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

Abstract

Phosphate ores can contain high levels of ²³⁸U and its decay products. Of these decay products ²²⁶Ra is an important environmental contaminant, while ²²⁸Ra from ²³² 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 ²²⁶Ra 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 ²²⁶Ra 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 ²²⁶Ra and ²²⁸Ra in New Zealand agricultural soils, as well as the fractionation of ²²⁶Ra into different soil phases. Activity concentrations for ²²⁶Ra of up to 1.6 kBq/kg were determined in phosphate-containing fertilisers used in New Zealand. In contrast, ²²⁸Ra 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 ²²⁶Ra and (21-102) Bq/kg ²²⁸Ra. 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 ²²⁶Ra was determined. These data indicated that ²²⁶Ra largely remains immobile in the residual phase of the soil. Calcium and available phosphate were significantly correlated with binding of ²²⁶Ra into labile and non-labile fractions. Barium is also hypothesised to play a significant role in co-precipitating ²²⁶Ra into non-labile soil fractions. While a high percentages of ²²⁶Ra 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.

Keywords: Barite; Fertiliser contaminant; Soil fractionation; Superphosphate.