CHARACTERIZING THE ENVIRONMENTAL FATE OF AQUATIC HERBICIDES BY CONNECTING QUANTIFICATION IN LAKES TO LABORATORY STUDIES

Abstract

Aquatic herbicides are a class of organic compounds that are used in surface water bodies to manage the persistence of invasive and nuisance aquatic plants. Understanding their degradation and transformation pathways are critical for managing herbicide effectiveness and target species resistance. This study examines the fate and transport of two aquatic herbicides, fluridone and florpyrauxifen-benzyl (FPB). A field campaign was conducted during the summer of 2022 to collect concentrations of the herbicides and their degradation products over time in three study lakes in Wisconsin. These field studies were followed by supplemental laboratory experiments to further analyze the dominant environmental transformation pathways for each herbicide. Photolysis, sorption, biodegradation, and hydrolysis can all be quantified separately under isolated conditions in the lab. Combined, the conclusions from these experiments along with mass balance modeling can illustrate which processes dictate the persistence of the herbicides in aquatic environments. Hydrolysis experiments show that FPB is transformed into its primary degradation product, florpyrauxifen, through base-promoted hydrolysis. Florpyrauxifen is then degraded microbially as determined by microcosm batch reactors. Photolysis experiments show that FPB, florpyrauxifen, and fluridone all undergo direct photodegradation and that indirect photodegradation is negligible. Fluridone differs from FPB in that it is very resistant to both abiotic and biotic processes, which is reflected by its longer half-life observed in lakes. Overall, a deeper understanding of aquatic herbicide behavior will allow resource managers to develop better application strategies, as well as have broader implications for further understanding processes that impact organic compounds in freshwater systems.