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Predicting the Long-Term Radiological and Agronomic Impacts of High Rates of Phosphogypsum Applied to Soils Under Bahiagrass Pasture – Part I

05-044-174Pt1Final

Abstract

This study developed data to support assessments of the radiological impacts of long-term application of phosphogypsum (PG) to agricultural lands. PG containing 21.4, 22.6, and 20.1 pCi g-1 226Ra, 210Pb, and 210Po, respectively, was applied at 10 and 20 Mg PG ha-1 in 1993 (FIPR Publ. No. 05-038-141) to two Florida soils cropped to bahiagrass. Radiological parameters were measured periodically for 5 ½ years. Levels of 226Ra, 210Pb, and 210Po in the top 5-cm soil layer increased with PG, and there was developing evidence of appearance in the 5-10 cm layer. Radon flux also increased and levels persisted. Effects on gamma radiation levels were slight and decreased after the first year. Effects on 226Ra, 210Pb, and 210Po in groundwater down to 90 cm were minimal and on ambient airborne 222Rn levels were inconclusive. The PG had a strong effect on 226Ra, 210Pb, and 210Po in the first post-treatment regrowth harvest at one site. PG-attributable 226Ra was observed in both mature hay and regrowth with no measurable decrease in uptake through the 6th growing season; PG-attributable 210Pb and 210Po was observed in mature hay only during the first two seasons. Mature hay generally had higher levels of radionuclides than the regrowth. Transfer factors (TFs), relating the measured radiological values to PG rate or radioactivity applied per unit area were calculated for use as predictive tools. Radiation doses and associated risks after 100 years of annual PG application at 0.4 Mg ha-1 to cattle-grazed bahiagrass pasture were estimated for various pathways from soil to humans.