Comparative Risk Assessment: Tools for Remedial Action Planning

Abstract

The US Environmental Protection Agency (USEPA) has chosen over 500 hazardous waste disposal sites for the National Priorities List (NPL) under the Comprehensive Environmental Response, Compensation and Liability Act. The priority sites were chosen utilizing the Hazard Response System (HRS) model that was designed to numerically score the relative health and environmental risks at uncontrolled hazardous waste sites. Although the HRS model assigns three scores to a hazardous waste facility (migration, fire and explosion, and direct contact), only the migration hazard mode score was used as a criterion for selection of sites for the NPL. The migration hazard mode score is a composite of separate scores for the potential migration of contaminants by routes involving groundwater, surface water, and air. This paper discusses the details of the HRS, and by way of example some of the methodological limitations and inconsistencies in the model that limit its usefulness. For example, the HRS does not consider hydraulic gradient in determining the groundwater route score. In addition, unlike the groundwater and surface water route scores, the air migration route score does not reflect the potential for release of contaminants to the air if direct evidence of release is lacking. Once sites are targeted for cleanup, decisions on the extent and method of cleanup must be made. USEPA has not yet developed a model similar to the HRS to aid in determining the extent of remediation necessary to protect public health once sites have been chosen for priority attention. The elements that must be considered in the development of such a model are discussed.