Modelling of the laboratory experiments of solute transport through a natural rock fracture

This report describes first part of a two year project that examines tracer retention in solute transport through a rock fracture. The work is carried out in close cooperation with the Laboratory of Radiochemistry of the University of Helsinki, where the experimental part of the project is performed. Objective of the whole project is to examine the processes that cause retention in solute transport through rock fractures. Especially, the focus of the work is on the matrix diffusion. The first part of the project, reported in the present report, aims to the characterisation of suitable flow fields to be used in the tracer experiment and interpretation of the pretests performed using nonsorbing tracers. In the examined fracture the natural direction of the flow is towards side 3 (fracture opens towards side 3). This means that, for a maximum length of the flow path, injection should be performed in borehole KR1. This configuration has also been used in the first tracer tests that were performed using uranine, technetium and sodium. The modelling implies that so far the flow rates have been rather high leading to advection dominated transport. Scoping calculations show that matrix diffusion begins to be observable for nonsorbing tracer when the flow rate is around 0.1 ..l/min for the column experiment and around 1 ..l/min for the fracture experiment.