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.

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