SOLUTE TRANSPORT PROCESSES IN A HIGHLY PERMEABLE FAULT ZONE OF LINDAUFRACTURED ROCK TEST-SITE (GERMANY)

The results of field tracer experiments performed in the Lindau fractu red rock test site (southern Black Forest, Germany) and subsequent mod eling 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 tunn els were used to investigate scaling effects: of solute transport proc esses in fractured rock. A series of tracer experiments using deuteriu m 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 t o 346 m) were performed under natural Bow field conditions, while smal l-scale tracer experiments (11 to 16 m) were performed under artificia lly induced radial-convergent and injection-withdrawal now fields. The tracer concentration curves observed in all experiments were strongly influenced by the matrix diffusion. The curves were evaluated with th e one-dimensional single fissure dispersion model (SFDM) (advective-di spersive transport in the fractures coupled with diffusive transport i n the adjacent rock matric) adjusted for the different Bow field condi tions. The fitting model parameters found determined the fracture aper ture, and matrix and fissure porosities. The determined fracture apert ure varied between the sections having different hydraulic conductivit ies (100 to 270 mu m and 430 to 580 mu m, respectively). The determine d values of matrix porosity (3 to 7%) seemed to be independent of the scale of the experiment. The modeled matrix porosities agreed well wit h 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 geomet ry of the artificially induced Bow fields. The dispersivities were fou nd to be independent of the scale of experiment but dependent on the B ow 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.