Anaerobic Dissipation of Herbicides in Microbiologically Induced Soil Redox Regimes

149 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1997.Herbicides may encounter anaerobiosis as a result of soil flooding, poor drainage, or offsite transport. Anaerobic dissipation of herbicides in the environment is poorly understood. The objective of this research was to characterize the anaerobic fates of herbicides atrazine, trifluralin, and dimethenamid in anaerobic flooded soil or sediment. Particular emphasis was placed on the effects of redox conditions and corresponding anaerobic microbial processes on herbicide mineralization and bound residue formation as routes of dissipation. Herbicide fate was evaluated by quantifying the distribution of radiolabeled material in the mineralized, aqueous (parent and metabolite), sorbed (extractable parent and metabolite), bound (unextractable), and volatile fractions of soil and sediment slurries over time. Microbial involvement was assessed by the disappearance of terminal electron acceptors nitrate and sulfate and the production of Fe(II), H$\sb2$S, and methane as compared with control microcosms. Redox potential and pH were also monitored. In a pure culture study, the bacterial isolate M91-3 metabolized (UL-$\sp{14}$C) atrazine to $\sp{14}$CO$\sb2$ and ($\sp{14}$C) hydroxyatrazine under denitrifying conditions. In anaerobic slurries, degradation of herbicides ($\sp{14}$C) atrazine, ($\sp{14}$C) trifluralin, and ($\sp{14}$C) dimethenamid was demonstrated. The abiotic or biological nature of degradation differed for each herbicide. Anaerobic microbial processes including denitrification, Fe(III) reduction, sulfate reduction, and methanogenesis each contributed to herbicide dissipation. Atrazine mineralization occurred under denitrifying and sulfate-reducing conditions and was enhanced under sulfate-reducing conditions. Abiotic hydroxyatrazine formation was apparent. Trifluralin degradation was abiotically catalyzed in the reduced environment created by anaerobic microbial metabolism, particularly by Fe(III) and sulfate reduction. Dimethenamid degradation in sediment was microbially-mediated. Dimethenamid and atrazine degradation appeared to be bioavailability-limited. The following half-lives were observed: trifluralin, 4-8 days, dimethenamid, 16-25 days, and atrazine, 16-25 days. In general, observed T$\sb{1/2}$'s were less that those predicted by first-order kinetics. The formation of soil-bound residues was a significant fate of all herbicides in anaerobic soil and sediment slurries. Bound residue formation increased with longer period of herbicide exposure to $>$50% of applied for trifluralin, dimethenamid, and atrazine by 3, 4, and 6 months, respectively. Results demonstrated that the herbicides studied were susceptible to anaerobic degradation in flooded soil and sediment and that anaerobic microbial processes contributed to dissipation. In general, more extensive herbicide transformations were noted in slurries that had been amended with terminal electron acceptors to permit the natural progression of increasingly reductive metabolism in microcosms.U of I OnlyRestricted to the U of I community indefinitely during batch ingest of legacy ETD