Redox-dependent behaviour of technetium-99 entering a permanently stratified anoxic fjord (Framvaren fjord, Norway)

Two sampling expeditions were undertaken at the highly stratified, anoxic Framvaren fjord to investigate the behaviour of redox sensitive 99Tc in the water column, its uptake onto particulate matter and removal to the sediments. Peak 99Tc concentrations from recent Sellafield discharges were observed at the fjord on both expeditions. Maximum 99Tc concentrations were observed at 6e8 m (salinity of 12.5e15) in Framvaren, indicating that inflowing water interleaves predominately at this depth. Below this, 99Tc transport down to the oxic/anoxic interface was found to be limited by the density gradient as, within error, all 99Tc above the interface is in the !10 kDa fraction. High Kd values were observed below the interface and therefore 99Tc uptake onto particulate matter occurred to the greatest extent at the particle concentration maximum. Particle-bound Tc was carried to the sediments, and the sediments hold a blurred record of European reprocessing discharges. However, a maximum of only 41% of the 99Tc was associated with particles and 20% with colloids, thus an observed minimum of 39% was present as ‘‘soluble’’ species in these strongly reducing waters. This is the first observation of ‘‘soluble’’ Tc species under highly reducing conditions in the natural environment. Subsequent uptake onto particles or colloids was not observed below this depth, and ‘‘soluble’’ Tc(IV) species persist in this system.