Mineralization of isoproturon, mecoprop and acetochlor in a deep unsaturated limestone and sandy aquifer
Résumé
Isoproturon (N N-dimethyl N'-[4-(1-methylethyl)phenyl]urea) mecoprop (MCPP) (2 (4-chloro-2-methylphenoxy)propanoic acid) and acetochlor (2-chloro-N-(2-ethoxymethyl)-N-(2-ethyl 6-methylphenyl)acetamide) are agricultural pesticides that may leach through the vadose zone down to groundwater Sediment samples were collected from intact sediment cores from 0 to 59 m below surface including soil unsaturated limestone and aquifer sand In the unsaturated limestone the initial pesticide concentrations (0 5-100 mu g kg(-1)) did not systematically affect the proportion of mineralized pesticides or the kinetics However in the aquifer mecoprop and to some degree isoproturon mineralization was found to increase with increasing initial concentration (05-100 mu g L-1 equivalent to 1-220 mu g kg(-1)) demonstrating the importance of using environmentally relevant concentrations when predicting pesticide fate The mineralization of isoproturon mecoprop and acetochlor was studied in 40 samples at low concentrations (1-3 mu g L-1) and specific pesticide-mineralizing bacteria were enumerated using C-14-MPN Presence of the mineralizers documented a degradation potential of the pesticides within the catchment The number of mineralizers varied from <0 18 to >16 000 g(-1) and was not found to correlate with depth Mecoprop isoproturon and acetochlor were substantially mineralized in the soils (19-44% after 8 months incubation at 1 mu g kg(-1)) in sub-surface unsaturated limestone samples (<= 2% for aceto chlor <= 21% for isoproturon and <= 31% for mecoprop) and in aquifer samples (4-28% for mecoprop <= 4 7% for isoproturon and <= 5 6% for acetochlor) The finding of isoproturon and acetochlor mineralization in deep aquifers is novel and important for the evaluation of the fate of these pesticides as even low mineralization rates can be important in aquifers exhibiting long residence times