Degradation of herbicides in two sandy aquifers under different redox conditions

We examined the potential for complete degradation (mineralisation) of the four [ring-U-¹⁴C]herbicides mecoprop, isoproturon, atrazine, and metsulphuron-methyl in two sandy aquifers representing aerobic, denitrifying, sulphate-reducing, and methanogenic conditions. Slurries with sediment and groundwater were set-up aerobically or anaerobically in the presence of the electron-acceptor prevailing at the sampling site, amended with 25 μg 1^-1 herbicide, and incubated ar 10°C. Considerable mineralisation was only observed in sediment from the plough layer incubated aerobically. Here, 30% of ¹⁴C-mecoprop was recovered as ¹⁴CO₂ after 15 days and 15% of isoproturon was recovered as ¹⁴CO₂ after 267 days. Only 7% of mecoprop was recovered as ¹⁴CO₂ after 313 days in sediment from the aquifer below sampled ar 1.95-3.00 mbs (m below the surface). In denitrifying and methanogenic slurries, 3% of ¹⁴C added as mecoprop was recovered as ¹⁴CO₂. Isoproturon was not mineralised except in the aerobic plough layer, and atrazine and metsulphuron-methyl were not mineralised under any of the conditions applied.