The use of enantiomeric ratios to identify natural attenuation of mecoprop in the Lincolnshire Limestone

Disposal of the phenoxyacid herbicide mecoprop into landfills in the Lincolnshire Limestone (UK) 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 downgradient 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 sulphate-reducing part of the landfill plume. In the iron- and nitrate-reducing zones of the plume, changes in the ER suggest faster degradation of (R)-mecoprop than (S)-mecoprop, or inversion of (R)- to (S)-. In the aerobic aquifer (S)-mecoprop degrades at a faster rate than (R)-mecoprop and the ER ratio reverses. The persistence of mecoprop in the confined Lincolnshire Limestone is explained by sulphate-reducing conditions which develop naturally, inhibiting degradation or inversion.