Under the Comprehensive Environmental Responsibility, Cleanup and Liability Act (Superfund), cleanups must follow two kinds of requirements:
applicable requirements meaning those that directly apply to the situation
relevant or appropriate requirements meaning those that apply to contaminants that are present at the site or apply to a contaminated medium, such as water, at the site
For example, the standards for cleaning up uranium and thorium processing facility sites are frequently considered "relevant and appropriate" for radiologically contaminated sites that did not conduct such processing.
ARARs can be federal, state, or local requirements.