Terrestrial Field Dissipation Studies - American Chemical Society

Chapter 4

Terrestrial Field Soil Dissipation Studies in Pesticide Environmental Risk Assessment

Downloaded by UNIV OF PITTSBURGH on December 6, 2015 | http://pubs.acs.org Publication Date: December 15, 2002 | doi: 10.1021/bk-2002-0842.ch004

Paul Hendley Syngenta Crop Protection Inc., 410 Swing Road, Greensboro, NC 27409

Field dissipation studies are a key data requirement for the registration of crop protection products. Guidelines for the conduct of these studies have been developed by many countries and have similar (but not quite identical) specifications. Recently a proposal to revise the United States Environmental Protection Agency (US EPA) and Canadian guidance has been put forward and promulgated as a potential Organization for Economic Cooperation and Development (OECD) guidance document; this proposal adds requirements for the studies to address the relative importance of all routes of dissipation and fully characterize the compound's dissipation in terms of "mass balance" by the incorporation of additional "modules" such as leaching and runoff. This has prompted a detailed review of the function of Terrestrial Field Soil Dissipation (TFSD) studies within regulatory risk assessment schemes from which it became apparent that, in the USA, the current TFSD study is not used significantly in risk assessment or to trigger further studies. Surprisingly, despite the high financial and reviewing costs of these studies for both regulators and industry, they provide little useful return on investment. While the proposal to design a more comprehensive study offers several advantages, a detailed review shows that, in most cases, the resulting study will not adequately address the objective of understanding mass

© 2003 American Chemical Society

In Terrestrial Field Dissipation Studies; Arthur, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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44 balance. Moreover, while some modules can easily be combined with field soil dissipation elements, some of the proposed add-in "modules" are incompatible with one another as well as with the traditional conduct of field soil dissipation studies such that attempts to incorporate them all are likely to result in unproductive compromises. It is proposed that a simple guideline be developed that describes a minimum set of data to measure surface soil dissipation. The study director should then be obliged to provide additional details as needed on a case-by-case basis in order to account for other potentially important dissipation mechanisms. These modules could be added to the TFSD study if appropriate or conducted as separate studies (as is generally done at present) at the discretion of the study director. Terrestrial field soil dissipation study findings would be compared with laboratory study and exposure modeling data to identify any discrepancies that require further field, laboratory or modeling effort. Ultimately, the combined laboratory, field and modeling data sets should be used to develop an environmental fate overview that adequately describes the environmental fate of the parent material and degradates and shows that the modeled estimated exposures used for risk assessments are supported by field dissipation data to demonstrate that modeled values are "reasonable".

What is the purpose of Terrestrial Field Soil Dissipation Studies? Before a meaningful discussion of optimal approaches for conducting Terrestrial Field Soil Dissipation (TFSD) studies on pesticides can take place, it is essential to understand their purpose. Unfortunately, this is not as straightforward as might be expected because various stakeholders in the regulatory process view the objectives somewhat differently.

Regulatory Considerations From a traditional US E P A regulator's perspective, multiple objectives exist for the TFSD study which include two primary elements; a) to provide information on the rate at which a parent pesticide dissipates in relevant soils under relevant weather and agronomic conditions and b) to provide information on the simultaneous formation and dissipation of significant soil dégradâtes. In addition, the study has long been regarded as the study whose results will trigger long term "build-up" and decline studies. Indeed, where it is expected that a



45 compound or significant degradate may be persistent, the plots used for the first season of a TFSD may be laid out to serve as the physical location for a ongoing "build-up" study program. Additionally, historically, the TFSD has been considered by some to provide useful whilst limited information on the potential for transport of parent or metabolites down the soil profile. In addition to these traditional perspectives, the recent North American proposal for draft OECD guidance for TFSD studies (1) has the stated objective to: "provide an integrated qualitative & quantitative environ mental fate assessment which characterizes the relative importance of each route of dissipation of the parent compound and major and/or toxicologically significant transformation products" In other words, the new draft guidance apparently substantially increases the scope of TFSD programs by requiring both qualitative and quantitative consideration of all the routes of dissipation. Presentations from some of the North American/OECD guidance document authors have suggested that this may only apply to significant dissipation routes, but this is not made clear in the current draft. Of probably even more significance for the redesign of TFSD studies is an additional stated intention that the new guidance anticipates that TFSD studies will "better address" the issue of mass balance. In other words, there is an expectation that in addition to estimating the rates of parent dissipation and the formation and decline rates of significant dégradâtes, the study should ideally be able to quantitatively account for ultimate disposition of the mass of the originally applied chemical. Clearly the scope of the TFSD as redefined in the recent draft guidance is now very broad. To effectively consider the value of the features added to the revised Terrestrial Field Soil Dissipation study guidance, it is first necessary to take a step backwards to develop a conceptual model of the processes that need to be accounted for. In addition it is important to understand the use made of the data generated in these studies in the pesticide regulatory process.

A Conceptual Model of Terrestrial Field Soil Dissipation Pesticide environmental fate studies have been conducted for forty or more years and, over that period, a considerable body of technology and refined scientific approaches has been amassed. However, for the purposes of this paper, the conceptual model needs to focus on general principles only; accordingly, the author apologizes for the unavoidable over-simplifications. Figure 1 depicts the key processes that can contribute to field dissipation. These include routes of transport away from the plot such as removal at harvest as crop residue, leaching out of the sampling zone, runoff from the plot and volatility, as well as degradative processes such as photolysis on various surfaces or biotic/abiotic degradation. While it is self-evident that for any given



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Figure 1. Conceptual model of key processes involved in terrestrialfieldsoil dissipation ofpesticides

combination of site, crop, pesticide and use pattern, the relative importance of these processes will differ, some general principles can be derived. In particular, given the emphasis of the new guidance on mass balance accounting, it is instructive to consider how readily one might expect to be able to quantify the individual contributions of the dissipation processes to the fate of the chemical. If the likely uncertainties associated with estimating the significance of each process are examined, it may be argued that the processes fall into three categories. 1.

Those where quantification is reasonably certain, provided reasonable care is taken in making field measurements- e.g. runoff from the plot and removal of pesticide and dégradâtes as deposits on the crop commodities and waste at harvest. Microbial and or chemical transformations in the soil to known dégradâtes would also fall in this category.

2.

Those where quantification will, even with reasonable care, be moderately uncertain. This category would include leaching below the sampling zone, volatilization from soil and/or crops and losses via tile drains.

3.

Those where quantification of significance will, at best, be very imprecise. For example, transport losses via wind blown soil,


47 macropores or interflow or, most significantly, losses associated with degradation to unknown products/ bound residues or losses due to mineralization to C 0 . 2

This evaluation leads to the conclusion that completing a "mass balance accounting sheet" is a goal that will generally be unattainable.


Terrestrial Field Soil Dissipation Studies in the Current Regulatory Milieu In the USA presently, data from a battery of laboratory studies (e.g. aerobic and anaerobic soil degradation studies, adsorption/desorption experiments, hydrolysis and photolysis degradation studies) are used to populate accepted and validated surface water and ground water exposure models [currently Pesticide Root Zone Model (PRZM) and PRZM-Exposure Analysis Model System (EXAMS)] based on conservative agronomic scenarios. The output from those models is related to laboratory toxicity information to determine i f further refinement of risk is needed. Frequently, the refinement of risk is best achieved by refining the exposure estimates or by performing monitoring studies to characterize the conservatism of modeled exposure estimates. Thus additional "higher tier" field or laboratory fate studies which help to refine exposure model predictions such as foliar washoff and dissipation, runoff, field leaching (lysimeters or the Prospective Ground Water study) are triggered as a result of using exposure models which use default laboratory data. The important finding is that (with the exception of the relatively insignificant "build-up" studies) the current TFSD study is rarely, i f ever, used to either trigger or rebut the need for any of the expensive higher tier studies.

What does the TFSD Study offer? Given that the TFSD currently has little or no direct bearing on environmental or ecological risk assessment, it is important to understand what it C A N offer the registrant and regulator, given the proposed increase in experimental cost. Table I summarizes the positives and negatives associated with the current study design. It is interesting to note that a major benefit of conducting a TFSD is to obtain field data from more than one site that helps to evaluate the "realism" of the regulatory exposure model predictions based on laboratory derived input parameters. It should be realized that this does not constitute a validation of the model since runoff and leaching are not measured in the current study design; however, it may be equally important to determine, for example, that while the use of the lab inputs predicts dissipation with a half-life of two to three weeks, the field dissipation measured values all have a half-life around 3 days. A

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In Terrestrial Field Dissipation Studies; Arthur, E., et al.; Washington. D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


Parent Dissipation rate(s) Degradate field formation/ decline rate(s) Info on vertical movement when analyte is at concentrations above soil L O D Data to justify triggering a field build-up study Inferences that dissipation processes are understood Field data to check realism of exposure model predictions based on Lab study inputs

Positives

Information specific to site/soil/ weather conditions No indication of mass balance Limited information on leaching of compounds moving in pore water (below soil LOD) Only infrequently impacts decision to request PGW No definitive data on impact of soil texture Possibly inaccurate info on dissipation in presence of crop

Negative/Neutral

Table I. Summary of the positive and negative/neutral aspects associated with the current TFSD study design



49 discrepancy like this should provide a strong indication to the risk assessor that the conventional modeling is failing to account for some key dissipation process. Examples of this might be a compound which has a fast soil photolysis rate or one where anaerobic A N D aerobic processes combine in the surface soil rather than the default P R Z M model which assumes that only aerobic processes are significant in the surface soil. This type of discrepancy has been at the root of the long-running and somewhat acrimonious debate between industry and E P A about whether laboratory or field dissipation data should be used as inputs for exposure models. The EPA Environmental Fate and Effects Division (EFED) guidance states that the laboratory values must be used since many complex co-occurring processes may have contributed to field half-life values. Assuming this continues to be the policy, using TFSD studies to identify discrepancies in modeling output may provide valuable justification for when it is appropriate to repeat modeling using modified input data-sets or model settings to account for additional dissipation processes. The above discussion considers key factors relating to the existing study design; it is instructive to perform the same analysis on the new design as shown in Table II. Undoubtedly, the additional effort involved in the proposed new design generates more data and thus provides a more complete data set for examination; however, there are obvious downsides associated with making the study more complex and expensive. Perhaps the most significant conclusion is that, as explained in the consideration of the conceptual model, despite the extra cost and effort, the enhanced TFSD study design does not necessarily provide improved information accounting for the disposition and the mass balance of the applied chemicals and/or metabolites.

Can Modules Be Effectively Combined Into An AllEncompassing Field Study?? An important statement in the list of issues associated with the new TFSD design is the fact that some of the suggested "modules" in the study are not compatible within the same study design. Table III shows why this is the case. For example, while in current TFSD studies the site is managed specifically to minimize the possibilities for losses via leaching or runoff, studies designed to measure runoff must simulate "reasonable worst-case" scenarios by the use of sloping sites with impermeable soils. In contrast, prospective ground water studies must be conducted using flat sites with highly permeable soils. Therefore, to incorporate both leaching and runoff modules successfully into the new TFSD design, seriously potential site incompatibilities would have to be addressed. However, this table does show that some modules are probably compatible within the same study design. For example, a TFSD study could successfully incorporate regular soil dissipation objectives alongside volatility and foliar washoff and foliar degradation components.


50 Table II. Summary of the positive and negative/neutral aspects associated with the proposed TFSD study design


Positive

Negative/Neutral

Only represents one combination of Provides understanding of interaction of more variables - may? Save money via soil/weather per site multi-tasking Can identity processes ignored in models & Non detections in TFSD may not rebut the quantify their impact presumption of exposure that triggers Prospective Ground Water (PGW) or water monitoring studies. Can help focus future studies and risk Much higher costs - more planning needed assessments In some cases may better address mass Several "modules" incompatible balance May provide a better data-setformodel Complexity adds GLP/technicalriskand validation risks potential distraction by minor issues Runoff depends on natural rain and is unlikely to be optimalforeco-risk Leaching potential not optimal due to soil textures? Site selection to meet multiple goals may make selection of agronomically relevant sites difficult Data still not used for modeling Mass balance mostly still poor

Are All "Modules" Equally Significant to Dissipation Study Design? Table IV examines the potential contribution that the key processes identified in the conceptual diagram are likely to make to overall dissipation of an active ingredient. It is clear that the significance of a process depends on the half life of the active ingredient; for example, for most compounds of low to moderate volatility, since volatilization tends to involve steady losses from day to day and can be weather dependent, the impact of volatile losses will increase where the stability of the compound of interest is greater. The most important conclusion from Table IV is that based on the review by Wauchope (2), runoff of all but the longest half-life compounds (assuming the plots to be used are reasonably flat) is not likely to contribute significantly to dissipation since even under worst-case conditions the amount of compound lost from the plot will be low relative to most other processes. Therefore it would appear that it is rarely, i f ever, necessary to add a "runoff module" to a field dissipation study. It is critical to realize that this is not to say that conducting separate runoff studies may not be vital to the regulatory process; for example, where the potential aquatic toxicity of a compound is significant, even small amounts of compound transport (e.g.

Terrestrial Field Dissipation Studies - American Chemical Society

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