The uranium concentration, and234U/238U and235U/238U activity ratios were measured by alpha spectrometry of surface water from the neighbourhood of the uranium mines El Lobo
and
El Pedregal
(Badajoz, Spain) before and after the rainy season. Changes were observed in all the three parameters studied. Leaching and dilution are the probable mechanisms for the seasonal changes in the concentration and in the234U/238U activity ratio as measured in open and dynamic systems (rivers).