RADIOLOGICAL IMPACT OF APPLYING PHOSPHOGYPSUM TO TILLED LAND CROPPED TO ANNUAL RYEGRASS1

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Abstract

The production of phosphoric acid by acidulation of rock phosphate with sulfuric acid produces phosphogypsum (PG) as a byproduct. Phosphogypsum is primarily CaSO4·2H2O and is a potential source of S and Ca for crops. Because of the presence of small amounts of 226Ra, the U.S. Environmental Protection Agency (EPA) has imposed severe restrictions on the use of PG. The objective of the study was to evaluate the radiological impact of applying phosphogypsum to tilled land. The experimental land spanned over two soils (Myakka-sandy, siliceous, hyperthermic Aeric Haplaquods and Pomona-sandy, siliceous, hyperthermic Ultic Haplaquods) tilled and cropped to annual ryegrass (Lolium multiflorum Lam.) for 3 crop-years. The PG with 792 226Ra, 836 210Pb, and 744 210Po Bq kg−1 was applied all at once at the start of the study at 2 and 4 Mg ha−1, with no PG as control, and harrowed into the top 15-cm layer of the soil. This layer initially contained 15.9, 28.1, and 14.1 Bq kg−1 of 226Ra, 210Pb, and 210Po, respectively. The 4 Mg PG ha−1 would have increased the initial soil 226Ra and 210Po by 9% each and 210Pb by 5%, assuming a soil bulk density of 1500 kg m−3. The results, averaged over crop-year and over the 3-crop-year period, showed no measurable increases in 226Ra, 210Pb, or 210Po in soil down to 90 cm sampled in layers of 15 cm, in groundwater sampled at 90 to 120 cm depth, in regrowth and mature forages, and in gamma radiation and airborne 222Rn both measured 1 m above the plots. However, 222Rn flux measured at the soil surface increased by 0.74 × 10−4 Bq m−2 s−1 per Mg PG ha−1. Thus, yearly application of 0.675 Mg ha−1 (upper level of annual application rates for gypsum as S or Ca source for crops) over a 100-year period of PG used in the study could increase 222Rn flux from the experimental land from 8.14 × 10−4 to 58.09 × 10−4 Bq m−2 s−1 at the end of said period, assuming no loss of 226Ra, the source of 222Rn in soil and PG. This 100-year-end value, however, would still be less than half of 160.0 × 10−4 Bq m−2 s−1, the average 222Rn flux for U.S. soils. However, because of the long half-life of 226Ra (1620 years), it is important that the experimental or field rates-of-loss of PG-attributable 226Ra in soil and soil surface 222Rn flux be determined. This can be facilitated by using much higher PG rates and more replicates than those used in the present study.

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