EPA 120/R-07/001 | March 2007 www.epa.gov/osa Framework for Metals Risk Assessment Office of the Science Advisor Risk Assessment Forum EPA 120/R-07/001 March, 2007 Framework for Metals Risk Assessment Office of the Science Advisor Risk Assessment Forum U.S. Environmental Protection Agency Washington, DC 20460 DISCLAIMER This document has been reviewed in accordance with U.S. Environmental Protection Agency policy. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. ii CONTENTS LIST OF TABLES....................................................................................................................... vii LIST OF FIGURES ..................................................................................................................... vii LIST OF ABBREVIATIONS AND ACRONYMS ...................................................................viii PREFACE...................................................................................................................................xiii AUTHORS.................................................................................................................................. xvi EXECUTIVE SUMMARY ....................................................................................................... xvii 1. INTRODUCTION .................................................................................................................1-1 1.1. PURPOSE AND AUDIENCE ................................................................................1-1 1.2. METALS FRAMEWORK SCOPE.........................................................................1-3 1.3. METALS ASSESSMENT CONTEXT...................................................................1-3 1.3.1. National Ranking and Categorization.........................................................1-4 1.3.2. National-Level Assessments.......................................................................1-6 1.3.3. Site-Specific Assessments ..........................................................................1-8 1.4. KEY PRINCIPLES TO CONSIDER IN METALS RISK ASSESSMENT ...........1-9 1.4.1. Metals are Naturally Occurring Constituents in the Environment and Vary in Concentrations Across Geographic Regions ................................1-9 1.4.2. All Environmental Media have Naturally Occurring Mixtures of Metals and Metals are Often Introduced into the Environment as Mixtures...................................................................................................1-10 1.4.3. Some Metals are Essential for Maintaining Proper Health of Humans, Animals, Plants, and Microorganisms .....................................................1-10 1.4.4. The Environmental Chemistry of Metals Strongly Influences Their Fate and Effects on Human and Ecological Receptors............................1-10 1.4.5. The Toxicokinetics and Toxicodynamics of Metals Depend on the Metal, the Form of the Metal or Metal Compound, and the Organism’s Ability to Regulate and/or Store the Metal ..........................1-11 2. FRAMEWORK FOR METALS RISK ASSESSMENT .......................................................2-1 2.1. HUMAN HEALTH AND ECOLOGICAL RISK ASSESSMENT: PLANNING AND PROBLEM FORMULATION ................................................2-1 2.2. METALS CONCEPTUAL MODEL ......................................................................2-4 2.3. ASSESSMENT PHASE........................................................................................2-10 2.3.1. Bioavailability...........................................................................................2-10 2.3.2. Exposure Characterization........................................................................2-11 2.3.3. Characterization of Effects/Hazard Analysis............................................2-14 2.4. RISK CHARACTERIZATION ............................................................................2-16 3. ENVIRONMENTAL CHEMISTRY, TRANSPORT AND FATE.......................................3-1 3.1. INTRODUCTION AND TERMINOLOGY...........................................................3-1 3.1.1. Hard and Soft Acids and Bases: The Stability of Complexes ...................3-2 3.1.2. Transformations..........................................................................................3-3 3.1.2.1. Biotic Methylation Transformations............................................3-5 3.1.2.2. Abiotic Transformations ..............................................................3-6 3.1.3. Aquatic Chemistry ......................................................................................3-7 3.1.3.1. Processes .....................................................................................3-7 3.1.3.2. Speciation and Complexes...........................................................3-7 iii 3.1.3.3. Importance of pH and Redox Conditions ....................................3-9 3.1.4. Ground Water and Metals Mobility............................................................3-9 3.1.4.1. Application of Partition Coefficients to Metal Mobility in Ground Water............................................................................3-11 3.1.5. Sediment Chemistry..................................................................................3-13 3.1.5.1. Metal Chemistry in Sediments ...................................................3-14 3.1.5.2. Estimating Metal Distribution in Sediments..............................3-16 3.1.6. Soil Chemistry ..........................................................................................3-17 3.1.6.1. Key Parameters Affecting Metal Bioavailability in Soils ..........3-18 3.1.6.2. Adsorption Behavior of the Metals of Concern .........................3-19 3.1.6.3. Aging of Metals in Soil ..............................................................3-19 3.1.6.4. Dissolution and Transformation of Metals................................3-20 3.1.6.5. Soil Metal Transfer to Plants.....................................................3-20 3.1.7. Atmospheric Behavior/Chemistry ............................................................3-21 3.2. METAL FATE AND TRANSPORT ....................................................................3-23 3.2.1. Aquatic and Terrestrial Transport Pathways for Metals...........................3-24 3.2.1.1. Aquatic Transport Models .........................................................3-26 3.2.2. Terrestrial Fate and Transport...................................................................3-29 3.3. ATMOSPHERIC FATE AND TRANSPORT......................................................3-29 4. HUMAN HEALTH RISK ASSESSMENT FOR METALS.................................................4-1 4.1. METALS PRINCIPLES..........................................................................................4-2 4.2. HUMAN EXPOSURE ASSESSMENT..................................................................4-3 4.2.1. Background Levels .....................................................................................4-3 4.2.2. Bioavailability.............................................................................................4-4 4.2.3. Susceptible Populations ..............................................................................4-5 4.2.3.1. Life Stage .....................................................................................4-6 4.2.3.2. Demographics..............................................................................4-6 4.2.3.3. Pregnancy and Lactation.............................................................4-6 4.2.3.4. Concurrent Damage or Disease ..................................................4-6 4.2.3.5. Nutritional State...........................................................................4-7 4.2.3.6. Genetic Polymorphisms and Variability......................................4-7 4.2.4. Environmental Release, Transport, and Fate..............................................4-8 4.2.5. Route-Specific Differences in Effects ........................................................4-8 4.2.5.1. Inhalation Exposure.....................................................................4-8 4.2.5.2. Dietary Exposure .......................................................................4-10 4.2.5.3. Incidental Soil Exposure............................................................4-11 4.2.5.4. Drinking Water Exposure ..........................................................4-11 4.2.5.5. Dermal Exposure .......................................................................4-12 4.2.6. Integrated Exposure ..................................................................................4-12 4.2.6.1. Modeling ....................................................................................4-12 4.2.7. Biomarkers................................................................................................4-13 4.3. HAZARD CHARACTERIZATION.....................................................................4-14 4.3.1. Mixtures and Interactions .........................................................................4-14 4.3.2. Essentiality................................................................................................4-16 4.3.3. Forms of Metals........................................................................................4-18 4.3.4. Toxicokinetics/Toxicodynamics...............................................................4-18 4.3.4.1. Absorption..................................................................................4-20