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 i n lakes and rivers in Switzerland is reported. The compounds most freq uently detected were the chiral 2-(4-chloro-2-methylphenoxy)propionic acid (mecoprop, MCPP) and the achiral 4-chloro-2-methylphenoxyacetic a cid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D), and dicamba, a ben zoic acid derivative. The compounds were generally present at concentr ations well below the ECE recommended drinking water tolerance level ( 100 ng/L), even in lakes situated in areas with intense agricultural a ctivities. The chiral 2-(2,4-dichlorophenoxy)propionic acid (dichlorpr op, DCPP) was hardly present, and none of the compounds was detected i n mountain lakes. In case of MCPP, both enantiomers (R and S) were pre sent, although only the technical material with the R enantiomer (meco prop-P) is registered and used as a herbicide in Switzerland. Previous studies indicated significant enantiomerization of MCPP and DCPP in s oil leading to residues enriched in R enantiomers independent of wheth er the racemic (RIS) or the enantiopure R compound was incubated. In s ome Swiss lakes now, the residues of MCPP showed compositions of R > S as expected from the soil degradation data, but in other lakes, surpr isingly, ''reversed'' compositions of S > R were found. This suggested the occurrence of additional biotic processes in the aquatic environm ent and/or contamination with racemic MCPP from another source. Labora tory incubation of MCPP and DCPP in lake and river water confirmed sig nificant enantiomerization. The enantiomerization is biologically medi ated and leads to residues of MCPP and DCPP in these waters, which are eventually enriched in the S enantiomers.