Müller, K. (Corresponding author)FZJ*

2006
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 3-89336-465-X, 978-3-89336-465-7

Abstract: The relevance of aquifer heterogeneity for flow and transport is broadly recognized. A characterization of aquifer properties and the local flow and transport processes is obviously hampered by the inaccessibility of the subsurface. To optimize the characterization of the spatial and temporal variability of transport in a heterogeneous aquifer tracer experiments were carried out. Monitoring tracer plumes during tracer tests solely with a grid of multilevel observation wells is often problematic due to large changes of concentrations over small distances. Using geo-electrical imaging methods concentration distributions can be mapped over a wide area. At the Krauthausen test site (Germany) two large-scale tracer experiments in a heterogeneous aquifer were conducted to investigate the potentiality of Electrical Resistivity Tomography (ERT) for imaging subsurface transport. Down-gradient to the tracer injection the breakthrough in three image planes, two planes perpendicular and one parallel to the mean flow direction, was monitored with ERT. First, a CaCl$_{2}$ tracer solution with a higher electrical conductivity than the groundwater was injected. One year later, the same experiment was repeated using a so-called ‘negative’ tracer with a lower conductivity than the groundwater. The conduction of two consecutive large-scale tracer experiments under similar boundary conditions and the application of a 'positive' tracer with density effects and a 'negative' tracer with lower density than the natural groundwater are the outstanding features of this work. With ERT changes in bulk electrical conductivity of the aquifer were monitored. Using calibration relations derived in laboratory experiments on columns packed with aquifer sediment, the bulk electrical conductivity measured with ERT was related to solute electrical conductivities measured with multilevel samplers (MLS). Although the injection well was screened over the entire aquifer thickness, both tracers were found in the middle and bottom part of the aquifer but not in the upper part where a high transport velocity was expected. The effect of density driven flow was clearly visible for the 'positive' tracer which showed a tendency towards the aquifer bottom boundary where it was spread out laterally. With ERT about 70 % of the injected tracer mass was recovered at the first cross section. The comparison between ERT and groundwater sampling showed that resistivity tomograms are an appropriate surrogate for traditional concentration maps at the Test site Krauthausen. Furthermore they allow the characterization of transport in heterogeneous aquifers with such a high spatial and temporal resolution that is otherwise impossible to obtain. The spatial variability of the transport processes in a heterogeneous aquifer is caused by the spatial variability of the hydraulic conductivity. Using cone penetration tests (CPT), the spatial distribution of a parameter that is related to the hydraulic conductivity was derived and empirical relations were used to derive the 3D distribution of the hydraulic conductivity in the tracer test zone. The 2D spatio-temporal information about the transport obtained with ERT in the image planes was compared with three-dimensional numerical flow and transport simulations using the TRACE and PARTRACE numerical models and an aquifer model based on cone penetration data. In order to compare the ERT and numerical simulation data sets, a stream tube model that represents transport in the aquifer by a set of 1D [...]

Keyword(s): tracer technique ; aquifer ; transport properties ; environmental transport ; pollutant transport

1. Agrosphäre (ICG-4)