Radium in New Zealand agricultural soils: Phosphate fertiliser inputs, soil activity concentrations and fractionation profiles

摘要

Phosphate ores can contain high levels of U-238 and its decay products. Of these decay products Ra-226 is an important environmental contaminant, while Ra-228 from Th-232 day may also be present, albeit at lower activity concentrations. Acid processing of phosphate ore to triple superphosphate elutes a large proportion of the Ra-226 from the final product. However, fertiliser production in New Zealand generally avoids acid processing and instead uses single superphosphate and reactive phosphate rock to maintain crop yields, meaning that Ra-226 is retained in the final product. As a first step towards characterising the human health impacts from fertiliser-borne radium, research was undertaken to identify loading and long-term accumulation of Ra-226 and Ra-228 in New Zealand agricultural soils, as well as the fractionation of Ra-226 into different soil phases.Activity concentrations for Ra-226 of up to 1.6 kBq/kg were determined in phosphate-containing fertilisers used in New Zealand. In contrast, Ra-228 did not exceed 75 Bq/kg. Analysis of 40 New Zealand soils, covering a range of agricultural uses, showed activities of between (27-88) Bq/kg Ra-226 and (21-102) Bq/kg Ra-228. Unexpectedly, there was also a strong correlation between the two radium isotopes. In 13 of the agricultural soils, all with very high available phosphate levels, the fractionation profile of Ra-226 was determined. These data indicated that Ra-226 largely remains immobile in the residual phase of the soil. Calcium and available phosphate were significantly correlated with binding of Ra-226 into labile and non-labile fractions. Barium is also hypothesised to play a significant role in co-precipitating Ra-226 into non-labile soil fractions. While a high percentages of Ra-226 immobile in the non-labile fraction would allow for marked accumulation over time it may limit the availability for uptake into crops and thus the ionising radiation dose for consumers.