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Imaging and characterization of spatial connectivity in a heterogeneous alluvial aquifer

Nils Gueting1, Andreas Englert2, Jan Vanderborght1, Jan van der Kruk1, Harry Vereecken1
1 IBG-3, Forschungszentrum Jülich
2 Angewandte Geologie, Ruhr-Universität Bochum

O 5.3 in Geophysikalische Methoden in der Hydrogeologie

29.05.2014, 14:40-15:00, H17, NW II

Predicting groundwater flow and contaminant transport in the subsurface is challenging because of the complex heterogeneity found in most geologic media. Of particular significance for flow and transport processes is the question if specific subsurface structures are spatially connected or not. Connected structures of high permeability act as preferential flow paths and lead to increased water fluxes and increased transport velocities. Connected structures of small permeability act as flow barriers and lead to decreased water fluxes and decreased transport velocities. Although knowledge of the connectivity structure is critical for flow and transport predictions, its characterization in natural aquifers is difficult because the subsurface is not easily accessible through measurements. In this study, we present an approach how to characterize spatial connectivity in the field based on high-resolution cone penetration tests (CPT) and full-waveform inversion of cross-hole ground penetrating radar (GPR) data. We apply the approach to field data from the uppermost alluvial aquifer at the Krauthausen test site, which was found to exhibit considerable heterogeneity in previous studies. Our approach consists of the following steps: First, we apply a cluster analysis with the CPT data and with the GPR data to distinguish different facies in the subsurface. While the CPT data set yields the detailed distribution of facies in vertical profiles, the GPR data set resolves the lateral distribution of facies in two-dimensional sections between boreholes. We develop a lithological interpretation of individual facies based on (1) soil properties derived from CPT, (2) electrical properties derived from GPR, and (3) grain size distributions measured in co-located boreholes. We characterize facies architecture on 2D planes between several boreholes and derive connectivity metrics for each facies. Using several planes with different orientations allows us to evaluate horizontal anisotropy and to draw conclusions about facies connectivity in 3D. Finally, we compare facies architecture and connectivity structure with the results of previously conducted pumping and tracer tests. Our work is still in progress, but first results look promising that the cluster analysis performed with CPT and GPR data allows to identify a set of distinct lithological facies in the subsurface. Boundary layers between individual facies appear to correlate with changes in grain size distribution measured in co-located boreholes. Moreover, a first comparison of the spatial distribution of facies derived independently from CPT and from GPR showed good agreement. Thus, we are confident that a detailed analysis of our data will provide important insights about facies connectivity in the subsurface, which is of critical importance for predicting flow and transport in heterogeneous aquifers.



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Letzte Änderung 31.10.2013