Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health ( 2000) United States Office of Water EPA 822 B 00 004 Environmental Protection Office of Science and Technology October 2000 Agency 4304 EPA 822 B 00 004 October 2000 Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health ( 2000)

Final Office of Science and Technology Office of Water U. S. Environmental Protection Agency Washington, DC 20460 NOTICE The policies and procedures set forth in this document are intended solely to describe EPA methods for developing or revising ambient water quality criteria to protect human health, pursuant to Section 304( a) of the Clean Water Act, and to serve as guidance to States and authorized Tribes for developing their own water quality criteria. This guidance does not substitute for the Clean Water Act or EPA's regulations; nor is it a regulation itself. Thus, it does not impose legally binding requirements on EPA, States, Tribes or the regulated community, and may not apply to a particular situation based upon the circumstances.

This document has been reviewed in accordance with U. S. Environmental Protection Agency policy and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

ii iii FOREWORD This document presents EPA's recommended Methodology for developing ambient water quality criteria as required under Section 304( a) of the Clean Water Act ( CWA). The Methodology is guidance for scientific human health assessments used by EPA to develop, publish, and from time to time revise, recommended criteria for water quality accurately reflecting the latest scientific knowledge. The recommended criteria serve States and Tribes' needs in their development of water quality standards under Section 303( c) of the CWA.

The term " water quality criteria" is used in two sections of the Clean Water Act, Section 304( a)( 1) and Section 303( c)( 2). The term has a different program impact in each section. In Section 304, the term represents a scientific assessment of ecological and human health effects that EPA recommends to States and authorized Tribes for establishing water quality standards that ultimately provide a basis for controlling discharges or releases of pollutants. Ambient water quality criteria associated with specific stream uses when adopted as State or Tribal water quality standards under Section 303 define the maximum levels of a pollutant necessary to protect designated uses in ambient waters. The water quality criteria adopted in the State or Tribal water quality standards could have the same numerical limits as the criteria developed under Section 304. However, in many situations States and authorized Tribes may want to adjust water quality criteria developed under Section 304 to reflect local environmental conditions and human exposure patterns before incorporation into water quality standards. When adopting their water quality criteria, States and authorized Tribes have four options: ( 1) adopt EPA's 304( a) recommendations; ( 2) adopt 304( a) criteria modified to reflect site specific conditions; ( 3) develop criteria based on other scientifically defensible methods; or ( 4) establish narrative criteria where numeric criteria cannot be determined.

EPA will use this Methodology to develop new ambient water quality criteria and to revise existing recommended water quality criteria. It also provides States and authorized Tribes the necessary guidance to adjust water quality criteria developed under Section 304 to reflect local conditions or to develop their own water quality criteria using scientifically defensible methods consistent with this Methodology. EPA encourages States and authorized Tribes to use this Methodology to develop or revise water quality criteria to appropriately reflect local conditions. EPA believes that ambient water quality criteria inherently require several risk management decisions that are, in many cases, better made at the State, Tribal, or regional level. Additional guidance to assist States and authorized Tribes in the modification of criteria based on the Methodology will accompany this document in the form of three companion Technical Support Documents on Risk Assessment, Exposure Assessment, and Bioaccumulation Assessment.

Geoffrey H. Grubbs Director Office of Science and Technology v ACKNOWLEDGMENTS Project Leader Denis Borum U. S. EPA Office of Science and Technology Coauthors Risk Assessment Joyce M. Donohue, Ph. D.* U. S. EPA Office of Science and Technology Julie T. Du, Ph. D.* U. S. EPA Office of Science and Technology Charles O. Abernathy, Ph. D. U. S. EPA Office of Science and Technology Exposure Denis Borum * U. S. EPA Office of Science and Technology Helen Jacobs, M. S. U. S. EPA Office of Science and Technology Henry Kahn, D. Sc. U. S. EPA Office of Science and Technology Bioaccumulation Keith G. Sappington, M. S.* U. S. EPA Office of Science and Technology Lawrence P. Burkhard, Ph. D. U. S. EPA Office of Research and Development Philip M. Cook, Ph. D. U. S. EPA Office of Research and Development Erik L. Winchester, M. S. U. S. EPA Office of Science and Technology U. S. EPA Technical Reviewers William Beckwith U. S. EPA Region 1 Jeff Bigler U. S. EPA Office of Science and Technology Sally Brough U. S. EPA Region 10 Karen Clark U. S. EPA Office of General Counsel Gregory Currey U. S. EPA Office of Wastewater Management Vicki Dellarco U. S. EPA Office of Prevention, Pesticides, and Toxic Substances Charles Delos U. S. EPA Office of Science and Technology Arnold Den U. S. EPA Region 9 Catherine Eiden U. S. EPA Office of Prevention, Pesticides, and Toxic Substances Michael Firestone U. S. EPA Office of Prevention, Pesticides, and Toxic Substances Steven Galson U. S. EPA Office of Prevention, Pesticides, and Toxic Substances Sue Gilbertson U. S. EPA Office of Science and Technology Denise Hakowski U. S. EPA Region 3 Joel Hansel U. S. EPA Region 4 Wayne Jackson U. S. EPA Region 2 Annie Jarabek U. S. EPA Office of Research and Development William Jordan U. S. EPA Office of Prevention, Pesticides, and Toxic Substances vi Margaret Kelly U. S. EPA Office of Children's Health Protection Henry Lee U. S. EPA Office of Research and Development Sharon Lin U. S. EPA Office of Wetlands, Oceans, and Watersheds Roseanne Lorenzana U. S. EPA Region 10 Gregory McCabe U. S. EPA Region 7 Jennifer Mclain U. S. EPA Office of Ground Water and Drinking Water Bruce Mintz U. S. EPA Office of Research and Development Dave Moon U. S. EPA Region 8 William Morrow U. S. EPA Office of Science and Technology Jacqueline Moya U. S. EPA Office of Research and Development Deirdre Murphy U. S. EPA Office of Air Quality Planning and Standards Joseph Nabholz U. S. EPA Office of Prevention, Pesticides, and Toxic Substances Russell Nelson U. S. EPA Region 6 Jennifer Orme Zavaleta U. S. EPA Office of Research and Development Lynn Papa U. S. EPA Office of Research and Development Robert Pepin U. S. EPA Region 5 David Pfeifer U. S. EPA Region 5 Rita Schoeny U. S. EPA Office of Science and Technology Charles Stephan U. S. EPA Office of Research and Development Linda Teuschler U. S. EPA Office of Research and Development David Tomey U. S. EPA Region 1 Fritz Wagener U. S. EPA Region 4 Jennifer Wigal U. S. EPA Office of Science and Technology Jeanette Wiltse U. S. EPA Office of Scence and Technology Gary Wolinsky U. S. EPA Region 9 Philip Woods U. S. EPA Region 9 William Wuerthele U. S. EPA Region 8 * Principal U. S. EPA Author and Contact vii EXTERNAL PEER REVIEW WORKGROUP The following professionals were part of the External Peer Review Workgroup that provided technical and scientific review regarding the content and technical approach in the July 1998 Draft Ambient Water Quality Criteria Derivation Methodology: Human Health. Their comments were reviewed and incorporated where appropriate to develop this final document.

Kenneth T. Bogen, Ph. D. Lawrence Livermore National Laboratory Paul E. Brubaker, Ph. D. P. E. Brubaker Associates Peter L. DeFur, Ph. D. Virginia Commonwealth University Karen Erstfeld, Ph. D. Rutgers University Bob Fares, Ph. D. Environmental Standards, Inc. Laura Green, Ph. D. Cambridge Environmental, Inc. Robert Hales, Ph. D. Virginia Institute of Marine Science Brendan Hickie, Ph. D. Trent University Ernest Hodgson, Ph. D. North Carolina State University Paul Locke, Ph. D. Johns Hopkins University Lynn S. McCarty, Ph. D. LS McCarty Scientific Research and Consulting Erik Rifkin, Ph. D. Rifkin and Associates, Inc. Damian Shea, Ph. D. North Carolina State University Nga Tran, Ph. D. Johns Hopkins University Curtis Travis, Ph. D. Project Performance Corp.

Potential areas for conflict of interest were investigated via direct inquiry with the peer reviews and review of their current affiliations. No conflicts of interest were identified. ix TABLE OF CONTENTS Page NOTICE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v EXTERNAL PEER REVIEW WORKGROUP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii CONTENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix TABLES AND FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiv LIST OF ACRONYMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv 1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 1.1 Water Quality Criteria and Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 1.2 Purpose of This Document . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 1.3 History of the Ambient Water Quality Criteria ( AWQC) Methodology . . . . . . . . . . . . 1 2 1.4 Relationship of Water Quality Standards to AWQC . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 1.5 Need for the AWQC Methodology Revisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 1.5.1 Group C Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 1.5.2 Consideration of Non Water Sources of Exposure . . . . . . . . . . . . . . . . . . . . . 1 7 1.5.3 Cancer Risk Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 8 1.6 Overview of the AWQC Methodology Revisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 9 1.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 13 2. CLARIFICATIONS ON THE METHODOLOGY, RISK CHARACTERIZATION, AND OTHER ISSUES FOR DEVELOPING CRITERIA . . . . . . . . . . . . . . . . . . . 2 1 2.1 Identifying the Population Subgroup that the AWQC Should Protect . . . . . . . . . . . . . 2 1 2.2 Science, Science Policy, and Risk Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 3 2.3 Setting Criteria to Protect Against Multiple Exposures From Multiple Chemicals ( Cumulative Risk) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 4 2.4 Cancer Risk Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 6 2.5 Microbiological Ambient Water Quality Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 7 2.6 Risk Characterization Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 9 2.7 Discussion of Uncertainty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 11 2.7.1 Observed Range of Toxicity Versus Range of Environmental Exposure . . . . 2 11 2.7.2 Continuum of Preferred Data/ Use of Defaults . . . . . . . . . . . . . . . . . . . . . . . . 2 11 2.7.3 Significant Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 11 2.8 Other Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 13 2.8.1 Minimum Data Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 13 2.8.2 Site Specific Criterion Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 13 2.8.3 Organoleptic Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 14 2.8.4 Criteria for Chemical Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 15 2.8.5 Criteria for Essential Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 16 2.9 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 16 x 3. RISK ASSESSMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1 3.1 Cancer Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1 3.1.1 Background on EPA Cancer Risk Assessment Guidelines . . . . . . . . . . . . . . . 3 1 3.1.2 EPA's Proposed Guidelines for Carcinogen Risk Assessment and the Subsequent July, 1999 Draft Revised Cancer Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 3.1.3 Methodology for Deriving AWQC by the 1999 Draft Revised Cancer Guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 4 <IP_ADDRESS> Weight of Evidence Narrative . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 5 <IP_ADDRESS> Mode of Action General Considerations and Framework for Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 6 <IP_ADDRESS> Dose Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 7 A. Determining the Human Equivalent Dose . . . . . . . . . . . . . . . . . . . 3 7 B. Dose Response Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 7 <IP_ADDRESS> Characterizing Dose Response Relationships in the Range of Observation and at Low Environmentally Relevant Doses . . . . . . . . . . . . . . . . . . . 3 8 A. Extrapolation to Low, Environmentally Relevant Doses . . . . . . . . 3 9 B. Biologically Based Modeling Approaches . . . . . . . . . . . . . . . . . . 3 9 C. Default Linear Extrapolation Approach . . . . . . . . . . . . . . . . . . 3 10 D. Default Nonlinear Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 11 E. Both Linear and Nonlinear Approaches . . . . . . . . . . . . . . . . . . 3 13 <IP_ADDRESS> AWQC Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 13 A. Linear Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 13 B. Nonlinear Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 14 <IP_ADDRESS> Risk Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 14 <IP_ADDRESS> Use of Toxicity Equivalence Factors ( TEF) and Relative Potency Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 15 3.1.4 References for Cancer Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 16 3.2 Noncancer Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 17 3.2.1 1980 AWQC National Guidelines for Noncancer Effects . . . . . . . . . . . . . . . 3 17 3.2.2 Noncancer Risk Assessment Developments Since 1980 . . . . . . . . . . . . . . . . 3 18 3.2.3 Issues and Recommendations Concerning the Derivation of AWQC for Noncarcinogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 20 <IP_ADDRESS> Using the Current NOAEL/ UF Based RfD Approach or Adopting More Quantitative Approaches for Noncancer Risk Assessment . . . . . . . . 3 20 A. The Benchmark Dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 22 B. Categorical Regression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 24 C. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 25 <IP_ADDRESS> Presenting the RfD as a Single Point or as a Range for Deriving AWQC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 25 <IP_ADDRESS> Guidelines to be Adopted for Derivation of Noncancer Health Effects Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 27 <IP_ADDRESS> Treatment of Uncertainty Factors/ Severity of Effects During the RfD Derivation and Verification Process . . . . . . . . . . . . . . . . . . . . . . . . . 3 27 xi <IP_ADDRESS> Use of Less Than 90 Day Studies to Derive RfDs . . . . . . . . . . . . . . 3 27 <IP_ADDRESS> Use of Reproductive/ Developmental, Immunotoxicity, and Neurotoxicity Data as the Basis for Deriving RfDs . . . . . . . . . . . . . . . . . . . . . . . . . 3 28 <IP_ADDRESS> Applicability of Toxicokinetic Data in Risk Assessment . . . . . . . . . . 3 28 <IP_ADDRESS> Consideration of Linearity ( or Lack of a Threshold) for Noncarcinogenic Chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 29 <IP_ADDRESS> Minimum Data Guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 29 3.2.4 References for Noncancer Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 30 4. EXPOSURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1 4.1. Exposure Policy Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1 4.1.1 Sources of Exposure Associated with Ambient Water . . . . . . . . . . . . . . . . . . . 4 2 <IP_ADDRESS> Appropriateness of Including the Drinking Water Pathway in AWQC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 <IP_ADDRESS> Setting Separate AWQC for Drinking Water and Fish Consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 <IP_ADDRESS> Incidental Ingestion from Ambient Surface Waters . . . . . . . . . . . . . . . 4 3 4.2. Consideration of Non Water Sources of Exposure When Setting AWQC . . . . . . . . . . 4 3 4.2.1 Policy Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 4.2.2 The Exposure Decision Tree Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 5 <IP_ADDRESS> Problem Formulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 9 <IP_ADDRESS> Data Adequacy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 10 <IP_ADDRESS> Regulatory Actions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 13 <IP_ADDRESS> Apportionment Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 13 4.2.3 Additional Points of Clarification on the Exposure Decision Tree Approach for Setting AWQC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 15 4.2.4 Quantification of Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 16 4.2.5 Inclusion of Inhalation and Dermal Exposures . . . . . . . . . . . . . . . . . . . . . . . 4 16 4.3 Exposure Factors Used in the AWQC Computation . . . . . . . . . . . . . . . . . . . . . . . . . 4 17 4.3.1 Human Body Weight Values for Dose Calculations . . . . . . . . . . . . . . . . . . . 4 18 <IP_ADDRESS> Rate Protective of Human Health from Chronic Exposure . . . . . . . . 4 19 <IP_ADDRESS> Rates Protective of Developmental Human Health Effects . . . . . . . . 4 20 4.3.2 Drinking Water Intake Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 21 <IP_ADDRESS> Rate Protective of Human Health from Chronic Exposure . . . . . . . . 4 23 <IP_ADDRESS> Rates Protective of Developmental Human Health Effects . . . . . . . . 4 24 <IP_ADDRESS> Rates Based on Combining Drinking Water Intake and Body Weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 24 4.3.3 Fish Intake Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 25 <IP_ADDRESS> Rates Protective of Human Health from Chronic Exposure . . . . . . . 4 25 <IP_ADDRESS> Rates Protective of Developmental Human Health Effects . . . . . . . . 4 29 <IP_ADDRESS> Rates Based on Combining Fish Intake and Body Weight . . . . . . . . 4 30 4.4 References for Exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 30 xii 5. BIOACCUMULATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1 5.1.1 Important Bioaccumulation and Bioconcentration Concepts . . . . . . . . . . . . . . 5 2 5.1.2 Goal of the National BAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 5.1.3 Changes to the 1980 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3 <IP_ADDRESS> Overall Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 <IP_ADDRESS> Lipid Normalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4 <IP_ADDRESS> Bioavailability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5 <IP_ADDRESS> Trophic Level Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5 <IP_ADDRESS> Site Specific Adjustments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5 5.1.4 Organization of This Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6 5.2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6 5.3 Framework for Determining National Bioaccumulation Factors . . . . . . . . . . . . . . . 5 10 5.3.1 Four Different Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 10 5.3.2 Overview of BAF Derivation Framework . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 12 5.3.3 Defining the Chemical of Concern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 14 5.3.4 Collecting and Reviewing Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 14 5.3.5 Classifying the Chemical of Concern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 15 5.4 National Bioaccumulation Factors for Nonionic Organic Chemicals . . . . . . . . . . . . . 5 16 5.4.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 16 5.4.2 Selecting the BAF Derivation Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 18 <IP_ADDRESS> Chemicals with Moderate to High Hydrophobicity . . . . . . . . . . . . . 5 18 <IP_ADDRESS> Chemicals with Low Hydrophobicity . . . . . . . . . . . . . . . . . . . . . . . . 5 19 <IP_ADDRESS> Assessing Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 20 5.4.3 Deriving National BAFs Using Procedure # 1 . . . . . . . . . . . . . . . . . . . . . . . . 5 22 <IP_ADDRESS> Calculating Individual Baseline BAF R f ds . . . . . . . . . . . . . . . . . . . . . . . 5 23 A. Baseline BAF R f d from Field Measured BAFs . . . . . . . . . . . . . . . . 5 23 B. Baseline BAF R f d Derived from BSAFs . . . . . . . . . . . . . . . . . . . . . 5 28 C. Baseline BAF R f d from a Laboratory Measured BCFtT and FCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 32 D. Baseline BAF R f d from a K ow and FCM . . . . . . . . . . . . . . . . . . . . . 5 38 <IP_ADDRESS> Selecting Final Baseline BAF R f ds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 39 <IP_ADDRESS> Calculating National BAFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 41 5.4.4 Deriving National BAFs Using Procedure # 2 . . . . . . . . . . . . . . . . . . . . . . . . 5 44 <IP_ADDRESS> Calculating Individual Baseline BAF R f ds . . . . . . . . . . . . . . . . . . . . . . . 5 45 A. Baseline BAF R f d from Field Measured BAFs . . . . . . . . . . . . . . . . 5 45 B. Baseline BAF R f d Derived from Field Measured BSAFs . . . . . . . . 5 46 C. Baseline BAF R f d from a Laboratory Measured BCF . . . . . . . . . . 5 46 <IP_ADDRESS> Selecting Final Baseline BAF R f ds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 46 <IP_ADDRESS> Calculating the National BAFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 47 5.4.5 Deriving National BAFs Using Procedure # 3 . . . . . . . . . . . . . . . . . . . . . . . . 5 47 <IP_ADDRESS> Calculating Individual Baseline BAF R f ds . . . . . . . . . . . . . . . . . . . . . . . 5 47 A. Baseline BAF R f d from Field Measured BAFs . . . . . . . . . . . . . . . . 5 48 xiii B. Baseline BAF R f d from a Laboratory Measured BCF . . . . . . . . . . 5 48 C. Baseline BAF R f d from a K ow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 49 <IP_ADDRESS> Selecting Final Baseline BAF R f ds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 49 <IP_ADDRESS> Calculating the National BAFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 50 5.4.6 Deriving National BAFs Using Procedure # 4 . . . . . . . . . . . . . . . . . . . . . . . . 5 51 <IP_ADDRESS> Calculating Individual Baseline BAF R f ds . . . . . . . . . . . . . . . . . . . . . . . 5 52 A. Baseline BAF R f d from Field measured BAFs . . . . . . . . . . . . . . . . 5 52 B. Baseline BAF R f d from a Laboratory measured BCF . . . . . . . . . . . 5 53 <IP_ADDRESS> Selecting Final Baseline BAF R f ds . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 53 <IP_ADDRESS> Calculating National BAFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 54 5.5 National Bioaccumulation Factors for Ionic Organic Chemicals . . . . . . . . . . . . . . . . 5 55 5.6 National Bioaccumulation Factors for Inorganic and Organometallic Chemicals . . . . 5 57 5.6.1 Selecting the BAF Derivation Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 57 5.6.2 Bioavailability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 5.6.3 Deriving BAFs Using Procedure # 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 <IP_ADDRESS> Determining Field Measured BAFs . . . . . . . . . . . . . . . . . . . . . . . . . 5 59 <IP_ADDRESS> Determining Laboratory Measured BCFs . . . . . . . . . . . . . . . . . . . . 5 60 <IP_ADDRESS> Determining the National BAFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 60 5.6.4 Deriving BAFs Using Procedure # 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 61 <IP_ADDRESS> Determining Field Measured BAFs . . . . . . . . . . . . . . . . . . . . . . . . . 5 62 <IP_ADDRESS> Determining Laboratory Measured BCFs . . . . . . . . . . . . . . . . . . . . 5 62 <IP_ADDRESS> Determining the National BAF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 62 5.7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 63 xiv TABLES AND FIGURES Page Table 3 1. Uncertainty Factors and the Modifying Factor . . . . . . . . . . . . . . . . . . . . . . . 3 19 Figure 4 1. Exposure Decision Tree for Defining Proposed RfD ( or POD/ UF) Apportionment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 8 Figure 5 1 Framework for Deriving a National BAF . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 13 Figure 5 2 BAF Derivation for Nonionic Organic Chemicals . . . . . . . . . . . . . . . . . . . . . 5 17 Table 5 1 Food Chain Multipliers for Trophic Levels 2, 3 and 4 . . . . . . . . . . . . . . . . . 5 36 xv LIST OF ACRONYMS ADI Acceptable Daily Intake ARAR Applicable or Relevant and Appropriate Requirements ASTM American Society of Testing and Materials AWQC Ambient Water Quality Criteria BAF Bioaccumulation Factor BAF R f d Baseline Bioaccumulation Factor BCF Bioconcentration Factor BCF R f d Baseline Bioconcentration Factor BCFtT Bioconcentration Factor Based on Total Concentrations in Tissue and Water BMD Benchmark Dose BMDL Lower Bound Confidence Limit on the BMD BMF Biomagnification Factor BMR Benchmark Response BSAF Biota Sediment Accumulation Factors BW Body Weight C R Lipid normalized Concentration C soc Organic Carbon normalized Concentration C t Concentration of the Chemical in the Specified Wet Tissue C w Concentration of the Chemical in Water CDC U. S. Centers for Disease Control and Prevention CSFII Continuing Survey of Food Intake by Individuals CWA Clean Water Act DDT 1,1,1 trichloro 2,2 bis( p chlorophenyl) ethane DDE 1,1 dichloro 2,2 bis( p chlorophenyl) ethylene DDD 1,1 dichloro 2,2 bis( p chlorophenyl) ethane DI Drinking Water Intake DNA Deoxyribonucleic Acid DNOC 2,4 dinitro o cresol DOC Dissolved Organic Carbon ED 10 Dose Associated with a 10 Percent Extra Risk EPA Environmental Protection Agency f fd Fraction Freely Dissolved f R Fraction Lipid FCM Food Chain Multiplier FEL Frank Effect Level FI Fish Intake FIFRA Federal Insecticide, Fungicide, and Rodenticide Act GLI Great Lakes Water Quality Initiative HCBD Hexachlorobutadiene IARC International Agency for Research on Cancer II Incidental Ingestion xvi ILSI International Life Sciences Institute IRIS Integration Risk Information System kg kilogram K ow Octanol Water Partition Coefficient L Liter LAS Linear Alkylbenzesulfonate LED 10 The Lower 95 Percent Confidence Limit on a Dose Associated with a 10 Percent Extra Risk LMS Linear Multistage Model LOAEL Lowest Observed Adverse Effect Level M R Mass of Lipid in Specified Tissue M t Mass of Specified Tissue ( Wet Weight) MCL Maximum Contaminant Level MCLG Maximum Contaminant Level Goal MF Modifying Factor mg Milligrams ml Milliliters MOA Mode of Action MOE Margin of Exposure NCHS National Center for Health Statistics NCI National Cancer Institute NFCS Nationwide Food Consumption Survey NHANES National Health and Nutrition Examination Survey NOAEL No Observed Adverse Effect Level NOEL No Observed Effect Level NPDES National Pollutant Discharge Elimination System PAH Polycyclic Aromatic Hydrocarbon PCB Polychlorinated Biphenyls POD Point of Departure POC Particulate Organic Carbon RDA Recommended Daily Allowance RfC Reference Concentration RfD Reference Dose RfD DT Reference Dose for Developmental Effects RPF Relative Potency Factor RSC Relative Source Contribution RSD Risk Specific Dose SAB Science Advisory Board SDWA Safe Drinking Water Act SF Safety Factor STORET Storage Retrieval TEAM Total Exposure Assessment Methodology TEF Toxicity Equivalency Factor TMDL Total Maximum Daily Load xvii TSD Technical Support Document USDA United States Department of Agriculture USEPA United States Environmental Protection Agency UF Uncertainty Factor WQBEL Water Quality Based Effluent Limits 1 1 1. INTRODUCTION 1.1 WATER QUALITY CRITERIA AND STANDARDS Pursuant to Section 304( a)( 1) of the Clean Water Act ( CWA), the U. S. Environmental Protection Agency ( EPA) is required to publish, and from time to time thereafter revise, criteria for water quality accurately reflecting the latest scientific knowledge on the kind and extent of all identifiable effects on human health which may be expected from the presence of pollutants in any body of water.

Historically, the ambient water quality criteria ( AWQC or 304( a) criteria) provided two essential types of information: ( 1) discussions of available scientific data on the effects of the pollutants on public health and welfare, aquatic life, and recreation; and ( 2) quantitative concentrations or qualitative assessments of the levels of pollutants in water which, if not exceeded, will generally ensure adequate water quality for a specified water use. Water quality criteria developed under Section 304( a) are based solely on data and scientific judgments on the relationship between pollutant concentrations and environmental and human health effects. The 304( a) criteria do not reflect consideration of economic impacts or the technological feasibility of meeting the criteria in ambient water. These 304( a) criteria may be used as guidance by States and authorized Tribes to establish water quality standards, which ultimately provide a basis for controlling discharges or releases of pollutants into ambient waters.

In 1980, AWQC were derived for 64 pollutants using guidelines developed by the Agency for calculating the impact of waterborne pollutants on aquatic organisms and on human health. Those guidelines consisted of systematic procedures for assessing valid and appropriate data concerning a pollutant's acute and chronic adverse effects on aquatic organisms, nonhuman mammals, and humans.

1.2 PURPOSE OF THIS DOCUMENT The Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health ( 2000) ( hereafter the " 2000 Human Health Methodology") addresses the development of AWQC to protect human health. The Agency intends to use the 2000 Human Health Methodology both to develop new AWQC for additional pollutants and to revise existing AWQC. Within the next several years, EPA intends to focus on deriving AWQC for chemicals of high priority ( including, but not limited to, mercury, arsenic, PCBs, and dioxin). Furthermore, EPA anticipates that 304( a) criteria development in the future will be for bioaccumulative chemicals and pollutants considered highest priority by the Agency. The 2000 Human Health Methodology is also intended to provide States and authorized Tribes flexibility in establishing water quality standards by providing scientifically valid options for developing their own water quality criteria that consider local conditions. States and authorized Tribes are strongly encouraged to use this Methodology to derive their own AWQC. However, the 2000 Human Health Methodology also defines the default factors EPA intends to use in evaluating and determining consistency of State water quality standards with the requirements of the CWA. The 1 2 Agency intends to use these default factors to calculate national water quality criteria under Section 304( a) of the Act. EPA will also use this Methodology as guidance when promulgating water quality standards for a State or Tribe under Section 303( c) of the CWA.

This Methodology does not substitute for the CWA or EPA's regulations; nor is it a regulation itself. Thus, the 2000 Human Health Methodology cannot impose legally binding requirements on EPA, States, Tribes or the regulated community, and may not apply to a particular situation based upon the circumstances. EPA and State/ Tribal decision makers retain the discretion to use different, scientifically defensible, methodologies to develop human health criteria on a case by case basis that differ from this Methodology where appropriate. EPA may change the Methodology in the future through intermittent refinements as advances in science or changes in Agency policy occur.

The 2000 Human Health Methodology incorporates scientific advancements made over the past two decades. The use of this Methodology is an important component of the Agency's efforts to improve the quality of the Nation's waters. EPA believes the Methodology will enhance the overall scientific basis of water quality criteria. Further, the Methodology should help States and Tribes address their unique water quality issues and risk management decisions, and afford them greater flexibility in developing their water quality programs.

There are three companion Technical Support Document ( TSD) volumes for the 2000 Human Health Methodology: a Risk Assessment TSD; an Exposure Assessment TSD; and a Bioaccumulation TSD. These documents are intended to further support States and Tribes in developing AWQC to reflect local conditions. The Risk Assessment TSD ( USEPA, 2000) is being published concurrently with this Methodology. Publication of the Exposure Assessment and Bioaccumulation TSDs are anticipated in 2001.

1.3 HISTORY OF THE AMBIENT WATER QUALITY CRITERIA ( AWQC) METHODOLOGY In 1980, EPA published AWQC for 64 pollutants/ pollutant classes identified in Section 307( a) of the CWA and provided a methodology for deriving the criteria ( USEPA, 1980). These 1980 AWQC National Guidelines ( or the " 1980 Methodology") for developing AWQC for the protection of human health addressed three types of endpoints: noncancer, cancer, and organoleptic ( taste and odor) effects. Criteria for protection against noncancer and cancer effects were estimated by using risk assessment based procedures, including extrapolation from animal toxicity or human epidemiological studies. Basic human exposure assumptions were applied to the criterion equation.

The risk assessment based procedures used to derive the AWQC to protect human health were specific to whether the endpoint was cancer or noncancer. When using cancer as the critical risk assessment endpoint ( which had been assumed not to have a threshold), the AWQC were 1Throughout this document, the term " risk level" regarding a cancer assessment using linear approach refers to an upper bound estimate of excess lifetime cancer risk.

1 3 presented as a range of concentrations associated with specified incremental lifetime risk levels1. When using noncancer effects as the critical endpoint, the AWQC reflected an assessment of a " no effect" level, since noncancer effects were assumed to have a threshold. The key features of each procedure are described briefly in the following paragraphs.

Cancer effects. If human or animal studies on a contaminant indicated that it induced a statistically significant carcinogenic response, the 1980 AWQC National Guidelines treated the contaminant as a carcinogen and derived a low dose cancer potency factor from available animal data using the linearized multistage model ( LMS). The LMS, which uses a linear, nonthreshold assumption for low dose risk, was used by the Agency as a science policy choice in protecting public health, and represented a plausible upper limit for low dose risk. The cancer potency factor, which expresses incremental, lifetime risk as a function of the rate of intake of the contaminant, was then combined with exposure assumptions to express that risk in terms of an ambient water concentration. In the 1980 AWQC National Guidelines, the Agency presented a range of contaminant concentrations corresponding to incremental cancer risks of 10 7 to 10 5 ( that is, a risk of one additional case of cancer in a population of ten million to one additional cancer case in a population of one hundred thousand, respectively).

Noncancer effects. If the pollutant was not considered to have the potential for causing cancer in humans ( later defined as a known, probable, or possible human carcinogen by the 1986 Guidelines for Carcinogen Risk Assessment, USEPA, 1986d), the 1980 AWQC National Guidelines treated the contaminant as a noncarcinogen; a criterion was derived using a threshold concentration for noncancer adverse effects. The criteria derived from noncancer data were based on the Acceptable Daily Intake ( ADI) ( now termed the reference dose [ RfD]). ADI values were generally derived using a no observed adverse effect level ( NOAEL) from animal studies, although human data were used whenever available. The ADI was calculated by dividing the NOAEL by an uncertainty factor to account for uncertainties inherent in extrapolating limited toxicological data to humans. In accordance with the National Research Council recommendations of 1977 ( NRC, 1977), safety factors ( SFs) ( later redefined as uncertainty factors) of 10, 100, or 1,000 were used, depending on the quality of the data.

Organoleptic effects. Organoleptic characteristics were also used in developing criteria for some contaminants to control undesirable taste and/ or odor imparted by them to ambient water. In some cases, a water quality criterion based on organoleptic effects would be more stringent than a criterion based on toxicologic endpoints. The 1980 AWQC National Guidelines emphasized that criteria derived for organoleptic endpoints are not based on toxicological information, have no direct relationship to adverse human health effects and, therefore, do not necessarily represent approximations of acceptable risk levels for humans. 1 4 1.4 RELATIONSHIP OF WATER QUALITY STANDARDS TO AWQC Under Section 303( c) of the CWA, States have the primary responsibility for establishing water quality standards, defined under the Act as designated beneficial uses of a water segment and the water quality criteria necessary to support those uses. Additionally, Native American Tribes authorized to administer the water quality standards program under 40 CFR 131.8 establish water quality standards for waters within their jurisdictions. This statutory framework allows States and authorized Tribes to work with local communities to adopt appropriate designated uses and to adopt criteria to protect those designated uses. Section 303( c) provides for EPA review of water quality standards and for promulgation of a superseding Federal rule in cases where State or Tribal standards are not consistent with the applicable requirements of the CWA and the implementing Federal regulations, or where the Agency determines Federal standards are necessary to meet the requirements of the Act. Section 303( c)( 2)( B) specifically requires States and authorized Tribes to adopt water quality criteria for toxics for which EPA has published criteria under Section 304( a) and for which the discharge or presence could reasonably be expected to interfere with the designated use adopted by the State or Tribe. In adopting such criteria, States and authorized Tribes must establish numerical values based on one of the following: ( 1) 304( a) criteria; ( 2) 304( a) criteria modified to reflect site specific conditions; or, ( 3) other scientifically defensible methods. In addition, States and authorized Tribes can establish narrative criteria where numeric criteria cannot be determined.

It must be recognized that the Act uses the term " criteria" in two different ways. In Section 303( c), the term is part of the definition of a water quality standard. Specifically, a water quality standard is composed of designated uses and the criteria necessary to protect those uses. Thus, States and authorized Tribes are required to adopt regulations which contain legally enforceable criteria. However, in Section 304( a) the term criteria is used to describe the scientific information that EPA develops to be used as guidance by States, authorized Tribes and EPA when establishing water quality standards pursuant to 303( c). Thus, two distinct purposes are served by the 304( a) criteria. The first is as guidance to the States and authorized Tribes in the development and adoption of water quality criteria which will protect designated uses, and the second is as the basis for promulgation of a superseding Federal rule when such action is necessary.

1.5 NEED FOR THE AWQC METHODOLOGY REVISIONS Since 1980, EPA risk assessment practices have evolved significantly in all of the major Methodology areas: that is, cancer and noncancer risk assessments, exposure assessments, and bioaccumulation. When the 1980 Methodology was developed, EPA had not yet developed formal cancer or noncancer risk assessment guidelines. Since then, EPA has published several risk assessment guidelines. In cancer risk assessment, there have been advances in the use of mode of action ( MOA) information to support both the identification of potential human carcinogens and the selection of procedures to characterize risk at low, environmentally relevant exposure levels. EPA published Proposed Guidelines for Carcinogen Risk Assessment ( USEPA, 1996a, hereafter the " 1996 proposed cancer guidelines"). These guidelines presented revised 1 5 procedures to quantify cancer risk at low doses, replacing the current default use of the LMS model. Following review by the Agency's Science Advisory Board ( SAB), EPA published the revised Guidelines for Carcinogen Risk Assessment   Review Draft in July 1999 ( USEPA, 1999a, hereafter the " 1999 draft revised cancer guidelines"). In noncancer risk assessment, the Agency is moving toward the use of the benchmark dose ( BMD) and other dose response approaches in place of the traditional NOAEL approach to estimate an RfD or Reference Concentration ( RfC). Guidelines for Mutagenicity Risk Assessment were published in 1986 ( USEPA, 1986b). In 1991, the Agency published Guidelines for Developmental Toxicity Risk Assessment ( USEPA, 1991), and it issued Guidelines for Reproductive Toxicity Risk Assessment in 1996 ( USEPA, 1996b). In 1998, EPA published final Guidelines for Neurotoxicity Risk Assessment ( USEPA, 1998), and in 1999 it issued the draft Guidance for Conducting Health Risk Assessment of Chemical Mixtures ( USEPA, 1999b).

In 1986, the Agency made available to the public the Integrated Risk Information System ( IRIS). IRIS is a database that contains risk information on the cancer and noncancer effects of chemicals. The IRIS assessments are peer reviewed and represent EPA consensus positions across the Agency's program and regional offices.

New studies have addressed water consumption and fish tissue consumption. These studies provide a more current and comprehensive description of national, regional, and specialpopulation consumption patterns that EPA has reflected in the 2000 Human Health Methodology. In addition, more formalized procedures are now available to account for human exposure from multiple sources when setting health goals such as AWQC that address only one exposure source. In 1986, the Agency published the Total Exposure Assessment Methodology ( TEAM) Study: Summary and Analysis, Volume I, Final Report ( USEPA, 1986c), which presents a process for conducting comprehensive evaluation of human exposures. In 1992, EPA published the revised Guidelines for Exposure Assessment ( USEPA, 1992), which describe general concepts of exposure assessment, including definitions and associated units, and provide guidance on planning and conducting an exposure assessment. The Exposure Factors Handbook was updated in 1997 ( USEPA, 1997a). Also in 1997, EPA developed Guiding Principles for Monte Carlo Analysis ( USEPA, 1997b) and published its Policy for Use of Probabilistic Analysis in Risk Assessment ( see http:// www. epa. gov/ ncea/ mcpolicy. htm). The Monte Carlo guidance can be applied to exposure assessments and risk assessments. The Agency has recently developed the Relative Source Contribution ( RSC) Policy for assessing total human exposure to a contaminant and apportioning the RfD among the media of concern, published for the first time in this Methodology.

The Agency has moved toward the use of a bioaccumulation factor ( BAF) to reflect the uptake of a contaminant from all sources ( e. g., ingestion, sediment) by fish and shellfish, rather than just from the water column as reflected by the use of a bioconcentration factor ( BCF) in the 1980 Methodology. The Agency has also developed detailed procedures and guidelines for estimating BAF values. 1 6 Another reason for the 2000 Human Health Methodology is the need to bridge the gap between the differences in the risk assessment and risk management approaches used by EPA's Office of Water for the derivation of AWQC under the authority of the CWA and Maximum Contaminant Level Goals ( MCLGs) under the Safe Drinking Water Act ( SDWA). Three notable differences are the treatment of chemicals designated as Group C, possible human carcinogens under the 1996 proposed cancer guidelines, the consideration of non water sources of exposure when setting an AWQC or MCLG for a noncarcinogen, and cancer risk ranges. Those three differences are described in the three subsections below, respectively.

1.5.1 Group C Chemicals Chemicals were typically classified as Group C   i. e., possible human carcinogens   under the existing ( 1986) EPA cancer classification scheme for any of the following reasons:

1) Carcinogenicity has been documented in only one test species and/ or only one cancer bioassay and the results do not meet the requirements of " sufficient evidence."

2) Tumor response is of marginal statistical significance due to inadequate design or reporting.

3) Benign, but not malignant, tumors occur with an agent showing no response in a variety of short term tests for mutagenicity.

4) There are responses of marginal statistical significance in a tissue known to have a high or variable background rate.

The 1986 Guidelines for Carcinogen Risk Assessment ( hereafter the " 1986 cancer guidelines") specifically recognized the need for flexibility with respect to quantifying the risk of Group C, possible human carcinogens. The 1986 cancer guidelines noted that agents judged to be in Group C, possible human carcinogens, may generally be regarded as suitable for quantitative risk assessment, but that case by case judgments may be made in this regard.

The EPA Office of Water has historically treated Group C chemicals differently under the CWA and the SDWA. It is important to note that the 1980 AWQC National Guidelines for setting AWQC under the CWA predated EPA's carcinogen classification system, which was proposed in 1984 ( USEPA, 1984) and finalized in 1986 ( USEPA, 1986a). The 1980 AWQC National Guidelines did not explicitly differentiate among agents with respect to the weight of evidence for characterizing them as likely to be carcinogenic to humans. For all pollutants judged as having adequate data for quantifying carcinogenic risk   including those now classified as Group C   AWQC were derived based on data on cancer incidence. In the 1980 AWQC National Guidelines, EPA emphasized that the AWQC for carcinogens should state that the recommended concentration for maximum protection of human health is zero. At the same time, the criteria 1 7 published for specific carcinogens presented water concentrations for these pollutants corresponding to individual lifetime excess cancer risk levels in the range of 10 7 to 10 5.

In the development of national primary drinking water regulations under the SDWA, EPA is required to promulgate a health based MCLG for each contaminant. The Agency policy has been to set the MCLG at zero for chemicals with strong evidence of carcinogenicity associated with exposure from water. For chemicals with limited evidence of carcinogenicity, including many Group C agents, the MCLG was usually obtained using an RfD based on the pollutant's noncancer effects with the application of an additional uncertainty factor of 1 to 10 to account for carcinogenic potential of the chemical. If valid noncancer data for a Group C agent were not available to establish an RfD but adequate data are available to quantify the cancer risk, then the MCLG was based upon a nominal lifetime excess cancer risk in the range of 10 6 to10 5 ( ranging from one case in a population of one million to one case in a population of one hundred thousand). Even in those cases where the RfD approach has been used for the derivation of the MCLG for a Group C agent, the drinking water concentrations associated with excess cancer risks in the range of 10 6 to 10 5 were also provided for comparison.

It should also be noted that EPA's pesticides program has applied both of the previously described methods for addressing Group C chemicals in actions taken under the Federal Insecticide, Fungicide, and Rodenticide Act ( FIFRA) and finds both methods applicable on a caseby case basis. Unlike the drinking water program, however, the pesticides program does not add an extra uncertainty factor to account for potential carcinogenicity when using the RfD approach.

In the 1999 draft revised cancer guidelines, there are no more alphanumeric categories. Instead, there will be longer narratives for hazard characterization that will use consistent descriptive terms when assessing cancer risk.

1.5.2 Consideration of Non water Sources of Exposure The 1980 AWQC National Guidelines recommended that contributions from non water sources, namely air and non fish dietary intake, be subtracted from the Acceptable Daily Intake ( ADI), thus reducing the amount of the ADI " available" for water related sources of intake. In practice, however, when calculating human health criteria, these other exposures were generally not considered because reliable data on these exposure pathways were not available. Consequently, the AWQC were usually derived such that drinking water and fish ingestion accounted for the entire ADI ( now called RfD).

In the drinking water program, a similar " subtraction" method was used in the derivation of MCLGs proposed and promulgated in drinking water regulations through the mid 1980s. More recently, the drinking water program has used a " percentage" method in the derivation of MCLGs for noncarcinogens. In this approach, the percentage of total exposure typically accounted for by drinking water, referred to as the relative source contribution ( RSC), is applied to the RfD to determine the maximum amount of the RfD " apportioned" to drinking water reflected by the MCLG value. In using this percentage procedure, the drinking water program 1 8 also applies a ceiling level of 80 percent of the RfD and a floor level of 20 percent of the RfD. That is, the MCLG cannot account for more than 80 percent of the RfD, nor less than 20 percent of the RfD.

The drinking water program usually takes a conservative approach to public health by applying an RSC factor of 20 percent to the RfD when adequate exposure data do not exist, assuming that the major portion ( 80 percent) of the total exposure comes from other sources, such as diet.

In the 2000 Human Health Methodology, guidance for the routine consideration of nonwater sources of exposure [ both ingestion exposures ( e. g., food) and exposures other than the oral route ( e. g., inhalation)] is presented. The approach is called the Exposure Decision Tree. Relative source contribution estimates will be made by EPA using this approach, which allows for use of either the subtraction or percentage methods, depending on chemical specific circumstances, within the 20 to 80 percent range described above.

1.5.3 Cancer Risk Ranges In addition to the different risk assessment approaches discussed above for deriving AWQC and MCLGs for Group C agents, there have been different risk management approaches by the drinking water and surface water programs on lifetime excess risk values when setting health based criteria for carcinogens. The surface water program has derived AWQC for carcinogens that generally corresponded to lifetime excess cancer risk levels of 10 7 to 10 5. The drinking water program has set MCLGs for Group C agents based on a slightly less stringent risk range of 10 6 to 10 5, while MCLGs for chemicals with strong evidence of carcinogenicity ( that is, classified as Group A, known, or B probable, human carcinogen) are set at zero. The drinking water program is now following the principles of the 1999 draft revised cancer guidelines to determine the type of low dose extrapolation based on mode of action.

It is also important to note that under the drinking water program, for those substances having an MCLG of zero, enforceable Maximum Contaminant Levels ( MCLs) have generally been promulgated to correspond with cancer risk levels ranging from 10 6 to 10 4. Unlike AWQC and MCLGs which are strictly health based criteria, MCLs are developed with consideration given to the costs and technological feasibility of reducing contaminant levels in water to meet those standards.

With the 2000 Human Health Methodology, EPA will publish its national 304( a) water quality criteria at a 10 6 risk level, which EPA considers appropriate for the general population. EPA is increasing the degree of consistency between the drinking water and ambient water programs, given the somewhat different requirements of the CWA and SDWA. 2 Although appearing in this equation as a factor to be multiplied, the RSC can also be an amount subtracted. Refer to the explanation key below the equations.

1 9 ( Equation 1 1)

( Equation 1 2) 1.6 OVERVIEW OF THE AWQC METHODOLOGY REVISIONS The following equations for deriving AWQC include toxicological and exposure assessment parameters which are derived from scientific analysis, science policy, and risk management decisions. For example, values for parameters such as a field measured BAF or a point of departure from an animal study [ in the form of a lowest observed adverse effect level ( LOAEL)/ no observed  adverse effect level ( NOAEL)/ lower 95 percent confidence limit on a dose associated with a 10 percent extra risk ( LED 10)] are empirically measured using scientific methods. By contrast, the decision to use animal effects as surrogates for human effects involves judgment on the part of the EPA ( and similarly, by other agencies) as to the best practice to follow when human data are lacking. Such a decision is, therefore, a matter of science policy. The choice of default fish consumption rates for protection of a certain percentage ( i. e., the 90th percentile) of the general population is clearly a risk management decision. In many cases, the Agency has selected parameter values using its best judgment regarding the overall protection afforded by the resulting AWQC when all parameters are combined. For a longer discussion of the differences between science, science policy, and risk management, please refer to Section 2 of this document. Section 2 also provides further details with regard to risk characterization for this Methodology, with emphasis placed on explaining the uncertainties in the overall risk assessment.

The generalized equations for deriving AWQC based on noncancer effects are:

Noncancer Effects2 Cancer Effects: Nonlinear Low Dose Extrapolation 1 10 ( Equation 1 3) Cancer Effects: Linear Low Dose Extrapolation where:

AWQC = Ambient Water Quality Criterion ( mg/ L) RfD = Reference dose for noncancer effects ( mg/ kg day) POD = Point of departure for carcinogens based on a nonlinear low dose extrapolation ( mg/ kg day), usually a LOAEL, NOAEL, or LED 10 UF = Uncertainty Factor for carcinogens based on a nonlinear low dose extrapolation ( unitless) RSD = Risk specific dose for carcinogens based on a linear low dose extrapolation ( mg/ kg day) ( dose associated with a target risk, such as 10 6) RSC = Relative source contribution factor to account for non water sources of exposure. ( Not used for linear carcinogens.) May be either a percentage ( multiplied) or amount subtracted, depending on whether multiple criteria are relevant to the chemical. BW = Human body weight ( default = 70 kg for adults) DI = Drinking water intake ( default = 2 L/ day for adults) FI i = Fish intake at trophic level ( TL) I ( I = 2, 3, and 4) ( defaults for total intake = 0.0175 kg/ day for general adult population and sport anglers, and 0.1424 kg/ day for subsistence fishers). Trophic level breakouts for the general adult population and sport anglers are: TL2 = 0.0038 kg/ day; TL3 = 0.0080 kg/ day; and TL4 = 0.0057 kg/ day. BAF i = Bioaccumulation factor at trophic level I ( I= 2, 3 and 4), lipid normalized ( L/ kg)

For highly bioaccumulative chemicals where ingestion from water might be considered negligible, EPA is currently evaluating the feasibility of developing and implementing AWQCs that are expressed in terms of concentrations in tissues of aquatic organisms. Such tissue residue criteria might be used as an alternative to AWQCs which are expressed as concentrations in water, particularly in situations where AWQCs are at or below the practical limits for quantifying a chemical in water. Even though tissue residue criteria would not require the use of a BAF in their derivation, implementing such criteria would still require a mechanism for relating chemical loads and concentrations in water and sediment to concentrations in tissues of appropriate fish and shellfish ( e. g., a BAF or bioaccumulation model). At this time, no revisions are planned to the Methodology to provide specific guidance on developing fish tissue based water quality criteria. 1 11 However, guidance may be provided in the future either as a separate document or integrated in a specific 304( a) water quality criteria document for a chemical that warrants such an approach.

AWQC for the protection of human health are designed to minimize the risk of adverse effects occurring to humans from chronic ( lifetime) exposure to substances through the ingestion of drinking water and consumption of fish obtained from surface waters. The Agency is not recommending the development of additional water quality criteria similar to the " drinking water health advisories" that focus on acute or short term effects; these are not seen as routinely having a meaningful role in the water quality criteria and standards program. However, as discussed below, there may be some instances where the consideration of acute or short term toxicity and exposure in the derivation of AWQC is warranted.

Although the AWQC are based on chronic health effects data ( both cancer and noncancer effects), the criteria are intended to also be protective against adverse effects that may reasonably be expected to occur as a result of elevated acute or short term exposures. That is, through the use of conservative assumptions with respect to both toxicity and exposure parameters, the resulting AWQC should provide adequate protection not only for the general population over a lifetime of exposure, but also for special subpopulations who, because of high water or fishintake rates, or because of biological sensitivities, have an increased risk of receiving a dose that would elicit adverse effects. The Agency recognizes that there may be some cases where the AWQC based on chronic toxicity may not provide adequate protection for a subpopulation at special risk from shorter term exposures. The Agency encourages States, Tribes, and others employing the 2000 Human Health Methodology to give consideration to such circumstances in deriving criteria to ensure that adequate protection is afforded to all identifiable subpopulations. ( See Section 4.3, Factors Used in the AWQC Computation, for additional discussion of these subpopulations.)

The EPA is in the process of revising its cancer guidelines, including its descriptions of human carcinogenic potential. Once final guidelines are published, they will be the basis for assessment under this methodology. In the meanwhile, the 1986 guidelines are used and extended with principles discussed in EPA's 1999 Guidelines for Carcinogen Risk Assessment  Review Draft ( hereafter " 1999 draft revised cancer guidelines"). These principles arise from new science about cancer discovered in the last 15 years and from EPA policy of recent years supporting full characterization of hazard and risk both for the general population and potentially sensitive groups such as children. These principles are incorporated in recent and ongoing assessments such as the reassessment of dioxin, consistent with the 1986 guidelines. Until final guidelines are published, information is presented to describe risk under both the old guidelines and draft revisions. Dose response assessment under the 1986 guidelines employs a linearized multistage model to extrapolate tumor dose response observed in animal or human studies down to zero dose, zero extra risk. The dose response assessment under EPA's 1999 draft revised cancer guidelines is a two step process. In the first step, the response data are modeled in the range of empirical observation. Modeling in the observed range is done with biologically based or appropriate curve fitting modeling. In the second step, extrapolation below the range of observation is accomplished by biologically based modeling if there are sufficient data or by a 1 12 default procedure ( linear, nonlinear, or both). A point of departure ( POD) for extrapolation is estimated from modeling observed data. The lower 95 percent confidence limit on a dose associated with 10 percent extra risk ( LED 10) is the standard POD for low dose extrapolation. The linear default procedure is a straight line extrapolation to the origin ( i. e., zero dose, zero extra risk) from the POD, which is the LED 10 identified in the observable response range. The result of this procedure is generally comparable ( within 2 fold) to that of using a linearized multistage model under existing, 1986 guidelines. The linear low dose extrapolation applies to agents that are best characterized by the assumption of linearity ( e. g., direct DNA reactive mutagens) for their MOA. A linear approach would also be applied when inadequate or no information is available to explain the carcinogenic MOA; this is a science policy choice in the interest of public health. If it is determined that the MOA understanding fully supports a nonlinear extrapolation, the AWQC is derived using the nonlinear default which is based on a margin of exposure ( MOE) analysis using the LED 10 as the POD and applying uncertainty factors ( UFs) to arrive at an acceptable MOE. There may be situations where it is appropriate to apply both the linear and nonlinear default procedures ( e. g., for an agent that is both DNA reactive and active as a promoter at higher doses).

For substances that are carcinogenic, particularly those for which the MOA suggests nonlinearity at low doses, the Agency recommends that an integrated approach be taken in looking at cancer and noncancer effects. If one effect does not predominate, AWQC values should be determined for both carcinogenic and noncarcinogenic endpoints. The lower of the resulting values should be used for the AWQC.

When deriving AWQC for noncarcinogens and carcinogens based on a nonlinear low dose extrapolation, a factor is included to account for other non water exposure sources [ both ingestion exposures ( e. g., food) and exposures other than the oral route ( e. g., inhalation)] so that the entire RfD, or POD/ UF, is not apportioned to drinking water and fish consumption alone. Guidance is provided in the 2000 Human Health Methodology for determining the factor ( i. e., the RSC) to be used for a particular chemical. The Agency is recommending the use of an Exposure Decision Tree procedure to support the determination of the appropriate RSC value for a given water contaminant. In the absence of data, the Agency intends to use 20 percent of the RfD ( or POD/ UF) as the default RSC in calculating 304( a) criteria or promulgating State or Tribal water quality standards under Section 303( c).

With AWQC derived for carcinogens based on a linear low dose extrapolation, the Agency will publish recommended criteria values at a 10 6 risk level. States and authorized Tribes can always choose a more stringent risk level, such as 10 7. EPA also believes that criteria based on a 10 5 risk level are acceptable for the general population as long as States and authorized Tribes ensure that the risk to more highly exposed subgroups ( sportfishers or subsistence fishers) does not exceed the 10 4 level. Clarification on this risk management decision is provided in Section 2 of this document.

The default fish consumption value for the general adult population in the 2000 Human Health Methodology is 17.5 grams/ day, which represents an estimate of the 90th percentile 1 13 consumption rate for the U. S. adult population based on the U. S. Department of Agriculture's ( USDA's) Continuing Survey of Food Intake by Individuals ( CSFII) 1994 96 data ( USDA, 1998). EPA will use this default intake rate with future national 304( a) criteria derivations or revisions. This default value is chosen to be protective of the majority of the general population. However, States and authorized Tribes are urged to use a fish intake level derived from local data on fish consumption in place of this default value when deriving AWQC, ensuring that the fish intake level chosen is protective of highly exposed individuals in the population. EPA has provided default values for States and authorized Tribes that do not have adequate information on local or regional consumption patterns, based on numerous studies that EPA has reviewed on sport anglers and subsistence fishers. EPA's defaults for these population groups are estimates of their average consumption. EPA recommends a default of 17.5 grams/ day for sport anglers as an approximation of their average consumption and 142.4 grams/ day for subsistence fishers, which falls within the range of averages for this group. Consumption rates for women of childbearing age and children younger than 14 are also provided to maximize protection in those cases where these subpopulations may be at greatest risk.

In the 2000 Human Health Methodology, criteria are derived using a BAF rather than a BCF. To derive the BAF, States and authorized Tribes may use EPA's Methodology or any method consistent with this Methodology. EPA's highest preference in developing BAFs are BAFs based on field measured data from local/ regional fish.

1.7 REFERENCES NRC ( National Research Council). 1977. Drinking Water and Health. Safe Drinking Water Committee. National Academy of Sciences, National Academy Press. Washington, DC.

USDA. 1998. U. S. Department of Agriculture. 1994   1996 Continuing Survey of Food Intakes by Individuals and 1994   1996 Diet and Health Knowledge Survey. Agricultural Research Service, USDA. NTIS CD   ROM, accession number PB98   500457.

USEPA ( U. S. Environmental Protection Agency). 1980. Guidelines and methodology used in the preparation of health effect assessment chapters of the consent decree water criteria documents. Federal Register 45: 79347, Appendix 3.

USEPA ( U. S. Environmental Protection Agency). 1984. Proposed guidelines for carcinogen risk assessment. Federal Register 49: 46294.

USEPA ( U. S. Environmental Protection Agency). 1986a. Guidelines for carcinogen risk assessment. Federal Register 51: 33992 34003.

USEPA ( U. S. Environmental Protection Agency). 1986b. Guidelines for mutagenicity risk assessment. Federal Register 51: 34006 34012. 1 14 USEPA ( U. S. Environmental Protection Agency). 1986c. Total Exposure Assessment Model ( TEAM) Study: Summary and Analysis, Volume I. Final Report. EPA/ 600/ 6 87/ 002a.

USEPA ( U. S. Environmental Protection Agency). 1986d. Guidelines for exposure assessment. Federal Register 51: 34042 34054.

USEPA ( U. S. Environmental Protection Agency). 1991. Guidelines for developmental toxicity risk assessment. Federal Register 56: 63789 63826.

USEPA ( U. S. Environmental Protection Agency). 1992. Guidelines for exposure assessment. Federal Register 57: 22888 22938.

USEPA ( U. S. Environmental Protection Agency). 1996a. Proposed guidelines for carcinogen risk assessment. Federal Register 61: 17960 18011.

USEPA ( U. S. Environmental Protection Agency). 1996b. Guidelines for reproductive toxicity risk assessment. Federal Register 61: 6274 56322.

USEPA ( U. S. Environmental Protection Agency). 1997a. Exposure Factors Handbook. Office of Research and Development. Washington, DC. EPA/ 600/ P 95/ 002Fa..

USEPA ( U. S. Environmental Protection Agency). 1997b. Guiding Principles for Monte Carlo Analysis. Risk Assessment Forum. Washington, DC. EPA/ 630/ R 97/ 001.

USEPA ( U. S. Environmental Protection Agency). 1998. Guidelines for neurotoxicity risk assessment. Federal Register 63: 26926.

USEPA ( U. S. Environmental Protection Agency). 1999a. 1999 Guidelines for Carcinogen Risk Assessment. Review Draft. Office of Research and Development. Washington, DC. NCEA F 0644.

USEPA ( U. S. Environmental Protection Agency). 1999b. Guidance for Conducting Health Risk Assessment of Chemical Mixtures. Final Draft. Risk Assessment Forum Technical Panel. Washington, DC. EPA/ NCEA C 0148. September. Website: http:// www. epa. gov/ ncea/ raf/ rafpub. htm USEPA ( U. S. Environmental Protection Agency). 2000. Methodology for Deriving Ambient Water Quality Criteria for the Protection of Human Health ( 2000). Technical Support Document Volume 1: Risk Assessment. Office of Science and Technology, Office of Water. Washington, DC. EPA 822 B 00 005. August. 2 1 2. CLARIFICATIONS ON THE METHODOLOGY, RISK CHARACTERIZATION, AND OTHER ISSUES FOR DEVELOPING CRITERIA 2.1 IDENTIFYING THE POPULATION SUBGROUP THAT THE AWQC SHOULD PROTECT Water quality criteria are derived to establish ambient concentrations of pollutants which, if not exceeded, will protect the general population from adverse health impacts from those pollutants due to consumption of aquatic organisms and water, including incidental water consumption related to recreational activities. For each pollutant, chronic criteria are derived to reflect long term consumption of food and water. An important decision to make when setting AWQC is the choice of the particular population to protect. For instance, criteria could be set to protect those individuals who have average or " typical" exposures, or the criteria could be set so that they offer greater protection to those individuals who are more highly exposed. EPA has selected default parameter values that are representative of several defined populations: adults in the general population; sport ( recreational) fishers; subsistence fishers; women of childbearing age ( defined as ages 15 44); and children ( up to the age of 14). In deciding on default parameter values, EPA is aware that multiple parameters are used in combination when calculating AWQC ( e. g., intake rates and body weight). EPA describes the estimated population percentiles that are represented by each of the default exposure parameter values in Section 4.

EPA's national 304( a) criteria are usually derived to protect the majority of the general population from chronic adverse health effects. EPA has used a combination of median values, mean values, and percentile estimates for the parameter value defaults to calculate its national 304( a) criteria. EPA believes that its assumptions afford an overall level of protection targeted at the high end of the general population ( i. e., the target population or the criteria basis population). EPA also believes that this is reasonably conservative and appropriate to meet the goals of the CWA and the 304( a) criteria program. EPA considers that its target protection goal is satisfied if the population as a whole will be adequately protected by the human health criteria when the criteria are met in ambient water. However, associating the derived criteria with a specific population percentile is far more difficult, and such a quantitative descriptor typically requires detailed distributional exposure and dose information. EPA's Guidelines For Exposure Assessment ( USEPA, 1992) describes the extreme difficulty in making accurate estimates of exposures and indicates that uncertainties at the more extreme ends of the distribution increase greatly. On quantifying population exposures/ risks, the guidelines specifically state:

In practice, it is difficult even to establish an accurate mean health effect risk for a population. This is due to many complications, including uncertainties in using animal data for human dose response relationships, nonlinearities in the doseresponse curve, projecting incidence data from one group to another dissimilar group, etc. Although it has been common practice to estimate the number of cases of disease, especially cancer, for populations exposed to chemicals, it should be understood that these estimates are not meant to be accurate estimates of real ( or actuarial) cases of disease. The estimate's value lies in framing 2 2 hypothetical risk in an understandable way rather than in any literal interpretation of the term " cases."

Although it is not possible to subject the estimates to such a rigorous analysis ( say, for example, to determine what criterion value provides protection of exactly the 90th percentile of the population), EPA believes that the combination of parameter value assumptions achieves its target goal, without being inordinately conservative. The standard assumptions made for the national 304( a) criteria are as follows. The assumed body weight value used is an arithmetic mean, as are the RSC intake estimates of other exposures ( e. g., non fish dietary), when data are available. The BAF component data ( e. g., for lipid values, for particulate and dissolved organic carbon) are based on median ( i. e., 50th percentile) values. The drinking water intake values are approximately 90th percentile estimates and fish intake values are 90th percentile estimates. EPA believes the use of these values will result in 304( a) criteria that are protective of a majority of the population; this is EPA's goal.

However, EPA also strongly believes that States and authorized Tribes should have the flexibility to develop criteria, on a site specific basis, that provide additional protection appropriate for highly exposed populations. EPA is aware that exposure patterns in general, and fish consumption in particular, vary substantially. EPA understands that highly exposed populations may be widely distributed geographically throughout a given State or Tribal area. EPA recommends that priority be given to identifying and adequately protecting the most highly exposed population. Thus, if the State or Tribe determines that a highly exposed population is at greater risk and would not be adequately protected by criteria based on the general population, and by the national 304( a) criteria in particular, EPA recommends that the State or Tribe adopt more stringent criteria using alternative exposure assumptions.

EPA has provided recommended default intake rates for various population groups for State and Tribal consideration. EPA does not intend for these alternative default values to be prescriptive. EPA strongly emphasizes its preference that States and Tribes use local or regional data over EPA's defaults, if they so choose, as being more representative of their population groups of concern.

In the course of updating the 2000 Human Health Methodology, EPA received some questions regarding the population groups for which the criteria would be developed. EPA does not intend to derive multiple 304( a) criteria for all subpopulation groups for every chemical. As stated above, criteria that address chronic adverse health effects are most applicable to the CWA Section 304( a) criteria program and the chemicals evaluated for this program. If EPA determined that pregnant women/ fetuses or young children were the target population ( or criteria basis population) of a chemical's RfD or POD/ UF, then the 304( a) criteria would be developed using exposure parameters for that subgroup. This would only be relevant for acute or subchronic toxicity situations. This does not conflict with the fact that chronic health effects potentially reflect a person's exposure during both childhood and adult years. 2 3 For RfD based and POD/ UF based chemicals, EPA's policy is that, in general, the RfD ( or POD/ UF) should not be exceeded and the exposure assumptions used should reflect the population of concern. It is recommended that when a State or authorized Tribe sets a waterbody specific AWQC, they consider the populations most exposed via water and fish. EPA's policy on cancer risk management goals is discussed in Section 2.4.

Health Risks to Children In recognition that children have a special vulnerability to many toxic substances, EPA's Administrator directed the Agency in 1995 to explicitly and consistently take into account environmental health risks to infants and children in all risk assessments, risk characterizations, and public health standards set for the United States. In April 1997, President Clinton signed Executive Order 13045 on the protection of children from environmental health risks, which assigned a high priority to addressing risks to children. In May 1997, EPA established the Office of Children's Health Protection to ensure the implementation of the President's Executive Order. EPA has increased efforts to ensure its guidance and regulations take into account risks to children. Circumstances where risks to children should be considered in the context of the 2000 Human Health Methodology are discussed in the Section 3.2, Noncancer Effects ( in terms of developmental and reproductive toxicity) and in Section 4, Exposure ( for appropriate exposure intake parameters).

Details on risk characterization and the guiding principles stated above are included in EPA's March 21, 1995 policy statement and the discussion of risk characterization ( USEPA, 1995) and the 1999 Guidelines for Carcinogen Risk Assessment. Review Draft ( USEPA, 1999a) and the Reproductive and Toxicity Risk Assessment Guidelines of 1996 ( USEPA, 1996b).

2.2 SCIENCE, SCIENCE POLICY, AND RISK MANAGEMENT An important part of risk characterization, as described later in Section 2.7, is to make risk assessments transparent. This means that conclusions drawn from the science are identified separately from policy judgments and risk management decisions, and that the use of default values or methods, as well as the use of assumptions in risk assessments, are clearly articulated. In this Methodology, EPA has attempted to separate scientific analysis from science policy and risk management decisions for clarity. This should allow States and Tribes ( who are also prospective users of this Methodology) to understand the elements of the Methodology accurately and clearly, and to easily separate out the scientific decisions from the science policy and risk management decisions. This is important so that when questions are asked regarding the scientific merit, validity, or apparent stringency or leniency of AWQC, the implementer of the criteria can clearly explain what judgments were made to develop the criterion in question and to what degree these judgments were based on science, science policy, or risk management. To some extent this process will also be displayed in future AWQC documents.

When EPA speaks of science or scientific analysis, it is referring to the extraction of data from toxicological or exposure studies and surveys with a minimum of judgment being used to 2 4 make inferences from the available evidence. For example, if EPA is describing a POD from an animal study ( e. g., a LOAEL), this is usually determined as a lowest dose that produces an observable adverse effect. This would constitute a scientific determination. Judgments applying science policy, however, may enter this determination. For example, several scientists may differ in their opinion of what is adverse, and this in turn can influence the selection of a LOAEL in a given study. The use of an animal study to predict effects in a human in the absence of human data is an inherent science policy decision. The selection of specific UFs when developing an RfD is another example of science policy. In any risk assessment, a number of decision points occur where risk to humans can only be inferred from the available evidence. Both scientific judgments and policy choices may be involved in selecting from among several possible inferences when conducting a risk assessment.

Risk management is the process of selecting the most appropriate guidance or regulatory actions by integrating the results of risk assessment with engineering data and with social, economic, and political concerns to reach a decision. In this Methodology, the choice of a default fish consumption rate which is protective of 90 percent of the general population is a risk management decision. The choice of an acceptable cancer risk by a State or Tribe is a risk management decision.

Many of the components in the 2000 Human Health Methodology are an amalgam of science, science policy, and/ or risk management. For example, most of the default values chosen by EPA are based on examination of scientific data and application of either science policy or risk management. This includes the default assumption of 2 liters a day of drinking water; the assumption of 70 kilograms for an adult body weight; the use of default percent lipid and particulate organic carbon/ dissolved organic carbon ( POC/ DOC) for developing national BAFs; the default fish consumption rates for the general population and sport and subsistence anglers; and the choice of a default cancer risk level. Some decisions are more grounded in science and science policy ( such as the choice of default BAFs) and others are more obviously risk management decisions ( such as the determination of default fish consumption rates and cancer risk levels). Throughout the 2000 Human Health Methodology, EPA has identified the kind of decision necessary to develop defaults and what the basis for the decision was. More details on the concepts of science analysis, science policy, risk management, and how they are introduced into risk assessments are included in Risk Assessment in the Federal Government: Managing the Process ( NRC, 1983).

2.3 SETTING CRITERIA TO PROTECT AGAINST MULTIPLE EXPOSURES FROM MULTIPLE CHEMICALS ( CUMULATIVE RISK)

EPA is very much aware of the complex issues and implications of cumulative risk and has endeavored to begin developing an overall approach at the Agency wide level. Assuming that multiple exposures to multiple chemicals are additive is scientifically sound if they exhibit the same toxic endpoints and modes of action. There are numerous publications relevant to cumulative risk that can assist States and Tribes in understanding the complex issues associated with cumulative risk. These include the following: 2 5 < Durkin, P. R., R. C. Hertzberg, W. Stiteler, and M. Mumtaz. 1995. The identification and testing of interaction patterns. Toxicol. Letters 79: 251 264.

< Hertzberg, R. C., G. Rice, and L. K. Teuschler. 1999. Methods for health risk assessment of combustion mixtures. In: Hazardous Waste Incineration: Evaluating the Human Health and Environmental Risks. S. Roberts, C. Teaf and J. Bean, ( eds). CRC Press LLC, Boca Raton, FL. Pp. 105 148.

< Rice, G., J. Swartout, E. Brady Roberts, D. Reisman, K. Mahaffey, and B. Lyon. 1999. Characterization of risks posed by combustor emissions. Drug and Chem. Tox. 22: 221 240.

< USEPA. 1999. Guidance for Conducting Health Risk Assessment of Chemical Mixtures. Final Draft. Risk Assessment Forum Technical Panel. Washington, DC. NCEA C 0148. September. Web site: http:// www. epa. gov/ ncea/ raf/ rafpub. htm < USEPA. 1998. Methodology for Assessing Health Risks Associated with Multiple Pathways of Exposure to Combustor Emissions. ( Update to EPA/ 600/ 6 90/ 003 Methodology for Assessing Health Risks Associated with Indirect Exposure to Combustor Emissions). National Center for Environmental Assessment. Washington, DC. EPA 600 R 98 137. Website http:// www. epa. gov/ ncea/ combust. htm < USEPA. 1996. PCBs: Cancer Dose Response Assessment and Application to Environmental Mixtures. National Center for Environmental Assessment. Washington, DC. EPA/ 600/ P 96/ 001F.

< USEPA. 1993. Review Draft Addendum to the Methodology for Assessing Health Risks Associated with Indirect Exposure to Combustor Emissions. Office of Health and Environmental Assessment, Office of Research and Development. Washington, DC. EPA/ 600/ AP 93/ 003. November.

< USEPA. 1993. Provisional Guidance for Quantitative Risk Assessment of Polycyclic Aromatic Hydrocarbons. Office of Research and Development. Washington, DC. EPA/ 600/ R 93/ 089. July.

< USEPA. 1990. Technical Support Document on Health Risk Assessment of Chemical Mixtures. Office of Research and Development. Washington, DC. EPA/ 600/ 8/ 90/ 064. August.

< USEPA. 1989a. Risk Assessment Guidance for Superfund. Vol. 1. Human Health Evaluation Manual ( Part A). Office of Emergency and Remedial Response. Washington, DC. EPA/ 540/ 1 89/ 002. 2 6 < USEPA. 1989b. Interim Procedures for Estimating Risks Associated with Exposures to Mixtures of Chlorinated Dibenzo p Dioxins and  Dibenzofurans ( CDDs and CDFs) and 1989 Update. Risk Assessment Forum. Washington, DC. EPA/ 625/ 3 89/ 016. March.

The Agency's program offices are also engaged in on going discussions of the great complexities, methodological challenges, data adequacy needs and other information gaps, as well as the science policy and risk management decisions that will need to be made, as they pursue developing a sound strategy and, eventually, specific guidance for addressing cumulative risks. As a matter of internal policy, EPA is committed to refining the Methodology as advances in relevant aspects of the science improve, as part of the water quality criteria program.

2.4 CANCER RISK RANGE For deriving 304( a) criteria or promulgating water quality criteria for States and Tribes under Section 303( c) based on the 2000 Human Health Methodology, EPA intends to use the 10 6 risk level, which the Agency believes reflects an appropriate risk for the general population. EPA's program office guidance and regulatory actions have evolved in recent years to target a 10 ! 6 risk level as an appropriate risk for the general population. EPA has recently reviewed the policies and regulatory language of other Agency mandates ( e. g., the Clean Air Act Amendments of 1990, the Food Quality Protection Act) and believes the target of a 10 ! 6 risk level is consistent with Agency wide practice.

EPA believes that both 10 ! 6 and 10 ! 5 may be acceptable for the general population and that highly exposed populations should not exceed a 10 ! 4 risk level. States or Tribes that have adopted standards based on criteria at the 10 ! 5 risk level can continue to do so, if the highly exposed groups would at least be protected at the 10 ! 4 risk level. However, EPA is not automatically assuming that 10 ! 5 will protect " the highest consumers" at the 10 ! 4 risk level. Nor is EPA advocating that States and Tribes automatically set criteria based on assumptions for highly exposed population groups at the 10 ! 4 risk level. The Agency is simply endeavoring to add that a specific determination should be made to ensure that highly exposed groups do not exceed a 10 ! 4 risk level. EPA understands that fish consumption rates vary considerably, especially among subsistence populations, and it is such great variation among these population groups that may make either 10 ! 6 or 10 ! 5 protective of those groups at a 10 ! 4 risk level. Therefore, depending on the consumption patterns in a given State or Tribal jurisdiction, a 10 ! 6 or 10 ! 5 risk level could be appropriate. In cases where fish consumption among highly exposed population groups is of a magnitude that a 10 ! 4 risk level would be exceeded, a more protective risk level should be chosen. Such determinations should be made by the State or Tribal authorities and are subject to EPA's review and approval or disapproval under Section 303( c) of the CWA.

Adoption of a 10 ! 6 or 10 ! 5 risk level, both of which States and authorized Tribes have chosen in adopting water quality standards to date, represents a generally acceptable risk management decision, and EPA intends to continue providing this flexibility to States and Tribes. EPA believes that such State or Tribal decisions are consistent with Section 303( c) if the State or authorized Tribe has identified the most highly exposed subpopulation, has demonstrated that the 2 7 chosen risk level is adequately protective of the most highly exposed subpopulation, and has completed all necessary public participation. States and authorized Tribes also have flexibility in how they demonstrate this protectiveness and obtain such information. A State or authorized Tribe may use existing information as well as collect new information in making this determination. In addition, if a State or authorized Tribe does not believe that the 10 ! 6 risk level adequately protects the exposed subpopulations, water quality criteria based on a more stringent risk level may be adopted. This discretion includes combining the 10 ! 6 risk level with fish consumption rates for highly exposed population groups.

It is important to understand that criteria for carcinogens are based on chosen risk levels that inherently reflect, in part, the exposure parameters used to derive those values. Therefore, changing the exposure parameters also changes the risk. Specifically, the incremental cancer risk levels are relative, meaning that any given criterion associated with a particular cancer risk level is also associated with specific exposure parameter assumptions ( e. g., intake rates, body weights). When these exposure parameter values change, so does the relative risk. For a criterion derived on the basis of a cancer risk level of 10 ! 6, individuals consuming up to 10 times the assumed fish intake rate would not exceed a 10 ! 5 risk level. Similarly, individuals consuming up to 100 times the assumed rate would not exceed a 10 ! 4 risk level. Thus, for a criterion based on EPA's default fish intake rate ( 17.5 gm/ day) and a risk level of 10 ! 6, those consuming a pound per day ( i. e., 454 grams/ day) would potentially experience between a 10 ! 5 and a 10 ! 4 risk level ( closer to a 10 ! 5 risk level). ( Note: Fish consumers of up to 1,750 gm/ day would not exceed the 10 ! 4 risk level.) If a criterion were based on high end intake rates and the relative risk of 10 ! 6, then an average fish consumer would be protected at a cancer risk level of approximately 10 ! 8. The point is that the risks for different population groups are not the same.

2.5 MICROBIOLOGICAL AMBIENT WATER QUALITY CRITERIA Guidance for deriving microbiological AWQC is not a part of this Methodology. In 1986, EPA published Ambient Water Quality Criteria for Bacteria  1986 ( USEPA, 1986a), which updated and revised bacteriological criteria previously published in 1976 in Quality Criteria for Water ( USEPA, 1976). The inclusion of guidance for deriving microbiological AWQC was considered in the 1992 national workshop that initiated the effort to revise the 1980 Methodology and was recommended by the SAB in 1993. Since that time, however, efforts separate from these Methodology revisions have addressed microbiological AWQC concerns. The purpose of this section is to briefly describe EPA's current recommendations and activities.

EPA's Ambient Water Quality Criteria for Bacteria  1986 recommends the use of Escherichia coli and enterococci rather than fecal coliforms ( USEPA, 1986a). EPA's criteria recommendations are:

° Fresh water: E. coli not to exceed 126/ 100 ml or enterococci not to exceed 33/ 100 ml; and ° Marine water: enterococci not to exceed 35/ 100 ml. 2 8 These criteria should be calculated as the geometric mean based on five equally spaced samples taken over a 30 day period.

In addition, EPA recommends that States adopt a single sample maximum, based on the expected frequency of use. No sample taken should exceed this value. EPA specifies appropriate single sample maximum values in the 1986 criteria document.

Current Activities and Plans for Future Work EPA has identified development of microbial water quality criteria as part of its strategy to control waterborne microbial disease, by controlling pathogens in waterbodies and by protecting designated uses, such as recreation and public water supplies. The program fosters an integrated approach to protect both ground water and surface water sources. EPA plans to conduct additional monitoring for Cryptosporidium parvum and E. coli, and determine action plans in accordance with the results of this monitoring.

EPA recommends no change at this time in the stringency of its bacterial criteria for recreational waters; existing criteria and methodologies from 1986 will still apply. The recommended methods for E. coli and enterococci have been improved. As outlined in the Action Plan for Beaches and Recreational Waters ( Beach Action Plan, see below), the Agency plans to conduct national studies on improving indicators together with epidemiology studies for new criteria development ( USEPA, 1999b). The Agency is also planning to establish improved temporal and spatial monitoring protocols.

In the Beach Action Plan, EPA identifies a multi year strategy for monitoring recreational water quality and communicating public health risks associated with potentially pathogencontaminated recreational rivers, lakes, and ocean beaches. It articulates the Agency's rationale and goals in addressing specific problems and integrates all associated program, policy, and research needs and directions. The Beach Action Plan also provides information on timing, products and lead organization for each activity. These include activities and products in the areas of program development, risk communication, water quality indicator research, modeling and monitoring research, and exposure and health effects research.

Recently, EPA approved new 24 hour E. coli and enterococcus tests for recreational waters that may be used as an alternative to the 48 hour test ( USEPA, 1997). EPA anticipates proposing these methods for inclusion in the 40 CRF 136 in the Fall of 2000. EPA has also published a video with accompanying manual on the original and newer methods for enterococci and E. coli ( USEPA, 2000).

As part of the Beach Action Plan, EPA made the following recommendations for further Agency study: 2 9 ° Future criteria development should consider the risk of diseases other than gastroenteritis. EPA intends to consider and evaluate such water related exposure routes as inhalation and dermal absorption when addressing microbial health effects. The nature and significance of other than the classical waterborne pathogens are to some degree tied to the particular type of waste sources.

° A new set of indicator organisms may need to be developed for tropical water if it is proven that the current fecal indicators can maintain viable cell populations in the soil and water for significant periods of time in uniform tropical conditions. Some potential alternative indicators to be fully explored are coliphage, other bacteriophage, and Clostridium perfringens.

° Because animal sources of pathogens of concern for human infection such as Giardia lamblia, Cryptosporidium parvum, and Escherichia coli 0157: H7 may be waterborne or washed into water and thus become a potential source for infection, they should not be ignored in risk assessment. A likely approach would be phylogenetic differentiation; that is, indicators that are specific to, or can discriminate among, animal sources.

° EPA intends to develop additional data on secondary infection routes and infection rates from prospective epidemiology studies and outbreaks from various types of exposure ( e. g., shellfish consumption, drinking water, recreational exposure).

° EPA needs to improve sampling strategies for recreational water monitoring including consideration of rainfall and pollution events to trigger sampling.

2.6 RISK CHARACTERIZATION CONSIDERATIONS On March 21, 1995, EPA's Administrator issued the EPA Risk Characterization Policy and Guidance ( USEPA, 1995). This policy and guidance is intended to ensure that characterization information from each stage of a risk assessment is used in forming conclusions about risk and that this information is communicated from risk assessors to risk managers, and from EPA to the public. The policy also provides the basis for greater clarity, transparency, reasonableness, and consistency in risk assessments across EPA programs. The fundamental principles which form the basis for a risk characterization are as follows:

° Risk assessments should be transparent, in that the conclusions drawn from the science are identified separately from policy judgments, and the use of default values or methods and the use of assumptions in the risk assessment are clearly articulated.

° Risk characterizations should include a summary of the key issues and conclusions of each of the other components of the risk assessments, as well as describe the likelihood of harm. The summary should include a description of the overall strengths and limitations ( including uncertainties) of the assessment and conclusions. 2 10 ° Risk characterizations should be consistent in general format, but recognize the unique characteristics of each specific situation.

° Risk characterizations should include, at least in a qualitative sense, a discussion of how a specific risk and its context compares with similar risks. This may be accomplished by comparisons with other pollutants or situations on which the Agency has decided to act, or other situations with which the public may be familiar. The discussion should highlight the limitations of such comparisons.

° Risk characterization is a key component of risk communication, which is an interactive process involving exchange of information and expert opinion among individuals, groups, and institutions.

Additional guiding principles include:

° The risk characterization integrates the information from the hazard identification, doseresponse and exposure assessments, using a combination of qualitative information, quantitative information, and information regarding uncertainties.

° The risk characterization includes a discussion of uncertainty and variability in the risk assessment.

° Well balanced risk characterizations present conclusions and information regarding the strengths and limitations of the assessment for other risk assessors, EPA decision makers, and the public.

In developing the methodology presented here, EPA has closely followed the risk characterization guiding principles listed above. As States and Tribes adopt criteria using the 2000 Human Health Methodology, they are strongly encouraged to follow EPA's risk characterization guidance. There are a number of areas within the Methodology and criteria development process where risk characterization principles apply:

° Integration of cancer and noncancer assessments with exposure assessments, including bioaccumulation potential determinations, in essence, weighing the strengths and weaknesses of the risk assessment as a whole when developing a criterion.

° Selecting a fish consumption rate, either locally derived or the national default value, within the context of a target population ( e. g., sensitive subpopulations) as compared to the general population.

° Presenting cancer and/ or noncancer risk assessment options.

° Describing the uncertainty and variability in the hazard identification, the dose response, and the exposure assessment. 2 11 2.7 DISCUSSION OF UNCERTAINTY 2.7.1 Observed Range of Toxicity Versus Range of Environmental Exposure When characterizing a risk assessment, an important distinction to make is between the observed range of adverse effects ( from an epidemiology or animal study) and the environmentally observed range of exposure ( or anticipated human exposure) to the contaminant. In many cases, EPA intends to apply default factors to account for uncertainties or incomplete knowledge in developing RfDs or cancer risk assessments using nonlinear low dose extrapolation to provide a margin of protection. In reality, the actual effect level and the environmental exposure levels may be separated by several orders of magnitude. The difference between the dose causing some observed response and the anticipated human exposure should be described by risk assessors and managers, especially when comparing criteria to environmental levels of a contaminant.

2.7.2 Continuum of Preferred Data/ Use of Defaults In both toxicological and exposure assessments, EPA has defined a continuum of preferred data for toxicological assessments ranging from a highest preference for chronic human data ( e. g., studies that examine a long term exposure of humans to a chemical, usually from occupational and/ or residential exposure) and actual field data for many of the exposure parameter values ( e. g., locally derived fish consumption rates, waterbody specific bioaccumulation rates), to default values which are at the lower end of the preference continuum. EPA has supplied default values for all of the risk assessment parameters in the 2000 Human Health Methodology; however, it is important to note that when default values are used, the uncertainty in the final risk assessment may be higher, and the final resulting criterion may not be as applicable to local conditions, than is a risk assessment derived from human/ field data. Using defaults assumes generalized conditions and may not capture the actual variability in the population ( e. g., sensitive subpopulations/ high end consumers). If defaults are chosen as the basis for criteria, these inherent uncertainties should be communicated to the risk manager and the public. While this continuum is an expression of preference on the part of EPA, it does not imply in any way that any of the choices are unacceptable or scientifically indefensible.

2.7.3 Significant Figures The number of significant figures in a numeric value is the number of certain digits plus one estimated digit. Digits should not be confused with decimal places. For example, 15.1, 0.0151, and 0.0150 all have 3 significant figures. Decimal places may have been used to maintain the correct number of significant figures, but in themselves they do not indicate significant figures ( Brinker, 1984). Since the number of significant figures must include only one estimated digit, the sources of input parameters ( e. g., fish consumption and water consumption rates) should be checked to determine the number of significant figures associated with data they provide. However, the original measured values may not be available to determine the number of significant figures in the input parameters. In these situations, EPA recommends utilizing the data as presented. 2 12 When developing criteria, EPA recommends rounding the number of significant figures at the end of the criterion calculation to the same number of significant figures in the least precise parameter. This is a generally accepted practice which can be found described in greater detail in APHA ( 1992) and Brinker ( 1984). The general rule is that for multiplication or division, the resulting value should not possess any more significant figures than is associated with the factor in the calculation with the least precision. When numbers are added or subtracted, the number that has the fewest decimal places, not necessarily the fewest significant figures, puts the limit on the number of places that justifiably may be carried in the sum or difference. Rounding off a number is the process of dropping one or more digits so that the value contains only those digits that are significant or necessary in subsequent computations ( Brinker, 1984). The following rounding procedures are recommended: ( 1) if the digit 6, 7, 8, or 9 is dropped, increase the preceding digit by one unit; ( 2) if the digit 0, 1, 2, 3, or 4 is dropped, do not alter the preceding digit; and ( 3) if the digit 5 is dropped, round off the preceding digit to the nearest even number ( e. g., 2.25 becomes 2.2 and 2.35 becomes 2.4) ( APHA, 1992; Brinker, 1984).

EPA recommends that calculations of water quality criteria be performed without rounding of intermediate step values. The resulting criterion may be rounded to a manageable number of decimal places. However, in no case should the number of digits presented exceed the number of significant figures implied in the data and calculations performed on them. The term " intermediate step values" refers to values of the parameters in Equations 1 1 through 1 3. The final step is considered the resulting AWQC. Although AWQC are, in turn, used for purposes of establishing water quality based effluent limits ( WQBELs) in National Pollutant Discharge Elimination System ( NPDES) permits, calculating total maximum daily loads ( TMDLs), and applicable or relevant and appropriate requirements ( ARARs) for Superfund, they are considered the final step of this Methodology and, for the purpose of this discussion, where the rounding should occur.

The determination of appropriate significant figures inevitably involves some judgment given that some of the equation parameters are adopted default exposure values. Specifically, the default drinking water intake rate of 2 L/ day is a value adopted to represent a majority of the population over the course of a lifetime. Although supported by drinking water consumption survey data, this value was adopted as a policy decision and, as such, does not have to be considered in determining the parameter with the least precision. That is, the resulting AWQC need not always be reduced to one significant digit. Similarly, the 70 kg adult body weight has been adopted Agency wide and represents a default policy decision.

The following example with a simplified AWQC equation illustrates the rule described above. The example is for hexachlorobutadiene ( HCBD), which EPA used to demonstrate the 1998 draft Methodology revisions ( USEPA, 1998b). The parameters that were calculated ( i. e., not policy adopted values) include values with significant figures of two ( the POD and RSC), three ( the UF), and four ( the FI and BAF). Based on the 2000 Human Health Methodology, the final criterion should be rounded to two significant figures. The bold numbers in parentheses indicate the number of significant figures and those with asterisks also indicate Agency adopted policy values. 2 13 ( Equation 2 1)

Example [ Refer to draft HCBD document for details on the POD/ UF, RSC and BAF data ( EPA 822 R 98 004). Also note that the fish intake rate in this example is the revised value.]:

AWQC = 7.3 × 10 5 mg/ L ( 0.073 µ g/ L, rounded from 7.285 × 10 2 µ g/ L) * represents Agency adopted policy value A number of the values used in the equation may result in intermediate step values that have more than four figures past the decimal place and may be carried throughout the calculation. However, carrying more than four figures past the decimal place ( equivalent to the most precise parameter) is unnecessary as it has no effect on the resulting criterion value.

2.8 OTHER CONSIDERATIONS 2.8.1 Minimum Data Considerations For many of the preceding technical areas, considerations have been presented for data quality in developing toxicological and exposure assessments. For greater detail and discussion of minimum data recommendations, the reader is referred to the specific sections in the Methodology on cancer and noncancer risk assessments ( and especially to the referenced EPA risk assessment guidelines documents), exposure assessment, and bioaccumulation assessment, in addition to the TSD volumes for each.

2.8.2 Site Specific Criterion Calculation The 2000 Human Health Methodology allows for site specific modifications by States and Tribes to reflect local environmental conditions and human exposure patterns. " Local" may refer to any appropriate geographic area where common aquatic environmental or exposure patterns exist. Thus " local" may signify Statewide, regional, a river reach, or an entire river.

Such site specific criteria may be developed as long as the site specific data, either toxicological or exposure related, is justifiable. For example, when using a site specific fish consumption rate, a State should use a value that represents at least the central tendency of the 2 14 population surveyed ( either sport or subsistence, or both). If a site specific fish consumption rate for sport anglers or subsistence anglers is lower than an EPA default value, it may be used in calculating AWQC. However, to justify such a level ( either higher or lower than EPA defaults), the State should assemble appropriate survey data to arrive at a defensible site specific fish consumption rate.

Such data must also be submitted to EPA for its review when approving or disapproving State or Tribal water quality standards under Section 303( c). The same conditions apply to sitespecific calculations of BAF, percent fish lipid, or the RSC. In the case of deviations from toxicological values ( i. e., IRIS values: verified noncancer and cancer assessments), EPA strongly recommends that the data upon which the deviation is based be presented to and approved by the Agency before a criterion is developed.

Additional guidance on site specific modifications to the 2000 Human Health Methodology is provided in each of the three TSD volumes.

2.8.3 Organoleptic Criteria Organoleptic criteria define concentrations of chemicals or materials which impart undesirable taste and/ or odor to water. Organoleptic effects, while significant from an aesthetic standpoint, are not a significant health concern. In developing and utilizing such criteria, two factors must be appreciated: ( 1) the limitations of most organoleptic data; and ( 2) the human health significance of organoleptic properties. In the past, EPA has developed organoleptic criteria if organoleptic data were available for a specific contaminant. The 1980 AWQC National Guidelines made a clear distinction that organoleptic criteria and toxicity based criteria are derived from completely different endpoints, and that organoleptic criteria have no demonstrated relationship to potential adverse human health effects because there is no toxicological basis. EPA acknowledges that if organoleptic effects ( i. e., objectionable taste and odor) cause people to reject the water and its designated uses, then the public is effectively deprived of the natural resource. It is also possible that intense organoleptic characteristics could result in depressed fluid intake which, in turn, might lead to an indirect human health effect via decreased fluid consumption. Although EPA has developed organoleptic criteria in the past and may potentially do so in the future, this will not be a significant part of the water quality criteria program. EPA encourages the development of organoleptic criteria when States and Tribes believe they are needed. However, EPA cautions States and Tribes that the quality of organoleptic data is often significantly less than that of toxicologic data used in establishing health based criteria. Therefore, a comprehensive evaluation of available organoleptic data should be made, and the selection of the most appropriate database for the criterion should be based on sound scientific judgment.

In 1980, EPA provided recommended criteria summary language when both types of data are available. The following format was used and is repeated here: 2 15 For comparison purposes, two approaches were used to derive criterion levels for ____. Based on available toxicity data, for the protection of public health the derived level is ____. Using available organoleptic data, for controlling undesirable taste and odor quality of ambient water the estimated level is ____. It should be recognized that organoleptic data as a basis for establishing a water quality criteria have no demonstrated relationship to potential adverse human health effects.

Similarly, the 1980 Methodology recommended that in those instances where a level to limit toxicity cannot be derived, the following statement should be provided:

Sufficient data are not available for ____ to derive a level which would protect against the potential toxicity of this compound.

2.8.4 Criteria for Chemical Classes The 2000 Human Health Methodology also allows for the development of a criterion for classes of chemicals, as long as a justification is provided through the analysis of mechanistic data, toxicokinetic data, structure activity relationship data, and limited acute and chronic toxicity data. When potency differences between members of a class is great ( such as in the case of chlorinated dioxins and furans), toxicity equivalency factors ( TEFs) may be more appropriately developed than one class criterion.

A chemical class is defined as any group of chemical compounds which are similar in chemical structure and biological activity, and which frequently occur together in the environment usually because they are generated by the same commercial process. In criterion development, isomers should be regarded as part of a chemical class rather than as a single compound. A class criterion, therefore, is an estimate of risk/ safety which applies to more than one member of a class. It involves the use of available data on one or more chemicals of a class to derive criteria for other compounds of the same class in the event that there are insufficient data available to derive compound specific criteria. The health based criterion may apply to the water concentration of each member of the class, or may apply to the sum of the water concentrations of the compounds within the class. Because relatively minor structural changes within the class of compounds can have pronounced effects on their biological activities, reliance on class criteria should be minimized depending on the data available.

The following guidance should also be followed when considering the development of a class criterion.

° A detailed review of the chemical and physical properties of the chemicals within the group should be made. A close relationship within the class with respect to chemical activity would suggest a similar potential to reach common biological sites within tissues. Likewise, similar lipid solubilities would suggest the possibility of comparable absorption and distribution. 2 16 ° Qualitative and quantitative toxicological data for chemicals within the group should be examined. Adequate toxicological data on a number of compounds within a group provides a more reasonable basis for extrapolation to other chemicals of the same class than minimal data on one chemical or a few chemicals within the group.

° Similarities in the nature of the toxicological response to chemicals in the class provides additional support for the prediction that the response to other members of the class may be similar. In contrast, where the biological response has been shown to differ markedly on a qualitative and quantitative basis for chemicals within a class, the extrapolation of a criterion to other members is not appropriate.

° Additional support for the validity of extrapolation of a criterion to other members of a class could be provided by evidence of similar metabolic and toxicokinetic data for some members of the class.

Additional guidance is described in the Technical Support Document on Health Risk Assessment of Chemical Mixtures ( USEPA, 1990).

2.9.5 Criteria for Essential Elements Developing criteria for essential elements, particularly metals, must be a balancing act between toxicity and the requirement for good health. The AWQC must consider essentiality and cannot be established at levels that would result in deficiency of the element in the human population. The difference between the recommended daily allowance ( RDA) and the daily doses causing a specified risk level for carcinogens or the RfDs for noncarcinogens defines the spread of daily doses within which the criterion may be derived. Because errors are inherent in defining both essential and adverse effect levels, the criterion is derived from a dose level near the center of such dose ranges.

The process for developing criteria for essential elements should be similar to that used for any other chemical with minor modifications. The RfD represents concern for one end of the exposure spectrum ( toxicity), whereas the RDA represents the other end ( minimum essentiality). While the RDA and RfD values might occasionally appear to be similar in magnitude to one another, it does not imply incompatibility of the two methodological approaches, nor does it imply inaccuracy or error in either calculation.

2.9 REFERENCES APHA. American Public Health Association. 1992. Standard Methods: For the Examination of Water and Wastewater. 18th Edition. Prepared and published jointly by: American Public Health Association, American Water Works Association, and Water Environment Federation. Washington, DC. 2 17 Brinker, R. C. 1984. Elementary Surveying. 7th Edition. Cliff Robichaud and Robert Greiner, Eds. Harper and Row Publishers, Inc. New York, NY.

NRC ( National Research Council). 1983. Risk Assessment in the Federal Government: Managing the Process. National Academy Press. Washington, DC.

USEPA ( U. S. Environmental Protection Agency). 1976. Quality Criteria for Water. Office of Water and Hazardous Materials. Washington, DC. July.

USEPA ( U. S. Environmental Protection Agency). 1986a. Ambient Water Quality Criteria for Bacteria   1986. Office of Water Regulations and Standards. Washington, DC. EPA/ 440/ 5 84/ 002. January.

USEPA ( U. S. Environmental Protection Agency). 1986b. Test Methods for Escherichia coli and Enterococci in Water by the Membrane Filter Procedure. Office of Research and Development. Cincinnati, OH. EPA/ 600/ 4 85/ 076.

USEPA ( U. S. Environmental Protection Agency). 1990. Technical Support Document on Health Risk Assessment of Chemical Mixtures. Office of Research and Development. Washington, DC. EPA/ 600/ 8 90/ 064. August.

USEPA ( U. S. Environmental Protection Agency). 1992. Guidelines for exposure assessment. Federal Register 57: 22888 22938.

USEPA ( U. S. Environmental Protection Agency). 1995. Policy for Risk Characterization. Memorandum of Carol M. Browner, Administrator. March 21, 1995. Washington, DC.

USEPA ( U. S. Environmental Protection Agency). 1996a. Draft revisions to guidelines for carcinogen risk assessment. Federal Register 61: 17960.

USEPA ( U. S. Environmental Protection Agency). 1996b. Guidelines for reproductive toxicity risk assessment. Federal Register 61: 6274 56322.

USEPA ( U. S. Environmental Protection Agency). 1997. Method 1600: Membrane Filter Test Method for Enterococci in Water. Office of Water. Washington, DC. EPA/ 821/ R 97/ 004. May.

USEPA ( U. S. Environmental Protection Agency). 1998a. Draft Water Quality Criteria Methodology: Human Health. Office of Water. Washington, DC. EPA 822 Z 98 001. ( Federal Register 63: 43756)

USEPA ( U. S. Environmental Protection Agency). 1998b. Ambient Water Quality Criteria for the Protection of Human Health. Hexachlorobutadiene ( HCBD). Draft. Office of Water. Washington, DC. EPA 882 R 98 004. July. 2 18 USEPA ( U. S. Environmental Protection Agency). 1999a. 1999 Guidelines for Carcinogen Risk Assessment. Review Draft. Office of Research and Development. Washington, DC. NCEA F 0644.

USEPA ( U. S. Environmental Protection Agency). 1999b. Action Plan for Beaches and Recreational Waters. Reducing Exposures to Waterborne Pathogens. Office of Research and Development and Office of Water. Washington, DC. EPA 600 R 98 079. March.

USEPA ( U. S. Environmental Protection Agency). 2000. Improved Enumeration Methods for the Recreational Water Quality Indicators: Enterococci and Escherichia coli. Office of Water, Office of Science and Technology. Washington, DC. EPA 821 R 97 004. March. 3 1 3. RISK ASSESSMENT This section describes the methods used to estimate ambient water quality criteria ( AWQC) for the protection of human health for carcinogenic chemicals ( Section 3.1) and for noncarcinogenic chemicals ( Section 3.2).

3.1 CANCER EFFECTS 3.1.1 Background on EPA Cancer Risk Assessment Guidelines The current EPA Guidelines for Carcinogen Risk Assessment were published in 1986 ( USEPA, 1986a, hereafter the " 1986 cancer guidelines"). The 1986 cancer guidelines categorize chemicals into alpha numerical Groups: A, known human carcinogen ( sufficient evidence from epidemiological studies or other human studies); B, probable human carcinogen ( sufficient evidence in animals and limited or inadequate evidence in humans); C, possible human carcinogen ( limited evidence of carcinogenicity in animals in the absence of human data); D, not classifiable ( inadequate or no animal evidence of carcinogenicity); and E, evidence of noncarcinogenicity for humans ( no evidence of carcinogenicity in at least two adequate animal tests in different species or in both adequate epidemiological and animal studies). Within Group B there are two subgroups, Groups B1 and B2. Group B1 is reserved for agents for which there is limited evidence of carcinogenicity from epidemiological studies. Group B2 is generally for agents for which there is sufficient evidence from animal studies and for which there is inadequate evidence or no data from epidemiological studies ( USEPA, 1986). The system was similar to that used by the International Agency for Research on Cancer ( IARC).

The 1986 cancer guidelines include guidance on what constitutes sufficient, limited, or inadequate evidence. In epidemiological studies, sufficient evidence indicates a causal relationship between the agent and human cancer; limited evidence indicates that a causal relationship is credible, but that alternative explanations, such as chance, bias, or confounding, could not adequately be excluded; inadequate evidence indicates either lack of pertinent data, or a causal interpretation is not credible. In general, although a single study may be indicative of a causeeffect relationship, confidence in inferring a causal association is increased when several independent studies are concordant in showing the association. In animal studies, sufficient evidence includes an increased incidence of malignant tumors or combined malignant and benign tumors:

° In multiple species or strains;

° In multiple experiments ( e. g., with different routes of administration or using different dose levels);

° To an unusual degree in a single experiment with regard to high incidence, unusual site or type of tumor, or early age at onset; 3 2 ° Additional data on dose response, short term tests, or structural activity relationships.