CHANGES IN THE ENANTIOMERIC RATIO OF (R)-MECOPROP TO (S)-MECOPROP INDICATE IN-SITU BIODEGRADATION OF THIS CHIRAL HERBICIDE IN A POLLUTED AQUIFER

Leachate samples from a waste disposal site in Switzerland and groundw ater samples downstream of the landfill were analyzed for residues of (R)- and (S)-mecoprop [(R)and (S)-2-(4-chloro-2-methylphenoxy)propioni c acid] by means of enantiomer-specific gas chromatography combined wi th mass spectrometry. (R)- And (S)-mecoprop were found at equal concen trations (up to 124 mu g/L) in the landfill leachate, indicating that a racemic mixture of mecoprop leached to the receiving groundwater. Gr oundwater samples downstream of the landfill contained from <0.001 to 975 mu g/L of mecoprop and 16 out of 31 samples showed a significant e xcess of (R)-mecoprop. Sorption of (R)- and (S)-mecoprop to aquifer ma trix and to various reference minerals was generally low and did not d iscriminate between the mecoprop enantiomers. We conclude that enantio selective microbial degradation increased the enantiomeric ratio of (R )- to (S)-mecoprop during groundwater passage of the landfill leachate . Most leachate-affected groundwaters are subject to a very complex in put pattern of contaminants in time and space. Thus, observations of c oncentration changes of contaminants with time or space are rarely ind icative of specific transformation processes in such aquifers. Under s uch conditions, analysis of the enantiomeric ratio of chiral contamina nts such as mecoprop can provide information about in situ biodegradat ion processes.