Investigations of solute transport in fractured rock aquifers often rely
on tracer test data acquired at a limited number of observation points.
Such data do not, by themselves, allow detailed assessments of the
spreading of the injected tracer plume. To better understand the
transport behavior in a granitic aquifer, we combine tracer test data
with single-hole ground-penetrating radar (GPR) reflection monitoring
data. Five successful tracer tests were performed under various
experimental conditions between two boreholes 6 m apart. For each
experiment, saline tracer was injected into a previously identified
packed-off transmissive fracture while repeatedly acquiring single-hole
GPR reflection profiles together with electrical conductivity logs in
the pumping borehole. By analyzing depth-migrated GPR difference images
together with tracer breakthrough curves and associated simplified flow
and transport modeling, we estimate (1) the number, the connectivity,
and the geometry of fractures that contribute to tracer transport, (2)
the velocity and the mass of tracer that was carried along each flow
path, and (3) the effective transport parameters of the identified flow
paths. We find a qualitative agreement when comparing the time evolution
of GPR reflectivity strengths at strategic locations in the formation
with those arising from simulated transport. The discrepancies are on
the same order as those between observed and simulated breakthrough
curves at the outflow locations. The rather subtle and repeatable GPR
signals provide useful and complementary information to tracer test data
acquired at the outflow locations and may help us to characterize
transport phenomena in fractured rock aquifers.