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

Phosphate ores can contain high levels of U and its decay products. Of these decay products Ra is an important environmental contaminant, while Ra from  Th 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 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 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 and Ra in New Zealand agricultural soils, as well as the fractionation of Ra into different soil phases. Activity concentrations for Ra of up to 1.6 kBq/kg were determined in phosphate-containing fertilisers used in New Zealand. In contrast, Ra 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 and (21-102) Bq/kg Ra. 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 was determined. These data indicated that Ra largely remains immobile in the residual phase of the soil. Calcium and available phosphate were significantly correlated with binding of Ra into labile and non-labile fractions. Barium is also hypothesised to play a significant role in co-precipitating Ra into non-labile soil fractions. While a high percentages of Ra 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.