Changes in the Enantiomeric Ratio of (R)- 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 groundwater samples downstream of the landfill were analyzed for residues of ( R )- and ( S )-mecoprop [( R )- and ( S )-2-(4-chloro-2-methylphenoxy)propionic acid] by means of enantiomer-specific gas chromatography combined with mass spectrometry. ( R )- And ( S )-mecoprop were found at equal concentrations (up to 124 μg/L) in the landfill leachate, indicating that a racemic mixture of mecoprop leached to the receiving groundwater. Groundwater samples downstream of the landfill contained from <0.001 to 975 μg/L of mecoprop and 16 out of 31 samples showed a significant excess of ( R )-mecoprop. Sorption of ( R )- and ( S )-mecoprop to aquifer matrix and to various reference minerals was generally low and did not discriminate between the mecoprop enantiomers. We conclude that enantioselective 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 input pattern of contaminants in time and space. Thus, observations of concentration changes of contaminants with time or space are rarely indicative of specific transformation processes in such aquifers. Under such conditions, analysis of the enantiomeric ratio of chiral contaminants such as mecoprop can provide information about in situ biodegradation processes.