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A Discussion with the FIFRA Scientific Advisory Panel Regarding the Terrestrial and Aquatic Level II Refined Risk Assessment Models (Version 2.0)

Executive Summary

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  1. Background

    In May 29 - 31, 1996, the Agency presented two ecological risk assessment case studies to the Scientific Advisory Panel (SAP) for review and comment. While recognizing and generally reaffirming the utility of EPA's current deterministic assessment process, the SAP offered a number of suggestions for improvement. Foremost among their suggestions was a recommendation to move beyond the present single point assessment process by developing the tools and methodologies necessary to conduct a probabilistic assessment of effects. Such an assessment would estimate the magnitude and probability of the expected impact and define the level of certainty and variation involved in the estimate.

    The recommendations of the SAP were consistent with issues that risk managers within EPA's Office of Pesticide Programs (OPP) raised in the past, namely questions about the magnitude of the risk described for a particular pesticide, the probability of the risk occurring, and the certainty of the evaluation.

    Following the recommendations of the SAP and in response to issues raised by OPP risk managers, the Agency began an initiative to refine the ecological risk assessment process for pesticides. The refined process focuses on evaluating the effects of pesticides to terrestrial and aquatic species within the context of FIFRA (Federal Insecticide, Fungicide, and Rodenticide Act), the main statutory authority for regulating pesticides at the federal level.

    The key goals and objectives of EPA's initiative include the following:

    • Develop a conceptual approach to refine the ecological assessment process;

    • Incorporate probabilistic tools and methods to provide an estimate on the magnitude and probability of effects;

    • Address the broad spectrum of responses to pesticide exposure;

    • Establish more realistic actual use scenarios and field conditions;

    • Build upon existing data requirements for registration;

    • Utilize, wherever possible, existing data bases and create new ones from existing data sources to minimize the need to generate additional data; and

    • Focus additional data requirements on reducing uncertainty in key areas.

    The initiative began with the formation of The Ecological Committee on FIFRA Risk Assessment Methods (ECOFRAM), a stakeholder workgroup, who was tasked with identifying and developing probabilistic tools and methods for terrestrial and aquatic assessments under the FIFRA regulatory framework. The conclusions and recommendations of the ECOFRAM were summarized in the draft reports, which were peer reviewed during two public workshops.

    Once the reports and the peer review workshops were completed, the Agency formed the Refined Risk Assessment Implementation Team (Implementation Team), who was charged with developing a plan to incorporate probabilistic tools and methods into the assessment process. After evaluating the ECOFRAM reports and workshop comments, the Implementation Team developed a conceptual approach for implementing changes to the current deterministic assessment process, using the reports and workshop comments as a starting point. This approach, which was evaluated and endorsed by the SAP in 2000, is based on a four-level risk assessment scheme.

    After proposing the four-level risk assessment scheme, the Implementation Team focused on developing pilot models (Version 1.0) and on conducting a case study. The refined aquatic risk assessment followed the approach outlined in the Agency's four-level risk assessment scheme for Level II. The refined terrestrial assessment went beyond the Level II assessment and fell somewhere between a Level II and III. The case study was presented to the SAP in 2001 as a generic chemical (Chem X).

    Following the case study with ChemX, the Agency refined the models based on the SAP comments made in 2001. In addition, the terrestrial Level II model was also refined to include dermal and inhalation exposure based on recommendations from ECOFRAM. These routes of exposure were also discussed during the 2000 SAP meeting. The Level II models (Version 2.0) are the subject of this SAP session.

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  2. Terrestrial Level II Model (Version 2.0)

    1. Overview

      The terrestrial Level II model (Version 2.0) is a multimedia exposure/effects model which evaluates acute mortality levels in generic or specific species over a user-defined exposure window. The spatial scale is at the field level, which includes the field and surrounding area. Both areas are assumed to meet the habitat requirements for each species, and contamination of edge or adjacent habitat from drift is assumed to be zero. It is anticipated that future modifications to the assessment model will address offsite transport of pesticide residues via drift.

      The major parameters addressed in the model are:

      • Multimedia ( vegetation, water, and air) estimates for oral, inhalation, and dermal routes of exposure;

      • Food habits of defined generic species or selected specific species that are proportioned for each food type consumed by the species;

      • Hourly ingestion/inhalation rates of food, water and air as a function of body weight, randomly assigned from species specific body weight distributions or defined generic species body weight distributions;

      • Hourly dermal residue transfer rates from contaminated vegetation as a function of body weight, randomly assigned from species (generic or specific) body weight distribution and frequency in contaminated area;

      • Frequency of feeding and drinking on the sprayed field, determined in hour time steps;

      • Distribution of residues on/in vegetation, water ( dew and puddles), and air as a function of application rates;

      • Degradation/dissipation rates of pesticides residues in each environmental media considered, i.e., food, vegetation, air and drinking water; and

      • Acute toxicity dose-response relationship based on a specific species (when data are available) or inter-species extrapolations from distributions fit to available effects data.

      For each individual bird considered in a run of the Level II model, a random selection of values is made for the major exposure input parameters to estimate an external (ingested) oral dose equivalent for that individual that accounts for body elimination rates in hourly time steps. The estimated external oral dose equivalent is compared to a randomly assigned tolerance for the individual preselected from the log probit dose/response distribution.

      The status of this individual (dead or not dead) for this time step is assigned by comparing the estimated oral dose equivalent to the random assigned tolerance. If the dose is greater than the tolerance, the individual is scored "dead". If the dose is less than or equal to the tolerance, the individual is scored "not dead". If scored "not dead", the loop is continued until the dose is greater than the tolerance or the user- defined model duration is reached. This procedure is repeated using Monte Carlo sampling. After multiple iterations of individuals, a probability density function of percent mortality is generated.

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    2. Major Modifications to the Level II Model (Version 1.0)

      The Level II model was modified based on changes suggested by the SAP in 2000 and 2001. The major changes suggested by the SAP in 2001 that have been incorporated are:

      1. Establishment of generic birds that represent species that occur in and around agro-environments. The model can use generic attributes to represent the more vulnerable species yet retains the ability to address specific focal species, when appropriate,

      2. Incorporation of a 1-hour exposure time step to allow the inclusion of a bimodal feeding pattern, as well as a higher resolution simulation of daily feeding behavior between treated and untreated areas,

      3. Incorporation of an algorithm (Markov Chain) to address serial correlation between sequential foraging events, and

      4. Development of a new model for estimating pesticide residues in on- field drinking water sources (puddles) that accounts for a number of parameters that affect puddling after a rainfall event. These parameters include rainfall amount, rainfall duration, soil infiltration rates, evaporation, degradation and the stochastic nature of field topography and its relation to puddle formation and duration.

      In addition, an inhalation and a dermal model were developed. Limited data are available regarding these routes of exposure, resulting in uncertainty in the estimates of risks. However, if these routes of exposure are ignored or assumed to be minimal, the uncertainty in risk estimates is not addressed. This is of concern because dermal and inhalation routes may contribute significantly to total dose in some situations. The incorporation of the dermal and inhalation exposure models provide an important initial step to evaluate the potential significance of these routes of exposure to the overall risk estimates.

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  3. Aquatic Level II Model (Version 2.0)

    The aquatic Level II model is a two-dimensional Monte Carlo risk model consisting of three main components: exposure, effects, and risk. The Version 1.0 model was presented to the SAP in 2001 and has been refined (Version 2.0) to include a varying volume surface water fate model (VVWM) to replace the previous fixed-volume surface water fate model.

    The exposure scenarios used at Level II are intended to provide estimates of vulnerable, edge-of-field situations across a wide range of geographical conditions under which the product is used. These surface waters are considered vulnerable because of their size and spatial relationship to the treated field. In addition, the soil series selected for use in crop scenarios are considered, based on best professional judgement, to be more vulnerable to soil erosion in the use area.

    The exposure component of the model simulates field-scale pesticide application, daily field run-off and erosion, and fate to a surrogate surface water. Version 1.0 of the aquatic model uses the Pesticide Root Zone Model (PRZM) to simulate temporal run-off and erosion of a pesticide from an agricultural field, and Exposure Analysis Modeling System (EXAMS) to simulate the fate of the pesticide in this run-off in a fixed volume of surface water. In Version 2.0, EPA is proposing to replace EXAMS with a new surface water fate model, VVWM, which simulates the fate and temporal hydrology in a receiving water using stochastic temporal variations in weather. For both versions, the simulated daily surface water concentrations provide typical 36-year maximum annual peaks for acute and chronic exposure. After calculating these peak values, the maximum annual peak data are fit to a mixed empirical distribution function.

    Thus, these surface waters would be expected to be among the highest exposure concentrations observed. However, this scenario is not unrealistic or overly conservative because these surface waters are expected to be present across the country. The challenge is to develop an aquatic exposure scenario that produces estimated environmental concentrations (EECs) that are not worst case, do not often underestimate pesticide concentrations, and present a realistic and scientifically defensible representation for vulnerable, edge-of-field surface waters across a wide range of geographical conditions.

    The Level II acute effects or toxicity component uses current acute toxicity tests for freshwater and saltwater fish and invertebrates required for a Level I risk assessment and includes slopes of the concentration-response curves. The Level I uses a single point estimate based on the most sensitive species tested. In contrast, the Level II uses a species sensitivity distribution constructed for each group and assumed to represent the range of responses likely to be encountered. The 5th, 50th, and 95th percentiles of the distribution, representing the generic 5th, 50th, and 95th most sensitive species LC50 (EC50) values, are identified.

    The Level II chronic effects assessment includes current early-life stage and sensitive partial and full life-cycle tests. Measurement endpoints are no observable adverse effects concentrations for survival, reproduction, and growth effects for invertebrates and embryo and larval/juvenile survival and larval/juvenile growth effects in fish. If a test contains more than one measurement endpoint (e.g., survival and growth), the lowest value for a given test is used. If there are sufficient chronic effects data, a species sensitivity distribution is constructed, and the 5th, 50th, and 95th percentiles are determined. However, in most cases, chronic data are limited, and the risk analyses are restricted to individual species.

    The Level II risk evaluation process yields estimates of likelihood and magnitude of effects for acute exposures. For the estimate of acute risks, a distribution of estimated exposure and a distribution of lethal effects are combined through a two-dimensional Monte Carlo analysis to obtain a distribution of joint probability functions. For the estimate of chronic risks, a distribution of exposure concentrations is compared to a chronic measurement endpoint. The risk analysis for chronic effects provides information only on the probability that the chronic endpoint assessed will be exceeded, not on the magnitude of the chronic effect expected. The assessment then proceeds to outline the consequences of the potential effects.

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