Interpretation of tracer tests performed in fractured rock of the Lange Bramke basin, Germany

Two multitracer tests performed in one of the major cross-fault zones of th e Lange Bramke basin (Harz Mountains, Germany) confirm the dominant role of the fault zone in groundwater flow and solute transport. Tracers having di fferent coefficients of molecular diffusion (deuterium, bromide, uranine, a nd eosine) yielded breakthrough curves that can only be explained by a mode l that couples the advective-dispersive transport in the fractures with the molecular diffusion exchange in the matrix. For the scale of the tests (ma ximum 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-frac ture approximation is acceptable and that matrix diffusion plays an importa nt role. The hydraulic conductivity of the fault zone obtained from the tra cer tests is about 1.5 x 10(-2) m/s, whereas the regional hydraulic conduct ivity of the fractured rock mass is about 3 x 10(-7) mis, as estimated from the tritium age and the matrix porosity of about 2%. These values show tha t the hydraulic conductivity along the fault is several orders of magnitude larger than that of the remaining fractured part of the aquifer, which con firms the dominant role of the fault zones as collectors of water and condu ctors of fast flow.