Occurrence and Transformation Reactions of Chiral and Achiral Phenoxyalkanoic Acid Herbicides in Lakes and Rivers in Switzerland

The occurrence of chiral and achiral phenoxyalkanoic acid herbicides in lakes and rivers in Switzerland is reported. The compounds most frequently detected were the chiral 2-(4-chloro-2-methylphenoxy)propionic acid (mecoprop, MCPP) and the achiral 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D), and dicamba, a benzoic acid derivative. The compounds were generally present at concentrations well below the ECE recommended drinking water tolerance level (100 ng/L), even in lakes situated in areas with intense agricultural activities. The chiral 2-(2,4-dichlorophenoxy)propionic acid (dichlorprop, DCPP) was hardly present, and none of the compounds was detected in mountain lakes. In case of MCPP, both enantiomers (R and S) were present, although only the technical material with the R enantiomer (mecoprop-P) is registered and used as a herbicide in Switzerland. Previous studies indicated significant enantiomerization of MCPP and DCPP in soil leading to residues enriched in R enantiomers independent of whether the racemic (R/S) or the enantiopure R compound was incubated. In some Swiss lakes now, the residues of MCPP showed compositions of R > S as expected from the soil degradation data, but in other lakes, surprisingly, “reversed” compositions of S > R were found. This suggested the occurrence of additional biotic processes in the aquatic environment and/or contamination with racemic MCPP from another source. Laboratory incubation of MCPP and DCPP in lake and river water confirmed significant enantiomerization. The enantiomerization is biologically mediated and leads to residues of MCPP and DCPP in these waters, which are eventually enriched in the S enantiomers.