The first step of the site assessment process is known as a preliminary assessment (PA). This assessment gathers historical and other readily available information on site conditions and surroundings to evaluate whether the site poses a potential threat to human health and the environment and/or whether further investigation is needed. Environmental samples are rarely collected during a PA.

The site investigation (SI) builds on the information gathered in the PA. EPA conducts a site investigation to determine whether the potential threat or threats identified in the PA actually exist. The SI includes the collection of environmental samples from areas identified in the preliminary assessment that have potential to contain hazardous substances or from areas where hazardous substances are present. The SI also determines whether hazardous substances are being released to the environment and are a threat to human health. Results from this sampling help to determine whether a site warrants short-term or long-term cleanup, or if a site requires no further action.