The Phase I Environmental Site Assessment looks at the historical records for a property to determine if the past use of the site might have had reason to use or store chemicals or products that could contaminate the environment or pose a risk to human health. This may involve talking to surrounding residences or businesses to get a better idea of what was on the property and how it was used in the past. Phase I assessments, typically, can be prepared quickly.
Phase II Environmental Site Assessments work off of the findings of Phase I, and involve soil or water sampling to determine if the suspected contaminant is present, and if so, in what amounts. The EPA works with scientists and medical professionals to set guidelines for how much of a contaminant is safe, and if a property is found to have more than acceptable amounts, the property requires clean-up to get it back within safe levels.
How Much is Acceptable
You may wonder why any amount of contaminant is considered acceptable. The answer is, it's only when the amounts of these contaminants get large and concentrated enough that they cause any harm. The limits set by the EPA are often very small, with concentrations measured in the parts per million, which is about four drops of dye in a 55-gallon drum of water. Some contaminants break down relatively quickly, and some stick around for a very long time. Phase II assessments provide a snapshot of a property and how the contamination levels look at 1 point in time. If contamination is found, the cleanup method selected will be tailored to the specific contaminant present.
All aspects of Brownfields assessment and cleanup require the oversight of an environmental professional (PDF), typically a professional geologist or engineer.