There are a number of different types of risk assessment that may be completed to inform remedial decisions; however, the risk assessment approach will depend primarily on the project’s regulatory framework.
Screening Level Risk Assessment
Whenever chemical concentration data is compared to numeric, risk-based screening levels, an informal assessment of risk takes place. A screening level risk assessment can be performed by calculating the ratio of the chemical concentration to the screening level and converting the result to a risk probability or hazard quotient. This type of risk assessment is the least complex and is used to quickly differentiate sites in need of regulatory attention from sites with negligible public health risks. Screening levels are calculated using generic, conservative exposure assumptions that may not be appropriate for every site; thus, the reduced complexity could result in unnecessary site cleanup.
Site-Specific Risk Assessment
Many state cleanup programs and the RCRA Corrective Action Program allow site-specific risk assessment. A site-specific risk assessment offers the flexibility to estimate the health effects associated with exposure consistent with current conditions or anticipated future use. A site-specific risk assessment begins with a conceptual site model (CSM) that identifies chemical sources, transport mechanisms, exposure media, human receptors and their potential exposure pathways (e.g., ingestion). The site-specific risk assessment can often incorporate existing or proposed institutional and engineering controls, and can be used to evaluate the health-protectiveness of a proposed remedy. When a site-specific risk assessment is conducted, the regulatory cancer risk and non-cancer hazard thresholds become the metric for determining if remedial action is warranted.
Baseline Risk Assessment
A baseline risk assessment evaluates risk in the absence of remediation and institutional controls, and is required to support development of Superfund remedial alternatives. EPA will use the results of a baseline risk assessment to determine if a site presents an “imminent or substantial” endangerment and make no-action decisions. To provide consistency across sites in the Superfund program, undeveloped areas are typically evaluated under a conservative, future residential land use assumption.
Each of the risk assessments described above are “forward” risk assessments that use chemical data to estimate potential health effects. It is also possible to conduct site-specific and baseline risk assessments in the “backward” direction to develop health-protective, numeric remediation goals.
About Emily Strake
Emily has nearly 20 years of experience in chemistry and human health risk assessment. Her experience includes assessing the potential adverse health effects to humans from exposure to contaminants in soil, sediment, groundwater, surface water, ambient and indoor air, and various types of animal, fish, and plant materials. She has been the primary author of risk assessment reports and screening evaluations for projects governed under USEPA RCRA and CERCLA, and state programs in Pennsylvania, California, Delaware, Connecticut, Oregon, and Maryland.