Riccardo Bersezio

Effects of sedimentary heterogeneity on groundwater flow in a Quaternary pro-glacial delta environment: joining facies analysis and numerical modelling

Abstract We present the sedimentological survey of Quaternary sediments developed in glacio-fluvial to glacio-lacustrine delta environments and exposed at an abandoned quarry in Northern Italy. This outcrop is considered an analogue of sedimentary structures that could form a real aquifer. We recognise the following sedimentary units: (I) colluvial sediments with human artefacts (less than 2 m thick); (II) gravelly sands with oversized pebbles (2.5–5.5 m thick), bounded at the base by a roughly flat composite disconformity surface; (III) and (IV) gravelly-sand foresets which form two bodies with different dips that are separated by an inclined, almost planar, erosional surface. From past observations in the surrounding area and a geoelectrical survey, we estimate that the minimum total thickness of the delta system is 20 m. We have analysed the sedimentary characteristics of two well exposed areas, which are located within units II, III and IV and covering areas of 115 and 95 m2, using detailed sedimentological surveys along six vertical sections. With the aid of a topographic survey we have reconstructed the distribution of textural units based on sedimentary facies and including information about grain-size distribution, sorting and packing. We have constructed a sedimentary model for the two areas using orthogonal cells of 10×10 cm, attributing the dominant textural unit to each cell. The distribution of sedimentary textural units is used to estimate the distribution of hydraulic conductivity for the two areas of this aquifer analogue; we have determined the hydraulic conductivity by assigning different values to the different textural units according to laboratory measurements of porosity and grain-size distribution, Kozeny–Carmans equation and literature data. We have modelled groundwater flow in the two areas approximating the balance equation with finite differences, using a regular grid with 10 cm spacing, and we have performed some numerical experiments. First, we have evaluated the equivalent conductivity tensor for the two areas. Secondly, we have considered a coarse grid, with 1.5 m spacing, and at this scale we have analysed the validation of the discrete form of Darcys law usually introduced in finite difference modelling. The results of the numerical experiments show the anisotropic behaviour of the medium, especially for unit II composed of sandy beds, some of which lead to a preferential path for groundwater flow in the horizontal direction. The deeper sedimentary units III and IV are characterised by a lower anisotropy ratio for hydraulic conductivity, because the layered structures of these units have different dips. Finally we show that the discrete form of Darcys law at a scale length of 1.5 m is a first-order approximation only.

Hydrogeophysical imaging of alluvial aquifers: electrostratigraphic units in the quaternary Po alluvial plain (Italy)

The integration of surface geological and geomorphological information with borehole point-data and geophysical (e.g., geoelectrical) images of the subsurface yields spatially consistent representations of alluvial aquifers heterogeneity at different scales, from depositional systems to basin fills. Such an approach requires a conceptual framework to match the stratigraphic units with their evidence from ground-based DC resistivity methods to effectively fill the gaps between sparse borehole data and to obtain valid representations of sedimentary heterogeneities. Such an approach is applied to characterize two sites of the Quaternary aquifers of the central Po Plain (Italy), which represent (1) the middle-upper Pleistocene braided to meandering river depositional systems sitting on Southalpine crust and (2) their down-current counterparts, where they are involved by the latest uplift and deformation due to the tectonic activity of the Apennine frontal thrusts. Electrical resistivity was considered as a proxy of the litho-textural properties of hydrofacies and their major hierarchical association at depth and was interpreted in accordance with the depth-decreasing resolution of ground-based resistivity methods. Thus, it was possible to identify the geophysical signature of hydrostratigraphic units through “Electrostratigraphic Units”, i.e., sedimentary volumes identified by resistivity contrasts that spatially preserve the vertical polarity. Hydrostratigraphy and electrostratigraphy were then joined together through a site-specific relationship between electrical resistivity and hydraulic conductivity, which takes into account the prevailing process of current conduction, the litho-textural properties of hydrofacies and the groundwater electrical conductivity. At the scales of aquifer systems and complexes, this approach permitted to establish the conceptual framework to match hydrostratigraphy, electrostratigraphy, average hydrodynamic properties and distribution of heterogeneities.

Relating electrical conduction of alluvial sediments to textural properties and pore-fluid conductivity

Electrical conductivity of alluvial sediments depends on litho-textural properties, fluid saturation and porewater conductivity. Therefore, for hydrostratigraphic applications of direct current resistivity methods in porous sedimentary aquifers, it can be useful to characterize the prevailing mechanisms of electrical conduction (electrolytic or shale conduction) according to the litho-textural properties and to the porewater characteristics. An experimental device and a measurement protocol were developed and applied to collect data on eight samples of alluvial sediments from the Po plain (Northern Italy), characterized by different grain-size distribution, and fully saturated with porewater of variable conductivity. The bulk electrical conductivities obtained with the laboratory tests were interpreted with a classical two-component model, which requires the identification of the intrinsic conductivity of clay particles and the effective porosity for each sample, and with a three-component model. The latter is based on the two endmember mechanisms, surface and electrolytic conduction, but takes into account also the interaction between dissolved ions in the pores and the fluid-grain interface. The experimental data and their interpretation with the phenomenological models show that the volumetric ratio between coarse and fine grains is a simple but effective parameter to determine the electrical behaviour of clastic hydrofacies at the scale of the representative elementary volume.

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.

Simulation of Fine-Scale Heterogeneity of Meandering River Aquifer Analogues: Comparing Different Approaches

We compare different approaches to fine scale simulation of aquifer heterogeneity of meandering river depositional elements, based on the study of a 3-D quarry exposure of historical point bar-channel sediments of the Lambro River (Po plain, Northern Italy). The starting point is a sedimentological and hydrostratigraphic hierarchic model obtained after mapping of five quarry faces with centimeter-scale detail. The vertical facies maps show the shape and size of two superimposed composite bars, of their component unit bars and channel fills and the distribution of the individual facies within them. Textural and poro-perm analyses allowed the definition of the properties of four basic hydrofacies (Open Framework Gravels, Gravelly Sands and Sandy Gravels, Clean Sands, Sandy Silts and Clays), with permeability contrasts by at least one order of magnitude \((1{0}^{-9} < \mathrm{K} < 1{0}^{-1})\). The correlation of hydrofacies has been quantified after discretization of the maps with square cells (side 0.05 m), by both transition-probability geostatistics and variographic analysis, to support 3-D pixel-oriented simulation of the volume. We found a high level of correspondence between the semivariogram ranges and the experimental transition probabilities computed on the entire dataset. Several realizations of 3-D conditioned simulations, that honour the vertical facies maps, were computed using Sequential Indicator Simulation (SIS) and T-Progs (transition-probability geostatistics software). Both methods yield more realistic results if the highest rank depositional elements are simulated separately than if the sedimentary volume is simulated on the whole. Image analyses on random sections through selected realizations shows that, in this specific case, SIS yields the most realistic simulations. However, both techniques are not capable of accounting for trends of depositional features that determine a non-stationary behaviour at the facies scale.

Hierarchical simulation of aquifer heterogeneity: implications of different simulation settings on solute-transport modeling

The fine-scale heterogeneity of porous media affects the large-scale transport of solutes and contaminants in groundwater and it can be reproduced by means of several geostatistical simulation tools. However, including the available geological information in these tools is often cumbersome. A hierarchical simulation procedure based on a binary tree is proposed and tested on two real-world blocks of alluvial sediments, of a few cubic meters volume, that represent small-scale aquifer analogs. The procedure is implemented using the sequential indicator simulation, but it is so general that it can be adapted to various geostatistical simulation tools, improving their capability to incorporate geological information, i.e., the sedimentological and architectural characterization of heterogeneity. When compared with a standard sequential indicator approach on bi-dimensional simulations, in terms of proportions and connectivity indicators, the proposed procedure yields reliable results, closer to the reference observations. Different ensembles of three-dimensional simulations based on different hierarchical sequences are used to perform numerical experiments of conservative solute transport and to obtain ensembles of equivalent pore velocity and dispersion coefficient at the scale length of the blocks (meter). Their statistics are used to estimate the impact of the variability of the transport properties of the simulated blocks on contaminant transport modeled on bigger domains (hectometer). This is investigated with a one-dimensional transport modeling based on the Kolmogorov-Dmitriev theory of branching stochastic processes. Applying the proposed approach with diverse binary trees and different simulation settings provides a great flexibility, which is revealed by the differences in the breakthrough curves.RésuméL’hétérogénéité à échelle fine des milieux poreux affecte le transport de solutés et de contaminants à grande échelle dans les eaux souterraines et elle peut être reproduite au moyen de nombreux outils de simulation géostatistique. Cependant, il est souvent difficile d’intégrer l’information géologique disponible avec ces outils. Une procédure de simulation hiérarchique basée sur un arbre binaire est proposée et testée sur deux blocs réels de sédiments alluviaux, d’un volume de quelques mètres cubes, qui représentent des analogues d’aquifère à petite échelle. La procédure est implémentée en utilisant la simulation d’un indicateur séquentiel. Néanmoins, elle est suffisamment générale pour pouvoir être adaptée à de nombreux outils de simulation géostatistique, améliorant ainsi leur facilité à incorporer de l’information géologique, i.e., la caractérisation de l’hétérogénéité en termes sédimentologique et d’architecture. Lorsqu’elle est comparée à une approche d’indicateur séquentiel standard pour des simulations en 2D, la procédure proposée fournit des résultats fiables, plus près des observations de référence, en termes d’indicateurs de proportions et de connectivité. Différents ensembles de simulations 3D, basées sur différentes séquences hiérarchiques, sont utilisées pour réaliser des expériences numériques de transport conservatif de solutés et pour obtenir des ensembles de vitesse de pore équivalente et de coefficient de dispersion à l’échelle de longueur des blocs (mètre). Leurs statistiques sont utilisées pour estimer l’impact de la variabilité des propriétés de transport des blocs simulés sur le transport de contaminants modélisé sur de plus grands domaines (hectomètre). Ceci est investigué avec un modèle de transport 1D basé sur la théorie de Kolmogorov-Dmitriev de processus stochastiques en branches. L’application de l’approche proposée avec différents arbres binaires et différentes configurations de simulation fournit une grande flexibilité, qui est mise en lumière par les différences des courbes de restitution.ResumenLa heterogeneidad a escala fina de loa medios porosos afecta el transporte a gran escala de los solutos y contaminantes en el agua subterránea y puede ser reproducida por medio de varias herramientas de simulación geoestadística. Sin embargo, la inclusión de la información geológica disponible en estas herramientas es a menudo engorrosa. Se propone y se prueba un procedimiento de simulación jerárquica basada en un árbol binario y en dos bloques de sedimentos aluviales de existencia real de un volumen de unos pocos metros cúbicos, que representan una analogía del acuífero en pequeña escala. El procedimiento se implementa utilizando un indicador de la simulación secuencial, pero es tan general que puede ser adaptado a diversas herramientas de simulación geoestadística, mejorando su capacidad para incorporar la información geológica, es decir, la caracterización sedimentológica y arquitectónica de la heterogeneidad. Cuando se los compara con el enfoque de un indicador secuencial estándar en simulaciones bidimensionales, en términos de proporciones e indicadores de conectividad, el procedimiento propuesto produce resultados confiables, más cercanos de las observaciones de referencia. Se usan diferentes conjuntos de simulaciones tridimensionales basadas en distintas secuencias jerárquicas para realizar experimentos numéricos de transporte conservativos de solutos y para obtener conjuntos de velocidad de equivalente poral y coeficientes de dispersión a escala de los bloques (metros). Sus estadísticas se utilizaron para estimar el impacto de la variabilidad de las propiedades de transporte de los bloques simulados sobre el transporte de contaminantes modelados en dominios más grandes (hectómetro). Esto se investigó con una modelización del transporte unidimensional basada en la teoría de Kolmogorov-Dmitriev de los procesos estocásticos ramificados. La aplicación del enfoque propuesto con distintos árboles binarios y diferentes configuraciones de simulación proporciona una gran flexibilidad, que está ilustrada por las diferencias en las curvas de rotura.摘要多孔介质小尺度异质性影响着地下水中溶质和污染物大规模迁移,可通过几个地质统计模拟工具再现此情景。然而,包含现有地质信息的这些工具常常是笨重的。本文提出了基于二元树的分等级模拟程序,并在两个现实生活中几立方的冲积沉积地块中进行了测试,这几立方的冲积沉积地块代表着小尺度的含水层类似物。采用连续指示物模拟完成了模拟过程,但过程非常普通,以至于可以用到各种地质统计模拟工具中,提高综合地质信息的能力,即沉积学和建筑学上的异质性描述。在与二维模拟标准的连续指示物方法相比,在比例和连通性指示物方面,提出的程序可得到可靠的结果,更接近参考观测值。基于不同分等级序列的三维模拟不同的总结果用于进行保守的溶质迁移数值实验,并获取地块标尺长度(米)相等孔隙速度和弥散系数的总结果。其统计结果用于估算模拟地块迁移特性的变化性对较大域(百米)污染物迁移的影响。利用基于分支随机程序Kolmogorov-Dmitriev理论的一维迁移模型对此进行了研究。采用各种各样的二元树和不同的模拟背景应用所提出的方法提供了很大的灵活性,而此灵活性通过突破曲线的差别得到显现。RiassuntoL’eterogeneità dei mezzi porosi a scala fine influenza il trasporto a larga scala di soluti e contaminanti negli acquiferi, e può essere riprodotta tramite diversi metodi di simulazione geostatistica. Tuttavia, non è sempre semplice includere informazioni geologiche in questi metodi di simulazione. Si propone quindi una procedura di simulazione gerarchica basata sul concetto di albero binario, e la si verifica su due blocchi di sedimenti alluvionali aventi un volume di pochi metri cubi e che rappresentano analoghi di acquiferi a piccola scala. La procedura è implementata tramite la simulazione sequenziale con indicatori, ma il principio è generale e può essere adattato ad altri metodi di simulazione geostatistica, migliorandone la capacità di includere informazioni geologiche legate per esempio alla caratterizzazione sedimentologica e architetturale dell’eterogeneità. Confrontata con una procedura di simulazione sequenziale con indicatori standard in termini di proporzioni e di indicatori di connettività, la procedura proposta restituisce risultati affidabili e più vicini alle osservazioni di riferimento. Diversi ensemble di simulazioni tridimensionali, basate su diverse sequenze gerarchiche, sono utilizzati in simulazioni numeriche di trasporto conservativo e per ottenere ensemble di velocità di poro equivalente e di coefficiente di dispersione alla scala dei blocchi (metri). Le loro statistiche sono usate per stimare l’impatto della variabilità delle proprietà di trasporto dei blocchi simulati sul trasporto di inquinanti modellato su domini più grandi (ettometri). Queste stime sono effettuate tramite simulazioni monodimensionali di trasporto basate sulla teoria dei processi stocastici ramificati di Kolmogorov-Dmitriev. Applicare l’approccio proposto utilizzando alberi binari differenti e diverse configurazioni di simulazione risulta in una notevole flessibilità, messa in evidenza dalle differenze osservate nelle curve di restituzione.ResumoA heterogeneidade em pequena escala do meio poroso afeta o transporte de solutos e contaminantes nas águas subterrâneas em grande escala e estes podem ser reproduzidos por diversas ferramentas de simulação geoestatísticas. Entretanto, incluir as informações geológicas disponíveis nestas ferramentas é frequentemente trabalhoso. Um procedimento de simulação hierárquica baseado em uma árvore binaria é proposta e testada em dois blocos de sedimentos aluviais reais com volume de poucos metros cúbicos, que representam aquíferos em pequena escala análogos. O procedimento é implementado usando simulação sequencial indicadora, porém isto é tão genérico que pode ser adaptado para diversas ferramentas geoestatísticas, melhorando a capacidade de incorporar as informações geológicas, por exemplo, a caracterização sedimentológica e arquitetural da heterogeneidade. Quando comparada com uma abordagem tradicional de simulação sequencial indicadora em bidimensionais, em termos de indicadores de proporções e conectividade, o procedimento proposto apresenta resultados confiáveis, próximos as observações de referência. Diferentes conjuntos de simulações tridimensionais baseadas em diferentes sequências hierárquicas são usados para gerar experimentos numéricos de transporte de solutos conservativos e para obter conjuntos de velocidade equivalente em poros e coeficientes de dispersão na escala do comprimento dos blocos (metros). Suas estatísticas são usadas para estimar o impacto da variabilidade das propriedades de transporte dos blocos simulados no trans

Geology of the San Colombano hill, a Quaternary isolated tectonic relief in the Po Plain of Lombardy (Northern Italy)

ABSTRACT The 1:10,000 geological map of the San Colombano hill covers 60 km2 in the Po Plain, south of Milan. The new and the historical surface geological data-sets are managed by a GeoDB aiming to contribute to re-interpret the Quaternary evolution at the Po Plain-Northern Apennine border. On the hill, the Calabrian shallow marine San Colombano Fm. unconformably overlies the truncated deeper-marine Miocene formations, up-thrusted by the external fronts of the Apennine Emilian Arc during Mio-Pliocene. Late Pleistocene alluvial units rest in unconformity above the marine succession both on the uplifted hilltop and on the surrounding plain. Fault-related offset of Late Pleistocene units, stratigraphic and morpho-structural evidences (facets, relic surfaces and drainage patterns), document the Quaternary tectonic history. Early to Middle Pleistocene ongoing thrust-folding at the northernmost buried reaches of the Emilian Arc was followed by Latest Pleistocene-Holocene transtension, possibly relating to the NNE striking Pavia-Casteggio lateral ramp.