Agricultural nonpoint source pollution, especially nitrate-N, is often a leading cause of US waterways being defined as impaired. Conservation practices have been implemented to remediate this problem; however, methods, such as tracking the lag time process responsible for the transport of agricultural nitrate, are needed to demonstrate that land management changes and agricultural practices are improving water quality. Metolachlor, a widely used pre-emergent herbicide in maize and soybean production, exists as 4 major enantiomers and is readily metabolized in soil to two abundant polar metabolites, metolachlor ethane sulfonic acid (MESA) and metolachlor oxanilic acid (MOXA). Both retain the parent stereochemistry and persist in agricultural groundwater reserves at high concentrations due to their high-water solubility. In 2000, the formulation of metolachlor changed from a racemic mixture to the S-form which is more biologically active. This chirality change provides a temporal stamp that can be used to date the age of ground and surface stream waters. In a previous study on a first order stream in Maryland, we observed a strong connection between nitrate-N and the degradation products of metolachlor. Here, we describe methods to separate all 4 of the major isomers of MESA and MOXA and expand on the use of these degradation products as transient tracers of nitrate-N and lag time processes. Sampling methods, such as time-sensitive grab sampling and time-weighted integrative sampling using passive samplers, were examined for their utility and applicability to provide additional information about groundwater flow paths. These data are being used to gain insight into the complexities of different watersheds which vary in size, types of drainage, environmental processes, and hydrology. This work will provide important data for modelers to assist producers and land managers in assessing the effectiveness of conservation practices.

Chiral separation of metolachlor metabolites (MESA and MOXA) as conservative tracers for agricultural nitrogen in water and water dating

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ACS Fall 2022

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