Current and future risks of radionuclide contamination to New Zealand’s food supply.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Identifying current and future food contamination risks is an important component of ensuring food security. Exposure to ionising radiation through dietary intake of radionuclides poses a potential source of food safety concern to New Zealand. Identifying and addressing current levels of exposure and future sources is necessary to ensure food security. An analytical survey of 40 common foods in the New Zealand diet was undertaken to establish current ranges for important radionuclides, including those of anthropogenic and natural origin. The derived activity concentrations for the thirteen radionuclides analysed were used to undertake dietary exposure modelling for ionising radiation doses to different age and gender groups of the New Zealand population. Exposure to naturally occurring radionuclides, in particular polonium-210, is the predominant source of the dose to the population, with seafood in the diet being the primary contributor to this dose. Anthropogenic radionuclides in contrast are present at only trace activity concentrations and present a low dietary exposure to ionising radiation dose. Exposure to caesium-137 and polonium-210 through seafood was identified as a source of near-term risk to increase the dietary burden to ionising radiation. Seafood samples from different species and harvested from different coastal regions were surveyed to determine the variability in activity concentrations. Activity concentrations of caesium-137 were low and showed little variation between regions. However, polonium-210 activities in shellfish were significantly higher than in other seafood types and displayed large variation between species. A dietary exposure model for seafood consumers identified the short-term risks from anthropogenic radionuclide exposure through seafood as a result of oceanic transport of caesium-137 are minimal in comparison to the dose of natural polonium 210 in these foods.
The agricultural use of phosphate fertilisers containing radium-226 impurities was examined as a potential source for a long-term increase to the dietary ionising radiation burden. Analysis of phosphate ore and fertiliser samples identified a range of radium-226 activity concentrations. Radium-226 activity concentrations in agricultural soils, and also unexpectedly radium-228, appear to have increased considerably against values reported in the 1970s. Soil partitioning results showed that the radium-226 is predominantly bound in non-labile fractions and is unlikely to be available for crop uptake.
Crop uptake in a number of common agricultural and horticultural crops was analysed to establish the dietary implications of an increase in soil radium-226 activity concentrations. The calculated concentration ratios correlate well with international default values for estimating crop uptake. Analysis of the concentration ratio in foliage at a site with a gradient of soil radium-226 established no increase in the crop activity concentration occurred. As a result, long-term loading of soil with radium-226 is unlikely to present a dietary risk. This conclusion was confirmed through conservative forecasting of the increased dose of radium-226 that might occur at current soil loading rates based on current fertiliser activity concentrations. The forecast model calculated that the increase to dietary ionising radiation burden is unlikely to reach thresholds requiring regulatory intervention for close to two millennia.
Both short- and long-term radiological risks to New Zealand food security are considered minimal. New Zealand is currently well insulated from the risks of radiological exposure through the diet.