The Indiana legislature’s passing of House Enrolled Act 1162 in April 2009 and IDEM’s adoption of the Remediation Closure Guide (RCG) in March 2012 put Indiana back on track for using the risk-based approaches for site closures that were more common before the previous Risk Integrated System of Closure (RISC) guidance took effect in early 2001. These changes restored our flexibility to use site-specific information to evaluate potential health risks associated with exposure to site contaminants.
Exposure risks can be evaluated and managed using several approaches, either singly or in combination. In order of increasing complexity, these are:
- Comparing contaminant concentrations to pathway-specific Screening Levels (SLs),
- Using site-specific qualitative and quantitative “lines of evidence”,
- Developing risk-management strategies (institutional or engineering controls) to reduce or eliminate exposures, and
- Comparing contaminant concentrations to calculated site- and pathway-specific screening levels or remediation objectives developed through health-based risk assessments and standard risk equations.
This article focuses on the last option – health-based risk assessments, which to the uninitiated may seem like a cryptic practice. Methods for assessing potential health risks from contaminated sites took root in the 1980s with the EPA’s Superfund program and have evolved into practical tools for guiding environmental remediation and risk-management decisions.
Experience with environmental remediations has shown that restoring contaminated sites to their pre-development conditions is often not possible, too costly, or even necessary in many cases. Risk assessments allow us to determine how much, or whether, remediation is necessary so that remediation costs can be balanced with acceptable reductions in exposure risk. Risk assessments:
- Consider site-specific physical, chemical, and exposure conditions,
- Require little information beyond that collected during a thorough site characterization,
- May establish defensible, site-specific cleanup goals,
- Allow the scope and cost of cleanups to be defined in advance of actual remediation, and
- Often reduce cleanup costs by limiting remediation to higher-risk contaminants and/or site areas or by supporting the adoption of institutional and engineering controls to manage exposure risk rather than eliminating it.
What Are Risk Assessments?
Risk assessments help us evaluate exposure to contaminants by answering these questions:
- Who may be exposed to the contaminants, now and in the future?
- How, how often, and for how long could the exposure occur?
- Which contaminants pose health risks?
- Do those contaminants occur at potentially harmful levels?
- How can the exposure risk be reduced or eliminated?
- What contaminant levels can safely remain at the site?
These questions are posed in the approximate order of their consideration during the risk assessment process.
The Risk Assessment Process.
Several conditions must occur simultaneously for risk from exposure to a contaminant to exist:
- The contaminant must be toxic or carcinogenic to receptors,
- The contaminant must occur in the environment at a level that is toxic or carcinogenic to receptors (human and non-human populations most likely to be exposed),
- A pathway must exist through which exposure to the contaminant is reasonably possible, and
- The frequency and duration of the exposure must exceed certain thresholds.
Requiring these conditions to exist at the same time makes risk assessment an inherently conservative process. Many contaminants are not toxic or carcinogenic at any level, and contaminants that occur at potentially harmful levels may not pose a health risk if they occur below certain concentrations or if exposure to them is unlikely or brief.
As suggested by the above questions and conditions, risk assessments are site- and contaminant-specific and proceed through a sequence of steps: Data collection and evaluation, exposure assessment, toxicity assessment, and risk characterization.
Data collection and evaluation occur throughout the investigation of a site as its physical characteristics are defined and the types, sources, transport mechanisms, and distributions of contaminants in soil, groundwater, surface water, and air or vapor are characterized. These are the basic elements of a Conceptual Site Model, or CSM (see Wilcox Blog “IDEM Strengthens Its Emphasis on Conceptual Site Models for Environmental Projects”, November 29, 2011).
The exposure assessment is the final part of CSM development when potential exposure pathways, potential receptors, scenarios through which exposure could occur, and exposure routes are identified. Common exposure scenarios are residential, commercial/industrial, recreational, and soil excavation. Each scenario is characterized by assumptions about the pathways, routes, frequency, and duration of exposure that are captured in standard risk equations developed for each scenario by the US EPA. The example equation below is for calculating non-carcinogenic Health Protective (screening) Levels (HPLs) for direct contact with soil through ingestion in a residential exposure scenario.
Exposure “pathways” and “routes” are not synonymous terms, and they are often erroneously used interchangeably. Pathways are environmental media or mechanisms, such as groundwater, soil, vapor intrusion, and soil excavation that bring contaminants into contact with receptors. Exposure routes are potential entry points of contaminants into the body such as ingestion, inhalation, and dermal contact.
Other sometimes confusing terms are “complete” and “incomplete” with respect to assessing exposure pathways. In the traditional risk-assessment sense, any contact with a contaminant results in a completed exposure pathway. Whether that exposure could result in a health risk depends on whether the contaminant is capable of producing harmful effects at the concentration, frequency, and duration at which the exposure occurs. It’s common to have completed pathways that don’t result in health risks. This aspect of risk assessment is the toxicity assessment, which is discussed below.
A thorough exposure assessment considers all potential on- and off-site exposure pathways and routes, though some may be eliminated during the assessment as inapplicable or complete but of low risk based on other lines of evidence. Other descriptors can be used to identify pathways for which risks will be managed using institutional controls or for which no remediation is necessary even if exposure would occur.
Toxicity assessments are performed through epidemiological, clinical, and animal research to identify the types and severity of health effects associated with exposure to specific chemicals. Consultants use the results of toxicity assessments in the risk-characterization step described below but normally don’t perform them. Within a toxicity assessment, a dose-response (DR) evaluation explores how the severity of adverse health effects varies with the concentration of a contaminant and the duration and frequency of exposure to it, ultimately identifying an exposure level (dose) below which adverse health effects are unlikely. DR evaluations may also distinguish between adults and children, who are particularly sensitive receptors and potentially prone to adverse health effects at lower doses, frequencies, and/or durations than adults. The DR evaluation may also differentiate between long-term (chronic) effects from exposure to low levels of contaminants over long periods and short-term (acute) effects from brief exposures to high levels of contaminants.
Risk characterization brings together information from the exposure and toxicity assessments to produce quantitative estimates of the risk of adverse health effects in populations exposed to the contaminants. The potential risk for each contaminant associated with each completed pathway is calculated using standard equations (see attached example). The individual compound-specific risk estimates are then summed to yield a quantitative estimate of total, cumulative risk for each pathway. The cumulative risks for toxic and carcinogenic compounds are compared with levels of acceptable risk established by the US EPA or state guidelines.
For non-carcinogenic contaminants, toxic effects are expressed as a Hazard Quotient (HQ) for each contaminant. The HQ compares the expected exposure level for the contaminant to a reference exposure level (dose) that is not expected to cause an adverse health effect. The HQs for all contaminants associated with an exposure pathway are summed to yield the Hazard Index (HI) for that pathway. The US EPA and IDEM consider individual HQs and cumulative HIs of less than 1.0 to be associated with low risk of non-carcinogenic, toxic effects.
For carcinogenic contaminants, calculated total excess lifetime cancer risks within the range of 10-4 to 10-6 are considered acceptable by the US EPA and IDEM. This range means that one additional incidence of cancer over the background cancer rate per 10,000 to 1,000,000 exposed individuals is considered acceptable. The screening levels for carcinogenic compounds in the IDEM Remediation Closure Guide are based on an intermediate excess lifetime cancer risk of 10-5, i.e., risk levels that would theoretically result in one additional incidence of cancer over the background cancer rate per 100,000 exposed individuals. While IDEM is supposedly open to a 10-4 risk level, the RCG offers no clear guidance for justifying this higher risk level.
If the toxicity assessment and risk characterization show that site contaminants are either not harmful or not present at harmful levels, the risk assessment may be halted. Often, a risk assessment will eliminate most site contaminants from further consideration while some higher-risk contaminants are retained, possibly for remediation or some other form of risk management. For example, ethylbenzene and xylenes may be eliminated as sources of risk at a petroleum site, but benzene, a known human carcinogen, may remain as a contaminant of concern.
Risk Assessments Lead to Site-Specific Risk-Management Decisions. Important uses of risk assessments are to determine if remediation is necessary and, if so, to establish cleanup goals for soil, groundwater, and vapor intrusion. If the risk assessment determines that the potential risk is already less than the maximum acceptable level (e.g., HQ/HI < 1.0 or carcinogenic risk < 10-5), the site or portions of it may not require remediation. If the potential risk exceeds the acceptable levels, the risk assessment can be used to “back calculate” protective remediation goals by identifying the highest concentration of each contaminant, or perhaps the most toxic or carcinogenic contaminants, that can be left in place without posing an unacceptable level of risk to potentially exposed receptors.
Groundwater Remediation Goals. The EPA has established or proposed health/risk-based drinking water standards for dozens of chemicals under the Safe Drinking Water Act. The Maximum Contaminant Levels (MCLs) and non-zero MCL Goals (MCLGs) generally serve as cleanup (screening) levels for groundwater that is currently or may be used as a residential drinking water source. Many chemicals, however, have no MCLs and their remediation goals may be established using risk assessment. Where contaminated groundwater discharges to surface water, remediation goals may be based on ecological protection and use of the surface water as a drinking water supply.
Soil Remediation Goals. Cleanup goals for soil depend on whether the use of a property is or will be residential, commercial or industrial, or recreational. Predicting the future use of a contaminated site is difficult and introduces uncertainty into the risk-assessment process.
Where current or future residential land use is anticipated, cleanup goals must allow unrestricted access and safe, unlimited exposure to site conditions. Soil cleanup goals for commercial and industrial land uses are generally less stringent than residential goals because adult workers are assumed to have less frequent and shorter periods of exposure to soil contaminants and to be less sensitive to exposures than children living and playing on a property. As a result, regulatory closures of commercial and industrial sites usually allow higher levels of contaminants to remain in place. Recreational land uses assume occasional, short-term exposures, which should theoretically allow higher contaminant concentrations to remain at recreational sites; however, children are typically more sensitive receptors than adults, and the political realities of allowing contamination to remain at parks and playgrounds are common factors in determining how risks are managed at contaminated recreational sites.
Using Risk-Based Thinking for Our Sites
Assessing and managing exposure risk at our clients’ sites should go beyond merely comparing analytical data to screening levels. We should also consider the exposure assumptions used to derive the screening levels and other factors (site zoning, accessibility, presence of exposure barriers, etc.) as site-specific lines of evidence to characterize the actual likelihood of exposure and adverse effects from exposure to contaminants at our sites. While a full, quantitative risk assessment as described above may not be warranted for a particular site, the same types of considerations and assumptions come into play when assessing risk in a more qualitative manner.
If you’d like more information about assessing, managing, or remediating health risks related to site contaminants, please contact me at (317) 472-0999 or email@example.com.