Probability of Unveiling
The Hazard Ranking System (HRS) is a systematic assessment developed by the Environmental Protection Agency (EPA) to evaluate the potential threat of contamination to human health and the environment from hazardous substances. This process is crucial in identifying sites that require immediate attention and cleanup.
Preliminary Assessment/Site Inspection (PA/SI)
The HRS process begins with a preliminary assessment or site inspection (PA/SI). Upon notification of a potentially hazardous site, the EPA conducts a thorough investigation, reviewing records, conducting interviews, and performing visual inspections and limited field sampling. The aim is to identify if hazardous substances have been released into groundwater, surface water, or air [1].
Data Collection and Evaluation
The HRS uses the information collected during the PA/SI to assess whether a release or threatened release has been observed. For groundwater and surface water pathways, this can include chemical sampling results showing contaminants above background or regulatory concern levels. For air releases, it involves air monitoring or emission data that establish elevated concentrations of hazardous substances or vapors [1][3].
Pathway Scoring and Ranking
The HRS applies mathematical formulas to the collected data to score the site’s contamination relative to human and ecological receptors. This includes considering the likelihood of release, the presence and concentrations of hazardous substances, and transport mechanisms in groundwater, surface water, and air pathways [1][3].
Special Considerations for Air Pathways
In 2017, the EPA incorporated the subsurface intrusion pathway (e.g., vapor intrusion) into the HRS scoring. This evaluates whether vapor-forming chemicals migrate from contaminated subsurface media into overlying buildings, posing acute or long-term health risks [3].
Regulatory Confirmations and Reporting
Releases are also subject to regulatory definitions and procedures under laws such as the Resource Conservation and Recovery Act (RCRA) and Underground Storage Tank (UST) regulations. These regulations confirm and report release events, including investigations and corrective actions for sites impacting soil, groundwater, or air [5].
Use of Emission Factors for Air Releases
For air pathway evaluations, emissions are often estimated using EPA emission factor databases (e.g., AP-42 or WebFIRE) or direct measurement to quantify hazardous substances released into the atmosphere [4].
In summary, establishing an observed release under the HRS involves a combination of site inspection, environmental sampling, data analysis, and pathway-specific evaluation according to the manual’s criteria to identify and quantify hazardous substance releases into groundwater, surface water, and air. This screening process is fundamental for scoring a site’s hazard level and potential listing on the National Priorities List (NPL) for cleanup [1][3].
The HRS defines an observed release as any release sample above the detection limit if the background is non-detect, or if the substance is detected in the background sample, the release sample concentration must be at least three times greater than the background level. An observed release can be documented for the source at the site if there is recorded information documenting that a substance attributable to the site has been entering the media of concern (Direct Observation), or if the detection in the media of one or more hazardous substances attributable to the site with concentrations significantly above background levels is found.
If an observed release cannot be documented, potential to release will always have a lower score, and actually contaminated (Level I or II) targets cannot be evaluated without an observed release. The procedures for determining potential to release are pathway-specific. The maximum potential to release value is 500 in all migration pathways.
The HRS Guidance Manual provides definitions and guidelines for key terms such as Observed Release, Attribution, and others. In the absence of an observed release, potential to release can be evaluated in the three migration pathways: Ground Water, Air, and Surface Water.
References:
[1] EPA.gov: Hazard Ranking System [2] EPA.gov: Hazardous Substance Release Detection [3] EPA.gov: Vapor Intrusion [4] EPA.gov: Emission Factors [5] EPA.gov: Resource Conservation and Recovery Act [6] EPA.gov: Underground Storage Tanks
- The preliminary assessment or site inspection (PA/SI) by the Environmental Protection Agency (EPA) might reveal that hazardous substances have seeped into the soil, affecting groundwater, surface water, or even permeating the air.
- The HRS uses data collected during the site inspection to determine whether a release or potential release of hazardous substances has occurred, accounting for chemicals in groundwater, surface water, or air that exceed background or regulatory concern levels.
- In the evaluation of air pathways, the EPA might estimate emissions using databases such as AP-42 or WebFIRE, or conduct direct measurements to quantify the amount of hazardous substances released into the atmosphere.
- The HRS Guidance Manual outlines definitions and guidelines for key terms like 'Observed Release' and 'Attribution,' emphasizing the importance of an observed release in determining the actual contamination levels in a site.
- The scientific field of environmental-science plays a significant role in the Hazard Ranking System (HRS), as it provides the foundation for understanding the potential impact of hazardous substances on the environment, human health, and even water and air resources, thereby informing policies and financing for cleanup operations.
