The effect of acid drainage on light rare earth elements (LREE) has been studied in the estuarine sediment of the Tinto and Odiel system, where the fluvial and marine water mixing cause an important pH and chlorinity gradient inside. Due to the high influence of pH over the LREE partitioning from dissolved toparticulate phases, an anomalous behaviour has been found on La content in these sediments. Large removal of LREE takes place at low chlorinity zones in unaffected estuaries, but in the studied system this natural process is moved out the outer zones of the estuary. So, the relative La depletion in the estuarine mixing zone is due to the low pH values (< 6) that prevents LREE removal in low chlorinity zones. Once the acid neutralization (pH ~ 7) occurs, some La enrichment can be observed in sediments of the marine estuary.