Evaluation of AGRO-2014 for Predicting Hydrophobic Organic Chemical Concentrations in Ponds.

Highly hydrophobic organic chemicals (HOCs), like pyrethroids, adsorb strongly to eroded soil and suspended sediment. Therefore, total suspended solids (TSS) concentration in the water column of receiving waters is important for determining the proportion of chemical in the sediment-sorbed vs. the dissolved (bioavailable) state. However, most current regulatory exposure models, such as the Exposure Analysis Modeling System (EXAMS) and Variable Volume Water Model (VVWM), do not include dynamic modeling of TSS. The objective of this study is to compare the performance of those models for simulating observed pesticide concentrations in small water bodies with an updated version of the AGRO model, called AGRO-2014, which includes dynamic sediment processes. The paper also evaluates the importance of explicitly modeling sediment dynamics for HOCs. We calibrated AGRO-2014 for small, static, water bodies using published pyrethroid mesocosm data. To improve the basis for intermodel comparison, AGRO-2014 includes the same algorithm for temperature-dependent degradation found in EXAMS and VVWM, direct acceptance of organic C partition coefficient () inputs, and acceptance of user-defined pesticide loading durations. Differences in sediment processes in AGRO-2014, EXAMS, and VVWM significantly affected predicted concentrations of high- compounds for standardized loading scenarios, whereas differences between the models were less evident for compounds with lower sorption to sediments. AGRO-2014 simulations of drift and slurry pyrethroid applications to ponds closely matched observed concentrations, while EXAMS and VVWM simulations underestimated the observations. The publicly available AGRO-2014 model offers improvements over other models for predicting concentrations of HOC compounds in small water bodies.