Ecological Risk Assessment Step 2
Sections of Step 2 Screening-level Exposure Estimate and Risk Calculation:
Step 2 of the Superfund Ecological Risk Assessment (ERA) process is the Screening Level Exposure Estimate and Risk Calculations. These calculations are done by 1) estimating to what level a plant or animal is exposed to a particular contaminant, and 2) comparing maximum contaminant concentrations to screening numbers (these comparisons result in Hazard Quotients). These are the last two phases of the Screening Level Ecological Risk Assessment (SLERA). The process concludes with a Scientific-Management Decision Point (SMDP) at which it is determined if:
- ecological threats are almost, or entirely, absent and therefore no further work is needed;
- the ecological risk assessment should continue to determine whether risk exists; or
- there is the possibility of adverse ecological effects, and a more detailed ecological risk assessment, with more information about the site, is needed.
Characterization of Exposure summarizes what is known of the extent of contamination at the site, and the measured or estimated uptake of the contaminants by the ecological receptors. The next part is Characterization of Risk in which the amount of exposure of the ecological receptors to the contaminants is compared with the dose associated with adverse effects. This comparison will help to determine whether the contamination at the site presents a potentially significant risk. An Uncertainty Section is included in all risk assessments to describe the uncertainties associated with the assumptions, extrapolations, and limitations of knowledge, and the possible effects of these uncertainties on the outcome.
Information known from Step 1 (Problem Formulation) of the Ecological Risk Assessment (ERA), including contaminant levels at the site and general information on the types of plants and animals that might be exposed to those contaminants, is used to estimate exposures for the screening-level ecological risk calculations. The ways that plants and animals can be exposed should be considered (these are called "exposure pathways"). For example, mercury (an inorganic contaminant) might be present in the sediment. It is taken up by benthic (bottom-dwelling) invertebrates which live in sediments, and then it is passed to fish when the fish eat the invertebrates, and finally to a Great Blue Heron that eats the contaminated fish..
For the exposure pathways that are described for this site, the highest measured or estimated on-site contaminant concentration for each environmental medium (sediments, water, or soil) should be used to estimate exposure to contaminants. This should help ensure that potential threats to the environment are not missed. Averages should not be used in the SLERA because they can underestimate the level of risk.
Conservative Assumptions Used in Screening Ecological Risk Assessments
In order to estimate exposures for which good site-specific information is lacking or difficult to develop, conservative assumptions should be used at this screening level. Examples of conservative assumptions are as follows:
- Area-use factor:
For terrestrial (land-dwelling) animals, it is assumed that the home ranges are fully within the contaminated area and thus the animals are exposed 100 percent of the time.
Bioavailability means that a particular chemical is present at the site in a form that can be taken in by a plant or animal (e.g., by inhalation, ingestion, or passing through the skin); assuming that this is 100 percent means that all of the contaminants present in a particular medium (e.g., soil, sediment, air, or water) are available for a plant or animal to take in.
- Life-stage (juvenile, adult, etc.):
It is assumed that the most sensitive life-stages are present at the site; the most sensitive life stages tend to be young and other pre-adult stages because they are smaller and have developed less resistance to foreign substances (including chemicals, bacteria, and viruses).
- Body weight and food ingestion rates:
Estimates of body weights and food ingestion rates should be conservative to maximize potential risk; body weights should be small because smaller organisms tend to be more sensitive to contamination; and food intake rates (how much an animal eats over a given period of time) should be high because the more an animal eats, the more exposure it receives; see the U.S. EPA's Wildlife Exposure Factors Handbook.
Bioaccumulation is the increase in concentration of contamination as one progresses through a food chain (i.e., from plants to herbivores to carnivores); for example, if ten mice each have one milligram of contaminant X in their bodies and an owl eats all ten, the owl will have ten milligrams of contaminant X in its body; how much contamination that is accumulated varies among species (from none to very high rates), but the rates chosen should be the highest; 100 percent bioaccumulation should be assumed.
- Dietary composition:
For a screening-level study, assume that the species in question feeds only on the most contaminated food type of all the types that it eats.
Professional judgment is needed to determine the uncertainty associated with information taken from the literature and any extrapolations used in developing a parameter to estimate exposures. All assumptions used to estimate exposures should be stated, including some description of the degree of bias possible in each. Where literature values are used, an indication of the range of values that could be considered appropriate also should be indicated.
Screening Level Risk Calculation
Ecological risk can be estimated numerically using the Hazard Quotient (HQ) approach. The HQ is a ratio, which can be used to estimate if risk to harmful effects is likely or not due to the contaminant in question. The HQ is calculated using one of the following equations:
Hazard Quotient Equations
- HQ = Dose / Screening Benchmark
- HQ = EEC / Screening Benchmark
- Dose = an estimated amount of how much contaminant is
taken in by a plant or animal, in terms of the body weight of
the plant or animal (e.g., mg contaminant/kg body weight per day);
- EEC = estimated (maximum) environmental contaminant concentration
at the site; how much contaminant is in the soil, sediment, or
water (e.g, mg contaminant/kg soil)
- Screening benchmark = generally a No-Adverse Effects Level concentration; if the contamination concentration is at or below this level, the contaminant is not likely to cause adverse effects.
After the calculation...
|HQ > 1.0||Harmful effects cannot be ruled out|
|HQ = 1.0||Contaminant alone is not likely to cause ecological risk|
|HQ < 1.0||Harmful effects are NOT likely|
|Final determination of risk from contaminants
in question is made in the Baseline
Ecological Risk Assessment (See Steps 3 - 7). If
a Hazard Quotient is calculated to be equal or greater than
one for a particular contaminant, that contaminant is then referred
to as a Contaminant
(or Chemical) of Potential Ecological Concern [COPEC; or sometimes as a Contaminant of Interest (COI) or an
Ecological Contaminant of Potential Concern (EcoCOPEC)].
Important Note: How large the HQ is (i.e., by how much it exceeds one) is not relevant to a Screening Ecological Risk Assessment. This is because U.S. EPA has not recognized any official means of evaluating the size of the results of these calculations, only whether or not the HQ exceeds one.
Scientific-Management Decision Point
The Scientific-Management Decision Point (SMDP) made at the end of the screening-level assessment will not set an initial cleanup goal. Instead, hazard quotients, derived in this step, are used to help determine potential risk. Thus, requiring a cleanup based solely on those values would not be technically feasible.
There are three possible decisions at this point:
- There is enough information to conclude that ecological risks are very low or nonexistent, and therefore there is no need to clean up the site on the basis of ecological risk;
- The information is not adequate to make a decision at this point, and the ecological risk assessment process will proceed; or
- The information indicates a potential for adverse ecological effects, and a more thorough study is necessary.
- If the screening-level calculations (Hazard Quotients) indicate the possibility of an ecological risk at a site, the risk assessment team and the risk manager should decide whether to proceed with further, in-depth, studies of the site. In addition, for those cases that proceed, information from the screening-level calculations can indicate and justify which contaminants and exposure pathways can be eliminated from further study because they are unlikely to pose a significant risk. At this stage, the decision made should be documented.