Distribution of natural radioactivity within an estuary affected by releases from the phosphate industry

Abstract The distribution and behaviour of the radionuclides 210 Pb, 210 Po and 226 Ra in the Odiel and Tinto river estuaries, southwest Spain, have been studied. This system receives large quantities of solid and liquid effluents in the form of phosphogypsum waste from the phosphate industry containing natural radionuclides. Ranges of activities from 20 to 5000 Bq kg −1 (d.w.) in riverine sediments were observed. The ratios of radionuclides were also determined (uncertainties quoted to 2 σ), showing clear disequilibria in the effluents from the factories ( 210 Pb/ 210 Po = 2.7±0.6, 210 Pb/ 226 Ra = 0.6±0.1, 210 Po/ 226 Ra = 0.24±.03) and from the phosphogypsum piles ( 210 Pb/ 210 Po = 2.1±0.6, 210 Pb/ 226 Ra = 1.5±0.6, 210 Po/ 226 Ra = 0.7±0.2). However, the sampling sites on the rivers affected directly by the discharges exhibit activity ratios other than those found in the effluents, manifesting their different behaviour in the estuarine environment. The estimated inventory in sediments for these radionuclides was 0.92±0.20 TBq (1993) which can be compared with the annual release of 0.52±.04 TBq (1993) from the phosphogypsum piles and factories. Preferential deposition of 210 Po and 210 Pb rather than 226 Ra onto the river bed was observed. The proportion of total sediment 210 Pb activity associated with particles less than 2 μm in diameter, which is one of the most resuspendable and transportable sizes, is 5%-15%.