Bioremediation of Herbicide-Contaminated Soil and Water

The contamination of soils, groundwater and surface waters by herbicides poses major clean-up problems. At present, the most common approach is containment, involving costly removal of highly contaminated soils to landfill sites. Bioremediation methods for in situ treatment are needed as alternative and/or supplementary approaches for cost-effective and residue-free clean-up. This project focuses on biodegradation of atrazine, but involves development of a general bioremediation protocol applicable to other herbicides. Atrazine is the most commonly detected organic contaminant in groundwater in Wisconsin and all other high corn production areas of the Midwest. Laboratory studies have demonstrated that atrazine can be degraded by the concerted activity of consortia (mutually compatible groups) of microorganisms. Isolation of microbial consortia by traditional enrichment-culture techniques has been ineffective in the case of atrazine degraders. This report describes an improved method for obtaining these cultures utilizing a conceptual computer model to facilitate interpreting, predicting, and controlling the performance of the bioremediation system. Preliminary studies have focused on development of analytical procedures to determine the concentration of atrazine and its metabolites, microbial biomass, and ammonium. Values derived from these experiments have been used to determine initial input parameters for the model using Pseudomonas sp. strain D. Experimental values obtained for growth rate (biomass/hr) and specific growth yield (gr protein/mole nitrogen consumed) under nitrogen limited and carbon limited conditions agreed closely with the results from computer model simulations. These findings confirmed the conceptual adequacy of the model. The model was used to identify the basis of unusually low growth yields under carbon limited conditions. In this case, accumulation of intermediate metabolites was predicted. This application of the model provided an example of its utility for interpreting system performance. The model was also used to simulate atrazine degradation under a condition prevailing in groundwater, i.e., low levels of dissolved oxygen. However, Pseudomonas sp. strain D did not have the metabolic capabilities to survive in this environment.