Changes in enantiomeric fraction as evidence of natural attenuation of mecoprop in a limestone aquifer

Natural attenuation of the chiral pesticide mecoprop 2-(2-methyl-4-chlorophenoxy)propionic acid] has been studied by determining changes in its enantiomeric fraction in different redox environments down gradient of a landfill in the Lincolnshire Limestone. Previous studies have shown that mecoprop degrades predominantly aerobically and that differences in the biological behaviour of the two enantiomers will change their relative proportions during biodegradation. Originally deposited as a racemic mixture, there has been no change in the enantiomeric fraction in the most polluted part of the landfill plume where conditions are sulphate reducing/methanogenic. In the nitrate-reducing zone, the proportion of (S)-mecoprop increases, suggesting preferential degradation of (R)-mecoprop; while in the aerobic zone, the proportion of (R)-mecoprop increases, suggesting faster degradation of (S)-mecoprop. Mecoprop persistence in the confined Lincolnshire Limestone further downdip is explained by inhibition of degradation in sulphate-reducing conditions, which develop naturally. Laboratory microcosm experiments using up to 10 mg l(-1) of mecoprop confirm these inferences and show that under aerobic conditions, (S)-mecoprop and (R)-mecoprop degrade with zero-order kinetics at rates of 1.90 and 1.32 mg l(-1) day(-1), respectively. Under nitrate-reducing conditions (S)-mecoprop does not degrade, but (R)-mecoprop degrades with zero-order kinetics at 0.65 mg l(-1) day(-1) to produce a stoichiometric equivalent amount of 4-chloro-2-methylphenol. This metabolite only degrades when the (R)-mecoprop has disappeared. The addition of nitrate to a dormant iron-reducing microcosm devoid of nitrate stimulated anaerobic degradation of (R)-mecoprop after a lag period of 21 days. There was no evidence for enantiomeric inversion. The study demonstrates the sensitivity of changes in enantiomeric fraction for detecting natural attenuation, and reveals subtle differences in mecoprop degradation in different redox environments within the Lincolnshire Limestone aquifer.