210Pb and 210Po in sediments and suspended matter in the Tagus estuary, Portugal. Local enhancement of natural levels by wastes from phosphate ore processing industry

Results of analyses of uranium series radionuclides in phosphate ore and in wastes released by the phosphate fertilizer industry confirm their potential for the enhancement of environmental radioactivity levels. Therefore, concentrations of 210Pb and 210Po were measured in bottom sediments and suspended matter in the Tagus estuary, Portugal, to assess the enhancement of radioactivity due to wastes from the phosphate industry. The concentration of 210Pb in surface sediments in the estuary increased inversely with sediment grain-size; conversely, increased percentage of sand has a dilution effect on the concentration of 210Pb measured in bulk sediment samples. By normalizing the data to the < 63-μm fraction, the naturally-occuring 210Pb in sediments was found to be 68 ± 19 Bq kg−1 (dry wt.) in background sediments. Higher 210Pb levels, up to 1580 Bq kg−1 (dry wt.), were measured in some bulk sediment samples. It was verified that this radionuclide has been introduced by the discharge of wastes from a phosphate fertilizer plant but enhanced concentrations are localized near the point of discharge. In other zones of the estuary, the concentrations of 210Pb in sediments and suspended matter were generally below those measured in the zone of phosphatic releases at the Barreiro Peninsula. Concentrations higher than the predicted average concentration of unsupported 210Pb from natural sources (atmospheric deposition, river input) were also measured in the upper estuary, both in bottom sediments and in suspended matter. It is suggested that these relatively elevated concentrations are due to the highly efficient scavenging of soluble naturally-occurring unsupported 210Pb onto suspended matter and to co-precipitation with iron-manganese hydroxides in the fresh water-salt water mixing zone.