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  • 1
    Language: English
    In: Advances in Water Resources, August, 2008, Vol.31(8), p.1074(13)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.advwatres.2008.04.010 Byline: Petros Gaganis (a), Leslie Smith (b) Keywords: Decision model; Risk assessment; Model error; Conceptual error; Model calibration; Bayesian analysis; Chernobyl; Strontium-90 migration Abstract: 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.sup.90Sr 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. Author Affiliation: (a) Department of Environment, University of the Aegean, Universty Hill, Xenia Building, 81100 Mytilene, Greece (b) Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada Article History: Received 16 August 2007; Revised 18 April 2008; Accepted 18 April 2008
    Keywords: Risk Assessment -- Models
    ISSN: 0309-1708
    Source: Cengage Learning, Inc.
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  • 2
    Language: English
    In: Science of the Total Environment, 01 June 2018, Vol.626, pp.875-886
    Description: Managed Aquifer Recharge (MAR) is becoming an attractive water management option, with more than 223 sites operating in European countries. The quality of the produced water, available for drinking or irrigation processes is strongly depended on the aquifer's hydrogeochemical characteristics and on the MAR system design and operation. The objective of this project is the assessment of the operation efficiency of a MAR system in Cyprus. The coupling of alternative methodologies is used such as water quality monitoring, micro-scale sediment sorption experiments, simulation of groundwater flow and phosphate and copper transport in the subsurface using the FEFLOW model and evaluation of the observed change in the chemical composition of water due to mixing using the geochemical model PHREEQC. The above methodology is tested in the Ezousa MAR project in Cyprus, where treated effluent from the Paphos Waste Water Treatment Plant, is recharged into the aquifer through five sets of artificial ponds along the riverbed. Additionally, groundwater is pumped for irrigation purposes from wells located nearby. A slight attenuation of nutrients is observed, whereas copper in groundwater is overcoming the EPA standards. The FEFLOW simulations reveal no effective mixing in some intermediate infiltration ponds, which is validated by the inverse modeling simulation of the PHREEQC model. Based on the results, better control of the infiltration capacity of some of the ponds and increased travel times are some suggestions that could improve the efficiency of the system.
    Keywords: Feflow ; Nitrates ; Cooper ; Phreeqc ; Managed Aquifer Recharge ; Coastal Aquifer ; Environmental Sciences ; Biology ; Public Health
    ISSN: 0048-9697
    E-ISSN: 1879-1026
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  • 3
    Language: English
    In: Climate and Development, 26 May 2016, Vol.8(3), pp.256-269
    Description: 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...
    Keywords: Adaptation ; Ecosystems ; Flood ; Fuzzy Cognitive Mapping ; Stakeholders ; Vulnerability ; Nadi River Basin ; Meteorology & Climatology ; Environmental Sciences
    ISSN: 1756-5529
    E-ISSN: 1756-5537
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  • 4
    Language: English
    In: Mathematical Geosciences, 2013, Vol.45(5), pp.531-556
    Description: Two methods for generating representative realizations from Gaussian and lognormal random field models are studied in this paper, with term representative implying realizations efficiently spanning the range of possible attribute values corresponding to the multivariate (log)normal probability distribution. The first method, already established in the geostatistical literature, is multivariate Latin hypercube sampling, a form of stratified random sampling aiming at marginal stratification of simulated values for each variable involved under the constraint of reproducing a known covariance matrix. The second method, scarcely known in the geostatistical literature, is stratified likelihood sampling, in which representative realizations are generated by exploring in a systematic way the structure of the multivariate distribution function itself. The two sampling methods are employed for generating unconditional realizations of saturated hydraulic conductivity in a hydrogeological context via a synthetic case study involving physically-based simulation of flow and transport in a heterogeneous porous medium; their performance is evaluated for different sample sizes (number of realizations) in terms of the reproduction of ensemble statistics of hydraulic conductivity and solute concentration computed from a very large ensemble set generated via simple random sampling. The results show that both Latin hypercube and stratified likelihood sampling are more efficient than simple random sampling, in that overall they can reproduce to a similar extent statistics of the conductivity and concentration fields, yet with smaller sampling variability than the simple random sampling.
    Keywords: Geostatistics ; Monte Carlo simulation ; Latin hypercube ; Mahalanobis distance ; Modflow ; MT3D
    ISSN: 1874-8961
    E-ISSN: 1874-8953
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  • 5
    Language: English
    In: Environmental Monitoring and Assessment, 2018, Vol.190(1), pp.1-20
    Description: 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 higher-relief 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.
    Keywords: Arsenic ; Groundwater ; Seasonal variation ; Spatial variation ; Volcanic rocks ; Greece
    ISSN: 0167-6369
    E-ISSN: 1573-2959
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  • 6
    Language: English
    In: Sustainable Water Resources Management, 2017, Vol.3(3), pp.283-295
    Description: In the Aegean Islands, the continued availability of freshwater resources is of fundamental concern. This study analyzes the freshwater system for the Island of Lesvos by simultaneously conceptualizing various issues surrounding it using vulnerability assessment as a quantitative tool. The endpoint approach to vulnerability assessment was applied by developing a numerical expression based on a set of 25 quantitative and qualitative indicators; the indicators were identified as proxies to reflect the various conspicuous and inconspicuous issues surrounding water resources of the Island. In addition, concurrent visualization of the indicators was carried out by plotting radar charts. The assessment indicated that the Lesvian hydrogeological system has significant vulnerabilities emanating from both natural and anthropogenic pressures in addition to a poor adaptive capacity to counter perturbations; this was corroborated by the composite water vulnerability index which was calculated to be 0.69. Based on the analysis of the assessment results, the priority management targets and existing management optimization tools, the authors propose a quantitative framework that could aid the development of an effective methodology for addressing problems in water resource management; this approach couples adaptive water management with vulnerability assessment. The proposed methodology may represent a tool for identification of better solutions to water management–decision problems and/or provide important insights during decision making in similar environments.
    Keywords: Water framework directive ; Vulnerability index ; Indicators ; Adaptive management ; Water resources ; Island systems
    ISSN: 2363-5037
    E-ISSN: 2363-5045
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  • 7
    Language: English
    In: Spatial Statistics, November 2015, Vol.14, pp.224-239
    Description: 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.
    Keywords: Spatial Variability ; Uncertainty Analysis ; Stratified Likelihood Sampling ; Latin Hypercube Sampling ; Stochastic Hydrogeology ; Statistics
    ISSN: 2211-6753
    E-ISSN: 2211-6753
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  • 8
    Language: English
    In: Water Resources Management, 2011, Vol.25(2), pp.509-522
    Description: 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.
    Keywords: Water consumption policies ; Non-economic social costs ; Social capital ; Citizens’ perceptions
    ISSN: 0920-4741
    E-ISSN: 1573-1650
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  • 9
    Language: English
    In: Water, Air, & Soil Pollution, 2015, Vol.226(9), pp.1-16
    Description: 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.
    Keywords: Arsenic ; Groundwater ; Rhyolites ; Geochemistry ; Lesvos ; Greece
    ISSN: 0049-6979
    E-ISSN: 1573-2932
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  • 10
    Language: English
    In: Mathematical Geosciences, 2018, Vol.50(2), pp.127-146
    Description: 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 of evaluating complex environmental models. Stratified sampling methods, including variants of Latin hypercube sampling, constitute more efficient sampling aternatives, often resulting in a more representative distribution of model outputs (e.g., solute concentration) with fewer model input realizations (e.g., hydraulic conductivity), thus reducing the computational cost of uncertainty analysis. The application of stratified and Latin hypercube sampling in a geostatistical simulation context, however, is not widespread, and, apart from a few exceptions, has been limited to the unconditional simulation case. This paper proposes methodological modifications for adopting existing methods for stratified sampling (including Latin hypercube sampling), employed to date in an unconditional geostatistical simulation context, for the purpose of efficient conditional simulation of Gaussian random fields. The proposed conditional simulation methods are compared to traditional geostatistical simulation, based on SR sampling, in the context of a hydrogeological flow and transport model via a synthetic case study. The results indicate that stratified sampling methods (including Latin hypercube sampling) are more efficient than SR, overall reproducing to a similar extent statistics of the conductivity (and subsequently concentration) fields, yet with smaller sampling variability. These findings suggest that the proposed efficient conditional sampling methods could contribute to the wider application of uncertainty analysis in spatially distributed environmental models using geostatistical simulation.
    Keywords: Geostatistics ; Monte Carlo simulation ; Uncertainty analysis ; Hydrogeology
    ISSN: 1874-8961
    E-ISSN: 1874-8953
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