Evaluating improvements to exposure estimates from fate and transport models by incorporating environmental sampling...

Widespread chemical contamination represents one of the largest threats of the Anthropocene. The Pesticide in Water Calculator (PWC) is a fate and transport model used by the Environmental Protection Agency and Health Canada to estimate pesticide exposures in lentic freshwater ecosystems and make pesticide registration decisions. Here, we show that maximum measured concentrations of 31% of herbicides and 42% of insecticides exceeded maximum estimated environmental concentrations (EECs) produced by the PWC, suggesting that EECs often do not represent worst-case exposure as they have been purported to do. Based on this observation, we generated statistical models using EECs and over 600,000 field measurements of 31 common insecticides and herbicides to document if the congruence of EECs and maximum field measurements could be improved by accounting for environmental sampling effort (number of times a pesticide is sampled) and contaminant application, factors commonly ignored in most fate and transport models. For lentic systems, variance in pesticide field measurements explained by EECs increased by 50% when sampling effort was included. For lotic systems, variance explained increased by only 4%, most likely because lotic systems are sampled over 4.9 times as much as lentic systems. Including use more than doubled the ability of the EECs to predict maximum pesticides concentrations in lentic systems. Our results suggest that exposure characterization in risk assessment can likely be improved by considering sampling effort and use, thus providing more defensible environmental standards and regulations.