Time variations of radon-222 concentrations in the water of the Lamalou, France, karst aquifer have been measured. The measurements have been performed at the spring outlet of the aquifer as well as in boreholes drilled from the surface. Radon concentrations variations recorded in the main outlet and in a well located next to it are compared. The response to rainfall depends strongly on the season when rainfall occurs. During summer, a rain episode is followed by a radon response in the well but not at the spring. Furthermore, the decay of the radon signal obeys strictly the radioactive decay law. Accordingly, during summer the water of the aquifer barely moves horizontaly. On the contrary, during the rainy season, every rainfall induces a signal both in the well and at the spring reflecting a different mechanism and a mixing process of the water in the main outlet.