Seasonal evolution of natural radionuclides in two rivers affected by acid mine drainage and phosphogypsum pollution

Abstract

The Odiel and Tinto rivers show singular characteristics due to the significant acid mine drainage (AMD) generated in the first section of their basins and the phosphogypsum (PG) stacks located on their common estuary. AMD leads to low pH and high redox potential, which keep high amounts of toxic elements and radionuclides in dissolution. The objective of this work was to analyse the seasonal evolution of U-Th isotopes and 210Po in these rivers and the estuarine mixing zone. Four sampling points were selected (a fluvial point and an estuarine one for each river) and water samples were collected monthly throughout a year. The concentrations of natural radionuclides in the dissolved and particulate phases were determined by alpha spectrometry. The Odiel and Tinto rivers show concentrations of U-Th isotopes and 210Po from one to three orders of magnitude higher than background continental waters due to the strong effect of AMD, and 234U/238U activity ratios up to 2.

The studied radionuclides show a clear seasonal behaviour in these rivers, with three different stages during the year: (1) concentration peaks observed during November and December due to the “washing effect” produced by the first rainfalls of the hydrological year, (2) a “dilution effect” by runoff in the rainy winter, and (3) a progressive “concentration effect” during the spring and summer. A non-conservative behaviour of the analysed radionuclides in the estuaries was demonstrated due to precipitation processes produced by the increase of pH. The polluted outflows from the PG stacks located in the Tinto estuary produce a significant radioactive impact, mainly during the rainiest months, increasing the concentration of U-isotopes and 210Po in the particulate phase.