Solute Transport Processes in a Highly Permeable Fault Zone of Lindau Fractured Rock Test Site (Germany)

Abstract

AbstractThe results of field tracer experiments performed in the Lindau fractured rock test site (southern Black Forest, Germany) and subsequent modeling are presented. A vertical, hydrothermally mineralized fault zone, with a permeability much greater than the surrounding granite mass, lies beneath a planned dam site. A dense network of boreholes and tunnels were used to investigate scaling effects of solute transport processes in fractured rock. A series of tracer experiments using deuterium and dye tracers (uranine, eosine, and pyranine) were performed over varying distances and under different testing procedures, resulting in different flow field conditions. Large‐scale tracer experiments (21 to 346 m) were performed under natural flow field conditions, while small‐scale tracer experiments (11 to 16 m) were performed under artificially induced radial‐convergent and injection‐withdrawal flow fields. The tracer concentration curves observed in all experiments were strongly influenced by the matrix diffusion. The curves were evaluated with the one‐dimensional single fissure dispersion model (SFDM) (advective‐dispersive transport in the fractures coupled with diffusive transport in the adjacent rock matrix) adjusted for the different flow field conditions. The fitting model parameters found determined the fracture aperture, and matrix and fissure porosities. The determined fracture aperture varied between the sections having different hydraulic conductivities (100 to 270 um and 430 to 580 um, respectively). The determined values of matrix porosity (3 to 7%) seemed to be independent of the scale of the experiment. The modeled matrix porosities agreed well with values determined in independent laboratory investigations of drill cores using mercury porosimetry. In situ fissure porosity, determined only in small‐scale experiments, was independent of the applied geometry of the artificially induced flow fields. The dispersivities were found to be independent of the scale of experiment but dependent on the flow conditions. The values found in forced gradient tests lie between 0.2 and 0.3 m, while values in experiments performed under natural flow conditions were one order of magnitude higher.