A SIMULATION STUDY OF THE EFFECTIVENESS OF LEGISLATION AND IMPROVED DOCKYARD PRACTICE IN REDUCING TBT CONCENTRATIONS IN THE TAMAR ESTUARY

Numerical simulations of the estuarine dispersal of tributyltin (TBT) are used in conjunction with available observations to assess the effect of reductions in TBT inputs to the Tamar Estuary in south-west England. Two alterations are considered; the ban of TBT antifoulants on boats under 25 m long from 1987, and the prevention of TBT release from the Royal Naval Dockyard at Devonport from 1988. Simulations indicate that the TBT level of 10-100 ng litre-1 observed in the Tamar is compatible with leaching from vessels at rates of about 1-mu-g/cm2/day, and indicate that removal of inputs from smaller craft would have approximately halved the concentration of TBT leached from vessels in the Estuary within a year. Dissolved TBT from dockyard washdown operations would be negligible, but it is calculated that about 1 tonne of paint-bound TBT might have been released per year. Preliminary simulations suggest that this could have become concentrated towards the region of the estuarine turbidity maximum.