Patrick Goblet

Module-oriented modeling of reactive transport with HYTEC

The paper introduces HYTEC, a coupled reactive transport code currently used for groundwater pollution studies, safety assessment of nuclear waste disposals, geochemical studies and interpretation of laboratory column experiments. Based on a known permeability field, HYTEC evaluates the groundwater flow paths, and simulates the migration of mobile matter (ions, organics, colloids) subject to geochemical reactions. The code forms part of a module-oriented structure which facilitates maintenance and improves coding flexibility. In particular, using the geochemical module CHESS as a common denominator for several reactive transport models significantly facilitates the development of new geochemical features which become automatically available to all models. A first example shows how the model can be used to assess migration of uranium from a sub-surface source under the effect of an oxidation front. The model also accounts for alteration of hydrodynamic parameters (local porosity, permeability) due to precipitation and dissolution of mineral phases, which potentially modifies the migration properties in general. The second example illustrates this feature.

Calibration of regional groundwater flow models: Working toward a better understanding of site-specific systems

[1]xa0This work highlights the lack of unique solutions for regional groundwater flow models and quantifies the degree of freedom concerning hydraulic conductivities for models calibrated on measured hydraulic heads. The potential of 4He as an independent tracer at reducing the nonuniqueness problem is tested. Four different calibrated groundwater flow scenarios are presented for the Carrizo aquifer and surrounding formations in Texas. It is shown that variations of hydraulic conductivities up to 2 orders of magnitude in the Carrizo aquifer and overlying confining layer lead to similar calculated hydraulic heads. No clear-cut arguments are present to invalidate one groundwater flow scenario over a different one. In contrast, when tested with a 4He transport conceptual model, all groundwater flow scenarios except one failed to reproduce a coherent 4He transport behavior in the system. This study exemplifies possible future contributions of 4He at discerning which model most closely replicates natural conditions.

Hydrologic and thermal modeling of an active volcano: the Piton de la Fournaise, Reunion

Abstract The hydrology of active volcanic islands and the interactions between cold groundwater flow and hot magmatic fluids are, as yet, poorly understood. In this study of the Piton de la Fournaise, Reunion, we try to estimate the different terms in the hydrologic balance and describe the nature and importance of groundwater flow in an active volcano. A challenging aspect of this study was that only limited data were available, a common feature of such systems. There were some volcanologic data, few hydrologic data and a deep geothermal borehole where a thermal profile had been established. First, we develop a global hydrologic model of the Piton de la Fournaise by making certain assumptions concerning groundwater pathways. This model is calibrated on the available hydrologic data. We then build a thermal model of the volcano based on hypotheses of its internal structure and on the groundwater flow pattern derived from the first model. This model is fitted on the thermal profile. We conclude that groundwater flow inside the volcano can be described as a succession of local perched aquifers, which eventually release water to a continuous basal aquifer close to the coast, with diffuse outlets to the ocean. The infiltrating water is kept away from the magma chamber by a thick impervious shell surrounding the chamber, probably created by silica precipitation in the fractured basalt. Heat is essentially conveyed to the ocean by groundwater flow. This model is likely to be valid for other active hot-spot volcanoes.

Large‐scale hydraulic conductivities inferred from three‐dimensional groundwater flow and 4He transport modeling in the Carrizo aquifer, Texas

[1]xa0Through a series of groundwater flow and 4He transport simulations, this study illustrates the conceptual and practical gains achieved by expanding a two-dimensional (2-D) model to a true 3-D one through an application in the Carrizo aquifer and surrounding formations in southwestern Texas. The 3-D simulations allow for a more detailed and accurate definition of the heterogeneities of the system by specifically identifying and differentiating processes that directly impact the three-dimensional hydraulic conductivity field. It is shown that while hydraulic conductivity decreases exponentially along the regional groundwater flow direction, such decrease is better described as a function of depth rather than recharge distance. This relationship reflects the combined influences of differential compaction of the media as well as downdip lithological change. The intrinsic permeability derived from this relationship agrees with field information and with previous findings obtained for the continental crust for depths ≤2 km, suggesting that for large scales, decrease rate of permeability with depth is independent of the media. Results also suggest that the solution for groundwater flow simulations based on calibration of hydraulic heads depends on the ratio between hydraulic conductivities of different formations, indicating that an infinite number of solutions are available for calibration of 3-D groundwater flow models. Understanding how geological processes directly affect the 3-D hydraulic conductivity field at the regional scale is essential not only to hydrogeological applications but also at improving our understanding of the Earths crust and mantle dynamics by allowing for a more accurate quantification of helium and heat fluxes.

Presentation and application of the reactive transport code HYTEC

Assessment of hazardous metal dissemination and remediation studies requires hydrogeological models able to describe the groundwater flow paths, migration along the paths and geochemical and/or biochemical reactions with mobile (colloidal) and immobile (mineral) phases of the medium. Both geochemical and hydrocdynamic facets are tightly linked together. Therefore, a strongly coupled modeling approach is required, solving the equations involved in geochemistry and hydrogeology simultaneously. HYTEC, currently used for groundwater pollution studies, safety assessment of nuclear waste disposals, geochemical studies and interpretation of laboratory column experiments, meets all these requirements. The code is based on a series of specialized modules, where among we can cite the geochemical code CHESS and differnet hydrodynamic transport codes communicating via a specific interface, currently based on MPI (Message Passing Interface). Several possibilities of the model is illustrated with the help of an operational application involving migration of toxic metals (zinc) in an aquifer. The example also demonstrates the impact of colloidal matter, capable of increasing the mobility under certain conditions.

Excess air in the noble gas groundwater paleothermometer: A new model based on diffusion in the gas phase

[1]xa0A key assumption for calculating paleotemperatures using noble gas concentrations in groundwater is that water equilibrates with standard air. However, if the unsaturated zone is depleted in O2, the noble gas partial pressures will be elevated, resulting in a bias of noble gas temperatures (NGTs) to low values. This oxygen depletion (OD) mechanism was used to explain low NGT values for a shallow aquifer in Michigan where new O2 saturation and CO2 measurements now confirm the OD model. Measured excess He, without an expected vertical concentration gradient in the water phase, suggests that the rate of noble gas equilibration at the base of the unsaturated zone is restricted, and that transport within the gas phase may be a rate-limiting step. A new NGT model is presented that uses the OD mechanism and that allows for partial re-equilibration of excess air via diffusion in the gas phase.

Noble gas thermometry and hydrologic ages: Evidence for late Holocene warming in Southwest Texas

[1]xa0Paleoclimatic reconstruction using noble gas concentrations in the Carrizo aquifer of southwest Texas and water ages determined through simulation of groundwater age reveals abrupt late Holocene temperature increases previously unidentified through 14C dating. Of particular interest is a temperature increase of up to 3.4°C in the first half of the last millennium following a cold period between ∼3.7 and 0.9 Kyrs BP. Wet, cool periods in the region are associated with El-Nino dominated conditions, while warm, arid events are linked to multi-decade La-Nina dominant events. The data shows a slow decrease in temperature between ∼1,200 and 200 Kyrs BP, a decrease that accelerated in the late Pleistocene and early Holocene. This decrease was followed by warming in the last millennium, that seems to be continuing today.

Hydraulic head interpolation in an aquifer unit using ANFIS and Ordinary Kriging

In this study, Ordinary Kriging (ok), and Adaptive Neuro Fuzzy based Inference System (anfis) are evaluated for assessing hydraulic head distribution in an aquifer unit covering 40 km2. Cartesian coordinates of the samples were used as inputs of anfis. Calibrated models are used to interpolate the hydraulic head distribution on a 50 m square - grid. Both simulations have realistic pattern (R2 > 0.97) even if ok performs slightly better than anfis at sampling location. The two methods capture different patterns. The Comparison of the two distributions allows for identifying area of estimate uncertainty, what can be used to improve the sampling network.

How basin model results enable the study of multi-layer aquifer response to pumping: the Paris Basin, France

The Albian aquifer of the Paris Basin (France) has been exploited since 1841 and shows drastic drawdown. A three-dimensional (3D) groundwater flow model is used to study the hydrodynamic response of the multi-layered aquifers to pumping activity in the Albian, at basin scale over 167 years. This 3D flow model uses geometry and hydrodynamic parameter distributions that are inherited from a genetic approach through basin modelling, the basin model creating a geometric pattern of hydrodynamic properties constrained by geological history. The paper aims to promote the use of the basin model approach (long time scale, 248 Ma) for the study of deep-aquifer response to anthropogenic perturbation (short time scale, 167 years) in situations for which hydrodynamic data are scarce but geological data are numerous. The results show that parameter distribution is insufficient to reproduce the Albian aquifer behaviour, notably highlighting a different meaning of the specific storage coefficient between basin modelling and groundwater-flow modelling. Dividing the storage coefficient by 100 and including available transmissivity data significantly improved the model/data comparison. The potential impact on a deep aquitard is then discussed. This study sheds light on the advantages and limitations of the basin model approach for groundwater-flow modelling in 3D.RésuméL’aquifère de l’Albien dans le Bassin Parisien (France) est exploité depuis 1841 et présente un rabattement important. Un modèle d’écoulement souterrain en trois dimensions est utilisé pour étudier la réponse hydrodynamique du système d’aquifères multicouche sur 167 ans, à l’échelle du bassin. Ce modèle hydrogéologique utilise la géométrie et les propriétés hydrodynamiques héritées d’une approche génétique via un modèle de bassin, qui crée une géométrie habillée en propriétés hydrodynamiques en fonction de l’histoire géologique du bassin. Cet article cherche à promouvoir l’intérêt de l’approche modèle de bassin (échelle de temps longue, 248xa0Ma) pour étudier la réponse des aquifères profonds à des perturbations anthropiques (échelle de temps courte, 167 ans) lorsque les données hydrodynamiques sont en nombre limité mais les données géologiques nombreuses. Les résultats montrent que la distribution des paramètres héritée de l’approche génétique ne permet pas de reproduire le comportement de l’aquifère de l’Albien, mettant notamment en exergue une différence de signification physique du coefficient d’emmagasinement spécifique entre modèle de bassin et modèle d’écoulement souterrain. La diminution de ce coefficient par un facteur 100 et l’implémentation des données de transmissivités disponibles pour l’aquifère de l’Albien améliorent significativement la comparaison modèle/données. Le potentiel impact sur un aquitard profond du bassin est ensuite discuté. Cette étude permet aussi de révéler l’intérêt et les limites de l’approche génétique fondée sur l’utilisation d’un modèle de bassin pour développer un modèle d’écoulement souterrain en 3D.ResumenEl acuífero Albiano de la cuenca de París (Francia) ha sido explotado desde 1841 y muestra un drástico agotamiento. Se utilizó un modelo de flujo de agua subterránea tridimensional (3D) para estudiar la respuesta hidrodinámica de acuíferos multicapas a la actividad de bombeo en el Albiano, a escala de cuenca durante 167 años. Este modelo de flujo 3D usa la geometría y las distribuciones de parámetros hidrodinámicos que son inherentes a una aproximación genética a través del modelado de la cuenca; el modelo de cuenca crea un patrón geométrico de las propiedades hidrodinámicas limitadas por la historia geológica. El objetivo del trabajo es promover el uso de la aproximación del modelo de cuenca (a escala de largo plazo, 248xa0Ma) para el estudio de la respuesta de los acuíferos profundos a la perturbación antropogénica (a escala de corto plazo, 167 años) en situaciones para las cuales los datos hidrodinámicos son escasos pero los geológicos son numerosos. Los resultados muestran que las distribución de parámetros es insuficiente para reproducir el comportamiento del acuífero Albiano, destacando sobre todo un distinto significado del coeficiente de almacenamiento específico entre el modelado de cuencas y la modelización del flujo de aguas subterráneas. La división del coeficiente de almacenamiento por 100 e incluyendo los datos de transmisividad disponibles mejoraron significativamente la comparación de datos/modelo. Luego se discute el impacto potencial sobre un acuitardo profundo. Este estudio arroja luz sobre las ventajas y limitaciones de una aproximación de modelo de cuenca para el modelado de flujo de agua subterránea en 3D.ResumoO aquífero Albiano da Bacia de Paris (França) tem sido explorado desde 1841 e mostra rebaixamentos drásticos. Um modelo tridimensional (3D) de escoamento de águas subterrâneas foi utilizado para estudar a resposta hidrodinâmica ao bombeamento dos aquíferos multicamadas do Albiano, à escala da bacia, ao longo de 167 anos. Este modelo de fluxo 3D usa a geometria e as distribuições dos parâmetros hidrodinâmicos, herdados de uma abordagem genética através da modelação da bacia, criando no modelo de bacia um padrão geométrico de propriedades hidrodinâmicas conformadas pela história geológica. O trabalho tem como objetivo promover o uso da abordagem de um modelo de bacia (escala de tempo, 248xa0Ma) para o estudo da resposta do aquífero profundo a perturbações antrópicas (escala de tempo curta, 167 anos) em situações para as quais os dados hidrodinâmicos são escassos, mas os dados geológicos são numerosos. Os resultados mostram que a distribuição de parâmetros não é suficiente para reproduzir o comportamento do aquífero Albiano, realçando nomeadamente um significado diferente do coeficiente de armazenamento específico entre o modelo de bacia e o modelo de escoamento de águas subterrâneas. Dividindo o coeficiente de armazenamento por 100 e incluindo os dados de transmissividade disponíveis, observou-se uma melhoria significativa na comparação modelo/dados. O impacte potencial sobre um aquitardo profundo é, então, discutido. Este estudo lança luz sobre as vantagens e limitações da abordagem do modelo de bacia para o estudo do escoamento de águas subterrâneas em 3D.

Hydrogeological features relevant to radionuclide migration in the natural environment

Abstract: This chapter discusses the principles of hydrogeology. The main laws of groundwater movement in rocks are described and the relevant equations are established for saturated and unsaturated media. Common aquifer systems are presented and an overview of the way these equations can be applied to them and solved is then given.