Relative abundance of atropisomer pairs in metolachlor metabolites, MESA and MOXA, vary with slope and hydric soils in subwatersheds of the Choptank River watershed, Maryland

Authors: Rice, Clifford; Fischel, Matthew; BIANCA, MARLA - US Department Of Agriculture (USDA); VINYARD, BRYAN - US Department Of Agriculture (USDA); MCCARTY, GREGORY - US Department Of Agriculture (USDA); Hapeman, Cathleen

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2025
Publication Date: 2/1/2025
Citation: Rice, C., Fischel, M.H., Bianca, M.R., Vinyard, B.T., Mccarty, G.W., Hapeman, C.J. 2025. Relative abundance of atropisomer pairs in metolachlor metabolites, MESA and MOXA, vary with slope and hydric soils in sub watersheds of the Choptank River in Maryland. Science of the Total Environment. 963. Article e178399. https://doi.org/10.1016/j.scitotenv.2025.178399.
DOI: https://doi.org/10.1016/j.scitotenv.2025.178399

Interpretive Summary: Many environmental pollutants contain several different isomers, with the same composition, but their shape is opposite. A pollutant's isomer shape can impact how toxic or benign the chemical is to plants and animals. We found that a common herbicide, metolachlor, and the chemicals that result from it breaking down in the environment, MESA, and MOXA, change shape based on the type of soil they flow through. We measured the specific pollutants' isomers in 15 subwatersheds forming the Upper Choptank Watershed. Subwatersheds with more wetland soils have a significant difference in the pollutants' shape compared to well-drained and drier soils. This study is the first time scientists reported pollutant isomer shape changes related to soil on the landscape scale and it is a new tool for environmental monitoring. These results impact policymakers, farmers, and scientists, who must understand the chemical shape to know how it will behave in the environment and agricultural systems.

Technical Abstract: Metolachlor is the most heavily used member of acetanilide herbicides, which are noted for forming highly soluble metabolites in root zone soils soon after field application. The two primary metabolites of metolachlor, metolachlor ethane sulfonic acid (MESA) and metolachlor oxanilic acid (MOXA), retain the same chiral chemistry as their source and are important tracers of nitrate loading from agricultural cropland. New analytical methods for separating the isomers of MESA and MOXA, enable studies assessing changes in the abundance of atropisomer pairs of the carbon chiral enantiomers in environmental samples. These changes were documented starting with the atropisomers in the parent metolachlor structure, leading to soil-degraded metabolites, and then in samples collected over 3 years from 15 subwatersheds in the Upper Choptank River Watershed. The influence of drainage differences, hydric soil and slope, across the watershed strongly correlate with shifts in atropisomer abundance ratios, especially for those enantiomers of MOXA and MESA with axial aS rotations. The hypothesis is that differentiating atropisomer chiral shifts occur as the compounds exit to receiving waters. These findings offer a novel tool to study the transport of these important tracers of cropland-influenced groundwater.