Sediment resuspension as a long-term secondary source of Chernobyl 137Cs in lake ecosystems: the example of Blacksåstjärn (Sweden)

Abstract

Abstract Thousands of Swedish lakes have been heavily contaminated by the radioactive fallout from the Chernobyl nuclear accident in 1986, especially with 137 Cs (up to 200 kBq m −2 ). The initially high concentrations of 137 Cs in small fish (up to 100 kBq kg −1 ww) have declined since 1987, but this decline has slowed down since 1989, and concentrations are virtually stagnant since 1990-92. Accordingly, the ecological half-life of 137 Cs in fish is not constant but gradually increasing. The recovery of the lakes is probably delayed by substantial mixing and resuspension of bottom sediments, which usually contain >90% (often >99%) of a lake's 137 Cs inventory. Primarily in shallow near-shore sediments, mixing processes carry buried contaminated particles back to the sediment surface, where they are frequently resuspended into the water column, followed by an even redeposition over the whole lake. This selective erosion has resulted in a pronounced focusing of 137 Cs towards deep lake areas. The significance of resuspension is supported by comparisons of 137 Cs concentrations in vertical sediment profiles, in surficial sediments, and in settling matter. Sediment resuspension can maintain high 137 Cs concentrations in lake waters, and resuspended as well as deposited particles are taken up by planktonic or benthic invertebrates, which are likely to assimilate associated 137 Cs. Food chain transfer of both sedimented and resuspended 137 Cs may thus substantially retard the natural decontamination of lacustrine fish communities. The observed slow burial of contaminated particles suggests that the effects of the Chernobyl accident will remain a major problem in lakes for several decades.