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In the area of a planned dam site in the southern Black Forest, an observation tunnel with boreholes drilled into an adjacent vertically orientated ore body offered nearly ideal conditions to investigate transport phenomena in a highly permeable fault and fracture zone. The experimental array, consisting of horizontal and inclined boreholes lying within distances of ten to twelve meters apart, gave the opportunity to perform forced gradient tracer tests over varying distances under fixed hydraulic boundary conditions. The breakthrough curves of the tracer experiments were analyzed using an adequate transport model. The fitting procedure yielded hydraulic parameters such as fissure and matrix porosities and first estimations of the average fracture aperture.

Two multitracer tests performed in one of the major cross-fault zones of the Lange Bramke basin (Harz Mountains, Germany) confirm the dominant role of the fault zone in groundwater flow and solute transport. Tracers having different coefficients of molecular diffusion (deuterium, bromide, uranine, and eosine) yielded breakthrough curves that can only be explained by a model that couples the advective–dispersive transport in the fractures with the molecular diffusion exchange in the matrix. For the scale of the tests (maximum distance of 225 m), an approximation was used in which the influence of adjacent fractures is neglected. That model yielded nearly the same rock and transport parameters for each tracer, which means that the single-fracture approximation is acceptable and that matrix diffusion plays an important role. The hydraulic conductivity of the fault zone obtained from the tracer tests is about 1.5×10–2 m/s, whereas the regional hydraulic conductivity of the fractured rock mass is about 3×10–7 m/s, as estimated from the tritium age and the matrix porosity of about 2%. These values show that the hydraulic conductivity along the fault is several orders of magnitude larger than that of the remaining fractured part of the aquifer, which confirms the dominant role of the fault zones as collectors of water and conductors of fast flow.