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

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; (m) and (TV) 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 m(2), 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 x 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 how 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 m and TV are characterised by a lower anisotropy ratio for hydraulic conductivity, because the layered structures of these units have different dips.