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 herbi cide enantiomerically enriched with the biologically active 1'S-isomers. Th is "chiral switch" is expected to result in lower concentrations and in a c hanged 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 resi dues from two lakes in Switzerland receiving inputs from agricultural activ ities. 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 epimerizati on of atropisomers to ratios 1:1 made these measurements possible despite i ncomplete enantiomer resolution. Surface water prior to 1998, and more rece nt waters from the deeper regions of these lakes, showed EE values of appro ximate to 0, indicating th at pre-1998 inputs were from the racemic product . In contrast, more recent surface water from these lakes showed a clear ex cess 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. Th e enantiomer composition of metolachlor loaded to the takes (EEinput) was d etermined from measured concentrations and EE values in the lake using comp uter models. EEinput values derived in this way were approximate to 0.35 in 1998 and approximate to 0.65 in 1999, corresponding to substitutions of ap proximate to 55 and approximate to 90% of racemic by S-metolachlor, respect ively. The data document the application of enantioselective determinations in environmental residues to apportion the relative contributions of old a nd new herbicide on the basis of enantiomer/ isomer composition. Furthermor e, the data show a rapid environmental response to the changed composition of the herbicide.