|
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:
- Hazard identification (which chemicals are important)
- Exposure assessment (where do the chemicals go, who might be
exposed, and how)
- Toxicity assessment (determining numerical indices of toxicity
for computing risk)
- 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.
Problem | Compliance
| Health & Safety | Sampling
| Analytical | Data
Quality
Site Assessment | Prediction
| Construction | GIS
| Monitoring & Assessment
|
