Use of the phosphogypsum wastes in agriculture soils : radiological impact

The phosphate fertilizer industry produce an important amount of phosphogypsum (PG) as a residue of its activity. Its is well known that such wastes contain significant amounts of natural radionuclides from the U, Th and K series. The raw material for the production (phosphate rock) has uranium activity concentrations of around 1 kBq Kg -1 from which about 15 % passes to the PG. At the Huelva industrial area (SW Spain) the wastes produced per year can reach some 3.10 6 Mg, but in spite of the recent scientific efforts its accumulation still being a problem of great concern for the area. In the other hand, reclamation of sodic soils for agriculture users requires a Ca amendment to diminish Na saturation. Then, PG (with a high proportion of CaSO4 -2H2O) is an effective amendment that has been widely used in the saline-sodic marsh soils from SW Spain. Using PG as an amendment dilutes the radionuclides down to background levels, becoming this practice a possible way to eliminate these wastes with a considerable additional value for the agricultural process. However, it is necessary to study the amount of radioisotopes that can move to water and plants to ensure the radiological safety of the amendment. PG has relatively high concentration of 226 Ra and other radionuclides, with an special concern due to the 22Rn emissions. These wastes could be used to improve the fertility of agriculture soils in a large former marsh area of the Guadalquivir river. Thus, it is interesting to study the levels and behaviour of natural radionuclides within this system to evaluate the radioactive impact if this amendment. An agronomical test is being conducted by one of the authors in an experimental farm in Lebrija (Seville). The soils are treated with 13 and 26 t ha-1 of PG, 30 t ha-1 of manure. Each treatment was repeated twice and continued for two years with beetroot and cotton plant production. We are measuring 226Ra (by alpha counting and gamma spectrometry) and U isotopes (by alpha spectrometry and ICP-MS analysis) in drainage waters, soils and vegetal-tissues samples. The PG used in the treatment has 620 ± 70 Bq Kg -1 of 226Ra. The drainage waters have 226Ra contents similar to those from non-contaminated natural waters, but the uranium concentrations are one order of magnitude higher. No significant levels of radionuclides were found in the vegetal tissues