Published in J. Environ. Qual. 34:114-121 (2005).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
The Evolving Science of Chemical Risk Assessment for Land-Applied Biosolids
Rosalind A. Schoof* and
Dana Houkal
Integral Consulting, Inc., 7900 SE 28th Street, Suite 300, Mercer Island, WA 98040
* Corresponding author (rschoof{at}integral-corp.com)
Received for publication March 8, 2004.
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ABSTRACT
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Biosolids, effluents, and manures are widely applied to agricultural land and other land with varying degrees of pretreatment or control. Regulations governing land application of biosolids take several broad forms in different countries, including limitations based on rates that do not lead to increases in background chemical concentrations or risk assessment approaches such as those used in the United States. Risk assessment is a process that is inherently limited by currently available information and practices, and consequently, risk-based land application limits must be reevaluated periodically. For complex mixtures such as biosolids, three principal categories of information will be affected by changing practices and scientific advances: (i) chemical constituents present in the material, (ii) the nature of expected exposures, and (iii) toxicity of the chemical constituents. New analytical methods and lower detection limits will affect chemical identification in wastes. Approaches to exposure assessment, such as increasing emphasis on probabilistic analyses, will continue to evolve, and exposure assumptions will change as new studies provide better data on factors such as soil ingestion, plant uptake of chemicals, and bioavailability of chemicals in soil. Similarly, toxicity assessments will be updated as new studies are conducted. The evolving science over the past decade is illustrated by comparing approaches used by the USEPA to assess human health and ecological risks for the Part 503 rule compared with the more recent evaluation of dioxins and related compounds in biosolids. While risks of chemicals in land-applied biosolids and other residuals need to be periodically reevaluated, such reevaluations may take forms other than full risk assessments.
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INTRODUCTION
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LAND APPLICATION of various forms of human and animal waste has been widely practiced around the world for millennia as one of the few practical methods of managing these materials. As population density and urbanization have increased it has become necessary to regulate land application of sewage sludge (referred to herein as biosolids) (National Research Council, 2002). Land application of other organic residuals is also coming under increasing scrutiny (Powers, 2003; USEPA, 2002a). Regulations governing land application typically take several broad forms, including risk assessment approaches or, alternatively, limitations based on rates that do not lead to increases in background chemical concentrations (McGrath et al., 1994).
The background-based approach is a form of the precautionary principle that may view any increase in chemical concentrations as unacceptable regardless of potential for harm (Marchant, 2003). Risk assessment–based approaches consider potential for harm in setting standards. Reliance on risk assessment is not inherently inconsistent with a precautionary approach to environmental decision-making; however, risk assessment alone does not provide all of the information needed to determine an appropriate degree of precaution. Marchant (2003) identifies nine attributes of individual risks that could help determine the appropriate level of precaution. Four of these attributes may be quantified through the use of risk assessment (i.e., reversibility, magnitude of possible consequences, probability of occurrence, and the amount and types of uncertainty associated with the risk). The other five attributes must be addressed as part of the risk management decision that considers the results of the risk assessment (i.e., societal benefits of the risk-creating activity, difficulty and cost of reducing the risk, potential alternatives to the risk-creating activity, potential risk–risk tradeoffs, and public perception of risk). This paper describes risk-based approaches for regulating land application of biosolids, and provides an analysis of the evolution of applicable risk assessment methods; however, we contend that a discussion of the appropriate risk-based limits for land-applied biosolids and other residuals should include explicit evaluation of all of the attributes identified by Marchant (2003).
While this paper focuses on the risks associated with land application of biosolids, it is also appropriate to consider the risks and feasibility of alternative management approaches. Failure to treat sewage has very clear associated risks. Ocean dumping of sewage sludge has been banned (National Research Council, 2002), and incineration is often perceived by the public to have high risks regardless of the degree to which emissions are controlled. Land-filling risks are largely controllable, but also suffer from public perception issues, and many areas would not have sufficient capacity to serve as a long-term option. Consideration of risks associated with land application should also be balanced by evaluation of biosolids as a potential resource. However, any consideration of land application benefits must clearly identify the "receptors" for benefits as well as risks. For example, use of biosolids as an amendment for contaminated soils to reduce metal bioavailability may have a benefit that outweighs risk for the same receptors. An example of this is the application of biosolids to soils highly contaminated with metals from mining or smelting operations. If amendment of these soils with biosolids reduces bioavailability and exposures of human and ecological receptors to the metals in the contaminated soils, overall risks to these receptors may be reduced above the cumulative risks associated with the biosolids application. In contrast, the benefit of increased agricultural productivity for a farmer may come at the cost of increased risks to neighboring receptors who do not receive the benefit.
The USEPA's current regulation governing land application of sewage sludge was promulgated in 1993 in 40 CFR Part 503 [under Section 405(d) of the Clean Water Act], and is commonly referred to as the Part 503 rule. The Part 503 rule was designed to protect human health and the environment from adverse effects of chemicals and to reduce pathogens and attraction of vectors that induce disease. The Part 503 rule land application limits for chemicals were based on a risk assessment (described in USEPA, 1995a). As a result of a 1999 proposal to add dioxins and related chemicals to the regulation, the USEPA (2002b) conducted a new risk assessment for these chemicals that incorporated new scientific information and risk assessment methodology. A probabilistic approach was used in a multipathway analysis of human health risks, in contrast with the deterministic, single-pathway method used to derive the existing land application limits. The new dioxin risk assessment can also be used to illustrate the evolution of the science of ecological risk assessment over the past decade, including changes such as consideration of multiple simultaneous exposure routes for individual receptors and altered perspectives on assessment of agricultural receptors vs. wild receptors. In the following sections, differences in the two risk assessments are examined to exemplify temporal changes in the risk assessment process.
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HUMAN HEALTH RISK ASSESSMENT
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Although the Part 503 rule was issued in 1993, the human health risk assessment approach was selected much earlier, in a similar timeframe to the USEPA's development of the Risk Assessment Guidance for Superfund (USEPA, 1989) that has served as the framework for most subsequent Agency risk assessments. Since the early 1990s, the USEPA has issued many volumes of guidance and data applicable to human health risk assessment. Exposure assessments have been markedly altered by the introduction of probabilistic approaches (USEPA, 1997b, 2001), as well as the massive compilation and evaluation of surveys of human behaviors and characteristics used to derive default exposure parameters such as soil ingestion, food consumption, and inhalation rates in the Exposure Factors Handbook (USEPA, 1997a). During this timeframe, methods for toxicity assessment have also undergone substantial changes, including development of benchmark dose approaches, in which the point of departure from the range of observed toxic doses is identified and used in extrapolating predicted toxicity (USEPA, 1995b) and new cancer risk assessment guidance (USEPA, 1999).
Guidance for conducting probabilistic risk assessment (PRA) for both human and ecological receptors was developed for the Superfund program (USEPA, 2001), but is broadly applicable in risk assessments across USEPA programs. The guidance focuses on Monte Carlo analysis as a method of quantifying variability and uncertainty in exposure assessment by substituting probability distributions for single point estimates for individual parameters such as soil ingestion rates and exposure duration (Fig. 1)
. This approach can minimize many of the concerns related to overestimating exposure due to the compounding nature of multiple assumptions of upper-bound point estimates for individual parameters (Finley and Paustenbach, 1994). A tiered approach is recommended by the USEPA (2001), beginning with a point estimate analysis or deterministic risk assessment, and progressing to probabilistic risk assessment as needed to satisfy decision-making requirements.
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Fig. 1. Probabilistic exposure assessment (modified from USEPA, 2001). In a probabilistic exposure assessment, distributions for each input parameter (e.g., V1, V2, to Vn) are combined to yield an overall exposure distribution. This distribution is then used to identify the reasonable maximum exposure (RME), which is defined as exposures corresponding to the 90th to 99.9th percentiles. This definition of RME is consistent between deterministic and probabilistic risk assessment.
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The Part 503 rule risk assessment was deterministic (i.e., point estimates of risk were generated). Independent risk estimates were generated for a highly exposed individual for each exposure pathway identified. Nine of the 14 exposure pathways evaluated considered human receptors, including home gardeners, a child ingesting biosolids, consumers of produce and livestock, residents inhaling airborne vapors or particulates or ingesting ground water, or recreational fishermen ingesting fish (USEPA, 1995a) (Table 1). An initial screening evaluation had limited the risk assessment to 10 inorganic chemicals and 12 organic chemicals or groups of structurally related organic chemicals. Regulations were developed only for the inorganic chemicals. All of the organic chemicals were screened out based on one or more of three criteria, including a previous ban on chemical production, limited detection in the 2001 National Sewage Sludge Survey, and/or reported concentrations well below risk-based limits (USEPA, 1995a). For each of the selected inorganic chemicals, regulations for land application were based on the outcome of the pathway showing the greatest risk. This approach has been criticized for potentially underestimating aggregate risks to a person exposed by more than one pathway (Harrison et al., 1999). However, for many of the chemicals evaluated, one pathway contributed a dominant share of the aggregate risks, minimizing this concern. For example, Pathway 3 (child ingesting biosolids) was the "limiting pathway" for five of the ten inorganic pollutants, and for eight of the ten inorganics, the Pathway 3 pollutant limit was more than an order of magnitude lower than the limits for other pathways for human receptors (USEPA, 1995a). This dominance of one pathway indicates that for those pollutants calculation of aggregate risks would not have led to lower application limits.
In contrast, the USEPA's recent human health risk assessment for dioxins and related chemicals used a probabilistic approach to evaluate risks to a farm family exposed to biosolids constituents by multiple pathways (USEPA, 2002b). This evaluation included 29 dioxin, dibenzofuran, and polychlorinated biphenyl (PCB) congeners (referred to herein as dioxins). A conceptual site model was developed to describe all the potential exposure pathways for these receptors (Fig. 2)
. Farm family children and adults were assumed to be exposed via inhalation of ambient air, and ingestion of soil, above- and belowground produce, beef and dairy products, and poultry and egg products. Potential exposure of breastfeeding infants and recreational fishers was also examined. These pathways include most of the general pathways included in the earlier Part 503 rule risk assessment, but often with substantially different assumptions. Several pathways not expected to be significant for dioxins were omitted, including ingestion of surface water and ground water. The USEPA's assumption that each receptor will be exposed by all of the identified exposure pathways has been repeatedly criticized as being overly conservative; however, as will be shown below, using this multipathway approach may not significantly affect the USEPA's interpretation of the results.
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Fig. 2. Farm family conceptual model (from USEPA, 2002b). Contaminants from biosolids applied to farmland are assumed to migrate from the application area along pathways indicated by arrows and by atmospheric deposition. The farm family is then assumed to contact contaminants either in the application area, in secondary media to which contaminants have migrated, and in biota that have contacted and absorbed the contaminants.
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Distributions of dioxin concentrations were obtained from the National Sewage Sludge Survey (USEPA, 2002b). The United States was divided into 41 climatic regions to provide distributions of climatic and meteorological data. Exposure point concentration distributions were then determined using source partition modeling of constituent releases, fate and transport modeling, and food chain models (Fig. 3)
. These distributions were combined with distributions for exposure factors to yield intake or dose distributions for various receptors.
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Fig. 3. Media concentration module (modified from USEPA, 2002b). The source partition model used the distribution of reported dioxin concentrations in sewage sludge to predict agricultural soil concentrations, and then used fate and transport and food chain models to estimate distributions of concentrations in other environmental media and in food. (Met data = meteorological data.)
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Toxic equivalency factors (TEFs) were used to adjust the concentration of each congener to reflect its toxic potency relative to the most potent congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). The resultant concentrations are termed toxic equivalency quotients (TEQs). Cancer risks are typically calculated by combining the intake or dose estimates with cancer slope factors to yield risk estimates. The oral cancer slope factor used was 1.56 x 10–1 (pg/kg d)–1. This value is presently unsettled due to scientific disputes regarding a reassessment issued by the USEPA (2000). Total multipathway risks were estimated to be one in a million (i.e., 1 x 10–6) for both adults and children at the 50th percentile, and 2 in 100000 (i.e., 2 x 10–5) and 1 in 100000 (1 x 10–5) for adults and children, respectively, at the 95th percentile (Table 2). The great majority of the risk was attributable to beef and milk ingestion. The fact that two exposure pathways contributed most of the risk suggests that the effect of adding together multiple exposure pathways that would not all be experienced by a single individual did not unduly influence the outcome of the risk assessment.
It is also noteworthy that the USEPA (68 FR 61084-61096) estimates the potentially affected population to be approximately 1600, with an upper bound of 11200, and that the predicted incremental risk for the 95th percentile of this highly exposed population would be only 10% of their risk attributable to background exposure to dioxins. Based on the results of the human health and ecological risk assessments (USEPA, 2002b), on 17 Oct. 2003, the USEPA (68 FR 61084-61096) announced a final decision not to regulate dioxins in land-applied sewage sludge, concluding that the information available on dioxin exposures, toxicity, and cancer risks supported a decision that no numeric limits or management practices were required to adequately protect human health and the environment from the adverse effects of dioxins in land-applied sewage sludge.
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ECOLOGICAL RISK ASSESSMENT
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There has been substantial progress in defining the methods and conduct of ecological risk assessment over the past 10 to 15 yr. The USEPA completed the revised risk assessments for the Part 503 rule in 1990–1992 at about the same time that it was developing the Framework for Ecological Risk Assessment (USEPA, 1992). Specific guidelines for planning and conducting ecological risk assessments were not finalized until 1998 (USEPA, 1998). These guidelines were followed in the recent ecological risk assessment for dioxins in sewage sludge (USEPA, 2002b). The approaches followed in the 1993 Part 503 rule and the new dioxin study are described below to illustrate the evolving approach to ecological risk assessment.
Five of 14 exposure pathways evaluated in the Part 503 rule considered ecological receptors (Table 3). In addition to agricultural lands, nonagricultural lands evaluated in the risk assessment included forest and range land, soil reclamation sites, and public contact sites (e.g., parks, golf courses). Animals evaluated as potential receptors included agricultural livestock (e.g., cattle, sheep, chickens) for Pathways 6 and 7, earthworms for Pathway 9 (evaluated because of data availability), and shrew, mole, and chicken for Pathway 10. The risk analysis yielded an allowable concentration of pollutant in the animal's diet or in the soil to derive a reference application rate of pollutant (kg pollutant per hectare) or a reference concentration in sewage sludge. For plants (Pathway 8) two approaches were used to derive reference application rates for a pollutant (kg pollutant per hectare). The lowest application rate of the two approaches was selected as the plant benchmark.
For Pathways 6 and 7, the USEPA used an allowable concentration of pollutant in the animal's diet to establish a reference application rate. For Pathway 6, allowable concentrations were obtained from the literature and soil-to-plant uptake regressions were derived to calculate the reference application rate. For plants (Pathway 8), the initial approach used results of laboratory plant toxicity tests at which a 50% yield reduction occurs to establish allowable plant tissue concentrations. This approach was hampered by reliance on toxicity data from corn, which is relatively insensitive to many inorganic pollutants. The second approach for plants was based on toxicity thresholds set at the first detectable yield reductions using literature data for sensitive plant species. Toxicity to soil organisms (Pathway 9) was assessed using results of published earthworm toxicity tests to establish a no-observed-adverse-effects level (NOAEL) in soil. For Pathway 10, an allowable concentration of pollutant in the animal's diet was used in conjunction with literature-based soil-to-earthworm bioaccumulation factors and chemical-specific bioavailability factors to establish a reference concentration in soil.
The approaches for assessing ecological risk were significantly modified in the dioxin study (USEPA, 2002b). A screening-level ecological risk assessment was conducted following the USEPA (1998) framework and was organized into problem formulation, analysis, and risk characterization phases. Habitats included in the assessment were field crop and pasture areas where biosolids were applied directly and freshwater body margins affected by migration of chemicals from adjacent fields (Fig. 4)
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Fig. 4. Ecological conceptual model for agricultural application (from USEPA, 2002b). Contaminants from biosolids applied to farmland are assumed to migrate from the application area along pathways indicated by arrows and by atmospheric deposition. Ecological receptors (see inserts) are then assumed to contact contaminants by the illustrated exposure routes.
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Principle receptors were avian and mammalian wildlife, chosen because dioxins are highly bioaccumulative and effects to higher trophic level receptors such as these are more likely. Toxicity and exposure information were available for a variety of avian and mammalian wildlife that could serve as appropriate receptors. Other receptors such as plants and soil invertebrates that had been the focus of the earlier risk assessment that derived pollutant standards for metals were of less concern due to their low sensitivity to dioxins.
The hazard quotient (HQ) approach was used to evaluate risk. A hazard quotient is the ratio of an exposure dose to toxicological benchmark at which adverse developmental and reproductive effects occur. Hazard quotients were calculated for wildlife receptors in 41 climatic regions in the contiguous 48 states.
Phase 1 was a deterministic ecological risk assessment performed as a conservative screen. Receptors included four maximally exposed species: the American robin (Turdus migratorius) for terrestrial habitat, and the osprey (Pandion haliaens), kingfisher (Ceryle alcyon), and mink (Mustela vison) for freshwater margin habitat. The conservative toxicity values used were no-observed-adverse-effect levels. Predicted exposures were maximized by assuming that soil concentrations were equal to 50th percentile, 90th percentile, and maximum biosolids concentrations, and each receptor's diet was assumed to consist entirely of a single food item that significantly bioaccumulates dioxins. In addition, the entire diet was assumed to come from contaminated media. Calculated hazard quotients for all receptors at all three concentrations of dioxins in biosolids exceeded 1. Consequently, another less-conservative screen was performed as Phase 2.
Phase 2 included 35 receptors representing a broad range of feeding guilds and trophic levels. Toxicity values selected were the geometric mean of the lowest-observed-adverse-effect level (LOAEL) and the no-observed-adverse-effect level. The 90th percentile chemical concentrations in soil, sediment, surface water, and plants were derived using fate and transport modeling and were used to generate exposure point concentrations. The results of Phase 2 showed that hazard quotients for all receptors were less than 1. Based on this finding, ecological risks were determined not to be of concern. As described above, the USEPA (68 FR 61084-61096; 24 Oct. 2003) used the risk assessment findings to conclude that neither numerical limitations nor requirements for management practices are currently needed to protect human health and the environment from reasonably anticipated adverse effects from dioxin and dioxin-like compounds in land-applied sewage sludge. The USEPA specifies that it "believes the screening ecological risk assessment is adequate to predict hazards to wildlife species from dioxins in land-applied sewage sludge." The USEPA further states that "[S]creening-level ecological risk assessments are designed to provide, for those chemicals and receptors that pass the screen (as in dioxins), a high level of confidence that there is a low probability of adverse effects to ecological receptors. The SERA provides insight into the potential for ecological effects from dioxins in land-applied sewage sludge. The approach used shows that the exposures to animals in terrestrial and water body margin habitats do not exceed protective ecological benchmarks (that is, hazard quotients do not exceed one), suggesting that dioxins in land-applied sewage sludge do not pose a high potential for adverse ecological effects."
Two important aspects of ecological risk assessment for evaluating land application of biosolids, effluents, and manures that changed between the Part 503 rule and the recent evaluation of dioxin are target habitats and receptors. During Part 503 rule development, a variety of habitats where biosolids were directly applied were evaluated. The habitats of concern were expanded in the dioxin risk assessment to include adjacent sensitive habitats (margins of surface water bodies) that could be affected by migration of pollutants from the site of application. Receptors used in the Part 503 rule development included agricultural plants and livestock as well as some wild animals. The dioxin risk assessment focused solely on wildlife. Both of these modifications point toward heightened concern for protection of valued components of the natural ecosystem, and reflect the bioaccumulative nature of dioxins. As was the case for the human health risk assessment, the different approaches used in the two ecological risk assessments reflect both the evolution of risk assessment methods and the different conceptual models and exposure pathways for metals compared with dioxins.
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CONCLUSIONS
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Risk assessment is inherently a process that is never complete. A single risk assessment is always constrained by limitations in available information and the current state of the science. For complex mixtures such as biosolids, effluents, and manures, three principal categories of information need to be periodically updated and reevaluated: (i) the nature of the chemical constituents in the material, (ii) the nature of the expected exposures, and (iii) the toxicity of the chemical constituents. For these materials both the chemical constituents and nature of exposures may change over time as new chemicals are introduced to waste streams, and as wastes are processed and disposed of in different ways. In addition, characterization of all three factors will be affected by scientific advances. New analytical methods and lower detection limits will affect chemical identification in wastes. Approaches to exposure assessment, such as increasing emphasis on probabilistic approaches, will continue to evolve, and exposure assumptions will change as new studies provide better data on factors such as soil ingestion, plant uptake of chemicals, and bioavailability of chemicals in soil. Similarly, toxicity assessments will be updated as new studies are conducted. It is noteworthy that many of these limitations and changes over time apply to risk assessment for pathogens in a manner directly analogous to their application to chemical risk assessment.
While not inherently flawed, the risk assessment approach used to support the Part 503 rule is now outdated. Our ability to characterize chemicals in biosolids has improved and the profile of chemicals present in biosolids, effluents, and manures has changed. Our understanding of chemical behavior has improved, including plant uptake and bioavailability to animals and humans. The knowledge base for toxicity and exposure parameters has also increased.
Despite all of these advances, it will not always be necessary to conduct new risk assessments. For many chemicals, such as metals, a new risk assessment would very likely lead to less stringent pollutant standards. When combined with a trend of decreasing metal concentrations in biosolids, it is possible that a new risk assessment would support a conclusion that no pollutant standards are needed for metals, analogous to the USEPA's finding regarding dioxins. On the other hand, there are some chemicals, such as polybrominated diphenyl ethers (Hale et al., 2001), that are being released to the environment at an increasing rate and may need a more formal assessment of potential risk as data become available.
The USEPA's dioxin risk assessment provides a useful model for performing additional risk assessments of other organic chemicals present in biosolids. Application of this model to other chemicals will be constrained by limited surveys of their concentrations in sewage sludge, as well as by limited data on fate and transport parameters and uptake into the food chain. Uncertainty assessments can be helpful in determining the relative importance of data limitations. One form of assessing sources of uncertainty is by use of a sensitivity analysis in which values of the model's inputs are varied, and the effect of such changes on the model output assessed (Saltelli, 2000; USEPA, 2001). The results can be used to quantitatively rank contributions of individual parameters to model output and variability. The sensitivity analysis approach used in the dioxin risk assessment could be applied to help focus research efforts on the most important fate and transport parameters and food chain pathways for future risk assessments. Inclusion of quantitative sensitivity analyses addressing all components of the risk assessments also provides greater transparency and facilitates future reviews to determine if the risk assessments need to be revised and updated. For example, if a sensitivity analysis indicates that the risk model output is relatively insensitive to a highly uncertain input parameter, that uncertainty does not seriously compromise the validity of the risk assessment.
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REFERENCES
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- Finley, B., and D. Paustenbach. 1994. The benefits of probabilistic exposure assessment: Three case studies involving contaminated air, water and soil. Risk Anal. 14:53–73.[Medline]
- Hale, R.C., M.J. LaGuardia, E.P. Harvey, M.O. Gaylor, T.M. Mainor, and W.H. Duff. 2001. Persistent pollutants in land-applied sludge. Nature (London) 412:140–141.[Medline]
- Harrison, E.Z., M.B. McBride, and D.R. Bouldin. 1999. Land application of sewage sludges: An appraisal of the U.S. regulations. Int. J. Environ. Pollut. 11:1–36.
- Marchant, G.E. 2003. From general policy to legal rule: The aspirations and limitations of the precautionary principle. Environ. Health Perspect. 111:1799–1803.[Medline]
- McGrath, S.P., A.C. Change, A.L. Page, and E. Witter. 1994. Land application of sewage sludge: Scientific perspectives of heavy metal loading limits in Europe and the United States. Environ. Rev. 2:108–118.
- National Research Council. 2002. Biosolids applied to land: Advancing standards and practices. The Natl. Academies Press, Washington, DC.
- Powers, W.J. 2003. Keeping science in environmental regulations: The role of the animal scientist. J. Dairy Sci. 86:1045–1051.[Abstract/Free Full Text]
- Saltelli, A. 2000. What is sensitivity analysis? p. 3–13. In A. Saltelli, K. Chan, and E.M. Scott (ed.) Sensitivity analysis. John Wiley & Sons, West Sussex, England.
- USEPA. 1989. Risk assessment guidance for Superfund. Volume I: Human health evaluation manual (Part A). USEPA/540/1-89/002. Office of Emergency and Remedial Response, USEPA, Washington, DC.
- USEPA. 1992. Framework for ecological risk assessment. EPA/630/R-92/001. USEPA, Washington, DC.
- USEPA. 1995a. A guide to the biosolids risk assessments for the EPA Part 503 Rule. EPA 832-B-93-005. Office of Wastewater Management, USEPA, Washington, DC.
- USEPA. 1995b. The use of the benchmark dose approach in health risk assessment. EPA/630/R-94/007. USEPA, Risk Assessment Forum, Washington, DC.
- USEPA. 1997a. Exposure factors handbook. EPA/600/P-95/002Fa-c [Online]. Available at www.epa.gov/ncea/exposfac.htm (verified 3 Aug. 2004). Natl. Center for Environ. Assessment, Office of Research and Development, USEPA, Washington, DC.
- USEPA. 1997b. Guiding principles for Monte Carlo analysis. EPA/630/R-97/001 [Online]. Available at http://cfpub2.epa.gov/ncea/cfm/recordisplay.cfm?deid=29596 (verified 3 Sept. 2004). Natl. Center for Environ. Assessment, Office of Research and Development, USEPA.
- USEPA. 1998. Guidelines for ecological risk assessment. EPA/630/R-95/002F [Online]. Available at www.epa.gov/superfund/programs/risk/tooleco.htm (verified 3 Aug. 2004). USEPA, Risk Assessment Forum, Washington, DC.
- USEPA. 1999. Guidelines for carcinogen risk assessment. NCEA-F-0644. Review draft [Online]. Available at www.epa.gov/ncea/raf/crasab.htm (verified 3 Aug. 2004). Natl. Center for Environ. Assessment, Risk Assessment Forum, USEPA, Washington, DC.
- USEPA. 2000. Exposure and human health reassessment of 2,3,7,8-tetrachlorobibenzo-p-dioxin (TCDD) and related compounds. USEPA/600/P-00/001Bg. Natl. Center for Environ. Assessment, Office of Research and Development, USEPA, Washington, DC.
- USEPA. 2001. Risk assessment guidance for Superfund. Volume III: Part A, Process for conducting probabilistic risk assessment. USEPA 540-R-02-002 [Online]. Available at www.epa.gov/superfund/programs/risk/rags3a/index.htm (verified 3 Sept. 2004). Office of Emergency and Remedial Response, USEPA, Washington, DC.
- USEPA. 2002a. Environmental and economic analysis of the final revisions to the discharge and elimination system regulations and effluent guidelines for concentrated animal feeding operations. EPA-821-R-03-003 [Online]. Available at www.epa.gov/npdes/pubs/cafo_benefit_p1.pdf (verified 4 Sept. 2004). Office of Sci. and Technol., USEPA, Washington, DC.
- USEPA. 2002b. Exposure analysis for dioxins, dibenzofurans, and coplanar polychlorinated biphenyls in sewage sludge. Tech. Background Document. Draft. Prepared by RTI, Research Triangle Park for Office of Water, USEPA, Washington, DC.
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ABSTRACT
INTRODUCTION
