Assessment of the Performance of Engineered Waste Containment Barriers (2007)

Environmental risk assessments are used to predict the impact that a barrier system might have on human health and the environment. Risk assessments may yield a variety of possible products:

• Incremental lifetime cancer risk for humans. Exposure to contaminants from the site may cause an incremental increase in the frequency of individuals who develop cancer over their lifetimes. The Environmental Protection Agency requires that this incremental increase in frequency be less than 1 × 10⁻⁶ to 1 × 10⁻⁴ for Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) and Resource Conservation and Recovery Act (RCRA) sites.

• Hazard index or reference dose for humans. A reference dose is a mass of chemical or millirem of radiation per unit of time that represents a threshold at which human health would be affected. For a given component, the ratio of the actual dose measured at the site divided by the reference dose is called the hazard quotient. The sum of hazard quotients for all substances is called the hazard index. A value of less than 1 for the hazard index is considered acceptable.

• Toxicological unit or hazard quotient for ecological systems. The concentration of a chemical measured at the site is compared with the chemical concentration that would cause an effect (like toxicity) in a receptor population (an assessment end point such as fish in a stream). The ratio of these two concentrations is called the hazard quotient or the toxicological unit. The sum of toxicological units for a particular receptor is used as a metric, with values less than 1 considered acceptable.

• Qualitative or lines of evidence for ecological systems. Because of the diversity and complexity of ecological systems, the ultimate product of a risk assessment is generally not a single number. Rather, the product includes conclusions about whether effects are occurring in different classes of assessment end points (e.g., fish, microorganisms, plants, wildlife) and a discussion of the supporting evidence.

The basic methodology for an environmental risk assessment is presented in NRC (1983) and summarized below. The methodology consists of four steps:

1. . Hazard identification. Identify chemicals of concern in disposed wastes and their potential to affect human or ecological health.

2. Exposure assessment. Establish the mass of chemicals of concern at specific locations. The product of the exposure assessment is a dose, such as the mass of chemical inhaled per unit time (referred to as an exposure profile for ecological risk assessments). For an engineered barrier, this step requires the following information:

1. Release rates of chemicals from the barrier (e.g., release of aqueous-phase chemicals into the groundwater through the leakage of leachate);

2. Fate and transport of released chemicals along pathways from the source to a receptor location (e.g., the transport of the aqueous-phase chemical to below a house foundation, partitioning of the chemical into the soil vapor phase, migration of the soil vapor into the house through cracks in the foundation); and

3. Means by which a receptor comes into contact with the chemicals at a receptor location (e.g., duration of exposure and inhalation rate for a receptor in the house).

3. Dose-response assessment. Establish the relationship between effects and doses for chemicals of concern. For humans, these relationships are expressed as cancer slope factors for carcinogens (a proportionality constant relating the incremental frequency of cancer incidence for a receptor that is exposed to the dose over their lifetime) or reference doses. For ecological systems the dose response is called a stressor-response profile, and it is expressed in a variety of ways, including as a point threshold value or as a distribution showing the percentage of a population showing effects as a

function of the dose. A significant difference between human health and ecological risk assessments is in the methodology used to establish the dose-response relationship. For humans these relationships are generally extrapolated from laboratory studies for animals. Ecological risk assessments, however, employ epidemiological studies in which tests are performed on samples from the medium and the organisms at the site. This step is generally the most difficult and controversial part of environmental risk assessment.

1. Risk characterization. Integrate the exposure assessment and the dose-response assessment to evaluate whether human health and the environment will be affected by the chemicals of concern. Risk characterization can be both quantitative and qualitative.

Typical pathways that would appear in a risk assessment for a barrier system include:

• leachate leakage→groundwater transport→groundwater pumping→water ingestion

• leachate leakage→groundwater transport→groundwater pumping→inhalation (showers and faucets)

• leachate leakage→groundwater transport→partitioning to vapor phase→vapor-phase transport to confined space (structures or excavations)→inhalation

• leachate leakage→surface water transport→direct contact/ingestion (both human and ecological receptors)

• leachate leakage→surface water transport→partitioning to sediment→singestion by ecological receptors

• leachate leakage→partitioning to soil particles→ ingestion

• gas leakage→partitioning to groundwater→all of the above pathways with groundwater transport

• gas leakage→vapor-phase transport to confined space (structure or excavation)→inhalation or explosion

• inadvertent intrusion through barrier→direct contact with waste or even transport of waste (e.g., inadvertently using wastes as fill materials for construction in the surrounding area)

With the exception of inadvertent intrusion, all of these pathways start with a source term expressing the mass flux of release from the barrier system as a function of time. However, inadvertent intrusion can be a significant pathway, particularly when the period for the assessment is so long (e.g., thousands of years) that institutional controls may no longer be effective.

Uncertainty is incorporated into an environmental risk assessment both implicitly and explicitly. Uncertainty is accounted for implicitly in practice by generally selecting conservative values for input variables (e.g., neglecting a depletion of the source term with time or using a maximum or a 95th percentile value instead of an average for the concentration of a chemical of concern). In some cases, uncertainty is accounted for explicitly by performing a probabilistic analysis and expressing a range or even a probability distribution of possible results.