Historical emissions of octachlorodibenzodioxin in a watershed in Queensland, Australia: estimation from field data and an environmental fate model.

An octachlorodibenzodioxin (OCDD)-dominated contamination is present along the coast of Queensland, Australia. Several findings indicate that this contamination originates from pesticide use, although due to limited information on OCDD levels in the pesticides used, estimating past and current emissions of OCDD solely from pesticide use data is unfeasible. We used all the qualitative and quantitative information available on OCDD in pesticides together with a previously validated chemical fate model for a catchment in the Queensland Wet Tropics to back-calculate the emissions of OCDD from measured soil concentrations. We estimate that under different emission scenarios an average of 2,500 kg of OCDD was emitted within the modelled 1,685 km2 (Tully river) catchment between 1950 and 2010. Because this catchment represents only approximately 0.85% of the whole coast of Queensland under a similar contamination, the total amount of OCDD released in this region is considerably larger. For all emission scenarios, we could show that the OCDD currently present in agricultural soil is a result of historical emissions, and current-day emissions are less important in comparison to past emissions. Overall 18% was lost by degradation and 62% was buried below the agricultural surface soil, as a result of facilitated transport.