Risk Assessments – Adding Focus
Often it is not economically feasible to conduct a detailed, definitive ecological risk assessment on all chemicals in all contaminated areas and with respect to all potential receptors. Screening-level ecological risk assessments are conducted to focus ecological risk assessments on the chemicals or other stressors, locations and receptors with the most potential for ecological risk. Traditionally, ecotoxicity ‘benchmarks’, also termed ‘reference values’, ‘screening values’ or ‘criteria’, have been used to limit definitive risk assessments to the most important exposure pathways. These values are concentrations of chemicals in environmental media that signify the boundary between potential and no potential risk. At least three recent reviews were conducted to attempt to identify these reference values or supporting data. (10) Unfortunately, the ecotoxicity data available on total petroleum hydrocarbons and total polycyclic aromatic hydrocarbons are not sufficient to establish general benchmark or reference values for effects of these chemicals on plants, invertebrates or toxic concentrations of mixtures of chemicals with similar modes of action (e.g. narcotics) may be estimated using mechanistic approaches. (11) The potentially large number of small brine and oil spills on E&P sites of high habitat value prompts the question of whether simple field criteria (e.g. threshold total area) may be used to exclude the spills from formal ecological risk assessment. Some US states have developed contaminant area limits that may be termed ‘exclusion criteria’ and are analogous to the ‘benchmarks’ described previously. (12) For example, the Pennsylvania Department of Environmental Protection (PADEP) assumes that two acres of surface soil contamination does not pose risk to vertebrate populations. (13) However, these values are policy decisions which are not based on population biology or landscape ecology considerations.
Where habitat area is a concern, remaining patches of suitable habitat may be compared with ‘critical patch size’ i.e. the contiguous habitat area needed to maintain a population. (14) Estimates of critical patch sizes are available for 33 species of small mammals, 36 species of large mammals, 77 species of birds and 44 species of herptiles. In general, critical patch size is largest for large omnivorous and carnivorous mammals, followed by small mammals, herptiles and birds, but the variance is largest for birds. If contiguous habitat patches at an E&P site have areas below the species-specific critical patch size, a wildlife population may not persist. It is anticipated that the results of the population models described above will also aid in future development of ‘exclusion criteria’ for leaving unrestored (but adequate) habitat at E&P sites.
Risk Assessments – Adding Benefits
Ecological risk assessment methodology may not be adequate for supporting some decisions at petroleum-contaminated sites. Although human health risk and economic cost are also considered, additional environmental information may be needed as well. For example, comparisons of remediation alternatives and ecological restoration options may require the estimation and comparison of net environmental benefits of each. A framework has been developed for net environmental benefit analysis (NEBA) of petroleum-contaminated sites that builds on principles of ecological risk assessment. (15) A NEBA for chemically contaminated sites typically involves comparison of several management alternatives:
- leaving contamination in place;
- physically, chemically or biologically remediating the site through traditional means;
- improving ecological value through onsite and offsite restoration alternatives that do not directly focus on removal of chemical contamination; or
- a combination of these alternatives.
Changes in ecological services (16) or populations through time are assessed for each alternative (see Figure 4). Principles of NEBA were first used to assess the impacts of marine oil spill dispersants. (17) NEBA has the potential to help land managers avoid the possibility that the selected remedial or ecological restoration alternative will provide no net environmental benefit over natural attenuation of contaminants and ecological recovery. Additional tools are needed to support NEBA – non-monetary environmental valuation methods, exposure-response models for chemicals and physical stressors such as roads and wells and associated habitat fragmentation, models of ecological recovery, and optimal strategies for ecological restoration.
Figure 4: Distribution of Corridors Between Meadow Habitats
for a Hypothetical Meadow-loving Species at Yellowstone National Park, USA
Patches of meadow (yellow) amid patches of various successional
stages of lodgepole pine (left) and wildlife corridors between meadow patches
(now shown in black), where most intense corridors are depicted in red and yellow (right).
Conclusion
Ecological risk assessments are benefiting from new tools that provide scientific credibility, focus the assessment on the exposure routes with the likeliest effects, consider impacts on wildlife from the population perspective, and examine the big picture, including benefits of proposed remedial actions. Risk assessment methods, in general, need to promote rigorous, relevant and recent science. Future research would enhance the value of these tools for the petroleum industry, including studies of all aspects of bioavailability of hydrocarbons and other chemical contaminants, field validation of population models and case studies of net environmental benefit analysis.
Figure 5: Hypothetical trajectory of environmental service or other ecological entity with time,
following a petroleum spill (contaminated reference state); conditions that would have been expected to
prevail in the absence of the spill (uncontaminated reference state); expected trajectory of the remediated
state; and expected trajectory of the restored state. A net environmental benefit analysis would compare these curves
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