Recommended Guidance for

Review of Existing National Primary

Drinking Water Regulations

Recommended by

The National Drinking Water Advisory Council (NDWAC) 

November 2000
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations i

TABLE OF CONTENTS

EXECUTIVE SUMMARY ................................................................ V

I. INTRODUCTION ................................................................... 1

A. BACKGROUND ..................................................................... 1

B. ABOUT THIS GUIDANCE DOCUMENT .................................................... 1

II. THE SIX-YEAR REVIEW PROCESS .................................................... 2

A. GOAL OF THE REVIEW ............................................................... 2

B. BASIC PRINCIPLES OF THE REVIEW PROCESS .............................................. 4

(i) Underlying Assumptions .......................................................... 4

(ii) Risk Management in the Review Process ............................................. 5

C. SCOPE OF THE REVIEW ............................................................... 6

(i) The Review Process ............................................................. 6

(ii) Potential Types of Regulatory Changes .............................................. 9

III. KEY ELEMENTS OF THE REVIEW PROCESS ......................................... 11

A. HEALTH EFFECTS ................................................................. 15

(i) Chemical Contaminants (including disinfectants and disinfection by-products) ............... 15

(ii) Radiological Contaminants ...................................................... 18

(iii) Microbial Pathogens .......................................................... 19

(iv) Review of Standards and Guidelines of Other Organizations ............................ 19

B. TECHNOLOGY ASSESSMENT .......................................................... 20

(i) Analytical Methods ............................................................ 20

(ii) Treatment Technologies ........................................................ 22

C. OCCURRENCE AND EXPOSURE ANALYSIS ................................................ 23

D. CONSIDERATION OF OTHER REGULATORY REVISIONS ...................................... 24

E. COST AND BENEFIT CONSIDERATIONS .................................................. 26

(i) Cost and Benefit Assumptions of Current NPDWR ..................................... 26

(ii) Cost and Benefit Estimation of Potential Regulatory Revisions ........................... 26

STAKEHOLDER INVOLVEMENT ....................................................... 27

A. KEY STAKEHOLDERS ............................................................... 27

B. STAKEHOLDER MEETINGS ........................................................... 27

C. OTHER STAKEHOLDER OUTREACH ..................................................... 28

APPENDICES ......................................................................... 29

APPENDIX 1: CONTAMINANTS REGULATED PRIOR TO AUGUST 1996 .............................. 30

APPENDIX 2: NATIONAL PRIMARY DRINKING WATER REGULATIONS (NPDWRS) PROMULGATED AFTER 

AUGUST 1996 AND NPDWRS ANTICIPATED TO BE PROMULGATED PRIOR TO AUGUST 2002 . . 33

APPENDIX 3: AREAS THAT WOULD BENEFIT FROM FUTURE RESEARCH ........................... 34

APPENDIX 4: DETERMINATION OF NATIONAL OCCURRENCE OF CONTAMINANTS .................... 36

APPENDIX 5: NON-REGULATORY APPROACHES ............................................. 44

APPENDIX 6: IRIS ASSESSMENTS ........................................................ 46

APPENDIX 7: OVERVIEW OF THE OFFICE OF PESTICIDE PROGRAMS PROCESS FOR TOXICITY ASSESSMENTS . 47

APPENDIX 8: ANALYTICAL METHODS .................................................... 49
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations ii

LIST OF FIGURES

Figure 1: Overview of the Review Process .................................................... 8

Figure 2: Review Process for NPDWRs ..................................................... 12
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations iii

ACRONYMS LIST

ANPRM – Advance Notice of Proposed Rule Making

ASTM -- American Society for Testing and Materials

ATP -- Alternate test procedure

ATSDR -- Agency for Toxic Substances and Disease Registry 

AWWARF -- American Water Works Association Research Foundation

BAT -- Best Available Technology

BEIR -- Biological Effects of Ionizing Radiation 

BMP -- Best management practice

CCL -- Contaminant Candidate List

D/DBP -- Disinfection/Disinfection By-Product

EPA -- Environmental Protection Agency

EPA-LV -- Las Vegas, Nevada EPA Laboratory

EQIP -- Environmental Quality Incentives Program

ESWTR -- Enhanced Surface Water Treatment Rule 

FIFRA -- Federal Insecticide, Fungicide, and Rodenticide Act 

FQPA -- Food Quality Protection Act

FR -- Federal Register

FRC -- Federal Radiation Council

IARC -- International Agency for Research on Cancer

ICR -- Information Collection Rule

ICRP -- International Commission on Radiological Protection

IOC -- Inorganic chemical

IRIS -- Integrated Risk Information System 

ISO -- International Standards Organization

LCR -- Lead and Copper Rule

LT2ESWTR -- Long-Term 2 Enhanced Surface Water Treatment Rule

MCL -- Maximum contaminant level

MCLG -- Maximum contaminant level goal

MDBP -- Microbial contaminants and disinfection by-products

MDL -- Method detection limit

ML -- Minimum Level

MOE -- Margin of Exposure

MRDL -- Maximum residual disinfectant level

MRDLG -- Maximum residual disinfectant level goal

MRL -- Minimum reporting level

NAS -- National Academy of Sciences

NAWQA -- National Water Quality Assessment Program

NCOD -- National Drinking Water Contaminant Occurrence Database

NCRP -- National Council on Radiation Protection and Measurement

NDWAC -- National Drinking Water Advisory Council 

NERL -- National Exposure Research Laboratory

NIST -- National Institute of Standards and Technology 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations iv

ACRONYMS (continued)

NOAEL -- No-Observed-Adverse-Effect Level

NIPDWR -- National Interim Primary Drinking Water Regulation

NPDWR -- National Primary Drinking Water Regulation

NRCS -- Natural Resource Conservation Service

NRPB -- National Radiation Protection Board

NRWA -- National Rural Water Association

NWIS -- National Survey of Water Information System

OGWDW -- Office of Ground Water and Drinking Water

ORIA -- Office of Radiation and Indoor Air

OPP -- Office of Pesticide Programs

OW -- Office of Water 

PAD -- Population Adjusted Dose (aPAD -- acute; cPAD -- chronic)

PE -- Performance evaluation

PQL -- Practical quantitation level

PT -- Proficiency Testing 

PWS -- Public water system

RfD -- Reference dose (aRfD -- acute effects; cRfD -- chronic effects)

RSC -- Relative source contribution

SDWA -- Safe Drinking Water Act

SDWIS -- Safe Drinking Water Information System

SM -- Standard Method

SOC -- Synthetic organic chemical

SWTR -- Surface Water Treatment Rule

THMs -- Trihalomethanes

TRI -- Toxic Release Inventory

TT -- Treatment technique

UNSCEAR -- United Nations Scientific Committee on the Effects of Atomic Radiation

URCIS -- Unregulated Contaminant Information System 

USDA -- United States Department of Agriculture

USDOI -- United States Department of the Interior

USGS -- United States Geological Survey

VOC -- Volatile organic chemical

WG -- Working group

WRAS -- Watershed Restoration Action Strategies

WS -- Water Supply
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations v

Recommended Guidance for 

Review of Existing National Primary 

Drinking Water Regulations

November 2000

EXECUTIVE SUMMARY

The Safe Drinking Water Act (SDWA), as amended in 1996, requires that the U.S.

Environmental Protection Agency (EPA) review existing National Primary Drinking Water

Regulations (NPDWRs) and, if appropriate, revise them, no less often than every six years. This

document describes the process and strategy for regulatory review that the National Drinking

Water Advisory Council (NDWAC) proposes that EPA use to meet this statutory requirement. 

The guidelines presented here are the NDWAC recommendations, based on its review of the EPA

May 2000 draft guidance. The EPA draft document had been developed through internal EPA

deliberations and discussions with the diverse stakeholders involved in drinking water and its

protection. 

EPA plans to complete the first review of NPDWRs by August 2002. That review will

include all NPDWRs promulgated prior to 1996. Those contaminants that are the subject of

current rulemaking activities or are being reviewed on a separate schedule will not be included in

the first six-year review cycle. Drinking water standards promulgated after 1996 will be reviewed

at a later date. 

For the first review cycle, EPA will propose and promulgate any revisions determined to

be appropriate after the completion of the review cycle (i.e., after August 2002). These will be

separate activities and will be completed consistent with EPA rulemaking policies and procedures. 

The actual details of the regulatory change (e.g., specific changes to the maximum contaminant

level goal (MCLG)) will be determined as a part of this rulemaking process and not as a part of

the review process. In subsequent six-year cycles, EPA should complete both the review and any

appropriate NPDWR revisions within a six-year window (i.e., specific regulatory changes will be

part of the review process).

As long as NPDWR revisions maintain or provide for greater protection of the health of

persons, the SDWA 1996 amendments give the Administrator discretion to determine if it is

appropriate to revise an existing NPDWR. In order to determine that a revision is appropriate,

EPA believes the revision must continue to meet the basic statutory requirements of the SDWA

(e.g., set the maximum contaminant level (MCL) as close to the MCLG as is feasible) and present

significant opportunities to: improve the level of public health protection; and/or to achieve cost

savings while maintaining, or improving, the level of public health protection. 
1 MRDLG -- Maximum residual disinfectant level goal. MRDL -- Maximum residual disinfectant level.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations vi

To define the scope of the six-year review process, this guidance identifies key

assumptions that the NDWAC recommends that EPA use in its approach to regulatory review. 

Under the defined scope of review, EPA should: 

• During the first six-year cycle, revisit data on which previous regulatory decisions

were made, effectively identifying data gaps that need to be filled;

• In future six-year review cycles, assume that existing regulations are adequate,

except where more current, reliable data are available;

• For the second and subsequent review cycles, group regulated contaminants for

review, in order to achieve some efficiencies in the review process;

• Rely on data available to the Agency in a timeframe that allows the Agency

sufficient time for the review process;

• Identify areas where significant data gaps exist and initiate activities to fill those

gaps for subsequent review rounds; and

• Require defensible, scientific methods to carry out a review leading to a revision.

In addition, EPA should apply the basic principles of risk management in order to make

the most protective and cost-effective decisions possible. 

The types of regulatory revisions that EPA plans to consider are based on the various

components of each primary drinking water regulation, and include possible changes to: 

MCLGs/MRDLGs; MCLs/MRDLs1; treatment techniques; other technologies (e.g., Best

Available Technology, or Compliance and Variance Technology); monitoring requirements; other

regulatory provisions; or, in rare instances, EPA may consider dropping a contaminant from

regulation. 

To most efficiently utilize limited resources, EPA plans to perform a series of analyses at

the beginning of each review cycle, intended to target those NPDWRs that are the most

appropriate candidates for revision. The Agency plans to use available, scientifically-sound data

to make decisions regarding whether or not to revise a regulation. EPA proposes to review the

following key information to make decisions regarding regulatory changes: health effects studies;

technology assessments; and, occurrence and exposure analyses. EPA may consider other

regulatory revisions not related to MCLG, MCL, or treatment technique requirements, such as

monitoring, or reporting requirements. The Agency should also consider options that may lead to

non-regulatory revisions, such as source water protection, to further enhance drinking water

quality. In addition, EPA plans to conduct rough analyses of costs and benefits as part of the

review process. Section III, Key Elements of the Review Process, provides a detailed discussion

of the information review process. 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 1

I. INTRODUCTION

A. Background

Under the Safe Drinking Water Act (SDWA), the U.S. Environmental Protection Agency

(EPA) must periodically review existing National Primary Drinking Water Regulations

(NPDWRs) and, if appropriate, revise them. This requirement is contained in Section 1412(b)(9)

of SDWA, as amended in 1996, which reads:

The Administrator shall, not less often than every 6 years, review and revise, as

appropriate, each national primary drinking water regulation promulgated under this

title. Any revision of a national primary drinking water regulation shall be promulgated

in accordance with this section, except that each revision shall maintain, or provide for

greater, protection of the health of persons.

Prior to the 1996 amendments, the SDWA required EPA to review NPDWRs at least

every three years to determine whether any changes in technology, treatment techniques (TT) or

other means might provide better health protection. EPA was required to publish its findings in

the Federal Register (FR) and provide an explanation, after opportunity for public comment, of

any finding that such new technology, treatment technique, or other means would not be feasible. 

Although the Agency did revise existing NPDWRs on occasion when new data became available,

EPA did not have a systematic process for reviewing NPDWRs on a regular basis. One of the

goals of the current effort is to establish a process for conducting regular reviews. 

B. About this Guidance Document

This document describes the process and strategy for reviewing existing NPDWRs that the

National Drinking Water Advisory Council (NDWAC) recommends that EPA use to meet this

statutory requirement during each six-year cycle. It provides the guidelines and tools needed to

conduct standardized, systematic reviews of NPDWRs during this and subsequent six-year cycles. 

It takes into account the most critical aspects of health protection and the setting of standards, as

we know them under the SDWA. In addition, this guidance allows for the fact that numerous

types of regulatory changes may be considered during a specific review cycle. Therefore the

guidance contains an element of flexibility to allow EPA reviewers the opportunity to consider a

range of possible issues. Further, revisions to the guidance may be appropriate from time to time

to address changes such as: the availability of new appropriate data sources (or the loss of

availability of an existing data source); revised policy guidelines; and significant fluctuations in

Agency resources available to conduct the reviews. For this reason, EPA should review this

guidance document periodically, modify it, when appropriate and with stakeholder involvement,

to reflect changing circumstances. 

Because of the advisory nature of the document, the term “should” denotes a

recommendation of the NDWAC. The terms “proposes” or “plans to” denote EPA’s planned

commitments. These planned commitments form the basis for the NDWAC to develop its

recommendations. Note that in addition, this document also describes current EPA procedures

that are related to the regulatory review process, as well as general description of activities that
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 2

are required of EPA, in order to fulfill its mandate under Section 1412 of the SDWA, as amended

in 1996.

The review process described in this document will culminate with decisions of whether or

not to revise each of the reviewed NPDWRs. This strategy has been developed through internal

deliberations, discussions with the diverse stakeholders involved in drinking water and its

protection, and recommendations of the NDWAC Working Group (WG) that have been approved

by the full NDWAC.

EPA plans to complete the first review by August 2002. That review will include all

NPDWRs promulgated prior to 1996. Those contaminants that are the subject of current

rulemaking activities or are being reviewed on a separate schedule will not be included in the first

six-year review cycle. See Appendix 1 for a full listing of contaminants regulated prior to the

SDWA Amendments of 1996. This Appendix indicates whether each contaminant falls within the

scope of this guidance, as it will be applied for the August 2002 review. Drinking water standards

promulgated after 1996 will be reviewed at a later date. Appendix 2 indicates the NPDWRs that

have been promulgated since 1996, or that are planned for promulgation by 2002.

EPA presented its draft guidance and related assumptions at a stakeholder meeting in

November 1999. The guidance was subsequently revised to reflect stakeholder suggestions. 

Some of the changes included: clarifying the role of research in this process; and discussion of the

potential need to review/revise a NPDWR. The guidance was subsequently reviewed and revised

by the NDWAC WG. During a series of meetings in the year 2000, the NDWAC WG developed

significant revisions and additions to the guidance document. The final recommendations, as

approved by the full NDWAC, are the result of these extensive efforts. 

II. THE SIX-YEAR REVIEW PROCESS

To most efficiently utilize limited resources, EPA plans to perform a series of analyses at

the beginning of each review cycle, intended to target those NPDWRs that are the most

appropriate candidates for revision. The Agency plans to use available, scientifically-sound data

to make decisions regarding whether or not to revise a regulation. Key information that EPA will

review to make decisions regarding regulatory changes include: health effects studies; technology

assessments; and, occurrence and exposure analyses. The Agency also plans to conduct a rough

analysis of costs and benefits as part of the review process. The NDWAC recommends that the

Agency consider non-regulatory options that could augment regulatory actions to achieve the

goal of public health protection. 

A. Goal of the Review

The goal of the review process is to identify priorities for regulatory revision. As a part of

the review process, EPA plans to evaluate regulated contaminants, based on sound science and

available data, to determine whether rule revisions are necessary and appropriate. For example,

new health effects findings and/or increased analytical method capabilities for analyzing a

contaminant may be available that suggest the need to revise a maximum contaminant level goal
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 3

(MCLG) and/or a maximum contaminant level (MCL). The key elements of the review described

in Section III (entitled “Key Elements of the Review Process”) include: health effects review,

technology assessment, occurrence and exposure analysis, consideration of regulatory revisions

that do not impact the MCLG, MCL, or TT requirements (such as monitoring and reporting), and

consideration of the costs and benefits of identified alternatives. EPA plans to use these key

elements to identify possible candidates for regulatory revision. Once preliminary candidates for

revision have been identified, EPA plans to further analyze those contaminants to target the

revisions that are most likely to result in improvements in the level of public health protection

and/or result in cost savings while at least maintaining the level of public health protection. 

For each NPDWR being evaluated, the review will end with a decision by EPA whether or

not to undertake rulemaking efforts to revise the NPDWR. A NPDWR conceivably could be

revised in one or more of the following ways: change the MCLG; change the MCL; modify

monitoring, treatment, and/or other requirements. In rare instances, EPA may determine that it is

appropriate to drop a contaminant from regulation if it is no longer a public health concern. As

required by the SDWA amendment, any revision to a regulation must maintain the level of public

health protection that exists under the current regulation, or must provide for greater public health

protection than the current regulation. Section II.C.(ii), entitled “Potential Types of Regulatory

Changes” more fully describes the conditions under which changes may be appropriate.

For the first review cycle, EPA plans to propose and promulgate any revisions determined

to be appropriate after the completion of the review cycle (i.e., after August 2002). This will be a

separate activity and will be completed consistent with EPA rulemaking policies and procedures. 

The actual details of the regulatory change (e.g., specific changes to the MCLG) will be

determined as a part of this rulemaking process and not as a part of the review process. In

subsequent six-year cycles, EPA plans to complete both the review and any appropriate NPDWR

revisions within a six-year window (i.e., specific regulatory changes will be part of the review

process).

As long as NPDWR revisions maintain or provide for greater protection of the health of persons,

the SDWA 1996 amendments give the Administrator discretion to determine if it is appropriate to

revise an existing NPDWR. In order to determine that a revision is appropriate, the revision must

continue to meet the basic statutory requirements of the SDWA (e.g., set the MCL as close to the

MCLG as is feasible) and:

• Present a significant opportunity to improve the level of public health protection;

and/or,

• Present a significant opportunity for cost savings while maintaining, or improving,

the level of public health protection. 

Risk management considerations should drive the final decision to proceed with a

potential regulatory revision. As a part of this guidance, the NDWAC has defined the risk

management considerations that the Agency should use in its decision-making process. These

considerations include: public health protection, and cost and benefit considerations.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 4

The first part of the review is an information-gathering effort that is science-based and

consists of several inter-related analyses designed to identify potential candidates for regulatory

revision. Some of these analyses examine the health basis and technology basis for setting an

MCLG and an MCL (or TT requirement) to determine whether there have been changes since the

NPDWR was promulgated (or revised). Provision is made for the consideration of other factors

that might identify appropriate revisions, such as changes in monitoring requirements. Other steps

in the decision-making process include review of occurrence and exposure information, and

consideration of potential benefits and costs associated with each prospective revision. Finally,

each prospective revision is assessed, based on the risk management considerations, to determine

the extent to which is it likely to improve public health protection and/or result in cost savings.

At each phase of the review process, some contaminants will be identified as not requiring

additional review during a cycle. These contaminants will be eliminated from further

consideration during that review cycle, since no new acceptable data are available that suggest the

need for regulatory revision. Other contaminants may be identified as a concern due to the

occurrence of the contaminant; due to a technology that may need to be developed or improved;

or; due to a health assessment under way that will not be available for the current review. Thus, a

full review may be needed but the Agency must await additional information to complete that

review. The review of these contaminants may “roll over” to the next review cycle. If the new

data, when available, are significant, EPA should make case-by-case decisions whether to

complete the review of that contaminant and initiate any appropriate rulemaking ahead of

schedule for that review cycle. In addition, EPA should identify data gaps for future research

needs that may support a later review.

Further, in developing this guidance, the NDWAC recognizes there are subject areas that

would benefit from further research. The NDWAC believes this research would make the review

process more conducive to future action by the Agency. A preliminary list of these research

areas, as they relate to all of the key elements of the review process (see Section III), is provided

in Appendix 3. Note that this list is not intended to be comprehensive.

B. Basic Principles of the Review Process

(i) Underlying Assumptions

In order to define the scope of the six-year review process, EPA should apply the

following key assumptions:

1. For the first six-year review period, EPA should review existing data on which the

MCL or TT was based (see description of individual key elements of the review in

Section III for a discussion of the level of scrutiny that will be applied). Thus,

EPA should revisit data on which previous regulatory decisions were made,

effectively identifying data gaps that need to be filled (see Assumption 4 for further

discussion of data gaps). In future six-year review cycles, EPA should assume that

existing regulations are adequate, except in those instances where reliable more

current data are available that indicate a need to re-evaluate an NPDWR (e.g.,

where a new health effects assessment has been conducted). Where new data are
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 5

available, EPA should utilize the guidance and any other established Agency

policies and procedures to determine whether changes to existing standards are

warranted in light of the new data. 

2. For the second and subsequent review cycles, EPA should group regulated

contaminants for review based on the following considerations: health risk, based

on the availability of new data (e.g., health effects, occurrence); other regulatory

activities that may affect contaminants that are in process or are already planned;

and contaminant type (e.g., volatile organic chemicals (VOCs), synthetic organic

chemicals (SOCs), radionuclides, microbes, etc.). 

3. EPA should, as proposed, primarily rely on data available to the Agency in a

timeframe that allows the Agency sufficient time to assess the information prior to

the completion of the review process. For example, for the first review to be

completed in August 2002, contaminants for which revised health effects

assessments are not completed by early calendar 2001 would not be reviewed for

possible MCLG/MCL changes during that cycle. In some cases, as noted in the

previous Section, EPA may decide to review a contaminant for which the Agency

has significant new data on an accelerated schedule. 

4. EPA does not plan to fill in significant data gaps through the review process, per

se. However, as a result of the review, the Agency should identify areas where

significant data gaps exist and initiate activities (e.g., a health re-assessment or

research) to fill those gaps for subsequent review rounds. One example of this

may be the identification of chemicals for which the most recent health effects

re-assessment is more than two review cycles old.

5. To the greatest extent possible, the Agency should use defensible, scientific

methods to carry out a review leading to a revision, including quality assurance

measures, to ensure that scientifically sound results are used in final decisions for

regulatory revisions. Also, as needed, the Agency’s peer review policy should be

utilized as a final check regarding any new analyses stemming from the review

and/or revision of an NPDWR.

(ii) Risk Management in the Review Process

Throughout the review and revision process, EPA should apply the basic principles of risk

management in order to make the most protective and cost-effective decisions possible. While at

different phases of the review process, the risk management assessment may be primarily

qualitative, consideration of the risk management factors -- outlined below -- can significantly aid

Agency decision-makers in the prioritization of potential revisions.

1. Identify the risk.

2. Identify any regulatory revisions that may reduce the risk, including: 
2

 Figure 2 illustrates how the separate analyses interrelate.

3

 See Appendix 4 for a description of the occurrence analysis that will be performed during the first review cycle

for chemical contaminants. Similar occurrence analysis will be performed for chemical and radiological

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 6

• Changes to MCLG, MCL, or TT requirements that appear feasible; and

• Changes to other regulatory requirements.

3. Identify the estimated costs and benefits of the regulatory revision option(s).

4. Identify feasible non-regulatory approaches, on a case-by-case basis, to be utilized under the

following conditions:

• As an interim measure until regulatory revisions take effect;

• To supplement and improve the protectiveness of regulatory revisions; and

• In lieu of regulatory revisions, only if no regulatory revisions can address the

problem, (e.g., the MCL already is as low as can be practically measured or would

be cost-prohibitive). 

C. Scope of the Review

(i) The Review Process

Resource constraints impose practical limitations on the Agency’s ability to conduct

in-depth reviews on an increasing number of NPDWRs every six years. For this reason, the

Agency plans to perform a series of analyses, intended to target those NPDWRs most likely to be

candidates for revision, in order to most efficiently utilize scarce resources to achieve maximum

public health benefits. 

Figure 1, at the end of Section II.C.(i), illustrates the flow of the review process.2 

Initially, all existing NPDWRs that are under review will be assessed. At the beginning of the

review process, EPA plans to focus on identifying those contaminants for which data are available

that are more current, and different than, the data used as the basis for promulgating or revising

the NPDWR. Key elements of this identification phase include an: 

• Initial technical review which is comprised of: 

- health effects data on which an NPDWR was based;

- current technology requirements (analytical methods and treatment) of the NPDWR;

and,

- potential regulatory changes that do not impact the MCLG, MCL, or TT

requirements, such as revisions to monitoring or reporting (also referred as other

regulatory revisions). 

• Initial occurrence and exposure analysis,3 and, 
contaminants in subsequent review cycles. However, EPA expects to have a more extensive occurrence database

for these reviews than will be available during the initial review cycle.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 7

• Identification of areas that may need further research.

Through these analyses, EPA plans to identify: 

• NPDWRs for which the Agency has revised health effects assessments suggesting

possible MCLG changes; 

• Contaminants where new or improved analytical methods or treatment would allow

the MCL to be established closer to the MCLG, or where adjustments in TT

requirements may be appropriate (see Section III.B of this document for discussion of

TT requirements); and 

• Contaminants where the Agency has identified other regulatory revisions that merit

further consideration. 

EPA plans to identify these NPDWRs as potential candidates for revision. 

EPA proposes to drop the other NPDWRs from consideration for possible revision during

the review round. Research needs that have been identified will be forwarded to the Office of

Ground Water and Drinking Water (OGWDW) research prioritization process for further

consideration.

EPA plans to conduct more in-depth analyses of those NPDWRs that were identified as

possible candidates for revision. EPA plans to conduct in-depth analysis of health effects data for

those NPDWRs identified as potential candidates for MCLG revision. Where the health effects

data suggest that a lower MCLG may be indicated, EPA plans to review the appropriate current

technologies to determine if technology will support a lower MCL or more efficient TT

requirements. As necessary, EPA plans to review other regulatory requirements (e.g.,

monitoring) to determine if any changes to these requirements are appropriate. Further, the

Agency plans to conduct a more in-depth occurrence and exposure assessment, and to perform a

rough cost/benefit analysis. After completing this review and analysis, EPA plans to identify those

NPDWRs the Agency believes should be revised. 

For the first review cycle, to be completed in August 2002, EPA plans to publish an

Advance Notice of Proposed Rule Making (ANPRM) in the Federal Register following this

in-depth analysis. The ANPRM will contain the Agency’s draft decisions, and the basis for those

decisions, for each NPDWR reviewed, including whether or not the Agency believes the NPDWR

should be revised. Based on the public comments received, EPA will revisit key elements of the

review, where necessary. The Agency will conclude this first six-year cycle of review by

publishing its final decisions in the Federal Register along with a schedule for proposing and

promulgating revisions. 

The process for public notice and rule revisions will be somewhat different for subsequent

review cycles, because the Agency plans to complete both the review and any necessary revisions
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 8

NPDWRs

under review

Initial Technical Review

♦ Health Effects Data

♦ Current Technology

♦ Other Regulatory

Revisions

♦ Occurrence/Exposure

Identify

research needs

NDPWRs -

no revisions

NDPWRs -

potential revisions

In-depth Technical

Analyses

♦ Health Effects Data

♦ Current Technology

♦ Other Regulatory

Revisions

♦ Occurrence/Exposure

NDPWRs -

no revisions

Review Public

Comments and

Relevant New Data

♦ Cost and Benefit Analysis

Review/Revise

conclusions within

context of new

information

Proposed Revisions

to NDPWRs

(subsequent cycles)

ANPRM

(1st 6-yr cycle)

Federal Register

notices:

Final List of

NPDWRs to be

revised

(1st 6-yr cycle)

Federal Register

notices:

Revised

NPDWRs

(subsequent

cycles)

within the six-year cycle. Therefore, in lieu of publishing an ANPRM, EPA plans to publish

proposed revisions. The preamble of the Federal Register notice containing the proposal will also

identify those NPDWRs that the Agency has reviewed and determined that no revisions are

appropriate. The Agency will develop final rule revisions based on public comments and any new

data. EPA expects to promulgate final revisions approximately 18 months following proposal.

Figure 1: Overview of the Review Process
4

 As with MCLGs, MRDLGs are established at the level at which no known or anticipated adverse effects on the

health of persons occur and which allows an adequate margin of safety. MRDLGs are non-enforceable health

goals based only on health effects and exposure information and do not reflect the benefit of the addition of the

chemical for control for waterborne microbial contaminants.

5 MRDLs are enforceable standards analogous to MCLs, which recognize the benefits of adding a disinfectant to

water on a continuous basis and to maintain a residual to control for pathogens in the distribution system. As with

MCLs, EPA sets the MRDLs as close to the MRDLGs as feasible.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 9

(ii) Potential Types of Regulatory Changes

As a part of the review, EPA will consider regulatory revisions, with the primary goal of

public health protection. The types of revisions considered are based on the various components

of each primary drinking water regulation. NPDWRs set enforceable MCLs for particular

contaminants in drinking water, or require ways to treat water to remove contaminants. Each

standard also includes requirements for water systems to test for contaminants in the water to

make sure standards are achieved, specifies recordkeeping and reporting requirements, defines

what constitutes compliance, and specifies language and delivery requirements for public

notification. Some regulatory revisions that are not listed below (e.g., revisions to approved

analytical methods) are already addressed through periodic rulemaking activities and will not be

included in the six-year review. In addition, there are other ongoing non-regulatory programs,

such as source water assessment and protection, which are central to the nation’s overall pollution

prevention strategy. These non-regulatory programs, which are described in Appendix 5, may

eventually help reduce the introduction and occurrence of contaminants to out nation’s waters,

therefore reducing the need for drinking water regulations. 

1. Changes to MCLGs/MRDLGs: The SDWA requires EPA to establish non-enforceable

health-based MCLGs. For disinfectants, EPA establishes maximum residual disinfectant

level goals (MRDLGs) instead of MCLGs to reflect the fact that these substances have

beneficial disinfection properties.4 As a part of each six-year review, EPA plans to

consider MCLG changes only in those instances where a health effects re-assessment has

been completed, since the MCLG was promulgated or last revised, and where the most

current assessment results in a revised reference dose (RfD) and/or cancer classification

that significantly affects the calculation of the MCLG. 

2. Changes to MCLs/MRDLs: An MCL is an enforceable standard for a contaminant. The

SDWA requires the MCL to be set as close to the MCLG as is feasible. For disinfectants,

EPA establishes maximum residual disinfectant levels (MRDLs) instead of MCLs.5 As a

part of the six-year review, EPA will consider MCL changes.

NOTE: In the rest of this document, unless otherwise noted, the term

“MCLG” should be interpreted to mean “MCLG or MRDLG, as appropriate”

NOTE: In the rest of this document, unless otherwise noted, the term “MCL”

should be interpreted to mean “MCL or MRDL, as appropriate”
6

The 1996 SDWA Amendments identify two classes of technologies for systems serving 10,000 and fewer

persons: compliance technologies and variance technologies. A compliance technology is defined in

§1412(b)(4)(E)(ii) as a technology or other means that is affordable and achieves compliance with an MCL or

satisfies a treatment technique requirement. Variance technologies, defined in §1412(b)(15)(A), are specified for

those system size category/source water quality combinations for which there are no listed compliance

technologies. Variance technologies, where they are permitted, may not achieve compliance with a particular

MCL or TT requirement; however, they must achieve the maximum reduction in inactivation efficiency that is

affordable, taking into consideration system size and source water quality. Variance technologies must also

achieve a level of contaminant reduction that is protective of public health. 

7

EPA conducted this review in 1998, and plans to do so again in 2001.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 10

3. Changes to treatment technique requirements: When it is not economically or

technically feasible to set an MCL, or when there is no reliable or economically feasible

method to detect contaminants in the water, EPA instead sets a TT requirement. A TT

specifies a type of treatment (e.g., filtration, disinfection, and/or chemical addition to limit

contamination) and means for ensuring adequate treatment performance (e.g., monitoring

of water quality to ensure treatment performance).

Generally, water treatment technology may improve to the point where more protective

drinking water standards may be considered. EPA plans to review new information as it

becomes available. However, as with MCLG/MCL-based revisions, data would need to

be scientifically valid. Moreover, before EPA would consider a revision to TT

requirements, the treatment technology would have to be generally available and must

have demonstrated consistent removal of the subject contaminant, at full scale. 

4. Changes to other technology:

a. Best Available Technology: When EPA sets an MCL, the NPDWR also contains Best

Available Technology (BAT) recommendations that address drinking water treatment

processes. Although not required for compliance purposes, EPA sets BATs that have

the capability to meet MCLs. 

EPA plans to review new information on water treatment technologies to determine if

a change to a BAT may be warranted. Again, these data would need to be

scientifically valid. Further, the technologies would need to be generally available and

demonstrate consistent removal of the subject contaminant, at full scale. 

b. Compliance and Variance Technology6: In addition, EPA should continue to

periodically review small system (i.e., systems serving up to 10,000 people)

compliance and variance technologies, for both the MCL-type and the TT-type rules

(however for microbiological contaminant regulations no variances are allowed).7 

5. Changes to monitoring requirements: As part of an NPDWR, EPA establishes

monitoring requirements, including the frequency and location of sampling. EPA should

consider possible revisions to monitoring requirements as a part of the six-year review in

those situations where such revisions are likely to significantly improve the level of public
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 11

health protection or result in a significant cost savings while at least maintaining the

current level of public health protection. 

• Changes to other regulatory provisions: NPDWRs also specify requirements such as

recordkeeping and reporting, what constitutes an out-of-compliance situation, and

mandatory language that must be used for public notification or public education

requirements. Based on actual implementation experience, and through written public

comments or public meetings, the Agency may identify adjustments that warrant

regulatory revision. 

(7) Dropping a contaminant from regulation: In rare instances, new data may demonstrate

that a contaminant does not, in fact, have adverse effects on the health of persons at levels

known or likely to occur in drinking water. If such a situation should arise, EPA should

consider whether it is appropriate to retain or drop the NPDWR for that contaminant. As

with other regulatory revision options, dropping a contaminant from regulation would

necessitate that EPA carry out the formal rulemaking process. 

III. KEY ELEMENTS OF THE REVIEW PROCESS

This Section discusses in detail, the key information that the NDWAC recommends that

EPA review to: (1) determine whether or not a rule revision is warranted, and (2) identify the

need for additional research projects. More specifically, EPA should review and/or conduct

health effects assessments, technology assessments, occurrence and exposure analyses, and a

rough analysis of costs and benefits. The progression of the review process, and the

interrelationships between the key elements of review are illustrated in the recommended decision

tree, presented below in Figure 2. 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 12

Are new health effects

data available or in

process?

Conduct review

when data

become available

Do new data

suggest possible

MCLG change?

No

Estimate analytical methods

limits; Determine if changes

to BAT are appropriate/

possible; Estimate

occurrence and exposure

Will new data be

available in time for

current review cycle?

Yes

Was current MCL

limited by analytical

method capabilities?

Estimate occurrence 

and exposure 

Is a change in

analytical method

feasible?

Is NPDWR

MCL-Based?

Yes

No

Yes

No

Yes

No

Yes

Go to 2

No

No

Yes

Go to 4

BEGIN REVIEW:

Corresponds to Health Effects [III.A] and Analytical Methods [III.B.i]

Go to 3

Was the current MCL

limited by treatment

capabilities?

No

Estimate occurrence 

and exposure

(if have not already done)

Is improved technology

available or feasible?

Yes

Was current MCL

limited by cost/benefit

considerations? *

Have parameters

changed that would

change the validity of

those estimates?

2

Yes

No

Yes

No

Yes

No

Corresponds to Water Treatment [III.B.ii] and Occurrence and Exposure [III.C]

Go to 3

Go to 3

* This question will primarily apply to post-1996 rules.

Key:

Decision Point Analysis Stop

Analysis Research Area

Figure 2: Review Process for NPDWRs 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 13

Based on number of PWSs

impacted and population exposed

estimate costs and benefits of 

each potential MCL change

3

No revisions

appropriate in 

this cycle

Are there other

potential regulatory

revisions?

Are data sufficient to support

rulemaking, and determine

that it will be no less protective

of public health?

Conduct risk management

assessment for all potential

changes, including cost/benefit

considerations and non-regulatory

options as interim measures

Were MCLG/MCL

changes recommended?

Corresponds to Cost and Benefit Considerations [III.E]

No

Yes No

Yes

No Yes

Determine whether changes to

treatment technique requirements

are possible/feasiblev;

Estimate occurrence and exposure;

Estimate costs/benefits of

any potential TT changes

Are new health effects

data available or in

process?

Conduct review

when data

become available

Do new data suggest

possible

MCLG change?

Will new data be

available in time for

current review cycle?

Is a change to the

treatment

requirement

possible/feasible?

Based on number of PWSs

affected and population

exposed, estimate costs and

benefits of each potential

TT change

Go to 5A

Corresponds to Treatment Technique NPDWRs: Health Effects [III.A] and Water

Treatment [III.B.ii]

No

No

No No

Yes

Yes

Yes

Yes

4

Go to 5B

v A TT revision may be appropriate if there have been advances in treatment technology;

advances in microbiological indicators; and/or information to show that the existing TT (in

whole or in part) is ineffective in reducing exposure.

Figure 2: Review Process for NPDWRs (continued)
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 14

Was current

treatment

requirement limited

by cost/benefit

considerations? *

No revisions

appropriate in

this cycle

No

Estimate occurrence

and exposure (if have

not already done)

Yes

Are there other

potential nontreatment-based

revisions?

Are data sufficient to support

rulemaking, and determine

that it will be no less protective

of public health?

Yes

Yes

No

Conduct risk management

assessment for all potential

changes, including cost/benefit

considerations and non-regulatory

options as interim measures

Were treatment

changes

recommended?

No

Yes

Have parameters

changed that would

effect the validity of

those estimates?

Yes

No

Corresponds to Treatment Technique NPDWRs: Cost and Benefit Considerations [III.E]

Go to 5B

above

5A 5B

No

* This question will primarily apply to post-1996 rules.

Figure 2: Review Process for NPDWRs (continued) 
8

The health effects for these contaminants are re-assessed no less frequently than every 15 years. Within EPA,

health effects assessments for pesticides are conducted by the OPP under authority of the Federal Insecticide,

Fungicide, and Rodenticide Act (FIFRA). A goal of FIFRA is that EPA review each pesticide's registration every

15 years. Under some circumstances, a pesticide's health effects may be re-assessed more frequently.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 15

A. Health Effects

The objectives for the examination of health effects under the six-year review are:

• To identify new health effects data for individual contaminants that could change the

MCLG for the contaminant in question and affirm or change the MCL, thus,

affording the same or greater protection of human health than provided by the

present MCLG.

• To use existing or ongoing Agency health effects assessments in accomplishing the

health effects data review.

• To ensure that the health effects data for each contaminant (with the exception of

pesticides that are still in active use8) are reviewed at least once in every two six-year

review cycles.

• To accomplish the review within the limitations imposed by Agency resources.

In order to accomplish the objectives outlined above, the review of health effects data

should be accomplished as described in sub-sections (i) - (iii) below. The outcome of the health

effects assessment should be a list of NPDWRs that are possible candidates for regulatory

revisions based on changes in health considerations. The list should be combined with the lists of

NPDWRs identified by other key elements of the review to develop a final list of NPDWRs that

are candidates for additional evaluation.

(i) Chemical Contaminants (including disinfectants and disinfection by-products)

If there is evidence that a chemical is mutagenic and may cause cancer, and there is no

dose below which the chemical is considered safe, the MCLG is set at zero. In these instances,

according to EPA policy, the MCL is based on technology (analytical methods/treatment) rather

than health effects. In these instances, health effects data have little impact on the MCL. If a

chemical is carcinogenic and acts by a well documented, nonmutagenic mode of action, the

MCLG may be set at a level above zero according to emerging Agency policy. This may provide

regulatory options for some carcinogens that were not available at the time of regulation. 

For non-carcinogenic chemicals, the MCLG is based on an oral RfD. The RfD is an

estimate (with uncertainty spanning perhaps an order of magnitude) of a daily oral exposure to the

human population (including sensitive subgroups) that is likely to be without an appreciable risk

of deleterious effects during a lifetime. A change in an Agency RfD for a chemical could

accordingly lead to a change in an MCLG and MCL. In deriving the MCLG for a

non-carcinogen, the Agency applies a Relative Source Contribution (RSC) factor to allocate a

restricted portion of the total allowable exposure to drinking water. The RSC is one factor which

will determine whether or not a change in RfD will lead to a change in the MCLG/MCL.
9

The Food Quality Protection Act (FQPA), which amended FIFRA in 1996, also requires the consideration of

special subpopulations in the risk assessment. This requirement has been met for most pesticides for which there is

an NPDWR, and OPP is scheduled to have met the requirement for all pesticides currently registered, by 2006.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 16

In the past, it has been the Agency policy to apply a risk management factor to the RfD for

chemicals with equivocal data on carcinogenicity. This policy is a second factor that must be

evaluated to determine the impact of a change in RfD on the MCLG/MCL for a noncarcinogen.

Against the background described above, the six-year review of chemical health effects

should be accomplished using the following screening steps. The following does not imply that

analyses should be done in a specified order. Analyses should proceed based upon availability of

appropriate data.

Step I

Recent health effects reviews for chemical contaminants completed under the following

programs should be examined. Where possible, an oral RfD or comparable value should be

derived and a carcinogen assessment from oral exposure should be conducted during these

reviews.

• EPA Integrated Risk Information System (IRIS) (see Appendix 6)

• Office of Pesticide Programs (OPP) registration (see Appendix 7)

• Agency for Toxic Substances and Disease Registry (ATSDR) Toxicological Profiles

• National Academy of Sciences (NAS) Assessments

In cases where there has been a change in the critical effect, or the dose-response pattern

for a regulated contaminant, and where that change could result in a change in the MCLG and

MCL, that contaminant will be selected as a candidate for possible regulatory revision.

Step II 

The 1996 revisions to SDWA mandate consideration of special subpopulations during the

regulation development process.9 As a reflection of that mandate, the Agency is performing

focused assessment of the risks to the fetus, infants and children during the development of new

regulations. For some regulated contaminants, particularly those regulated prior to 1990, the data

for developmental and reproductive effects were limited. Accordingly, the Agency should

conduct a literature search for peer-reviewed papers published over a period from five years prior

to regulation to the present, to determine if there are new data which identify a developmental or

reproductive effect as the critical effect from exposure or if the data for developmental or

reproductive effects are inadequate to support an assessment of this endpoint. Any contaminant

for which developmental or reproductive effects now appear to be the critical effect, or where

there was no consideration of development or reproductive data gaps during the regulatory risk

assessment, should be considered as a candidate for possible regulatory revision. 
10 As noted above, FIFRA has a goal that each pesticide be reviewed every 15 years. In addition, FIFRA has

provision to permit the evaluation of a registration through various conditions such as the submission of additional

data, at which time EPA would re-evaluate the toxicity and risk. Finally, the Administrator can initiate a Special

Review of any pesticide that may pose an unreasonable risk to humans or the environment. This request may be

initiated by the Administrator or at the suggestion of any interested party. Therefore, a pesticide that would appear

to be an unlikely candidate during a current six-year review cycle, because its review is not imminent under the

15-year FIFRA review goal, could in fact be a candidate because of one or more special conditions.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 17

Pesticides that are still in use will be reviewed using a different process.10 As a condition

of registration and/or re-registration, the pesticide program requires the submission of

reproduction and developmental toxicity studies for most pesticides. OPP also uses open

literature data for weight of evidence when determining a developmental or reproductive effect. 

Once a pesticide that is being considered for regulatory revision is determined to have a

developmental or reproductive effect, OPP, through agreements under the Office of Water

(OW)/OPP Coordinating Steering Committee, will provide health effects data to OW’s various

programs for decision making.

Step III

Some regulated chemicals (including disinfectants and disinfection by-products) may not

have been the subject of recent health effects assessment under the programs identified above and

may not have developmental or reproductive effects as their critical effect or as a data gap. The

reviews for these contaminants should be prioritized as follows.

1. In cases where an IRIS or OPP review has been, or will be, initiated during the review

period, the six-year review should be deferred until the completion of the IRIS/OPP

review. This may move some of the contaminants to the next six-year cycle (see

Assumption 3 in Section II.B.(i)).

2. A literature search for new data should be executed for all chemicals (other than pesticides

that still are in active use) that have not been either selected or removed from

consideration in Screening Steps I and II, and are not the topics of an IRIS or OPP

review-in-process. If the literature search indicates that there is a change in either the

critical effect of dose-response characteristics that could result in a change in the MCLG

and MCL, that contaminant will be nominated for an IRIS review by OW. This may move

some of the contaminants to the next six-year cycle.

Once the final list of NPDWRs requiring review is established, EPA should assess the

health effects data that “triggered” any change in RfD, cancer classification, or cancer slope

factor. As required by the 1996 SDWA amendments, EPA will continue its program of studies to

identify special subpopulations that may be at greater risk than the general population to adverse

health effects from exposure to contaminants in drinking water. The health data should be

integrated with available RSC and risk management (policy) adjustments for possible

carcinogenicity used in the original EPA assessment to determine if the revised health 
11 Risk assessment options could include NOAEL/LOAEL approach, benchmark dose approach, categorical

regression approach, linear multistage approach, point of departure approach, and mode of action approach. The

nature of the effects and the characteristics of the data are used in determining which approach is best for a

particular data set.

12 Radiological contaminants that are regulated, or proposed to be regulated, in drinking water, as of November 

2000 include the following categories: radium-226 and radium-228 (combined), gross alpha particle radioactivity,

man-made beta particle and photon radioactivity, uranium, and radon.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 18

assessment is likely to alter the present MCLG.11 Where necessary, and with an adequate

allotment of time within the six-year review process, new studies published after the RfD or

cancer change should be reviewed for their impact on the MCLG.

(ii) Radiological Contaminants12

Reviews for radiological contaminants should be treated differently than reviews for

chemical contaminants. This is because sources of scientific data, and the reviews by US and

international agencies, which are used in developing EPA risk coefficients are different than are

used for chemicals, as described in Section A.(i) above. A brief summary of radiation health

effects reviews is provided below: this guidance should reflect the fact that EPA has a program

and mechanisms in place to utilize these radiation assessments. These help EPA incorporate

peer-reviewed scientific data into the Agency risk coefficients used in its analyses. When such

risk coefficients and analyses change, this would by necessity lead the Agency to a review of the

affected drinking water standard. Therefore, this guidance incorporates the analyses that are

described below.

All radiological contaminants can be addressed at the same time, since all emit ionizing

radiation. There is a constant influx of papers on ionizing radiation and on radionuclides,

addressing exposures, dose-response relationships, types of effects, etc. There are also periodic

reviews of new information by international groups, such as the United Nations Scientific

Committee on the Effects of Atomic Radiation (UNSCEAR) and the International Commission on

Radiation Protection (ICRP), national groups such as National Committee on Radiation

Protection (NCRP) and National Radiation Protection Board (NRPB), and others. When there

has been enough new information, the Agency has tasked the NAS to provide a review of new

data and develop models to estimate numeric risk of radiation exposure. The Agency then uses

the new information and models to develop the models the Agency will use in risk assessments

involving exposure to ionizing radiation or radionuclides.

Within EPA, the Office of Radiation and Indoor Air (ORIA) is responsible for assessing

the health risks for radioactive contaminants (radionuclides) in the environment. ORIA has

initiated major re-assessments approximately every 10 years, depending on the availability of new

data and resources. After these re-assessments, the estimated risk per unit intake of radionuclides

is revised.

Ionizing radiation is a known human carcinogen. Since all emit ionizing radiation, all

radiological contaminants are classified as human carcinogens. Consistent with the SDWA,
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 19

MCLGs for known or probable carcinogens are typically set at zero because it is assumed, in the

absence of conclusive data to the contrary, that there is no “safe” level of exposure. 

In estimating the health effects from radiological contaminants in drinking water, EPA

subscribes to the linear, nonthreshold model, which assumes that any exposure to ionizing

radiation has a potential to produce deleterious effects on human health, and that the magnitude of

the effects are directly proportional to the exposure level. The Agency further believes that the

extent of such harm can be estimated by extrapolating effects on human health that have been

observed at higher doses and dose rates to those likely to be encountered from environmental

sources of radiation. The risks associated with radiation exposure are extrapolated from a large

base of human data. EPA recognizes the inherent uncertainties that exist in estimating health

impact at the low levels of exposure and exposure rates expected to be present in the

environment. EPA also recognizes that, at these levels, the actual health impact from ingested

radionuclides will be difficult, if not impossible, to distinguish from natural disease incidences,

even using very large epidemiological studies employing sophisticated statistical analyses. 

However, in the absence of other data, the Agency continues to support the use of the linear,

nonthreshold model in assessing risks associated with all carcinogens. 

In assessing health risks for radionuclides other than radon, EPA relies primarily on data

and models presented in reports from the Federal Radiation Council (FRC); the National

Academy of Science Committee on the Biological Effects of Ionizing Radiation (BEIR); the

UNSCEAR; the ICRP; and published, peer-reviewed epidemiological and animal studies. 

In assessing health risks for radon, EPA relies primarily on data and models presented in

reports from BEIR, the U.S. Department of Health and Human Services, and the World Health

Organization’s International Agency for Research on Cancer (IARC).

(iii) Microbial Pathogens

The MCLG is set at zero for any regulated organism that has caused, or has the potential

of causing, a waterborne disease outbreak, since a probability exists that low doses may cause

infection in some percentage of special subpopulations. To control the pathogen, EPA either sets

a TT requirement or an MCL for an easily and reliably measured surrogate (e.g., total coliforms). 

If human infectious dose data are available, they will be used to review the level of treatment

needed and the associated risk of exposure and disease. 

(iv) Review of Standards and Guidelines of Other Organizations

EPA plans to include in its review of NPDWRs an update of standards and guidelines of

other organizations, such as the World Health Organization, the European Union and others. 

Where any difference in health standards or goals is found to exist, EPA plans to, where possible,

determine the rationale for that difference. While some differences may clearly be matters of

policy, others may be science- (or technology-) based, and therefore require further analysis.

To best assess the health effects of a contaminant, scientifically valid studies must be

conducted and/or reviewed from the peer-reviewed literature. Some areas of general research for
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 20

health effects are listed in Appendix 3. The list is not exhaustive, but studies in those areas may

provide the necessary information to best assess risk. 

B. Technology Assessment

The SDWA generally requires that MCLs be set as close to the MCLG as is feasible. 

When determining feasibility, factors that the Agency considers are: the cost and ability of the

analytical and treatment methods; and, the availability of these technologies. In some cases,

particularly when the Agency sets a zero MCLG, EPA establishes the MCL based on the

limitations of analytical methods or treatment. Where these constraints apply to the current MCL,

EPA should assess whether changes in approved methods and/or the availability of new treatment

technologies would support a lower MCL.

(i) Analytical Methods

As described in Appendix 8, EPA already has a program in place to approve new

analytical methods for drinking water contaminants. The review and approval of potential new

methods, therefore, is outside the scope of the six-year review. In those instances where the

MCL has been established based on analytical capability limitations and/or the health effects

analysis suggests that the MCLG/MCL should be lowered, EPA should review the existing

approved methods for the contaminant to determine whether the currently approved methods

provide sufficient analytical capability to reliably measure the contaminant at levels lower than the

current MCL. If it appears that the currently approved method capabilities are the limiting factor

for revising an MCL, EPA should include a request in the Federal Register for potential new

and/or improved methods which are technologically and economically feasible to address a lower

level of quantitation. It should be noted that before a method can be used as a basis for revision

of an MCL, however, the method must be approved, affordable, and be in general use with

sufficient laboratory capacity existing. Therefore, it is unlikely that any method submitted in

response to the Federal Register notice would be used to revise an MCL in the same review cycle

in which the method was submitted. 

(a) Chemical Contaminants

OGWDW uses the Practical Quantitation Level (PQL) to estimate the level at which

laboratories can routinely measure a chemical contaminant in drinking water. The process for

setting the PQL is presented in Appendix 8. Historically, OGWDW has used two main

approaches to determine a PQL for SDWA analytes. The preferred approach uses data from

Water Supply (WS) studies (which have mainly been used to certify drinking water laboratories). 

In most cases, OGWDW uses the WS method when sufficient WS data are available to calculate a

PQL. In the absence of sufficient WS data, OGWDW uses the multiplier method, in which the

PQL is calculated by multiplying the EPA-derived method detection limit (MDL) by a factor of 5

or 10. Because analytical methods may have been updated and/or measurement capabilities by

laboratories may have improved since an MCL was promulgated/last revised, EPA should

re-evaluate the PQLs for some regulated contaminants.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 21

The analytical capabilities should be reviewed for contaminants which are: (1) not

undergoing a health effects review, but the MCL was previously set at the lower level of

quantitation level (or PQL); and (2) undergoing a health effects review and the potential new

MCLG is less than the current MCL. Initially the re-evaluation process should identify the

contaminants (not undergoing a health effects review), whose MCL was limited by the

measurement capabilities (i.e., where the MCL was set at the PQL). EPA should compare the

analytical method capabilities that were available at the time the MCL was set to currently

approved analytical methods. This method comparison should delineate if improvements in

measurement capabilities have occurred (this could be an indication that the PQL may have

changed). In addition and if necessary, EPA should use more recent WS studies to determine if

the PQL may have changed. For contaminants undergoing health effects review (where a

potentially new MCLG is less than the current MCL), the Agency also should use the method

comparison and WS analysis approaches to assess the limits of our analytical capabilities. The

method comparisons and the analysis of WS data should be used to identify contaminants that

need further PQL assessment. The purpose of this analysis is to determine whether the analytical

method capabilities would support a lower MCL.

(b) Radiological Contaminants

Appendix 8.G describes the process EPA has used in the past to assess analytical

feasibility for radiological contaminants. Historically, the determination of measurement feasibility

for radionuclides has not relied on a PQL convention as is used with chemical analytes. Instead,

measurement reliability was computed using the grand average of the means and reproducibility

(total error) of the study population of laboratories that participated in Radiochemical

Performance Evaluation (PE) studies. The distribution of laboratory averages (around the known

value) reflected both random uncertainty and bias introduced by a laboratory. This distribution

around the known value (or reproducibility) represented an empirical measure of the reliability or

feasibility of laboratories to measure a nuclide, all factors considered. The goal of determining the

reproducibility for the population of laboratories was to provide an upper limit on accuracy for

any laboratory result provided by a certified laboratory and is not obtainable except by multiple

laboratory testing. 

Although the feasibility of measurement for radionuclides in drinking water will not be

assessed under the current six-year process, EPA plans to include them as part of future six-year

reviews. In reviewing measurement feasibility for these future reviews, the first step should be to

assess if new radiochemical methodologies or method modifications have been approved which

could potentially lower the measurement feasibility. Because the PE program has been

externalized, the Agency should determine the means to assess the multiple laboratory data as

needed for the determination of measurement feasibility for the various program needs. Before

the next six-year review, the Agency should detail a more specific process to determine

measurement feasibility for radionuclides on a large scale. 

(c) Microbiological Contaminants

For microbes, EPA does not have, or currently envision, a routine pathogen monitoring

requirement, but rather employs indicators of water quality (e.g., total coliforms, E. coli). PQLs
13 However, it is not clear whether the process, as described in Appendix 8, for setting the PQL for chemicals is

applicable to microorganisms or related parameters. In principle, the process for setting PQLs for microorganisms

should be similar to that of chemicals. However methods for determining microbial density suffer greatly from

inherent problems associated with the organisms. These include, but are not limited to, the level of injury, uneven

distribution of the organism in the environment, the ability to selectively isolate the target microorganism, the lack

of method precision, and the difficulty to determine infectivity and/or disease causation.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 22

have not been used for microbial indicators because, for approval, the method must be able to

detect a single cell (i.e., MDL and PQL must both be one cell) in a 100 ml water sample. In

addition, the false-positive and false-negative (i.e., recovery) rates must be reasonable. EPA is

considering whether to define “reasonable” in numerical terms. 

Occasionally, EPA may require limited quantitative pathogen monitoring in source water

(e.g., Cryptosporidium). In such a case, it may be possible to determine a method’s MDL and

PQL, because the best available methods may not be sensitive enough to detect a single cell. 

However, EPA normally uses MDLs and PQLs to set an MCL. Because the Agency does not set

MCLs for pathogens in the source water, it might be inappropriate to set MDLs and PQLs for this

purpose. It may be appropriate to determine an MDL and PQL for some required non-microbial

measurements associated with microbial water quality such as turbidity, disinfection residual and

algal microcystins.13 

(ii) Treatment Technologies

As discussed previously, an NPDWR either identifies the BAT for meeting the MCL (even

though BAT is not required for compliance purposes), or establishes enforceable TT

requirements. In those instances where lowering an MCL for a contaminant may be warranted,

based on improved analytical method capabilities or health effects data that support lowering an

MCLG, EPA should review the BAT for that contaminant. In some cases, EPA may also

consider revisions that clarify existing BAT and/or TT requirements where existing requirements

are confusing or incorrectly specified.

If new information requires EPA consideration of a more stringent regulated level, to a

point that an MCL would no longer be feasible, the SDWA requires that EPA set a TT

requirement for that contaminant. 

For the treatment analysis, EPA plans to utilize the same sources that have been the

primary resources in development of EPA regulations and guidance. These include EPA’s

treatment technologies and cost reports, peer review journals, and other technology sources,

including information received from stakeholders.

EPA plans to include in its review of NPDWRs, any updates of standards and guidelines

of other parties, such as the World Health Organization, the European Union and others. Where

any difference in technology-related standards or goals are found to exist, EPA plans to, where

possible, determine the rationale for that difference. While some differences may clearly be

matters of policy, others may be science- (or technology-) based and therefore require further

analysis. 
14 While occurrence and exposure estimates factor into the derivation of an RSC a much more important factor is

exposure information for other media (air, food, etc.) relative to that for water. Exposure information for other

media will be assessed as a part of the health effects assessment described in Section III.A. of this document, and

not as a part of the occurrence and exposure assessment described in this section. For most contaminants, (e.g.,

those with zero MCLGs and those where the RSC was set by default at the 20% floor), recalculation of the RSC

may not be necessary.

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 23

Research in the area of technology assessment should focus on areas of water treatment

and analytical methodologies. Scientifically valid studies should be conducted and/or reviewed

from the peer-reviewed literature. Some areas of general research for technology assessment are

listed below in Appendix 3. The list is meant as a guide to begin research into those areas that

may best provide protection for public health. Studies in these areas may provide the necessary

information to best assess occurrence and possibly to control the contaminant. 

C. Occurrence and Exposure Analysis

The goal of the occurrence and exposure analysis during the review process is to generally

quantify the extent of regulated contaminant occurrence and human exposure levels. Results of

this analysis include estimated numbers of PWSs and populations impacted by a contaminant in

water. For some microbes, this may not be the case. Combined with results of the other technical

analyses described in this Section (e.g., health effects related to contaminant exposure) these

results will be used to help determine which revisions are most likely to provide the greatest

public benefit. In some cases, these results may also be used as a factor when recalculating

relative source contributions (RSCs).14 EPA plans to perform further, in-depth, occurrence and

exposure analysis prior to any proposed revision to an NPDWR.

This guidance document Section describes the occurrence and exposure analytical

processes that are more specific to the first round of review, i.e., that which is to be completed by

2002. EPA expects to make refinements to this review in subsequent rounds as more data

become available and as the methodology evolves. Thus, the guidance mentions future reviews

but does not contain a detailed description of how these reviews will be conducted. 

For the chemicals to be reviewed in the first round of review, Appendix 4 describes: (1)

the current analyses of State regulated contaminant data sets; (2) the development of a nationally

representative sample, including analysis and ranking of States’ pollution potential and geographic

diversity; (3) the resultant 8 ‘cross-section States’; (4) the potential addition of State data sets

based primarily on data availability, pollution potential ranking, and geographic representation; (5)

the limitations of these data; and, (6) alternative approaches to producing a national occurrence

estimate. The alternatives described are founded on methodologies utilized in 1999 for the

analysis of occurrence of regulated chemical contaminants, and in 2000 of arsenic in public water

supplies. Some pros and cons of the 1999 analysis and the recent arsenic analysis are mentioned,

as is the general problem of uncertainties of extrapolation of results from a limited database to a

national estimate. EPA plans to use the analyses as described and additional analyses as

appropriate. However, the above noted analysis of chemical contaminant occurrence in public

water supplies, which was peer reviewed in 1999, is considered the basis for the first six-year

review occurrence analysis.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 24

Ambient water data, if available, may supply the Agency with a correlation to drinking

water systems’ finished water levels. This may be of some value in providing a raw/finished water

comparison, indicative of water treatment effectiveness in removing contamination from water. 

Future review cycles should benefit from the statutorily required National Drinking Water

Contaminant Occurrence Database (NCOD). In addition to this, EPA maintains other data

systems containing information on microbial contaminants and disinfection/disinfection

by-products (D/DBPs). The NCOD, its current limitations, and other data systems are described

more fully in Appendix 4.

In addition, EPA continues to assess methods for extrapolating quality data sets to assess

national-scale, contaminant levels for use in the regulatory program. Thus, the current guidance

cannot establish a sole methodology for estimating national occurrence/exposure, since methods

are expected to evolve during the early years of the six-year NPDWR review program.

Where cost and benefit considerations were major factors in setting MCLs, EPA should

assess if occurrence and exposure are substantially different from what was assumed during the

original rulemaking. A revision to the NPDWR should be considered if new occurrence and

exposure estimates show that public health is affected to a greater degree than EPA had

anticipated in the original cost and benefit analysis.

Research to date in the occurrence and exposure area has been limited. The problem

being, not so much that the contaminants have not been found, but that there has not been a

concise study of these contaminants nationally, at levels related to the MCL. In research, the

issue has often been the accuracy of the testing method, rather than an issue of actual occurrence. 

As noted in this guidance, State agencies often supply data on contaminant occurrence

which is of some value to EPA. In addition, United States Geological Survey (USGS) source

water testing may be one of the most valuable sources of occurrence data from untreated water,

especially when it provides accurate temporal data. Related to this, some areas that would benefit

from future research are listed in Appendix 3.

D. Consideration of Other Regulatory Revisions

In addition to possible revisions to MCLGs, MCLs, and treatment techniques, EPA should

also consider other regulatory revisions not related to these requirements, (such as monitoring, or

reporting requirements), and should consider options that may lead to non-regulatory revisions,

(such as source water protection and others, as outlined in Appendix 5). These options should be

considered independently, and in addition to, possible MCLG, MCL, or TT revisions. If new

information is available that may demonstrate the effectiveness of a non-MCL, non-MCLG, or

non-TT approach in improving public health protection with respect to a contaminant or group of

contaminants, then this information should be included in the risk management assessment. These

options could be especially important if the costs of other regulatory compliance are considered to

be too high, or if interim measures are needed pending promulgation of a rule. As with other

aspects of the six-year review, input from stakeholders should be sought regarding these potential
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 25

changes. Within the constraints of the SDWA, EPA strive to create an integrated package of

drinking water regulations that complement each other. EPA should also consider simplifying

rules to reduce the regulatory burden on States and water suppliers if such simplification can be

shown to be equally protective of public health. 

Changes to monitoring requirements: As part of an NPDWR, EPA establishes

monitoring requirements, including the frequency and location of sampling. EPA should consider

possible revisions to monitoring requirements as a part of the six-year review in those situations

where such revisions are likely to significantly improve the level of public health protection or

result in a significant cost savings while at least maintaining the current level of public health

protection. Monitoring revisions may include (but are not limited to) changes in one or more of

the following: frequency of monitoring; timing of sample collection; sample site location; or

sample volume. In some situations, monitoring changes may affect an entire class of contaminants

(e.g., all SOCs); in other cases, adjustments to the monitoring requirements for individual

contaminants may be appropriate. As a part of evaluating potential monitoring revisions, the

Agency proposes to perform in-depth statistical testing, to determine the probability that the

potential modification would best capture and not bias representation of contaminant occurrence

in water. 

Occurrence data from both finished drinking water (such as EPA’s recent report on

drinking water contaminants in finished water) and from source water (such as USGS’ National

Water Quality Assessment Program (NAWQA) data) show that many contaminants, especially

pesticides, have strong seasonal trends in occurrence. It may be more appropriate to require

monthly sampling for some of these compounds during the season of greatest use, and little or no

sampling during the rest of the year. Occurrence data can also be used to demonstrate a lack of

vulnerability of certain intakes to certain contaminants, and this information can be useful in

granting monitoring waivers.

Changes in data reporting requirements: NPDWRs also specify requirements, such as

recordkeeping and reporting, to EPA and to the public. States, which are EPA’s partners in

ensuring safe drinking water, are willing to submit necessary data elements to meet this need, but

EPA data reporting requirements may go beyond State capabilities. Some of the required data

elements may not be necessary, and based on past experience, may not even be used by the

Agency. We recommend that EPA with assistance from States and other stakeholders evaluate

what data are the minimum data elements that must be reported in order to safeguard public

health. Data management should be considered at the time of rule development, not after the rule

is adopted. An implementation plan should be developed at the same time that the rule is drafted,

so States and public water systems (PWSs) can see the impact of the proposed rule. 

Multi-media mitigation: Opportunities exist for improving public health by regulating

exposure to drinking water contaminants that also occur in other media. One example is the

radon program, where PWSs are to be involved in promoting changes in building codes in order

to have new homes constructed with radon venting.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 26

Contaminant reduction can be achieved through other environmental programs that

regulate use, storage, or transfer of the contaminant. Examples include the air/water cooperative

program for radon, the recent policy memo of Charles Fox, EPA Assistant Administrator, to

achieve reduction of waterborne pathogens through cooperative efforts of wastewater and

drinking water programs, and the pesticide registration and regulatory program.

Some possible areas of research into other regulatory revisions (e.g., non-MCL/TT, and

non-MCLG regulations) are listed in Appendix 3. The list is meant as a guide to begin research

into those areas that may best provide protection for public health. 

E. Cost and Benefit Considerations 

(i) Cost and Benefit Assumptions of Current NPDWR

In some instances for NPDWRs promulgated after August 1996, cost and benefit

considerations may have been a factor in setting the regulatory requirements. For these

“post-1996" NPDWRs, if other technical assessments described in this guidance do not identify an

additional need to revisit an NPDWR, then EPA should review the cost and benefit assumptions

of the current NPDWR to determine whether they still are valid. If the original assumptions no

longer are valid in light of more current data, EPA should assess whether changes to the

regulatory requirements might be appropriate as a result of adjustments to the assumptions. 

These NPDWRs will be identified as candidates for possible regulatory revision.

(ii) Cost and Benefit Estimation of Potential Regulatory Revisions

Analyses of costs and benefits may play a role in risk management decisions associated

with an NPDWR review. However, it is not anticipated that the level of analysis will be

comparable to that cost and benefit studies that must accompany a formal regulation development

process. 

Where possible, the Agency should first develop a qualitative listing of potential costs and

benefits associated with a proposed revision. Where data such as occurrence and exposure

information are available, the Agency should develop associated quantitative, albeit possibly

preliminary, estimates of costs and benefits. ‘Major’ items such as health effects averted

(monetized where feasible), and construction and installation of treatment facilities should be

estimated. Quantification would depend upon the availability of national health, chemical

occurrence, exposure and cost data. 

At a minimum, EPA’s cost assessment should include consideration of cost for treatment

associated with the MCLs being considered, including: capital costs, operation and maintenance

costs, monitoring costs, and administrative costs (both to the water supplier and the State and

Federal agencies). 

Where possible and appropriate, EPA’s benefit assessment should include, but not be

limited to, an estimate of the number of illnesses and/or diseases prevented by reducing the MCL

or improving the TT. The assessment should include discussion of the following: 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 27

• Infants, children, the elderly and immunocompromised, and any other known sensitive

subpopulations;

• Reductions in adverse, synergistic health effects caused by a contaminant of concern

existing in combination with other co-occurring contaminants;

• Illness or disease prevented by the simultaneous reductions of levels in other regulated

contaminants by treatment process(es) necessary to control the contaminant of

concern; and

• Reductions in adverse health effects caused by an accompanying reduction of a

contaminant’s degradation products co-existing with the contaminant. 

Other “benefits” that be considered are any expected reductions to monitoring and administrative

costs (both to water suppliers and State agencies) that are associated with the regulatory changes

that are being considered. 

During the regulatory review period, there may be uncertainty regarding actual targets,

(i.e., to what level an MCL or MCLG may later be proposed), pending additional technical

review. If it is known that a regulatory level may change, although to an unknown level, costs

and benefits should be estimated, or listed, for a series of possible levels, for consideration in the

decision-making process. 

STAKEHOLDER INVOLVEMENT

A. Key Stakeholders

EPA plans extensive opportunity for stakeholder involvement in the six-year review

process. Key stakeholders include (but are not limited to) members of the following:

• The general public • Public interest groups

• Members of Congress • Public water suppliers

• Other Federal agencies • National trade associations

• State, Tribal, and local officials • Other EPA offices

• Public Health/Health Care Providers • Environmental groups

• Manufacturers • Agricultural Producers

B. Stakeholder Meetings

For the first review cycle, EPA plans to hold three stakeholder meetings during the review

process. The purpose of each meeting will be to inform stakeholders of the Agency’s progress

and to solicit additional input and advice. The general topics to be covered at each of the meeting

is as follows: 

• First Meeting: the NPDWRs to be included in the review; the general analytical approach

to be utilized; and the primary data sources for the review round.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 28

• Second Meeting: the guidance recommended by NDWAC; the preliminary results of

analysis to date; and EPA’s preliminary selection of NPDWRs for revision.

• Third Meeting: follow-up comments to the ANPRM.

During subsequent review cycles, EPA plans to hold two stakeholder meetings prior to

proposing any NPDWR revisions. The purpose of the first meeting will be to discuss the

NPDWRs to be included in the review and any refinements to the Six-Year Review Guidance that

may be appropriate (e.g., adjustments to general analytical approach and/or primary data sources). 

The second meeting will discuss the preliminary results of analysis to date and EPA’s preliminary

selection of NPDWRs for revisions. If appropriate, EPA also may hold a third stakeholder

meeting following proposal of revisions to discuss public comments received in response to the

proposal.

If EPA has available travel funds, the Agency will try to hold at least one of these

meetings in a location other than the East Coast.

C. Other Stakeholder Outreach

In addition to the stakeholder meetings, EPA intends to keep stakeholders informed and

involved through the following mechanisms.

• Participation in national meetings, workshops, technical forums.

• Informal meetings with groups and association and individual discussions with

technical experts at least once a year. In some cases, EPA may benefit from such

discussions in terms of gaining greater awareness of specific issues, and/or direct

transfer of applicable data from stakeholders or other public entities.

• Information posted on the OGWDW web page. Issue papers, dates and locations of

public meetings, summaries of stakeholder meetings, summaries of informal meetings,

etc., will be placed on the web page. This information will be updated periodically.

• Federal Register Notices pertaining to the six-year review.

• Supporting documents explaining the Agency’s rationale and approach to the

regulatory review and decision-making process. The Agency will request input

through the peer review process. 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 29

APPENDICES
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 30

Appendix 1: Contaminants Regulated Prior to August 1996

Note: The purpose of this table is to indicate those NPDWRs which will be reviewed under the

initial six-year review period that ends August 2002.

Contaminant Pre 8/96 Rule Within Scope of 1st

Review Cycle

Reason for Not

Including

Inorganic Chemicals

 Antimony Phase V Rule Yes

 Arsenic Pre 1986 NIPDWR No Subject of ongoing

rulemaking activity

 Asbestos Phase II Rule Yes

 Barium Phase IIB Rule Yes

 Beryllium Phase V Rule Yes

 Cadmium Phase II Rule Yes

 Chromium Phase II Rule Yes

 Copper Lead and Copper Rule

(LCR)

 No Reviewed March

2000 

 Cyanide Phase V Rule Yes

 Fluoride Phase II Rule Yes

 Lead LCR Yes

 Inorganic Mercury Phase II Rule Yes

 Nitrate (as N) Phase II Rule Yes

 Nitrite (as N) Phase II Rule Yes

 Selenium Phase II Rule Yes

 Thallium Phase V Rule Yes

Organic Chemicals

 Acrylamide Phase II Rule Yes

 Alachlor Phase II Rule Yes

 Atrazine Phase II Rule No Subject of ongoing

review

 Benzene VOC Rule Yes

 Benzo(a)pyrene Phase V Rule Yes

 Carbofuran Phase II Rule Yes

 Carbon Tetrachloride Phase II Rule Yes

 Chlordane Phase II Rule Yes

 2,4-D Phase II Rule Yes

 Dalapon Phase V Rule Yes
Contaminant Pre 8/96 Rule Within Scope of 1st

Review Cycle

Reason for Not

Including

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 31

 1,2-Dibromo-3- 

chloropropane (DBCP)

 Phase II Rule Yes

 o-Dichlorobenzene Phase II Rule Yes

 p-Dichlorobenzene VOC Rule Yes

 1,2-Dichloroethane VOC Rule Yes

 1,1-Dichloroethylene VOC Rule Yes

 Cis-1,2-Dichloroethylene Phase II Rule Yes

 trans-1,2-Dichloroethylene Phase II Rule Yes

 Dichloromethane Phase V Rule Yes

 1,2-Dichloropropane Phase II Rule Yes

 Di(2-ethylhexyl)adipate Phase V Rule Yes

 Di(2-ethylhexyl)phthalate Phase II Rule Yes

 Dinoseb Phase V Rule Yes

 Dioxin (2,3,7,8-TCDD) Phase V Rule Yes

 Diquat Phase V Rule Yes

 Endothall Phase V Rule Yes

 Endrin Phase V Rule Yes

 Epichlorohydrin Phase II Rule Yes

 Ethylbenzene Phase II Rule Yes

 Ethylene Dibromide Phase II Rule Yes

 Glyphosate Phase V Rule Yes

 Heptachlor Phase II Rule Yes

 Heptachlor epoxide Phase II Rule Yes

 Hexachlorobenzene Phase V Rule Yes

 Hexachlorocyclopentadiene Phase V Rule Yes

 Lindane Phase II Rule Yes

 Methoxychlor Phase II Rule Yes

 Monochlorobenzene Phase II Rule Yes

 Oxamyl (Vydate) Phase V Rule Yes

 Polychlorinated biphenyls

(PCBs) Phase II Rule Yes

 Pentachlorophenol Phase IIB Rule Yes

 Picloram Phase V Rule Yes

 Simazine Phase V Rule Yes

 Styrene Phase II Rule Yes
Contaminant Pre 8/96 Rule Within Scope of 1st

Review Cycle

Reason for Not

Including

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 32

 Tetrachloroethylene Phase II Rule Yes

 Toluene Phase II Rule Yes

 Total Trihalomethanes

(THMs) Stage I DBP Rule No Subject of ongoing

rulemaking activity

 Toxaphene Phase II Rule Yes

 2,4,5-TP (Silvex) Phase II Rule Yes

 1,2,4-Trichlorobenzene Phase V Rule Yes

 1,1,1-Trichloroethane Phase V Rule Yes

 1,1,2-Trichloroethane Phase V Rule Yes

 Trichloroethylene VOC Rule Yes

 Vinyl chloride VOC Rule Yes

 Xylenes Phase II Rule Yes

Radionuclides

Beta particles and photon

emitters Pre 1986 NIPDWRs No Subject of ongoingrulemaking activity

Gross alpha particle activity Pre 1986 NIPDWRs No Subject of ongoing

rulemaking activity

Radium-226/228 (combined) Pre 1986 NIPDWRs No Subject of ongoing

rulemaking activity

Microorganisms

Giardia lambia

Surface Water

Treatment Rule

(SWTR)

 No Subject of ongoing

rulemaking activity

Heterotrophic plate count

(HPC) SWTR No Subject of ongoingrulemaking activity

Legionella No Subject of ongoing

rulemaking activity

Total Coliforms (including

fecal coliform and E. coli)

 Total Coliforms Rule Yes

NOTE: EPA may

publish the revise/not

revise decision in a

separate FR notice

 Turbidity SWTR No Subject of ongoing

rulemaking activity

Viruses SWTR No Subject of ongoing

rulemaking activity
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 33

Appendix 2: National Primary Drinking Water Regulations (NPDWRs) Promulgated After

August 1996 and NPDWRs Anticipated to be Promulgated Prior to August 2002

Note: The purpose of this table is to indicate those NPDWRs which will not be reviewed under

the initial six-year review as these regulations will have not been in effect for 6 years by August

2002.

Regulation Date of Promulgation

Disinfectants and Disinfection By-Products Rule (Stage 1) December 1999

Interim Enhanced Surface Water Treatment Rule December 1999

Radon Early 2001* 

Radium, Gross Alpha and Beta/Photon Emitters November 2000*

Uranium November 2000*

Filter Backwash Recycling Rule December 2000* 

Enhanced Surface Water Treatment Rule (Long Term 1) December 2000* 

Arsenic June 2001* 

Ground Water Rule May 2002*

Disinfectants and Disinfection By-Products Rule (Stage 2) May 2002*

Enhanced Surface Water Treatment Rule (Long Term 2) May 2002*

* Indicates anticipated date of promulgation as of November 2000.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 34

Appendix 3: Areas that Would Benefit from Future Research

In developing this guidance, the NDWAC recognizes there are subject areas that would

benefit from further research. NDWAC believes this research would make the review process

more conducive to future action by the Agency. Below, is a preliminary list of these research

areas, as they relate to all of the key elements of the review process (see Section III). Note that

this list is not intended to be comprehensive.

Health Effects

• General toxicity studies.

• Collecting data on “banned” contaminants.

• Data to assess the synergistic effects of contaminants when assessing health effects.

• Dose response profiles for microbial pathogens.

• Studies to identify new, and previously unrecognized waterborne disease agents, for

assessing the relative contribution from water in the incidence of disease and for

assessing the level of endemic disease.

• Studies on the ecology of pathogens (e.g., survival, replication, interaction with other

microbial pathogens).

Technology Assessment

Research in the area of technology assessment should include focus areas of water

treatment and analytical methodologies. Scientifically valid studies should be conducted and/or

reviewed from the peer-reviewed literature. Some areas of general research for technology

assessment are listed below. The list is meant as a guide to begin research into those areas that

may best provide protection for public health. 

• Data, information and technology.

• New methods for monitoring to generate occurrence data.

• New method for selected pathogens (or chemicals) or surrogates.

Occurrence and Exposure Analysis

Research to date in the occurrence and exposure area has been limited. The problem

being, not so much that the contaminants have not been found, but that there has not been a

concise study of these contaminants nationally, at levels related to the MCL. In research, the

issue has often been the accuracy of the testing method, rather than an issue of actual occurrence. 

As noted in this guidance, State agencies often supply data on contaminant occurrence which is of

some value to EPA. In addition, United States Geological Survey (USGS) source water testing

may be one of the most valuable sources of occurrence data from untreated water, especially
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 35

when it provides accurate temporal data. Related to this, some areas that would benefit from

future research are listed below.

• Research on contaminants exhibiting temporal occurrence patterns including: the

necessity to characterize chemical application/use; seasonal weather patterns; physical

or chemical degradation; and resultant impacts on occurrence levels. 

• In unique cases, research to determine relationships between raw and finished water

contaminant levels such that projection of finished water (i.e., regulated water) may be

improved. Such studies may also help in understanding of treatment and/or

monitoring issues.

• Research, e.g., pathway absorption research, may assist in the Agency’s understanding

of human exposure to chemicals, perhaps singly or by chemical group as needed. 

Exposure via alternative routes may be an area of concern for certain contaminants.

• Research in advanced statistical methodologies may allow development of alternate

models for more accurately estimating national chemical occurrence.

Other Regulatory Revisions

Research is needed on optimization of monitoring design in terms of parameter selection

and timing of sampling, to provide maximum representation of actual variation in relevant water

quality indicators at minimum cost. Research is also needed on innovative sampling, analytical,

and monitoring technology, including real-time sensors.

• Surrogate parameters: Research is needed to identify appropriate surrogate

parameters for drinking water contaminants that are difficult to measure such as

pathogens and disinfection by-product formation potential.

• Data reporting requirements: Research is needed on data reporting requirements that

will provide the appropriate information to State and Federal regulators, in relation to

the ultimate questions that need to be answered, without excessive cost or burden to

water providers.

• Treatment technology: Research is needed on new drinking water treatment

techniques that will provide greater protection from multiple contaminants at less cost.

• Effectiveness of source water protection measures: Research is needed on the

effectiveness of various source water protection measures in improving water quality.

• Proper water system operation: Many aspects of water system operation can help

ensure high water quality. Research needs include identifying what percent of

backflow devices are tested each year, and what are the success/failure rates of these

backflow devices. Other topic areas include: Re-growth, Pressure Surges, etc.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 36

Appendix 4: Determination of National Occurrence of Contaminants

In reviewing existing chemical occurrence data, the Agency will determine which NPDWR

contaminants are occurring at significantly high or low levels and, furthermore, develop

nationally-representative contaminant occurrence and exposure profiles. The Agency may be able

to utilize results of these analyses in combination with the analytical results of the primary

health-based and technology analyses described within this guidance for the purpose of

determining the level of priority of potential regulatory revisions (for some microbial contaminants

this type of analysis may not apply).

EPA will use simple statistical tools to identify contaminants which occur at either high or

low levels and then use these findings to estimate the number of people exposed to various levels

of each contaminant. These initial occurrence and exposure findings will be evaluated together

with other health-based or technology considerations (e.g., contaminants considered for MCL or

MCLG changes). These combined evaluations will serve to identify a list of contaminants for

which EPA will analyze available occurrence data in greater detail to produce occurrence and

exposure profiles that are more statistically representative of the nation.

The result of this effort will be improved EPA occurrence and human exposure

assessments for priority chemical contaminants. The Agency has already completed part of the

initial occurrence analysis using the analytical approach as described below (Section A). The

Agency may augment these initial analyses with other analyses as appropriate (see Section B).

In future reviews, the Agency expects to have a larger, centralized database providing

access to high quality regulated and unregulated contaminant monitoring information. The

National Drinking Water Contaminant Occurrence Database (NCOD) is being developed to

satisfy the statutory requirements set by Congress in the 1996 SDWA amendments. The purpose

of NCOD is to provide support for EPA’s decisions regarding contaminants to be selected for

regulation; for subsequent regulation development; and, for the six-year regulatory review

process. 

NCOD is expected to contain occurrence data from both PWSs and other sources such as

the US Geological Survey’s National Survey Water Information System (NWIS). NCOD does

not currently contain occurrence data from all water systems and all States, and as such, will not

be supportive of the first cycle (2002) of NPDWR review. The only PWS data contained in

NCOD is compliance status information that has been reported voluntarily by States to the Safe

Drinking Water Information System (SDWIS). Occurrence data at levels below and above

current MCLs, are expected to populate NCOD. As of August 2000, the NCOD is operational,

although in a developmental stage. It is not yet available to support nationwide estimates of

occurrence of regulated contaminants: regulated contaminant occurrence data would need to be

collected and entered into the appropriate database. As noted below, the first six-year review

cycle is relying on analyses of available data that was provided by State agencies, outside of the

context of NCOD development.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 37

The NCOD is not structured to store data on microbial and disinfection by-products

(MDBP) contaminant levels in water. For MDBP data, the EPA drinking water program relies on

several sources:

• The EPA Information Collection Rule (ICR) of May1996 includes source and finished

water sampling at large PWSs. Included in the ICR are extensive monitoring

requirements for 300 PWSs operating 500 treatment plants. Analytes include:

Cryptosporidium and Giardia oocysts/cysts, enteric viruses, and chemical by-products

of treatment (as well as treatment information). Data from this effort have been placed

in ICR Auxiliary Databases.

• The “ICR Supplemental Survey” monitoring included protozoa (Cryptosporidium and

Giardia), DBP precursors, E. coli, fecal coliform, and total coliform data from

medium/large systems (47 large, 40 medium systems). This survey also collected

occurrence information (except for protozoa) from small systems. This survey

represents a 12-month data set. The survey data, like the ICR Auxiliary Database, is

contained in an Microsoft 97 Access file (July 2000, for EPA internal use); it is to be

made available to the public, in condensed form, in Fall 2000.

• The Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), to be

proposed in 2001, would as drafted require 1,800 medium/large systems to test for

Cryptosporidium, E. coli, and turbidity. These data would be collected, entered into a

data system, and used to place systems into defined ‘bins’ that will relate to treatment

requirements.

• Small systems monitoring: The National Rural Water Association (NRWA) is taking

on the task of collecting DBP precursor information at small systems in United States. 

The survey data are to be stored in a Microsoft Access database. 

Note that the above listing of MDBP occurrence collection efforts is not necessarily an

exhaustive list of data sources available to the Agency; nor does it describe MDBP data analyses

which have or will have occurred in future regulatory actions (e.g., DBP II, LT2ESWTR) and

future MDBP reviews.

A. Current Analyses of State Chemical Occurrence Data

Several sources of data were used in the current occurrence analysis. The primary source

of data, used as the basis of most of these analyses, was compliance monitoring data from PWSs

as volunteered by States. More than 70% of these data, referred to as the “State data sets,” date

from approximately 1993 to 1997. The secondary source of data was other SDWA compliance

monitoring data from public drinking water systems, dating from 1983 to 1992. These are

referred to as the “Unregulated Contaminant Information System (URCIS) Round 1 Data” and

are from EPA sources. The URCIS data were used for comparison analyses for eight regulated

VOCs and two regulated SOCs. Other privately- and publicly-available data sources were also

used for comparative purposes. For greater detail regarding the proposed approach refer to the

reader to A Review of Contaminant Occurrence in Public Water Systems (1999), also known as

the “Drinking Water Contaminant Occurrence Report,” EPA 816-R-99-006. (The full document
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 38

can be found at the EPA website: www.epa.gov/safewater/occur/occur.html. From the Office of

Ground Water and Drinking Water website, readers may also search on the term “contaminant

occurrence.”)

State data sets. The most important source of data used in the contaminant occurrence

analyses were State-derived data sets. These data sets were compiled by State drinking water

agencies. The eight (8) State data sets used represent over 25% of the U.S. population served by

PWSs, and over 20% of the PWSs, a substantial sample. These data represent more than 10.7

million analytical results from nearly 26,000 PWSs. For the initial stage of analyses, an overview

of contaminant occurrence was prepared based on all data (including the full group of 12 State

data sets as well as supplemental data). The 12 State data sets were used to identify the range of

contaminant occurrence findings. Data sets from 8 States were selected for use in a national

analysis as providing the best data quality and completeness, and for providing a balanced national

cross-section of occurrence data. The States included in the national cross-section provide a

nationally balanced geographic coverage, and a balanced representation of States distributed

across the full range of State pollution potential rankings. (See the “National Representation”

section below for more detail.) This group of 8 States, referred to as the “8 cross-section States,”

reflects a national cross-section of States that is indicative (though not strictly statistically

representative) of national contaminant occurrence. This compilation and analysis of data can be

found in the “Drinking Water Contaminant Occurrence Report.” 

Selection of appropriate State data sets and management of the data (handling, editing,

formatting, “cleaning,” etc.) was necessary for the simple non-parametric analyses conducted. 

The primary objective regarding the data used in these contaminant occurrence analyses was

development of a consistent and repeatable data management approach that would allow valid

comparisons between and among the various data sets, and allow the State data sets to be

evaluated in aggregate to provide an overview of occurrence patterns at the national level. 

URCIS data. The URCIS database includes information on 62 regulated contaminants,

from 40 U.S. States or Territories. These records represent data for contaminants monitored

from 1983 through April 1992 (including, 56 VOCs, 2 SOCs, and 4 trihalomethanes (THMs)). 

The majority of the data are from the first round of required unregulated contaminant monitoring

that began in 1987. Because of the age of the data, especially in relation to subsequent significant

improvements made in data processing systems, the quality of data received by EPA for URCIS is

highly variable. Approximately 3.5 million records, primarily of VOC occurrence, were of

adequate quality for analytical uses. This data from EPA’s URCIS database was analyzed to

provide a supplemental comparison (primarily for VOC data) to the occurrence findings for the

State data sets. 

National representation. Two broad factors were considered in development of a

nationally representative compilation of State data sets: geographic or spatial diversity, and

pollution potential. Consideration of States that together provide a geographic diversity was a

means by which to include contaminant occurrence data from the wide, national range of climatic

and hydrologic conditions across the United States. The representative group of State data sets
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 39

was also selected to represent the range of indicators of pollution potential for manufacturing and

agricultural sectors.

The pollution potential for each State was evaluated in a number of ways. For a number

of reasons it was decided, that the total manufacturing facilities per square mile (and EPA’s Toxic

Release Inventory (TRI) releases as a secondary indicator) and total dollars spent on agricultural

chemicals (excluding fertilizers) were the best indicators of potential contamination from volatile

and synthetic organic contaminants. (The reasoning for selection of these indicators is described

in the section, “More on Pollution Potential Indicators” below and is fully described in the

referenced 1999 “Drinking Water Contaminant Occurrence Report.”) The 50 States were ranked

based on these broad pollution potential indicators and divided into quartiles. The rankings were

reviewed to select States in approximate balance from each quartile. The 8 cross-section States

that were selected provided a broad distribution, geographically, and across the pollution potential

rankings. The broad geographic coverage was also intended to provide a national distribution

across the range of potential inorganic contaminant occurrence conditions.

The data from the cross-section States were used to compute aggregate occurrence

values, i.e., the percentage of water systems that had a detection of contaminant X, as an

approximation of contaminant occurrence at a national level. Analyses were conducted to

establish the number and percentage of PWSs that had a sample detection less than the minimum

reporting level (MRL), greater than one-half the maximum contaminant limit (1/2 the MCL) and

greater than the MCL. 

Using the pollution potential indicators and geographic distribution, the cross-section of

States were selected to represent both the central tendency and the range of national pollutant

occurrence. However, any extrapolation from eight States to a national estimate is inherently

uncertain. These occurrence estimates cannot be considered a truly statistically “representative”

sample of nation-wide occurrence for any given contaminant. As a preliminary evaluation of the

national “representativeness” of the 8 cross-section States, the aggregated occurrence data were

compared to three sources of data to provide perspective: (1) the URCIS data; (2) the atrazine

and simazine studies by Novartis (see references); and (3) data from the USGS. While the

URCIS data has data quality limitations it also has greater State representation (some data from

39 States and 1 Territory) than the cross-section States. Recognizing the differences between

these data sets, the comparison to URCIS data showed that, in the majority of cases (about 67%),

the cross-section had a slightly higher proportion of systems with detections of a contaminant and

for about one-third of contaminants, URCIS shows an equal or slightly greater percentage. In all

cases, the values are comparable; no values for systems with detections stand out as markedly or

unexpectedly different. In summary, the aggregated national cross section of contaminant

occurrence data compiled in the “Drinking Water Contaminant Occurrence Report” appears to

provide a slightly high, but reasonable approximation of national occurrence values.

To expand the coverage of the existing occurrence analysis based on the cross-section

States, additional State data sets will be obtained (as available) and included in the analyses. The

addition of representative State data sets will be evaluated as above, through a process that

includes: 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 40

• Ranking of States’ pollution potential; 

• Dividing States into quartiles based on the pollution potential rankings; and 

• Selecting States that are representative geographically and that equally represent the

different quartiles. 

By adding State data sets based on the same pollution potential indicators and geographic

criteria previously used to define the original 8 cross-section States, additional analyses of

contaminant occurrence under the first six-year review may be conducted in a manner that

maintains consistency with, and builds upon, the 1999 “Drinking Water Contaminant Occurrence

Report.”

Another analysis that could expand the understanding is comparison of source water

quality data to the occurrence analyses. The extent of this comparison, and other possible

analyses (such as co-occurrence, trend analyses, and relation of occurrence to various land use

and contaminant source parameters) will depend on, and may be limited by, the availability of

source water quality data.

More on Pollution Potential Indicators. Many past EPA and USGS studies have shown

that some simple measures such as population (or population density) are closely associated with

pollution. This is intuitively (as well as empirically) apparent, since it is human activity and its

related land use -- be it manufacturing or agricultural activity -- that is the source of most

pollutants, particularly the organic chemicals. 

The primary indicators used in the current analysis, as reported in the 1999 EPA “Drinking

Water Contaminant Occurrence Report,” ranks each State by the potential for pollution from

manufacturing and agriculture. In general, manufacturing/industrial activities typically associated

with population density are considered the major sources of many VOCs (degreasers, solvents,

petroleum compounds). Most SOCs are pesticides, and agriculture is the largest user of these

compounds. While inorganic chemicals (IOCs) can have various uses in manufacturing, they also

occur naturally. Ambient concentrations of IOCs also can be enhanced by mining or other diffuse

activities. Natural geologic sources of IOCs were not directly considered in the assessment for

representativeness, in part because whole States needed to be evaluated and such geologic

sources are often localized. However, by including geographic or spatial coverage across the

United States as a factor, from New Jersey to Montana for example, a range of geologic

conditions are inherently included in addition to a broad range of hydrogeologic and climatic

variation.

Numerous factors were considered as potential indicators of manufacturing-related

pollution, including EPA’s TRI (including total releases, releases per square mile, and releases

excluding air releases), the number of manufacturing establishments, the number of manufacturing

establishments per square mile, the number of manufacturing employees, the value added by

manufacturers, and the value added per capita (see Annual Survey of Manufacturers, 1995;

Census of Manufacturers, 1992; and Toxic Release Inventory, 1995). These factors were each
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 41

considered in terms of their inherent value as pollution potential indicators, their range and

variance (in providing a relative ranking of the States), and their inter-relationships. 

The total TRI releases per square mile, number of manufacturing establishments per

square mile, and value added per capita were considered the three most useful indicators for the

pollution potential associated with VOCs. The TRI was considered useful because it is a measure

of how many pounds of toxic chemicals are released within the State. While there are problems

with the TRI (e.g., some inconsistent release estimation techniques; omission of many small

establishments, or unreported releases below specified thresholds), it validly can be used as a

direct indicator of potential pollutants released. The number of manufacturing establishments per

square mile takes into account how many factories are actually engaged in manufacturing and thus

how many establishments potentially contribute to pollution. The final factor that was considered

to be viable was the value added to products by manufacturers on a per capita basis. Initially this

seemed to be a well-suited measure because of the presumed correlation between value added and

the level of production (and by-product pollution) within the State. The problem with this

measure (and also with the measure of number of manufacturing establishments per square mile),

is that it does not take into account the variation in pollution released by different industries. For

example, an industry that adds a lot of value to a product may cause little pollution while another

industry that does not add much value may contribute more pollution.

The data evaluated in the “Drinking Water Contaminant Occurrence Report” showed a

close correlation between the number of manufacturing establishments per square mile and the

population density in each State, as well as a clear linear association with the total TRI pounds

released/square mile, number of manufacturing employees, and total value added. Hence, the

number of manufacturing establishments per square mile was used as the primary indicator. The

other key reason for choosing this factor was that it is a simple measure of how many

establishments are actually engaged in manufacturing and thus are potentially polluting sources of

drinking water. The TRI total pounds released per square mile was used as a secondary factor in

determining representativeness. Squillace and others (1995) found a significant correlation

between VOC occurrence in ambient ground water and population density in a USGS national

NAWQA study. As noted, population density and manufacturing density are highly correlated. 

Manufacturing density and TRI data were used in this ranking because they were considered more

direct measures of pollution potential for this study.

There is no complete measure of pesticide usage by States that is readily available. So, a

variety of factors were considered to assess potential organic chemical pollution from agriculture

in each State as indicators of pollution potential from SOCs. These factors included the percent

of the State’s population that is classified as rural, the percent of land in the State that is crop

land, the percent of land that is grassland pasture and rangeland (a possible inverse indicator), and

total farm agricultural chemical expenses. Like the manufacturing factors, these agricultural

variables were considered in terms of their value in indicating potential sources of pollution and

were plotted against one another to determine how closely they were correlated.

Of these factors, total farm agricultural chemical expenses was considered to be the best

indicator of potential pollution. The percent of the State’s population that lives in rural areas does

not necessarily relate to agricultural chemical use or crop land. There is, of course, a correlation
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 42

between crop land and agricultural chemical use, but there are notable exceptions such as Florida

and California which use a large amount of agricultural chemicals despite having more limited

crop land area. While there are some incomplete surveys of pesticide use, the Census of

Agriculture (1992) measure of dollars spent on agricultural chemicals is a more consistent and

complete measure. 

In summary, three specific measures were selected as reasonable indicators of pollution

potential:

• The number of manufacturing facilities per square mile, to reflect the range of potential

VOC occurrence;

• Total expenditures on farm agricultural chemicals, to reflect the range of potential

SOC occurrence; and 

• TRI releases, in total pounds per square mile, to reflect the releases of any type of

chemical into the environment. 

These measures were used to assess the pollution potential characteristics of the States. In

addition, and as mentioned previously, in the development of a nationally representative group of

States the geographic distribution of States must reflect the range of hydrologic, geologic, and

climatic conditions.

B. Alternative Approaches to a National Occurrence Estimate

The current method for estimating national contaminant occurrence uses data from a

cross-section of eight of the States from which data were available. Some other State data sets

were available, but are excluded from the analysis, either because of poor data quality, or because

they are too similar to other States already in the cross-section. For example, the cross-section

excludes Ohio and Indiana, even though data is available from these States, because they are

similar in location and pollution potential to Illinois and Michigan, which are already in the

cross-section. This 0/1, or exclude/include approach has the advantage of simplicity, and it has

been approved by external peer reviewers of the report. However, it has the weakness of not

using potentially useful data.

An alternative to the exclude/include strategy is to assign a more general weight to each

State, determined by the number of nearby or similar States in the data set, the size of the State,

and/or the quality and size of the State’s data set. A recent example of such a strategy is EPA’s

draft analysis of arsenic occurrence in drinking water (US EPA, 2000). This report used a

geographical weighting scheme, in which State data sets were combined into regional occurrence

estimates, and regions were combined into a national estimate. The effect was to use all of the

available data, but to assign lower weight to States in regions with more data. Other weighting

schemes could use pollution potential indicators in a similar way, assigning lower weight to States

with pollution indicators that are similar to those of other States in the data set. Geographical and

pollution-based weights could also be combined into a single weighting scheme. 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 43

The advantage of these more general weighting schemes is that they use all of the available

data. If carefully designed, they can therefore be expected to reduce both the bias and variance of

national occurrence estimates. These schemes have important disadvantages, however. First,

they lose the simplicity of the 0/1 approach, and may be hard to explain or justify to the public. 

Second, a large amount of analysis may be required in order to make a reasonable and defensible

choice of weights. This is especially true because each contaminant will probably require a

separate set of State weights, because of differences in the quality of the data sets. Third, the

choice of weights requires assumptions that may be hard to check or justify. For example, the

arsenic analysis postulated certain geographic regions, in order to assign the distributions of

“nearby” States to States without data. 

Extrapolation from a sample of eight or more States to a national occurrence estimate is

inherently uncertain. Any method of extrapolation, including the 0/1 approach and the more

general weighting schemes described above, requires assumptions that may be challenged. At

least as important as choosing a method of extrapolation, will be for EPA to, first, estimate the

uncertainty due to the extrapolation, and second, obtain as much data as possible outside of the

State data sets, in order to validate the results of its analysis. 

References Cited

Novartis Crop Protection, Inc. Voluntary Atrazine Monitoring Program at Selected Community

Water Systems: Iowa 1996. Technical Report 6-97, Environmental and Public Affairs Department

(1997).

Squillace, P.J., D.A. Pope, and C.V. Price. Occurrence of the Gasoline Additive MTBE in

Shallow Ground Water in Urban and Agricultural Areas. Fact Sheet 114-95, U.S. Geological

Survey (1995).

US EPA (1999). A Review of Contaminant Occurrence in Public Water Systems, Office of

Ground Water and Drinking Water, Washington, DC. EPA 816-R-99-006 (November 1999). 

US EPA (2000). Arsenic Occurrence in Public Drinking Water Supplies (Draft), Office of

Ground Water and Drinking Water, Washington, DC (May 2000).
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 44

Appendix 5: Non-Regulatory Approaches

While the six-year review is a regulatory process, NDWAC believes it is appropriate to

point out the importance of incorporating non-regulatory efforts to work in concert with the

regulatory program in safeguarding drinking water. Ongoing non-regulatory programs are not a

substitute for drinking water regulations, but may help to reduce the introduction and occurrence

of contaminants to our nation’s waters, therefore, reducing the need for drinking water

regulations. Both new and existing programs can take advantage of education, voluntary actions,

incentives, and program integration to channel resources and efforts toward safe drinking water

goals. 

Education, guidance and research: Public education can increase the general level of

understanding about safe drinking water, and raise expectations for high-quality water. Special

education programs should be aimed at healthcare providers, who are heavily involved in drinking

water issues, have frequent contact with patients, and enjoy a high degree of trust. EPA can

publish guidance documents on topics such as source water protection and data handling to help

direct voluntary efforts at risk reduction. Operator certification and training programs can help

assure proper operation of water and wastewater treatment facilities. Drinking water managers

should have a convenient forum for sharing success stories of risk reduction. Drinking water

managers should also have ample opportunities for influencing the direction of relevant research. 

EPA will carefully consider information gaps related to health effects and occurrence, and

promote research to fill the gaps. Public education could be improved by taking advantage of

recent advances in knowledge-based software. For example, a program developed under a recent

American Water Works Association Research Foundation (AWWARF) grant helps to notify

sensitive subpopulations when they may be at risk from their drinking water.

Safe drinking water can also be enhanced through guidance that promotes integration of

source water assessments, hydrologic sensitivity analysis, Phase II/V waiver assessments, and

radionuclide vulnerability assessments.

Voluntary actions: At all geographic levels, voluntary actions can help protect safe

drinking water. For example, water systems can voluntarily adopt International Standards

Organization (ISO) standards to improve water quality. These programs include ISO 9000 which

sets goals of improving drinking water quality through treatment procedures, and ISO 14000

which improves the environmental health and safety policies and organizational structure of water

systems. Water systems can also join the EPA Safe Water Partnership, which sets turbidity goals

and self-assessment of operations to improve water quality. States can voluntarily offer

monitoring waivers to systems known to be low risks for certain types of contamination. 

Volunteers can play a key role in source water monitoring and assessment.

Incentives: Monetary and non-monetary incentives can encourage voluntary efforts to

protect water. One example comes from Europe, where water systems pay farmers to avoid

pesticides and use organic farming methods in source areas. Similar payments could be offered to

keep livestock away from streams. Another example is that where lower interest rates are offered

through the State Revolving Loans for small systems.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 45

Program integration: Some of the best opportunities for non-regulatory approaches

involve joining forces with existing programs aimed at source water protection and

watershed restoration. These programs, which can be established incrementally for cost-effective

management, include:

• State, Tribal, and local efforts developed as part of source water assessment and

protection plans. These are typically coordinated through State drinking water

agencies and EPA regional offices.

• Watershed Restoration Action Strategies (WRAS) developed by States, Tribes, local

governments, and watershed groups as part of the Clean Water Action Plan. These

are typically coordinated through State clean water agencies. Many (but not all) are

part of the EPA Clean Water Act Section 319 program.

• United States Department of Agriculture (USDA) programs such as Environmental

Quality Incentives Program (EQIP) and others, which encourage agricultural

producers to install best management practices (BMPs). These are typically

coordinated through the State Technical Advisory Committees, under the Natural

Resource Conservation Service (NRCS) State Conservationist.

• United States Department of Interior (USDOI) programs such as the Fish and Wildlife

Service’s Partners for Fish and Wildlife, the Office of Surface Mining’s Appalachian

Clean Streams Initiative, and the Bureau of Land Management’s Abandoned

Minelands Initiative. 

More information on these programs, and others, can be found on the following web sites: Clean

Water Action Plan: www.cleanwater.gov and Surf Your Watershed: www.epa.gov/surf.
15 IRIS contains chemical specific health effects information. Information on synergistic effects of chemical

mixtures is scarce and is seldom available for inclusion in IRIS. 

Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 46

Appendix 6: IRIS Assessments

The Integrated Risk Information System (IRIS) is an EPA database containing Agency

consensus scientific positions on potential adverse human health effects that may result from

chronic exposure to chemical substances found in the environment.15 Assessments by IRIS

undergo internal and external peer reviews by health scientists.

The main reasons for including a chemical in the IRIS program are (1) Agency statutory,

regulatory, or program implementation needs, and (2) availability of new scientific information or

methodology that might significantly change current IRIS assessment.

The Office of Water (OW), as well as other EPA offices, periodically nominates priority

chemical substances requiring new or updated assessments in IRIS. Several chemicals nominated

by OW are on the IRIS agenda. For example, in order to meet the statutory requirements of the

1996 SDWA, a number of disinfection by-products have been included in IRIS. New information

has become available for several regulated chemicals (e.g. cadmium, xylenes, tetra- and

trichloroethylenes, di(2-ethylhexyl)phthalate, etc.) , thus, these chemicals have been selected for

updated IRIS reviews. 

IRIS assessments are based solely on scientifically valid studies. Evaluations of original

toxicological and epidemiological studies conducted by the National Toxicology Program,

National Cancer Institute, National Institute of Environmental Health Sciences, EPA’s National

Center for Environmental Assessment, industry, universities, etc., are all used in risk assessment. 

These studies are individually evaluated for their soundness, methodological strength and

weaknesses, and whether or not they have been conducted according to current quality standards.

IRIS reviews are not based on secondary sources such as reviews conducted by other

national or international organizations (e.g. State of California, World Health Organization or the

International Agency for Research on Cancer), although such assessments are often examined as

part of the IRIS review.

A full list of chemicals assessed in IRIS and those for which assessments are planned can

be found on IRIS web site (http://www.epa.gov/iris). A large number of these IRIS assessments

are of direct relevance to the regulatory function of OW and more specifically to the six-year

review.
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NDWAC Recommendations 47

Appendix 7: Overview of the Office of Pesticide Programs Process for Toxicity

Assessments

Under the requirements of Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), a

registrant (manufacturer) is required to submit animal toxicity data on the potential human health

effects that may be posed by pesticide chemicals. Toxicity data are provided during the initial

registration of a pesticide as well as during the periodic re-registration review of the pesticide as

required by FIFRA. The schedule, priority, for when an existing pesticide enters a re-review is set

in part by regulatory requirements which include provisions to give priority to certain active

ingredients. The Office of Pesticides (OPP) will establish the review schedule taking into account

the procedures outlined in the Act. A more complete discussion of the re-registration process can

be found in Section 4(a)-(f) of FIFRA.

In 1998, OW’s OGWDW and the OPP established major areas of coordination on

cross-cutting scientific issues. Included in the major efforts was the harmonization of the human

health hazard assessments and dose response relationships for pesticides. The two offices have

agreed to share health effects data and coordinate activities on the issues such as end-point

selection, dose response information, and quantifying risks. Therefore, the OW and OPP are

working closely on establishing consistency in health effects end-points through resource and

information sharing.

The OPP receives health effects data that are generated under specific scientific guidelines

established by the Agency and conducted under the requirements of Good Laboratory Practices. 

These guidelines are available on the EPA’s Internet site at the following location: 

http://www.epa.gov/opptsfrs/OPPTS_Harmonized/870_Health_Effects_Test_Guidelines/indexx.h

tml. 

In addition to the required guideline studies, the OPP will obtain and review open

literature data on adverse effects to test species. Although these studies are not used in

establishing health end-points (RfDs and cancer potency or threshold values), they are used in

establishing the “weight of evidence” for an adverse effect. Data source include, but are not

limited to, published, peer reviewed journal articles in the open literature and toxicity data

submitted to other U.S. federal or international agencies that do not conform to the OPP’s test

guidelines.

Below is a brief overview of end-point selection. 

Toxicity Assessment

Non-Cancer Effects: 

Reference Dose. For non-cancer effects, toxicity is represented by a reference dose; it

may be calculated for acute effects (aRfD) and chronic effects (cRfD). RfDs are calculated by

determining the No-Observed-Adverse-Effect Level (NOAEL) from either acute or chronic

toxicity studies (the choice of study depends on which type of RfD is being calculated - aRfD or

cRfD) and dividing it by the appropriate uncertainty factors. Typically, an uncertainty factor is
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 48

applied to account for variation within the human population (i.e., intraspecies); and an additional

uncertainty factor is applied to account for the differences between humans and animals as the

animal data are translated to humans (interspecies). 

If the RfD will be used in dietary risk assessment, then it is adjusted to take into account

the FQPA Safety Factor for infants and children. Such an adjusted RfD is called a Population

Adjusted Dose (PAD). Like the RfD, it may be acute (aPAD) or chronic (cPAD). In making the

decision regarding the FQPA Safety Factor, the Agency takes into account both information on

the toxicity of the pesticide and the completeness of the toxicity and exposure databases. For

more information on how the Agency applies the FQPA Factor, see the document “Standard

Operating Procedures for use of FQPA Safety Factor,” April 26, 1999 at

http://www.epa.gov/pesticides/trac/science/.

Cancer Effects: 

Linear Effect - Cancer Potency Factor (q1*). The cancer potency factor, which is

commonly known as a q1*, is the relative strength of a carcinogen. The bigger the q1*, the more

potent the carcinogen. It is calculated using a computer model that assumes linearity at doses

below which the effect occurred in the studies.

Non-Linear Effect - Margin of Exposure. For some carcinogenic pesticides, it is not

considered appropriate to calculate a potency factor. In these cases, the cancer effect is assumed

to have a threshold, as for non-cancer effects, and as such, a Margin of Exposure (MOE) is

derived. The MOE is a ratio, calculated by dividing the toxicity Point of Departure (such as a

NOAEL) by the estimated or calculated exposure level. We have not yet established a policy on

the level of risk that is of no concern for non-linear cancer risk assessment.

During the review of the toxicity data and the dose-response assessment, the pesticide

being evaluated undergoes review by several in-house peer review committees. 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 49

Appendix 8: Analytical Methods

A. What Section of SDWA Requires the Agency to Specify Analytical Methods?

Section 1401 of SDWA directs EPA to promulgate NPDWRs which specify either MCLs

or TTs for drinking water contaminants (42 USC 300g-1). SDWA requires EPA to set an MCL

“if, in the judgement of the Administrator, it is economically and technologically feasible to

ascertain the level of a contaminant in water in public water systems” [SDWA section

1401(1)(C)(i)]. Alternatively, if it is not economically or technologically feasible to so determine

the level of a contaminant, the Administrator may identify known TTs, which sufficiently reduce

the contaminant in drinking water, in lieu of an MCL [SDWA section 1401(1)(C)(ii)]. In

addition, SDWA requires an NPDWR to include “criteria and procedures to assure a supply of

drinking water which dependably complies with such maximum contaminant levels; including

accepted methods for quality control and testing procedures to insure compliance with such

levels” [SDWA section 1401(1)(D)]. 

B. What is the Typical Process for Approving Methods for SDWA Analytes?

Methods are initially approved as a part of an MCL or monitoring requirement

rulemaking. Thereafter, as revisions to the approved methods are published or as new

technologies are developed, the Agency, from time-to-time, will group a set of methods for

proposal in a methods update rule. There is no set schedule for method updates, the drivers being

the relative urgency for a revised or new compliance method, and Agency resources that can be

diverted from other SDWA regulatory mandates and activities. The time from start to finish of a

methods update rule is generally 18-24 months. This can increase significantly if there is adverse

public comment on a proposed method. 

The revised or new methods included in a methods update rule may be from EPA, other

Federal or State agencies, or standards organizations (e.g. American Society for Testing and

Materials (ASTM) or Standard Methods (SM)). These non-EPA entities have independent

review and/or collaborative testing requirements. In addition, methods may also be developed by

private laboratories, vendors or groups. Independent review and collaborative testing of these

privately developed methods is accomplished by requiring submission of the method to the

Agency under the alternate test procedure (ATP) program. Privately developed methods must

pass the ATP process before they can be included in an EPA methods update rulemaking. 

Initially, many ATP applications are missing data. Once a completed ATP application is recorded

by the Agency, the ATP pass/fail decision generally takes three to four months. For successful

ATPs, this period is followed by the formal rulemaking process, which was described above as

taking 18-24 months.

C. What Factors Does the Agency Consider in Approving Analytical Methods?

In deciding whether an analytical method is economically and technologically feasible to

determine the level of a contaminant in drinking water, the Agency considers the following

factors.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 50

• Is the method sensitive enough to address the level of concern (i.e., the MCL)?

• Does the method give reliable analytical results at the MCL? What is the precision (or

reproducibility) and the bias (accuracy or recovery)?

• Is the method specific? Does the method identify the contaminant of concern in the

presence of potential interferences?

• Is the availability of certified laboratories, equipment and trained personnel sufficient

to conduct compliance monitoring?

• Is the method rapid enough to permit routine use in compliance monitoring?

• What is the cost of the analysis to Water Supply systems?

Regarding the first criteria (i.e., sensitivity), the MDL and the PQL are two performance

measures used by EPA to estimate the limits of performance of analytic chemistry methods for

measuring contaminants in drinking water. For SDWA analytes, EPA defines the MDL as “the

minimum concentration of a substance that can be measured and reported with 99% confidence

that the analyte concentration is greater than zero”(40 CFR Part 136 Appendix B). MDLs can be

operator, method, laboratory, and matrix specific. MDLs are not necessarily reproducible within

a laboratory or between laboratories on a daily basis due to the day-to-day analytical variability

that can occur and the difficulty of measuring an analyte at very low concentrations. In an effort

to integrate this analytical chemistry data into regulation development, the Agency uses the PQL

to estimate or evaluate the minimum, reliable quantitation level that most laboratories can be

expected to meet during day-to-day operations. EPA’s Drinking Water program defines the PQL

as “the lowest concentration of an analyte that can be reliably measured within specified limits of

precision and accuracy during routine laboratory operating conditions (50 FR 46906, November

13, 1985).” For several SDWA analytes, EPA set the MCL at the PQL. 

D. How Are PQLs Typically Determined for SDWA Contaminants?

Historically, EPA’s OGWDW uses two main approaches to determine a PQL for SDWA

analytes. The preferred approach, the WS method, uses data from WS studies to calculate the

lower limit of quantitation. The WS method is used in most cases when sufficient WS data are

available to calculate a PQL. In the absence of WS data, the second approach that EPA uses is

the multiplier method. In this approach, the PQL is calculated by multiplying the EPA-derived

MDL by a factor of 5 or 10. The 5 or 10 multiplier is used to account for the variability and

uncertainty that can occur at the MDL. 

1. How Were Water Supply Studies Conducted?

Water supply laboratory PE studies have been an integral part of EPA’s certification

program for drinking water laboratories for over 20 years. Historically, EPA’s National Exposure

Research Laboratory (NERL) in Cincinnati, Ohio conducted WS studies for all current and

proposed drinking water contaminants. Although EPA conducted the WS studies semi-annually,

for certification purposes, laboratories were only required to demonstrate acceptable performance

once a year. 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 51

Each WS study included WS samples (or sample concentrates) that were analyzed for

both SDWA analytes and analytes being considered for regulation under the SDWA. During

these WS studies, EPA-NERL sent participating laboratories a set of the stable WS sample

concentrates in sealed glass ampules, a data reporting form, and appropriate instructions. 

EPA-NERL sent WS samples to all laboratories that conducted drinking water analyses, including

utility laboratories, commercial laboratories, and State and EPA Regional laboratories. With

appropriate dilution, the laboratory then analyzed the WS samples using the specified procedures. 

Afterwards, the laboratory sent the completed reporting form to EPA for evaluation. After

evaluation, EPA returned a fully detailed report to each participating laboratory. 

At this point in time, WS PE studies are no longer performed by EPA. Due to resource

limitations, on July 18, 1996 (61 FR 37464; EPA 1996b), EPA proposed options for the

externalization of the PE studies program (now referred to as the Proficiency Testing or PT

program). After evaluating public comment, in the June 12, 1997 final notice (62 FR 32112; EPA

1997b) the Agency:

decided on a program where EPA would issue standards for the operation of the

program, the National Institute of Standards and Technology (NIST) would develop

standards for private sector PE (PT) suppliers and would evaluate and accredit PE

suppliers, and the private sector would develop and manufacture PE (PT) materials and

conduct PE (PT) studies. In addition, as part of the program, the PE (PT) providers

would report the results of the studies to the study participants and to those organizations

that have responsibility for administering programs supported by the studies.

Since the last WS studies performed by EPA were done in the Fall of 1999, the

externalization should not effect the data needed for the first six-year review process

(1996-2002). 

2. PQL Determinations - How Are WS Studies Evaluated and What Criteria Are Used?

a. Evaluation of WS Studies

For each analyte in the WS study, EPA evaluates the results using Kafadar’s biweight

estimates of the mean and standard deviation from all the study data, and separately, for the data

reported by EPA and State laboratories. Where acceptance limits are not specified by regulations,

and where there are data from at least 13 EPA and State laboratories, the biweight estimates from

the EPA and State laboratory data are used to calculate a 95 percent prediction interval. If there

are no regulatory limits, but fewer than 13 EPA and State laboratories reported data, then the

prediction interval is calculated from the biweight estimates made from all the study data reported

for the analyte. 

For each analyte, 95 percent of the study data from laboratories operating in a State of

statistical control, i.e., “in control,” should theoretically be within the 95 percent prediction

interval. Since 1986, such prediction intervals, or the limits set in regulations, have been used as

the acceptance limits to judge laboratory performance in WS studies.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 52

% recovery '

measured concentration

spiked concentration

× 100

The recovery of an analyte is defined as the estimated biweight mean divided by the true

concentration of the analyte in the study, and can be calculated as follows:

Using the recovery of an analyte instead of the mean concentration facilitates comparisons across

WS studies performed at different true concentrations.

The statistical derivation of the PQL involves determining the concentration of an analyte

at which a set percentage of the laboratories achieve results within a specified range of the spiked

value. Historically, the percentage of laboratories was set at 75 percent, while a range of

acceptance limits around the spiked value were used. In many cases, EPA derived PQLs only

from the data submitted by the EPA Regional and State laboratories that participate in the WS

studies. 

A PQL derived from WS data in such a manner is considered a stringent target for routine

laboratory performance because:

• WS samples are prepared in reagent water and therefore do not contain the matrix

interferences that may occur in field samples.

• Laboratories analyze only a small number of samples for the study and are aware that

the samples are for the purposes of PE (i.e., they are not “blind” samples).

In deriving a PQL from WS study data, the Agency sets a fixed percentage acceptance

window around the spiked value of the WS samples and plots the percentage of laboratories

achieving results within that window (y-axis) against the spiked concentration of the WS study

samples (x-axis). While the acceptance limits for inorganics typically range from 15 to 30

percent, the acceptance limits for organics generally range from 40 to 50 percent. Several SDWA

analytes have acceptance limits of 2 sigma. The data are subjected to a linear regression analysis

to determine the concentration at which 75 percent of EPA Regional and State laboratories

achieve acceptable results. 

E. What Approaches Will EPA Use to Re-evaluate the PQLs of Contaminants

Identified from the Six-Year Review Process?

For the six-year review process, several approaches could be used for the re-assessment or

re-evaluation of the PQLs for selected chemical contaminants. To be consistent with the process

that the Agency has used in the past, only the “WS data method” and the “MDL 5 or 10

Multiplier method” will be considered for this six-year review process. Of these two approaches,

the Agency prefers to use the WS data approach. The advantages and disadvantages for each of

these PQL derivation methods are listed below. 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 53

1. Analysis of WS Data - Using the data from more recent WS studies, a new PQL will be

derived and compared to the old PQL (i.e. the one that is currently in place). 

The advantages of the WS Data method of deriving a PQL:

• Uses inter-laboratory data collected at concentrations near the MCL. 

• More representative of what methods are being used for the analysis of that

contaminant.

• May be the preferred approach for contaminants with MCLGs of zero.

The disadvantages of the WS Data method of deriving a PQL:

• The PQL derived for each contaminant is affected by the Agency’s choice of an

acceptable level of precision. Because the acceptance limits for many of the currently

regulated SDWA contaminants are already set, it will not be necessary to derive new

acceptance limits for this or future six-year reviews. 

• The PQL may be influenced by the WS data used, i.e., all data or only data from EPA

State and Regional laboratories.

• Some feel that laboratory performance on WS data may be skewed, because WS

samples may be treated as special samples that are critical for laboratory certification.

• The derivation of PQLs from WS data is a resource- and time-intensive process.

• Because the WS samples are designed to test precision and accuracy around the MCL,

the WS data may not cover concentrations several orders of magnitude below the

current MCL. Hence, for some analytes, data points at lower levels may not be

represented.

2. The MDL-Multiplier Approach - Using the MDL of the currently approved method(s) for

each contaminant, a 5 or 10 multiplier method will be used to estimate the PQL. This

value would then be compared to the PQL that was derived before the 1996 SDWA

amendments. 

The advantages of the PQL-by-MDL Multiplier approach:

• It is a relatively easy and clear process.

The disadvantages of the PQL-by-MDL Multiplier approach:

• The WS studies test laboratory performance near the MCL not near the MDL. A PQL

derived from the MDL method may not be representative, because the reproducibility

of a result obtained at the MDL is often not as good as that obtained near the MCL.

• Because several methods may be approved for the same contaminant, it can be difficult

to decide which MDL to select for the PQL calculation.
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 54

F. Other Methods of Deriving a PQL

The discharge permit and pretreatment program for wastewater uses the Minimum Level

(ML) MDL-Multiplier method to derive a PQL. The ML is calculated by multiplying a specified

MDL for a contaminant by 3.18 and rounding to an integral number. Because the 3.18 ML

method has not been peer reviewed for drinking water, it will not be used by OGWDW to derive

PQLs for six-year review process. The EPA OGWDW will only use the wastewater ML method

for comparison purposes (to see how this method of deriving a PQL compares to OGWDW’s WS

method of deriving PQLs). 

G. Historical Determination of Radionuclides Analytical Feasibility

Since 1976, EPA has conducted a Radiochemistry PE Studies Program at the Las Vegas,

Nevada EPA Laboratory (EPA-LV). The EPA-LV PE program was used to set standards for

radiochemical methods, generate reports that compared the data from all radiochemical

laboratories, and to define data quality using the data from these PE studies. All radiochemistry

laboratories (both US and foreign) participated in these PE studies. This amount of participation

by a wide range of laboratories permitted method and laboratory assessment on an unprecedented

scale and was to be the backbone of performance-based radiochemistry methods. Any sample

type of interest to the Agency (water, milk, food, soil, air, urine) was measured by all laboratories

wishing to demonstrate proficiency with a given regulated nuclide or group of nuclides in a matrix

of interest. The number of laboratories per study ranged from 50 to over 200 depending on the

parameter of the PE study. The results from the studies were reduced to statistical parameters

and scored relative to performance data quality objectives and reported relative to all the

laboratories performing the same measurement. Performance accuracy was reported as the ratio

of the relative difference of the reported mean to the known concentration (determined

gravimetrically and verified analytically) and the normal deviate. The normal deviate was a

weighted parameter taking into account random uncertainty and reasonable introduced errors. A

ratio between 0 and 2 (95 percent confidence interval), between 2 and 3 in the warning range, and

greater that 3, out of control and failure. The criteria were objective, empirical, not based on the

population distribution (all, some, or none of the labs could pass), and defined measurement data

quality on a sustained basis. The PE program’s scope normalized performance expectations and

challenged laboratories to the highest standards for accuracy because the performance standards

were independent of any study group. PE studies were conducted 17 to 20 times per year and the

data generated under these controlled conditions were used to demonstrate measurement data

quality and method feasibility in support of regulatory MCLs and action levels. 

With regards to radiochemical data quality and measurement feasibility, radiochemical

measurements were (and still are) typically characterized by: 

(1) system background,

(2) the counting time of the sample and background, 

(3) the sample volume, 

(4) counter efficiency, 
Guidance for Review of NPDWRs; November 2000

NDWAC Recommendations 55

(5) similarity of the standard to the target nuclides, 

(6) chemical yield, if separation chemistry was employed, 

(7) replicate precision (repeatability) within laboratories, and 

(8) between laboratory accuracy (reproducibility) relative to a known concentration (this

factor includes any systematic error(s) introduced by the laboratory).

Repeatability, both individually and collectively, demonstrated that within a laboratory, the

dominant source of uncertainty was that due to the randomness of radioactive decay, the counting

uncertainty. Because the uncertainty is characterized by the Poisson Distribution, repeatability

could be compared to the expected, computed uncertainty when randomness dominated, (3 to 5

times background). Reproducibility was the index of data quality used to determine the limits of

data quality and the feasibility of resolving differences between nuclide concentrations. With the

externalization of the PE program, data quality monitoring and control is no longer possible. 

Future performance of methods and data quality can only be estimated from historical

performance.

As opposed to chemical analytes, the determination of measurement feasibility for

radionuclides do not rely on a PQL convention. Because of the aforementioned factors that

contribute to measurement feasibility for radionuclides, EPA-LV determined that a different,

statistically valid and scientifically defensible approach was necessary for determining

measurement reliability for radiochemicals. In determining measurement reliability for

radionuclides, the grand average of the means and reproducibility (total error) of the study

population are computed. The distribution of laboratory averages reflected both random

uncertainty and bias introduced by the laboratory and represented an empirical yard stick of the

reliability or feasibility of laboratories to measure a nuclide, all factors considered. This

distribution around the known value was the reproducibility (data quality) of the population for

the nuclide in water, the focus here. The goal of determining the reproducibility for the

population of laboratories was to provide an upper limit on accuracy for any result provided by a

certified laboratory. The reproducibility of a population of laboratories (i.e., the measurement

feasibility) is not obtainable except by multiple laboratory testing.

Reference:

Kafadar, Karen. “A biweight approach to the one-sample problem.” Journal of the American

Statistical Association. 77 416-424. 1982.