Living with the Toxic Legacy of DDT, Floods Accelerating the Pathway from Catchments to the Coast

Past environmental decisions continue to present challenges in remediating and controlling contaminants decades later. Organochloride pesticides (OCP) were vital to increasing crop yields and profitability for agricultural communities. Dichlorodiphenyltrichloroethane (DDT) became a prevalent pesticide in the 1950s, with the US producing up to 100,000 tons per annum before studies began to link DDT to predatory bird decline in 1962. Within Australia DDT was the most widely used OCP and attributed to decline in Peregrine Falcon population, despite DDT being banned in 1986, population recovery has taken decades and concentration of DDT and its metabolites (DDE, DDD) are still present in terrestrial and marine sediments. OCP bioaccumulates and has low solubility allowing concentrations to persist bound to sediments for decades, creating a need to understand the transportation pathways of the sediments-bound OCPs. This study aimed to better understand OCP distribution within sediments from the Brisbane River and catchment in southeast Queensland, Australia. Sampling was conducted at 81 sites from the Brisbane River mouth to upper catchment. Results found DDT, DDE, DDD and dieldrin above detection limits at 43% of sampled sites with combined concentrations up to 50 mg/kg as well as DDT concentrations up to 18 mg/kg. This study has found that while these pesticides are no longer being used in agricultures processes, the historic intensive use has created a sediment-bound legacy which is mobilised to estuarine and coastal systems through sediment transport processes. Long term studies within the lower Brisbane River identified elevated concentrations after major flood events, creating a need to better understand how the upper catchment pesticide storage and export processes.