ABSTRACT
Understanding and simulating the fate and transport of pesticides from a field to adjacent receiving water bodies is critical for estimating long-term environmental exposure concentrations (EECs) in regulatory higher-tier environmental exposure assessments (EEA). The potential of field mitigation practices like vegetative filter strips (VFS) to reduce pesticide pollution is receiving increasing attention. Previous research has proposed a modeling framework that links the US Environmental Protection Agency's (US-EPA) PRZM/EXAMS higher-tier EEA with a process-based VFS model (VFSMOD). This framework was updated to consider degradation and carryover of pesticide residue trapped in the VFS. However, there is disagreement on pesticide degradation assumptions among different regional EEA regulations (i.e. US or European Union), and in particular on how temperature and soil moisture dynamics may affect EECs. This research updated the VFS modeling framework to consider four degradation assumptions and determine if VFS residues and/or EECs differed with each assumption. Two model pesticides (mobile-labile and immobile-persistent) were evaluated for three distinct agroecological scenarios (continental row-crop agriculture, wet maritime agriculture, and dry Mediterranean intensive horticulture) with receiving water bodies and VFS lengths from 0 to 9m. The degradation assumption was important in long-term assessments to predict VFS pesticide residues (statistically different at p<0.01). However, due to the relatively small contribution of residues on the total pesticide mass moving through the VFS, degradation assumptions had a negligible impact on EECs. This indicates that, while important differences exist between EU or US EEAs, the choice of pesticide degradation assumption is not a main source of these differences.
ABSTRACT
Vegetative filter strips (VFS) are a widely adopted practice for limiting pesticide transport from adjacent fields to receiving waterbodies. The efficacy of VFS depends on site-specific input factors. To elucidate the complex and non-linear relationships among these factors requires a process-based modeling framework. Previous research proposed linking existing higher-tier environmental exposure models with a well-tested VFS model (VFSMOD). However, the framework assumed pesticide mass stored in the VFS was not available for transport in subsequent storm events. A new pesticide mass balance component was developed to estimate surface pesticide residue trapped in the VFS and its degradation between consecutive runoff events. The influence and necessity of the updated framework on acute and chronic estimated environmental concentrations (EECs) and percent reductions in EECs were investigated across three, 30-year U.S. EPA scenarios: Illinois corn, California tomato, and Oregon wheat. The updated framework with degradation predicted higher EECs than the existing framework without degradation for scenarios with greater sediment transport, longer VFS lengths, and highly sorbing and persistent pesticides. Global sensitivity analysis (GSA) assessed the relative importance of mass balance and degradation processes in the context of other input factors like VFS length (VL), organic-carbon sorption coefficient (Koc), and soil and water half-lives. Considering VFS pesticide residue and degradation was not important if single, large runoff events controlled transport, as is typical for higher percentiles considered in exposure assessments. Degradation processes become more important when considering percent reductions in acute or chronic EECs, especially under scenarios with lower pesticide losses.
