((a)) ** Threshold determinations** An agency is not required to prepare an environmental document with respect to a proposed agency action if—
((1)) the proposed agency action is not a final agency action within the meaning of such term in chapter 5 of title 5;
((2)) the proposed agency action is excluded pursuant to one of the agency’s categorical exclusions, another agency’s categorical exclusions consistent with , or another provision of law;
((3)) the preparation of such document would clearly and fundamentally conflict with the requirements of another provision of law; or
((4)) the proposed agency action is a nondiscretionary action with respect to which such agency does not have authority to take environmental factors into consideration in determining whether to take the proposed action.
((b)) ** Levels of review**
((1)) ** Environmental impact statement** An agency shall issue an environmental impact statement with respect to a proposed agency action requiring an environmental document that has a reasonably foreseeable significant effect on the quality of the human environment.
((2)) ** Environmental assessment** An agency shall prepare an environmental assessment with respect to a proposed agency action that does not have a reasonably foreseeable significant effect on the quality of the human environment, or if the significance of such effect is unknown, unless the agency finds that the proposed agency action is excluded pursuant to one of the agency’s categorical exclusions, another agency’s categorical exclusions consistent with , or another provision of law. Such environmental assessment shall be a concise public document prepared by a Federal agency to set forth the basis of such agency’s finding of no significant impact or determination that an environmental impact statement is necessary.section 4336c of this title
((3)) ** Sources of information** In making a determination under this subsection, an agency—
((A)) may make use of any reliable data source; and
((B)) is not required to undertake new scientific or technical research unless the new scientific or technical research is essential to a reasoned choice among alternatives, and the overall costs and time frame of obtaining it are not unreasonable.