Ecological Risk Assessment for Contaminated Sites - Chapter 7 doc

—Francis Bacon, On Seditions Following the CERCLA remedial investigation, the feasibility study FS is ducted to analyze the benefits i.e., risk reduction, costs, and risks associated wit

9 Remedial Decisions

Good policy analysis recognizes that physical truth may be poorly or incompletely known Its objective is to evaluate, order, and structure incomplete knowledge so as to allow decisions to be made with as complete an understanding as possible of the current state of knowledge, its limitations, and implications.

—Granger Morgan (1978)

The remedy is worse than the disease.

—Francis Bacon, On Seditions

Following the CERCLA remedial investigation, the feasibility study (FS) is ducted to analyze the benefits (i.e., risk reduction), costs, and risks associated withremedial alternatives The use of ecological risk assessment should not end with thebaseline risk assessment for the site or even with the recommendation of remedialgoal options in the remedial investigation (Chapter 8) Risk assessment is integral

con-to (1) the analysis of individual remedial alternatives; (2) the ultimate, balancedremedial decision for the site; (3) prioritization of the remediation sequence formultiple sites; and (4) the assessment of the efficacy of remediation The baselineassessment in the RI addresses only the no-action alternative This chapter addressesthe use of ecological risk assessment in the FS and in the subsequent decision-making process

In remedial alternative analysis, the following questions should be asked:(1) How will present and future risks associated with contaminants be mitigated byeach alternative? (2) What new risks may be associated with each alternative? Thefirst question can be answered based on the baseline risk characterization, remedialgoals, and the proposed remedial alternatives No new risk assessment is necessary.The analysis required to answer the second question fully should often be a completerisk assessment: problem formulation, exposure assessment, effects assessment, riskcharacterization, and description of uncertainties Stressors (most often physical) arenew, and some assessment endpoints and exposure pathways are likely to be differentfrom those in the original assessment Some of the hazards associated with remedialactions are listed in Table 9.1 Recovery is an important part of the risk character-ization for effects of stressors associated with remedial actions Unfortunately, reme-dial risks are rarely given due attention in the feasibility study because (1) the FS

is often under a severe time constraint; (2) the FS is often performed by the engineerswho design the remedial alternatives, not risk assessors; and (3) in the United States,regulators do not require or expect rigorous assessments of remedial actions TheEPA guidance for assessment of human health risks of remedial actions is muchless demanding than that for baseline risk assessments, and it makes quantitativeassessment optional (EPA, 1991d) The guidance for assessment of ecological risksfrom remedial actions is less than a page in length (Sprenger and Charters, 1997)

Following the remedial alternative analysis, risk managers must finally decidewhich remedial option is best Risks from remediation must be balanced against thebaseline risks that would be mitigated It is also advisable for the final remedy tobalance human health and ecological risk—that is, for the remedial action to be asprotective of ecological receptors as it is of human health Finally, the decision, ofnecessity, includes the costs of each alternative

At large facilities such as the DOE Oak Ridge Reservation, multiple nated sites require remediation The process of prioritizing these sites should incor-porate principles of ecological risk assessment For example, if all sites cannot beremediated immediately, it may be appropriate to evaluate the risk associated with

contami-a delcontami-ay in remedicontami-ation of econtami-ach site

9.1 REMEDIAL ALTERNATIVE ANALYSIS

The best remedial option is chosen by balancing costs and benefits of the variousalternatives, the latter including reduction of the ecological risks described in theremedial investigation According to the National Contingency Plan, the detailedanalysis of alternatives consists of using nine criteria to evaluate each one, and

TABLE 9.1 Examples of Hazards Posed by Remedial Actions

From Chemicals:

Mobilization by dredging of contaminants buried in sediments Increased availability of contaminants due to use of chelating agents Exposure of consumers to high contaminant levels in hyperaccumulator plants Release of contaminants during incineration or thermal desorption

Use of biocides to eliminate contaminated communities

From Physical Disturbance:

Destruction of benthic communities by dredging Destruction of terrestrial ecosystems by:

Removal of contaminated soil Creation of roads, parking areas, laydown areas, and other support facilities Creation or expansion of waste burial grounds

Creation or expansion of borrow pits for caps or fill Mowing to maintain lawns

Paving to eliminate hydrological and biotic exposure to soil Rerouting, channelization, and lining of streams

Destruction of soil structure by cleaning Soil erosion and compaction

From Biotic Introductions:

Revegetation with exotic species Invasion of ecosystems by microbes or plants introduced for bioremediation

Indirect:

Opening the site to human use and development Encouraging development in the surrounding area by removing the stigma of contamination

then identifying relative advantages and disadvantages of each (EPA, 1990a) Thecriteria are

1 Protection of health and the environment

2 Compliance with federal applicable or relevant and appropriate ments (ARARs)

require-3 Long-term effectiveness and permanence

4 Reduction of toxicity, mobility, or volume through treatment

The first two criteria are threshold criteria and should be weighed more heavilythan others (Sprenger and Charters, 1997) Clearly, the first criterion should includeeffects of the remedial action Few references to risks from remediation are made inthe National Contingency Plan, although potential negative impacts of remediationare alluded to in a discussion regarding the EPA expectation that the preferred alter-native will often be treatment of contaminated media Treatment will be limited when

“implementation of a treatment-based remedy would result in greater overall risk tohuman health and the environment due to risks posed to workers and the surroundingcommunity during implementation” and “severe effects across environmental mediaresulting from implementation would occur” (EPA, 1990a) The third criterionaddresses “any residual risk remaining at the site after completion of the remedialaction” (discussed in Chapter 10) Short-term effectiveness, the fifth criterion, refers

to adverse and beneficial effects of the action during implementation and construction.State acceptance and community acceptance criteria relate to ecological riskassessment to the extent that the state regulatory agency and community valueecological receptors Indeed, for three units on the Oak Ridge Reservation, LowerEast Fork Poplar Creek and two ponds at the K-25 site, members of the publicinsisted that the DOE balance risks from the proposed remedial action against risksidentified in the remedial investigation to choose the appropriate alternative Theserepresentatives of the community were in favor of maximum ecological protection

at reasonable cost It is also notable that state and local communities may acceptthe risks associated with minimal clean-up if the site is designated a “brownfield”(i.e., an industrial land-use site), particularly if the alternative is to construct a facility

on cleaner land

Finally, the risk manager chooses the most appropriate remedy for the site Thebalancing of factors involved in this decision is discussed in Section 9.2 Althoughfinal remedial actions always require the consideration of the factors above, interimactions do not At any time during the remedial investigation and feasibility study

process, the risk manager may decide that a chemical release or the threat of a release

of pollutants necessitates an interim action This time-critical response, termed a

removal action, is not a comprehensive or final remedy for the site Thus the ninecriteria do not apply (EPA, 1990a) For example, a decision was made to treat aTCE plume beneath the K-25 site on the Oak Ridge Reservation, based on humanhealth concerns and the potential for the plume to migrate off-site Ecologicalconcerns, such as the impacts of the reduced flow to a stream, were not required to

be considered prior to the removal action decision

9.1.1 R ISKS A SSOCIATED WITH R EMEDIAL A LTERNATIVES

The conventions of ecological risk assessment are rarely followed to identify andcharacterize ecological risks that may be associated with remedial alternatives Thus,risk assessment associated with remediation is discussed at length here During nego-tiations among stakeholders concerning remediation, it is often expected that healthand safety issues will arise, including risks to construction workers, risks to the publicfrom incinerator emissions, and risks to the public from dump truck traffic However,the prospect of new ecological risks is rarely a concern It is probably assumed thatany ecological risks from remediation are short-lived EPA and state regulatory agen-cies do not typically require well-structured, prospective ecological risk assessments

as part of Superfund remedial feasibility studies Although the EPA definition ofstressor in its “Guidelines for Ecological Risk Assessment” is broad and includesphysical stressors, risks associated with CERCLA remediation are not a focus of thedocument (EPA, 1998) Nonetheless, the “Ecological Risk Assessment Guidance forSuperfund” (Sprenger and Charters, 1997) states that the ecological impacts of reme-dial options are an important aspect of protecting the environment Given the oftenhaphazard ecological analyses in feasibility studies, decision makers are at risk ofunknowingly substituting ecological risks from remedial alternatives for human healthand ecological risks that have been identified in the remedial investigation

Ecological risks from remediation may be classified into two categories: (1) theexacerbation of existing contaminant risks or (2) the physical destruction or trans-formation of ecological habitats and associated ecological communities In the firstinstance, a removal action may cause further contamination of groundwater andsurface water, or remedial technologies may increase the bioavailability of contam-inants An addendum to the baseline exposure assessment may be necessary tocharacterize these risks from chemicals In contrast, it is primarily the problemformulation phase of the risk assessment that must be improved if ecological riskassessment is to contribute to the assessment of nonchemical remedial alternatives,such as excavation and dredging Components of the problem formulation that meritdiscussion are the identification of stressors and assessment endpoints and thedevelopment of conceptual models Exposure and exposure–effects relationshipsmay be obvious if ecosystems or portions of them are eliminated by physicaldisturbance In addition, the risk characterization for physical stressors associatedwith remediation should evaluate the recovery of the affected ecological receptors

If a large number of activities are associated with a single remedial action, or

if multiple remedial actions are undertaken concurrently and in close proximity, it

may be necessary to use an ecological risk assessment framework that has beendeveloped for multiple activities The standard ecological risk assessment frameworkwas developed for assessments of individual chemicals and other individual agentsand does not incorporate a logical structure for assessing multiple agents and inte-grating their risks (EPA, 1998) Similarly, indirect or secondary effects are notaddressed Suter (1999b) developed a framework for the assessment of militarytesting and training programs that would be applicable to complex remedial actions.Suter recommends that impacts of each activity be assessed separately and integrated

in the risk characterization, and provides a conceptual approach for addressingcombined effects

The sections below are organized according to the EPA ecological risk ment framework Although environmental impacts of remedial actions are required

assess-to be assessed in the feasibility study, the EPA framework is not required assess-to be used.Nonetheless, the authors believe that the framework is helpful in organizing theprocess of analyzing and characterizing risks from remediation

9.1.2 P ROBLEM F ORMULATION

9.1.2.1 The Nature of Stressors

Physical, chemical, and biological stressors may be introduced as a result of ticular remedial actions Technologies that may introduce new chemical stressorsinclude microbial bioremediation, phytoremediation, solvent extraction, chemicaloxidation, and poisoning of contaminated fish prior to removing them Chemicalstressors associated with bioremediation could include toxic metabolites of theprocess (e.g., vinyl chloride from TCE), nutrients added to enhance the process,surfactants added to enhance the process, and peroxide added to provide a source

par-of oxygen to bacteria The microorganisms themselves could be biological stressors,

if their multiplication and dispersal would constitute a hazard Similarly, someplants introduced for phytoremediation or revegetation could become weeds Asummary of chemical emissions from conventional remedial technologies is pre-sented in EPA (1991d)

Physical stressors associated with remediation might be the most harmful, atleast in the short term Examples include removal of vegetation and topsoil and soilcompaction by heavy equipment and human activity Similarly, the maintenance oflawn would be a stressor to the plant community and wildlife populations In aquaticsystems, changes in water flow, erosion of stream banks, dredging of sediments, anddecreased riparian vegetation would be potential stressors In all environments, theremoval of habitat is a stressor that would be expected to result from a physicalremoval action

9.1.2.2 Conceptual Models for Alternatives Assessment

The presentation of conceptual models could potentially increase the clarity andrigor of the alternatives assessments For no-action alternatives or alternatives thatare intended as human health rather than as ecological remedies (e.g., fences, fishingadvisories, land-use controls), conceptual models for the baseline risk assessments

are applicable In addition, these conceptual models may be used if the remedialaction may mobilize chemical contaminants However, the remedial alternatives thatinvolve removal, isolation, or treatment of soil or sediment require disturbance notonly of the contaminated areas but also of uncontaminated areas used for roads,structures, laydown areas, borrow pits, landfills, or treatment facilities Hamby(1996) reviews common remedial technologies for soils, surface water, and ground-water The Federal Remedial Technologies Roundtable provides a Web site listing

in situ and ex situ technologies that have been used in over 100 case-study ations (http://www.frr.gov)

remedi-Generic conceptual models for potential impacts of these activities on nents of terrestrial ecosystems, aquatic ecosystems, and wetlands are presented inFigures 9.1, 9.2, and 9.3, respectively These conceptual models for physical distur-bance differ from typical models for chemicals in that the arrows represent chains

compo-of causal processes rather than flows compo-of chemicals Additionally, the receptors aredefined broadly because the consequences of physical disturbances tend to be lessdiscriminatory than those of chemicals Because of the great diversity of physicaldisturbances that could occur during remediation, these generic models requiresubstantial adaptation to specific cases The generic models should be modified asremediated sites are monitored and unexpected links emerge For example, on theOak Ridge Reservation a TCE-contaminated aquifer is being remediated through apump-and-treat technology To prevent Mitchell Branch, a neighboring stream, frombeing drained by the remedial measure, a length of the stream has been altered to

a culvert Damage to the stream, soil compaction, and the trampling of the ripariancommunity along the stream should be included in the conceptual model or modelsfor the remedial action

Large-scale physical or chemical remedial measures may impact neighboringsites Thus, remedial decisions should be considered in the context of the manage-ment of neighboring sites For example, on the Oak Ridge Reservation land managershave proposed draining a contaminated pond to mitigate risks to trespassing fisher-men and avian piscivores Hydrologically connected to this pond is a waste burialground, contaminated groundwater, and an associated spring All of these elementsshould be components of the conceptual model for the remedial action For example,rotenone added to the pond to kill PCB-contaminated fish may escape from the pond

to hydrologically connected water bodies Similarly, Garten (1999) has found thatforest vegetation mitigates leaching of strontium-90 from soils at locations wheretransport is controlled mainly by subsurface flow Thus, a conceptual model for theremoval of trees from a similar strontium-90-contaminated site should include apathway to groundwater and possibly to surface water and aquatic organisms.More indirect effects of remediation may interfere with goals for protection ofecological receptors For example, chelation agents added to soil to aid in phytore-mediation may strip the soil of particular nutrients (Entry et al., 1996), thus causingadverse effects on the plant community These agents could also increase the uptake

of contaminants by soil invertebrates and increase food web transfer Thermal ing of soil destroys its structure and organic matter, raises its pH, and has a sterilizingeffect (Tamis and Udo de Haes, 1995) Not only are the native soil flora and faunaaffected, but plants seeded on the cleaned soil are likely to be adversely impacted

clean-A more complex example involves the remediation of the Rocky Mountain clean-Arsenal,including the demolition of chemical factories at the site This reduction in the stigma

of contamination and the improvement in aesthetics are leading to increased opment on nearby lands The development threatens the wildlife habitat that exists

devel-in the vicdevel-inity of the site and provides a habitat corridor between the site and theFront Range (Baron, 1997) Risk assessors and managers must decide which stres-sors have been created indirectly by the remedial action and pose likely or potentiallyhigh-magnitude risks

If conceptual models are consistent with remedial alternatives, the models mayinclude the ultimate environmental fate of the contaminated medium Where willdredged sediments be deposited? Is treated soil proposed to be returned to its site

of origin? Tamis and Udo de Haes (1995) note that in the Netherlands cleaned soilhas a stigma associated with it Soil that has been cleaned through thermal processes

is generally used as fill in construction because of the loss of structure and organicmatter (Tamis and Udo de Haes, 1995) Soil cleaned through the use of chemicalextraction is used in the concrete and asphalt industries Biologically remediatedsoil is often used to cover waste dumps (Tamis and Udo de Haes, 1995)

Suter (1999a) notes that conceptual models that represent multiple activities,multiple agents, nonchemical agents, and indirect effects, all of which may beassociated with remedial actions, can be difficult to develop Because these concep-tual models do not simply represent flows of contaminants, it is advisable to definethe processes that link the physical components of the models Because these modelsmay become quite complex, it is often desirable to structure them hierarchically in

FIGURE 9.1 Generic conceptual model of the effects of physical disturbance on trial ecosystems.

terres-both detailed and aggregated form Suter (1999a) also recommends that risk sors create modular component models that can be reused in different combinationsfor different assessments.

asses-9.1.2.3 Assessment Endpoints

As stated in Section 2.5, ecological assessment endpoints are statements of mental values, i.e., entities and associated properties that are to be protected Can-didate assessment endpoints in the remedial feasibility study should include boththose that were selected as endpoints for the baseline ecological risk assessment andthose that were excluded because they were not deemed to be exposed to contami-nants at the site Some of these receptors could be at risk from the physical distur-bances associated with remediation In addition, assessment endpoints shouldinclude receptors at neighboring sites, such as terrestrial or aquatic communities

environ-FIGURE 9.2 Generic conceptual model of the effects of physical disturbance on aquatic ecosystems.

affected by off-site activities such as road construction and creation of borrow pitsand waste disposal sites As in the baseline ecological risk assessment, ecosystemsand organisms with special regulatory status, such as wetlands and threatened andendangered species, should be assessment endpoints if an exposure pathway exists.Some physical remedial actions are likely to severely disrupt habitat for endpointpopulations or communities by virtue of their severity or large spatial scales.Although hazardous waste sites that are entirely denuded of vegetation because ofcontamination are rare, the removal of soil and the associated plant community as

a remedial measure is common Thus, the physical disruption of habitat, which couldput associated populations of organisms at risk, should dictate that these populations

be selected as assessment endpoints Appropriate assessment endpoints in the context

of physical disturbance might include diversity of the plant community and lations of wildlife that might be affected by the new arrangement of patches ofhabitat and forage vegetation (see Figure 9.1) The boundary of a proposed actionmay determine whether a rare or highly valued plant community would be entirelyremoved It is notable that some of the more subtle properties of assessment end-points for contaminated sites, such as reproductive potential, would not be appro-priate for locations where the removal of an entire community is planned

equivalent) for the feasibility study Two alternatives are possible: (1) an inated and relatively undisturbed reference site or (2) the contaminated site prior toremediation If baseline risks from the contaminants are balanced against remedialrisks, as is suggested in Section 9.2 below, then the conditions resulting from bothtypes of risks should be compared with the reference conditions (e.g., backgroundsoils) that are used in the baseline risk assessment (Section 2.7.3).

uncontam-9.1.3 E XPOSURE A SSESSMENT

The feasibility study should include an estimate of exposure of all assessmentendpoints to all significant stressors for each remedial alternative If any new expo-sure pathways are identified in the conceptual model for the remedial action, a newexposure assessment may be required An example is the introduction of new chem-icals into soil or water, either as reactants in the remedial technology or as degra-dation products of the initial toxicant The volatilization of organic contaminantsduring water evaporation from dredged sediments is another example (Chiarenzelli

et al., 1998) If contaminated media are moved (e.g., dredge spoil transported uplandfor disposal), new assessment endpoints may be appropriate In the case of dredgespoil, the models used to estimate uptake of contaminants by wildlife foods in soil(Section 3.5.2) may not be appropriate for estimating accumulation of chemicalsfrom disposed sediment (Edwards et al., 1998)

If the remedial technology has the potential to increase the bioavailability ofthe remaining contaminants, an amendment to the baseline exposure assessment isrequired For example, the hyperaccumulation of contaminants by plants in phy-toremediation could increase the availability of the contaminants to herbivores.Solvent extraction, if performed in situ, could increase the bioavailability of agedorganic chemicals or reduce the ability of the soil to support a community ofmicroorganisms that could otherwise degrade the chemical For example, Inoue andHorikoshi (1991) found that in liquid culture, none of 61 bacteria tested could grow

in the presence of organic solvents with values for the log octanol–water partitioncoefficient (log Kow) less than 3.1, and most could not grow in the presence of asolvent of log Kow less than 4.0 Also, dredging suspends contaminated and anoxicsediments in the water column, increasing exposure of aquatic organisms in thewater column to contamination

Information concerning the release of contaminants or changes in their form due

to remediation may be obtained from the results of treatability studies These arebench-scale or small field trials of proposed technologies using the actual contam-inated medium from the site Another source of information is monitoring conducted

at sites where the remedial technology has been previously applied

The estimated exposure of assessment endpoint receptors to physical stressors(such as soil removal or trampling) may consist of only a description of spatialextent, intensity, frequency, and duration, if known Exposures to any indirect stres-sors, such as impacts on habitat, should be considered, even if only qualitativeassessment is possible Only quantities that can be related to effects are used toestimate risks; other exposures are noted as having uncertain impacts in the riskcharacterization Because an exposure assessment for a remedial action would be a

prospective assessment, actual data reflecting exposure would not be available, andsome estimates of exposure could be highly uncertain.

9.1.4 E FFECTS A SSESSMENT

If the relationship between exposure and effects is not obvious for the variousremedial technologies, a formal effects assessment may be performed This mayinclude toxicity tests of soil from bench-scale tests of treatment technologies Ofcourse, toxicity tests are more useful than concentrations of contaminants in soilalone as evidence of risk to most population- or community-level assessment end-points Thus, toxicity tests may be thought of as measures of the effectiveness of aremedial technology as well as evidence for new risks (The assessment of efficacyfollowing the implementation of remedial actions is described in Section 10.2.4.)Surveys of biota cannot be performed at the site of concern in a prospective riskassessment for a remedial action However, the spatial extent of the remedial action

is known, so a significant level of effects can be defined spatially (Box 9.1) Inaddition, records of the monitoring of sites following historical uses of the particulartechnologies or actions may contribute to the effects assessment Also, habitatsuitability models may be used to compare the suitability of wildlife habitat beforeand after a proposed remedial action or to compare alternative remedial actions(Rand and Newman, 1998) In the United States, habitat evaluation procedure modelsare available for more than 100 terrestrial and aquatic species which can be used toestimate changes in habitat suitability or abundance of the endpoint species (U.S.Fish and Wildlife Service, 1988) Alternatively, habitat models may be developed

ad hoc For chemical stressors, the use of exposure-response models to describe therelationship between contaminant concentrations in environmental media and effects

is advisable (Chapter 4)

Typical exposure–response relationships are based on severity rather than tion The duration of effects has not normally been considered in the regulation ofecological risks (Suter, 1993a) However, the acceptability of the no-action alterna-tive, also known as “natural attenuation” (i.e., natural dilution and degradation ofchemicals), depends on the acceptability of the duration of effects Similarly, theacceptability of engineered remedial alternatives may depend on the time to recovery

dura-of the remediated ecosystem (Section 9.1.5) Therefore, the duration dura-of effects is amore important dimension of ecological risk in remedial assessments than in assess-ments of most purely toxicological risks

9.1.5 R ISK C HARACTERIZATION

As in other risk characterizations (Chapter 6), the available evidence concerningexposure to and effects of each remedial action on associated assessment endpointsshould be weighed in the feasibility study The weight of evidence process has beendescribed in Section 6.5, and the discussion does not need to be repeated here.However, two concepts that merit attention are the spatial context of risk and timerequired for recovery of assessment endpoints affected by physical disturbance Theconcept of recovery is discussed at length because the recovery times of affected

populations, communities, or ecosystem processes affect the comparison of risksassociated with contaminants and those induced by remedial actions.

9.1.5.1 Spatial Considerations

Several spatial considerations should arise during the characterization of remedialrisks in the feasibility study This spatial context of risk is particularly critical forproposed remedial actions that include a physical disturbance component, such asexcavation of soil or dredging of sediments If the plant community is an assessment

BOX 9.1

The de Minimus Effects Level with Respect to Spatial area

In Chapter 2, a 20% criterion was discussed as the upper bound for a de minimis

level of ecological effects The criterion was used to refer to a level of effects on

a population or other assessment endpoint that was deemed significant by latory agencies There are no known regulatory precedents for establishing a 20%criterion for identifying a significant area affected by physical disturbance, such

regu-as that regu-associated with a remedial action Nonetheless, the 20% rule for severity

of effects may be applicable to areal scales of ecological effects by analogy Inparticular, the loss of all individuals from 20% of the range of a population can

be considered equivalent to loss of 20% of individuals from the entire range of

a population (if the issue of relative rates of recovery is temporarily ignored).Therefore, for actions such as dredging, which lead to loss of all members of apopulation or community within a prescribed area, the 20% criterion may beassumed to apply to spatial area Capping, dredging, or paving of >20% of therange a population or community would be a potentially significant loss Theeffects assessment should reflect this assumption if federal and local regulatoryagencies and other risk managers approve

The use of the 20% severity criterion to signify a de minimus level of disturbance

to particular areas may be questioned for two reasons First, the regulatoryprecedents for the criterion are based on toxic effects that are not readily observed

or measured In contrast, when the 20% criterion is applied to areas physicallydisturbed rather than areas experiencing toxic effects, the results are likely to bereadily observed Hence, because physical disturbances are more apparent thantoxic effects, the 20% criterion may be less acceptable for the former Second,studies of risk perception indicate that familiar risks are more acceptable thanunfamiliar risks (Slovic, 1987) Indeed, McDaniels et al (1997) used a principalcomponents analysis to identify four factors that characterize perceived ecologicalrisk: ecological and human impact, human benefit, controllability, and knowledge.Therefore, the familiar clearing of 20% of a forest may be more acceptable thanunfamiliar toxic effects in 20% of that same forest Thus, we have simply assumedthat in the case of physical disturbance, the observability and risk perceptionfactors described above are negligible or cancel, permitting the use of a 20%criterion for physical disturbance