applications of background data in ecological risk assessment: various shades of gray

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Applications of Background Data in Ecological RiskAssessment: Various Shades of GrayMark E. Stelljes a & Rosemary R. Wood aa SLR International Corp, Concord, California, USAPublished online: 18 Jun 2010.

To cite this article: Mark E. Stelljes & Rosemary R. Wood (2003) Applications of Background Data in Ecological RiskAssessment: Various Shades of Gray, Human and Ecological Risk Assessment: An International Journal, 9:7, 1609-1621, DOI:10.1080/714044785

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November 27, 2003 11:25 Human and Ecological Risk Assessment TJ883-06

Human and Ecological Risk Assessment, 9: 1609–1621, 2003Copyright C© ASPISSN: 1080-7039 printDOI: 10.1080/10807030390260245

Applications of Background Data in Ecological RiskAssessment: Various Shades of Gray

Mark E. Stelljes and Rosemary R. WoodSLR International Corp, Concord, California, USA

ABSTRACTThis paper discusses general approaches for evaluating the utility and manner of

conducting background analyses in soil for ecological risk assessments. The typesand sources of background data are discussed, and advantages and disadvantagesof using literature-based versus site-specific background data are presented. Thevalue of background evaluations is discussed with regard to the goals and objectivesfor a project. A comparison of literature-based ecological soil screening levels withgeneric metal background concentrations is presented to illustrate a typical problemin incorporating background data in ecological risk assessments, which is that manygeneric background concentrations are higher than ecological screening levels. Thisbrings into question both the relevance of ecological screening levels and genericbackground levels. These issues are discussed along with cost-benefit considerationsin an attempt to provide recommendations for determining the most appropriatetype of background evaluation to conduct at a given site.

Key Words: background evaluation, ecological risk assessment, statistics, ecologi-cal soil-screening levels.

INTRODUCTION

Many chemicals commonly found at contaminated sites are also found in non-impacted areas. Accurately quantifying site-specific background concentrations ofchemicals in soil is important, especially if the ambient concentrations exceed risk-based soil screening levels. Background analyses, which evaluate whether or notchemicals detected at a contaminated site reflect ambient levels, are often conductedas part of human health and ecological risk assessments. There are two types of “back-ground” data: 1) naturally occurring chemicals, such as inorganics, present in theenvironment that have not been influenced by human activity, and 2) anthropogenicchemicals, such as polycyclic aromatic hydrocarbons (PAH), which are present inthe environment due to human activities, but are unrelated to the contaminated sitein question.

Address correspondence to Mark E. Stelljes, SLR International Corp, 1430 Willow Pass Road,Suite 230, Concord, California 94520, USA. E-mail: [email protected]

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M. E. Stelljes and R. R. Wood

In 1989, the U.S. Environmental Protection Agency’s (USEPA’s) key guidance,Risk Assessment Guidance for Superfund, outlined objectives and suggested methodsfor background evaluations (USEPA 1989). Since then, guidance recommendingspecific statistical approaches has been published by the U.S. Environmental Protec-tion Agency (USEPA 2002a,b) and the California Environmental Protection Agency(CalEPA 1997). With increasing risk assessment experience, it is clear that back-ground analyses are not always straight forward, particularly in view of the risk-basedscreening levels now published by USEPA and other entities for ecological receptors.Frequently, adequate site-specific data are not available for comparison to site-relatedchemical data. In addition, depending on regional lithology, background levels ofmetals in soil may be higher than screening levels. Ecological screening levels forchemicals are frequently lower than screening levels for human receptors, takingthem closer to background. Having ecological screening levels that are at or be-low background necessitates close evaluation of screening methods used to identifychemicals of potential ecological concern, and examination of the relative meritsof obtaining site-specific background versus exposure data for ecological risk assess-ments. In view of USEPA’s increasing emphasis on risk assessment of toxic metals,particularly their ecotoxicity, background analysis is becoming even more impor-tant (USEPA 2002c). This paper discusses general approaches for evaluating theutility and manner of conducting background analyses in soil for ecological riskassessments and addresses problems that may be encountered.

USES OF BACKGROUND DATA

In risk assessment, background data serve two general functions. One is to de-fine an “unimpacted” or “reference” area, which can be used to discern the areaoutside the zone of contamination at a given site. The second general function isto allow for a comparison of naturally occurring concentrations with site-specificdata to assist in defining the scope of a risk assessment. A former industrial site inthe San Francisco Bay Area with apparently elevated concentrations of 11 metals insoil that could have been present as a result of site activities is a good example ofhow the second general use of background data was applied. Although backgrounddata for the pertinent county are available in the published literature, there wereproblems relating site-specific lithology to these data (Scott 1991). Therefore, a site-specific background evaluation was performed. To identify metals likely present insoil as a result of industrial activities, two approaches were taken: first, the distribu-tion of site-related soil data was evaluated and the presence of any obvious outlierswas noted. Outlier data were evaluated separately from other site-related data. Sec-ond, local background soil samples were collected from shallow soil at the samedepths as site-related soil samples and in corresponding soil types. Care was takento ensure both background and site-related samples were collected from native soil(bay mud and clay), and were analyzed using identical methods. Statistical analysisof the resulting data indicated that 8 of the 11 metals were likely present at back-ground concentrations. After this evaluation, only three metals that were detectedin shallow soil, including arsenic, were considered likely present as a result of site ac-tivities. This site-specific background evaluation was particularly important because

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Ecological Risk Assessment: Applications of Background Data

naturally-occurring arsenic concentrations are elevated in many parts of NorthernCalifornia, frequently exceeding both human and ecological risk-based screeninglevels. At this site, however, a cursory comparison of site and background data madeit clear that arsenic had likely been released to the site subsurface. This exampleunderscores the importance of conducting background evaluations to differentiatebetween site-related and background sources of metals.

ISSUES RELATED TO BACKGROUND DATA COLLECTIONAND APPLICATION

Although the example presented above dealt with a simple site situation, many is-sues were involved in designing and implementing even this simple protocol. Theseissues involve identifying appropriate sources (e .g ., literature or site-specific) andtypes of quality of background data (e .g ., which chemicals, how measured, presenceof outliers), working within the overlying regulatory environment, and identifyingrepresentative background concentrations. These issues are further discussed in thefollowing text.

Sources of Background Data

In order to conduct a background evaluation at a site, relevant concentrationsneed to be defined. There are two general sources of background data, the literatureand site-specific data collection. These sources both have their advantages and dis-advantages, and deciding which sources to use, and when, are important decisionsin conducting an effective background analysis.

The literature provides off-the-shelf background concentrations for most inor-ganic elements. Relying on these literature-derived values is quick and cheap. How-ever, these values are typically generic, and cover large land areas. For example,Shacklette and Boerngen (1984) of the United States Geological Service (USGS)compiled background concentrations for metals for the entire conterminous UnitedStates, and also reported concentrations separately for the western and eastern por-tions of the United States. These generic levels have several advantages:

� Cover a variety of soil types and land uses� Inexpensive and quick to compile� Statistical analyses have already been conducted, saving the user time and money

to compile statistically based concentrations.

However, there are disadvantages to using generic data:

� Site-specific soil may be naturally enriched in certain elements, which wouldlead to (erroneously) concluding that concentrations exceed generic back-ground (for instance, arsenic in areas with geothermal activity, common inNorthern California and southern Alaska, and nickel and chromium in SanFrancisco Bay Area serpentinite soils)

� The site-specific soil type may not have been included in the generic dataset� Regulatory agencies often require site-specific background, minimizing the

value of such generic data, except for qualitative comparisons.

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Prior to collecting site-specific data, however, an additional refinement can bemade to a generic background dataset by focusing on a subset of the generic datathat was collected either 1) in proximity to the site of interest or 2) in the samesoil type. The first of these approaches focuses on the goal of using site-specificdata, but without the costs associated with mobilizing field crews and collectingsite background samples. Depending on the size of the dataset and the locationof generic background samples, this may or may not be a useful approach at agiven site. Such an approach could be used with data from either Shacklette andBoerngen (1984) or Bradford et al. (1996), which are both relevant for California.These sources provide maps indicating where samples contained in the dataset werecollected.

The second refinement to generic background analysis focuses on the assumptionthat elemental concentrations should be relatively consistent for a given soil type,regardless of location. This requires that the generic background data be reportedin such a way that lithologies can be identified. Because lithologic information is notprovided by Shacklette and Boerngen (1984), such an approach cannot be used ifthis source is used for background. However, the Bradford et al. (1996) report cate-gorizes samples by both soil type and county. In addition, lithology-specific statisti-cal analyses are provided. For soils nationwide, Kabata-Pendias and Pendias (1985)provide lithology-specific data, which have been compiled in the Risk AssessmentInformation System (RAIS) by the Oak Ridge National Laboratory (ORNL 2002).This database provides no information on sampling location for the specified soiltypes. These options will be further evaluated in the discussion section of this paper.

In recent years an increasing number of regional background studies have beenconducted in response to the continuing need to evaluate and identify contam-inated properties. In California alone, in addition to the Bradford et al. (1986)report, which is statewide, there are several studies of smaller regions. For soil back-ground in the San Francisco Bay Area, two studies are particularly useful. Scott (1991)studied background for inorganics in northern Santa Clara County. In addition,the Lawrence Berkeley Laboratory (LBNL 1995) provided a very useful, lithology-specific background study for the area east of San Francisco Bay. This study includeslithology-specific statistical analyses. The City of Oakland (2002) has also compiledbackground data specific to Oakland, and background levels of arsenic, nickel, andselenium in the southern San Francisco Bay Area have been evaluated (Andersen1998). If necessary, region- and lithology-specific data can be used to augment site-specific background data, where appropriate. USEPA (2000) has provided a goodoverview of regional and state background soil studies nationwide, in addition to anationwide database of background data obtained from Superfund sites (referred toas the CERCLIS database). One regional study found a reasonably good correlationbetween state-specific and nationwide USGS data (Alkhatib and O’Connor 1998;Shacklette and Boerngen 1984). Although the authors found that geometric meansof common Rhode Island metals were substantially lower than the correspondingUSGS regional results, a review of the entire dataset indicated that almost all theRhode Island data fell within the range of USGS results.

The second major source of background data is true site-specific data collection.On first glance, this would provide the most useful information upon which to con-duct a background evaluation for a given site. However, this is a more costly approach


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than using generic background data, and requires that appropriate backgroundlocations can be identified. Although this may be straightforward at some sites (e .g .,at an undeveloped site with mine tailings, background could be collected from anarea upgradient to the tailings), it is very complex at other sites (e .g ., a paved indus-trial site, with fill in many areas, that is surrounded by other industrial sites that mayhave had historical air emissions). The value of collecting site-specific backgrounddata should, therefore, depend mostly on the goals of the assessment in order toweigh the various factors discussed above. Whether site-specific or regional back-ground data are used, naturally occurring levels of inorganics in soil may exceedecological screening levels.

Types of Background Data

In addition to the source of background data, the type of background data alsoneeds to be considered. For example, metals are naturally occurring, but some areessential nutrients for animals or plants (e .g ., zinc, selenium, and iron). When col-lecting background data on elements, should essential elements be included? Thisquestion needs to be addressed at each site. If the site of interest is a former agri-cultural area, for example, background considerations may not focus on essentialnutrients, because pesticides used in agriculture typically exclude known essentialnutrients in their formulations. However, such sites may demonstrate elevated levelsof nitrates or other chemicals used as fertilizers. At such a site, background concen-trations of arsenic and other metals should be evaluated because they have been usedin pesticide formulations, and could, therefore, be site-related chemicals. However,for a site that was a former metals recycling facility, iron and zinc are likely to beelevated from site activities. For such a site, background concentrations of essentialnutrients could be useful to determine the portion of these elemental concentrationsthat can be ascribed to site-specific activities. These data could be used to identifywhether there could be toxicity from an excess of an essential nutrient.

The depth of background samples is also an important consideration. Ambientlevels are typically not consistent with depth, so the background concentrationswill likely change depending on the depth of the samples. Site and backgroundsamples should be collected at the same depths to ensure this variable is adequatelyaddressed.

When considering anthropogenic ambient levels (e .g ., PAHs), the issue becomesmore complex. PAHs in low concentrations are ubiquitous in surficial soils through-out the country due to anthropogenic sources like automobile emissions and forestfires. When there is a need to identify anthropogenic background concentrations,the selection of background locations becomes critical. Although it may be tempt-ing to collect shallow soil samples from the median strip of an interstate to showrelatively high “background” levels, this may not be an appropriate approach if thesite of interest is not adjacent to the same interstate. The selection of backgroundlocations needs to be consistent with the habitat and soil types at the site of interest,and as close to the site as possible in an unimpacted area for maximum value. Thiscould lead to collection of metals background from one location, and collection ofanthropogenic background at a completely different location.


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Regulatory Environment

Overlying these issues is the regulatory agency overseeing the risk assessment ac-tivities at a specific site. The agency may not allow for background evaluation ofanthropogenic chemicals. They may require involvement with identifying relevantbackground concentrations, and could reject the assessment if they are not involved.This adds both time and cost to the assessment. Often, the more agencies and trusteesare involved, the more time and money are needed to reach a consensus regardingbackground.

The following discussion provides an example of collection and application ofsite-specific background data. Fort Ord, a former light infantry U.S. Army basein Monterey County, California, underwent a Remedial Investigation/Feasibility(RI/FS) study in the 1990s under the requirements of the Comprehensive Environ-mental Response, Compensation, and Liability Act (CERCLA). Because historicalbase uses indicated that inorganic chemicals could have been released to the envi-ronment, a background study was conducted for the base to generate backgrounddata for use in the RI/FS. Lithology- and depth-specific background concentrationsfor metals were evaluated for thirteen priority pollutant metals. The backgrounddataset was adjusted for outliers, whose inclusion could have artificially raised therange of ambient concentrations, resulting in a less conservative analysis. Four sep-arate sets of background metal concentrations were established for pertinent soiltypes and depths.

In the baseline phase of the risk assessment, site-specific concentrations (i.e., thoseassumed to be associated with historical releases) were compared to the maximumbackground concentration for the pertinent soil type and depth interval. In the post-remediation phase of the risk assessment, more complex background evaluationswere conducted including a statistical comparison between site and backgrounddatasets to identify whether or not they were significantly different (i.e., WilcoxonRank Sum test). This background study demonstrated 1) the necessity for site-specificdata that are also lithology-specific, at a large site with varying lithology; 2) the useof a simple comparison of site and background data, and 3) the application ofmore refined analyses to identify which detected chemicals were present at ambientconcentrations.

Identification of Representative Background Values

Once the details regarding the type of background data, nature of chemicals tobe addressed, and site-specific sampling locations and depths are identified, thereis still the issue of identifying a representative background concentration for eachchemical. There are many approaches in the literature, and developed by specificagencies, for calculating representative background values. These approaches rangefrom simply using a mean or maximum value of background concentrations, to usingthe mean plus 2 or 3 times the standard deviation of the dataset, and use of con-fidence or tolerance limits. More refined statistical analyses including parametricand non-parametric comparisons based on the distribution of site and backgrounddatasets are variously recommended (USEPA 2002b; CalEPA 1997). Nonparametrictests are often preferred in background evaluations because they are independentof the true underlying distribution. This enables direct comparison of datasets with


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different distributions. Regardless of the approach used, interaction with the reg-ulatory agency is critical to ensure that the results will be acceptable for makingremediation decisions at a site.

DISCUSSION

USEPA (2001) stated in a recent Eco Update: “While contaminants of concern maybe removed from further assessment through comparison with toxicological bench-marks, comparison with background levels generally cannot be used to remove con-taminants of concern owing to the need to fully characterize site risk.” There aretwo problems with this statement. First, soil cannot be cleaned up to concentrationsbelow background. This is true whether or not background concentrations (alone,or combined with site concentrations) are associated with an elevation in risk ata site. Second, ecological screening levels for soil may be lower than background.Contrary to human health risk assessment, where risk-based screening levels andsoil remediation goals generally exceed background (with the frequent exceptionof arsenic), the closeness of ecological risk-based levels and background concen-trations makes it essential to conduct a detailed background analysis for ecologicalrisk assessments of metals in soils (and other media such as sediments). As with ar-senic for human health, USEPA has repeatedly communicated that, although thereis no practical means of cleaning up below background, effective and proactive riskcommunication requires discussion of these risks and uncertainties.

Value of Background Data in Ecological Risk Assessment

Given this background and USEPA’s statement, what then is the value of back-ground data in ecological risk assessment? The answer, of course, is that it depends.It depends on the goals and objectives of the project. A background evaluation isnot done to exclude chemicals that may represent a toxicity issue, but rather to savethe expense of including background level chemicals in a quantitative ecologicalrisk assessment (ERA). While this expense is minimal for a human health risk as-sessment, where toxicity values and exposure assumptions are readily available, itis considerably more labor-intensive for an ERA, for which toxicity values for targetreceptors may need to be derived from the literature, and exposure information mayneed to be compiled for the relevant receptors. In these situations, it is often morecost-effective to conduct a focused, site-specific background evaluation in advanceof implementing a full, quantitative ERA.

Understanding this concern, USEPA (2000) conducted an extensive backgroundevaluation as part of their development of ecological soil screening levels (Eco-SSLs). CERCLIS databases associated with Superfund sites, USGS reports, state-specific studies, and the published literature were consulted to develop a backgrounddatabase. This database provides the range and statistical values (both arithmetic andgeometric) for seventeen metals nationwide. Soil types and sampling locations werenot specified. Therefore, this dataset is of limited utility in conducting site-specificrisk assessments. However, USEPA, in comparing Eco-SSLs to national background,demonstrated that various regions of the United States have naturally occurringsoil metal concentrations greater than Eco-SSLs. Based on this observation, USEPA


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(2000) emphasized that background analyses should be conducted in such areas todistinguish site-related and naturally occurring inorganics.

For instance, the lowest Eco-SSL for chromium, which corresponds to plant toxic-ity (5 milligrams per kilogram [mg/kg]; chromium not speciated), is lower than thenationwide arithmetic mean background value for chromium of 24.8 mg/kg (USEPA2000). This brings into question the appropriateness of Eco-SSLs. If chromium con-centrations above 5 mg/kg were indeed toxic, then the majority of plants acrossthe country should show signs of toxicity. To reduce these concerns, USEPA (2000)emphasized the importance of background by providing a comparison to nation-wide, eastern, and western background levels alongside several published Eco-SSLs.Where background exceeds ecological screening levels, USEPA (2000) recommendsan alternative screening-level approach to modify the Eco-SSLs on the basis of site-specific consideration such as receptors, exposure assumptions, dietary composition,area use factors, site characteristics, and bioavailability. Alternately, a site-specific riskassessment can be conducted with the development of receptor-specific hazard quo-tients (HQs). Whichever approach is taken, some kind of background evaluationshould be conducted, particularly if HQs are elevated and site remediation is con-sidered. From our perspective, Eco-SSLs that are below background, requiring asite-specific background evaluation wherever the pertinent element is detected, arenot a valuable addition to an ecological risk assessor’s toolbox. Instead, they compli-cate the process by requiring modification of “published screening values,” whichis typically difficult and time-consuming to both conduct and obtain regulatory ap-proval. This issue is further highlighted below.

Ecological soil screening levels including Eco-SSLs and Preliminary RemediationGoals (PRGs) were compiled for inorganic chemicals listed in Table 1. The metalsrepresented in the table are those for which USEPA (2000) compiled backgrounddata. The lowest published screening level is provided for each metal, whether devel-oped for plant, soil invertebrate, or wildlife receptors. The PRGs were developed forsoil by Efroymson et al. (1997) for the Oak Ridge National Laboratory. Efroymsonet al. stated that soil PRGs for ecological receptors are more uncertain than those forwater and sediments due to limitations in the types of organisms (i.e., plants, inverte-brates, etc.), number of studies, biological endpoints, and soil-mammal or soil-plantuptake models. As a result, confidence in the PRGs is generally low. Limitations inthe database also affects availability of Eco-SSLs, most of which were published foronly one or two receptor types. Although a detailed discussion of the scope andmethods used to develop Eco-SSLs and PRGs is beyond the scope of this paper, thelimitations in the database led to use of uncertainty factors that are overly conserva-tive. As a result, the Eco-SSLs are also more conservative (and therefore lower) thanthey might be if a stronger database of toxicity information were available.

Nationwide background levels were also compiled in Table 1 from Schacklette andBoerngen (1984) and statewide levels for California were compiled from Bradfordet al. (1996). As shown in Table 1, the Eco-SSL for only chromium was lower thanall of these published background values. Compared with Eco-SSLs, a greater num-ber of the PRGs (six) were lower than at least one of the listed geometric meanbackground values: barium, chromium, nickel, selenium, vanadium, and zinc. Bycontrast, a comparison to arithmetic mean background values for the sources inTable 1 demonstrated a greater number of screening levels lower than background.


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Table 1. Comparison of reported background levels in soil withecological screening levels.

Ecological screeningBackground concentration (mg/kg) level (mg/kg)d

Analytes Nationwidea Californiab Nationwidec Eco-SSLd PRGe

Ag — 0.41 0.051 — 2Al 47,000 71,000 7148 — —As 5.2 2.8 2.25 37 9.9Ba 440 468 20.3 — 283Be 0.63 1.1 0.36 — 10Cd — 0.26 0.021 29 4Co 6.7 12.6 8.93 32 20Cr 37 76 10.5 5 0.4Cu 17 24 7.45 61 60Fe 18,000 34,000 3211 — —Mn 330 592 62.5 — —Ni 13 36 5.69 — 30Pb 16 21.7 13.6 — 40.5Sb 0.48 0.5 0.332 21 5Se 0.26 0.03 0.543 — 0.21V 58 101 12.6 — 2Zn 48 145 18.1 120 8.5

Eco-SSL-Ecological Soil Screening Level.PRG-Preliminary Remediation Goal.aGeometric mean value. From: Shacklette and Boerngen (1984).b Geometric mean value. From: Bradford et al. (1996).c Geometric mean value. From: USEPA (2000).dLowest screening level for all receptors. From: USEPA (2000).e From: Efroymson et al. (1997).

This illustrates the importance of utilizing representative background values basedon the appropriate data distribution.

Similar to USEPA (2000), Efroymson et al. (1997) recommended adapting thePRGs using site-specific variables where problems were encountered (such asbackground levels greater than PRGs). These modifications could be based on landuse and exposure assumptions, habitat considerations such as habitable fraction ofarea and habitat quality, soil parameters, and other factors. The cost-benefit issuesassociated with such additional evaluations, compared to collection of site-specificbackground data, are discussed later in this paper.

Figure 1 graphically illustrates the comparison of ecological screening levelsto background concentrations. Background data from the three sources listed inTable 1 were normalized by averaging the geometric means from the three back-ground datasets against the lowest background concentration in Table 1 (i.e., forcadmium). This allows for direct comparison of ecological screening values withbackground levels using the same scale. The ratios of the normalized backgroundconcentrations published for each metal to the lower of the Eco-SSLs and PRGsare shown as histograms in the figure. If a histogram bar is above the normalized


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Figure 1. Comparison of normalized soil background concentrations and ecolog-ical screening levels.

background concentration, this indicates that the ecological screening level is be-low the background concentration. The higher the bar is above the backgroundline, the greater the discrepancy between the screening level and background. Mostecological screening levels (i.e., arsenic, barium, chromium, cobalt, copper, nickel,lead, selenium, vanadium, and zinc) are less than the normalized background value.For chromium, vanadium, and zinc, the ratios range from 58 to 730. This means thatthe ecological screening levels for these three elements are between one and threeorders of magnitude lower than “typical” background levels. Screening levels of onlyfour elements, antimony, beryllium, cadmium, and silver, are higher than normal-ized background (i.e., ratio of background to screening level less than one). Thisimplies that use of literature-based background concentrations is unlikely to be par-ticularly useful for most ecological risk assessments where elements other than thesefour are detected.

Cost-Benefit Considerations

Finally, cost-benefit considerations should play a primary role in determining thetype and nature of background evaluation to conduct at a given site. For example,at one site, metals in shallow soil were the primary chemicals of concern. After con-ducting the data evaluation and excluding outliers, twelve metals were detected inat least one of 33 samples from the site. Based on cursory inspection of the site con-centrations compared with literature-based background concentrations, 10 of the 12metals would have been retained as chemicals of potential concern. Therefore thescope of the risk assessment would include quantitative evaluation of 10 metals. Dueto the low ecological screening values (as shown in Table 1), it was likely that the re-sults of this assessment would indicate a potential problem. However, concentrations


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were not substantially greater than literature-based background. We wanted to avoidmaking a Type I error at this site (i.e., false positive), leading to remediation andassociated costs that might not be warranted to protect the environment. Therefore,we decided to collect 13 site-specific background samples to refine the chemical ofpotential concern selection process. This effort cost under $10,000 (including mo-bilization costs and analytical fees). Based on the detailed background evaluationconducted using these site-specific background concentrations, only two metals (ar-senic and nickel) were shown to be above local background. Therefore, collectionand analysis of site-specific background reduced the number of chemicals requiringquantitative evaluation from 10 to 2. This led to a cost savings in conducting therisk assessment, and greatly reduced the extent of site remediation compared withthe generic approach. If the generic approach were used, additional costs would beneeded to compile receptor-specific exposure data, which could involve activitiessuch as animal trapping, tissue sampling, and toxicity testing, and result in muchhigher costs than would be incurred conducting a site-specific background analysis.This illustrates that overall project costs need to be weighed against both the incre-mental cost of conducting a site-specific background evaluation, and the potentialbenefits of such an evaluation.

CONCLUSIONS AND RECOMMENDATIONS

The goal of “conducting a background evaluation,” which is to separately identifychemicals present at a site at naturally-occurring concentrations and those presentabove background, is quite straightforward in principal but complex in practice.As discussed above, many issues need to be addressed for each site before makingdecisions about the need for, and value of, background analysis. Because each siteneeds to be individually evaluated, we can only provide general recommendationsthat should be considered when evaluating background-related concerns.

� Consider the goals and objectives of the project before deciding how (and if)to evaluate background

� When using literature-based background concentrations, match the back-ground soil lithology and sampling depth with samples from the site, and uselocal data wherever available

� Identifying a reference location to collect site-specific background data can becontentious and difficult, and should be done as part of project planning to en-sure site-specific background data are usable and relevant to the site of interest

� Regulatory and community input should be incorporated into decisions regard-ing site-specific background data collection; the importance of this input growswith the size and complexity of the site

� If metals are the focus of an ecological risk assessment, literature-based back-ground data are not likely to provide much value because ecological screeninglevels are frequently below these concentrations

� Literature-based background may be useful as a screening step, particularlyfor sites where antimony, beryllium, cadmium, and silver are detected becausegeneric background is below ecological screening levels for these metals


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� If project resources are limited, conducting a site-specific background evalu-ation may be cost-effective when viewed in light of the overall project (e .g .,remediation cost savings versus additional data collection and analysis, espe-cially related to collection of receptor-specific exposure or toxicity data).

Conducting a well designed, site-specific background evaluation will always provideuseful and defensible data when conducting an ecological risk assessment; it is therelative value of the data to the project goals that should drive decisions regardingevaluation of background.

REFERENCES

Alkhatib E and O’Connor T. 1998. Background Levels of Priority Pollutants in Soil. AmericanEnvironmental Laboratory 10:6–9. International Scientific Communications, Inc., Shelton,CT, USA

Andersen DW. 1998. Natural Levels of Nickel, Selenium, and Arsenic in the Southern SanFrancisco Bay Area. Department of Geology, San Jose State University, San Jose, CA, USA

Bradford GR, Chang AC, Page AL, et al. 1996. Background Concentrations of Trace and MajorElements in California Soils. Kearney Foundation of Soil Science, Division of Agricultureand Natural Resources, University of California, Riverside, CA, USA

CalEPA (California Environmental Protection Agency). 1997. Selecting Inorganic Consti-tuents as Chemicals of Potential Concern at Risk Assessments at Hazardous Waste Sitesand Permitted Facilities. Final Policy. Human and Ecological Risk Division, Department ofToxic Substances Control, Sacramento, CA, USA

City of Oakland. 2002. Survey of Background Metal Concentration Studies. Oakland UrbanLand Redevelopment Program, Department of Public Works, Oakland, CA. Available athttp://www.oaklandpw.com/ulrprogram/metals.pdf

Efromyson RA, Suter II GW, Sample BE, et al. 1997. Preliminary Remediation Goals for Ecolog-ical Endpoints. ES/ER/TM-162/R2. US Department of Energy, Office of EnvironmentalManagement, Oak Ridge National Laboratory, Oak Ridge, TN, USA

Kabata-Pendias A and Pendias H. 1985. Trace Elements in Soils and Plants. CRC Press, Inc.,Boca Raton, FL, USA. Cited in: ORNL, 2002

LBNL (Lawrence Berkeley National Laboratory). 1995. Protocol for DeterminingBackground Concentrations of Metals in Soil at Lawrence Berkeley National Laboratory.University of California, Environmental Restoration Program, Berkeley, CA, USA

ORNL (Oak Ridge National Laboratory). 2002. Risk Assessment Information System. GenericSoil Background Values. Available at http://risk/lsd.ornl.gov/cgi-bin/background/generic

Scott CM. 1991. Background metal concentrations in soils in northern Santa Clara County,California. In: Recent Geologic Studies in the San Francisco Bay Area. Pacific Section,Society for Sedimentary Geology, Book 76. SEPM, 6128 East 38th Street, Suite 308, Tulsa,OK 74135-5814, USA. Available at: www.sepm.org

Shacklette HT and Boerngen JG. 1984. Element Concentrations in Soils and Other SurficialMaterials of the Conterminous United States. United States Geological Survey ProfessionalPaper 1270. United States Government Printing Office, Washington, DC, USA

USEPA (US Environmental Protection Agency). 2000. Ecological Soil Screening Level Guid-ance (ECO-SSL). Office of Emergency and Remedial Response, Washington, DC, USA.Available at: http://www.epa.gov/superfund/programs/risk/ecorisk/guidance.pdf

USEPA (US Environmental Protection Agency). 2001. ECO Update. The Role of Screening-Level Risk Assessment and Refining Contaminants of Concern in Baseline Ecological Risk


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Assessments. Intermittent Bulletin. Publication 9345.0-14, EPA 540/F-01/014. Office ofSolid Waste and Emergency Response, Washington, DC, USA

USEPA (US Environmental Protection Agency). 2002a. Transmittal of Policy Statement:Role of Background in the CERCLA Cleanup Program. Memorandum from Michael B.Cook (OERR) to Superfund National Policy Managers. Office of Solid Waste and Emer-gency Response, Washington, DC, USA. OSWER 9285.6-07P. May 1

USEPA (US Environmental Protection Agency). 2002b. Guidance for Comparing Backgroundand Chemical Concentrations in Soil for CERCLA Sites. Office of Emergency and RemedialResponse, EPA 540-R-01-003, OSWER 9285.7-41, Washington, DC, USA. September

USEPA (US Environmental Protection Agency). 2002c. Draft Action Plan. Development ofa Framework for Metals Assessment and Guidance for Characterizing and Ranking Met-als. EPA/630/P02/003A. External Review Draft, Office of Research and Development,Washington, DC, USA


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applications of background data in ecological risk assessment: various shades of gray

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