1 Department of Environmental Engineering, Technical University of Denmark2 Water Resources Engineering, Department of Environmental Engineering, Technical University of Denmark3 Geological Survey of Denmark and Greenland4 Geological Survey of Denmark and Greenland
The Role of Sand Lenses
The topic of this PhD thesis is an integrated investigation of sand lenses in glacial diamictons. Sand lenses indicate various deposition regimes and glaciotectonic deformation styles and are as such important features in studies of glacial sediments. In a hydrogeological framework, sand lenses further constitute conductive facies within low-permeability tills and are suspected to affect hydraulic conductivity fields and subsurface transport behaviour. The purpose of the study is to characterize sand lenses in terms of occurrence, geometry and connectivity and to assess their importance for contaminant transport in clayey tills. Sand lenses are considered enigmatic geological features resulting from complex interplay of glacial deposition and deformation. The subglacial hydraulic conditions and the predominant deforming forces are determining the appearance of sand lenses. Despite the abundance and the variability of occurrence, there is only sparse systematic information reported in the literature. As an example there is no consistent nomenclature for the different types of sand and gravel deposits in tills. In this study, the specific geometry of sand lenses was characterized by means of a field observation study and a literature survey. A number of geometric parameters (length, thickness, anisotropy, orientation, etc.) were selected to describe the size and shape of sand lenses. The resulting characteristic measures were used to define a classification scheme and to categorize five types of sand lenses. These are: sand layers, sand sheets, sand bodies, sand pockets and sand stringers. The scheme is a useful tool to include sand lenses in future till investigations and it supports rapid identification of till types. The spatial distribution of sand lenses is variable because of generally complex architectures of till successions. On the other hand, it is a relevant parameter to describe because mean lengths and spacing determine the connectivity between lenses. Pixel-based mapping of geological cross-sections was performed to facilitate geostatistical analyses of spatial variability. Variogram models yield nonstationary patterns including trending in vertical direction, variable size of lenses and strong geometric anisotropy. Non-stationarity complicates the identification of correlation functions and hampers the simulation of facies distribution. Transition probability-based geostatistics applied abundantly in modeling complex facies architecture were used in this study to simulate the variability of sand lenses in tills. Multiple-point statistics, however, showed enhanced capabilities to reproduce characteristic geological structures. Especially strong anisotropy and variable size of sand lenses were best represented in multiple-point realizations. Stochastic models enable the identification of connectivity functions and can be used to simulate heterogeneity at poorly or unsampled locations. Once the specific structures of sand lenses are reproduced to satisfaction, hydraulic parameters can be assigned to the different geological facies. The average hydraulic conductivity between the sand lenses and the clayey matrix differ by three to four orders of magnitude. The influence of sand lenses on the transport regime thus depends primarily on the connectivity between lenses. Three-dimensional realizations indicate clear channel networks, whereas only limited connectivity was found for the two-dimensional case. This is an important aspect because it emphasizes the need to collect data and to represent this type of heterogeneity in 3D. The physical response of sand lens heterogeneity was evaluated performing solute transport modeling mimicking leaching from a contaminated site in clayey till. It emphasized the need to include geological heterogeneity even if occurring at the finest scale. Compared to average or random conductivity fields, simulated sand lenses with specific hydraulic properties enhance the horizontal spreading of contaminants without a significant increase of the equivalent permeability in the till. Overall, sand lenses occur in all types of glacial sediments and with a broad range of shapes and hydraulic properties. Geometric characterization enabled classification of the most common types. Geostatistical analyses suggested that sand lenses in tills create connected channel networks in 3D. Consequently, sand lens heterogeneity is an important aspect when modeling transport processes of contaminants or performing risk assessment in clayey till settings. In either case it is recommended to consider and to include detailed representations of heterogeneity and sand lenses.