The susceptibility determination is designed to evaluate potential sources of contamination within a public water system’s delineated assessment area. Once potential sources have been located, site conditions and "chemicals of concern" associated with each potential source will be identified. Chemicals of concern are those chemicals expected to be associated with the activities of the potential source. For example, the chemical benzene is associated with retail fuel facilities. Therefore, benzene would be considered the chemical of concern for the underground storage tanks at a retail gasoline station.

The susceptibility determination assumes that any contaminant released to the ground surface has the potential to enter a public water supply system. Susceptibility is determined based on the following factors:

• Health effects (toxicity and potential cancer risk) of a chemical of concern,
• Leaching potential (mobility) of a chemical of concern,
• Protection provided by the underlying geologic materials, and
• Operating practices and design of the potential source(s) of contamination.

Each of these factors will be assigned a value and evaluated. The potential contaminant source will then be given a score and assessed as posing a low, moderate, or high concern to the source water, based upon the score outcome. This provides information that can be used to quickly screen the potential sources of contamination to identify those that may pose a threat to drinking water sources.