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%

Estimating groundwater recharge uncertainty from joint application of an aquifer test and the water-table fluctuation method

Specific yield and groundwater recharge of unconfined aquifers are both essential parameters for groundwater modeling and sustainable groundwater development, yet the collection of reliable estimates of these parameters remains challenging. Here, a joint approach combining an aquifer test with application of the water-table fluctuation (WTF) method is presented to estimate these parameters and quantify their uncertainty. The approach requires two wells: an observation well instrumented with a pressure probe for long-term monitoring and a pumping well, located in the vicinity, for the aquifer test. The derivative of observed drawdown levels highlights the necessity to represent delayed drainage from the unsaturated zone when interpreting the aquifer test results. Groundwater recharge is estimated with an event-based WTF method in order to minimize the transient effects of flow dynamics in the unsaturated zone. The uncertainty on groundwater recharge is obtained by the propagation of the uncertainties on specific yield (Bayesian inference) and groundwater recession dynamics (regression analysis) through the WTF equation. A major portion of the uncertainty on groundwater recharge originates from the uncertainty on the specific yield. The approach was applied to a site in Bordeaux (France). Groundwater recharge was estimated to be 335 mm with an associated uncertainty of 86.6 mm at 2σ. By the use of cost-effective instrumentation and parsimonious methods of interpretation, the replication of such a joint approach should be encouraged to provide reliable estimates of specific yield and groundwater recharge over a region of interest. This is necessary to reduce the predictive uncertainty of groundwater management model

Otimização adaptativa de um campo de poços vulnerável

International audienceThe contamination of groundwater resources is a challenge for drinking water supplies. To meet water quality standards, well-field operators need practical solutions to reduce the vulnerability of production wells. Strategies for several combinations ofmanagement variables such as well flow rate or water level in drains, are usually possible to satisfy the required production rate.However, these strategies may lead to contamination issues for the abstracted groundwater. A surrogate transport model wasimplemented in a well field vulnerable to a contaminated stream. An adaptive multi-objective optimization approach is proposed.The objective is to maximize the water production at the well field while minimizing the proportion of stream water abstracted.The optimization problem is adaptive to the stream level, which is a key parameter describing hydrological conditions. Asystematic exploration of management settings is conducted and a three-dimensional Pareto front is extracted. From theseoptimum settings, a practical easy-to-use approach is developed. The well-field operator can adjust production settings tooptimum conditions as a function of the observed stream water level and desired production rate.La contamination de la ressource en eaux souterraine est un défi pour l’approvisionnement en eau potable. Pour respecter les normes de qualité de l’eau potable, les exploitants ont besoin de solutions pratiques pour réduire la vulnérabilité des puits de production. La mise en place de stratégies jouant sur les schémas de gestion, telles que le débit des puits ou le niveau d’eau dans les drains, sont généralement possibles pour atteindre le débit de production requis. Cependant, ces stratégies peuvent entraîner des problèmes de contamination des eaux souterraines captées. Un modèle équivalent de transport a été mis en œuvre sur un champ captant vulnérable à un cours d’eau contaminé. Une approche d’optimisation multi-objectifs adaptative est proposée. L’objectif est de maximiser la production d’eau tout en minimisant la proportion d’eau du cours d’eau captée. Le problème d’optimisation est adaptatif au niveau du cours d’eau, paramètre clé décrivant les conditions hydrologiques. Une exploration systématique des paramètres de gestion est effectuée et un front de Pareto tridimensionnel est extrait. À partir de ces paramètres optimaux, une approche pratique et facile à utiliser est développée. L’opérateur peut ajuster les paramètres de production aux conditions optimales en fonction du niveau d’eau du cours d’eau observé et du débit de production souhaité

Agglomerative hierarchical clustering of airborne electromagnetic data for multi-scale geological studies

International audienceAirborne electromagnetic methods provide detailed subsurface resistivity imaging over extensive areas. The inversion of electromagnetic measurements can be conducted with a quasi-3D spatially constrained inversion scheme, which yields numerous vertical resistivity soundings. So as to conduct the interpretation, these soundings can be interpolated to obtain a 3D resistivity model. However, large surveys result in huge resistivity models, which can be challenging to interpret with 2D or even 3D views. We propose a complementary approach for the interpretation of airborne electromagnetic surveys based on agglomerative hierarchical clustering. With this statistical classification method, the numerous 1D vertical resistivity profiles distributed over the area of interest can be summarized in a 2D horizontal map. Inverted electromagnetic soundings are aggregated into clusters according to their vertical resistivity profiles. These clusters define areas with a consistent vertical resistivity response, i.e. to the first order, areas with comparable vertical (hydro)-geological structure. The clustering method is illustrated with an extensive airborne electromagnetic survey conducted over Reunion Island. After a description of the method, we show that the proposed classification facilitates the confrontation between airborne geophysical results and geological or hydrogeological observations/data. We highlight that hierarchical clustering is of high interest for multi-scale studies, from regional to local scales. This approach introduces a new way to use geophysical surveys to map areas with specific geological/hydrogeological behaviors