randall wentsel, ph.d. 7 september, 2011. background problems pbt process is based on principles...
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Randall Wentsel, Ph.D.7 September, 2011
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Background Problems
PBT process is based on principles developed for organic substances that do not apply to metals
PBT process lacks discriminatory power for metals
BCF for metals – will be discussed The US EPA therefore initiated a process
to address issues associated with metals
to provide opportunities for external input, peer review and cross-Agency
involvement
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Collaboration Development of the Framework involved
national and international experts in two workshops and five issue papers that supported the development of the document
US EPA was active with MERAG development in Europe attending workshops and reviewing documents
US EPA was active in Canadian Metals in the Human Environment Research Network
Co-sponsored a SETAC workshop on metals issues
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Purpose Present key guiding principles based on
the unique attributes of metals Describe how metals-specific attributes
and principles may then be applied in the context of existing US EPA risk assessment guidance and practices
Outline key metal principles and how they should be considered in existing human health and ecological risk assessment practices
Foster consistency across US EPA programs and regions
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Principles Metals are naturally occurring
constituents in the environment and vary in concentrations across geographic regions
All environmental media have naturally occurring mixtures of metals, and metals often are introduced into the environment as mixtures
Some metals are essential for maintaining proper health of humans, animals, plants, and microorganisms
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Principles Unlike organic chemicals, metals are neither
created nor destroyed by biological or chemical processesthey can transform from one species to
another (valence states) and can convert them between inorganic and organic forms
The absorption, distribution, transformation, and excretion of a metal (toxicokinetics) within an organism depends on:the metalthe form of the metal or metal compoundthe organism’s ability to regulate and/or
store the metal
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Metals FrameworkUtilized the ecological risk assessment
processListed questions assessors should
consider in the phases of risk assessmentIncluded chapters on
Introduction (bioavailability)Environmental chemistryAquatic eco-risk assessmentTerrestrial eco-risk assessment Human health
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Environmental Issues in the Framework
Limited use of BAF/BCF Incorporation of Bioavailability Background Toxicity – BLM; AVS-SEM Secondary Poisoning Environmental Chemistry
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BAF/BCF Issues Certain metal compounds are known to
bioaccumulate in tissues and this bioaccumulation can be related to their toxicity
BCFs for metals vary with species, environmental
conditions, generally show an inverse relationship with media concentration, and are not a predictor of toxicity
For soil invertebrates and most plants, metal BAFs are typically less than 1 and usually are based on the total metal in soil and tissue that do not account for bioavailability differences
The latest scientific data on bioaccumulation do not currently support the use of bioconcentration factor (BCF) or bioaccumulation factor (BAF) values when applied as generic threshold criteria for the hazard potential of inorganic metals
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Bioavailability Issues Bioavailability of metals and the
associated risk vary widely according to the physical, chemical, and biological conditions under which an organism is exposed
Bioavailability should be explicitly incorporated into all risk assessments
Where data or models are insufficient, assumptions should be clearly articulated
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Soil pH
Soil Organic Matter
Low organic matter
(<2%)
Medium organic matter
(2 to 6%)
Highorganic matter
(6 to 10%)
4 < Soil pH < 5.5 Very high High Medium
5.5 < Soil pH < 7 High Medium Low
7 < Soil pH < 8.5 Medium Low Very low
Qualitative bioavailability of metal cations in natural soils to plants and soil invertebrates
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EPA's Bioavailability Committee Initiated in March, 2007Develops new guidance concerning site assessment and cleanup at hazardous waste sites
Evaluates new methods and supports site specific assessments
Identifies research needs to address data gaps relevant to contaminant bioavailability in soil site assessment activities
EPA’s Bioavailability Committee: http://www.epa.gov/superfund/health/contaminants/bioavailability/trw.htm
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BackgroundBackground levels refers to those concentrations
of metals that derive from natural as well as anthropogenic sources that are not the focus of the risk assessment
Metal concentrations vary widely over space and time owing to differences in geology, hydrology, anthropogenic and natural loads from “nontarget” sources, and other factors
It is recommended that, when appropriate, regional- or national- level ecological risk assessments be subdivided into metal-related ecoregions, referred to as metalloregions (McLaughlin and Smolders, 2001)
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Ecological ToxicityFor most metals, the free ionic form is most
responsible for toxicityFree-ion activity models are useful for
establishing relative toxicity among metals in different mediaBLM – EPA Water Quality CriteriaFIAM
Sediment toxicity is reduced by acid volatile sulfides, organic carbon and other factors that bind free ions and decrease bioavailability
Soil toxicity is affected by pH, CEC, and % organic matter
Al and Fe: soil chemistry vs toxicity database
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Secondary PoisoningInorganic metal compounds rarely
biomagnify across three or more trophic levels
Soil- Plant Barrier issues(Chaney, 1980, Chaney et al, 2000)Can assume that most plant species do not
bioconcentrate metals (i.e. <1)Pb, As, Cr and Co are not taken up by plants in
measurable quantitiesWildlife
Incidental soil ingestion is a proportionally more important pathway for herbivores than for carnivores or invertivores.
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Environmental Chemistry Metal speciation affects metal behavior in
environmental media
pH and redox potential affect speciation
Kd values – a coefficient for mobility in soilslimited use of single values
Aging of metals in media reduces bioavailability
Metal sorption behavior affects bioavailability
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Metals Framework, March, 2007 http://www.epa.gov/raf/metalsframework
Fairbrother et al., 2007. Ecotoxicology and Environmental Safety. 68: 145-227
EPA Ecological Soil Screening Levels epa.gov/ecotox/ecossl/
Issue papers August 2004:
epa.gov/raf/publications/paper risk assessment-metals.htm
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