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–Carman's 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 Darcy's 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 Darcy's law at a scale length of 1.5 m is a first-order approximation only.

Introduction

The distribution of sedimentary facies controls the heterogeneity of hydrogeological properties of porous sedimentary aquifers at different scales. The arrangement of individual facies and their porosity and permeability determine the path of groundwater flow across sedimentary bodies. Therefore the capability to forecast hydrogeological heterogeneity due to facies changes helps to improve solutions of flow and diffusion problems in this kind of aquifer. When real aquifers are studied, it is impossible to model groundwater flow at a scale such that we can take into account the effects of fine-scale sedimentary heterogeneity; in fact this would require a precise knowledge of the sedimentary bodies that cannot be obtained from sparse data at some wells and this would be prohibitive for the required computing power. Therefore the fine scale heterogeneity is usually `upscaled' and the heterogeneous real medium is substituted at a larger scale with an equivalent often anisotropic medium, whose parameters allow the reproduction of the average flow of the real heterogeneous sedimentary structure.

Several papers dealt with the problem of describing water flow at different scales introducing the appropriate physical laws and the appropriate physical parameters. We do not claim to present a thorough review of the literature on the subject, which can be found, for instance, in Koltermann and Gorelick (1996)and Anderson (1997). However we recall that several authors have applied techniques for the calculation of equivalent hydraulic conductivities for sand–shale sequences (see, among the others, Desbarats, 1987; Begg et al., 1989; Bachu and Cuthiell, 1990) and for heterogeneous confining layers (Bierkens, 1996). Several papers are devoted to the characterisation of groundwater flow in porous aquifers by studying outcropping sedimentary structures that are considered analogues of real aquifers (see, e.g., Bierkens and Weerts, 1994; Jussel et al., 1994; Whittaker and Teutsch, 1996).

Several papers present theoretical results about the problem of finding equivalent physical parameters for sedimentary bodies; see, for instance, the review by Renard and de Marsily (1997)and the papers by Warren and Price (1961), White and Horne (1987), Gómez Hernández and Journel (1991) and Dykaar and Kitanidis (1992). The issue of scaling physical laws and physical parameters has been discussed in relation with the characteristics of field measurements, e.g., by Cushman (1986)and Beckie (1996), whereas the problem of the determination of physical parameters describing groundwater flow at a given scale by solution of inverse problems is discussed, for example, in Ginn and Cushman (1992)and Giudici et al. (1995).

To test the influence of the distribution of facies heterogeneity on groundwater flow at different scales, we are studying several outcrops of Quaternary continental sedimentary units at the northern border of the Po plain (Northern Italy). We selected some cases that are representative of different sedimentary features typical of the aquifers in the alluvial Po plain and in the tributary valleys on the Alpine side. For this we join the traditional sedimentological method of facies analysis with numerical modelling of flow. Following the definitions of Koltermann and Gorelick (1996), we apply a descriptive approach using a zonation with homogeneous hydrofacies, based on geological information.

Our objective is to test the influence of sedimentary heterogeneity on the anisotropical behaviour of the aquifer at different scales, from the lower-rank architectural units (microbedforms resting above first order surfaces, according to Miall, 1996) up to the sediment packages that build the elements of the sedimentary architecture (e.g. accretion macroforms and/or minor channels). In this paper, we present a study of a quarry exposure of Quaternary gravelly-sand units which developed in glacio-fluvial to glacio-lacustrine delta environments, to study the effects of different features, such as sandy delta foresets vs. sheeted or channelled sands and gravels. The abandoned quarry is known as the `Bassone quarry' and is located near Como (Lombardy, Northern Italy; Fig. 1).

In Section 2we describe the methods applied in this work, and there we will discuss the link between sedimentological and modelling research. 3 Geological and sedimentological results, 4 Two-dimensional groundwater flow modelare devoted to the geological and sedimentological results and to the numerical modelling experiments, respectively. Namely, in Section 3we present the results of the sedimentological survey, which are the basis for the reconstruction of the K distribution and the numerical modelling experiments, described in Section 4. A discussion about some limitations of the method used for this study is found in Section 5together with the main conclusive remarks, which show the importance of joining sedimentological analysis and ground water flow models.