ORIGINAL ABSTRACT

Glyphosate is widely applied in the Great Barrier Reef catchment area as a knock-down alternative to residual photosystem II herbicides. Although glyphosate and its metabolite aminomethylphosphonic acid (AMPA) are periodically monitored, they are excluded from routine water quality monitoring under the Great Barrier Reef Catchment Loads Monitoring Program and are therefore absent from key reporting and risk assessment tools such as the Pesticide Reporting Portal and the Pesticide Risk Metric. This study evaluated whether that exclusion is justified by comparing detected surface water concentrations against water quality guidelines for aquatic ecosystem protection. Across 18 sites from 2006 to 2024, glyphosate was detected above the limit of reporting (LOR; 0.25 µg/L) in 48 of the 272 samples at only five sites, with a maximum concentration of 11 µg/L-approximately 16 times lower than the current default guideline value for 99% species protection of 180 µg/L. AMPA was detected above the LOR (0.25 µg/L) in 60 of the 262 water samples at only four sites from 2011 to 2024, with a maximum concentration of 5 µg/L. The detected concentrations, combined with the low aquatic toxicity of glyphosate and its strong soil adsorption, indicate a minimal ecological risk under current Australian and New Zealand guidelines. These findings support the continued exclusion of glyphosate from routine reporting and monitoring frameworks, although periodic monitoring remains important to detect emerging concerns.