Submitted to: Environmental Toxicology and Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/11/2011
Publication Date: 8/2/2011
Citation: Webb, R.M., Sandstrom, M.W., Krutz, L.J., Shaner, D.L. 2011. Simulation of branched serial first-order decay of atrazine and metabolites in adapted and nonadapted soils. Environmental Toxicology and Chemistry. 30:1973-1981. Interpretive Summary: Regulatory agencies rely on modeling to draw inferences about pesticide effects on the environment. A key input variable for modelers is pesticide persistence in the environment. A collaborative project between USGS and USDA-ARS has resulted in a model that can help regulatory agencies and research scientists better predict the persistence of pesticides and their degradation products in the environment. Future calibration of this new model for soils throughout the United States can reduce the uncertainty, relative to that of traditional modeling approaches, in predicting pesticide fate and transport, thereby supporting improved agricultural management schemes for reducing risk to the environment.
Technical Abstract: Pesticides are applied to fields in amounts calculated to provide maximum control of the insects or weeds while minimizing the health threats to humans and the environment. A key variable in these calculations is the half life, or persistence, of the product and its degradates, also known as daughter products, each with unique physical and chemical properties and toxicity. Traditional models simulate the decay of pesticides as serial first-order decay from parent to daughter to granddaughter. This approach can produce erroneous results when simulating branched decay where the parent breaks down into multiple daughter products at the same time. A common error is the failure to conserve mass when the disappearance rate of the parent is not consistent with the production rates of the daughters. This potential error can be overcome by incorporating equations developed to describe the branched serial first-order decay (BSFOD) of compounds. In this paper, a BSFOD model is used to derive transformation rates describing the decay of a common herbicide, atrazine, and its degradates observed in soils adapted to previous atrazine applications and in soils with no history of atrazine applications. The results support two conclusions: (1) atrazine and daughter products are less persistent in adapted soils than in nonadapted soils; and (2) hydroxyatrazine was the dominant daughter product in most of the soils tested. A method to simulate BSFOD in a one-dimensional solute-transport unsaturated zone model is presented. Calibration of BSFOD models for soils throughout the country can reduce the uncertainty, relative to that of traditional models, in predicting the fate and transport of pesticides and their daughter products and thus support improved agricultural management schemes for reducing threats to the environment.