Chernobyl's legacy in food and water

Abstract

Radiocaesium (¹³⁷Cs) from the 1986 Chernobyl accident has persisted in freshwater fish in a Scandinavian lake for much longer than was expected¹. On the basis of new data generalizing this observation, we propose that the continuing mobility of ¹³⁷Cs in the environment is due to the so-called ‘fixation’ process of radiocaesium in the soil tending towards a reversible steady state. Our results enable the contamination of foodstuffs by Chernobyl fallout to be predicted over the coming decades. Restrictions in the United Kingdom, for example, may need to be retained for a further 10–15 years — more than 100 times longer than originally estimated.

Main

We have measured ¹³⁷Cs activity concentrations (Q_t) in terrestrial vegetation (seven sites, including data from ref. 2), lake water (dissolved phase, two lakes) and mature fish (three species) in Cumbria, UK, over the same period as the Norwegian study¹. Our results for vegetation and water contamination (examples in Fig. 1) show the same two-component exponential decline (Q_t = Q₁e^−k,t+Q ₂e^−k,t) observed for immature fish¹. The decline in ¹³⁷Cs in mature fish was influenced by slower biological uptake rates during the initial period after Chernobyl^1,3, so only the second component of the decline is shown for our fish data (Fig. 1).

Figure 1: Long-term changes in ¹³⁷Cs in brown trout in Norway (from ref. 1), and in perch, terrestrial vegetation and water in Cumbria, UK.

The decline in ¹³⁷Cs in immature fish, water and vegetation during the first five years has an effective ecological half-life (T_eff) of 1–4 years^1,4 as a result of ‘fixation’. Dotted lines indicate the hypothetical continuation of irreversible fixation. Fits of the two-exponential model to our data, indicating the reversibility of ‘fixation’, are shown as solid lines. Long-term declines in all three Cumbrian systems are similar (T_eff range of 6–30 years) and are in quantitative agreement with results from the Norwegian fish study¹.

Our results show that the effective ecological half-life (T_eff, the time for the ¹³⁷Cs concentration to reduce by 50%) in young fish, water and terrestrial vegetation has increased from 1–4 years during the first five years after Chernobyl^1,4 to 6–30 years in recent years. The common rate of decline in ¹³⁷Cs concentration in lake water, fish and vegetation suggests that it is controlled by the same process in all three pools. This is consistent with a controlling influence of changes in chemical availability of ¹³⁷Cs in soil (in these lakes, long-term ¹³⁷Cs in the water originates in catchment runoff⁵).

The decline in ¹³⁷Cs mobility and bioavailability over the first few years after fallout is believed to be controlled by slow diffusion of ¹³⁷Cs into the illitic clay mineral lattice⁴. This ‘fixation’ process controls the amount of radiocaesium in soil water and therefore its availability to terrestrial biota and for transfer to rivers and lakes⁴. Studies of ¹³⁷Cs in contaminated sediments^6,7, however, indicate that this process may be reversible. From the persisting mobility of radiocaesium, and particularly the increase of T_eff towards the physical decay rate of ¹³⁷Cs (T_1/2 = 30.2 years), we conclude that the sorption–desorption process of radiocaesium in soils and sediments is tending towards a reversible steady state.

The continuing mobility of ¹³⁷Cs in the environment means that foodstuffs will remain contaminated for much longer than was first expected. In the United Kingdom, restrictions on the sale and slaughter of sheep are currently in place on 389 upland farms (with about 232,000 sheep) on which some sheep have ¹³⁷Cs activity concentrations above the UK limit for the entry of meat into the food-chain (1,000 Bq kg⁻¹). During our studies on three restricted farms in 1991–93 (ref. 8), the maximum ¹³⁷Cs level in sheep meat was 1,870 Bq kg⁻¹.

Assuming that this is typical of restricted farms within the UK and using the rates of long-term decline we have estimated, restrictions may need to remain in place on some farms for a total of 30 years after the Chernobyl accident, which is more than 100 times longer than initially expected. In some areas of the former Soviet Union, consumption of forest berries, fungi⁹ and fish¹⁰ (present ¹³⁷Cs content, 10–100 kBq kg⁻¹), which contribute significantly to people's radiation exposure, will need to be restricted for at least a further 50 years.