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

Abstract

The 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.

Tracer tests in fractured rocks with a new fluorescent dye—pyrene-1,3,6,8-tetra sulphonic acid (PTS) / Tests de traçage en roches fracturées avec un nouveau produit fluorescent—l'acide pyrène 1.3.6.8 tétra sulfonique (PTS)

“…The distance between the injection (BL8) and detection boreholes (BL10) is 11.2 m. An instantaneous injection of the tracer cocktail containing uranine and PTS was performed. According to the results of Himmelsbach et al (1998), the thickness of the ore dike is approximately 2 m, while the hydraulic conductivity is 2.4 × 10 -5 m s -1 (2.07 m day -1 ). A constant hydraulic head was adjusted by opening borehole BL10, which led to a constant flow rate of 0.23 l s -1 .…”

“…Rock parameters found at the LRL were compared to those previously obtained by Himmelsbach et al (1998) who used deuterium, pyranine and uranine.…”

“…The ore dike is nearly vertical with a thickness of 0.3-3.0 m. The tunnel system of the rock laboratory is about 90 m below the ground surface. Details on the test site and the experimental set up for the LRL can be found in Himmelsbach et al (1998). In the LRL a tracer experiment was performed under a radial convergent flow field (monopole test).…”

“…Seiler et al, 1989;Abelin et al, 1991;Behrens et al, 1997;Ptak, 1997), or in fissured rocks (e.g. Neretnieks et al, 1982;Novakowski et al, 1985;Maloszewski & Zuber, 1993;Himmelsbach et al, 1998;Hoehn et al, 1998;Reimus et al, 1999;Meigs et al, 2001;Sanford et al, 2002, Witthüser et al, 2003, Mazurek et al, 2003.…”

“…After successful tests of PTS in flow-through laboratory columns (Einsiedl, 1999), the present study focuses on the examination of PTS transport behaviour at the field scale at two test sites. The first study was performed at the Lindau Rock Laboratory (LRL), southern Black Forest, Germany, where a 600-m long observation tunnel was excavated to investigate the hydraulic effect of a highly permeable fault zone (ore dike) at a planned dam site (Himmelsbach et al, 1998). The second test was carried out at the Grimsel Test Site (GTS), which is the NAGRA (National Cooperative for the Disposal of Radioactive Waste) underground rock laboratory in the Swiss Alps (Haslital, Berner Oberland).…”