Relating atrazine degradation rate in soil to environmental conditions: implications for global fate modeling

With the ready availability of georeferenced environmental data, regional and global chemical fate models have become increasingly spatially explicit. However, the description of how chemical fate properties such as degradation rate constants and partition coefficients depend on environmental conditions has not kept up with these developments. Consequently, model results are often subject to large uncertainty stemming from inherent variability in these properties. Here, we present an extensive meta-analysis of soil degradation of one exemplary compound, the herbicide atrazine. In the first part of the paper, we present the results of an in-depth statistical analysis of the dependence of atrazine degradation rate constants on various environmental factors. In the second part, the resulting estimation equation for atrazine degradation rate constants is implemented in CliMoChem, a model for the prediction of global chemical fate, which we supplemented with spatial information on various soil descriptors, such as temperature, sand and clay content, organic carbon content, and pH. Estimates of polar accumulation, an important indicator of global chemical fate, were then compared between this model setup and estimates obtained when the degradation rate constant is represented by a single value or as being dependent on temperature only. Results for the three rate estimation methods demonstrate that a spatially explicit description of the soil degradation process results in 4-fold higher estimates of polar accumulation, while reducing uncertainty in the prediction of this endpoint by more than 40%.