Chernobyl radioactivity persists in fish

Abstract

After the accident at the Chernobyl nuclear reactor in 1986, the concentration of radioactive caesium (¹³⁴Cs and ¹³⁷Cs) in fish was expected to decline rapidly. The estimated ecological half-life (the time needed to reduce the average caesium concentration by 50%) was 0.3 to 4.6 years¹,². Since 1986, we have measured radiocaesium in brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus), both of which are widely eaten in Scandinavia, in a lake contaminated by Chernobyl fallout³,⁴. We have measured radiocaesium in nearly 4,000 fish, taking samples 2-4 times every year from spring to autumn. We find that the decline in radiocaesium was initially rapid for 3-4 years and was then much slower. About 10% of the initial peak radioactivity declines with an ecological half-life of as long as 8-22 years.

Main

The concentration of ¹³⁷Cs, the long-lived radioactive caesium isotope with a physical half-life of 30.2 years, peaked in 1986. The radioactivity was three times higher in brown trout than in Arctic charr (geometric means: 10,468 and 3,097 Bq kg⁻¹). The decline in ¹³⁷Cs from its maximum in 1986 to 1998 is modelled by single- and two-component decay functions: Q _t=Q e^−kt and $Q_t=Q_1{\text{e}}^{-k_{1}t}+Q_2{\text{e}}^{-k_{2}t}$, where Q is the caesium concentration, k is the decay rate and t is time in years after the peak. The ecological half-lives are ln2/k, and are an indication of how long it will take the fish to rid themselves of radioactivity. The proportional contribution of the maximum radioactivity with slow decay rates was estimated as Q ₂/(Q ₁+Q ₂).

The decline in ¹³⁷Cs was rapid during the first three (brown trout) and four (Arctic charr) years, and was then slower. Based on the initial rapid decline, ecological half-lives were estimated using a single-component decay function at 1.0 and 1.5 years for brown trout and Arctic charr, respectively, as in other post-Chernobyl studies¹,², but this underestimates the time that ¹³⁷Cs persists in the fish. A two-component decay function gives better model fits (extra sum of squares test, P ≪0.001) than single-component models. The two-component models explain 90% and 92% of the individual variance in caesium concentration in brown trout and Arctic charr, respectively. Seasonal dynamics from 1988 (ref. 5) and size dependency⁴ in caesium levels meant that modelling should be done on all young fish (aged 2 and 3 years) until 1988, and then only on spring samples. Ecological half-lives were estimated at 0.6 and 7.7 years for the first and second components for brown trout, and 1.1 and 22.4 years for Arctic charr (Fig. 1). The second component constituted 11.5% and 10.7% of the initial peak ¹³⁷Cs activity for brown trout and Arctic charr, respectively.

Figure 1: ¹³⁷Cs concentrations in fish in the study lake from peak levels in 1986 through to 1998.

a, Brown trout; b, Arctic charr. Observed (points, ±95% confidence limits) and predicted (solid lines) ¹³⁷Cs concentrations are shown. Early predictions of ¹³⁷Cs decline based on data from peak levels until 1989 are shown for comparison (broken lines).

The two-component nature of the decay indicates that the fish may be affected by two contaminant pools. The first is a rapidly declining pool caused by caesium deposited on the lake surface and washed out from the catchment before being sorbed to catchment soils⁶. The caesium in the lake declined quickly as a result of hydraulic dilution, accumulation in bottom sediments, reduced run-off from the catchments, and loss of ¹³⁷Cs through outflow⁶,⁷. The second is a slowly declining pool of ¹³⁷Cs leaking from the lake catchment⁶,⁸ and caesium recycling within the lake⁹.

The relative importance of the secondary pools depends on catchment and lake characteristics. The ¹³⁷Cs concentration in the environment may approach a steady state¹⁰, declining only as a result of decay (half-life, 30 years). This may apply generally for radioactive elements entering the biogeochemical cycles, and is supported by our estimates of ecological decay for ¹³⁷Cs in Arctic charr. The different accumulation and ecological decay of caesium in Arctic charr and brown trout is probably due to their different ecological niches: they segregate in habitat and diet, both of which influence caesium turnover³.