The Impact Sedimentary Anisotropy on Solute Mixing in Groundwater
O 15.8 in Forum Junge Hydrogeologen
13.04.2016, 14:15-14:30, Plank Hörsaal, Geb. 40.32
The importance of realistic sedimentary architectures for flow and transport in gravel deposits has been analyzed in aquifer-analog studies (e.g., Bayer et. al., 2011). A common approach is to take outcrop data, identify structural elements with similar hydraulic characteristics (“hydrofacies”), assign hydraulic conductivity values to each hydrofacies, and analyse metrics of groundwater flow (e.g., effective hydraulic-conductivity tensors) and solute transport (e.g., macrodispersion coefficients). However, in many studies the internal anisotropy of the sedimentary units has been neglected. This anisotropy is discontinuous at the interfaces of the hydrofacies.
Bakker and Hemker (2004), and more recently Cirpka et al. (2015), have demonstrated that changes in the orientation of hydraulic anisotropy can cause twisting in groundwater velocity fields, which impacts transverse mixing and thus the length of mixing-controlled contaminant plumes. In these studies, however, extended stripes of aquifer material with particular anisotropy orientations have been assumed, which do not resemble real sedimentary architectures.
In the present study, we consider a virtual, three-dimensional gravel aquifer composed of trough-fill structures with blockwise homogeneous, anisotropic hydraulic conductivity, mimicking glaciofluvial sediments reported in the upper Rhine valley. We compare four cases: (1) the homogeneous, isotropic base case; (2) variability of isotropic hydraulic conductivity between the individual trough fills; (3) variability in the orientation of anisotropic hydraulic conductivity among the trough fills, in which the principal values of hydraulic conductivity are identical in all blocks; (4) the combination of variability in the absolute value of hydraulic conductivity and the orientation of anisotropy. For cases 2-4, we generate multiple realisations, perform flow simulations in such a way that the mean flow velocity vector is identical among all realisations in all cases, and conduct transport simulations on streamline-oriented grids.
The results clearly show that variability in the orientation of anisotropy is the key factor for variability in transverse flow components and thus controls transverse mixing. By contrast, the variability in the absolute value of hydraulic conductivity controls the variability of the longitudinal velocity fluctuations and thus longitudinal macrodispersion.
We conclude that estimating the small-scale anisotropy of sedimentary units is important for the prediction of transverse mixing. In most practical applications such information is difficult to gather by field surveys, but extended aquifer-analog studies may help in estimating the strength of anisotropy and the range of variability in its orientation.
BAKKER, M., HEMKER, K. (2004), Analytic solutions for groundwater whirls in box-shaped, layered anisotropic aquifers, Adv. Water Res., 27(11), 1075–1086.
BAYER, P., HUGGENBERGER, P., RENARD, P., COMUNIAN, A. (2011), Three-dimensional high resolution fluvio-glacial aquifer analog: Part 1: Field study, J. Hydrol., 405(1–2), 1–9.
CIRPKA, O. A., CHIOGNA, G. , ROLLE, M., BELLIN, A. (2015), Transverse mixing in three-dimensional nonstationary anisotropic heterogeneous porous media, Water Resour. Res., 51, 241–260.
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