An ensemble-based approach for pumping optimization in an island aquifer considering parameter, observation and climate uncertainty

In coastal zones, a major objective of groundwater management is often to determine sustainable pumping rates which avoid well salinization. Understanding how model and climate uncertainties affect optimal management solutions is essential for providing groundwater managers with information about salinization risk and is facilitated by the use of optimization under uncertainty (OUU) methods. However, guidelines are missing for the widespread implementation of OUU in real-world coastal aquifers and for the incorporation of climate uncertainty into OUU approaches. An ensemble-based OUU approach was developed considering parameter, observation and climate uncertainty and was implemented in a real-world island aquifer in the Magdalen Islands (Quebec, Canada). A sharp-interface seawater intrusion model was developed using MODFLOW-SWI2 and a prior parameter ensemble was generated containing multiple equally plausible realizations. Ensemble-based history matching was conducted using an iterative ensemble smoother which yielded a posterior parameter ensemble conveying both parameter and observation uncertainty. Sea level and recharge ensembles were generated for the year 2050 and were then used to generate a predictive parameter ensemble conveying parameter, observation and climate uncertainty. Multi-objective OUU was then conducted, aiming to both maximize pumping rates and minimize the probability of well salinization. As a result, the optimal trade-off between pumping and the probability of salinization was quantified considering parameter, historical observation and future climate uncertainty simultaneously. The multi-objective, ensemble-based OUU led to optimal pumping rates that were very different from a previous deterministic OUU and close to the current and projected water demand for risk-averse stances. Incorporating climate uncertainty into the OUU was also critical since it reduced the maximum allowable pumping rates for users with a risk-averse stance. The workflow used tools adapted to very high-dimensional, nonlinear models and optimization problems to facilitate its implementation in a wide range of real-world settings.</p

A framework for parameter estimation using sharp-interface seawater intrusion models

Funding : This work was supported by Quebec’s Ministère de l'Environnement et de la Lutte contre les changements climatiques (MELCC) [project « Acquisition de connaissances sur les eaux souterraines dans la région des Îles-de-la-Madeleine » (Groundwater characterization project in the Magdalen Islands region)]; and the Fonds québécois de la recherche sur la nature et les technologies (FRQNT) [International internship program accessed through CentrEau, the Quebec Water Research Center]. The authors would like to thank the Municipality of Les Îles-de-la-Madeleine for providing pumping datasets and information on current and historical groundwater management. They would also like to thank the team at Université Laval working on the Magdalen Islands project, for their help acquiring datasets and for field logistics, John Molson, for proofreading, and finally the two anonymous reviewers for their valuable comments. The authors would also like to thank Vincent Post for discussions on deep open boreholes, and Francesca Lotti and John Doherty for discussions on seawater intrusion modeling and data assimilation. J-C Comte and O Banton acknowledge the financial support from the Fonds d'Action Québécois pour le Développement Durable for the ERT data collection, undertaken as part of the Madelin'Eau consortium (Ageos-Enviro'Puits-Hydriad), and further thank the Municipality of Les Îles-de-la-Madeleine for fieldwork logistical and technical support.Peer reviewedproo

Hydrogéologie des milieux volcaniques insulaires: apports d'une étude intégrée aux îles Galapagos

With a growing population and limited freshwater resources, the hydrogeology of the Galapagos Islands remains to a great extent unknown. Rainfall is relatively weak and unevenly distributed in space and time. The economical center of the archipelago, Santa Cruz Island, has only small intermittent streams, while several permanent streams are present on San Cristóbal Island. In the frame of this study, an interdisciplinary approach is used to characterize the hydrogeology of the archipelago. A new method has been developed to enhance the interpretation of airborne electromagnetics surveys. With geostatistical interpolation techniques, this method allows the construction of a 3D grid of resistivity. SkyTEM surveys completed in Galapagos were processed with this technique, and confronted to surface analysis with remote sensing and field work. The first hydrogeological conceptual model is proposed for San Cristóbal Island. Climatic conditions have been investigated with the installation of a monitoring network along the windward side of Santa Cruz Island. The occurrence of fog during six months of the year presents an additional input in the water budget and increases groundwater recharge. This input has been quantified with a physically based canopy interception model. The basal aquifer of Santa Cruz Island has been investigated from the analysis of tidal signal propagation, hydraulic tests, as well as fault and fracture mapping. Results show that young basalts, densely fractured by cooling joints, are highly permeable. Because they are poorly connected, faults have a limited impact over regional groundwater flow. Contrasting hydrogeological configurations in the Galapagos Islands are explained by an evolution pattern. In relatively young islands, such as Santa Cruz, basalts are fractured and permeable. Seawater intrusion is strong and freshwater rapidely flows to the ocean. On the opposite, conditions are more favorable for groundwater storage in older volcanic islands, where the regional permeability is smaller and valley incision leads to the existence of springs.Avec une population croissante et des ressources en eau de surface limitées, l'hydrogéologie des îles Galápagos reste pour une grande part inconnue. Les précipitations sont relativement faibles et inégalement réparties dans le temps et l'espace. Le centre économique de l'archipel, l'île de Santa Cruz, ne dispose que de quelques cours d'eau intermittents tandis que l'île de San Cristóbal présente plusieurs rivières pérennes. Dans le cadre de ce travail, une approche pluridisciplinaire est utilisée afin de caractériser l'hydrogéologie de l'Archipel. Une nouvelle technique d'interprétation des sondages életro\-magné\-tiques héliportés a été développée. Grâce à des méthodes géostat\-istiques, elle permet la construction d'une grille 3D de la résistivité électrique. Les données issues de la mission SkyTEM réalisée sur les îles de Santa Cruz et San Cristóbal en 2006 sont mises en perspectives. Confrontée aux observations de surface collectées par télédétection et sur le terrain, la géophysique permet de proposer un modèle conceptuel pour l'île de San Cristóbal. Les conditions climatiques ont été suivies avec la mise en place de stations météorologiques le long du versant au vent de l'île de Santa Cruz. La présence de brouillard pendant 6 mois de l'année représente un apport supplémentaire pour la recharge des aquifères. Cet apport a été quantifié avec une méthode basée sur un modèle d'interception à base physique. L'aquifère de base de l'île de Santa Cruz a été étudié avec la propagation du signal de marée, des essais de pompage, et la cartographie de la fracturation. Ces travaux montrent que les basaltes ''jeunes'' fracturés ont une forte perméabilité. En revanche, les failles n'ont qu'un effet limité sur l'hydrogéologie régionale, car elles sont peu connectées. Les configurations hydrologiques contrastées au sein de l'archipel sont expliquées par un schéma d'évolution. Dans les îles relativement jeunes, telles que Santa Cruz, les basaltes fracturés sont perméables. Ils offrent peu de résistance à l'intrusion saline et l'eau douce est rapidement drainée jusqu'à la mer. Les conditions sont plus favorables dans les îles plus âgées. Les perméabilités plus faibles et l'incision des vallées permettent l'émergence de sources

Hydrogéologie des milieux volcaniques insulaires (apport d une étude intégrée aux îles Galápagos, Équateur)

La population de l Archipel des Galápagos est croissante, tandis que les ressources en eau sont limitées et inégalement réparties. Dans le cadre de ce travail, une approche intégrée combinant géophysique, géologie structurale, suivis hydro-climatologique et piézométrique est utilisée pour caractériser les modèles conceptuels hydrogéologiques d une île à l autre et identifier d éventuelles ressources complémentaires. La géophysique permet d identifier des zones d intérêt hydrogéologique. Les données acquises lors de la mission géophysique SkyTEM réalisée en 2006 sur les îles de Santa Cruz et San Cristóbal ont été mises en perspective avec une nouvelle technique d'interprétation. Avec des outils géostatistiques, cette méthode a permis la construction d'une grille 3D de la résistivité électrique pour chacune des deux îles. Des zones d'intérêts ont été cartographiées en 3D et confrontées aux observations structurale, géomorphologiques et hydrologiques. Un paramètre déterminant en hydrogéologie est la recharge des aquifères. Sur les principales îles de l'Archipel, la présence semi-permanente de brouillard pendant six mois de l'année présente un apport supplémentaire aux précipitations classiques. Pour quantifier cet apport, un réseau de suivi hydrométéorologique a été mis en place sur le versant au vent de l'île de Santa Cruz. L apport du brouillard a été quantifié avec une méthode basée sur un modèle d'interception à base physique. La vitesse du vent est identifiée comme un facteur déterminant pour l interception du brouillard. A Santa Cruz, un suivi hydrogéologique de l'aquifère de base a été réalisé. Affecté par l intrusion saline, cet aquifère est soumis aux variations du niveau de la mer et à la dynamique de recharge. La cartographie de la fracturation a été réalisée sur des affleurements sélectionnés. Combinée à l'analyse de la propagation du signal de marée, elle a permis de caractériser les propriétés hydrodynamiques des basaltes.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Optimality Versus Viability in Groundwater Management with Environmental Flows

The protection of environmental flows is a main challenge pursued by water regulating agencies in their groundwater management policies. A stylised hydro-economic model with natural drainage is used to compare the outcome of the optimal control approach in which environmental flows are introduced as an externality with the viable approach in which environmental flows are modelled as a constraint to satisfy. The optimal and viable paths for the water table, water extraction for irrigation and environmental flows are analytically derived together with their long-term values. We show how results are sensitive to some key parameters like the discount factor and the monetary value of the externality in the optimal control approach. We show how the value of the environmental flows target in the viable approach can be derived from the optimal control approach. Numerical simulations based on the Western La Mancha aquifer illustrate the main results of the study.French National Research Agency (ANR) within the Cluster of Excellence COTE [ANR-10-LABX-45]24 month embargo; published online: 27 March 2019This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Particle tracking as a vulnerability assessment tool for drinking water production

International audienceThe simulation of concentration values and use of such data for historymatching is often impeded by the computation time of groundwater transport models based on the resolution of the advection-dispersion equation. This is unfortunate because such data are often rich in information and the prediction of concentration values is of great interest for decision making. Particle tracking can be used as an efficient alternative under a series of simplifying assumptions, which are often reasonable at groundwater sinks (wells and drains). Our approach consists of seeding particles around a sink and tracking particles backward, up to the source boundary condition, such as a contaminated stream. This particle tracking approach allows the use of parameter estimation and optimization methods requiring numerous model calls. We present a Python module facilitating the pre-and post-processing operations of a modeling workflow based on the widely used USGS MODFLOW6 and MODPATH7 programs. The module handles particle seeding around the sink and estimation of the mixing ratio of water withdrawn from the sink. This ratio is computed with a mixing law from the particle endpoints, accounting for particle velocities and mixing in the source model cells. We investigate the best practice to obtain robust derivatives with this approach, which is a benefit for the screening methods based on linear analysis. We illustrate the interest of the approach with a real world case study, considering a drinking water well field vulnerable to a contaminated stream. The configuration is typical of many other drinking water production sites. The modeling workflow is fully script-based to make the approach easily reproducible in similar cases

Uncertainty Quantification of Contaminated Soil Volume with Deep Neural Networks and Predictive Models

International audienceThe estimation of the soil volume exceeding a contamination threshold over decommissioned industrial sites is critical for the design of remediation strategies. In practice, the volume calculation is mostly based on preliminary sampling surveys and the use of interpolation methods. However, if the volume is not estimated correctly, this can lead to environmental and economic risks. Geostatistical-oriented methodologies have been developed to better assess the volume using uncertainty ranges. In our study, we propose a methodology entitled “Evol” to better estimate the volume and reduce the uncertainty ranges with a combination of classic non-parametrical interpolation techniques and deep learning. Evol consists of generating a synthetic model from a real polluted site, extracting descriptive variables (features) from multiple sample sets, and evaluating the error in the volume calculation. A Deep Neural Network model is then trained with the features to estimate the volume and uncertainty range for any sample set. Our methodology demonstrated high accuracy in error estimation, as evidenced by a low RMSE of 0.008 across most sample sets. Additionally, the confidence volume intervals produced by our approach were narrower than those generated by classic techniques, resulting in interval size reductions of up to 89%