Aerobic soil degradation studies are a core data requirement to support the registration of pesticides across the world. Such studies are used to determine the rate of degradation of the active substance, the formation of metabolites and the processes by which the residues of the active substance are dissipated from the environment. During the metabolism of the parent molecule, it is not unusual to see strong adsorption of pesticide residues to the soil. This strong adsorption is due to interaction and/or entrapment of the residues within the soil particles and associated organic material leading to poor extraction from the soil. Historically, Non-Extractable Residues (NER) were treated as a sink and classified as a dissipation process; however, this approach is changing since residues may be released over longer time scales and therefore pose potential risks to the environment in subsequent years. Regulatory authorities worldwide take differing approaches to NER and the subsequent risk to the environment posed by such residues. Accordingly, strategies to characterize and identify the components present in NER are becoming increasingly important to enable robust and accurate risk assessments and the long-term safety of the product.  A recent aerobic soil study performed on a pipeline herbicide showed high NER throughout the 120-day experiment. Following the extraction methodology framework from the European Centre for Ecotoxicology and Toxicology of Chemicals and the U.S. Environmental Protection Agency NER guidance document, a series of solvents and harsh conditions were tested. Solvents of different polarities were unsuccessful at improving the extractability and harsh conditions were shown to break down the metabolites, making definitive identification extremely difficult. Significant improvement was observed using EDTA, which is a chelating agent that has been shown to displace complexations and release residues from the soil into the extraction solvent. The addition of EDTA to the extraction media resulted in dramatically improved extractability at ambient temperature and pressure, enabling chromatographic analysis to characterize and identify the pesticide residues that were released from the soil.  These results indicate that the use of chelating agents can facilitate the extraction of strongly sorbed pesticide residues from soil and their subsequent identification; thus enabling a more reliable risk assessment of the pesticide to be conducted.

Development of new strategies to address NER in soil

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