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Abstract

The first impounding of the Bumbuna dam was subject to unexpected high seepages discharge (up to 550 l/s) from the foundation drains. Fluorescent tracer tests, commonly used for hydrogeological study of porous and karst aquifer, were performed to locate the seepage zones. Solutions of 3 different fluorescent tracers (Uranine, Sulforhodamine-B and Tinopal CBS-X) were injected in the reservoir at regularly spaced points down to a depth of 80 m. Tracer concentration in the seepage water from the foundation drains was continuously monitored by high resolution fluorometers with detection limit as low as 0.02 ppb.A total of 107 tests were performed in two separate campaigns conducted in January–February and October–December 2009. Concentration curves recorded from each test were analyzed to determine the time of first response to the injection (detection time) and the quantity of tracer returned from each test (restitution). Maps of minimum detection time and maximum tracer restitution were generated to identify the position of leakages.Seepage mitigation works were planned on the base of test results and completed in the dry season after the drawdown of the reservoir. Works consisted in the additional grouting along the grout curtain, sealing of fissures on the bituminous layer and installation of a PVC membrane on the dam upstream face.Inspection of the upstream dam face confirmed the presence of damages in the areas identified by tracer tests. Since completion of seepage mitigation works the dam operated satisfactorily with maximum seepage from the foundation of 70 l/s.