U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Chemistry, Degradation, and Transport of Triazine Herbicide Metabolites in Surface Water

Abstract

Deethylatrazine and deisopropylatrazine are two major metabolites of the triazine herbicides that occur in surface water of the midwestern United States. They may originate from the decomposition of several original parent compounds. Atrazine, the major triazine herbicide used in the Midwest, degrades systematically to both deethylatrazine and deisopropylatrazine with a ratio of approximately 0.40 for runoff water from cornfields. Ground water usually contains only trace concentrations of deisopropylatrazine compared to deethylatrazine because of the more rapid decomposition of deisopropylatrazine in the unsaturated zone. Thus, the deisopropylatrazine-to-deethylatrazine ratio (D²R) may be used to distinguish surface-water runoff from ground water that discharges to surface water. However, other parent herbicides may affect the D²R. For example, cyanazine may degrade to deisopropylatrazine by the loss of the cyano-isopropyl group. Because cyanazine is the fourth most frequently used herbicide in the Midwest, the D²R in surface water often is considerably greater than 0.40 (from 0.60-0.80). Simazine, another parent triazine herbicide, also may degrade to deisopropylatrazine. Simazine is used for weed control along highways and in orchards where considerably larger concentrations are applied. The resulting runoff contains deisopropylatrazine as a major dealkylated degradation product. Propazine, a parent triazine herbicide that is used only occasionally on grain sorghum, degrades to yield the deethylatrazine metabolite. Finally, terbuthylazine, a triazine not marketed in the United States, but used extensively in Europe, also may degrade to deisopropylatrazine via the loss of a t-butyl group. Thus, these parent compounds should be considered when using metabolite ratios as indicators of flow path.