  Identifying transport pathways in fractured rock is extremely challenging as
flow is often organized in a few fractures that occupy a very small portion of
the rock volume. We demonstrate that saline tracer experiments combined with
single-hole ground penetrating radar (GPR) reflection imaging can be used to
monitor saline tracer movement within mm-aperture fractures. A dipole tracer
test was performed in a granitic aquifer by injecting a saline solution in a
known fracture, while repeatedly acquiring single-hole GPR sections in the
pumping borehole located 6 m away. The final depth-migrated difference sections
make it possible to identify consistent temporal changes over a 30 m depth
interval at locations corresponding to fractures previously imaged in GPR
sections acquired under natural flow and tracer-free conditions. The experiment
allows determining the dominant flow paths of the injected tracer and the
velocity (0.4-0.7 m/min) of the tracer front.
