Pesticides undergo many transformation and transport processes in the environment. One process is the formation of non-extractable residues (NERs), a fraction of pesticides, including transformation products, that becomes tightly bound to soil and sediment with low potential to remobilize. As such, NERs have negligible contribution to environmental exposure, and the Pest Management Regulatory Agency (PMRA) considers their formation a route of dissipation. In laboratory studies, pesticide residues can be difficult to remove from soil or sediment samples using conventional solvent extraction protocols. When unextracted residues from inadequate extractions are included with the NER fraction, degradation rate constants can be overestimated, and, consequently, persistence and estimated environmental concentrations are underestimated. From a Canadian regulatory perspective, the PMRA, therefore, takes a cautious approach by treating unextracted residues resulting from inadequate extractions as residues of concern and including them in half-life calculations. Historically, extraction protocols have often used solvents with similar polarities under ambient laboratory conditions. However, increasingly, advanced extraction technologies are being used to characterize bound residues in combination with multiple solvents of varying polarity. In 2014, the US EPA published the <i>Guidance for Addressing Unextracted Pesticide Residues in Laboratory Studies</i>, recommending that multiple polar and nonpolar solvents are tested with each soil or sediment and across the duration of the study, as parent and transformation products vary in chemical properties and may interact with soil or sediment differently. The PMRA is developing a guidance for evaluating the quality of solvent extraction protocols that builds on the US EPA guidance and addresses unique Canadian policy issues. The PMRA guidance will describe current perspectives on NERs, steps for evaluating solvent choice and extraction methodologies, and, ultimately, the determination of unextracted versus non-extractable residues. The PMRA approach is elaborated with examples that illustrate the impact of suitable extraction protocols on regulatory conclusions for persistence in the environment and exposure to pesticides from drinking water sources and other ecosystems.

Unextracted or non-extractable: A Canadian regulatory perspective

ACS Fall 2020 Virtual Meeting

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