Catchment to coast contaminant transport to a protected marine park on the East coast of Australia.

Coastal contamination is an ongoing global concern where metal, nutrient and agrochemical runoff are enhanced by human development along coasts and in catchments. Identification of contaminant mobilisation and transportation behaviours can provide valuable insights into pollutant sources in regions impacted by multi-source pollution disturbance scenarios. Because there are substantial knowledge gaps surrounding contaminant behaviour in coastal regions impacted by geogenic and anthropogenic contaminants, we investigated contaminant profiles and transport pathways at five freshwater sites, two coastal lagoons, two groundwater wells, five beach locations and four sites along a marine transect within a developing coastal catchment. The focus of this study was on contaminant delivery to an adjacent marine park located on the East Coast of Australia. In this study, four pesticides were identified over the research period, including imidacloprid, diuron and chlorantraniliprole, during the high flow events in 2022, and tebuconazole, during low flow periods in 2023. This study also identified zinc (up to 267 µg L-1) and copper (up to 2.46 µg L-1), as well as total nitrogen (up to 744 µg L-1) and phosphates (to 163 µg L-1) to be exceeding local water quality guidelines at multiple locations within the catchment. Copper fluxes were found to surpass guidelines during high rainfall events, whereas zinc remained above guidelines in 86% of sampling regimes along the catchment, in groundwater, and the coastal ocean. Pollutant levels suggest that pesticides primarily originate from horticultural industries, while metal and nutrient contaminants likely arise from a combination of geogenic and anthropogenic inputs. The relative contribution of each source varies with water flow conditions and interannual variability. The results found here suggest that multiple inputs should be considered when determining sources and contaminant transport pathways to the coastal ocean.