Solute transport and retention in fractured rock

This work reviews and compiles the analysis of tracer tests that were performed in the Äspö Hard Rock Laboratory in Sweden over a period of several years. The tracer tests were carried out in fractured rock at a depth of about 400 metres below the ground surface. The experimental programme progressed from a simple flow path in a single fracture to multiple flow paths through a network of fractures. The modelling of the tracer tests on different scales shows that matrix diffusion is an important retention process of solute transport in fractured rock. The tracer breakthrough curves show indications of the diffusive mass exchange between mobile and immobile zones, although in many cases in situ tracer tests should be performed under boundary conditions that favour advective solute transport. The analysis of the flow paths in a single fracture and in a network of fractures indicates that an increase in the complexity of the flow field and extending flow paths over several fractures does not introduce new retention processes. The same retention processes that are applicable for a single fracture can be applied over longer pathways that are composed of several different fractures. Modelling the tracer tests on a block scale indicates that this is the case even when the hydraulic structures along the flow path are geologically of different origin. Ultimately, the retention properties are governed by the average properties along the flow path. The main outcome of the study is that the assessment of in situ solute retention properties requires adequate information on the immobile zones and flow paths. The heterogeneity of the immobile zone retention properties affects the effective tracer retention properties. The retention properties estimated for a given tracer test set-up cannot be directly transferred to another flow field without considering how the changes in the flow field will affect the contributions of the different immobile zones to the overall retention.