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Modifying EPA Criteria Development for Intermittent Exposure Conditions: Atrazine Case Study

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Atrazine is one of the most widely used agrochemicals in the country - an herbicide used in the production of corn and sorghum. Consequently, trace levels of this substance are found throughout the Midwest and in other areas across the country as a result of wet weather induced runoff. Not unexpectedly, the exposure magnitude and duration are controlled by the type of system (riverine versus loctic), the seasonal application, and runoff rates. River concentrations may exceed 20 ug/l for short periods (days or weeks) and are essentially zero in the non-growing season. Lake and pond exposures are typically much lower than stream exposures, but are of a greater duration. These different scenarios raise very different environmental concerns.

EPA has been in the process of developing Section 304 (a) criteria to ensure that the adverse impacts of this beneficial agrochemical are avoided. This is EPA's first attempt to develop criteria for an herbicide, which presents a number of new development and implementation issues. Plants, not animals, are expected to have the most sensitive response to this parameter.

EPA's traditional approach to criteria development is described in the Guidelines for Deriving Numerical National Water Quality Criteria for the Protection of Aquatic Organisms and Their Uses (USEPA, January 1985) and were based primarily upon EPA's experience in assessing adverse impacts on animals, not plants. Moreover, it is apparent that the underlying assumptions used to develop federal criteria do not apply well to pollutants that only enter the environment through wet weather induced flows. A number of key assumptions, including the likelihood of continuous exposure at levels inducing adverse effects and the need for an extensive recovery period from elevated exposures, do not fit the pattern of exposure or the recovery period for atrazine.

Unlike many other agrochemicals, e.g., pesticides, atrazine poses virtually no threat of acute toxicity or impacts due to bioaccumulation/bioconcentration at levels commonly encountered in the environment. Atrazine is also not demonstrated to cause chronic toxicity to sensitive fishes or invertebrates at expected instream concentrations or durations of exposure. Chronic impacts on fish growth have been documented to be temporary, with a rapid recovery once elevated exposures are no longer present. This chemical may have impacts on plant growth at levels lower than those that could adversely affect aquatic life; however, such impacts are temporary and are highly dependent on the magnitude and duration of exposure. Thus, proper regulation of atrazine will require a refined consideration of the magnitude/duration of exposure that must exist for the pollutant to cause adverse effects on the environment.

Contrary to the available information, EPA's proposed atrazine criteria document establishes restrictive chronic criteria that (1) appear inconsistent with procedures for calculating chronic criteria; (2) rely upon test results that do not reasonably reflect expected acute or chronic impact levels; and (perhaps most importantly) (3) do not reflect the type of environmental exposures, anticipated impacts, and recovery times that are well-documented to be associated with the constituent that justify a less restrictive and more flexible criteria. The continuous exposure assumption that led directly to the derivation of stringent chronic criteria is not applicable to the vast majority of circumstances where atrazine exposures occur. Because the criteria derivation did not reflect the anticipated environmental exposure scenarios or recovery times, application of the proposed criteria will lead to the imposition of limitations significantly more restrictive than justified by the underlying technical information. Assuming that certain exposure conditions will exist when EPA's own analysis confirms that the assumptions do not reasonably apply in the real world will result in the agency's decision being vacated. Chemical Mfrs. Ass'n v. EPA, 28 F.3d 1259 (D.C. Cir. 1994); see also, American Iron & Steel Inst. v. EPA, 115 F.3d 979, 1005 (D.C. Cir. 1997).

This paper looks at the issued raised by EPA's draft atrazine criteria and in particular how it is necessary to modify the traditional criteria development approach to fit the type of exposures with which this pollutant will be associated. This paper also raises questions regarding organism recovery which is an important assumption underlying the development of most criteria.
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Document Type: Research Article

Publication date: 2002-01-01

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