Potential mineralization of four herbicides in a ground water-fed wetland area

Herbicides may leach from agricultural fields into ground water feeding adjacent wetlands. However, only little is known of the fate of herbicides in wetland areas. The purpose of the study was to examine the potential of a riparian fen to mineralize herbicides that could leach from an adjacent catchment area. Slurries were prepared from sediment and ground water collected from different parts of a wetland representing different redox conditions. The slurries were amended with O₂, NO₃^-, SO₄^2-, and CO₂, or CO₂ alone as electron acceptors to simulate the in situ conditions and their ability to mineralize the herbicides mecoprop, metsulfuron-methyl, isoproturon and atrazine. In addition, the abundance of bacteria able to utilize O₂, NO₃^-, SO₄^2-+ CO₂, and CO₂ as electron acceptors was investigated along with the O₂-reducing and methanogenic potential of the sediment. The recalcitrance to bacterial degradation depended on both the type of herbicide and the redox conditions pertaining. Mecoprop was the most readily degraded herbicide, with 36% of [ring-U-¹⁴C]mecoprop being mineralized to ¹⁴CO₂ under aerobic conditions after 473 d. In comparison, approximately 29% of [phenyl-U-¹⁴C]metsulfuron-methyl and 16% of [ring-U-¹⁴C]isoproturon mineralized in aerobic slurries during the same period. Surprisingly, 8 to 13% of mecoprop also mineralized under anaerobic conditions. Neither metsulfuron-methyl nor isoproturon were mineralized under anaerobic conditions and atrazine was not mineralized under any of the redox conditions examined. The present study is the first to report mineralization of mecoprop in ground water in a wetland area, and the first to report mineralization of a phenoxyalcanoic acid herbicide under both aerobic and anaerobic conditions.