A field verification of the Toxic Organic Substance Transport and Bioaccumulation Model (TOXIC) was conducted using the insecticide dieldrin and the herbicides alachlor and atrazine as the test compounds. The test sites were two Iowa reservoirs. The verification procedure included both steady-state analyses and quasi-dynamic simulations using time-variable flows and pollutant loadings along with model coefficients derived from laboratory and literature data.
Laboratory measurements were used in simulations of alachlor, atrazine and dieldrin, and model predictions were well within an order of magnitude of field observations. For the herbicide alachlor, for example, laboratory protocol measurements were used directly in model simulations with excellent agreement between model predictions and measured concentrations.
The TOXIC model, therefore, was considered to be field verified. Moreover, the successful field verification supports the validity of EPA's Exposure Analysis Modeling System (EXAMS), which handles pollutant transport and transformation kinetics in an almost identical manner.