Phosphogypsum amendment effect on radionuclide content in drainage water and marsh soils from southwestern Spain

Phosphogypsum (PG) is a residue of the phosphate fertilizer industry that has relatively high concentrations of Ra-226 and other radionuclides. Thus, it is interesting to study the effect of PG applied as a Ca amendment on the levels and behavior of radionuclides in agricultural soils. A study involving treatments with 13 and 26 Mg ha(-1) of PG and 30 Mg ha(-1) of manure was performed, measuring Ra-226 and U isotopes in drainage water, soil, and plant samples. The PG used in the treatment had 510 +/- 40 Bq kg(-1) of Ra-226. The Ra-226 concentrations in drainage waters from PG-amended plots were similar (between 2.6 and 7.2 mBq L-1) to that reported for noncontaminated waters. Although no significant effect due to PG was observed, the U concentrations in drainage waters (200 mBq L-1 for U-238) were one order of magnitude higher than those described in noncontaminated waters. This high content in U can be ascribed to desorption processes mainly related to the natural adsorbed pool in soil (25 Bq kg(-1) of U-238). This is supported by the U-234 to U-238 isotopic ratio of 1.16 in drainage waters versus secular equilibrium in PG and P fertilizers. The progressive enrichment in Ra-226 concentration in soils due to PG treatment cannot be concluded from our present data. This PG treatment does not determine any significant difference in Ra-226 concentration in drainage waters or in plant material [cotton (Gossipium hirsutum L.) leaves]. No significant levels of radionuclides except K-40 were found in the vegetal tissues.