Description

Transport processes in fractured aquifer are characterised by a couple of transport mechanisms within the fracture and the neighbouring matrix. To understand the possible transport mechanisms of tracers in a fractured chalk block, laboratory single- and multi-tracer tests were carried out in two blocks under defined boundary conditions. The breakthrough curves (BTC's) of the tracers are governed by sharp peaks due to advective transport and dispersive respectively diffusive transport processes in the tailing part of the BTC's. The simulation of the BTC's with the single fissure dispersion model could not reasonably fit the BTC's. A multichannel SFDM with the superposition of BTC's of at least two different flow paths, resulted in an acceptable fit (by calculating the cumulative tracer breakthrough and recovery curves). Those simulations indicate the existence of flow channelling effects within the fracture. The results showed that matrix diffusion process plays only a minor role in the determination of the fate of nitrate in the groundwater aquifers. The key factor controlling the fate of nitrate in the groundwater is the redox process.