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Regulatory Compliance

TECH GUIDE

Risk Assessment

Risk assessment is an important tool used by land reclamation practitioners for understanding the human health and environmental hazards associated with the waste generated in the mining process. Risk is defined as the probability of suffering harm or loss. Risk is also dependent on the severity of the consequence. If the probability is the same that two events will occur but the second event has more severe consequences, the second event presents a higher risk.

Scientific principles are the basis for evaluating risk at a mine site. The results of the risk analysis help toxicologists make environmental decisions that will protect human health and the environment. In addition to risk assessment, political, social, economic, and other factors also drive decision-making. Risk assessment is very helpful as a substitute when numerical standards for a specific contaminant are not established. In these cases, risk assessment can be used to determine when the concentration and exposure time of a given contaminant poses an unacceptable threat to human health or the environment.

The following sections discuss human and ecological risk assessment guidelines. For more information of risk assessment, see LaGrega et al. (2001). Also, check out the EPA Superfund Risk Assessment Website.

Human Health

Specific guidelines have been established by the Environmental Protection Agency (EPA) for the preparation of hazardous waste site risk assessments for human health. There are three manuals used: Risk Assessment Guidance for Superfund (RAGS), Exposure Factors Handbook, and Superfund Exposure Assessment Manual. These manuals follow a four-stage process for quantitatively estimating risk to human health:

  1. Hazard identification (which chemicals are important)
  2. Exposure assessment (where do the chemicals go, who might be exposed, and how)
  3. Toxicity assessment (determining numerical indices of toxicity for computing risk)
  4. Risk characterization (estimating the magnitude of risk, and the uncertainty of the estimate)
Hazard Identification
In this first stage, contaminants of concern are established. Information that should be collected during this step includes: site history; land use; contaminant levels in each media (air, groundwater, surface water, soils, tailings, and waste rock); environmental characteristics affecting contaminant fate and transport (geology, hydrogeology, and topography); potentially affected population(s); and potentially affected biota.
Exposure Assessment
In the exposure assessment stage, it is determined what the risk is to a given population exposed to the contaminants of concern. Much of this stage consists of fate and transport analyses that provide a means for understanding the pathways (i.e. groundwater, surface water, air, etc.) by which contaminants from the site can reach the population. From the fate and transport analysis, the following elements are determined: the source(s) of the contaminants; the chemical release mechanism(s) (i.e. leaching); transport mechanisms (i.e. groundwater); transfer mechanisms (i.e. transfer of contaminants from the aqueous phase to chemical sorption onto soil particles); transformation mechanisms (i.e. biodegradation); exposure point (i.e. residential well); receptors (i.e. residential consumers of drinking water); and exposure route (i.e. ingestion).

The next step of exposure assessment is to determine the potentially exposed populations including present and future populations, sub-populations of special concern (i.e. young children), and potential on-site workers during remediation/reclamation. Once potential populations and exposure pathways have been determined, different scenarios (i.e. residential use, worker, recreational use, etc.) are developed to characterize the conditions under which the populations may be potentially exposed. The next step is to estimate the concentration of contaminants at the exposure points, including all pathways - air, ground and surface water, soils, tailings, waste rock, and food (i.e. plants and fish). For example, contaminant concentrations should be obtained from drinking water wells in the vicinity of the site. Estimating future concentrations at different locations is also important and generally requires fate and transport modeling.

The final step in the exposure assessment stage is to estimate the doses of the different contaminants to which a population are potentially exposed at the exposure points. Dose is a measure of the contaminant mass that the body intakes by all exposure routes per unit of body weight per unit of time. There are three types of doses - the administered dose (the amount ingested, inhaled, or in contact with the skin), the intake dose (the amount absorbed by the body), and the target dose (the amount reaching the target organ). The administered dose is easy to calculate given the concentration of the contaminant at the exposure point is known. Calculations of absorbed and target doses are not so straightforward. Many parameters that have been developed from years of research are used in the calculations. Some examples of these parameters are skin surface areas, soil ingestion rates, and inhalation rates. Information on exposure frequency and duration should be documented on a case-by-case basis.
Toxicity Assessment
The toxicity assessment stage defines the toxicity for each contaminant. If a person is susceptible to a certain dose, what will the response be? Will the person have a chance of developing cancer or will the dose simply cause a minor effect such as skin irritation? The results of the toxicity assessment take the form of mathematical constants that can be inserted into risk calculation equations. Included in this stage should be an analysis for calculating the uncertainty inherent in the mathematical constants.
Estimation of Risk
The final stage of the four-stage human health risk assessment is to estimate risks. The results are quantitative estimates of both carcinogenic (cancer causing) and non-carcinogenic risks to populations for all exposure scenarios considered. Estimates are typically calculated for all three exposure routes and for the maximally exposed individual (MEI) as well as the most probable exposed population. Calculations of the risk estimates are fairly straightforward but interpretation of the estimates presents more of a challenge. Estimates should be properly interpreted so that good decisions can be made regarding the situation. Questions may be posed such as "what is an acceptable risk"? Acceptability is the bottom line. It is influenced greatly by public concern and input. For good decisions to be made, informed members of the public are imperative.

Ecological

The above discussion was based solely on risk assessment concerning human health. Equally important is the potential risk of impacting the environment. The introduction of contaminants to the environment can have direct effects on wildlife, water quality, microorganisms, soil quality, etc. An ecological risk assessment follows a similar four-stage process as for human health risk assessment. A survey is conducted to evaluate potentially impacted ecosystems and their components. In the ecological risk assessment process, risks are evaluated for their potential effects on wildlife, plants, and microorganisms.

 

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