Multiyear LC-MS profiles of groundwater DOM reveal re-emergence of recent and legacy contaminants in a fractured-rock catchment

This is the authors' abstract. We don't add key points for this paper.

-toluamide (DEET), the coniferous resin acid 7-oxodehydroabietic acid (7-ODAA), the legacy herbicide simazine, the triazine transformation product hydroxypropazine, and triphenyl phosphate (TPP), a flame retardant and lubricant-across perched (7 m) and phreatic (∼90 m) groundwater zones. DEET and 7-ODAA exhibited seasonal concentration patterns consistent with usage-related release, while simazine, hydroxypropazine, and TPP occurred episodically, with notable re-emergence during extreme events, such as the 2018 drought and a 5 year groundwater highstand. Our findings highlight that both recent and legacy organic contaminants undergo dynamic mobilisation processes, influenced by recharge patterns and climate extremes. These results underscore the importance of sustained, high-frequency monitoring to capture episodic events and trace compound-specific responses across the hydrogeological system. By identifying persistent and transient compounds as potential flow tracers, this study advances understanding of organic matter transport in the Critical Zone and offers insights into groundwater protection strategies under changing environmental conditions.