Fulvia Baratelli

Connectivity and single/dual domain transport models: tests on a point-bar/channel aquifer analogue

In porous aquifers, groundwater flow and solute transport strongly depend on the sedimentary facies distribution at fine scale, which determines the heterogeneity of the conductivity field; in particular, connected permeable sediments could form preferential flow paths. Therefore, properly defined statistics, e.g. total and intrinsic facies connectivity, should be correlated with transport features. In order to improve the assessment of the relevance of this relationship, some tests are conducted on two ensembles of equiprobable realizations, obtained with two different geostatistical simulation methods—sequential indicator simulation and multiple point simulation (MPS)—from the same dataset, which refers to an aquifer analogue of sediments deposited in a fluvial point-bar/channel association. The ensembles show different features; simulations with MPS are more structured and characterised by preferential flow paths. This is confirmed by the analysis of transport connectivities and by the interpretation of data from numerical experiments of conservative solute transport with single and dual domain models. The use of two ensembles permits (1) previous results obtained for single realizations to be consolidated on a more firm statistical basis and (2) the application of principal component analysis to assess which quantities are statistically the most relevant for the relationship between connectivity indicators and flow and transport properties.RésuméDans les aquifères poreux, l’écoulement d’eau souterraine et le transport de solutés dépend fortement de la distribution des faciès sédimentaires à l’échelle fine, ce qui détermine l’hétérogénéité du champ de conductivité hydraulique; en particulier, les sédiments perméables connectés peuvent donner lieu à des écoulements préférentiels. Ainsi, la connectivité totale et intrinsèque de faciès définie correctement de manière statistique devrait être corrélée avec les propriétés du transport. Afin d’améliorer l’évaluation de la signification de cette relation, des tests ont été menés sur deux ensembles de réalisations équiprobables, obtenues à partir de deux méthodes différentes de simulation numérique—simulation séquentielle à indicateurs et simulation en points multiples (SPM)—construites à l’aide du même jeu de données se rapportant à un analogue aquifère de sédiments déposés dans un environnement fluvial avec une association de barres et de chenaux ponctuels. Les ensembles montrent différentes caractéristiques ; les simulations avec SPM sont plus structurées et caractérisées par des écoulements préférentiels. Ceci est confirmé par l’analyse des connectivités lors du transport et par l’interprétation des données expérimentales numériques de transport conservatif issues de modèles en domaine simple et dual. L’utilisation des deux ensembles permet (1) de consolider des résultats obtenus au préalable pour des réalisations simples sur une base statistique plus solide et (2) l’application de l’analyse en composante principale pour évaluer quelles quantités sont les plus significatives du point de vue statistique pour la relation entre les indicateurs de connectivité et les propriétés d’écoulement et de transport.ResumenEn acuíferos porosos, el flujo del agua subterránea y el transporte de soluto dependen fuertemente de la distribución de las facies sedimentarias a una escala fina, lo cual determina la heterogeneidad del campo de conductividad; en particular los sedimentos permeables conectados pueden forman trayectorias preferenciales de flujo. Por lo tanto, la estadística correctamente definida, por ejemplo la conectividad total e intrínseca de las facies, debe ser correlacionada con las características del transporte. Con el objeto de mejorar la evaluación de la relevancia de esta relación, se llevaron a cabo algunos ensayos en dos conjuntos de realizaciones equiprobables, obtenidas con dos métodos de simulación geoestadística diferente—simulación de indicador secuencial y simulación de múltiples puntos (MPS)—a partir de un mismo conjunto de datos, los cuales se refieren a una analogía de acuífero sedimentos depositados en una asociación albardón/canal fluvial. Los conjuntos muestran diferentes características; las simulaciones con MPS son más estructuradas y caracterizadas por trayectorias preferenciales de flujo. Esto está confirmado por el análisis de las conectividades del transporte y por la interpretación de datos de experimentos numéricos del transporte conservativo de soluto con modelos de dominios simples y dobles. El uso de los dos conjuntos permite (1) consolidar, sobre una base estadística más firme, los resultados previos obtenidos para realizaciones simples y (2) la aplicación del análisis de la componente principal para evaluar cuales cantidades son estadísticamente más relevantes para la relación entre los indicadores de conectividad y las propiedades de flujo y transporte.摘要在孔隙含水层中地下水流和溶质运移强烈依赖于沉积物中颗粒精细分布的特征,它确定了传导场地的不均匀性,特别是相关联的可渗透沉积层形成优先的水流。因而,适当确定统计资料,例如:总的和固有的连通特质,需要关联其运移特征。以便评价其关系,这些实验产生了两个同等的认识,根据相同的资料以两种不同的地质统计方法—序次指标模拟和多重点模拟(MPS)—获得。其利用了沉积地层在水流的点状阻障/通道中含水层模拟评价。整体效果显示出不同的特征,MPS模拟更突出了优先水流的结构和特点。它根据分析传导连通性和稳定溶质在单/双域传导模型数值实验数据解译确认。应用在两个总体认识上:(1) 原结果在观测单域内的认识是以更牢靠的统计数据为基数,(2) 主成分分析的应用定量统计了在连通指标和流量及传导特性之间的相互关系。ResumoEm aquíferos porosos, o fluxo de água subterrânea e o transporte de solutos dependem fortemente da distribuição das fácies sedimentares à escala fina, que determina a heterogeneidade do campo de condutividades; em particular, os sedimentos permeáveis interconetados que podem formar caminhos de fluxo preferenciais. Por essa razão, é de esperar que certas estatísticas, definidas de forma adequada, tais como a conetividade total e intrínseca das fácies, se correlacionem com as caraterísticas de transporte. A fim de melhorar a avaliação da importância desta correlação, efetuaram-se alguns testes em dois conjuntos de realizações equiprováveis, obtidos com dois métodos geoestatísticos de simulação diferentes—simulação sequencial de indicadores e simulação por múltiplos pontos (MPS)—a partir do mesmo conjunto de dados, referente a um análogo de um aquífero de sedimentos depositados numa sequência fluvial de meandro/canal. Os dois conjuntos apresentam caraterísticas diferentes; as simulações com MPS são mais estruturadas e caraterizam-se por caminhos de fluxo preferenciais. Este facto é confirmado pela análise de conetividades de transporte e pela interpretação de dados de experiências numéricas de transporte de solutos conservativos com modelos de domínio único e duplo. O uso de dois conjuntos permite (1) que os resultados anteriores obtidos por realizações únicas possam ser consolidados numa base estatística mais firme e (2) a aplicação da análise de componentes principais para avaliar quais as quantidades estatisticamente mais relevantes para a relação entre os indicadores de conetividade e as propriedades de fluxo e transporte.

A sensitivity analysis for an evolution model of the Antarctic ice sheet

The evolution of the Antarctic ice sheet for the last 200,000 years is simulated with a finite difference thermomechanical model based on the shallow ice approximation. The model depends on the surface temperature, the ice accumulation rate, the geothermal heat flux and the basal sliding coefficient, which are estimated with large uncertainty. A second-order approximation of the model in a neighborhood of the reference values for these parameters permits the computation of both local and variance-based sensitivity indices. The results show the dominant effect of the surface temperature on the model predictions.

Single- and Dual-domain Models of Solute Transport in Alluvial Sediments: the Effects of Heterogeneity Structure and Spatial Scale

Fine-scale heterogeneity of alluvial aquifers controls solute transport in groundwater at the scales relevant for practical applications: the architecture of sedimentary structures might create preferential flow paths (PFPs) or hydraulic barriers, which affect the breakthrough curves (BTCs). Objective of this paper was the assessment of the relevance of single- and dual-domain models for different heterogeneity patterns and scale lengths in alluvial sediments. Three case studies have been analysed with a classical single-domain model (SDM) and with three dual-domain models (DDMs): a dual-porosity model (DPorM) and two dual-permeability models (DPerM), which differ for the presence or the absence of solute exchange between the two domains. The first case study includes numerical tracer tests in metre-scale blocks of alluvial sediments; the second is a laboratory experiment of tracer injection in a decimetre-scale column of homogeneous sand; the third is a field tracer test performed at hectometre scale at the Cape Cod site. The relevance of the solute exchange in the DDMs is analysed with the characteristic advection and exchange times and with the Péclet and Damköhler numbers. The SDM is satisfactory for alluvial sediments with unstructured heterogeneity. The uncoupled DPerM is shown to be a better approach than the DPorM in sediments with PFPs; in this case, the coupled DPerM does not improve significantly the results of the uncoupled DPerM. A minor difference between the results of the three DDMs is observed for sediments in which the non-Fickian behaviour is not clearly determined by the presence of PFPs.

Exposing high-school students to Geosciences through seminars, laboratory and field demonstrations

To familiarize high school students with the richness of geosciences and their practical applications, Universita degli Studi di Milano has been conducting a number of activities in co-operation with the Liceo G. Bruno (Melzo, MI, Italy). This contribution discusses the number of activities conducted including classroom seminars, laboratory experiments and in-the-field demonstrations.