Risk Assessment
Risk assessments are used to evaluate the concentrations of site contaminants relative to human health or ecological exposure criteria. This evaluation is, in turn, used to determine what, if any, remedial action(s) will be necessary to protect human health and the environment.
The USEPA identifies four elements in a risk assessment as follows:
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Identification of constituents of interest;
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Exposure assessment;
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Toxicity assessment; and
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Risk characterization.
Constituents of Interest are identified through screening detected compound concentrations against conservative screening criteria. Compounds that exceed their screening criteria are carried forward into the quantitative risk assessment.
The Exposure Assessment evaluates the potential exposure pathways of receptors to exposure media (e.g. soil, groundwater, surface water, air). An exposure pathway has four elements which must be present for the pathway to be complete. These include:
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A source of contamination
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A transport mechanism to the receptor;
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A point of potential contact; and
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An exposure route (e.g. ingestion, inhalation, dermal contact).
Below is a Site Conceptual Exposure Model from an Exposure Assessment:
Numerical values are then assigned to the various exposure parameters (e.g. soil ingestion, water ingestion, exposure time and duration, exposed surface area, etc.) in order to quantify the total exposure (or Dose).
The Toxicity Assessment assigns toxicity values to the constituents of interest based on interpretation of toxicological studies. The majority of the toxicity values used are published in the Integrated Risk Information System (IRIS) maintained by the USEPA.
Finally, the Risk Characterization combines the assigned values from the Exposure Assessment and the Toxicity Assessment to calculate the potential for adverse effects to the receptor.
As the risk assessment is developed, the risk assessor strives to minimize the amount of Uncertainty for each parameter. However, uncertainty is inherent in the risk assessment process and the uncertainties are identified and their potential to impact the overall predicted risks is evaluated and documented.
Many risk assessments are performed assuming "worst case" exposure conditions to err on the side of caution. However, assigning "worst case" conditions to each parameter results in an unrealistically conservative assessment, which unnecessarily drives up the cost of remediation. Furthermore, risk assessments do not consider that real, and potentially much higher risks are inherent in remedial construction. I have many years of successful experience conveying these issues to regulators and eliminating unnecessary conservatism in risk assessments. I accomplish this by actually observing and evaluating the potential site exposures based on the actual site use patterns. Elimination of unrealistic exposure assumptions still leaves adequate conservatism in the risk assessment due to the nature of toxicity value development as well as conservatism in assumptions regarding dose calculations and contamination absorption in the body.