A big fertilizer industrial complex and a vast extension of phosphogypsum piles (12 km2), sited in the estuary formed by the Odiel and Tinto river mouths (south-west of Spain), are producing an unambiguous radioactive impact in their sourrounding aquatic environment, through radionuclides from the U-series. The levels and distribution of radionuclides in the different water phases of this estuarine system (filtered and particulate fractions) have been determined. The analyses of radionuclide concentrations and activity ratios have provided us with interesting information with which to evaluate the extension, degree and routes of the radioactive impact, as well with the knowledge of the different pathways followed for the radioactive contamination which disturbs this natural system. The obtained results indicate that the main pathway of radioactive contamination of the estuary is through the dissolution in its waters of the radionuclides released by the industrial activities and their later fixation on the particulate materials. On the other hand, the tidal activity plays an important role in the transport and homogeneization along the estuary of the radioactivity released from the fertilizer plants. Additionally, the results obtained for the different studied natural radionuclides (U-isotopes, Th-isotopes and 210Po) in the filtered and particulate fractions have provided very rich information about their different environmental behaviour in this aquatic system.