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Using Multiple-Point Geostatistics for Tracer Test Modeling in a Clay-Drape Environment with Spatially Variable Conductivity and Sorption Coefficient |
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| Authors: | Marijke Huysmans Philippe Orban Elke Cochet Mathias Possemiers Benedicta Ronchi Katherine Lauriks Okke Batelaan Alain Dassargues |
| Institution: | 1. Department of Earth and Environmental Sciences, KU Leuven, Celestijnenlaan 200 E, 3001, Heverlee, Belgium 2. Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050, Brussels, Belgium 3. Hydrogeology and Environmental Geology, Department of Architecture, Geology, Environment, and Civil Engineering (ArGEnCo), Université de Liège, B.52/3 Sart-Tilman, 4000, Liège, Belgium 4. School of the Environment, Flinders University, GPO Box 2100, Adelaide, 5001, Australia |
| Abstract: | This study investigates the effect of fine-scale clay drapes on tracer transport. A tracer test was performed in a sandbar deposit consisting of cross-bedded sandy units intercalated with many fine-scale clay drapes. The heterogeneous spatial distribution of the clay drapes causes a spatially variable hydraulic conductivity and sorption coefficient. A fluorescent tracer (sodium naphthionate) was injected in two injection wells and ground water was sampled and analyzed from five pumping wells. To determine (1) whether the fine-scale clay drapes have a significant effect on the measured concentrations and (2) whether application of multiple-point geostatistics can improve interpretation of tracer tests in media with complex geological heterogeneity, this tracer test is analyzed with a local three-dimensional ground-water flow and transport model in which fine-scale sedimentary heterogeneity is modeled using multiple-point geostatistics. To reduce memory needs and calculation time for the multiple-point geostatistical simulation step, this study uses the technique of direct multiple-point geostatistical simulation of edge properties. Instead of simulating pixel values, model cell edge properties indicating the presence of irregularly shaped surfaces are simulated using multiple-point geostatistical simulations. Results of a sensitivity analysis show under which conditions clay drapes have a significant effect on the concentration distribution. Calibration of the model against measured concentrations from the tracer tests reduces the uncertainty on the clay-drape parameters. The calibrated model shows which features of the breakthrough curves can be attributed to the geological heterogeneity of the aquifer and which features are caused by other processes. |
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