Tracers and Modelling in Hydrogeology (Proceedings of the TraM'2000 Conference held at Liège, Belgium, May 2000). IAHS Publ. no. 262, 2000. 325 Transport of heavy metals in a fractured porous block: experiments and a 3-D model THOMAS HIMMELSBACH Federal Institute for Geosciences and Natural Resources, Stilleweg 2, D-30655 Hannover, Germany e-mail: himmelsbach@bgr.de EDSON WENDLAND CEPETRO/DEP/UNICAMP, Caixa Postal 6052, 13081-970 Campinas SP, Brasil Abstract Exploited coal seams are used to deposit industrial residues from thermal energy and waste incineration plants in North Rhine Westfalia, Germany. Use of such an underground repository requires proof that dissolved contaminants cannot reach the biosphere. Numerical models may be used as tool to predict the local and temporal migration of pollutants. In order to investigate the migration of heavy metals in a porous sandstone block containing a single fracture, two tracer experiments were performed under precisely defined hydraulic flow conditions. In the experiments, pyranine, cadmium and lead were applied, and the hydrochemical conditions corresponded to a depth of 1000 m below ground surface. The fracture was considered as a two dimensional parallel plate conduit and the rock matrix was treated as a three dimensional porous medium. Together with rock properties determined in the laboratory, the modelling yielded a good agreement between measured and simulated breakthrough curves by calibrating the fracture aperture and longitudinal dispersion. INTRODUCTION In the early 1990s the concept of subsurface storage of industrial residues in coal mines became an alternative to surface waste dump sites (Jâger et al., 1990). The residues consist of filter ashes and desulphurization residues, which are injected as a hydraulic suspension into exploited coal seams. Numerical simulations showed that dissolved contaminants should only return to the biosphere when the mines are closed and the pumping of groundwater stops (Himmelsbach & Kônig, 1997). Flow processes in fractured rocks were initially considered to occur in parallel plate conduits of parallel plates separated by constant apertures. Experimental (Witherspoon et al, 1980) and theoretical investigations showed that this simplification may be inadequate due to channelling processes (Neuzil & Tracy, 1981; Rasmuson & Neretnieks, 1986; Tsang & Tsang, 1987). Most studies neglected the matrix diffusion. A numerical model for transport in a one dimensional fracture with diffusion into the matrix was presented by Grisak & Pickens (1992), while the transport equation was published by Tang et al. (1981). This paper describes a bench scale experiment which addresses matrix diffusion and advective dispersive flow in the fracture.