September 01, 2016
The Organisation for Economic Co-operation and Development (OECD) recently published a harmonized international guidance document for conducting pesticide terrestrial field dissipation (TFD) studies. This guidance, designed to help avoid unnecessarily large numbers of TFD studies in different countries, and support the global joint review process, was jointly developed by the US Environmental Protection Agency (EPA), the Canadian Pest Management Regulatory Agency (PMRA) and the OECD. An overview of this new guidance was recently presented at the Fresenius conference in Mainz, Germany, and at the ACS National Meeting in August 2016.
The new guidance affords the comparison of similar ecoregions in Europe and North America so that a study conducted in Europe can be acceptable to North America and vice versa and thus can help reduce unnecessary, costly, and time-consuming duplication of studies. Similar global ecoregions are identified with the help of the Ecoregion Crosswalk tool (Europe-North America Soil Geographic Information for Pesticide Studies, or ENASGIPS) to assist in site selection across regions based on environmental conditions and the conceptual pesticide dissipation model developed from laboratory fate studies.
The ENASGIPS model allows the calculation of similarity of ecoregions, or similarity between the conditions of a given TFD site and an ecoregion, based on the following five environmental parameters. Annual average temperature and precipitation, topsoil texture, pH and organic carbon content are used to generate a similarity score. If the similarity score is sufficiently high (≥ 80%), the TFD study results from one ecoregion (or study site) are assumed to be representative for another, and can be considered by other countries and regulatory agencies. This model also helps with selection of sites based on concerns (or worst case scenarios) identified by the conceptual model and has the advantage that another field study may not be required if there is a use expansion to new areas or new crops. However, this tool has not been fully tested and there is no guidance on how to apply it and how to interpret the results.
The new guidance includes proposed experimental design changes from North American and/or EU practices as summarized below. Note, however, that the triggering criteria for TFD studies in the EU regulations remain unchanged. The additional DegT50 module may still be conducted as a higher-tier option even when TFD studies have not been triggered by the criteria in the EU regulations.
- Field plot design: test plots should be level without any slope in order to minimize all processes (other than formation of transformation products or extracted residues) that can affect the fate of the chemical as far as possible.
- Application: it is recommended that the maximum total annual or seasonal dose is applied in a single application in order to obtain robust calculation of dissipation of active substance and metabolites, if appropriate.
- Study duration: the duration of the TFD study has historically taken up to two years to complete. The new GD proposes a duration that is sufficiently long to determine the DT90 of the parent compound as well as the pattern of formation and decline of major transformation products in the soil.
- Irrigation: the study design should include sufficient water to meet the crop need in timing and quantity (110% of crop need) and in accordance with normal agronomic practices for the region in which the site is situated.
- Soil sampling: at least three subplots should be used as the basis for the replicated sampling strategy. Soil cores should be collected to a standard sampling depth of 1 m (except for day zero where a sampling depth of 0 – 30 cm is proposed) and divided into six or more depth increments for analysis; with a requirement to have one residue-free horizon at the bottom. In other words, not all samples need to be analyzed. Sampling of at least five soil cores per sub-plot is recommended, resulting in 15 soil cores per sampling event.
As with all field studies, the utility of the experimental results is limited to the climatic, pedological, agronomic (crop, timing, application rate etc.) conditions and spatial/temporal scales pertinent to the studies conducted. As a result, a modeling exercise on the data generated in these studies will provide the appropriate regulatory context and implication for potential groundwater exposure. “This information is important because the duration and spatial scale of any experiment will rarely match the important temporal and spatial scale parameters that will drive actual groundwater concentrations, even in small unconfined vulnerable shallow aquifers”.
Based on the foregoing, a pragmatic application of the new guidance is recommended in order to meet the regulatory objectives of the studies. Therefore, applicants are strongly encouraged to consult with the appropriate regulatory authorities during the design phase, with respect to (a) approval of selected sites, and number of sites; (b) study duration; (c) which formulation types to test if it is anticipated that the final end-use formulation(s) may influence dissipation; (d) whether to include the DegT50 module or not and (e) any other consideration.
To discuss these and other testing strategies in detail, contact:
Dr. Kalumbu Malekani