The aim was to characterize hydrochemical changes that take place in the Sancho reservoir, which receives waters contaminated by acid mine drainage (AMD) from the Meca River (Iberian Pyrite Belt, SW Spain). The sampling procedure comprised bimonthly campaigns carried out between October 2011 and May 2012, in the entrance of the reservoir and in the input river waters. In addition to in situ parameters, heavy metals and sulphate were analysed in the laboratory by photometry and atomic absorption spectrometry, respectively. The database was submitted to statistic treatment through the STATGRAPHICS Centurion XVI.I software. The results show a clear relationship between the sampling site in the entrance of the reservoir and in the watercourse, with an almost instantaneous cross-correlation. At the beginning of the sampling period a small rise in pH at both sites was detected, coinciding with a reduction in the concentration of heavy metals. This attenuation results from the dilution process caused by rain as well as the sorption by iron oxyhydroxysulphates that precipitate as a consequence of the above-mentioned rise in pH. The time changes that were detected in this system are due to the influence of input waters from the waste dumps of the Tharsis mines.