Exposure and combined risk of pesticide mixtures in tropical wetland waters, Australia

Located within the Great Barrier Reef Catchment Area (GBRCA), the Herbert River Floodplain is designated as an area of nationally important wetlands. Furthermore, these wetlands provide a range of critical ecosystem services which protect the Great Barrier Reef World Heritage area ecosystem from land-based effects including agricultural runoff. The surrounding intensive agricultural land use puts these wetlands at risk of excessive loading of pesticides, which may result in species decline, loss of value and reduced function of ecosystem services. The aim of this study is to investigate the combined risk from the co-occurrence of pesticides in wetlands of the Herbert River Floodplain. Waters of five freshwater wetlands were monitored for 55 pesticides using a combination of grab and passive sampling techniques between February 2021 and March 2023. The combined mixture toxicity for up to 19 commonly detected pesticide active ingredients (PAIs) was estimated using the pesticide risk metric (PRM) model, calculated as the combined percentage of species affected (PAF). Thirty-six pesticides were detected in waters across five wetlands, eight of which exceeded regulatory guidelines at least once. Site-specific species protection goals were exceeded at least once at four of the five studied wetlands. Average monthly PAF (from passive sampling) was found to be increased during the wet season, though no increase in instantaneous PAF (from grab sampling) was detected in the wet season. Our findings indicate wetlands of the broader GBRCA are likely to be at risk due to pesticide exposure, and that this risk occurs during both wet and dry seasons. Risk periods in wetlands are not consistent with those previously observed in nearby rivers and creeks and should be monitored accordingly.