Environmental Engineering Reference
In-Depth Information
these apply as well to individual species in an ecosystem,
but even the simplest ecosystem has several species. The
NRC's risk publications for HRA (1983, 1993) were followed
by three major modifi cations: (1) the adaptation and modi-
fi cation of the HRA to fi t the needs of different agencies,
(2) the adaptation of the risk-assessment paradigm to ecore-
ceptors and ecosystems, and (3) the inclusion of stakehold-
ers and public policy (NRC, 1986, 2009; Norton et al., 1992;
Bachman et al., 1998).
ERA evolved from the convergence of human health
risk assessment, ecology, and ecotoxicology to provide
data for environmental management, decision mak-
ers, and the public. Generally, ERA estimates the effects
of stressors (e.g. chemical, biologic, physical) on ecologic
end points to support decision making (Barnthouse, 1994;
Suter, 2001). ERA is inherently more diffi cult than HRA
because of the complexity of ecosystems. HRA evaluates
the risk to humans, while ERA must consider the poten-
tial risk to every population or species in an ecosystem,
which can range from hundreds to thousands (if one con-
siders microbes). An ecosystem is a spatial entity consist-
ing of abiotic (e.g. air, soil, sediment, water) and biotic
(e.g. bacteria, fungi, plants, animals) components through
which energy fl ows, nutrients move, and matter is cycled
(Odum, 1957; Payne, 1966; Sheehan, 1984; Wilson, 1986;
Hunsaker et al. 1990). Ecosystems involve complex food
webs through which contaminants can move. Further,
ecosystems have both structure (e.g. species and popula-
tions, different levels and types of vegetation) and func-
tion (e.g. energy fl ow, nutrient fl ow), and these aspects
must also be evaluated. Evaluating the risks to ecosystems
from contaminants such as mercury is monumental, and
it requires the careful selection of indicators and measure-
ment end points assessment endpoints.
One key feature of the NRC (1983, 1993) HRA paradigm was
the separation of the public, public policy, and cost consid-
erations from the risk-assessment process, which was meant
to be pure science. Excluding stakeholders often led to skep-
ticism and rejection of risk estimates. This failure to include
a range of stakeholders in the risk-assessment process was
emphasized by the Presidential/Congressional Commission
on Risk Assessment and Risk Management (PCCRARM,
1997), and led to myriad risk-assessment approaches tailored
to fi t the needs of particular agencies, many of which dealt
with the problem of mercury in the environment (USDOI,
1986; USEPA, 1986; Bascietto et al., 1990; NRC, 1994, 2009;
USACOE, 1996; USEPA, 1997; Washburn et al., 1998; Burger
and Gochfeld, 1997a) (Figures 12.2 to 12.4). The PCCRARM
(1997) report encouraged managing risks in a broader con-
text, using an iterative approach rather than merely a for-
malized risk assessment paradigm (Figure 12.5). Thus, the
NRC (1983, 1993) paradigm was adapted and modifi ed to
fi t the particular environmental concerns of each agency.
In the NRC (1983) formulation, risk assessment was isolated
from risk management so that stakeholder and cost-benefi t
concerns were not superimposed on the “pure science” of
Risk-assessment phase
Hazard
identification
Policy
Science,
including
monitoring
Exposure:
response
assessment
Exposure
assessment
Risk
characterization
Risk management
FIGURE 12.1
The National Research Council's modifi ed model of
Human Risk Assessment to include both human health and
ecosystems. (
Source:
Adapted from NRC, 1983, 1993.)
Risk-assessment phase
Problem formulati
on
A
n
a
l
y
s
i
s
Characterization
of exposure
Characterization
of effects
Risk characterization
Discussion between the risk
assessor and risk manager
Risk management
FIGURE 12.2
The Environmental Protection Agency's (EPA) ecologic
risk evaluation model. (
Source:
Adapted from Norton et al., 1992.)
(from epidemiologic or laboratory studies), establishing the
relationship between levels and effects (dose-response),
establishing how people are exposed (pathways and routes),
and identifying the target or at-risk populations. All of
