Degradation of herbicides in shallow Danish aquifers: an integrated laboratory and field study

Degradation of pesticides in aquifers has been evaluated based on a number of co-ordinated field and laboratory studies carried out in Danish aquifers . These studies included investigations of vertical and horizontal variabil ity in degradation rates from the vadose zone to an aquifer, the effects of aerobic versus anaerobic conditions, and the importance of concentration o n degradation kinetics for a selected range of herbicides. The studies were based on different experimental approaches ranging from simple batch exper iments to column studies to field injection experiments and, where appropri ate, results were compared. Some herbicides were degraded under aerobic con ditions (some phenoxy acids, DNOC and glyphosate) and others under aerobic conditions (other phenoxy acids, DNOC; there was some indication of atrazin e transformation). Certain pesticides were not degraded in any investigatio ns (dichlobenil, the dichlobenil metabolite 2,6-dichlorobenzamide (BAM), be ntazone, isoproturon, metarnitron and metsulfuron-methyl). The spatial vari ability was substantial, since hardly any of the investigated pesticides we re degraded in all comparable samples. This means that it is very difficult to claim that a given pesticide is readily degradable in aquifers. However , the experimental approaches used (with incubations lasting more than a ye ar) may not be sensitive enough to verify the low degradation rates that ma y be significant as a result of the long retention time of groundwaters. (C ) 2001 Society of Chemical Industry.