[Federal Register Volume 82, Number 7 (Wednesday, January 11, 2017)]
[Proposed Rules]
[Pages 3518-3552]
[FR Doc No: 2016-31262]

Vol. 82

Wednesday,

No. 7

January 11, 2017

Part III

Environmental Protection Agency

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40 CFR Part 141

National Primary Drinking Water Regulations; Announcement of the
Results of EPA's Review of Existing Drinking Water Standards and
Request for Public Comment and/or Information on Related Issues;
Proposed Rule

Federal Register / Vol. 82 , No. 7 / Wednesday, January 11, 2017 /
Proposed Rules

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ENVIRONMENTAL PROTECTION AGENCY

[EPA-HQ-OW-2016-0627; FRL-9957-49-OW]

40 CFR Part 141

RIN 2040-ZA26

National Primary Drinking Water Regulations; Announcement of the
Results of EPA's Review of Existing Drinking Water Standards and
Request for Public Comment and/or Information on Related Issues

AGENCY: Environmental Protection Agency (EPA).

ACTION: Request for public comments.

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SUMMARY: The Safe Drinking Water Act (SDWA) requires the U.S.
Environmental Protection Agency (EPA) to conduct a review every six
years of existing national primary drinking water regulations (NPDWRs)
and determine which, if any, need to be revised. The purpose of the
review, called the Six-Year Review, is to evaluate current information
for regulated contaminants to determine if there is new information on
health effects, treatment technologies, analytical methods, occurrence
and exposure, implementation and/or other factors that provides a
health or technical basis to support a regulatory revision that will
improve or strengthen public health protection. EPA has completed a
detailed review of 76 NPDWRs and at this time has determined that eight
NPDWRs are candidates for regulatory revision. The eight NPDWRs are
included in the Stage 1 and the Stage 2 Disinfectants and Disinfection
Byproducts Rules, the Surface Water Treatment Rule, the Interim
Enhanced Surface Water Treatment Rule and the Long Term 1 Enhanced
Surface Water Treatment Rule. EPA requests comments on the eight NPDWRs
identified as candidates for revision and will consider comments and
data as it proceeds with determining whether further action is needed.
In addition, as part of this Six-Year Review, EPA identified 12 other
NPDWRs that were or continue to be addressed in recently completed,
ongoing or pending regulatory actions. EPA thus excluded those 12
NPDWRs from detailed review. This document is not a final regulatory
decision, but rather the initiation of a process that will involve more
detailed analyses of factors relevant to deciding whether a rulemaking
to revise an NPDWR should be initiated.

DATES: Comments must be received on or before March 13, 2017.

ADDRESSES: Submit your comments, identified by Docket ID No. EPA-HQ-OW-
2016-0627, to the Federal eRulemaking Portal: http://www.regulations.gov. Follow the online instructions for submitting
comments. Once submitted, comments cannot be edited or withdrawn. EPA
may publish any comment received to its public docket. Do not submit
electronically any information you consider to be Confidential Business
Information (CBI) or other information whose disclosure is restricted
by statute. Multimedia submissions (audio, video, etc.) must be
accompanied by a written comment. The written comment is considered the
official comment and should include discussion of all points you wish
to make. EPA will generally not consider comments or comment contents
located outside of the primary submission (i.e. on the web, cloud, or
other file sharing system). For additional submission methods, the full
EPA public comment policy, information about CBI or multimedia
submissions, and general guidance on making effective comments, please
visit http://www2.epa.gov/dockets/commenting-epa-dockets.
    Mail: Water Docket, Environmental Protection Agency, Mail code:
2822T, 1200 Pennsylvania Ave. NW., Washington, DC 20460.
    Hand Delivery: EPA Docket Center Public Reading Room, EPA
Headquarters West, Room 3334, 1301 Constitution Ave. NW., Washington,
DC. Hand deliveries are only accepted during the Docket's normal hours
of operation, and special arrangements should be made for deliveries of
boxed information.

FOR FURTHER INFORMATION CONTACT: For technical inquiries contact:
Richard Weisman,<PHONE_NUMBER>, or Kesha Forrest,<PHONE_NUMBER>,
Office of Ground Water and Drinking Water, Environmental Protection
Agency. For general information about the existing NPDWRs discussed in
this action, contact the Safe Drinking Water Hotline. Callers within
the United States may reach the Hotline at<PHONE_NUMBER>. The Hotline
is open Monday through Friday, excluding Federal holidays, from 10 a.m.
to 5:30 p.m. Eastern Time.

SUPPLEMENTARY INFORMATION:

Abbreviations and Acronyms Used in This Action

ADWR--Aircraft Drinking Water Rule
AGI--Acute Gastrointestinal Illness
AOC--Assimilable Organic Carbon
ASDWA--Association of State Drinking Water Administrators
ATSDR--Agency for Toxic Substances and Disease Registry
AWWA--American Water Works Association
BAT--Best Available Technology
CBI--Confidential Business Information
CDC--Centers for Disease Control and Prevention
CFR--Code of Federal Regulations
CT--Concentration x Contact Time
cVOCs--Carcinogenic Volatile Organic Compounds
CWS--Community Water System
DBCP--1,2-Dibromo-3-Chloropropane
DBP--Disinfection Byproducts
D/DBP--Disinfectants/Disinfection Byproducts
D/DBPR--Disinfectants/Disinfection Byproducts Rule
DEHA--Di(2-ethylhexyl)adipate
DEHP--Di(2-ethylhexyl)phthalate
DOC--Dissolved Organic Carbon
DPD--N,N-diethyl-p-phenylenediamine
EDB--Ethylene Dibromide
EJ--Environmental Justice
EO--Executive Order
EPA--U.S. Environmental Protection Agency
EQL--Estimated Quantitation Level
FAC--Federal Advisory Committee
FBRR--Filter Backwash Recycling Rule
FDA--U.S. Food and Drug Administration
FRN--Federal Register Notice
GAC--Granulated Activated Carbon
GWR--Ground Water Rule
GWUDI--Ground Water Under the Direct Influence of Surface Water
HAA5--Haloacetic Acids (five) (sum of monochloroacetic acid,
dichloroacetic acid, trichloroacetic acid, monobromoacetic acid and
dibromoacetic acid)
HAAs--Haloacetic Acids
HAV--Hepatitis A Virus
HPC--Heterotrophic Plate Count
IARC--International Agency for Research on Cancer
ICR--Information Collection Request
IESWTR--Interim Enhanced Surface Water Treatment Rule
IRIS--Integrated Risk Information System
LT1--Long-Term 1 Enhanced Surface Water Treatment Rule
LT2--Long-Term 2 Enhanced Surface Water Treatment Rule
MCL--Maximum Contaminant Level
MCLG--Maximum Contaminant Level Goal
MDBP--Microbial and Disinfection Byproducts
MDL--Method Detection Limit
MRDL--Maximum Residual Disinfectant Level
MRDLG--Maximum Residual Disinfectant Level Goal
MRL--Minimum Reporting Level
NAS--National Academy of Sciences
NCWS--Non-Community Water System
NDMA--N-Nitrosodimethylamine
NDWAC--National Drinking Water Advisory Council
NIH--National Institutes of Health
NPDWR--National Primary Drinking Water Regulation
NRC--National Research Council
NTNCWS--Non-Transient Non-Community Water System
NTP--National Toxicology Program
PCBs--Polychlorinated Biphenyls
PCE--Tetrachloroethylene
PHS--U.S. Public Health Service

PT--Proficiency Testing
PQL--Practical Quantitation Limit
PWS--Public Water System
qPCR--Quantitative Polymerase Chain Reaction
RfD--Reference Dose
RICP--Research and Information Collection Partnership
RSC--Relative Source Contribution
RTCR--Revised Total Coliform Rule
SDWA--Safe Drinking Water Act
SMCL--Secondary Maximum Contaminant Level
SOC--Synthetic Organic Chemical
SWTR--Surface Water Treatment Rule
SWTRs--Surface Water Treatment Rules (including SWTR, IESWTR and
LT1)
SYR--Six-Year Review
TCE--Trichloroethylene
TC/EC--Total Coliforms/E. coli
TCR--Total Coliform Rule
THM--Trihalomethanes
TTHM--Total Trihalomethanes (sum of four THMs: chloroform,
bromodichloromethane, dibromochloromethane and bromoform)
TNCWS--Transient Non-Community Water System
TOC--Total Organic Carbon
TT--Treatment Technique
UCFWR--Uncovered Finished Water Reservoirs
UCMR--Unregulated Contaminant Monitoring Rule
USGS--U.S. Geological Survey
UV--Ultraviolet
WBDOSS--Waterborne Disease Outbreak Surveillance System
WHO--World Health Organization

Table of Contents

I. General Information
    A. Does this action apply to me?
    B. What should I consider as I prepare my comments for EPA?
II. Statutory Requirements for the Six-Year Review
III. Stakeholder Involvement in the Six-Year Review Process
IV. Regulations Included in the Six-Year Review 3
V. EPA's Protocol for Reviewing the NPDWRs Included in This Action
    A. What was EPA's review process?
    B. How did EPA conduct the review of the NPDWRs?
    1. Initial Review
    2. Health Effects
    3. Analytical Feasibility
    4. Occurrence and Exposure Analysis
    5. Treatment Feasibility
    6. Risk-Balancing
    7. Other Regulatory Revisions
    C. How did EPA factor children's health concerns into the
review?
    D. How did EPA factor environmental justice concerns into the
review?
VI. Results of EPA's Review of NPDWRs
    A. What are the review result categories?
    1. The NPDWR is Not Appropriate for Revision at This Time
    2. The NPDWR is a Candidate for Revision
    B. What are the detailed results of EPA's third six-year review
cycle?
    1. Chemical Phase Rules/Radionuclides Rules
    2. Fluoride
    3. Disinfectants/Disinfection Byproducts Rules (D/DBPRs)
    4. Microbial Contaminants Regulations
VII. EPA's Request for Comments
References

I. General Information

A. Does this action apply to me?

    This action itself does not impose any requirements on individual
people or entities. Instead, it notifies interested parties of EPA's
review of existing national primary drinking water regulations (NPDWRs)
and its conclusions about which of these NPDWRs may warrant new
regulatory action at this time. EPA requests public comment on the
eight NPDWRs identified as candidates for revision. EPA will consider
comments received as the Agency moves forward with determining whether
regulatory actions are necessary for the eight NPDWRs.

B. What should I consider as I prepare my comments for EPA?

    Please see Section VII for the topic areas related to this document
for which EPA requests comment and/or information. EPA will accept
written or electronic comments (please do not send both). Instructions
for submitting comments can be found in the ADDRESSES section of this
document. EPA prefers electronic comments. No facsimiles (faxes) will
be accepted. Commenters who want EPA to acknowledge receipt of their
written comments should also send a self-addressed, stamped envelope.
    You may find the following suggestions helpful when preparing your
comments:
     Explain your views as clearly as possible.
     Describe any assumptions that you used.
     Provide any technical information and/or data you used
that support your views.
     If you estimate potential burden or costs, explain how you
arrived at your estimate.
     Provide specific examples to illustrate your concerns.
     Offer alternatives.
     Make sure to submit your comments by the comment period
deadline.
    To ensure proper receipt by EPA, identify the appropriate docket
identification number in the subject line on the first page of your
response. It would also be helpful if you provide the name, date, and
volume/page numbers of the Federal Register document you are commenting
on.

II. Six-Year Review--Statutory Requirements and Next Steps

    Under the Safe Drinking Water Act (SDWA), as amended in 1996, EPA
must periodically review existing NPDWRs and, if appropriate, revise
them. Section 1412(b)(9) of the SDWA states: ``The Administrator shall,
not less often than every six 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.''
    Pursuant to the 1996 SDWA Amendments, EPA completed and published
the results of its first Six-Year Review (Six-Year Review 1) on July
18, 2003 (68 FR 42908, USEPA, 2003b) and the second Six-Year Review
(Six-Year Review 2) on March 29, 2010 (75 FR 15500, USEPA, 2010h),
after developing a systematic approach, or protocol, for the review of
NPDWRs.
    In this document EPA is announcing the results of the third Six-
Year Review (Six-Year Review 3). Consistent with the process applied in
the Six-Year Review 2, EPA is requesting comments on this document and
will consider the public comments and/or any new, relevant data
submitted for the eight NPDWRs listed as candidates for revision as the
Agency proceeds with determining whether revisions of these regulations
are necessary. The announcement whether or not the Agency intends to
revise an NPDWR (pursuant to SDWA Sec.  1412(b)(9)) is not a regulatory
decision. Instead, it initiates a process that will involve more
detailed analyses of health effects, analytical and treatment
feasibility, occurrence, benefits, costs and other regulatory matters
relevant to deciding whether a rulemaking to revise an NPDWR should be
initiated. The Six-Year Review results do not obligate the Agency to
revise an NPDWR in the event that EPA determines during the regulatory
process that revisions are no longer appropriate and discontinues
further efforts to revise the NPDWR. Similarly, the fact that an NPDWR
has not been selected for revision means only that EPA believes that
regulatory changes to a particular NPDWR are not appropriate at this
time for the reasons given in this action; future reviews may identify
information that leads to an initiation of the revision process.
    The reasons that EPA has identified an NPDWR as a ``candidate for
revision''

is that, at a minimum, the revision 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.

III. Stakeholder Involvement in the Six-Year Review Process

    The Agency has involved interested stakeholders in the Six-Year
Review 3 process. Below are examples of such involvement:

     In November 2014, EPA briefed the National Drinking
Water Advisory Council (NDWAC) on the Six-Year Review protocol and
the key elements of that protocol as they relate to the microbial
and disinfection byproducts (MDBP) rules. The briefing included
information on how EPA is implementing NDWAC's previous
recommendations (NDWAC, 2000) on the Six-Year Review process in
review of the MDBP rules;
     In January 2015, states provided input (through the
Association of State Drinking Water Administrators (ASDWA)) on rule
implementation issues related to the NPDWRs being reviewed as part
of the Six-Year Review 3 (ASDWA, 2016);
     EPA initiated a series of public stakeholder meetings
about the review of the Long Term 2 Enhanced Surface Water Treatment
Rule (LT2). These meetings were held in accordance with the
recommendation of the MDBP Federal Advisory Committee (FAC) \1\ to
have public meetings following the first round of monitoring under
the LT2, and as a result of the Executive Order (E.O.) 13563
``Improving Regulation and Regulatory Review.'' \2\ E.O. 13563
states that regulations shall be based ``on the open exchange of
information and perspectives among state, local, and tribal
officials, experts in relevant disciplines, affected stakeholders in
the private sector, and the public as a whole.'' Some affected
stakeholders recommended that EPA include the LT2 among the Agency's
top priorities for review under E.O. 13563. EPA included the LT2 in
its ``Improving our Regulations: Final Plan for Periodic
Retrospective Review of Existing Regulations'' (USEPA, 2011). EPA
agreed to ``assess and analyze new data/information regarding
occurrence, treatment, analytical methods, health effects, and risk
from all relevant waterborne pathogens to evaluate whether there are
new or additional ways to manage risk while assuring equivalent or
improved protection, including with respect to the covering of
finished water reservoirs'' (USEPA, 2011). EPA hosted three public
meetings in Washington, DC, on December 7, 2011, April 24, 2012 and
November 15, 2012. EPA presented information about: The LT2
requirements, monitoring data collected under the LT2, analytical
methods, forecasts about the second round of monitoring and the
treatment technique requirements. In addition to presentations to
educate the public, the meetings included public statements, panel
discussions, question and answer sessions and requests by EPA to
provide data and information about the implementation of the LT2 to
inform the regulatory review.
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    \1\ https://www.epa.gov/sites/production/files/2015-11/documents/stage_2_m-dbp_agreement_in_principle.pdf.
    \2\ E.O. 13563 requires federal agencies to ``consider how best
to promote retrospective analysis of rules that may be outmoded,
ineffective, insufficient, or excessively burdensome, and to modify,
streamline, expand, or repeal them in accordance with what has been
learned.'' The order required each federal agency to develop a plan
``consistent with law and its resources and regulatory priorities.''
https://www.gpo.gov/fdsys/pkg/FR-2011-01-21/pdf/2011-1385.pdf.
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IV. Regulations Included in the Six-Year Review 3

    Table IV-1 lists all 88 NPDWRs established to date. The table also
reports the maximum contaminant level goal (MCLG) and the maximum
contaminant level (MCL). The MCLG is ``set 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'' (SDWA Sec.  1412(b)(4)).
The MCL is the maximum permissible level of a contaminant in water
delivered to any user of a public water system (PWS) and generally ``is
as close to the maximum contaminant level goal as is feasible'' (SDWA
Sec.  1412(b)(4)(B)).\3\ Where it is not ``economically or technically
feasible'' to set an MCL, EPA can establish a treatment technique (TT),
which must prevent adverse health effects ``to the extent feasible''
(SDWA Sec.  1412(b)(7)(A)). In the case of disinfectants (e.g.,
chlorine, chloramines and chlorine dioxide), the values reported in the
table are not MCLGs and MCLs, but maximum residual disinfectant level
goals (MRDLGs) and maximum residual disinfectant levels (MRDLs).
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    \3\ Under limited circumstances, SDWA Sec.  1412(b)(6)(A) also
gives the Administrator the discretion to promulgate an MCL that is
less stringent than the feasible level and that ``maximizes health
risk reduction benefits at a cost that is justified by the
benefits.''
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    Table IV-1 also includes NPDWRs that EPA identified as candidates
for revision in past Six-Year Reviews. During the Six-Year Review 1,
EPA identified the Total Coliform Rule (TCR) as a candidate for
revision.\4\ EPA published the Revised Total Coliform Rule (RTCR) in
2013 (78 FR 10270, USEPA, 2013a). Four additional NPDWRs for
acrylamide, epichlorohydrin, tetrachloroethylene (PCE) and
trichloroethylene (TCE) were identified as candidates for revision
during the Six-Year Review 2. Of the 88 NPDWRs, EPA identified 12 as
part of recently completed, ongoing or pending regulatory actions; as a
result, these 12 are not subject to a detailed review for the Six-Year
Review 3. This action involves the remaining 76 NPDWRs. EPA applied the
same protocol used for previous Six-Year Reviews, with minor
clarifications (USEPA, 2016f), to the Six-Year Review 3 process.
Section V of this action describes the revised protocol used for the
Six-Year Review 3 and Section VI describes the results of the review of
the NPDWRs.
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    \4\ The NPDWRs apply to specific contaminants/parameters or
groups of contaminants. Historically, when issuing new or revised
standards for these contaminants/parameters, EPA has often grouped
the standards together in more general regulations, such as the
Total Coliform Rule, the Surface Water Treatment Rule or the Phase V
rules. In this action, however, for clarity, EPA discusses the
drinking water standards as they apply to each specific regulated
contaminant/parameter (or group of contaminants), not the more
general regulation in which the contaminant/parameter was regulated.
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    In addition to the regulated chemicals, radiological and
microbiological contaminants included in the previous reviews, this
document also includes the review of the MDBP regulations that were
promulgated under the following actions: The Ground Water Rule (GWR);
the Surface Water Treatment Rules (SWTRs); the Disinfectants and
Disinfection Byproducts (D/DBP) Rules; and the Filter Backwash
Recycling Rule (FBRR). EPA reviewed the LT2 in response to EO 13563
(USEPA, 2011) and as part of the Six-Year Review 3 process.

                                                    Table IV-1--NPDWRs Included in Six-Year Review 3
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                                                               MCL or TT (mg/L) \1\ \2\     Contaminants/                           MCL or TT (mg/L) \2\
    Contaminants/parameters         MCLG (mg/L) \1\ \3\                  \3\                 parameters      MCLG (mg/L) \1\ \3\            \3\
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Acrylamide....................  0..........................  TT.........................  Ethylbenzene....  0.7..................  0.7
Alachlor......................  0..........................  0.002......................  Ethylene          0....................  0.00005
                                                                                dibromide (EDB).
Alpha/photon emitters.........  0 (pCi/L)..................  15 (pCi/L).................  Fluoride........  4.0..................  4.0
Antimony......................  0.006......................  0.006......................  Giardia lamblia   0....................  TT
                                                                                \4\.
Arsenic.......................  0..........................  0.010......................  Glyphosate......  0.7..................  0.7

Asbestos......................  7 (million fibers/L).......  7 (million fibers/L).......  Haloacetic acids  n/a \5\..............  0.060
                                                                                (HAA5).
Atrazine......................  0.003......................  0.003......................  Heptachlor......  0....................  0.0004
Barium........................  2..........................  2..........................  Heptachlor        0....................  0.0002
                                                                                epoxide.
Benzene.......................  0..........................  0.005......................  Heterotrophic     n/a..................  TT
                                                                                bacteria \6\.
Benzo[a]pyrene................  0..........................  0.0002.....................  Hexachlorobenzen  0....................  0.001
                                                                                e.
Beryllium.....................  0.004......................  0.004......................  Hexachlorocyclop  0.05.................  0.05
                                                                                entadiene.
Beta/photon emitters..........  0 (millirems/yr)...........  4 (millirems/yr)...........  Lead............  0....................  TT
Bromate.......................  0..........................  0.010......................  Legionella......  0....................  TT
Cadmium.......................  0.005......................  0.005......................  Lindane.........  0.0002...............  0.0002
Carbofuran....................  0.04.......................  0.04.......................  Mercury           0.002................  0.002
                                                                                (inorganic).
Carbon tetrachloride..........  0..........................  0.005......................  Methoxychlor....  0.04.................  0.04
Chloramines...................  4..........................  4.0........................  Monochlorobenzen  0.1..................  0.1
                                                                                e
                                                                                (Chlorobenzene).
Chlordane.....................  0..........................  0.002......................  Nitrate (as N)..  10...................  10
Chlorine......................  4..........................  4.0........................  Nitrite (as N)..  1....................  1
Chlorine dioxide..............  0.8........................  0.8........................  Oxamyl (Vydate).  0.2..................  0.2
Chlorite......................  0.8........................  1.0........................  Pentachloropheno  0....................  0.001
                                                                                l.
Chromium (total)..............  0.1........................  0.1........................  Picloram........  0.5..................  0.5
Copper........................  1.3........................  TT.........................  Polychlorinated   0....................  0.0005
                                                                                biphenyls
                                                                                (PCBs).
Cryptosporidium...............  0..........................  TT.........................  Radium..........  0 (pCi/L)............  5 (pCi/L)
Cyanide.......................  0.2........................  0.2........................  Selenium........  0.05.................  0.05
2,4-Dichlorophenoxyacetic acid  0.07.......................  0.07.......................  Simazine........  0.004................  0.004
 (2,4-D).
Dalapon.......................  0.2........................  0.2........................  Styrene.........  0.1..................  0.1
Di(2-ethylhexyl)adipate (DEHA)  0.4........................  0.4........................  2,3,7,8-TCDD      0....................  3.00E-08
                                                                                (Dioxin).
Di(2-ethylhexyl)phthalate       0..........................  0.006......................  Tetrachloroethyl  0....................  0.005
 (DEHP).                                                                                ene.
1,2-Dibromo-3-chloropropane     0..........................  0.0002.....................  Thallium........  0.0005...............  0.002
 (DBCP).
1,2-Dichlorobenzene (o-         0.6........................  0.6........................  Toluene.........  1....................  1
 Dichlorobenzene).
1,4-Dichlorobenzene (p-         0.075......................  0.075......................  Total coliforms   n/a..................  TT
 Dichlorobenzene).                                                                         (under ADWR \7\
                                                                                and RTCR \8\).
1,2-Dichloroethane (Ethylene    0..........................  0.005......................  Total             n/a \9\..............  0.080
 dichloride).                                                                              Trihalomethanes
                                                                                (TTHM).
1,1-Dichloroethylene..........  0.007......................  0.007......................  Toxaphene.......  0....................  0.003
cis-1,2-Dichloroethylene......  0.07.......................  0.07.......................  2,4,5-TP          0.05.................  0.05
                                                                                (Silvex).
trans-1,2-Dichloroethylene....  0.1........................  0.1........................  1,2,4-            0.07.................  0.07
                                                                                Trichlorobenzen
                                                                                e.
Dichloromethane (Methylene      0..........................  0.005......................  1,1,1-            0.20.................  0.2
 chloride).                                                                                Trichloroethane.
1,2-Dichloropropane...........  0..........................  0.005......................  1,1,2-            0.003................  0.005
                                                                                Trichloroethane.
Dinoseb.......................  0.007......................  0.007......................  Trichloroethylen  0....................  0.005
                                                                                e.
Diquat........................  0.02.......................  0.02.......................  Turbidity \6\...  n/a..................  TT
E. coli.......................  0..........................  MCL \10\ and TT \8\........  Uranium.........  0....................  0.030
Endothall.....................  0.1........................  0.1........................  Vinyl Chloride..  0....................  0.002
Endrin........................  0.002......................  0.002......................  Viruses.........  0....................  TT
Epichlorohydrin...............  0..........................  TT.........................  Xylenes (total).  10...................  10
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. MCLG: The maximum level of a contaminant in drinking water at which no known or anticipated adverse effect on the health of persons would occur,
  allowing an adequate margin of safety.
2. MCL: The maximum level allowed of a contaminant in water which is delivered to any user of a public water system.
TT: An enforceable procedure or level of technological performance which public water systems must follow to ensure control of a contaminant.
3. Units are in milligrams per liter (mg/L) unless otherwise noted. Milligrams per liter are equivalent to parts per million. For chlorine, chloramines
  and chlorine dioxide, values presented are MRDLG and MRDL.
4. The current preferred taxonomic name is Giardia duodenalis, with Giardia lamblia and Giardia intestinalis as synonymous names. However, Giardia
  lamblia was the name used to establish the MCLG in 1989. Elsewhere in this document, this pathogen will be referred to as Giardia spp. or simply
  Giardia unless discussing information on an individual species.
5. 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.
6. Includes indicators that are used in lieu of direct measurements (e.g., of heterotrophic bacteria, turbidity).
7. The Aircraft Drinking Water Rule (ADWR) 40 CFR part 141 Subpart X, promulgated October 19, 2009, covers total coliforms.
8. Under the RTCR, a PWS is required to conduct an assessment if it exceeded any of the TT triggers identified in 40 CFR 141.859(a). It is also required
  to correct any sanitary defects found through the assessment.
9. There is no MCLG for total trihalomethanes (TTHM). MCLGs for some of the individual contaminants are: Bromodichloromethane (zero), bromoform (zero),
  dibromochloromethane (0.06 mg/L), and chloroform (0.07 mg/L).
10. A PWS is in compliance with the E. coli MCL unless any of the conditions identified under 40 CFR 141.63(c) occur.

V. EPA's Protocol for Reviewing the NPDWRs Included in This Action

A. What was EPA's review process?

Overview
    This section provides an overview of the process the Agency used to
review the NPDWRs discussed in this action. The protocol document,
``EPA Protocol for the Third Review of Existing National Primary
Drinking Water Regulations,'' contains a detailed description of the
process the Agency used to review the NPDWRs (USEPA, 2016f). The
foundation of this protocol was developed for the Six-Year Review 1
based on the recommendations of the NDWAC (2000). The Six-Year Review 3
process is very similar to the process implemented during the Six-Year
Review 1 and the Six-Year Review 2, with some clarifications to the
elements related to the review of NPDWRs included in the MDBP rules.
Figure V-1 presents an overview of the Six-Year review protocol and
review outcomes.
    The primary goal of the Six-Year Review process is to identify and
prioritize NPDWRs for possible regulatory revision. The two major
outcomes of the detailed review are either:
    1. The NPDWR is not appropriate for revision and no action is
necessary at this time.
    2. The NPDWR is a candidate for revision.
    The reasons for a Six-Year Review outcome of ``not appropriate for
revision at this time'' can include:
     Regulatory action--recently completed, ongoing or pending.
The NPDWR was recently completed, is being reviewed in an ongoing
action, or is subject to a pending action.
     Ongoing or planned health effects assessment. The NPDWR
has an ongoing health effects assessment (i.e., especially for those
NPDWRs with an MCL set at the MCLG or where the MCL is based on the
SDWA cost benefit provision), or EPA is considering whether a new
health effects assessment is needed.
     No new information. EPA did not identify any new, relevant
information that indicates changes to the NPDWR.
     Data gaps/emerging information. There are data gaps or
emerging information that need to be evaluated.
     Low priority and/or no meaningful opportunity. New
information indicates a possible change to the MCLG and/or MCL but
changes to the NPDWR are not warranted due to one or more of the
following reasons: (1) Possible changes present negligible gains in
public health protection; (2) possible changes present limited
opportunity for cost savings while maintaining the same or greater
level of health protection; and (3) possible changes are a low priority
because of competing workload priorities, limited return on the
administrative costs associated with rulemaking and the burden on
states and the regulated community associated with implementing any
regulatory change that would result.
    Alternatively, the reasons for a Six-Year Review outcome that an
NPDWR is a ``candidate for revision'' are that, at a minimum, the
revision 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.
    Individual regulatory provisions of NPDWRs that are evaluated as
part of the Six-Year Review are: MCLG, MCL, MRDLG, MRDL, TT, other
treatment technologies such as best available technology (BAT), and
regulatory requirements, such as monitoring requirements.
    For example, the microbial regulations include TT requirements
because there is no reliable method that is economically and
technically feasible to measure the microbial contaminants covered by
those regulations. These TT requirements rely on the use of indicators
that can be measured in drinking water, such as the concentration of a
disinfectant, to provide public health protection. As part of the Six-
Year Review 3, EPA evaluated new information related to the use of
those indicators to determine if there is a meaningful opportunity to
improve the level of public health protection. Results of EPA's review
of the MDBP regulations are presented in Sections VI.B.3 and VI.B.4.
    For the purpose of this document (except where noted for clarity),
discussions of the review of MCLGs and MCLs should be assumed to also
apply to the review of MRDLGs and MRDLs for disinfectants.
Basic Principles
    EPA applied a number of basic principles to the Six-Year Review
process:
     The Agency sought to avoid redundant review efforts.
Because EPA has reviewed information for certain NPDWRs as part of
recently completed, ongoing or pending regulatory actions, these NPDWRs
are not subject to the detailed review in this document.
     The Agency does not believe it is appropriate to consider
revisions to NPDWRs for contaminants with an ongoing or planned health
effect assessment and for which the MCL is set equal to the MCLG or
based on benefit-cost 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 MCL.
Therefore, EPA noted that these NPDWRs are not appropriate for revision
and no action is necessary at this time if the health effects
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' cost-benefit provision. If
the health effects assessment is completed before the next Six-Year
Review, EPA will consider these NPDWRs at that time.
     In evaluating the potential for new information to affect
NPDWRs, EPA assumed no change to existing policies and procedures for
developing NPDWRs. For example, in determining whether new information
affected the feasibility of analytical methods for a contaminant, the
Agency assumed no change to current policies and procedures for
calculating practical quantitation levels.
     EPA considered new information from health effects
assessments that were completed by the information cutoff date.
Assessments completed after this cutoff date will be reviewed by EPA
during the next review cycle or (if applicable) during the revision of
an NPDWR. The information cutoff date for the Six-Year Review 3 was
December 2015.
     During the review, EPA identified areas where information
is inadequate or unavailable (data gaps) or emerging and is needed to
determine whether revision to an NPDWR is appropriate. To the extent
EPA is able to fill data gaps or fully evaluate the emerging
information, the Agency will consider the information as part of the
next review cycle.
     EPA may consider accelerating review and potential
revision for a particular NPDWR before the next review cycle when
justified by new public health risk information.
     Finally, EPA assured scientific analyses supporting the
review were consistent with the Agency's peer review policy (USEPA,
2015a).

[GRAPHIC] [TIFF OMITTED] TP11JA17.006

B. How did EPA conduct the review of the NPDWRs?

    The protocol for the Six-Year Review 3 is broken down into a series
of questions that can inform a decision about the appropriateness of
revising an NPDWR. These questions are logically ordered into a
decision tree. This section provides an overview of each of the review
elements that EPA considered for each NPDWR during the Six-Year Review
3, including the following: Initial review, health effects, analytical
feasibility, occurrence and exposure, treatment feasibility, risk
balancing and other regulatory revisions. The final review combines the
findings from all of these review elements to recommend whether an
NPDWR is a candidate for revision. Further information about the review
elements is described in the protocol document (USEPA, 2016f). Results
from the review of these elements are presented in Section VI.
1. Initial Review
    EPA's initial review of all the contaminants included in the Six-
Year Review 3 involved a simple identification of the NPDWRs that have
either been recently completed, or are being reviewed in an ongoing or
pending action since the last Six-Year Review (cutoff date was August
2008). In addition, the initial review also identified contaminants
with ongoing health effects assessments that have an MCL equal to the
MCLG. Excluding such contaminants from the Six-Year Review 3 prevents
duplicative agency efforts.
2. Health Effects
    The principal objectives of the health effects review are to
identify: (1) Contaminants for which a new health effects assessment
indicates that a change in the MCLG might be appropriate (e.g., because
of a change in cancer classification or a change in reference dose
(RfD)), and (2) contaminants for which new health effects information
indicates a need to initiate a new health effects assessment.
    To meet the first objective, EPA reviewed the results of health
effects assessments completed before December 2015, the information
cutoff date for the Six-Year Review 3.
    To meet the second objective, the Agency conducted an extensive
literature review to identify peer-reviewed studies published before
December 2015. The Agency reviewed the studies to determine whether
there was new health effects information, such as reproductive and
developmental toxicity data, that could potentially affect the MCLG, or
otherwise change the Agency's understanding of the health effects of
contaminants under consideration. EPA then evaluated the need to plan
the initiation of a new health effects assessment.
3. Analytical Feasibility
    When establishing an NPDWR, EPA identifies a practical quantitation
limit (PQL), which is ``the lowest achievable level of analytical
quantitation during routine laboratory operating conditions within
specified limits of precision and accuracy'', as noted in the November
13, 1985, Federal Register proposed rule (50 FR 46880, USEPA, 1985).
EPA has a separate process in place to approve new analytical methods
for drinking water contaminants; therefore, review and approval of
potential new methods is outside the scope of the Six-Year Review
protocol. EPA recognizes, however, that the approval and adoption in
recent years of new and/or improved analytical methods may enable
laboratories to quantify contaminants at lower levels than was possible
when NPDWRs were originally promulgated. This ability of laboratories
to measure a contaminant at lower levels could affect its PQL, the
value at which an MCL is set when it is limited

by analytical feasibility. Therefore, the Six-Year Review process
includes an examination of whether there have been changes in
analytical feasibility that could possibly change the PQL for the
subset of the NPDWRs that reached this stage of the review.
    To determine if changes in analytical feasibility could possibly
support changes to PQLs, EPA relied primarily on two alternate
approaches to develop an estimated quantitation limit (EQL): an
approach based on the minimum reporting levels (MRLs) obtained as part
of the Six-Year Review 3 Information Collection Request (ICR), and an
approach based on method detection limits (MDLs).
    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. The MRL values provide direct evidence from actual
monitoring results about whether quantitation below the PQL using
current analytical methods is feasible. An MDL is a measure of
analytical method sensitivity. MDLs have been used in the past to
derive PQLs for regulated contaminants.
    EPA used the EQL as a threshold for occurrence analysis to help the
Agency determine if there may be a meaningful opportunity to improve
public health protection. It should be noted, however, that the use of
an EQL does not necessarily indicate the Agency's intention to
promulgate a new PQL. Any revision to PQLs will be part of future
rulemaking efforts if EPA has determined that an NPDWR is a candidate
for revision.
4. Occurrence and Exposure Analysis
    The occurrence and exposure analysis is conducted in conjunction
with other review elements to determine if there is a meaningful
opportunity to revise an NPDWR by:
     Estimating the extent of contaminant occurrence, i.e., the
number of PWSs in which contaminants occur at levels of interest
(health-effects-based thresholds or analytical method limits), and
     Evaluating the number of people potentially exposed to
contaminants at these levels.
    To evaluate national contaminant occurrence under the Six-Year
Review 3, EPA reviewed data from the Six-Year Review 3 ICR database
(SYR3 ICR database), the UCMR datasets (USEPA, 2016j) and other
relevant sources.
    For the Six-Year Review 3, EPA collected SDWA compliance monitoring
data through use of an ICR (75 FR 6023, USEPA, 2010a). EPA requested
that all states and primacy entities (tribes and territories)
voluntarily submit their compliance monitoring data for regulated
contaminants in public drinking water systems. Specifically, EPA
requested the submission of compliance monitoring data and related
information collected between January 2006 and December 2011 for
regulated contaminants and related parameters (e.g., water quality
indicators). Forty-six states plus eight primacy agencies provided
data. The assembled data constitute the largest, most comprehensive set
of drinking water compliance monitoring data ever compiled and analyzed
by EPA to inform decision making, containing almost 47 million records
from approximately 167,000 PWSs, serving approximately 290 million
people nationally. Through extensive data management efforts, quality
assurance evaluations, and communications with state data management
staff, EPA established the SYR3 ICR database (USEPA, 2016i). The number
of states and PWSs represented in the dataset varies across
contaminants because of variability in state data submissions and
contaminant monitoring schedules. Except as noted in Section VI, EPA
believes that these data are of sufficient quality to inform an
understanding of the national occurrence of regulated contaminants and
related parameters. Details of the data management and data quality
assurance evaluations are available in the supporting document (USEPA,
2016q). The resulting database is available online on the Six-Year
Review Web site (https://www.epa.gov/dwsixyearreview).
5. Treatment Feasibility
    An NPDWR either identifies the BAT for meeting an MCL, or
establishes enforceable TT requirements. EPA reviews treatment
feasibility to ascertain if there are technologies that meet BAT
criteria for a hypothetical more stringent MCL, or if there is new
information that demonstrates an opportunity to improve public health
protection through revision of an NPDWR TT requirement.
    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. The result of this review element is a
determination of whether treatment feasibility would pose a limitation
to revising an MCL or provide an opportunity to revise the TT
requirement.
6. Risk-Balancing
    EPA reviews risk-balancing to examine how the Six-Year Review can
address tradeoffs in risks among different NPDWRs and take into account
unregulated contaminants as well. Under this review, EPA considers
whether a change to an MCL and/or TT will increase the public health
risk posed by one or more contaminants, and, if so, the Agency
considers revisions that will balance overall risks. This review
element is relevant only to the NPDWRs included in the MDBP rules,
which were promulgated to address risk-balancing between microbial and
DBP requirements, and among differing types of DBPs. The risk-balancing
approach was based on the SDWA requirements that EPA ``minimize the
overall risk of adverse health effects by balancing the risk from the
contaminant and the risk from other contaminants the concentrations of
which may be affected by the use of a TT or process that would be
employed to attain the maximum contaminant level or levels'' (SDWA
Sec.  1412(b)(5)(B)(i)).
    EPA reviewed risk-balancing between microbial and DBP contaminants.
For example, EPA considered the potential impact on DBP concentrations
should there be a consideration to increase the stringency of microbial
NPDWRs. This approach also was used during the development of more
recent MDBP rules such as the LT2 rule and the Stage 2 Disinfectants/
Disinfection Byproducts Rule (D/DBPR) rule. In addition, EPA reviewed
risk-balancing between different types of DBP contaminants. Depending
on the stringency of potential DBP regulations, compliance strategies
used by the regulated community might have the effect of increasing the
concentrations of other types of contaminants, both regulated and
unregulated. EPA considered these potential compliance strategies when
conducting its Six-Year Review 3 with a goal to balance the overall
health risks.
7. Other Regulatory Revisions
    In addition to possible revisions to MCLGs, MCLs and TTs, EPA
evaluated whether other revisions are needed to regulatory provisions,
such as monitoring and system reporting requirements. EPA focused this
review element on issues that were not already being addressed through
alternative mechanisms, such as a recently completed, ongoing or
pending regulatory action. 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 that could be shown to either clearly improve the
level of public health protection, or represent a meaningful
opportunity for achieving cost savings while maintaining the same level
of public health protection. The result of this review element is a
determination regarding whether EPA should consider revisions to the
monitoring and/or reporting requirements of an NPDWR.

C. How did EPA factor children's health concerns into the review?

    The 1996 amendments to SDWA require special consideration of
sensitive life stages and populations (e.g., infants, children,
pregnant women, elderly and individuals with a history of serious
illness) in the development of drinking water regulations (SDWA Sec.
1412(b)(3)(C)(V)). As a part of the Six-Year Review 3, EPA completed a
literature search covering developmental and reproductive endpoints
(e.g., fertility, embryo survival, developmental delays, birth defects
and endocrine effects) for information published as of December 2015
for regulated chemicals that had not been the subject of a health
effects assessment during this review period. EPA reviewed the results
of the literature searches to identify any studies that might suggest a
need to revise MCLGs. These studies were considered in EPA's review of
NPDWRs, which is discussed in Section VI.

D. How did EPA factor environmental justice concerns into the review?

    Executive Order (E.O.) 12898, ``Federal Actions to Address
Environmental Justice in Minority Populations or Low-Income
Populations,'' establishes a federal policy for incorporating
environmental justice (EJ) into federal agency missions by directing
agencies to identify and address disproportionately high and adverse
human health or environmental effects of its programs, policies and
activities on minority and low-income populations. EPA evaluates
potential EJ concerns when developing regulations. This Six-Year Review
was developed in compliance with E.O. 12898. Should the Six-Year Review
lead to a decision to revise an NPDWR, any subsequent rulemakings will
include an EJ component and an opportunity for public comment.

VI. Results of EPA's Review of NPDWRs

    Table VI-1 lists the results of EPA's review for each of the 76
NPDWRs discussed in this section of this action, along with the
principal rationale for the review outcomes. Table VI-1 also includes a
list of the 12 NPDWRs that have been recently completed, or have
ongoing or pending regulatory actions.

                                Table VI-1--Summary of Six-Year Review 3 Results
----------------------------------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------------------------------
Not Appropriate for Revision at     Recently completed,    1,2-Dichloroethane           E. coli.
 this Time.                          ongoing or pending     (Ethylene dichloride).      Lead.
                                     regulatory action.    1,2-Dichloropropane........  Tetrachloroethylene
                                                           Benzene....................   (PCE).
                                                           Carbon Tetrachloride.......  Total coliforms (under
                                                                                ADWR and RTCR).
                                                           Copper                       Trichloroethylene (TCE)
                                                           Dichloromethane (Methylene   Vinyl chloride.
                                                            chloride).
Not Appropriate for Revision at     Health effects         Alpha/photon emitters......  Mercury \1\
 this Time \2\.                      assessment in         Arsenic....................  Nitrate \1\
                                     process (as of        Atrazine...................  Nitrite \1\
                                     December 2015) or     Benzo(a)pyrene (PAHs)......  o-Dichlorobenzene \1\
                                     contaminant           Beta/photon emitters.......  p-Dichlorobenzene \1\
                                     nominated for health  Cadmium \1\................  Polychlorinated
                                     assessment.           Chromium...................   biphenyls (PCBs).
                                                           Di(2-ethylhexyl) phthalate   Radium.
                                                            (DEHP) \1\.                 Simazine.
                                                           Ethylbenzene...............  Uranium \1\
                                                           Glyphosate.................
                                    No new information,    1,2-Dibromo-3-chloropropane  Dalapon.
                                     NPDWR remains          (DBCP).                     Di(2-ethylhexyl)adipate
                                     appropriate after     2,4,5-TP (Silvex)..........   (DEHA).
                                     review.               Antimony...................  Dinoseb.
                                                           Asbestos...................  Endrin.
                                                           Bromate....................  Ethylene dibromide.
                                                           Chloramines (under D/DBPR).  Pentachlorophenol.
                                                           Chlorine (under D/DBPR)....  Thallium.
                                                           Chlorine dioxide...........  trans-1,2-
                                                           Chlorobenzene                 Dichloroethylene.
                                                            (monochlorobenzene).        Turbidity.
                                    Low priority and/or    1,1,1-Trichloroethane......  Epichlorohydrin.
                                     no meaningful         1,1,2-Trichloroethane......  Fluoride.
                                     opportunity.          1,1-Dichloroethylene.......  Heptachlor.
                                                           1,2,4-Trichlorobenzene.....  Heptachlor epoxide.
                                                           2,3,7,8-TCDD (Dioxin)......  Hexachlorobenzene.
                                                           2,4-D......................  Hexachlorocyclopentadien
                                                           Acrylamide.................   e.
                                                           Alachlor...................  Lindane.
                                                                                Methoxychlor.
                                                           Barium                       Oxamyl (Vydate).
                                                           Beryllium..................  Picloram.
                                                           Carbofuran.................  Selenium.
                                                           Chlordane..................  Styrene.
                                                           cis-1,2-Dichloroethylene...  Toluene.
                                                           Cyanide....................  Toxaphene.
                                                           Diquat.....................  Xylenes.
                                                           Endothall..................

Candidate for Revision............  New information......  Chlorite...................  Heterotrophic Bacteria.
                                                           Cryptosporidium (under       Legionella.
                                                            SWTR, IESWTR, LT1).         TTHM.
                                                           Giardia lamblia............  Viruses (under SWTR).
                                                           Haloacetic Acids (HAA5)....
----------------------------------------------------------------------------------------------------------------
\1\ Contaminants nominated for Integrated Risk Information System (IRIS) assessments per SYR Protocol.
\2\ LT2, FBRR, and GWR also identified as not appropriate for revision at this time. See Section VI.B.4 for
  additional information on the results of EPA's review of these regulations.

A. What are the review result categories?

    For each of the 76 NPDWRs discussed in detail in the following
sections of this action, the review outcomes fall in one of the
following categories:
1. The NPDWR is Not Appropriate for Revision at This Time
    The current NPDWR remains appropriate and no action is necessary at
this time. In this category, NPDWRs are grouped under the following
subcategories:
     Health effects assessment in process (as of December 2015)
or contaminant nominated for health assessment,
     No new information and NPDWR remains appropriate after
review,
     Data gaps/emerging information, and
     No meaningful opportunity.
2. The NPDWR Is a Candidate for Revision
    The NPDWR is a candidate for revision based on the review of new
information.

B. What are the detailed results of EPA's third six-year review cycle?

1. Chemical Phase Rules/Radionuclides Rules
Background
    The NPDWRs for chemical contaminants, collectively called the Phase
Rules, were promulgated between 1987 and 1992 (after the 1986 SDWA
amendments). In December 2000, EPA promulgated final radionuclide
regulations, which were issued as interim rules in July 1976.
Information related to the review for fluoride is discussed separately
in Section VI.B.2.
Summary of Review Results
    EPA has decided that it is not appropriate at this time to revise
any of the NPDWRs covered under the Phase Rules or Radionuclide Rules.
These NPDWRs were determined not to be candidates for revision for one
or more of the following reasons: There was no new information to
suggest possible changes in MCLG/MCL; new information did not present a
meaningful opportunity for health risk reduction or cost savings while
maintaining/improving public health protection; or there was an ongoing
or pending regulatory action. Details related to the review of all
Phase Rules and Radionuclide Rules contaminants can be found in the
``Chemical Contaminant Summaries for the Third Six-Year Review of
National Primary Drinking Water Regulations'' (USEPA, 2016b).
Initial Review
    The initial review identified 12 chemical contaminants with NPDWRs
under the Chemical Phase Rules that were being considered as part of
ongoing or pending regulatory actions, and 61 chemical or radionuclide
NPDWRs were identified as appropriate for review. The NPDWRs with
ongoing or pending regulatory actions included eight carcinogenic
volatile organic compounds (cVOCs), lead, copper, acrylamide and
epichlorohydrin.
    In 2011, EPA announced its plans to address a group of regulated
and unregulated cVOCs in a single regulatory effort. The eight
regulated VOCs being currently evaluated for a potential cVOCs group
regulation include: Benzene; carbon tetrachloride; 1,2-dichloroethane;
1,2-dichloropropane; dichloromethane; PCE; TCE; and vinyl chloride. The
regulatory revisions to TCE and PCE, initiated as an outcome of the
Six-Year Review 2, are also being considered as part of the group
regulatory effort. Since a regulatory effort is ongoing for these eight
contaminants, they were excluded from a detailed review as part of the
third Six-Year Review.
    The NPDWRs for acrylamide and epichlorohydrin were also previously
identified as candidates for regulatory revision and were pending
regulatory action. The polyacrylamides and epichlorohydrin-based
polymers available today for water treatment have lower residual
monomer content than when EPA promulgated residual content as a TT
(USEPA, 2016s). For example, the 90th percentile concentration of
acrylamide residual monomer levels was approximately one-half the
residual level listed in the current TT and no residual epichlorohydrin
was detected. The health benefits associated with the lower impurity
levels are already being realized by communities throughout the
country; therefore, a regulatory revision will minimally affect health
risk. Given resource limitations, competing workload priorities, and
administrative costs and burden to states to adopt any regulatory
changes associated with the rulemaking, as well as limited potential
health benefits, these NPDWRs are considered a low priority and no
longer candidates for revision at this time.
    EPA is also currently considering Long-Term Revisions to the Lead
and Copper Rule; and therefore, evaluation of that NPDWR under the Six-
Year Review process would be redundant.
Health Effects
    The principal objectives of the health effects review are to
identify: (1) Contaminants for which a new health effects assessment
indicates that a change in MCLG might be appropriate (e.g., because of
a change in cancer classification or an RfD), and (2) contaminants for
which the Agency has identified new health effects information
suggesting a need to initiate a new health effects assessment.
    Before identifying chemical NPDWR contaminants for which an updated
MCLG may be appropriate, EPA first identified chemicals with ongoing or
planned EPA health effects assessments. As of December 31, 2015, 19
chemical/radiological contaminants reviewed had ongoing or planned
formal EPA health effects assessments. Table VI-2 below lists the 19
contaminants with ongoing or planned EPA assessments and the status of
those reviews.

  Table VI-2--Six-Year Review Chemical/Radiological Contaminants With Ongoing or Planned EPA Health Assessments
----------------------------------------------------------------------------------------------------------------
          Chemical/radionuclide                                            Status
----------------------------------------------------------------------------------------------------------------
Alpha/photon emitters....................  EPA is conducting a review of alpha and beta photo emitters.
Arsenic, inorganic.......................  Inorganic arsenic is being assessed by the EPA IRIS Program. The
                                            assessment status can be found at: (https://cfpub.epa.gov/ncea/iris2/atoz.cfm).
Atrazine.................................  Atrazine and simazine are being assessed under EPA's pesticide
                                            registration review process.
Benzo(a)pyrene...........................  Benzo(a)pyrene is being assessed by the EPA IRIS Program. The
                                            assessment status can be found at: (https://cfpub.epa.gov/ncea/iris2/atoz.cfm).
Beta/photon emitters.....................  EPA is conducting a review of alpha and beta photo emitters.
Cadmium..................................  Cadmium is included in the EPA IRIS Multi-Year Agenda.
Chromium (VI) as part of total Cr).......  Chromium VI is being assessed by the EPA IRIS Program. The assessment
                                            status can be found at: (https://cfpub.epa.gov/ncea/iris2/atoz.cfm).
DEHP.....................................  DEHP is included in the EPA IRIS Multi-Year Agenda.
Ethylbenzene.............................  Ethylbenzene is being assessed by the EPA IRIS Program. The
                                            assessment status can be found at: (https://cfpub.epa.gov/ncea/iris2/atoz.cfm).
Glyphosate...............................  GlyphosateGlyphosate is being assessed under EPA's pesticide
                                            registration review process.
Mercury..................................  Mercury is included in the EPA IRIS Multi-Year Agenda.
Nitrate..................................  Nitrate is included in the EPA IRIS Multi-Year Agenda.
Nitrite..................................  Nitrite is included in the EPA IRIS Multi-Year Agenda.
o-Dichlorobenzene........................  o-Dichlorobenzene is included in the EPA IRIS Multi-Year Agenda.
p-Dichlorobenzene........................  p-Dichlorobenzene is included in the EPA IRIS Multi-Year Agenda.
PCBs.....................................  PCBs are being assessed by the EPA IRIS Program. The assessment
                                            status can be found at: (https://cfpub.epa.gov/ncea/iris2/atoz.cfm).
Radium (226, 228)........................  EPA is conducting a review of radium.
Simazine.................................  Atrazine and simazine are being assessed under EPA's pesticide
                                            registration review process.
Uranium..................................  Uranium is included in the EPA IRIS Multi-Year Agenda.
----------------------------------------------------------------------------------------------------------------

    For chemicals that were not excluded due to an ongoing or planned
health effects assessment by EPA, or by the National Academy of
Sciences (NAS), commissioned by EPA, a more detailed review was
undertaken. Of the chemicals that underwent a more detailed review, EPA
identified 21 for which there have been official Agency changes in the
RfD and/or in the cancer risk assessment from oral exposure or new
relevant non-EPA assessments that might support a change to the MCLG.
These 21 chemicals were further evaluated as part of the Six-Year
Review 3 to determine whether they were candidates for regulatory
revision. Table VI-3 lists the 21 chemicals with available new health
effects information and the sources of the relevant new information. As
shown in this table, 11 chemical contaminants have information that
could support a lower MCLG and 10 contaminants have new information
that could support a higher MCLG.

  Table VI-3--Chemicals With Available New Health Assessment That Could
                        Support a Change in MCLG
------------------------------------------------------------------------
             Chemical                     Relevant new assessment
------------------------------------------------------------------------
                       Potential Decrease in MCLG
------------------------------------------------------------------------
Carbofuran.......................  USEPA, 2008a (OPP).
Cyanide..........................  USEPA, 2010e (IRIS).
cis-1,2-Dichloroethyelene........  USEPA, 2010d (IRIS).
Endothal.........................  USEPA, 2005f (OPP).
Hexachloropentadiene.............  USEPA, 2001a (IRIS).
Methoxychlor.....................  CalEPA 2010a.
Oxamyl...........................  USEPA, 2010f (OPP).
Selenium.........................  Health Canada 2014.
Styrene..........................  CalEPA 2010b.
Toluene..........................  USEPA, 2005c (IRIS).
Xylenes..........................  USEPA, 2003a (IRIS).
------------------------------------------------------------------------
                       Potential Increase in MCLG
------------------------------------------------------------------------
Alachlor.........................  USEPA, 2006a (OPP).
Barium...........................  USEPA, 2005b (IRIS).
Beryllium........................  USEPA, 1998a (IRIS).
1,1-Dichloroethylene.............  USEPA, 2002b (IRIS).
2,4 Dichlorophenoxy-acetic Acid..  USEPA, 2013b (OPP).
Diquat...........................  USEPA, 2002a (OPP).
Lindane..........................  USEPA, 2002d (OPP).
Picloram.........................  USEPA, 1995 (OPP).
1,1,1-Trichloroethane............  USEPA, 2007a (IRIS).
1,2,4-Trichlorobenzene...........  ATSDR, 2010.
------------------------------------------------------------------------

    Details of the health effects review of the chemical and
radiological contaminants are documented in the ``Six-Year Review 3--
Health Effects Assessment for Existing Chemical and Radionuclides
National Primary Drinking Water Regulations--Summary Report'' (USEPA,
2016h).
Analytical Feasibility
    EPA performed analytical feasibility analyses for the contaminants
that reached this portion of the review. These contaminants included
the 11 chemical contaminants identified under the health effects review
as having potential for a lower MCLG and an additional 14 contaminants
with MCLs based on analytical feasibility and MCLs higher than the
current MCLGs. The document ``Analytical Feasibility Support Document
for the Third Six-Year Review of National Primary Drinking Water
Regulations: Chemical Phase Rules and Radionuclides Rules'' (USEPA,
2016a) describes the first step in the process EPA used to evaluate
whether changes in PQL are possible in those instances where the MCL is
limited, or may be limited, by analytical feasibility. The EQL analysis
is documented in the '' Development of Estimated Quantitation Levels
for the Third Six-Year Review of National Primary Drinking Water
Regulations (Chemical Phase Rules)'' (USEPA, 2016d).
    Table VI-4 shows the outcomes of EPA's analytical feasibility
review for two general categories of drinking water contaminants:
Contaminants where health effects assessments indicate potential for
lower MCLGs; and contaminants where existing MCLs are based on
analytical feasibility.
     A health effects assessment indicates potential for lower
MCLG. This category includes the 11 contaminants identified in the
health effects review as having information indicating the potential
for a lower MCLG. EPA reviewed analytical feasibility to determine if
analytical feasibility could limit the potential for MCL revisions. For
six contaminants (carbofuran, cyanide, endothall, methoxychlor, oxamyl
and styrene), the current PQL is higher than the potential new MCLG
identified in the health effects review. For these contaminants, the
PQL assessment did not support reduction of the current PQL, or data
were inconclusive or insufficient to reach a conclusion. Consequently,
analytical feasibility could be a limiting factor for setting the MCL
equal to the potential new MCLG. The current PQL is not a limiting
factor for the remaining five contaminants identified by the health
effects review for possible changes in their MCLG (i.e., cis-1,2-
dichloroethylene, hexachlorocyclopentadiene, selenium, toluene and
xylene).
     Contaminants for which existing MCLs are based on
analytical feasibility. This category includes 14 contaminants with
existing MCLs that are greater than their MCLGs because they are
limited by analytical feasibility. Two of the contaminants (thallium
and 1,1,2-trichloroethanetrichloroethane) are non-carcinogenic and have
a non-zero MCLG and the remaining 12 contaminants are carcinogens with
MCLGs equal to zero. EPA evaluated whether the PQL could be lowered for
each of these contaminants. For one contaminant, 1,1,2-trichloroethane,
EPA concluded that new information from Proficiency Testing (PT)
studies, along with MRL and MDL data, indicate the potential to revise
the PQL. For two contaminants (dioxin and PCBs), data from PT studies
were inconclusive, but MRL and MDL data indicated the potential to
revise the PQL. For five contaminants (chlordane, heptachlor,
heptachlor epoxide, hexachlorobenzene and toxaphene) data from PT and
MRL studies were inconclusive, but MDL data indicate the potential to
revise the PQL. For the remaining five contaminants, either EPA did not
have sufficient new information to evaluate analytical feasibility or
EPA concluded that new information does not indicate the potential for
a PQL revision.
    Where these evaluations indicated the potential for a PQL
reduction, Table VI-4 lists the type of data that support this
conclusion. The notation ``PT'' indicates that the PQL reassessment
based on PT data (USEPA, 2016a) supports the reduction. The notations
``MRL'' and ``MDL'' indicates that these two approaches support PQL
reduction. The findings based on PT offer more certainty. When the PQL
reassessment outcome is that the current PQL remains appropriate, Table
VI-4 shows the result ``Data do not support PQL reduction.''

 Table VI-4--NPDWRs Included in Analytical Feasibility Reassessment and
                        Result of That Assessment
------------------------------------------------------------------------
                                   Current PQL    Analytical feasibility
          Contaminant               ([mu]g/L)      reassessment result
------------------------------------------------------------------------
  11 Contaminants Identified Under the Health Effects Review as Having
                        Potential for Lower MCLG
------------------------------------------------------------------------
Carbofuran.....................               7  Data do not support PQL
                                                  reduction.
Cyanide........................             100  Data do not support PQL
                                                  reduction.
cis-1,2-Dichloroethylene.......               5  PQL not limiting.
Endothall......................              90  PQL reduction supported
                                                  (MRL, MDL).
Hexachlorocyclopentadiene......               1  PQL not limiting.
Methoxychlor...................              10  PQL reduction supported
                                                  (PT, MRL).
Oxamyl.........................              20  PQL reduction supported
                                                  (MRL, MDL).
Selenium.......................              10  PQL not limiting.
Styrene........................               5  PQL reduction supported
                                                  (PT, MRL, MDL).
Toluene........................               5  PQL not limiting.
Xylene.........................               5  PQL not limiting.
------------------------------------------------------------------------
  14 Contaminants With MCLs Based on Analytical Feasibility and Higher
                               Than MCLGs
------------------------------------------------------------------------
Benzo(a)pyrene.................             0.2  Data do not support PQL
                                                  reduction.
Chlordane......................               2  PQL reduction supported
                                                  (MRL, MDL).
1,2-Dibromo-3-chloropropane                 0.2  Data do not support PQL
 (DBCP).                                          reduction.
Di(2-ethylhexyl)phthalate                     6  Data do not support PQL
 (DEHP).                                          reduction.
Ethylene dibromide (EDB).......            0.05  Data do not support PQL
                                                  reduction.
Heptachlor.....................             0.4  PQL reduction supported
                                                  (MDL).
Heptachlor Epoxide.............             0.2  PQL reduction supported
                                                  (MDL).
Hexachlorobenzene..............               1  PQL reduction supported
                                                  (PT, MDL).
Pentachlorophenol..............               1  Data do not support PQL
                                                  reduction.

PCBs...........................             0.5  Data do not support PQL
                                                  reduction.
Dioxin.........................      3.0 x 10-5  PQL reduction supported
                                                  (MRL, MDL).
Thallium.......................               2  Data do not support PQL
                                                  reduction.
Toxaphene......................               3  PQL reduction supported
                                                  (MDL).
1,1,2-Trichloroethane..........               5  PQL reduction supported
                                                  (PT, MRL, MDL).
------------------------------------------------------------------------

Occurrence and Exposure
    Using the SYR3 ICR database, EPA conducted an assessment to
evaluate national occurrence of regulated contaminants and estimate the
potential population exposed to these contaminants. The details of the
current chemical occurrence analysis are documented in ``The Analysis
of Regulated Contaminant Occurrence Data from Public Water Systems in
Support of the Third Six-Year Review of National Primary Drinking Water
Regulations: Chemical Phase Rules and Radionuclides Rules'' (USEPA,
2016p). Based on benchmarks identified in the health effects and
analytical feasibility analyses, EPA conducted the occurrence and
exposure analysis for 18 contaminants.
    This analysis shows that these 18 contaminants occur at levels
above the identified benchmark in a very small percentage of systems,
which serve a very small percentage of the population, indicating that
revisions to NPDWRs are unlikely to provide a meaningful opportunity to
improve public health protection across the nation. Therefore, these
contaminants were not identified as candidates for regulatory revision.
Table VI-5 lists the benchmarks used to conduct the occurrence
analysis, the total number of systems with mean concentrations
exceeding a benchmark and the estimated population served by those
systems.

                   Table VI-5--Occurrence and Potential Exposure Analysis for Chemical NPDWRs
----------------------------------------------------------------------------------------------------------------
                                                                                Population served by
                                                                Number (and percentage)    systems with a mean
                                                 Benchmark \1\   of systems with a mean    concentration higher
                  Contaminant                       (ug/L)        concentration higher     than benchmarks (and
                                                                    than benchmarks        percentage of total
                                                                                population)
----------------------------------------------------------------------------------------------------------------
           Contaminants Identified Under the Health Effects Review as Having Potential for Lower MCLG
----------------------------------------------------------------------------------------------------------------
Carbofuran....................................              >5                1 (0.00%)            993 (0.0004%)
Cyanide.......................................             >50               98 (0.27%)          574,038 (0.27%)
cis-1,2-Dichloroethylene......................             >10                4 (0.01%)            5,569 (0.00%)
Endothall.....................................             >50                1 (0.01%)             993 (0.001%)
Hexachlorocyclopentadiene.....................             >40                0 (0.00%)                0 (0.00%)
Methoxychlor..................................              >1               1 (0.003%)             993 (0.000%)
Oxamyl........................................              >9                2 (0.01%)           9,742 (0.004%)
Selenium......................................             >40               49 (0.10%)          135,685 (0.05%)
Styrene.......................................            >0.5             117 (0.210%)         571,425 (0.217%)
Toluene.......................................            >600                0 (0.00%)                0 (0.00%)
Xylene........................................          >1,000               2 (0.004%)            825 (0.0003%)
----------------------------------------------------------------------------------------------------------------
                  Contaminants With MCLs Based on Analytical Feasibility and Higher Than MCLGs
----------------------------------------------------------------------------------------------------------------
Chlordane.....................................              >1                3 (0.01%)           1,353 (0.001%)
Heptachlor....................................            >0.1                3 (0.01%)            1,643 (0.00%)
Heptachlor Epoxide............................           >0.04               14 (0.04%)          11,659 (0.005%)
Hexachlorobenzene.............................            >0.1               6 (0.016%)           8,703 (0.004%)
2,3,7,8-TCDD (Dioxin).........................       >0.000005                2 (0.06%)           1,450 (0.002%)
Toxaphene.....................................              >1                6 (0.02%)          715,106 (0.32%)
1,1,2-Trichloroethane.........................              >3                0 (0.00%)                0 (0.00%)
----------------------------------------------------------------------------------------------------------------

    In addition, EPA performed a source water occurrence analysis for
the 10 chemical contaminants in which updated health effects
assessments indicated the possibility to increase (i.e., render less
stringent) the MCLG values. EPA conducted this analysis to determine if
there was a meaningful opportunity to achieve cost savings while
maintaining or improving the level of public health protection. The
data available to characterize contaminant occurrence was limited
because there is no comprehensive dataset that characterizes source
water quality for drinking water systems. Data from the U.S. Geological
Survey (USGS) National Water Quality Assessment program and the U.S.
Department of Agriculture Pesticide Data Program water monitoring
survey provide useful insights into potential contaminant occurrence in
source water. The analysis of the available contaminant occurrence data
for potential drinking water sources indicated relatively low
contaminant occurrence in the concentration ranges of interest. As a
consequence, EPA could not conclude that there is a meaningful
opportunity for system cost savings by increasing the MCLG and/or MCL
for these 10 contaminants. The results of this

analysis were documented in ``Occurrence Analysis for Potential Source
Waters for the Third Six-Year Review of National Primary Drinking Water
Regulations'' (USEPA, 2016e).
Treatment Feasibility
    Currently, all of the MCLs for chemical and radiological
contaminants are set equal to the MCLGs or PQLs or are based on
benefit-cost analysis; none are currently limited by treatment
feasibility. EPA considers treatment feasibility after identifying
contaminants with the potential to lower the MCLG/MCL that constitute a
meaningful opportunity to improve public health. No such contaminants
were identified in the occurrence and exposure analysis described
above.
Other Regulatory Revisions
    In addition to possible revisions to MCLGs, MCLs and TTs, EPA
considered whether other regulatory revisions are needed to address
implementation issues, such as revisions to monitoring and system
reporting requirements, as a part of the Six-Year Review 3. EPA used
the protocol to evaluate which implementation issues to consider
(USEPA, 2016f). EPA's protocol focused on items that were not already
being addressed, or had not been addressed, through alternative
mechanisms (e.g., as a part of a recent or ongoing rulemaking).
Implementation Issues Identified for the Six-Year Review 3
    EPA compiled information on implementation related issues
associated with the Chemical Phase Rules. EPA also identified
unresolved implementation issues/concerns from previous Six-Year
Reviews. EPA shared the list of identified potential implementation
issues with a group of state representatives convened by ASDWA to
obtain input from state drinking water agencies concerning the
significance and relevance of the issues (ASDWA, 2016). The complete
list of implementation issues related to the Phase Rules and
Radionuclide Rules is presented in ``Consideration of Other Regulatory
Revisions in Support of the Third Six-Year Review of the National
Primary Drinking Water Regulations: Chemical Phase Rules and
Radionuclide Rules'' (USEPA, 2016c).
    The Agency determined that the following three issues, identified
by state stakeholders, were within the scope of NPDWR review and were
the most substantive:
    a. Nitrogen monitoring in consecutive systems and the distribution
system,
    b. Alternative nitrate-nitrogen MCL of 20 mg/L for non-community
water systems (NCWSs), and
    c. Synthetic organic chemical (SOC) detection limits.
    Table VI-6 provides a brief description of the three issues and the
Agency's findings to date.

  Table VI-6--Chemical Rule Implementation Issues Identified That Fall
                   Within the Scope of an NPDWR Review
------------------------------------------------------------------------
       Implementation issue               Description and findings
------------------------------------------------------------------------
Nitrogen Monitoring in Consecutive  Current nitrite and nitrate
 Systems and the Distribution        standards are measured at the point
 System.                             of entry to the distribution
                                     system. Under some conditions,
                                     nitrification of ammonia in water
                                     system distribution networks could
                                     potentially result in increased
                                     total nitrite or nitrate
                                     concentrations at the point of use.
                                    To address the concern, certain
                                     water systems could develop and
                                     implement a nitrification
                                     monitoring program, which would
                                     include changing or adding
                                     additional monitoring locations.
                                    Research is needed to further
                                     evaluate the extent of this
                                     potential issue, including
                                     development of criteria to identify
                                     the specific systems where
                                     distribution system monitoring
                                     could be targeted. If the outcome
                                     of the research suggests that the
                                     magnitude of the problem represents
                                     a meaningful opportunity to improve
                                     public health protection, the
                                     regulation could be considered for
                                     revision.
Alternative Nitrate-Nitrogen MCL    EPA evaluated the possibility of
 of 20 mg/L for NCWS.                removing or further restricting the
                                     option for some NCWSs to use an
                                     alternative nitrate-nitrogen MCL of
                                     up to 20 mg/L. The nitrate-nitrogen
                                     MCL in PWSs is 10 mg/L. However,
                                     Sec.   141.11 of the Code of
                                     Federal Regulations (CFR) provides
                                     that states have the discretion to
                                     allow some NCWSs to use an
                                     alternative nitrate-nitrogen MCL of
                                     up to 20 mg/L if certain conditions
                                     are met, including conditions where
                                     water will not be available to
                                     children under six months of age.
                                    Other provisions related to this
                                     issue are included in Sec.   141.23
                                     of the CFR, which pertains to
                                     monitoring. This section states:
                                     ``Transient, non-community water
                                     systems shall conduct monitoring to
                                     determine compliance with the
                                     nitrate and nitrite MCL in Sec.
                                     Sec.   141.11 and 141.62 (as
                                     appropriate) in accordance with
                                     this section.'' The monitoring
                                     section does not address non-
                                     transient non-community water
                                     systems (NTNCWSs) eligibility to
                                     use an alternative nitrate MCL.
                                    Two potential concerns identified
                                     with the current rule provisions
                                     are:
                                       Potential health concerns
                                       other than methemoglobinemia
                                       associated with the ingestion of
                                       nitrate-nitrogen, such as
                                       possible effects on fetal
                                       development.
                                       The fact that the
                                       alternative MCL was initially
                                       intended to be used by entities
                                       such as industrial plants that do
                                       not provide drinking water to
                                       children under six months of age
                                       (44 FR 42254, USEPA, 1979).
                                       Industrial plants are generally
                                       considered to be NTNCWSs.
                                       Therefore, it is possible the
                                       alternative MCL was intended to
                                       apply specifically to NTNCWSs and
                                       not transient non-community water
                                       systems (TNCWSs).
                                    The Agency has nominated nitrate and
                                     nitrite for an IRIS assessment as a
                                     result of the Six-Year Review
                                     process, and both of these
                                     contaminants are listed in the IRIS
                                     multi-year plan. An updated
                                     assessment is needed that evaluates
                                     health effects other than
                                     methemoglobinemia. Specifically, an
                                     assessment is needed that evaluates
                                     potential health effects of nitrate-
                                     nitrogen at levels between 10 and
                                     20 mg/L on adult populations. When
                                     completed, the IRIS assessment may
                                     support initiation of a rule
                                     revision if potential adverse
                                     health effects were identified at
                                     drinking water concentrations below
                                     the alternative nitrate MCL of 20
                                     mg/L for populations other than
                                     infants less than six-months of
                                     age.

Synthetic Organic Chemical (SOC)    According to states, some
 Detection Limits.                   laboratories have reported
                                     difficulty in achieving the
                                     detection limits for some SOCs on a
                                     regular basis. Section 40 CFR
                                     141.24(h)18 provides detection
                                     limits for the SOCs, including some
                                     pesticides. PWSs that do not detect
                                     a SOC contaminant above these
                                     concentrations may qualify for
                                     reduced monitoring frequency for
                                     individual contaminants. It was
                                     reported that some SOCs may have
                                     detection limits that are lower
                                     than levels that can be
                                     economically and efficiently
                                     achieved by laboratories using
                                     approved methods. Thus, some water
                                     systems may not be able to qualify
                                     for reduced monitoring if the
                                     laboratories cannot achieve the
                                     listed detection limits. This issue
                                     was also identified as a concern by
                                     the states during the Six-Year
                                     Review 2.
                                    To address the SOC method detection
                                     limits, the Agency investigated the
                                     MRL values for SOCs from the SYR 3
                                     ICR and found there was an existing
                                     approved analytical method for each
                                     SOC that laboratories can use to
                                     achieve the appropriate detection
                                     limits in order to reduce
                                     monitoring requirements.
                                    Using the MRL values, the Agency
                                     evaluated the percentage of records
                                     in the ICR database at or below the
                                     detection limit. EPA considered
                                     this percentage as an indication of
                                     laboratories' collective ability to
                                     detect contaminant concentrations
                                     at or below these levels. The
                                     Agency found that for most of the
                                     SOCs, nearly half of the records
                                     were at or below the detection
                                     limit listed in the regulation
                                     while other SOCs had a sufficient
                                     number of records below the
                                     detection limit to determine that
                                     there was an approved analytical
                                     method that could be used.
------------------------------------------------------------------------

2. Fluoride

Background

    Fluoride can occur naturally in drinking water as a result of the
geological composition of soils and bedrock. Some areas of the country
have high levels of naturally occurring fluoride. EPA established the
current NPDWR to reduce the public health risk associated with exposure
to high levels of naturally occurring fluoride in drinking water
sources.
    Low levels of fluoride are frequently added to drinking water
systems as a public health protection measure for reducing the
incidence of cavities. The decision to fluoridate a community water
supply is made by the state or local municipality, and is not mandated
by EPA or any other federal entity. The U.S. Public Health Service
(PHS) recommendation for community water fluoridation is 0.7 mg/L (U.S.
Department of Health and Human Services, 2015). Fluoride is also added
to various consumer products (such as toothpaste and mouthwash) because
of its beneficial effects at low level exposures.
    EPA published the current NPDWR on April 2, 1986 (51 FR 11396,
USEPA, 1986) to reduce the public health risk associated with exposure
to high levels of naturally occurring fluoride in drinking water
sources. The current NPDWR established an MCLG and MCL of 4.0 mg/L to
protect against the most severe stage of skeletal fluorosis (referred
to as the ``crippling'' stage) (NRC, 2006a). EPA also established a
secondary maximum contaminant level (SMCL) for fluoride of 2.0 mg/L to
protect against moderate and severe dental fluorosis, which was
considered at the time to be a cosmetic effect. As provided under the
statute, the SMCL is not enforceable in the same manner as the MCL.
Public notification is required when PWSs exceed the MCL or SMCL.
    EPA has reviewed the NPDWR for fluoride in previous Six-Year Review
cycles. As a result of the first Six-Year Review (68 FR 42908, USEPA,
2003b), EPA requested that the National Research Council (NRC) of the
National Academies of Sciences (NAS) conduct a review of the health and
exposure data on orally ingested fluoride. In 2006, the NRC published
the results of its review and concluded that severe dental fluorosis is
an adverse health effect when it causes both a thinning and pitting of
the enamel, a situation that compromises the function of the enamel in
protecting against decay and infection (NRC, 2006a). The NRC
recommended that EPA develop a dose-response assessment for severe
dental fluorosis as the critical effect and update an assessment of
fluoride exposure from all sources.
    During the Six-Year Review 2, the Agency was in the process of
developing a dose-response assessment of the non-cancer impacts of
fluoride on severe dental fluorosis and the skeletal system. In
addition, EPA was in the process of updating its evaluation of the
relative source contribution (RSC) of drinking water to total fluoride
exposure considering the contributions from dental products, foods,
pesticide residues, and other sources such as ambient air and
medications. These assessments were not completed at the time of the
Six-Year Review 2; thus, no action was taken under the Six-Year Review
2 (75 FR 15500, USEPA, 2010h).
    In 2010, EPA published fluoride health assessments. The ``Dose
Response Analysis for Non-Cancer Effects'' (USEPA, 2010b) identified an
oral RfD for fluoride of 0.08 milligrams per kilograms per day (mg/kg/
day) based on studies of severe dental fluorosis among children in the
six months to 14 year age group (USEPA, 2010b). The ``Exposure and
Relative Source Contribution Analysis'' (USEPA, 2010c) concluded that
the RSC values for drinking water range from 40 to 70 percent, with the
higher values associated with infants fed with powdered formula or
concentrate reconstituted with residential tap water (70%) and with
adults (60%). The major contributors to total daily fluoride intakes
for these age groups are drinking water, commercial beverages, solid
foods and swallowed fluoride-containing toothpaste (USEPA, 2010c).
Summary of Review Results
    The Agency has determined that a revision to the NPDWR for fluoride
is not appropriate at this time. EPA acknowledges information regarding
the exposure and health effects of fluoride (as discussed later in the
``Health Effects'' and ``Occurrence and Exposure'' sections). However,
with EPA's identification of several other significant NPDWRs as
candidates for near-term revision (see Sections VI.B.3 and VI.B.4),
potential revision of the fluoride NPDWR is a lower priority that would
divert significant resources from the higher priority candidates for
revision that the Agency has identified, as well as other high priority
work within the drinking water office. These other candidates for
revision include the Stage 1 and Stage 2 Disinfectants and Disinfection
Byproducts Rules (D/DBPRs) that apply to approximately 42,000 PWSs, and
for which EPA has identified the potential to further reduce

bladder cancer risks attributed to exposure to DBPs; the Surface Water
Treatment Rules, for which the Agency has identified the potential to
further reduce risks from a myriad of serious waterborne diseases
(e.g., giardiasis, cryptosporidiosis, legionellosis, hepatitis,
meningitis and encephalitis) for approximately 12,000 surface water
systems; and the pending revisions to the lead and copper NPDWR which
apply to approximately 68,000 PWSs.
    While EPA has evaluated the available health effects and exposure
information related to fluoride (as discussed later in the ``Health
Effects'' and ``Occurrence and Exposure'' sections), the Agency also
recognizes that new studies on fluoride are currently being performed.
These include new studies that address health endpoints of concern
other than dental fluorosis. Based on the NRC recommendations, EPA
evaluated dental fluorosis for the purposes of this action. EPA will
continue to monitor the evolving science, and, when appropriate, will
reconsider the fluoride NPDWR's relative priority for revision and take
any other available and appropriate action to address fluoride risks
under SDWA.
    Finally, most community water systems (CWSs) that provide
fluoridation of their drinking water have already lowered their
fluoridation level to a single level of 0.7 mg/L from a previous range
of 0.7 to 1.2 mg/L to accommodate the updated PHS recommendation (U.S.
Department of Health and Human Services, 2015). The U.S. Food and Drug
Administration (FDA) also issued a letter to bottled water
manufacturers recommending that they not add fluoride to bottled water
in excess of the revised PHS recommendations (FDA, 2015). In addition,
the FDA stated it intends to revise the quality standard regulation for
fluoride added to bottled water to be consistent with the updated PHS
recommendation. Therefore, EPA anticipates that a significant portion
of the population's exposure to fluoride in drinking water, as well as
some commercial beverages that use fluoridated water from CWSs and
certain bottled water, has already been or will be reduced.
Notwithstanding this action's decision, EPA will continue to address
risk associated with fluoride in drinking water, with a specific focus
on the small systems with naturally occurring fluoride in their source
waters.
Initial Review
    EPA did not identify any recent, ongoing or pending action on
fluoride that would exclude fluoride from the Six-Year Review 3.
Health Effects
    The NRC (2006a) evaluated the impact of fluoride on reproduction
and development, neurotoxicity and behavior, the endocrine system,
genotoxicity, cancer and other effects, in addition to the tooth and
bone effects. At fluoride levels below 4.0 mg/L, the NRC found no
evidence substantial enough to support adverse effects other than
severe dental fluorosis and skeletal fractures. The NRC concluded that
the available data were inadequate to determine if a risk of effects on
other endpoints exists at an MCLG of 4.0 mg/L and made recommendations
for additional research.
    EPA assessments (USEPA, 2010b; 2010c) found that the RSC values are
lower than the RSC of 100 percent used to derive the original MCLG of
4.0 mg/L, where EPA assumed that drinking water was the sole source of
exposure to fluoride. EPA has concluded that information on the dose-
response and exposure assessment may support lowering the MCLG to
reflect levels that would protect against risk of severe dental
fluorosis and skeletal fractures.
    As part of this Six-Year Review, EPA reviewed health effects data
on the impact of fluoride on reproduction and development,
neurotoxicity and behavior, the endocrine system, genotoxicity, cancer
and other effects that were identified by the NRC as requiring
additional research (NRC, 2006a). EPA noted limitations in some of
these studies such as lack of details and confounding factors. Overall,
the new data were insufficient to alter the NRC conclusion that severe
dental fluorosis is the critical health effects endpoint for the MCLG.
    Based upon the recommendations of the NRC, EPA has evaluated dental
fluorosis as a critical endpoint of concern for this Six-Year Review
(USEPA, 2010b; 2010c). However new studies are underway to examine
other health endpoints (i.e., developmental neurobehavior effects,
endocrine disruption and genotoxicity). One example is an ongoing
National Toxicology Program (NTP) systematic review of animal studies
that examined the impact of fluoride on learning and memory (NTP,
2016). For more information about fluoride developmental neurotoxicity
visit the National Toxicology Program Web site at https://ntp.niehs.nih.gov/pubhealth/hat/noms/fluoride/neuro-index.html.
Additional information related to the review of the fluoride NPDWR is
provided in the ``Six-Year Review 3 Health Effects Assessment Summary
Report'' (USEPA, 2016h).
Analytical Feasibility
    The current PQL for fluoride is 0.5 mg/L (USEPA, 2009a). EPA has
not identified any changes in analytical feasibility that could limit
its ability to revise the MCL/MCLG for fluoride.
Occurrence and Exposure
    EPA analyzed fluoride occurrence using the SYR3 ICR database, which
contains fluoride analytical results from approximately 47,000 PWSs in
49 states/entities from 2006 to 2011. Sample records for fluoridated
water (i.e., in which a system adds fluoride to maintain a
concentration in the 0.7 to 1.2 mg/L range) were omitted from the
analysis because the fluoridated systems would not be impacted by
revisions to the fluoride NPDWR. EPA estimated the number and percent
of systems that have mean fluoride concentrations exceeding various
benchmarks and the corresponding estimates of population served by
those systems. The data indicated that about 130 systems (0.3 percent),
serving approximately 60,000 people (0.03 percent), had an estimated
system mean concentration exceeding the current MCL of 4.0 mg/L,
whereas more than 900 systems (2 percent), serving approximately 1.5
million people (0.8 percent), had an estimated system mean
concentration greater than the SMCL of 2.0 mg/L. Among these systems,
many are small systems (serving fewer than 10,000 people) and very
small systems (serving fewer than 500 people). Evaluations based on
mean (or average) fluoride concentrations generally reflect an
approximation of chronic (long-term) exposure. It is important to note
that these average concentration-based evaluations help to inform Six-
Year Review results, but do not assess compliance with regulatory
standards nor should be viewed as compliance forecasts for PWSs.
Treatment Feasibility
    A BAT or small system compliance technology for fluoride was not
established in the Code of Federal Regulations (40 CFR 141.62).
However, EPA (1998d) identified activated alumina and reverse osmosis
as BATs for fluoride.
    Activated alumina is the most commonly used treatment technology
for fluoride removal. It is capable of removing fluoride to
concentrations well below the MCL of 4.0 mg/L, but with a shortened
media life at lower target concentrations. Membrane technologies, such
as reverse osmosis, nanofiltration, and electrodialysis, are

also capable of removing fluoride to very low levels (<0.3 mg/L). They
are often used to remove fluoride along with other contaminants such as
total dissolved solids, arsenic, and uranium. In general, these
technologies are costly and complex to operate--and thus likewise
present potential challenges for small water systems (USEPA, 2014a).
3. Disinfectants/Disinfection Byproducts Rules (D/DBPRs)
Background
    The D/DBPRs were promulgated in two stages--Stage 1 in 1998 (63 FR
69390, USEPA, 1998b) and Stage 2 in 2006 (71 FR 388, USEPA, 2006d).
Disinfection byproducts (DBPs) are formed when the disinfectants
commonly used in PWSs to kill microorganisms react with organic and
inorganic matter in source water. DBPs have been associated with
potential adverse health effects, including cancer and developmental
and reproductive effects. Monitoring parameters within the D/DBPRs
consist of the following: DBPs--TTHM, HAA5, bromate and chlorite;
disinfectants--chlorine, chloramines and chlorine dioxide; and water
quality indicators--total organic carbon (TOC) and alkalinity. The
rules include MCLGs/MRDLGs, as well as MCLs/MRDLs and TT requirements,
which were developed for individual parameters considering their health
risks.
    For organic DBPs, the concern is potential increased risk of cancer
and short-term adverse reproductive and developmental effects. For
bromate, the concern is potential increased risk of cancer. Chlorite (a
regulated DBP) and chlorine dioxide (a disinfectant) are associated
with methemoglobinemia, and for infants, young children and pregnant
women, effects on the thyroid are also of concern. For chlorine and
chloramines, health effects include eye/nose irritation and stomach
discomfort (for chloramines, also anemia).
    The D/DBPRs apply to all sizes of CWSs and non-transient non-
community water systems (NTNCWSs) that chemically disinfect their water
or receive chemically disinfected water (that is, involving any
disinfectants other than ultraviolet (UV) light), as well as transient
non-community water systems (TNCWSs) that add chlorine dioxide. The
rules require that these systems comply with established MCLs, TTs,
operational evaluation levels for DBPs and MRDLs for disinfectants.
    A major challenge for water suppliers is balancing the risks from
microbial pathogens and DBPs. The risk-balancing tradeoff approach was
intended to lower the overall risks from DBP mixtures while continuing
to provide public health protection from microbial risks.
Summary of Review Results
    EPA has identified the following NPDWRs within the D/DBPRs as
candidates for revision under this Six-Year Review cycle because of the
opportunity to further reduce public health risk from exposure to DBPs:
Chlorite, HAA5 and TTHM. This result is based on a scientific review of
publicly available information. EPA's review process follows the
protocol described in Section V of this document. New information has
strengthened the weight of evidence supporting an association between
chlorination DBPs and bladder cancer risk compared to the information
available during development of the existing D/DBPRs. New information
also is available related to the reproductive/developmental effects
discussed in the Stage 2 D/DBPR. In addition, new toxicological data
are available to support the development of MCLGs for some individual
DBPs currently lacking MCLGs (for example, dibromoacetic acid).
    This result will also provide for additional opportunity to address
concerns with unregulated DBPs: For example, nitrosamines and chlorate.
In the Federal Register document for Preliminary Regulatory
Determination 3 (79 FR 62715, USEPA, 2014b), the Agency stated that
``because chlorate and nitrosamines are DBPs that can be introduced or
formed in PWSs partly because of disinfection practices, the Agency
believes it is important to evaluate these unregulated DBPs in the
context of the review of the existing DBP regulations. DBPs need to be
evaluated collectively, because the potential exists that the strategy
used to control a specific DBP could increase the concentrations of
other DBPs. Therefore, the Agency is not making a regulatory
determination for chlorate and nitrosamines at this time.''
    Chlorate and chlorite are two different oxidation states of
chlorine and are chemically inter-convertible. They occur, and can co-
occur, when hypochlorite solution and/or chlorine dioxide are applied
during the drinking water treatment process. Chlorite is a regulated
DBP. New information has shown that the relative source contribution
for chlorite could be lower than previously estimated in the existing
D/DBPRs, which could lead to a lower MCLG, and that there are common
health endpoints associated with exposure to chlorite and chlorate.
    Compliance monitoring data evaluated for the Six-Year Review 3 show
widespread occurrence of DBPs and their organic precursors (as measured
as TOC) in drinking water. Research that has been published since the
development of the Stage 2 D/DBPR has improved EPA's understanding of
the effectiveness of and limitations associated with various treatment
approaches, such as those for removal of precursors, use of
disinfectants other than chlorine and localized treatment.
    Given that this is the first time EPA is conducting a Six-Year
Review of the D/DBPRs, extensive information about review findings is
provided below, with further information provided in EPA's ``Six-Year
Review 3 Technical Support Document for Disinfectants/Disinfection
Byproducts Rules'' (USEPA, 2016l). Additional information related to
the review of D/DBPRs is provided in the ``Six-Year Review 3 Technical
Support Document for Chlorate'' (USEPA, 2016k) and the ``Six-Year
Review 3 Technical Support Document for Nitrosamines'' (USEPA, 2016o).
Initial Review
    There are no recently completed, ongoing or pending regulatory
actions on the D/DBPRs that would exclude them from the Six-Year Review
3.
Health Effects
    Under the Stage 1 and 2 D/DBPRs, toxicology studies for specific
DBPs and disinfectant residuals were used to inform MCLGs (and cancer
potency factors where MCLGs are zero) and MRDLGs. Epidemiology studies
were used to estimate potential risks from DBP mixtures (due to cancer
and developmental/reproductive effects) and support the benefits
analysis. Epidemiology studies supported a potential association
between exposures to elevated THM4 levels in chlorinated drinking water
and cancer, but the evidence was insufficient to establish a causal
relationship. The most consistent evidence was for bladder cancer. For
the development of the benefits analysis for both the Stage 1 and the
Stage 2 D/DBPRs, EPA used five bladder cancer case-control epidemiology
studies that were conducted in the 1980s and 1990s (Cantor et al.,
1985; 1987; McGeehin et al., 1993; King and Marrett, 1996; Freedman et
al., 1997; Cantor et al., 1998). In addition, EPA used one meta-
analysis (Villanueva et al., 2003) and one pooled analysis (Villanueva
et al., 2004). The five case-control studies used similar (though not
identical) exposure metrics based on years of exposure to chlorinated
drinking water (primarily chlorinated surface water) to

estimate odds ratios. All five studies showed an increase in the odds
ratio for bladder cancer incidence with an increased duration of
exposure. Using the published odds ratio results from these five
studies, EPA calculated an estimate for the lifetime cancer risk
(population attributable risk) that ranged from 2 to 17 percent;
between 2 and 17 percent of bladder cancers occurring in the U.S. could
be attributed to long-term exposure to chlorinated drinking water at
the time of the Stage 1 D/DBPR. Detailed explanations of these
calculations can be found in the benefits analysis for the Stage 2 D/
DBPR (USEPA, 2005a). The evidence from the studies in 1985 to 1998, the
meta-analysis in 2003 and the pooled analysis in 2004 was strong enough
to support the benefit analysis with several thousand potential bladder
cancer cases per year estimated as being avoided from the combined
effects of the Stage 1 and Stage 2 D/DBPRs (USEPA, 2005a).
    Studies from the 1970s to 2005 also suggested a possible
association between adverse developmental/reproductive health effects
and exposure to chlorinated drinking water. Effects were observed in
all areas but lacked consistency across studies and did not provide
enough of a basis to quantify risks or benefits. The adverse
developmental/reproductive effects consisted of effects on fetal growth
(small for gestational age, low birth weight and pre-term delivery),
effects on viability (spontaneous abortion, stillbirth) and
malformations (neural tube, oral cleft, cardiac or urinary defects).
    Since the development of the Stage 2 D/DBPR, EPA has identified
additional sources of information related to health effects of DBPs.
New toxicological information could be used to develop MCLGs for the
following regulated DBPs (within HAA5): Dibromoacetic acid (NTP, 2007),
other brominated haloacetic acids not currently regulated, including
bromochloroacetic acid (NTP, 2009) and bromodichloroacetic acid (NTP,
2014), plus additional unregulated DBPs such as nitrosamines and
chlorate (USEPA, 2016k; 2016o).
    EPA has identified new epidemiological, pharmacokinetic and
pharmacodynamic studies that, considered together with studies
available during the development of the Stage 2 D/DBPR, add to the
weight of evidence for bladder cancer being associated with exposure to
chlorination DBPs (notably those containing bromine) in drinking water.
    Pharmacokinetic and pharmacodynamic studies (Ross and Pegram, 2003;
2004; Leavens et al., 2007; Stayner et al., 2014; Kenyon et al., 2015),
in conjunction with epidemiology studies (Villanueva et al., 2007;
Kogevinas et al., 2010; Cantor et al., 2010), indicate that non-
ingestion routes of exposure (dermal and inhalation) from some
brominated DBPs may play a significant role in influencing increased
bladder cancer risk, and that there may be greater concern about sub-
populations with certain genetic characteristics (polymorphisms). EPA's
``Six-Year Review 3 Technical Support Document for Disinfectants/
Disinfection Byproducts Rules'' (USEPA, 2016l) characterizes the
research that informs the mode of action by which brominated DBPs may
be contributing to bladder cancer.
    While uncertainties remain regarding the degree to which specific
DBPs contributed to the bladder cancer incidence observed in
epidemiology studies, the collective data suggest a stronger case for
causality than when the Stage 2 D/DBPR was promulgated (Regli et al.,
2015; USEPA, 2016l). However, the Agency recognizes there are also
different perspectives on this issue, including suggestions about areas
for additional research (Hrudey et al., 2015).
    Further, the Agency has identified new information about health
effects from unregulated DBPs. This includes health effects information
on chlorate and nitrosamines that, along with occurrence/exposure
information, was previously noted in the Preliminary Regulatory
Determination 3 (79 FR 62715, USEPA, 2014b). The Agency is considering
the health effects of chlorate and nitrosamines within the broader
context of the health effects of regulated DBPs (USEPA, 2016k; 2016o).
    EPA also identified information about the relative cytotoxicity and
genotoxicity of many other unregulated DBPs (Richardson et al., 2007;
Richardson et al., 2008; Plewa and Wagner 2009; Plewa et al., 2010;
Fern[aacute]ndez et al., 2010; Richardson and Postigo, 2011; Yang et
al., 2014). Data from in vitro mammalian cell testing, which compared
the cytotoxicity and genotoxicity of iodinated, brominated, and
chlorinated DBPs, showed that the iodinated DBPs (those containing
iodine) were generally more toxic than the brominated DBPs (those
containing bromine), which were in turn more toxic than the chlorinated
DBPs (those containing chlorine). Nitrogen-containing DBPs, including
haloacetonitriles, haloacetamides and halonitromethanes, were more
cytotoxic and genotoxic than the haloacids and halomethanes that did
not contain nitrogen.
    Approximately 40 new studies about developmental/reproductive
effects have become available since the development of the Stage 2 D/
DBPR. These studies address endpoints such as fetal growth (low birth
weight, small for gestational age and pre-term delivery), congenital
anomalies and male reproductive outcomes. These studies continue to
support a potential health concern, though, as discussed above, the
relationship of DBP exposure to these types of adverse outcomes may not
be well enough understood to permit quantification of risks or
benefits. A recent ``four-lab study'' on the effects of DBP mixtures on
animals, conducted by EPA researchers (Narotsky et al., 2011; 2013;
2015), suggests diminished concern for many developmental/reproductive
endpoints.
    EPA also examined data about health effects for inorganic DBPs,
including information showing that the RSC for chlorite could be lower
than 80 percent (which could potentially support lowering the MCLG)
because there is more dietary exposure than previously assumed due to
the increased use of chlorine dioxide and acidified sodium chlorite as
disinfectants in the processing of foods (U.S. EPA, 2006e; WHO, 2008).
In addition, chlorate, chlorite and chlorine dioxide may share common
health endpoints, namely hematological and thyroid effects (Couri and
Abdel-Rahman, 1980; Bercz et al., 1982; Moore and Calabrese, 1982;
Abdel-Rahman et al., 1984; Khan et al., 2005; Orme et al., 1985; NTP,
2005; USEPA, 2006e; WHO, 2008; Lee et al, 2013; Nguyen et al, 2014).
    The Agency did not identify any relevant data that suggest an
opportunity to revise the MCLG for bromate, or the MRDLG for chlorine
or chloramines.
Analytical Feasibility
    The Agency has not identified any improvements to analytical
feasibility that could lead to improvements to the NPDWRs included in
the D/DBPRs. Development of these rules was not constrained by the
availability of analytical methods, and new EPA-approved methods that
would revise this finding have not been identified. Should new, EPA-
approved methods for one or more D/DBPRs be identified, that
information might be able to help inform potential future regulatory
development efforts.
Occurrence and Exposure
    In this Six-Year Review evaluation of D/DBP occurrence and
exposure, EPA

evaluated compliance monitoring information collected under the SYR3
ICR, which was previously discussed in Section V.B.4. EPA also
evaluated information from the DBP ICR database (USEPA, 2000a) that had
been used to prepare the original D/DBPRs. Additionally, EPA used data
from the third monitoring cycle of the Unregulated Contaminant
Monitoring Rule (UCMR3) to evaluate chlorate occurrence in 2013-2015,
and data from the UCMR2 to evaluate nitrosamine occurrence in 2008-
2010. This information is briefly described below, with additional
information in EPA's ``Six-Year Review 3 Technical Support Document for
Disinfectants/Disinfection Byproducts Rules'' (USEPA, 2016l).
    It is important to note that the information collected through the
SYR3 ICR spans the years 2006-2011. As such, it primarily reflects
occurrence following the effective date for the Stage 1 D/DBPR, but
prior to the effective date for the Stage 2 D/DBPR. These evaluations
help to inform Six-Year Review results but do not assess compliance
with regulatory standards.
    New information since the promulgation of the Stage 2 D/DBPR has
improved our understanding on DBP formation and occurrence. As part of
this Six-Year Review, EPA has identified literature describing more
than 600 specific DBPs that have been found in drinking water (e.g.,
Richardson et al., 2007); these include chlorinated, brominated and
iodinated DBPs, as well as nitrogenous compounds. Additionally, EPA
identified literature on the sources of precursors (both organic and
inorganic), as well as the influence that different precursors have on
DBP formation. For example, some of this literature discusses the