Geostatistical simulation and numerical upscaling, to model ground-water flow in a sandy-gravel, braided river, aquifer analogue

An integrated approach combining sedimentological, geostatistical, and numerical analyses has been applied to model groundwater flow through a glaciofluvial, braided-stream, gravel-sand aquifer of Pleistocene age, exposed in a gravel pit in the eastern Piedmont (northern Italy). The aquifer under investigation consists of a stack of braided bar and minor channel fills and scour rills, where different gravel-sand bedforms migrated. It represents an analogue of buried sediments, which host some of the most important aquifer complexes and water resources of northern Italy. The work has been performed with three main steps: (1) drawing and discretization of the geological models of two orthogonal exposures and estimate of conductivity of every facies; (2) geostatistical simulation of the 3D facies heterogeneity, based on semivariogram analysis of the geological models; (3) application of a 3D numerical model of flow to upscale the conductivity field. Semivariogram analysis developed from facies indicator functions has shown that: (a) the variability in the data set in the horizontal direction is a representation of the oriented distribution of the sedimentary facies, corresponding to the average N-S elongation of sediment units, according to the paleoflow direction; (b) the vertical correlation length approximates the average thickness of the bedsets bounded by fourth-order erosional surfaces (unit bars, scour fill, and small channel fill associations) and may represent the thickness of an individual hydrostratigraphic unit; (c) the conductivity and facies distributions are more variable in the vertical direction than in the horizontal one, with a ratio between vertical and horizontal ranges of about 1:8. The 3D conductivity distribution within the considered deposits has been determined using a sequential indicator simulation, whereas transition-probability analysis has been used to confirm the correlation model obtained with semivariogram analysis and to check the result of the simulation. The equivalent hydraulic-conductivity tensor has been computed with finite-difference modeling of groundwater flow for cubic blocks of the aquifer analogue with side length of about 13 in. At this scale the medium is almost homogeneous, but anisotropic, with the lowest values of hydraulic conductivity in the vertical direction and the greatest values along the elongation of the sedimentary units, i.e., almost parallel to average paleoflow.