PB-210 AND PO-210 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 a nd in wastes released by the phosphate fertilizer industry confirm the ir potential for the enhancement of environmental radioactivity levels . Therefore, concentrations of Pb-210 and Po-210 were measured in bott om sediments and suspended matter in the Tagus estuary, Portugal, to a ssess the enhancement of radioactivity due to wastes from the phosphat e industry. The concentration of Pb-210 in surface sediments in the es tuary increased inversely with sediment grain-size; conversely, increa sed percentage of sand has a dilution effect on the concentration of P b-210 measured in bulk sediment samples. By normalizing the data to th e < 63-mum fraction, the naturally-occurring Pb-210 in sediments was f ound to be 68 +/- 19 Bq kg-1 (dry wt.) in background sediments. Higher Pb-210 levels, up to 1580 Bq kg-1 (dry wt.), were measured in some bu lk sediment samples. It was verified that this radionuclide has been i ntroduced 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 Pb-210 in sedim ents and suspended matter were generally below those measured in the z one of phosphatic releases at the Barreiro Peninsula. Concentrations h igher than the predicted average concentration of unsupported Pb-210 f rom natural sources (atmospheric deposition, river input) were also me asured 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-occur ring unsupported Pb-210 onto suspended matter and to co-precipitation with iron-manganese hydroxides in the fresh water-salt water mixing zo ne.