United States 

 Environmental Protection 

 Agency 

EPA Protocol for the 

Second Review of Existing National Primary 

Drinking Water Regulations (Updated)
Office of Water (4607M) 

EPA 815-B-09-002 

October 2009 

www.epa.gov/safewater 
EPA-OGWDW EPA Protocol for the Second Review of Existing EPA 815-B-09-002 

National Primary Drinking Water Regulations October 2009 

Table of Contents 

Executive Summary...................................................................................................................ES-1 

1 Introduction.......................................................................................................................... 1-1 

1.1 Basic Principles............................................................................................................ 1-1 

1.2 Scope of Review .......................................................................................................... 1-2 

1.3 Organization and Contents of this Document.............................................................. 1-5 

2 Overview of Six-Year Review Protocol ..............................................................................2-1 

2.1 Protocol Refinements for Six-Year Review 2 .............................................................2-1 

2.2 Elements of the Six-Year Review 2 Decision Tree ..................................................... 2-2 

3 Detailed Discussion of Decision Tree Implementing the Protocol...................................... 3-1 

3.1 Initial Review Branch ..................................................................................................3-1 

3.1.1 Inputs to the Initial Review..................................................................................3-1 

3.1.2 Output of Initial Review ......................................................................................3-3 

3.2 Health Effects and MCLG Branch...............................................................................3-3 

3.2.1 Inputs to Health Effects and MCLG Review.......................................................3-4 

3.2.2 Outputs from Health Effects and MCLG Review................................................ 3-6 

3.3 Maximum Contaminant Level (MCL) Branches......................................................... 3-6 

3.3.1 Inputs to Maximum Contaminant Level (MCL) Review .................................... 3-8 

3.3.2 Outputs from Maximum Contaminant Level (MCL) Review ............................. 3-9 

3.4 Treatment Technique Branch....................................................................................... 3-9 

3.4.1 Inputs to Treatment Technique Review............................................................. 3-10 

3.4.2 Outputs from Treatment Technique Review...................................................... 3-10 

3.5 Treatment Technique Analysis Branch...................................................................... 3-10 

3.5.1 Inputs to Treatment Technique Analysis Review..............................................3-10 

3.5.2 Outputs from Treatment Technique Analysis Review....................................... 3-11 

3.6 Methods Branch......................................................................................................... 3-11

3.6.1 Inputs to Methods Review .................................................................................3-13 

3.6.2 Output from Methods Review ........................................................................... 3-14 

3.7 Occurrence Branch .................................................................................................... 3-14 

3.7.1 Inputs to Occurrence Review.............................................................................3-15 

3.7.2 Output from Occurrence Review....................................................................... 3-17 

3.8 Treatment Branch....................................................................................................... 3-17

3.8.1 Inputs to Treatment Technology Review........................................................... 3-18 

3.8.2 Output of Treatment Technology Review ......................................................... 3-19 

3.9 Implementation Branch.............................................................................................. 3-19 

3.9.1 Inputs to Implementation Review......................................................................3-19 

3.9.2 Outputs from Implementation Review............................................................... 3-20 

4 References............................................................................................................................ 4-1 

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National Primary Drinking Water Regulations October 2009 

Table of Exhibits 

Exhibit 1-1. NPDWR Contaminants Included in Six-Year Review 2......................................... 1-3 

Exhibit 1-2. NPDWR Contaminants with Recent or Ongoing Actions....................................... 1-5 

Exhibit 2-1. Overview of Six-Year Review Decision Tree .........................................................2-3 

Exhibit 3-1. Initial Review Branch.............................................................................................. 3-2 

Exhibit 3-2. Health Effects and MCLG Branch ..........................................................................3-4 

Exhibit 3-3. Maximum Contaminant Level Branch 1 (Potential for MCLG Revision) ..............3-7 

Exhibit 3-4. Maximum Contaminant Level Branch 2 (No Potential for MCLG Revision) ........3-8 

Exhibit 3-5. Treatment Technique Branch .................................................................................. 3-9 

Exhibit 3-6. Treatment Technique Analysis Branch ................................................................. 3-11 

Exhibit 3-7. Analytical Methods Branch ...................................................................................3-12 

Exhibit 3-8. Occurrence Branch ................................................................................................3-15 

Exhibit 3-9. Treatment Branch ..................................................................................................3-18 

Exhibit 3-10. Implementation Branch .......................................................................................3-19 

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Abbreviations and Acronyms 

2,4-D 2,4-dichlorophenoxyacetic acid 

ATSDR Agency for Toxic Substances and Disease Registry 

BAT best available technology 

CalEPA California Environmental Protection Agency 

CCRIS Chemical Carcinogenesis Research Information System 

CICADS European Commission Concise International Chemical Assessment 

Documents 

DART Developmental and Reproductive Toxicology 

DBCP 1,2-dibromo-3-chloropropane 

DBPR Disinfectants and Disinfection Byproducts Rule 

DEHA di(2-ethylhexyl)adipate 

DEHP di(2-ethylhexyl)phthalate 

EDB ethylene dibromide 

EPA U.S. Environmental Protection Agency 

EQL estimated quantitation level 

FAO Food and Agriculture Organization of the United Nations 

GWR Ground Water Rule 

HAA5 haloacetic acids 

HSDB Hazardous Substances Data Bank 

IARC International Agency for Research on Cancer 

ICR Information Collection Request 

IPCS/EHC International Programme on Chemical Safety/Environmental Health 

Criteria 

IRIS Integrated Risk Information System 

JECFA Health Canada, Joint Expert Committee on Food Additives 

JMPR Joint FAO/WHO Meeting on Pesticide Residues 

LCR Lead and Copper Rule 

LT2 Long-Term 2 Enhanced Surface Water Treatment Rule 

MCL maximum contaminant level 

MCLG maximum contaminant level goal 

MDL method detection limit 

mg/L milligrams per liter 

MRL minimum reporting level 

N nitrogen 

NAS National Academy of Sciences 

NAWQA National Water Quality Assessment 

NCOD National Drinking Water Contaminant Occurrence Database 

NDWAC National Drinking Water Advisory Council 

NELAC National Environmental Laboratory Accreditation Conference 

NIEHS National Institute of Environmental Health Sciences 

NPDWR National Primary Drinking Water Regulation 

NTP National Toxicology Program 

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OGWDW Office of Ground Water and Drinking Water 

OPP Office of Pesticide Programs 

OW Office of Water 

OST Office of Science and Technology 

PCBs polychlorinated biphenyls 

PQL practical quantitation level 

PE performance evaluation 

PT proficiency testing 

PWS public water system 

R2S2 Regulatory Review Support Spreadsheet 

RfD reference dose 

SDWA Safe Drinking Water Act 

SSCT small system compliance technology 

TT treatment technique 

TTHM total trihalomethanes 

WHO World Health Organization 

WS water supply 

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Executive Summary 

The 1996 Safe Drinking Water Act (SDWA) Amendments require the U.S. Environmental 

Protection Agency (EPA or the Agency) to review and revise National Primary Drinking Water 

Regulations (NPDWRs) at least once every six years as appropriate to maintain or improve 

human health protection. EPA completed and published the results of its first Six-Year Review 

(Six-Year Review 1) July 18, 2003 (68 FR 42908) after developing a systematic approach, or 

protocol, for the review of NPDWRs. As described in this document, EPA has applied the same 

protocol with minor refinements (revised protocol) to its second Six-Year Review of NPDWRs 

(Six-Year Review 2). 

In Six-Year Review 2, EPA addressed the following: 

• Maximum Contaminant Level Goals (MCLGs; the health goal) – for some contaminants 

new health effects assessments completed since the MCLG was promulgated or last revised 

include revised reference doses (RfD) and/or cancer classification. 

• Maximum Contaminant Levels (MCLs; the enforceable standard) – for some contaminants, 

the MCL is equal to the MCLG, and the health effects assessment indicates potential to 

revise the MCLG. Improvements in analytical or treatment feasibility may also indicate 

feasibility to set the MCL closer to the MCLG. 

• Treatment Technique (TT; sometimes established in lieu of an MCL) – new information on 

health effects, analytical feasibility, or treatment feasibility may suggests a possibility to 

revise TT. 

• Other Treatment Technology (NPDWRs contain Best Available Technologies, or BATs, 

capable of achieving MCLs) – Changes to BAT recommendations may be appropriate for 

revised MCLs. 

• Other Regulatory Requirements (Monitoring) –Other regulatory revisions may be 

appropriate if information suggest that changes in monitoring standards (e.g., frequency) 

could reduce health risks or costs while maintaining or improving the level of public health 

protection. 

This comprehensive review comprised 85 NPDWRs. To facilitate the process, EPA developed a 

Six-Year Review 2 decision tree that structures a series of questions about whether there is new 

information suggesting that it is possible to revise one or more NPDWR elements in a logical 

order. The order of the questions within the tree reflects the sequential relationships between the 

different NPDWR elements and thus avoids unnecessary analyses. 

For example, EPA must generally set the MCL as close to the MCLG as feasible. Consequently, 

if the MCL is equal to the MCLG, EPA must make decisions regarding the availability and 

adequacy of new information relevant to the potential to revise the MCLG before decisions 

regarding the potential to revise the MCL. Also, if there is no potential to revise the MCLG and 

the MCL is already equal to the MCLG, then there is no basis for revising the MCL. In this 

instance, the “branch” of the decision tree containing questions about revising the MCL is not 

reached, and it is not necessary to review information related to analytical feasibility. 

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The first branch of the decision tree is an Initial Review Branch, with the purpose of identifying 

contaminants for which further review of detailed technical data is premature (the contaminant is 

the subject of recent or ongoing rulemaking, or there is an ongoing health effects assessment). 

Excluding such contaminants from subsequent review prevents duplicative Agency efforts. For 

contaminants for which there is an ongoing health effects assessment and the MCL is above the 

MCLG, the review proceeds to branches that evaluate whether there is potential to lower the 

MCL. The Agency’s review of new information that may affect the MCL for these contaminants 

is one of several refinements of the protocol. During Six-Year Review 1, EPA took no further 

action on any contaminants with ongoing health effects assessments. This refinement addresses 

the SDWA requirement that EPA set each MCL as close to the MCLG as feasible. 

The additional branches of the decision tree are: 

• Health effects and MCLG 

• MCL 

• Treatment technique 

• Analytical methods 

• Occurrence and exposure 

• Treatment feasibility. 

A series of technical support documents (U.S. EPA, 2009a-g) provide data and analysis to 

support EPA’s decisions in each branch. EPA also developed an automated tool called the 

Regulatory Review Support Spreadsheet (R2S2) to track the review results. This tool enhances 

transparency, automates the decision process, and facilitates the Agency’s reporting of its 

recommendations. The Agency will continue to refine the Six-Year Review protocol during 

subsequent reviews to address changing circumstances. 

As for Six-Year Review 1, Six-Year Review 2 results consist of recommendations to revise some 

NPDWRs, and to take no action at this time for the remaining NPDWRs. A recommendation to 

revise an NPDWR starts a regulatory process that involves more detailed analyses concerning 

health effects, costs, benefits, occurrence, and other matters relevant to deciding whether and 

how an NPDWR should be revised. At any point in this process, EPA may find that regulatory 

revisions are not appropriate and may discontinue regulatory revision efforts. Review of that 

NPDWR would, however, continue in future Six-Year Reviews. 

Similarly, a recommendation to “take no action at this time” means only that EPA does not 

believe that regulatory changes to a particular NPDWR are appropriate based on health effects, 

analytical methods, treatment data, ongoing scientific reviews, priority, or other reasons. 

Reviews of these contaminants in future Six-Year Reviews may lead to a recommendation that 

regulatory changes are appropriate. 

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1 Introduction 

The 1996 Safe Drinking Water Act (SDWA) Amendments require the U.S. Environmental 

Protection Agency (EPA or the Agency) to periodically review existing National Primary 

Drinking Water Regulations (NPDWRs). Section 1412(b)(9) of SDWA reads: 

 ...[t]he Administrator shall, not less than every 6 years, review and revise, as 

appropriate, each 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. 

Pursuant to the 1996 SDWA Amendments, EPA completed and published the results of its first 

Six-Year Review (Six-Year Review 1) July 18, 2003 (68 FR 42908) after developing a 

systematic approach, or protocol, for the review of NPDWRs. EPA has applied the same 

protocol with minor refinements (revised protocol) to the second Six-Year Review of NPDWRs 

(Six-Year Review 2). Section 2 provides an overview of the protocol and Section 3 describes the 

protocol and the minor refinements used for the Six-Year Review 2. The Agency will continue to 

refine the protocol during subsequent six-year reviews to address changing circumstances. 

1.1 Basic Principles 

The primary goal of the Six-Year Review process is to identify NPDWRs for possible regulatory 

revision. Although the statute does not define when a revision is “appropriate,” as a general 

benchmark, EPA considered a possible revision to be “appropriate” if, at a minimum, it presents 

a meaningful opportunity to: 

• improve the level of public health protection, and/or 

• achieve cost savings while maintaining or improving the level of public health protection. 

Toward this end, EPA applied a number of basic principles in reviewing NPDWRs. First, the 

Agency sought to avoid redundant review efforts. Therefore, EPA classified NPDWRs that were 

the subject of other rulemaking actions either ongoing or completed during this review period as 

having “ongoing actions” or “recent actions” and not subject to further technical review under 

Six-Year Review 2. 

Second, EPA evaluated the potential for new information to affect NPDWRs in a manner 

consistent with its existing policies and procedures for developing NPDWRs. For example, in 

determining whether a possible change in analytical feasibility existed, the Agency applied the 

current policy and procedures for calculating the practical quantitation level for drinking water 

contaminants. 

Third, the Agency does not believe it is appropriate to consider revisions to NPDWRs for 

contaminants with an ongoing health effect assessment for which the maximum contaminant 

level (MCL) is set equal to the maximum contaminant level goal (MCLG) or based on benefitcost analysis. This principle stems from the fact that any new health effects information could 

affect the MCL via a change in the MCLG or the assessment of the benefits associated with the 

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MCL. Therefore, EPA made a “take no action” recommendation if the health effect assessment 

would not be completed during the review period for each contaminant that has either an MCL 

that is equal to its MCLG or an MCL that is based on the 1996 SDWA Amendments’ costbenefit provision. 

Fourth, EPA will address new information from health effect assessments completed after the 

information cutoff date (March 1, 2009) for the Six-Year Review 2 and any new conclusions or 

additional information associated with the contaminant during the next review cycle. The 

Agency may consider accelerating a review and possible revision for a particular NPDWR 

before the next review cycle when justified by new public health risk information. 

Fifth, EPA identified areas of inadequate or unavailable data (data gaps) or emerging data that is 

needed to determine whether revision to an NPDWR is appropriate. If EPA is able to fill such 

gaps or fully evaluate the emerging information after completing Six-Year Review 2, the Agency 

will consider the information as part of the next review cycle. EPA may consider accelerating a 

review and possible revision for a particular NPDWR if a review and possible revision is 

justified by new public health risk information. 

Finally, EPA applied the Agency’s peer review policy (USEPA, 2000), where appropriate, to 

any new analyses. 

1.2 Scope of Review 

As for Six-Year Review 1, Six-Year Review 2 encompassed the individual elements of 

NPDWRs, as follows: 

• MCLG changes – EPA generally considered changes to the MCLG (the health goal) only in 

instances of a new health effects assessment completed since the MCLG was promulgated or 

last revised, which resulted in a revised reference dose (RfD) and/or cancer classification 

justifying a revised MCLG. 

• MCL changes – EPA generally considered changes to the MCL (the enforceable standard) 

whenever: (1) the health effects assessment justifies a possible change to the MCLG and the 

existing MCL is set at the MCLG, or (2) the current MCL was limited by analytical or 

treatment feasibility and the review of these capabilities indicates that it may now be feasible 

to set the MCL closer to the MCLG.1

• Treatment Technique (TT)2 changes – Treatment techniques can improve to the point where 

more protective drinking water standards may be considered. EPA generally considered 

revisions to TT requirements whenever there was new information on health effects, 

analytical feasibility, or treatment feasibility that suggests a possibility to revise the TT. 

• Changes to Other Treatment Technology – When EPA sets an MCL, the NPDWR also 

contains Best Available Technology (BAT) recommendations that address drinking water 

 

1

 Although the 1996 SDWA Amendments allow EPA in certain circumstances to set the MCL at a level higher than 

the feasible level if the benefits do not justify the costs, SDWA also precludes the Agency from making an existing 

standard less stringent solely on economic considerations. 

2

 A TT specifies a type of treatment (e.g., filtration, disinfection, or other methods of control to limit contamination 

in drinking water) and means for ensuring adequate treatment performance (e.g., monitoring of water quality to 

ensure treatment performance). 

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

have the capability to meet MCLs. EPA generally limited review of BAT recommendations 

for those NPDWRs with possible MCL revisions. 

• Changes to Other Regulatory Requirements – EPA generally considered other regulatory 

revisions, such as changes to monitoring requirements, if other possible NPDWR revisions or 

health effects information suggest that changes in monitoring standards (e.g., increased 

frequency in monitoring) could reduce health risks or costs while maintaining or improving 

the level of public health protection. This part of the review focused on implementationrelated issues that are not being addressed, or have not been addressed, through alternative 

mechanisms (e.g., as part of a recent or ongoing rulemaking). Where appropriate alternative 

mechanisms do not exist, EPA generally considered implementation-related concerns if the 

possible revision met the following criteria: 

• The possible revision indicates a possible change to an NPDWR, as defined under section 

1401 of SDWA. 

• The possible revision was “ready” for rulemaking – that is, the problem to be resolved 

has been clearly identified and specific option(s) formulated to address the problem. 

• The possible revision could improve the level of public health protection or represent a 

cost savings while maintaining or improving public health protection. 

For Six-Year Review 2, EPA reviewed the chemical, microbiological, and radiological 

NPDWRs for the 85 contaminants shown in Exhibit 1-1. Of the 85 NPDWRs, EPA is reviewing 

or has revised 14 through recent or ongoing rulemakings (see Exhibit 1-2). 

Exhibit 1-1. NPDWR Contaminants Included in Six-Year Review 2 

Contaminants 

MCLG 

(mg/L)1

MCL 

(mg/L)1 Contaminants 

MCLG 

(mg/L)1

MCL 

(mg/L)1

Acrylamide 0 TT Epichlorohydrin 0 TT 

Alachlor 0 0.002 Ethylbenzene 0.7 0.7 

Alpha particles 0 (pCi/L) 15 (pCi/L) Ethylene dibromide (EDB) 0 0.00005 

Antimony 0.006 0.006 Fluoride 4 4 

Arsenic 0 0.01 Giardia lamblia 0 TT 

Asbestos 7 (million 

fibers/L) 

7 (million 

fibers/L) Glyphosate 0.7 0.7 

Atrazine 0.003 0.003 Haloacetic acids (HAA5) n/a3 0.06 

Barium 2 2 Heptachlor 0 0.0004 

Benzene 0 0.005 Heptachlor epoxide 0 0.0002 

Benzo[a]pyrene 0 0.0002 Hexachlorobenzene 0 0.001 

Beryllium 0.004 0.004 Hexachlorocyclopentadiene 0.05 0.05 

Beta particles 0 (millirems/yr) 4 (millirems/yr) Lead 0 TT 

Bromate 0 0.01 Legionella 0 TT 

Cadmium 0.005 0.005 Lindane 0.0002 0.0002 

Carbofuran 0.04 0.04 Mercury (inorganic) 0.002 0.002 

Carbon tetrachloride 0 0.005 Methoxychlor 0.04 0.04 

Chloramines 4 4 Monochlorobenzene 

(Chlorobenzene) 0.1 0.1 

Chlordane 0 0.002 Nitrate (as N) 10 10 

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Exhibit 1-1. NPDWR Contaminants Included in Six-Year Review 2 

Contaminants 

MCLG 

(mg/L)1

MCL 

(mg/L)1 Contaminants 

MCLG MCL 

(mg/L)1 (mg/L)1

Chlorine 4 4 Nitrite (as N) 1 1 

Chlorine dioxide 0.8 0.8 Oxamyl (Vydate) 0.2 0.2 

Chlorite 0.8 1 Pentachlorophenol 0 0.001 

Chromium (total) 0.1 0.1 Picloram 0.5 0.5 

Coliform 0%2 5%2 Polychlorinated biphenyls 

(PCBs) 0 0.0005 

Copper 1.3 TT Radium 0 (pCi/L) 5 (pCi/L) 

Cryptosporidium 0 TT Selenium 0.05 0.05 

Cyanide 0.2 0.2 Simazine 0.004 0.004 

2,4-D 0.07 0.07 Styrene 0.1 0.1 

Dalapon 0.2 0.2 2,3,7,8-TCDD (Dioxin) 0 3.00E-08 

Di(2-ethylhexyl)adipate 

(DEHA) 0.4 0.4 Tetrachloroethylene 0 0.005 

Di(2-

ethylhexyl)phthalate 

(DEHP) 

0 0.006 Thallium 0.0005 0.002 

1,2-Dibromo-3-

chloropropane (DBCP) 0 0.0002 Toluene 1 1 

1,2-Dichlorobenzene 

(o-Dichlorobenzene) 0.6 0.6 Total Trihalomethanes (TTHMs) n/a4 0.08 

1,4-Dichlorobenzene 

(p-Dichlorobenzene) 0.075 0.075 Toxaphene 0 0.003 

1,2-Dichloroethane 

(Ethylene dichloride) 0 0.005 2,4,5-TP (Silvex) 0.05 0.05 

1,1-Dichloroethylene 0.007 0.007 1,2,4-Trichlorobenzene 0.07 0.07 

cis-1,2-

Dichloroethylene 0.07 0.07 1,1,1-Trichloroethane 0.2 0.2 

trans-1,2-

Dichloroethylene 0.1 0.1 1,1,2-Trichloroethane 0.003 0.005 

Dichloromethane 

(Methylene chloride) 0 0.005 Trichloroethylene 0 0.005 

1,2-Dichloropropane 0 0.005 Uranium 0 (μg/L) 30 (μg/L) 

Dinoseb 0.007 0.007 Vinyl Chloride 0 0.002 

Diquat 0.02 0.02 Viruses 0 TT 

Endothall 0.1 0.1 

Endrin 0.002 0.002 Xylenes (total) 10 10 

1. Units are in milligrams per liter (mg/L) unless otherwise noted. Milligrams per liter are equivalent to parts per million. 

2. No more than 5.0% samples total coliform-positive in a month. 

3. There is no MCLG for all five haloacetic acids. MCLGs for some of the individual contaminants are: dichloroacetic acid (zero), 

trichloroacetic acid (0.02 mg/L), and monochloroacetic acid (0.07 mg/L). Bromoacetic acid and dibromoacetic acid are regulated 

with this group, but have no MCLGs. 

4. There is no MCLG for total trihalomethanes. MCLGs for some of the individual contaminants are: bromodichloromethane 

(zero), bromoform (zero), dibromochloromethane (0.06 mg/L), and chloroform (0.07mg/L). 

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Exhibit 1-2. NPDWR Contaminants with Recent or Ongoing Actions 

Contaminant/Indicator Recent or Ongoing Action 

Disinfection Byproducts 

Bromate Stage 2 DBPR (January 4, 2006) 

Chlorite Stage 2 DBPR (January 4, 2006) 

TTHMs: chloroform, bromodichloromethane, 

dibromochloromethane, bromoform Stage 2 DBPR (January 4, 2006) 

HAA5: monochloroacetic acid, dichloroacetic 

acid trichloroacetic acid, monobromoacetic 

acid dibromoacetic acid Stage 2 DBPR (January 4, 2006) 

Disinfectant Residuals 

Chlorine GWR (November 8, 2006) 

Chloramines GWR (November 8, 2006) 

Chlorine dioxide Stage 2 DBPR (January 4, 2006) 

Inorganics 

Lead 

Short-Term Revisions (October 10, 2007) 

Long-Term Revisions currently underway 

Copper Long-Term Revisions currently underway 

Microorganisms 

Cryptosporidium LT2 (January 5, 2006) 

Giardia lamblia LT2 (January 5, 2006) 

Legionella GWR (November 8, 2006) 

Viruses (enteric) 

LT2 (January 5, 2006) 

GWR (November 8, 2006) 

Coliform Total Coliform Rule-making currently underway 

Dates of promulgation are as follows: 

Stage 2 DBPR: 71 FR 388 (January 4, 2006) 

LT2 Rule: 71 FR 654 (January 5, 2006) 

GWR: 71 FR 65574 (November 8, 2006) 

LCR Short-Term Regulatory Revisions: 72 FR 57782 (October 10, 2007) 

1.3 Organization and Contents of this Document 

This document describes the review process for Six-Year Review 2: 

• Section 2 provides an overview of the Six-Year Review protocol and the decision tree EPA 

developed to implement it for Six-Year Review 2. 

• Section 3 provides detailed description of the individual branches of the decision tree 

implementing the Six-Year Review 2 protocol. 

EPA has developed additional technical support documents, noted in the text, to provide detailed 

discussion of each element of the review.

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2 Overview of Six-Year Review Protocol 

During Six-Year Review 1, the Agency developed a systematic approach or protocol to review 

existing NPDWRs (USEPA, 2003c). The Agency based this protocol on the recommendations of 

the National Drinking Water Advisory Council (NDWAC), through internal Agency 

deliberations, and discussions with a diverse group of stakeholders involved in drinking water 

and its protection. 

For Six-Year Review 2, EPA assessed this protocol and determined that it remained appropriate 

and suitable for the second review. Thus, the information requirements and decision-making 

process of the Six-Year Review 2 protocol are essentially the same as those implemented during 

Six-Year Review 1, with some minor refinements to enhance the Agency’s effectiveness in 

applying the protocol to the review of NPDWRs. 

The Six-Year Review 2 protocol addresses critical aspects of health protection and the setting of 

standards under the SDWA. Similar to the Six-Year Review 1, the results of the Six-Year 

Review 2 protocol are recommendations to revise some NPDWRs, and to take no action at this 

time for the remaining NPDWRs. 

The publication of a recommendation to revise pursuant to a Six-Year Review under Section 

1412(b)(9) is not the end of the regulatory process, but is the beginning of one. A 

recommendation to revise starts a regulatory process that involves more detailed analyses 

concerning health effects, costs, benefits, occurrence, and other matters relevant to deciding 

whether and how an NPDWR should be revised. At any point in this process, EPA may find that 

regulatory revisions are not appropriate and may discontinue regulatory revision efforts. Review 

of that NPDWR would, however, continue in future Six-Year Reviews. 

Similarly, a recommendation to “take no action at this time” means only that EPA does not 

believe that regulatory changes to a particular NPDWR are appropriate due to: a lack of new 

health effects, analytical methods, or treatment data; ongoing scientific reviews; low priority; or 

other reasons. Reviews of these contaminants in future Six-Year Reviews may lead to a 

recommendation that regulatory changes are appropriate. 

The Agency will continue to refine the Six-Year Review protocol during subsequent reviews to 

address changing circumstances. 

2.1 Protocol Refinements for Six-Year Review 2 

During Six-Year Review 2, EPA refined the protocol to implement a more detailed “decision 

tree” than it used during Six-Year Review 1. The revised protocol can be broken down into a 

series of questions about whether there is new information for a contaminant that suggests it is 

possible to revise one or more of the NPDWR elements. These questions can be logically 

ordered into a decision tree that incorporates the sequential relationships between the different 

NPDWR elements. For example, EPA must generally set the MCL as close to the MCLG as 

feasible. Consequently, if the MCL is equal to the MCLG, EPA must make decisions regarding 

the availability and adequacy of new information relevant to the potential to revise the MCLG 

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before decisions regarding the potential to revise the MCL. Also, if there is no potential to revise 

the MCLG and the MCL is already equal to the MCLG, then there is no basis for revising the 

MCL. In this instance, the MCL branch of the decision tree is not reached, and it is not necessary 

to make related decisions such as whether the practical quantitation level (PQL) can be revised. 

EPA developed an automated tool called the Regulatory Review Support Spreadsheet (R2S2) to 

track the review process for each contaminant that leads to the revise/take no action 

recommendations. This tool enhances transparency of the review results. The automation also 

streamlines the decision process and facilitates the Agency’s reporting of its recommendations. 

Exhibit 2-1 shows the decision tree structure for the revised protocol. 

2.2 Elements of the Six-Year Review 2 Decision Tree 

The Six-Year Review decision tree contains a “branch” with multiple questions for each review 

topic. Information flows between these branches as shown in Exhibit 2-1. Each branch 

corresponds to a specific technical review of an NPDWR element that EPA conducted during 

Six-Year Review 2. These branches include: 

• Initial review 

• Health effects and MCLG 

• MCL 

• Treatment technique 

• Analytical methods 

• Occurrence and exposure 

• Treatment feasibility 

• Implementation. 

The following sections describe each branch and provide detailed descriptions of EPA’s data 

requirements, analyses, and decision-making process.

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Exhibit 2-1. Overview of Six-Year Review Decision Tree 

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3 Detailed Discussion of Decision Tree Implementing the 

Protocol 

This section describes the individual branches of the decision tree in detail, including the 

purpose, inputs, and outputs of each branch. 

3.1 Initial Review Branch 

The first branch of the decision tree is an Initial Review Branch (Exhibit 3-1), with the purpose 

of identifying contaminants meeting one of three conditions for which there is a recent or 

ongoing action or for which further review of detailed technical data is premature. The three 

conditions are: 

• EPA has recently reviewed and revised the NPDWR (i.e., since August 2002) 

• EPA is conducting an ongoing regulatory revision 

• EPA is performing a formal health effects assessment of the regulated contaminant, the 

results are due after the cutoff data for the review, or EPA completed a health effects 

assessment, but then identified new information with potential to affect the MCLG and the 

MCL is set equal to the MCLG. 

Excluding such contaminants from subsequent steps in the NPDWR review decision tree 

improves the efficiency and effectiveness of the review process. It prevents duplicative Agency 

data collection and analysis efforts for recent or ongoing actions to review and revise NPDWRs. 

It also avoids recommendations based on inadequate information for contaminants with ongoing 

or pending health effects assessments that already have MCLs set equal to their respective 

MCLGs. For a contaminant that has an ongoing health effects assessment and an MCL above its 

MCLG, EPA’s review continues to branches that evaluate whether there is potential to lower the 

MCL. The Agency’s review of new information that may affect the MCL for these contaminants 

is one of several refinements of the protocol. During Six-Year Review 1, EPA took no further 

action on any contaminants with ongoing health effects assessments. The refinement addresses 

the SDWA requirement that EPA set each MCL as close to the MCLG as feasible; a common 

limitation is the analytical capability at the time the NPDWR is promulgated, especially for a 

contaminant with an MCLG equal to zero. 

3.1.1 Inputs to the Initial Review 

The questions in the Initial Review Branch are screening-level questions. For Six-Year Review 

2, EPA answered these questions for each contaminant covered by a NPDWR. The first two 

questions in the branch require information regarding whether a contaminant is the subject of 

recent or ongoing rules. For Six-Year Review 2, the regulatory schedule for the Office of Ground 

Water and Drinking Water (OGWDW) provided the inputs to these two decisions. Exhibit 1-2

lists the contaminants that are subject to recent or ongoing rules. 

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Exhibit 3-1. Initial Review Branch 

Is a new HEA [including 

reproductive and 

developmental effects] in 

progress and expected 

after March 2009, or is 

there new information* after 

completion of a new HEA?

Is a regulatory review or 

revision in progress or 

planned through 2009?

Action 

taken or 

ongoing

Continue 

review

Was a regulatory action 

undertaken since August 

2002?

Key:

No

Yes

GO TO

Health Effects and 

MCLG Branch

Is MCLG < MCL, or 

does MCLG=0?

GO TO 

MCL 

Branch 2

Take no 

action

Continue 

review

HEA = health effects assessment

*New information that could affect the MCLG. May need to renominate for IRIS/OPP/OW review based on this new information. 

Is a new HEA [including 

reproductive and 

developmental effects] in 

progress and expected 

after March 2009, or is 

there new information* after 

completion of a new HEA?

Is a regulatory review or 

revision in progress or 

planned through 2009?

Action 

taken or 

ongoing

Continue 

review

Was a regulatory action 

undertaken since August 

2002?

Key:

No

Yes

GO TO

Health Effects and 

MCLG Branch

Is MCLG < MCL, or 

does MCLG=0?

GO TO 

MCL 

Branch 2

Take no 

action

Continue 

review

HEA = health effects assessment

*New information that could affect the MCLG. May need to renominate for IRIS/OPP/OW review based on this new information. 

The third question requires information regarding whether a formal Agency health effects 

assessment is in progress, and if results will be available by the cutoff date for the review (March 

1, 2009). Health effects assessments used to develop NPDWRs are usually performed under the 

following EPA programs: Integrated Risk Information System (IRIS), Office of Pesticide 

Programs (OPP), the Office of Water (OW), and the National Academy of Sciences (NAS) when 

commissioned by EPA. The question expands this “No Action” category to include any 

contaminant for which a health effects assessment was completed during the current review 

round, but subsequent new information has the potential to affect its MCLG. 

Health effects assessments are conducted outside the scope of the Six-Year Review process and 

follow EPA guidelines established to assess risks for different health effects, different exposure 

routes, and in different sensitive population groups and life stages including children. EPA’s 

Office of Science and Technology (OST) tracks the status of health effects assessments and 

provides summaries to OGWDW that identify the contaminants with ongoing health effects 

assessments and their expected completion dates. 

For Six-Year Review 2, OST collected health effects information for contaminants that were not 

part of an ongoing or recent rule. These contaminants were in one of two lists for the purpose of 

tracking health effects information: 

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• Contaminants with ongoing formal EPA health effects assessments 

• All other regulated drinking water contaminants that reached this decision tree point.

OGWDW established a cutoff date (March 1, 2009) after which it would not be feasible to fully 

review and evaluate the potential to revise a contaminant’s MCLG during Six-Year Review 2. 

3.1.2 Output of Initial Review 

The outputs of the initial review branch are: (1) a list of regulated contaminants excluded from 

further review branches during the current cycle3, (2) a list of contaminants that proceed to the 

Health Effects and MCLG branch for questions about the potential to revise the MCLG, and (3) 

a list of contaminants that proceed to the MCL Branch 2 despite ongoing health effects 

assessments because they have MCLs that are greater than their respective MCLGs. 

3.2 Health Effects and MCLG Branch 

The primary purpose of the Health Effects and MCLG Branch (Exhibit 3-2) is to identify the 

NPDWRs for which there is potential to revise the MCLG. To do this, the protocol requires that: 

• A revised or new health effects assessment be completed during the current cycle before 

March 1, 2009 

• The assessment results in a change to the RfD or cancer risk. 

Another refinement to the protocol for the second cycle provides an option to revisit Agency 

decisions to take no action for contaminants that had a new health effects assessment that 

indicated potential for an MCLG revision during the prior cycle. 

The Health Effects and MCLG Branch also identifies whether there is new health effects 

information identified during a review of peer-reviewed literature that leads to a nomination for a 

new health effects assessment for those contaminants for which there are no recent or ongoing 

assessments. 

 

3

 Contaminants that have a “Take no action” result on the Initial Review may still be affected by a cross-cutting 

issue affecting multiple contaminants that qualifies for consideration under the conditions described for other 

regulatory revisions. 

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Exhibit 3-2. Health Effects and MCLG Branch 

HEA since Round 1 cutoff of 

August 2002 (July 18, 2003 

FR 42907]?

Consider 

MCLG 

revision 

Any change in the current 

health effects assessment 

(i.e., RfD change or nonlinear 

mode of action) that suggests 

MCLG change?

HEA = health effects assessment

*New information that could affect the MCLG

Perform full toxicological search or 

update Round 1 information. New 

information*?

Take no 

action on 

MCLG

GO TO 

MCL 

Branch 2

Nominate for IRIS/OPP/OW review

Is the MCLG > 0?

Is there new information* 

indicating a nonlinear mode of 

action or potential 

reproductive/dev. effects?

Key:

No

Yes

GO TO 

MCL 

Branch 1

Did a previous round suggest 

MCLG revision but reject it because 

of constraints on the MCL?

HEA since Round 1 cutoff of 

August 2002 (July 18, 2003 

FR 42907]?

Consider 

MCLG 

revision 

Any change in the current 

health effects assessment 

(i.e., RfD change or nonlinear 

mode of action) that suggests 

MCLG change?

HEA = health effects assessment

*New information that could affect the MCLG

Perform full toxicological search or 

update Round 1 information. New 

information*?

Take no 

action on 

MCLG

GO TO 

MCL 

Branch 2

Nominate for IRIS/OPP/OW review

Is the MCLG > 0?

Is there new information* 

indicating a nonlinear mode of 

action or potential 

reproductive/dev. effects?

Key:

No

Yes

GO TO 

MCL 

Branch 1

Did a previous round suggest 

MCLG revision but reject it because 

of constraints on the MCL?

3.2.1 Inputs to Health Effects and MCLG Review

The first question in the Health Effects and MCLG Branch identifies the contaminants having a 

formal health effects and toxicological assessment completed during the current review cycle 

and by the cutoff date (March 1, 2009). For Six-Year Review 2, OGWDW used the status 

summaries provided by OST to answer this question. 

For contaminants that have a new health effects assessment, this branch asks whether there was a 

change in toxicological parameters that affect the MCLG. For Six-Year Review 2, OST provided 

this information in a summary report that indicated: 

• Whether the assessment resulted in changes to the RfD or cancer classification 

• Whether these changes would potentially affect the MCLG. 

OST obtained RfD and cancer classification information from the formal health effects 

assessment documents developed by sources such as IRIS, OPP, and OW. OST took the 

following steps to derive a possible MCLG: 

• Classified the contaminant in one of three OW Categories based on cancer risk 

classifications 

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• Derived a possible MCLG value (or range of values) using the method associated with each 

OW Category. 

The document, “Six-Year Review 2 – Health Effects Assessment – Summary Report” (USEPA, 

2009e) describes how OST assigned the OW Categories to contaminants and derived MCLG 

values. 

The next question in this branch divides contaminants that did not have a recent health effects 

assessment into two categories for the purpose of a literature search: (1) those with nonzero 

MCLGs, and (2) those with MCLGs of zero. Subsequent questions address whether literature 

searches indicate a need for a new formal health effects assessment. For Six-Year Review 2, 

OST conducted a review of the peer-reviewed literature on relevant health effects (i.e., general 

toxicity, reproductive and developmental toxicity, and cancer risk via an oral route for the 

general population and sensitive subpopulation groups including children) to search for new 

health effects information that indicates potential that the current RfD values or cancer risk 

categories are not adequately representing health risks. 

OST’s review for each chemical began with the authoritative reviews or assessments by IRIS, 

OPP, the National Academy of Sciences (NAS), the Agency for Toxic Substances and Disease 

Registry (ATSDR), the National Toxicology Program (NTP), National Institute of 

Environmental Health Sciences (NIEHS), California EPA (CalEPA), World Health Organization 

(WHO), European Commission Concise International Chemical Assessment Documents 

(CICADS), International Programme on Chemical Safety/Environmental Health Criteria 

(IPCS/EHC), International Agency for Research on Cancer (IARC), Health Canada, Joint Expert 

Committee on Food Additives (JECFA), and Joint FAO/WHO Meeting on Pesticide Residues 

(JMPR). OST obtained each organization’s most recent assessment available. OST also 

conducted literature searches to identify primary literature to supplement the information in the 

authoritative reviews. The searches utilized the following databases: TOXLINE, MEDLINE®, 

Developmental and Reproductive Toxicology (DART®), Chemical Carcinogenesis Research 

Information System (CCRIS), and Hazardous Substances Data Bank (HSDB). 

For contaminants for which health effects assessments were completed during Six-Year Review 

2, OST also conducted supplemental literature searches covering the period from either the two 

years preceding the publication date of a final IRIS, OPP, or NAS assessment or the three years 

prior to the publication of an ATSDR Toxicological Profile. 

The purpose of the literature search was to identify: 

• Whether there was new health effects information indicating a nonlinear mode of action or 

potential reproductive/developmental or other toxicological effects for contaminants at 

concentrations at or below the MCL when the MCLG is zero 

• Whether there was new cancer data or toxicological information in the literature that 

potentially affects the RfDs for contaminants without a recent health effects assessment and a 

nonzero MCLG. 

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OST provided OGWDW with summary information from the literature review, including a 

recommendation about whether new information might lead OW to nominate the contaminant 

for a formal health effects assessment. 

The final question in this branch identifies contaminants for which there was not a new health 

effects assessment in the current review cycle, but there was one during the previous review 

cycle that included a change in the RfD. During Six-Year Review 1, EPA took no action to 

revise the NPDWR for some of these contaminants for one of the following reasons: 

• The possible revision would not have provided a meaningful opportunity to reduce health 

risks 

• The possible revision would not have provided a meaningful opportunity to reduce costs 

while maintaining the same or greater level of health protection 

• The possible revision would have been a low priority because of competing workload 

priorities, the administrative costs associated with rulemaking, and the burden on States and 

the regulated community to implement any regulatory change that resulted. 

During the Six-Year Review 2, EPA revisited these decisions to consider whether new 

information (e.g., changes in analytical feasibility or new occurrence and exposure analyses) 

may affect that decision. 

3.2.2 Outputs from Health Effects and MCLG Review

The Health Effects and MCLG Branch sorts the contaminants into the following categories based 

on health effects information: 

• Contaminants for which there is potential to revise the MCLG based on the availability of 

new Agency health effects information, or contaminants for which there was a potential to 

revise the MCLG during the first Six-Year Review, but for which EPA took no action 

• Contaminants for which a literature review indicates a potential change in health effects 

information and that should, therefore, be nominated for a formal heath effects assessment 

through OW, IRIS, or OPP 

• Contaminants for which there is no potential to revise the MCLG during Six-Year Review 2. 

The decision tree directs the first category of contaminants to the MCL branch that reflects 

potential for MCLG revision (MCL Branch 1). It directs the second and third categories of 

contaminants to a second MCL branch that reflects no action will be taken regarding MCLG 

revision (MCL Branch 2). 

3.3 Maximum Contaminant Level (MCL) Branches 

The purpose of each MCL Branch is to identify NPDWRs for which new information indicates 

potential to revise the MCL. The SDWA requires that EPA generally set the MCL as close to the 

MCLG as feasible [Section 1412(b)(4)(B)]. Feasibility refers to both the ability to treat water to 

meet the MCL, as well as monitor water quality at the MCL. For most contaminants for which 

the MCLG is greater than zero, the MCL equals the MCLG, which indicates that neither 

analytical method quantitation nor treatment capabilities limit the ability to achieve the MCLG. 

Conversely, when the MCLG equals zero, the MCL is usually set equal to the practical 

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quantitation limit (PQL), which is based on the detection capability that most laboratories can 

reliably and consistently achieve using approved analytical methods within specified limits of 

precision and accuracy. Thus, the PQL is the most common limiting factor with respect to 

feasibility. Consequently, the MCL branches address analytical feasibility before treatment 

feasibility. 

The decision tree includes two MCL Branches: one for contaminants with a possible MCLG 

revision (MCL Branch 1; Exhibit 3-3), and the other for contaminants with no action regarding 

the MCLG (MCL Branch 2; Exhibit 3-4). 

Exhibit 3-3. Maximum Contaminant Level Branch 1 (Potential for MCLG Revision) 

Does new information 

indicate lower MCL is 

feasible?

Is MCL based on 

benefit-cost analysis?

Is MCL based on 

treatment feasibility?

Is MCL based on PQL?

GO TO Methods 

Branch and return 

here

Is PQL<MCL or 

change indicated?

Is standard an 

MCL?

Is there new benefits 

or cost information?

Take no 

action on 

MCL or 

MCLG

Consider 

MCL 

revision

GO TO Treatment 

Branch and return 

here.

Is there sufficient 

information to 

identify BATs?

GO TO Occurrence 

Branch and return 

here. 

Would MCL revision 

provide a meaningful 

opportunity for health 

risk or cost reduction?

GO TO Treatment 

Technique Branch

Key:

No

Yes/ has potential

Is MCL =MCLG? Is higher MCLG indicated?

GO TO 

Implementation 

Branch

Issue health 

advisory if needed.

Does new information 

indicate lower MCL is 

feasible?

Is MCL based on 

benefit-cost analysis?

Is MCL based on 

treatment feasibility?

Is MCL based on PQL?

GO TO Methods 

Branch and return 

here

Is PQL<MCL or 

change indicated?

Is standard an 

MCL?

Is there new benefits 

or cost information?

Take no 

action on 

MCL or 

MCLG

Consider 

MCL 

revision

GO TO Treatment 

Branch and return 

here.

Is there sufficient 

information to 

identify BATs?

GO TO Occurrence 

Branch and return 

here. 

Would MCL revision 

provide a meaningful 

opportunity for health 

risk or cost reduction?

GO TO Treatment 

Technique Branch

Key:

No

Yes/ has potential

Is MCL =MCLG? Is higher MCLG indicated?

GO TO 

Implementation 

Branch

Issue health 

advisory if needed.

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Exhibit 3-4. Maximum Contaminant Level Branch 2 (No Potential for MCLG 

Revision) 

Does new information 

indicate lower MCL is 

feasible?

Is MCL based on 

benefit-cost analysis?

Is MCL based on 

treatment feasibility?

Is MCL based on 

PQL?

GO TO Methods 

Branch and return 

here.

Is PQL<MCL or 

change indicated?

Is standard an 

MCL?

Is there new benefit or 

cost information?

Take no 

action on 

MCL

Consider 

MCL 

revision

GO TO Treatment 

Branch and return 

here.

Is there sufficient 

information to 

identify BATs?

GO TO Occurrence 

Branch and return 

here. 

Would MCL revision 

provide a meaningful 

opportunity for health 

risk or cost reduction?

GO TO Treatment 

Technique Branch

Key:

No

Yes/ has potential

GO TO 

Implementation 

Branch

Does new information 

indicate lower MCL is 

feasible?

Is MCL based on 

benefit-cost analysis?

Is MCL based on 

treatment feasibility?

Is MCL based on 

PQL?

GO TO Methods 

Branch and return 

here.

Is PQL<MCL or 

change indicated?

Is standard an 

MCL?

Is there new benefit or 

cost information?

Take no 

action on 

MCL

Consider 

MCL 

revision

GO TO Treatment 

Branch and return 

here.

Is there sufficient 

information to 

identify BATs?

GO TO Occurrence 

Branch and return 

here. 

Would MCL revision 

provide a meaningful 

opportunity for health 

risk or cost reduction?

GO TO Treatment 

Technique Branch

Key:

No

Yes/ has potential

GO TO 

Implementation 

Branch

3.3.1 Inputs to Maximum Contaminant Level (MCL) Review

The two MCL branches have similar questions and differ in that one poses the questions for 

contaminants with a possible MCLG revision (MCL Branch 1), and the other poses the questions 

for contaminants with no action regarding the MCLG (MCL Branch 2). For example, MCL 

Branch 1 has an additional question to identify and address circumstances where the health 

effects information indicates potential to revise the MCLG upward, which would affect the MCL 

if the MCL is equal to the MCLG. 

The initial questions on the MCL branches pertain to following: 

• Whether the standard is an MCL or a TT 

• Whether a higher or lower MCLG is indicated, if applicable 

• The basis for the current MCL. 

For Six-Year Review 2, OGWDW used the NPDWRs and supporting rule documents to answer 

the first and third questions. OGWDW used health effects information provided by OST to 

determine whether there is potential for a higher or lower MCLG. 

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Subsequent questions on the MCL branches involve subordinate branches for analytical 

methods, occurrence, and treatment analysis that explore the availability of new information that 

could affect OGWDW’s recommendation regarding an MCL revision. Later sections of this 

document address the specific data requirements of these subordinate branches and describe the 

analyses that EPA conducted as part of these branches. The MCL branches combine the findings 

from these subordinate branches into an overall MCL recommendation. 

3.3.2 Outputs from Maximum Contaminant Level (MCL) Review

The MCL branches identify contaminants for which the review did not identify any new 

information indicating potential for MCL revision and those for which new information indicates 

EPA should consider revising the MCL. After completing an MCL branch, the decision tree 

directs the review to the Implementation Branch. 

3.4 Treatment Technique Branch 

When a contaminant has a TT standard instead of an MCL, the protocol uses the Treatment 

Technique Branch of the decision tree (Exhibit 3-5), instead of either of the MCL Branches. The 

purpose of the Treatment Technique Branch is to identify whether there is potential to revise a 

TT standard. 

Exhibit 3-5. Treatment Technique Branch 

Key:

No

Yes

Take no 

action on 

TT

Consider 

TT revision

GO TO TT Analysis 

Branch and return 

here. Does the new 

information indicate 

that a meaningful 

opportunity exists for

health risk or cost 

reduction?

Does new health 

risk, analytical 

methods, or 

treatment technique 

information by 

March 2009 indicate 

possible TT 

revision?

GO TO 

Implementation 

Branch

Key:

No

Yes

Take no 

action on 

TT

Consider 

TT revision

GO TO TT Analysis 

Branch and return 

here. Does the new 

information indicate 

that a meaningful 

opportunity exists for

health risk or cost 

reduction?

Does new health 

risk, analytical 

methods, or 

treatment technique 

information by 

March 2009 indicate 

possible TT 

revision?

GO TO 

Implementation 

Branch

Take no 

action on 

TT

Consider 

TT revision

GO TO TT Analysis 

Branch and return 

here. Does the new 

information indicate 

that a meaningful 

opportunity exists for

health risk or cost 

reduction?

Does new health 

risk, analytical 

methods, or 

treatment technique 

information by 

March 2009 indicate 

possible TT 

revision?

GO TO 

Implementation 

Branch

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3.4.1 Inputs to Treatment Technique Review 

The TT Branch includes the following questions: 

• Does new information in the following areas indicate potential for TT revision: health risk, 

analytical methods, or treatment technique? 

• Based on the decisions on the Treatment Technique Analysis Branch, does a meaningful 

opportunity exist for health risk or cost reduction? 

The following contaminants have a TT in lieu of an MCL: acrylamide, copper, Cryptosporidium, 

epichlorohydrin, Giardia lamblia, lead, Legionella, and viruses. Of these contaminants, all 

except acrylamide and epichlorohydrin were affected by recent or ongoing rule revisions during 

Six-Year Review 2. USEPA (2009f) describes the information that the Agency obtained during 

its review of the TT for acrylamide and epichlorohydrin. 

3.4.2 Outputs from Treatment Technique Review 

The Treatment Technique Branch identifies contaminants for which EPA should consider 

revisions to a TT standard because all of the following apply: 

• New health, methods, or treatment information are available that suggest revision 

• There is a meaningful opportunity to lower health risks or costs. 

The decision tree then directs the review to the Implementation branch. 

3.5 Treatment Technique Analysis Branch 

The purpose of the Treatment Technique Analysis Branch (Exhibit 3-6) is to determine whether 

the new information that could affect the TT standard has the potential to constitute a meaningful 

opportunity to revise the TT standard. 

3.5.1 Inputs to Treatment Technique Analysis Review 

The Treatment Technique Analysis Branch includes the following questions: 

• Is there a significant increase in health risk estimated from exposure to the contaminant? 

• Is there a significant improvement in analytical feasibility or treatment technique? 

The first question identifies whether new health effects information indicates health risks that are 

significantly different from those considered at the time EPA promulgated the NPDWR. The 

second question addresses whether there are significant changes in analytical feasibility 

constraints that might have originally led to a contaminant having a TT standard in lieu of an 

MCL. It also addresses whether significant changes in treatment feasibility indicate potential for 

revision to the TT standard. 

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Exhibit 3-6. Treatment Technique Analysis Branch 

Return to TT 

branch

Is there a significant 

increase in health risk 

estimated from 

exposure to the 

contaminant?

Meaningful 

opportunity 

exists for TT 

revision. 

Key:

No

Yes

No 

meaningful 

opportunity 

exists for TT 

revision. 

Is there a significant 

improvement in 

analytical feasibility or 

treatment 

techniques?

Return to TT 

branch

Is there a significant 

increase in health risk 

estimated from 

exposure to the 

contaminant?

Meaningful 

opportunity 

exists for TT 

revision. 

Key:

No

Yes

No 

meaningful 

opportunity 

exists for TT 

revision. 

Is there a significant 

improvement in 

analytical feasibility or 

treatment 

techniques?

3.5.2 Outputs from Treatment Technique Analysis Review 

The Treatment Technique Analysis Branch identifies contaminants for which new information 

has the potential to constitute a meaningful opportunity to lower health risks or costs through a 

TT revision. The decision tree then directs the review back to the main Treatment Technique 

Branch. 

3.6 Methods Branch 

The purpose of the Methods Branch (Exhibit 3-7) is to determine whether there is potential to 

revise the PQL for a regulated contaminant. The PQL is the level at which laboratories can 

reliably and consistently measure a chemical contaminant in drinking water. This is usually 

interpreted as the analyte concentration at which 75% of laboratories can measure concentration 

within the promulgated acceptance criteria. 

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Exhibit 3-7. Analytical Methods Branch 

Return to source 

(MCL 1 or MCL 2)

Do MDL and/or MRL 

data suggest 

quantitation levels 

below the PQL?

Do PT data available 

by March 2009 

support quantitation 

levels below the 

PQL?

Are new analysis 

data available by 

March 2009?

Did analysis in prior 

round indicate 

potential to revise 

PQL?

Cannot 

revise 

PQL. 

There is 

potential to 

revise PQL. 

Key:

No

Yes

Return to source 

(MCL 1 or MCL 2)

Do MDL and/or MRL 

data suggest 

quantitation levels 

below the PQL?

Do PT data available 

by March 2009 

support quantitation 

levels below the 

PQL?

Are new analysis 

data available by 

March 2009?

Did analysis in prior 

round indicate 

potential to revise 

PQL?

Cannot 

revise 

PQL. 

There is 

potential to 

revise PQL. 

Key:

No

Yes

The branch considers two categories of contaminants: 

• Contaminants for which the MCL is limited by analytical feasibility (i.e., the MCL is set at 

the PQL), and the MCLG is still appropriate. 

• Contaminants for which the health effects review indicated potential to change the MCLG 

and the current PQL is above possible MCLG values. 

EPA reviews and approves analytical methods under a separate regulatory process. Therefore, 

Six-Year Review 2 did not include a review to determine whether the approved analytical 

methods, themselves, can be revised. Historically, EPA has used two main approaches to 

determine a PQL for SDWA analytes: (1) Performance Evaluation (PE) data from Water Supply 

(WS) studies is the preferred alternative when sufficient data are available; or (2) a multiplier 

method, in which the PQL is calculated by multiplying the EPA-derived method detection limit 

(MDL) by a factor of 5 or 10 [50 FR 46880 (November 13, 1985); 52 FR 25690 (July 8, 1987); 

54 FR 22062 (May 22, 1989)]. Using PE data to derive the PQL for chemical NPDWRs involves 

determining the concentration of an analyte at which 75% of EPA Regional and State 

laboratories achieve results within a specified acceptance window [see 54 FR 22062 (May 22, 

1989)]. 

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3.6.1 Inputs to Methods Review 

The Methods Branch includes the following questions: 

• Are new data available by the cutoff date (March 1, 2009) that EPA selected for Six-Year 

Review 2? 

• Do the new analytical methods data indicate potential to revise the PQL? 

• Do other new data such as method detection limit (MDL) and/or minimum reporting level 

(MRL) information indicate potential to revise the PQL? 

• Do previous data or analyses (i.e., Six-Year Review 1) indicate potential to revise the PQL? 

The document, “Analytical Feasibility Support Document for the Second Six-Year Review of 

Existing National Primary Drinking Water Regulations” (USEPA, 2009b), describes the process 

EPA used to evaluate the potential to revise PQL values and provides the results for each 

contaminant. The protocol developed for Six-Year Review 1 primarily utilized PE data from WS 

studies. These were laboratory accreditation studies conducted under EPA oversight until 1999, 

when the program was privatized. Now, the National Environmental Laboratory Accreditation 

Conference (NELAC) conducts the accreditation program via Proficiency Testing (PT) studies. 

EPA could not obtain actual PT study data from NELAC or any PT providers. One PT provider, 

however, made pass/fail rates from its PT studies available to EPA. This provider accounts for 

approximately 50% of the PT results nationwide (USEPA, 2009b). Because current PE data or 

comparable PT data were not available for this review, EPA modified the review process. 

The PQL reassessments for Six-Year Review 2 use a variety of data. The primary data sources 

are: 

• PE data available through late 1999, for which EPA derived passing rates during Six-Year 

Review 1 

• Laboratory passing rates based PT data (i.e., the percent of laboratories passing a proficiency 

test for a given study) from late 1999 through 2004 for a single PT provider. 

EPA relied primarily on the PT passing rate results at and below the current PQL to indicate 

potential for PQL revision. Passing rates for the older PE data provided supplemental 

information, especially when there were no PT studies with true values below the current PQL. 

EPA placed contaminants into one of three categories based on whether the PT and PE data 

supported, may support, or did not support a lower PQL. For example, EPA placed contaminants 

with passing rates above 75% for PT studies with true values below the PQL in the “PQL 

reassessment supports reduction of the current PQL” category. USEPA (2009b) provides a 

complete summary of the data and results by contaminant. 

When the analysis of PT and PE data did not provide conclusive indications regarding whether 

there was potential to revise a PQL, EPA reviewed two other sources of information. The first 

source was the minimum reporting levels in the Information Collection Request (ICR) database 

(USEPA, 2009c). An MRL is the lowest level or contaminant concentration that a laboratory can 

reliably achieve within specified limits of precision and accuracy under routine laboratory 

operating conditions using a given method (USEPA, 2009c). EPA received voluntary 

submissions of compliance monitoring data for public water systems from 51 States and entities. 

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The data contain a large number of analytical non-detection records with accompanying MRLs 

for regulated contaminants (see Section 3.7.1). EPA evaluated the distribution of MRL values for 

each contaminant to identify the mode or value occurring most frequently for that contaminant 

(“modal MRL”). The use of modal MRLs to provide additional insight into whether there is 

potential to revise a PQL is another refinement of the protocol, necessitated by limited 

availability of PT and PE data below the current PQL and made possible by the extensive 

amount of information included in the ICR database. 

The second type of information that EPA reviewed to evaluate potential to change the PQL was 

the MDLs for analytical methods approved by EPA for drinking water. In using MDLs, EPA 

followed the multiplier approach used to derive some PQLs. This approach was also used to 

identify possible analytical feasibility levels for Six-Year Review 1 (USEPA, 2003a). USEPA 

(2009b) provides the MDLs reviewed during Six-Year Review 2. Based on these MDL values, 

EPA used an MDL multiplier to estimate where the possible lower limit of quantitation may 

currently lie. The multiplier is 10 for most contaminants; the exception is contaminants for 

which EPA developed a PQL using a multiplier of 5 (e.g., dioxin). 

EPA also used the modal MRL and MDL-based estimates when it derived estimated quantitation 

levels (EQLs) for the occurrence analysis to help the Agency determine if there is a meaningful 

opportunity for health risk reduction. The report “Development of Estimated Quantitation Levels 

for the Second Six-Year Review of National Primary Drinking Water Regulations”(USEPA, 

2009c) describes the method that EPA used to develop EQLs. The EQL does not, however, 

represent the Agency’s intent to calculate new PQL at this time. Because of lack of data, EPA 

did not recalculate PQLs during Six-Year Review 2. 

3.6.2 Output from Methods Review 

The output of the Methods Branch is a decision regarding whether new information or 

information from an earlier cycle indicates a potential to lower the PQL for a contaminant. The 

decision tree then returns the review to the MCL Branch for subsequent questions. 

3.7 Occurrence Branch 

The purpose of the Occurrence Branch (Exhibit 3-8) is to determine whether the potential to 

revise an MCL presents a meaningful opportunity to: 

• Improve the level of public health protection 

• Achieve cost savings while maintaining or improving the level of public health protection. 

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Exhibit 3-8. Occurrence Branch 

Return to source 

(MCL 1 or MCL 

2)

Can the data provide 

occurrence estimates 

at alternative MCLs?

Are new monitoring 

data available by 

March 2009?

Are there Round 1 

occurrence data for 

alternative MCLs?

Data gap 

prevents 

analysis. 

Use

Round 1 

occurrence 

data. 

Key:

No

Yes

Estimate 

affected 

systems 

and pop. 

Would MCL revision 

provide a meaningful 

opportunity for health 

risk or cost 

reduction?

Return to source 

(MCL 1 or MCL 

2)

Can the data provide 

occurrence estimates 

at alternative MCLs?

Are new monitoring 

data available by 

March 2009?

Are there Round 1 

occurrence data for 

alternative MCLs?

Data gap 

prevents 

analysis. 

Use

Round 1 

occurrence 

data. 

Key:

No

Yes

Estimate 

affected 

systems 

and pop. 

Would MCL revision 

provide a meaningful 

opportunity for health 

risk or cost 

reduction?

EPA’s goal in evaluating contaminant occurrence is to: 

• Estimate the number of public water systems (PWSs) in which contaminants occur at levels 

of interest based on health effects or analytical methods information 

• Evaluate the number of people potentially exposed to these levels. 

This occurrence and exposure information indicates how changing an MCL may affect health 

risks and compliance costs. 

3.7.1 Inputs to Occurrence Review 

The initial questions elicit information regarding the availability of monitoring data for 

estimating occurrence at alternate thresholds (e.g., MCLs and EQLs). For Six-Year Review 2, 

the responses to these questions reflected new data that OGWDW received. EPA issued an ICR 

as a one-time request for States to submit historical monitoring data (covering the years 1998 

through 2005) for regulated contaminants voluntarily to EPA. A total of 51 States and entities 

provided compliance monitoring data that included all analytical detection and non-detection 

records. These data represent the national occurrence of regulated contaminants in public 

drinking water systems. USEPA (2009a) provides a detailed description of the extensive data 

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management efforts, quality assurance evaluations, and communications with State data 

management staff. 

The ICR contaminant occurrence dataset comprises more than 17 million analytical records from 

approximately 136,000 PWSs in 45 States. Approximately 265 million people are served by 

these PWSs nationally. The number of States and PWSs represented in the dataset varies across 

contaminants because of variability in voluntary State data submissions and contaminant 

monitoring schedules. This is the largest, most comprehensive set of drinking water compliance 

monitoring data ever compiled and analyzed by EPA. 

EPA used a two-stage analytical approach to analyze these data and characterize the national 

occurrence of contaminants. The “stage 1” analysis is a simple, non-parametric count of 

occurrence or regulated contaminants in public water systems. A typical stage 1 analysis 

generates a count of the number (or percentage) of systems with at least one analytical detection 

of a specific contaminant, or with at least one analytical detection with a concentration greater 

than a concentration threshold of interest, i.e., a possible MCLG or EQL. This approach provides 

information on peak occurrence levels, which are relevant for contaminants with acute health 

effects. It generates conservative (i.e., potentially upwardly biased) occurrence estimates, 

however, for contaminants with chronic health effects. 

For the contaminants with chronic health effects, EPA developed “stage 2” analysis estimates by 

generating estimated long-term mean concentrations at each system in the ICR dataset. A 

complete description of the two-stage analytical approach and a detailed presentation of 

occurrence estimates are in USEPA (2009a). EPA calculated the system means for the stage 2 

analysis using a simple arithmetic average of all detection and non-detection data for each public 

water system. Because the contaminant concentrations associated with the non-detection data are 

unknown, EPA assigned three different values to the non-detect results to estimate a range of 

system-level means, which then allowed EPA to estimate the number and percent of systems 

with estimated means exceeding selected threshold values. Two of the three values are based on 

the MRL values that accompany the non-detect results in the ICR dataset. The MRL is the 

lowest level that can be reliably achieved within specified limits of precision and accuracy under 

routine laboratory operating conditions using a given method. The three values that EPA 

substituted for non-detect results were MRL, one-half of the MRL, and zero. 

The most conservative approach was to assume that all non-detect results were equal to the 

MRL. This approach yields an upper-bound estimate of each system’s level of exposure. EPA 

also explored the less conservative assumption that concentrations for the non-detected results 

were uniformly distributed between the MRL and zero, thereby substituting one-half of the MRL 

for all non-detected results. Finally, EPA considered the assumption that the actual concentration 

for each non-detected result was typically much smaller than the MRL, supporting the use of 

zero to represent each non-detect. This method yielded a lower-bound estimate of the system’s 

mean and also reflects the approach that may be used to calculate annual averages for 

compliance. This simplified review method differs from the stage 2 approach in the Six-Year 

Review 1, which used more sophisticated modeling methods to address the non-detected results. 

That analysis, however, was based on a substantially smaller dataset (i.e., data from 16 States 

instead of 45 States). EPA used the three non-detection replacement assumptions in the stage 2 

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analyses to obtain reasonable bounds on the actual system mean concentrations. After EPA 

calculated system means for each of the three substitution methods, it compared the results to the 

various thresholds of interest to estimate the number and percent of systems with a mean 

concentration above a health threshold of concern and corresponding populations. 

Another refinement for Six-Year Review 2 was to include information on potential source water 

quality for the contaminants with possible MCLG increases. Because the ICR data represent 

water quality at entry points to the distribution system, the stage 1 and stage 2 occurrence 

analysis results are not adequate to evaluate the cost savings potential for contaminants with the 

potential for higher MCLG values. Therefore, for Six-Year Review 2, OGWDW also evaluated 

source water quality information for these contaminants. This information came from two 

national data sources: the National Water Quality Assessment (NAWQA) program conducted by 

the U.S. Geological Survey (USGS), and EPA’s STORET (short for STOrage and RETrieval) 

data system, which are part of OGWDW’s National Contaminant Occurrence Database (NCOD). 

The document, “Occurrence Analysis for Data in Potential Source Waters for the Second SixYear Review of NPDWRs” (USEPA 2009d), provides additional details on this review. 

Regardless of the occurrence data source and analysis method, EPA must determine whether the 

extent of occurrence represents a meaningful opportunity to reduce health risks or costs. There is 

no single quantitative threshold that applies to all contaminants. The EPA Administrator has the 

discretion to determine which revisions are appropriate, and may consider a variety of factors 

including but not limited to the type of health effects for the general population and sensitive 

populations and lifestages including children, the geographical distribution of the affected 

systems and populations, the size of the affected populations, and competing Agency priorities 

and resource constraints. 

3.7.2 Output from Occurrence Review 

The output of the Occurrence Review Branch is the identification of contaminants for which 

MCL revision would provide a meaningful opportunity for health risk reduction or cost savings 

while maintaining or improving the level of public health protection. An additional result is the 

identification of contaminants for which data gaps prevent an occurrence review. The decision 

tree then returns the review to the MCL Branch for subsequent questions. 

3.8 Treatment Branch 

When EPA promulgates an MCL, the NPDWR also contains BAT recommendations for 

drinking water treatment processes. To be a BAT, the treatment technology must meet several 

criteria such as having demonstrated consistent removal of the target contaminant under field 

conditions. Although treatment feasibility and analytical feasibility together address the technical 

feasibility requirement for an MCL, historically treatment feasibility has not been a limiting 

factor for MCLs. Thus, the purpose of the Treatment Review Branch (Exhibit 3-9) is to ascertain 

that there are technologies that meet BAT criteria when an MCL can be lowered and doing so 

presents a meaningful opportunity to reduce health risks. 

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Exhibit 3-9. Treatment Branch 

Can BATs and 

SSCTs meet 

alternative MCLs?

Can new 

technologies 

identified by March 

2009 meet alternative 

MCLs?

Cannot 

achieve 

revised 

MCL. 

Can 

achieve 

revised 

MCL 

Identify 

potential 

BATs & 

SSCT s

Return to source 

(MCL 1 or MCL 2)

Key:

No

Yes

Can BATs and 

SSCTs meet 

alternative MCLs?

Can new 

technologies 

identified by March 

2009 meet alternative 

MCLs?

Cannot 

achieve 

revised 

MCL. 

Can 

achieve 

revised 

MCL 

Identify 

potential 

BATs & 

SSCT s

Return to source 

(MCL 1 or MCL 2)

Can BATs and 

SSCTs meet 

alternative MCLs?

Can new 

technologies 

identified by March 

2009 meet alternative 

MCLs?

Cannot 

achieve 

revised 

MCL. 

Can 

achieve 

revised 

MCL 

Identify 

potential 

BATs & 

SSCT s

Return to source 

(MCL 1 or MCL 2)

Key:

No

Yes

3.8.1 Inputs to Treatment Technology Review 

The Treatment Technology Branch includes the following questions: 

• Can the BATs and small system compliance technologies (SSCTs) meet alternative MCLs? 

• Can new technologies identified by the cutoff date (March 1, 2009) meet alternative MCLs? 

For Six-Year Review 2, EPA limited its review of BATs to those NPDWRs for which it was 

considering possible revisions to the MCL based on the health effects or analytical feasibility 

reviews. To address both questions, OGWDW conducted a review of treatment performance 

studies for all technologies that are applicable for the contaminant in question. OGWDW used 

the same sources that it has relied on in the past to develop regulations and guidance, including 

published EPA treatment reports, peer-reviewed journals, and other sources of technology 

performance (e.g., pilot and demonstration project reports), as well as information received from 

EPA stakeholders. OGWDW evaluated whether these treatment studies indicate that current 

BATs are capable of achieving possibly lower MCLs and whether newer treatment technologies 

potentially meet BAT criteria. The document, “Water Treatment Technology Feasibility Support 

Document for Chemical Contaminants for the Second Six-Year Review of National Primary 

Drinking Water Regulations” (USEPA, 2009f), provides additional data and analysis details. 

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3.8.2 Output of Treatment Technology Review 

The output of the Treatment Technology Branch is a determination of whether treatment 

feasibility would pose a limitation to revising an MCL. The decision tree then returns the review 

to one of the MCL Branches for subsequent questions. 

3.9 Implementation Branch 

The purpose of the Implementation Branch (Exhibit 3-10) is to evaluate potential revisions 

pertaining to “other” regulatory requirements, such as monitoring and system reporting. 

Regulatory revisions to MCLs or TTs may affect the monitoring requirements for a contaminant 

and new health risk information may also warrant revisions. 

Exhibit 3-10. Implementation Branch 

Will the change affect 

monitoring/reporting 

requirements?

Will it affect 

monitoring/reporting 

requirements?

Take no 

action on 

other 

implementation issues. 

Consider 

revisions. 

End

Did MCL or TT 

change?

Is there new health risk 

information on 

reproductive or 

developmental toxicity?

Key:

No

Yes

Will the change affect 

monitoring/reporting 

requirements?

Will it affect 

monitoring/reporting 

requirements?

Take no 

action on 

other 

implementation issues. 

Consider 

revisions. 

End

Did MCL or TT 

change?

Is there new health risk 

information on 

reproductive or 

developmental toxicity?

Key:

No

Yes 

3.9.1 Inputs to Implementation Review 

The Implementation Branch requires information regarding whether a change in a contaminant’s 

MCL or TT, or new health effects information will affect the monitoring requirements for a 

particular contaminant. For Six-Year Review 2, EPA focused this review on issues that were not 

already being addressed through alternative mechanisms, such as a part of a recent or ongoing 

rulemaking. EPA also reviewed implementation-related NPDWR concerns that were “ready” for 

rulemaking – that is, the problem to be resolved had been clearly identified, along with specific 

options to address the problem, and shown to either clearly improve the level of public health 

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protection, or represent a meaningful opportunity for cost savings while maintaining the same 

level of public health protection. The report “Consideration of Other Regulatory Revisions in 

Support of the Second Six-Year Review of the National Primary Drinking Water Regulations” 

(USEPA, 2009g) provides a description of the stakeholder process that EPA used to identify 

inputs for the implementation review. 

3.9.2 Outputs from Implementation Review 

The output of the Implementation Branch is a determination regarding whether EPA should 

consider revisions to the monitoring requirements of an NPDWR. It is the final branch of the 

decision tree. 

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4 References 

USEPA. 2000. Science Policy Council Handbook: Peer Review, 2nd Edition. EPA Report 100-

B-00-001. Washington, DC: Office of Science Policy, Office of Research and Development. 

December 2000. Available on the Internet at: http://www.epa.gov/OSA/spc/pdfs/prhandbk.pdf. 

USEPA. 2003a. Analytical Feasibility Support Document for the Six-Year Review of Existing 

National Primary Drinking Water Regulations (Reassessment of Feasibility for Chemical 

Contaminants). EPA 815-R-03-003. March 2003. 

USEPA. 2003b. Occurrence Estimation Methodology and Occurrence Findings Report for the 

Six-Year Review of Existing National Primary Drinking Water Regulations. EPA 815-R-03-006. 

June 2003. 

USEPA. 2003c. EPA Protocol for Review of Existing National Primary Drinking Water 

Regulations. EPA Report 815-R-03-002. Washington, DC: Office of Ground Water and Drinking 

Water. June 2003. Available on the Internet at: 

http://www.epa.gov/safewater/standard/review/pdfs/support_6yr_protocal_final.pdf. 

USEPA. 2009a. Analysis of Occurrence Data from the Second Six-Year Review of Existing 

National Primary Drinking Water Regulations. EPA 815-B-09-006. 

USEPA. 2009b. Analytical Feasibility Support Document for the Second Six-Year Review of 

Existing National Primary Drinking Water Regulations. EPA 815-B-09-003. 

USEPA. 2009c. Development of Estimated Quantitation Levels for the Second Six-Year Review 

of National Primary Drinking Water Regulations. EPA 815-B-09-005. 

USEPA. 2009d. Occurrence Analysis for Potential Source Waters for the Second Six-Year 

Review of National Primary Drinking Water Regulations. EPA 815-B-09-004. 

USEPA. 2009e. Six-Year Review 2 – Health Effects Assessment – Summary Report. EPA 822-

R-09-006. 

USEPA. 2009f. Water Treatment Technology Feasibility Support Document for Chemical 

Contaminants for the Second Six-Year Review of National Primary Drinking Water Regulations. 

EPA 815-B-09-007. 

USEPA. 2009g. Consideration of Other Regulatory Revisions in Support of the Second Six-Year 

Review of the National Primary Drinking Water Regulations. EPA 815-B-09-008. 

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