Acid mine drainage (AMD) remains one of the major environmental problems for the mining industry. When AMD is produced on a mine site, passive treatment techniques such as limestone drains (LDs) can be used to improve water quality, particularly when the effluent is relatively small. In 1999, LDs were installed in combination with a cover with capillary barrier effects to rehabilitate the abandoned acid-generating Lorraine mine site in Quebéc, Canada. However, the quality of the water exiting the LD does not meet the local regulations (even if a significant improvement has been observed). To better understand the behaviour of the drains, the hydraulic residence time (HRT) was evaluated using various tracer tests. Tests’ results indicate that the HRT in the Lorraine LDs is close to the minimum value targeted at the design stage but is significantly different than those estimated from the geometrical characteristics and porosity of the drains and the water flow discharge.