Jump to main content.

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:

  1. ecological threats are almost, or entirely, absent and therefore no further work is needed;
  2. the ecological risk assessment should continue to determine whether risk exists; or
  3. 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.

Estimating Exposure

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: 

Top of page

Uncertainty Analysis

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
  1. HQ = Dose / Screening Benchmark
  2. HQ = EEC / Screening Benchmark

After the calculation...
If... Then...
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.

Top of page

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:

  1. 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;
  2. The information is not adequate to make a decision at this point, and the ecological risk assessment process will proceed; or
  3. The information indicates a potential for adverse ecological effects, and a more thorough study is necessary.

Top of page

Local Navigation

Jump to main content.