Changed Enantiomer Composition of Metolachlor in Surface Water Following the Introduction of the Enantiomerically Enriched Product to the Market

Racemic metolachlor is currently being replaced by S-metolachlor, the herbicide enantiomerically enriched with the biologically active 1‘S-isomers. This “chiral switch” is expected to result in lower concentrations and in a changed enantiomer/isomer composition of the residues in the environment. To verify this switch, enantioselective gas chromatography/mass spectrometry was used to determine the enantiomer/isomer composition of metolachlor residues from two lakes in Switzerland receiving inputs from agricultural activities. As a measure of this composition, we used the enantiomer excess (EE) defined as the excess of the 1‘S- over the 1‘R-isomers. Thermal epimerization of atropisomers to ratios 1:1 made these measurements possible despite incomplete enantiomer resolution. Surface water prior to 1998, and more recent waters from the deeper regions of these lakes, showed EE values of ≈0, indicating that pre-1998 inputs were from the racemic product. In contrast, more recent surface water from these lakes showed a clear excess of the 1‘S-isomers with EE values up to 0.25 in 1998 and up to 0.64 in 1999, indicating significant contributions from the nonracemic product. The enantiomer composition of metolachlor loaded to the lakes (EE_input) was determined from measured concentrations and EE values in the lake using computer models. EE_input values derived in this way were ≈0.35 in 1998 and ≈0.65 in 1999, corresponding to substitutions of ≈55 and ≈90% of racemic by S-metolachlor, respectively. The data document the application of enantioselective determinations in environmental residues to apportion the relative contributions of old and new herbicide on the basis of enantiomer/isomer composition. Furthermore, the data show a rapid environmental response to the changed composition of the herbicide.