gained in updating the prior information on the model parameters is used to assess the correctness of the model structure, which is defined ...
Abstract Geostatistical simulation using controlled or stratified sampling methods, namely Latin ... more Abstract Geostatistical simulation using controlled or stratified sampling methods, namely Latin hypercube and stratified likelihood sampling, are capable of generating representative realizations from (log)Gaussian random fields, i.e., spanning efficiently the range of values corresponding to the (log)Gaussian multivariate probability distribution. Although such realizations often serve as parameters for physical process simulators, existing controlled sampling methods do not account for model sensitivity; hence, they need not yield representative realizations of model outputs. To address this shortcoming, controlled sampling methods are embedded within a two-step simulation procedure. The first step involves stratified sampling at a set of control points where attribute values are expected to exert a large impact on model predictions and/or where uncertainty in such predictions is expected to be largest. In the second step, control point samples are used to generate attribute realizations over the entire study region using classical geostatistical simulation. The application of the proposed controlled, two-step, geostatistical simulation procedure is illustrated in a hydrogeological context via a synthetic case study involving physically-based simulation of flow and transport in a porous medium with known boundary and initial conditions over a simple geometrical domain.
... Petros Gaganis a , Peter Kjeldsen b and Vasilis N. Burganos a ,* a Institute of Chemical Engi... more ... Petros Gaganis a , Peter Kjeldsen b and Vasilis N. Burganos a ,* a Institute of Chemical Engineering and High Temperature Chemical ProcessesFoundation for Research and Technology, Hellas (ICE/HT-FORTH), PO Box 1414, GR 26504 Patras, Greece b Environment ...
Proceedings of the 16th International Conference on Environmental Science and Technology, Nov 19, 2022
A research conducted in water wells located in the rhyolithic volcanic area of Mandamados, Lesvos... more A research conducted in water wells located in the rhyolithic volcanic area of Mandamados, Lesvos Island, Greece, indicated that significant seasonal variation of arsenic concentration in groundwater exists mainly in wells near the coastal zone. However, there were differences among those coastal wells with regard to the processes and factors responsible for the observed seasonal variability of the element, although they are all located in a small homogeneous area. On the other hand, in wells located in higher relief regions, the concentration of As in groundwater followed a fairly constant pattern throughout the year, which is probably related to the faster groundwater flow in this region due to a higher hydraulic gradient. In general, seasonal variation of As in groundwater in the study area was found to be related to geology, recharge rate, topography-distance from coast, and well depth.
Two recently developed approaches to quantification of model (conceptual) error in a single groun... more Two recently developed approaches to quantification of model (conceptual) error in a single groundwater model, a per-datum calibration methodology and a Bayesian model error analysis, were applied to a problem of 90 Sr migration to water wells at Chernobyl, Ukraine. The intent of this composition is to demonstrate their utility to accounting for the uncertainty due to model error in estimating risks (or costs) in decision models. Bayesian model error analysis resulted in a more conservative estimate of the probability of the Pripyat Town well field contamination than did the per-datum calibration approach. This difference in risk estimates is a result of the conceptual differences between the two methods. Per-datum calibration relies primarily on information on model error contained in the measurements of the dependent variables to quantify its effect on model predictions. The Bayesian model error analysis assigns equal importance to prior information on the parameters and measurements of the dependent variable, thus allowing the incorporation of a more informative description of parameter distributions, as well as subjective judgement into a risk analysis. The suitability of either of the two methods, when applied to a specific problem, may be determined based on the nature and quantity of available data.
Environmental Monitoring and Assessment, Dec 23, 2017
A survey conducted in water wells located in the rhyolithic volcanic area of Mandamados, Lesvos I... more A survey conducted in water wells located in the rhyolithic volcanic area of Mandamados, Lesvos Island, Greece, indicated that significant seasonal variation of arsenic concentration in groundwater exists mainly in wells near the coastal zone. However, there were differences among those coastal wells with regard to the processes and factors responsible for the observed seasonal variability of the element, although they are all located in a small homogeneous area. These processes and factors include (a) a higher rate of silicate weathering and ion exchange during the dry period followed by the dilution by the recharge water during the wet period, (b) enhanced desorption promoted by higher pH in summer and subsequent dilution of As by rainwater infiltration during the wet period, and (c) reductive dissolution of Mn during the wet period and by desorption under high pH values during the dry period. On the other hand, in wells located in higherrelief regions, the concentration of As in groundwater followed a fairly constant pattern throughout the year, which is probably related to the faster flow of groundwater in this part of the area due to a higher hydraulic gradient. In general, seasonal variation of As in groundwater in the study area was found to be related to geology, recharge rate, topography-distance from coast, and well depth.
The effect of systematic model error on the model predictions varies in space and time, and diffe... more The effect of systematic model error on the model predictions varies in space and time, and differs for the flow and solute trans port components of a groundwater model. The classical single-objective formulation of the inverse problem by its nature cannot capture these characteristics of model error. We introduce an inverse approach that allows the spatial and temporal variability of model error to be evaluated in the parameter space. A set of solutions for model parameters are obtained by this new method that almost exactly satisfies the model equation at each observation point (per-datum calibration). This set of parameter estimates are then used to define a posterior parameter space that may be translated into a probabilistic description of model output to rep resent the level of confidence in model performance. It is shown that this approach can provide useful information regarding the strengths and limitations of a model as well as the performance of classical calibration procedures.
Despite the growing discussion on vulnerability and adaptation in urban areas, there is limited r... more Despite the growing discussion on vulnerability and adaptation in urban areas, there is limited research on how smaller towns and cities in Small Island Developing States are being affected by and responding to climate change impacts. This study uses fuzzy cognitive mapping (FCM), field visits and semi-structured interviews with 40 stakeholders across 6 different stakeholder groups in the Nadi River Basin, Fiji Islands to identify, analyse and deconstruct climate change vulnerability and adaptation options to manage increasing flood risks. The research evidence suggests that vulnerability to floods in the basin is on the rise due to a complex mesh of three intersecting factors. Firstly, non-climatic pressures such as development, drainage, social change, agriculture, tourism growth and deforestation combine, juxtapose and interact in a rather unique way with global climate variability (interdependent systems) to increase the stress on the river and coastal ecosystems. Secondly, the most vulnerable or at-risk populations like the farmers, squatter households and in particular women within the community have weak coping capacity due to a combination of demographic and social characteristics. Thirdly, vulnerability is on the rise due to climate factors as well as the flurry of unplanned development, redevelopment and degradation of catchment resources. The research findings have implications for adaptation policies. In particular, the basin stakeholders should integrate climate change within sectorial planning processes, actively engage the vulnerable groups, promote knowledge, awareness and social learning, and invest in adaptive management across all levels of decision-making. Structural policy changes to land-use planning and insurance financing schemes are also necessary to address growing risks. These have the potential to enhance local capacities of communities to adapt to climate-induced floods and improve ecosystem integrity for resilience building.
ABSTRACT A major difficulty that is typically encountered when real contamination problems are in... more ABSTRACT A major difficulty that is typically encountered when real contamination problems are investigated is the typically large number of constituents in the actual VOC mixture compared to the number of species that can be handled numerically. An approach to dealing with complex mixtures of individual hydrocarbons while reducing the computational cost is to define a small number of effective composite constituents to simulate the average behavior and fate of groups of individual hydrocarbons in the subsurface. However, there are two additional sources of uncertainty associated with this technique that may influence the reliability of model predictions, namely, the time dependence of the effective properties of a composite constituent due to temporal composition changes, and the approximation in determining their effective property values. An approach to limit these sources of uncertainty was developed recently by the authors based on experimental data from a lysimeter study that involved artificially mixed kerosene. The minimization of the time dependence of the effective thermodynamic properties of the composite constituents is achieved through a methodology for optimizing component grouping, based on the minimization of an objective function. Simple arithmetic averages are found to provide poor approximations to the effective group properties and alternative algorithms are proposed to express the effective property values of composite constituents in terms of property values of the member components. The applicability of this approach to large-scale contamination problems is demonstrated here using data from field studies. Compared to the case of simulating all organics as individual compounds, our approach offers the advantage of a greatly reduced computational cost, reduced data requirements, as well as a smaller impact of the uncertainty in the compound properties on modeling results because effective values are used for each composite constituent, which follow a pdf with reduced variance compared to that of the individual values.
ABSTRACT A computationally cost effective approach to modeling transport and fate of a mixture of... more ABSTRACT A computationally cost effective approach to modeling transport and fate of a mixture of a large number of volatile organic contaminants consists of simulating specific target contaminants as individual constituents while the influence of the mixture composition on the transport process is taken into account by defining a small number of composite constituents to represent the cumulative effect of groups of individual hydrocarbons. Compared to the case of individual compounds, there are two additional sources of uncertainty associated with this approach, namely, the identification of the effective thermodynamic property values of a composite constitutent, and their time dependence due to temporal composition variability. In this work, an approach to limit these sources of uncertainty is described and demonstrated with data from a lysimeter experiment, that involved an artificially mixed kerosene. The mixture component that represents the main threat of groundwater contamination is selected as a target contaminant while the rest of the hydrocarbons are grouped into four composite constitutents. The minimization of the time dependence of the effective thermodynamic properties of the composite constituents is achieved through a selection of an appropriate grouping criterion. A methodology that is based on the minimization of an objective function is introduced for selecting the optimal grouping criterion, which may be different for different hydrogeologic settings and compositions of the organic mixture. Given the assumption of equilibrium partitioning among phases, the appropriate effective property values of the composite constituents in relation to the property values of the member hydrocarbons are then identified through a step-wise inverse analysis.
Petroleum products involve mixtures of different volatile organic compounds (VOCs) that may repre... more Petroleum products involve mixtures of different volatile organic compounds (VOCs) that may represent a risk for groundwater after a spill to the unsaturated zone. However, the majority of the published approaches and codes for unsaturated zone modeling handle only either a single or a limited number of compounds in order to cope with the complexity of processes involved in VOC attenuation. This work aimed at simulating complex VOC scenarios using a grouping procedure, which results in the definition of a small number of effective pseudospecies with carefully specified effective properties. Grouping criteria are developed for a hypothetical scenario that involves a mixture of seven VOCs with properties covering a wide range of values. These criteria are assessed through a comparison of group-based calculations with rigorous calculations using the individual components. Groups of species with similar solubility resulted in concentration profiles that compared very satisfactorily with the rigorous results. The approach is validated with data from a large-scale lysimeter experiment. The simulation results compare satisfactorily with the experimental data for several days and for the vast majority of the mixture components. The agreement was significantly improved when temporally varying solubility values were used, justified by the difference in the compound volatility and the concomitant alteration of the VOC mixture composition with time.
High-resolution single source-solute transport experiments in glass-etched pore networks presente... more High-resolution single source-solute transport experiments in glass-etched pore networks presented in a previous publication are used for quantifying the hydrodynamic dispersion as a function of Peclet number (Pe). This type of artificial porous media is ideal for understanding the effects of pore-scale phenomena on solute dispersion thanks to its well characterized topology and pore geometry. The hydrodynamic dispersion coefficients are estimated by matching the spatial and temporal distributions of the solute concentration over various regions of the network with the numerical solution of the advection-dispersion equation and using a parameter space analysis to ensure well conditioning of the parameters. The source release function and assumptions on boundary conditions are found to affect significantly the reliability of the dispersion coefficient estimates. The estimated longitudinal dispersion coefficients are in close quantitative agreement with literature data for dispersion in porous media. The extracted transverse dispersivity (a T) values indicate an apparent decrease of a T with increasing flow velocity, which is at variance of constant dispersivity approximation adopted commonly in soil hydrology models and is attributed to incomplete diffusive mixing at the pore intersections. Calculated values of an effective mixing ratio at pore junctions are in agreement with published theoretical estimates and reveal a transition from partial mixing to streamline routing as Pe increases within the range investigated here.
Typical contaminants found in the unsaturated zone (diesel oil, unleaded gasoline, etc.) are mixt... more Typical contaminants found in the unsaturated zone (diesel oil, unleaded gasoline, etc.) are mixtures of several volatile organic compounds (e.g., toluene, xylene, etc.) having variable properties. A large number of models and codes dealing with the description of contaminant attenuation in the subsurface have appeared in the literature. In the present work, several representative codes capable of unsaturated zone modeling are presented. Only a limited number of these codes consider vapor phase diffusion, despite its significance in practical applications. These codes are discussed here in more detail. Three of these codes treat contaminants as mixtures of more than one compound with variable properties. However, only two of them include degradation, which is also an important attenuation mechanism. New approaches trying to overcome code limitations-in terms of compound number they can handle-are also discussed.
A field experiment was conducted in Denmark in order to evaluate the fate of 13 volatile organic ... more A field experiment was conducted in Denmark in order to evaluate the fate of 13 volatile organic compounds (VOCs) that were buried as an artificial fuel source in the unsaturated zone. Compound-specific isotope analysis showed distinct phases in the 13 C/ 12 C ratio evolution in VOC vapors within 3 m from the source over 114 days. At day 3 and to a lesser extent at day 6, the compounds were depleted in 13 C by up to-5.7‰ with increasing distance from the source compared to the initial source values. This trend can be explained by faster outward diffusion of the molecules with 12 C only compared to molecules with a 13 C. Then, the isotope profile leveled out, and several compounds started to become enriched in 13 C by up to 9.5‰ with increasing distance from the source, due to preferential removal of the molecules with 12 C only, through biodegradation. Finally, as the amount of a compound diminished in the source, a 13 C enrichment was also observed close to the source. The magnitude of isotope fractionation tended to be larger the smaller the mass of the molecule was. This study demonstrates that, in the unsaturated zone, carbon isotope ratios of hydrocarbons are affected by gas-phase diffusion in addition to biodegradation, which was confirmed using a numerical model. Gas-phase diffusion led to shifts in δ 13 C >1‰ during the initial days after the spill, and again during the final stages of source volatilization after >75% of a compound had been removed. In between, diffusion has less of an effect, and thus isotope data can be used as an indicator for hydrocarbon biodegradation.
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Papers by Petros Gaganis