Bioavailability of 99Tc in Fucus vesiculosus: mechanisms and patterns of release.

Abstract

About 1665 TBq of the highly mobile radionuclide, technetium-99 (99Tc), has been discharged into the Irish Sea by BNFL, Sellafield since 1994. This long-lived radionuclide with a half-life of 213,000 years has the potential to contaminate the marine environment for many generations. As 99Tc is highly soluble in the aerobic surface waters, it remains in the water column making it available for uptake by marine organisms. High 99Tc activity concentrations up to of 270 Bq kg-1 (dry weight) in lobsters and 60 kBq kg-1 (wet weight) in brown seaweed species have been reported in the literature. The use of contaminated seaweed as a soil conditioner in coastal areas, including Cumbria, creates a potentially important pathway for the transfer of 99Tc from marine to terrestrial ecosystems and thence to man. While there is some evidence that this practice can lead to contamination of some crops, the mechanisms and dynamics of the seaweed-to-soil-toplant pathway have so far been neglected. The key objectives for this research programme were to determine the rate of release of 99Tc from environmentally contaminated brown seaweed (Fucus vesiculosus) into soil and soil solution over time, to identify the mechanisms involved and to quantify the availability of the released 99Tc to crop plants. The results from a pot experiment, using seaweed collected from the shore close to Sellafield incorporated into a sandy coastal soil, established that between 54% and 71% of the 99Tc within the seaweed had accumulated in the soil 15 weeks after application, despite low temperature conditions. Concurrent CO2 monitoring (used as a measure of microbial decomposition) suggested the initial 99Tc release (up to 40% in the first eight weeks) was due to leaching and that microbial decomposition was responsible for the release of the remainder in the latter phase. A second, larger scale experiment confirmed the release of 99Tc from Fucus vesiculosus into soil solution with around 3% of the 99Tc added within the seaweed present in the soil solution after 18 weeks. In the initial six weeks the rate of release of 99Tc was faster from seaweed cut into 4-6 cm pieces that from those cut into 20-25 cm pieces. Monitoring of the CO2 production indicated that there was no significant difference between the two treatments suggesting that a factor other than microbial decomposition was influencing the rate of 99Tc release. The predominant mechanism appeared to be leaching. Around 46% of the 99Tc contained within the seaweed was released into the soil over 21 months, of which around 11% was present in the soil solution. A proportion of 99Tc released into the soil was readily taken up by spinach plants with an average concentration ratio calculated as 120 (Range 10 - 414). This suggested that some 99Tc was present in the soil solution as pertechnetate. Around 36% of 99Tc could be leached out of the seaweed when shaken in rain water over a six day period, the majority being lost between days two and four. This was similar to the time period when soluble sugars were lost. While this suggested a considerable proportion of 99Tc was present in a soluble form the percentage was markedly lower than was expected from the results of published experiments carried out with artificially contaminated seaweed. Investigation of the chemical forms of 99Tc present in the soil solution using gel chromatography revealed that one month after seaweed addition to the soil between 36% and 50% of the 99Tc in the soil solution was present as pertechnetate. This confirmed that a substantial amount of 99Tc released from Fucus vesiculosus into soil would be available for plant uptake. The high rate of 99Tc release from Fucus vesiculosus into the soil and the subsequent presence of bioavailable pertechnetate in the soil solution shown by these experiments confirm that the practice of using contaminated seaweed as a soil conditioner is a potentially important pathway for the sea-to-land-to-plant transfer of 99Tc. There is the potential, therefore, for human exposure to 99Tc on consumption of vegetables grown on plots conditioned with contaminated seaweed.