The EPA method of risk assessment uses long term or chronic exposure as a basis for determining the excess cancer risk at a Superfund site. Oftentimes, the risk from exposure to contaminated groundwater is inappropriately calculated from the single highest confirmed concentration found in a groundwater well. This approach is mathematically and conceptually indefensible since a single measurement cannot represent the contamination in an entire plume at a Superfund site. Instead, a sufficient database is required to effectively represent site risk during a lifetime of exposure.

The larger database serves to reduce the uncertainty inherent in risk analysis, and the Remedial Project Manager is provided with a more scientifically sound risk evaluation on which to trigger a remedial decision. While this approach applies to most Superfund sites, factors such as calculation method, well placement and use of the historical database attain particular importance at sites where groundwater contamination is not clearly established.

This guidance is intended to improve the quality and consistency of deriving exposure point concentrations in groundwater in risk assessments performed in Region III. (EPA/903/8-91/002)

Related Information:

OSWER Directive 9283.1-42, February 2014: Memorandum for Determining Groundwater Exposure Point Concentrations, Supplemental Guidance

• Exposure Point Concentrations In Groundwater (PDF)(5 pp, 32 K, 1991-11-13, EPA/903/8-91/002)
  This guidance is intended to improve the quality and consistency of deriving exposure point concentrations in groundwater in risk assessments performed in Region III.