The use of enantiomeric ratios to assess the fate of mecoprop in groundwater

Disposal of the chiral phenoxyacid herbicide mecoprop into landfills in the Lincolnshire Limestone has polluted an abstraction well 2.5 km away. Differences in the biological behaviour of the two mirror image structures of mecoprop (or enantiomers), means that changes in the enantiomeric ratio (ER) can help identify the extent of biodegradation down gradient of the landfill. Deposited as a racemic mixture (i.e. 50% of each enantiomer), there has been no change in the ER in the most polluted part of the landfill plume where conditions are sulphate reducing/methanogenic, indicating no degradation. In the iron and nitrate reducing zones of the plume (S)-mecoprop dominates suggesting either inversion of the (R)-mecoprop to (S)-mecoprop, or faster degradation of (R)-mecoprop. In the aerobic aquifer the gradual increase in the ER in favour of (R)-mecoprop suggests faster degradation of (S)-mecoprop. The persistence of mecoprop in the confined Lincolnshire Limestone further down dip is explained by degradation being inhibited by sulphate reducing conditions that develop naturally