Influence of grain fabric and lamination on the anisotropy of hydraulic conductivity in unconsolidated dune sands

Abstract

In local-scale groundwater flow problems, like for instance in contaminated groundwater studies, it becomes increasingly important for proper prediction of contaminant migration to incorporate small-scale geological and associated parameter heterogeneity in groundwater flow models. Of the micro-scale geological heterogeneities, the effect of grain fabric and lamination in unconsolidated sediments on the vertical anisotropy of effective hydraulic conductivity has received relatively little attention. This is mainly due to the low conductivity contrast between laminae in combination with the friable nature of unconsolidated sediment which makes it sensitive to disturbances during experimental procedures. As an alternative, digital image analysis of sediment thin sections was applied to separate and quantify both causes of anisotropy. The effective value of vertical anisotropy ratio of unconsolidated eolian dune sand as determined experimentally on distributed, orientated core plugs ranges between 1.10 and 1.50. This wide range is partly caused by artificial noise introduced by distributed sampling in a heterogeneous medium and assuming thereby that the sampled material is homogeneous. By applying digital image analysis of thin sections, high-resolution logs of hydraulic conductivity were generated. Simple upscaling of these values to effective conductivity parallel and perpendicular to sedimentary laminae provided quantitative estimates of the vertical anisotropy caused by lamination. Anisotropy values range between 1.0 and 1.075. The anisotropy ratio due to grain fabric ranges in value between 1.05 and 1.45 assuming that both anisotropy due to grain fabric and lamination are orientated identically. If this is not the case the effective anisotropy is the weighted mean of anisotropies of the constituent layer types. This study shows that for eolian sands the anisotropy owing to grain fabric dominates the effective value of anisotropy. If conductivity contrast between laminae increases, the contribution of anisotropy caused by grain fabric to the effective value of anisotropy decreases.