Technology                              Residual disinfectant                          Residual  disinfectant
                                                         --------------------------------   All systems  --------------------------------   All systems
                                                             Chlorine       Chloramine       (percent)       Chlorine       Chloramine       (percent)
                                                             (percent)       (percent)                       (percent)       (percent)
--------------------------------------------------------------------------------------------------------------------------------------------------------
Enhanced Coagulation (EC)...............................            73.5            76.9            74.8            57.2            65.4            60.4
EC (no pre-disinfection)................................            73.4            88.0            78.4            44.1            62.7            50.5
EC & GAC10..............................................             100            97.1            99.1             100            95.7            98.6

EC & GAC20..............................................             100             100             100             100             100             100
EC & All Chloramines....................................              NA            83.9              NA              NA            73.6             NA
--------------------------------------------------------------------------------------------------------------------------------------------------------
Note: Enhanced coagulation/softening is required under the Stage 1 DBPR for conventional plants.
Source: Seidel (2001).

    The BAT requirements for large consecutive systems are the same as
proposed, but the requirements have changed for small consecutive
systems. EPA believes that the best compliance strategy for consecutive
systems is to collaborate with wholesalers on the water quality they
need. For consecutive systems that are having difficulty meeting the
MCLs, EPA is specifying a BAT of chloramination with management of
hydraulic flow and storage to minimize residence time in the
distribution system for systems serving at least 10,000 and management
of hydraulic flow and storage to minimize residence time in the
distribution system for systems serving fewer than 10,000. EPA believes
that small consecutive systems can use this BAT to comply with the
Stage 2 DBPR, but if they cannot, then they can apply to the State for
a variance.
    Chloramination has been used for residual disinfection for many
years to minimize the formation of chlorination DBPs, including TTHM
and HAA5 (USEPA 2003d). EPA estimates that over 50 percent of large
subpart H systems serving at least 10,000 use chloramination for Stage
1. The BAT provision to manage hydraulic flow and minimize residence
time in the distribution system is to facilitate the maintenance of the
chloramine residual and minimize the likelihood for nitrification. EPA
has not included chloramination for consecutive systems as part of the
BAT for systems serving fewer than 10,000 due to concerns about their
ability to properly control the process, given that many have no
treatment capability or expertise and the Agency's concern about such
systems having operational difficulties such as distribution system
nitrification.
    EPA believes that the BATs for nonconsecutive systems are not
appropriate for consecutive systems because their efficacy in
controlling DBPs is based on precursor removal. Consecutive systems
face the unique challenge of receiving waters in which DBPs are already
present if the wholesale system has used a residual disinfectant, which
the BATs for non-consecutive systems do not effectively remove. GAC is
not cost-effective for removing DBPs. Nanofiltration is only moderately
effective at removing THMs or HAAs if membranes with a very low
molecular weight cutoff (and very high cost of operation are employed).
Therefore, GAC and nanofiltration are not appropriate BATs for
consecutive systems.
3. Summary of Major Comments
    Commenters concurred with EPA's identification of BATs for non-
consecutive systems but expressed concern about the BAT for consecutive
systems. Many commenters agreed that Stage 2 compliance for consecutive
systems would usually best be achieved by improved treatment by the
wholesale system. However, they noted that the proposed BAT may not be
practical for compliance if water delivered to the consecutive system
is at or near DBP MCLs. In addition, chloramination requires operator
supervision and adjustment and many consecutive systems that buy water
may be reluctant to operate chemical feed systems. Therefore, EPA
included chloramines as part of the BAT in today's rule only for
systems serving at least 10,000 because of the operator attention it
requires and concerns with safety and nitrification. While some
commenters believed that having a BAT for consecutive systems
contradicts the premise of the Stage 1 DBPR that DBPs are best
controlled through TOC removal and optimizing disinfection processes,
the SDWA requires EPA to identify a BAT for all systems required to
meet an MCL. No commenter recommended an alternative BAT. EPA still
believes that precursor removal remains a highly effective strategy to
reduce DBPs. Thus, EPA encourages States to work with wholesale systems
and consecutive systems to identify strategies to ensure compliance,
especially those systems with DBP levels close to the MCL.

E. Compliance Schedules

1. Today's Rule
    This section specifies compliance dates for the IDSE and MCL
compliance requirements in today's rule. As described elsewhere in
Section IV of this preamble, today's rule requires PWSs to carry out
the following activities:
     Conduct initial distribution system evaluations (IDSEs) on
a required schedule. Systems may comply by using any of four approaches
for which they qualify (standard monitoring, system specific study, 40/
30 certification, or very small system waiver).
     Determine Stage 2 monitoring locations based on the IDSE.
     Comply with Stage 2 MCLs on a required schedule.
    Compliance dates for these activities vary by PWS size. Table IV.E-
1 and Figure IV.E-1 specify IDSE and Stage 2 compliance dates.
Consecutive systems of any size must comply with the requirements of
the Stage 2 DBPR on the same schedule as required for the largest
system in the combined distribution system.

                                                    Table IV.E-1.--IDSE and Stage 2 Compliance Dates
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                 Compliance dates by PWS size (retail population served) \1\
                                    --------------------------------------------------------------------------------------------------------------------
            Requirement                 CWSs and NTNCWSs
                                        serving at least        CWSs and NTNCWSs        CWSs and NTNCWSs     CWSs serving < 10,000     NTNCWSs serving
                                             100,000          serving 50,000-99,999  serving 10,000-49,999                                < 10,000
--------------------------------------------------------------------------------------------------------------------------------------------------------
Submit IDSE monitoring plan OR.....  October 1, 2006.......  April 1, 2007.........  October 1, 2007......  April 1, 2008........  Not applicable.
Submit IDSE system specific study
 plan OR.
Submit 40/30 certification OR......
Receive very small system waiver
 from State.
Complete standard monitoring or      September 30, 2008....  March 31, 2009........  September 30, 2009...  March 31, 2010.......  Not applicable.
 system specific study.
Submit IDSE Report.................  January 1, 2009.......  July 1, 2009..........  January 1, 2010......  July 1, 2010.........  Not applicable.
Begin subpart V (Stage 2)            April 1, 2012.........  October 1, 2012.......  October 1, 2013......  October 1, 2013
 compliance monitoring \2\.                                                                                (October 1, 2014 if
                                                                                Crypto- sporidium
                                                                                monitoring is
                                                                                required under
                                                                                Subpart W)..
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Wholesale and consecutive systems that are part of a combined distribution system must comply based on the schedule required of the largest system
  in the combined distribution system.
\2\ States may grant up to an additional 2 years for systems making capital improvements.

BILLING CODE 6560-50-P

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BILLING CODE 6560-50-C

2. Background and Analysis
    The compliance schedule in today's final rule stems from the risk-
targeted approach of the rule, wherein PWSs conduct initial monitoring
to determine locations and concentrations of high DBPs. A primary
objective of this schedule is to ensure that PWSs identify locations
with high DBP concentrations and provide appropriate additional
treatment in a timely manner for high risk areas, while not requiring
low risk systems to add additional treatment. The compliance schedule
balances the objective of early risk-targeted monitoring with adequate
time for PWSs and the State or primacy agency to assure full
implementation and compliance. EPA is establishing concurrent
compliance schedules under the Stage 2 DBPR for all systems (both
wholesale systems and consecutive systems) in a particular combined
distribution system because this will assure comparable risk-based
targeting information being available at the same time for all PWSs
that are part of a combined distribution system and thereby allow for
more cost-effective compliance with TTHM and HAA5 MCLs.
    SDWA section 1412(b)(10) states that a drinking water regulation
shall take effect 3 years from the promulgation date unless the
Administrator determines that an earlier date is practicable. Today's
rule requires PWSs to begin monitoring prior to 3 years from the
promulgation date. Based on EPA's assessment and recommendations of the
Advisory Committee, as described in this section, EPA has determined
that these monitoring start dates are practicable and appropriate.
    Systems must submit their IDSE plans (monitoring plans for standard
monitoring, study plans for system specific studies) to the primacy
agency for review and approval. The State or primacy agency will then
have 12 months to review, and, as necessary, consult with the system. A
number of PWSs will then conduct one year of distribution system
monitoring for TTHM and HAA5 at locations other than those currently
used for Stage 1 DBPR compliance monitoring. At the conclusion of this
monitoring, these PWSs have three months to evaluate analysis and
monitoring results and submit Stage 2 compliance monitoring locations
and schedules to the State or primacy agency. Where required, PWSs must
provide the necessary level of treatment to comply with the Stage 2
MCLs within three years of the completion of State or primacy agency
review of the IDSE report, though States may allow an additional two
years for PWSs making capital improvements.
    EPA has modified the proposed compliance schedule to stagger
monitoring start dates for PWSs serving 10,000 to 99,999 people and to
allow more time for development and review of IDSE monitoring plans
prior to the start of monitoring. The following discussion addresses
these changes from the proposal.
    The proposed rule required all PWSs serving at least 10,000 people
(plus smaller systems that are part of a combined distribution system
with a PWS that serves at least 10,000 people) to complete IDSE
monitoring and submit IDSE reports (including recommended Stage 2
compliance monitoring locations) two years after rule promulgation,
followed by one year for review of IDSE reports, after which systems
had three years to come into compliance with Stage 2B MCLs.
    Under today's final rule, PWSs serving at least 100,000 people
(plus smaller systems that are part of the combined distribution
system) will meet the same Stage 2 compliance deadlines as proposed.
However, the timing of the IDSE has been changed to allow for a more
even workload and a greater opportunity for primacy agency involvement
(e.g., through monitoring plan review and approval). The IDSE plan
submission dates for PWSs serving 50,000 to 99,999 people (plus smaller
systems that are part of the combined distribution system) will be 12
months after the effective date; for PWSs serving 10,000 to 49,999
(plus smaller systems that are part of the combined distribution
system), the IDSE plan submission dates will be 18 months after the
effective date. The Stage 2 compliance schedule for systems serving
fewer than 10,000 people remains the same as proposed. Stage 2 MCL
compliance dates are modified accordingly.
    This staggering of IDSE start dates for PWSs serving 10,000 to
99,999 people is advantageous in several respects:

     Provides PWSs greater assurance that IDSEs are properly
conducted by requiring IDSE plan review prior to conducting the IDSE.
     Provides additional time to develop budgets and establish
contracts with laboratories.
     Spreads out the workload for technical assistance and
guidance. The staggered schedule will allow States and EPA to provide
more support to individual PWSs as needed.
     Provides time for DBP analytical laboratories to build
capacity as needed to accommodate the sample analysis needs of PWSs and
extends and smooths the demand for laboratory services.
     Maintains simultaneous rule compliance with the LT2ESWTR
as recommended by the Stage 2 M-DBP Advisory Committee and as mandated
by the 1996 SDWA Amendments, which require 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 treatment technique or process
that would be employed to attain the maximum contaminant level'' (Sec.
1412(b)(5)(B)(i)).

    The Advisory Committee recommended the Initial Distribution System
Evaluation, as discussed in Section IV.F, and EPA is finalizing an IDSE
schedule generally consistent with the Advisory Committee timeframe
recommendation, but modified to stagger the schedule for systems
serving more than 10,000 but less than 100,000, and to address public
comments on the IDSE requirements.
    For all systems, the IDSE schedule has been revised to allow
systems to submit and States or primacy agencies to review (and revise,
if necessary) systems' recommendations for IDSE and Stage 2 monitoring
locations, while still allowing systems three years after completion of
the State or primacy agency review of Stage 2 compliance monitoring
locations to make necessary treatment and operational changes to comply
with Stage 2 MCLs.
    Figure IV.E-2 illustrates compliance schedules for examples of
three combined distribution systems, with the schedule dictated by the
retail population served by the largest system.

                   Figure IV.E-2.--Schedule Examples.
------------------------------------------------------------------------

-------------------------------------------------------------------------
--Wholesale system (pop. 64,000) with three consecutive systems (pops.
 21,000; 15,000; 5,000):
    --IDSE monitoring plan due for all systems April 1, 2007 since
     wholesale system serves 50,000-99,999
    --Stage 2 compliance beginning October 1, 2012 for all systems
--Wholesale system (pop. 4,000) with three consecutive systems (pops.
 21,000; 5,000; 5,000):

    --IDSE monitoring plan due for all systems October 1, 2007 since the
     largest system in combined distribution system serves 10,000-49,999
    --Stage 2 compliance beginning October 1, 2013 for all systems
--Wholesale system (pop. 4,000) with three consecutive systems (pops.
 8,000; 5,000; 5,000):
    --IDSE monitoring plan due for all systems April 1, 2008 since no
     individual system in combined distribution system exceeds 10,000
     (even though total population exceeds 10,000)
    --Stage 2 compliance beginning October 1, 2013 if no Cryptosporidium
     monitoring under the LT2ESWTR is required or beginning October 1,
     2014 if Cryptosporidium monitoring under the LT2ESWTR is required
------------------------------------------------------------------------

    This schedule requires wholesale systems and consecutive systems
that are part of a combined distribution system with at least one
system with an earlier compliance deadline to conduct their IDSE
simultaneously so that the wholesale system will be aware of compliance
challenges facing the consecutive systems and will be able to implement
treatment plant, capital, and operational improvements as necessary to
ensure compliance of both the wholesale and consecutive systems. The
Advisory Committee and EPA both recognized that DBPs, once formed, are
difficult to remove and are generally best addressed by treatment plant
improvements, typically through precursor removal or use of alternative
disinfectants. For a wholesale system to make the best decisions
concerning the treatment steps necessary to meet TTHM and HAA5 LRAAs
under the Stage 2 DBPR, both in its own distribution system and in the
distribution systems of consecutive systems it serves, the wholesale
system must know the DBP levels throughout the combined distribution
system. Without this information, the wholesale system may design
treatment changes that allow the wholesale system to achieve
compliance, but leave the consecutive system out of compliance.
    In summary, the compliance schedule for today's rule maintains the
earliest compliance dates recommended by the Advisory Committee for
PWSs serving at least 100,000 people (plus smaller systems that are
part of the combined distribution system). These PWSs serve the
majority of people. The schedule also maintains the latest compliance
dates the Advisory Committee recommended, which apply to PWSs serving
fewer than 10,000 people. EPA has staggered compliance schedules for
PWSs between these two size categories in order to facilitate
implementation of the rule. This staggered schedule is consistent with
the schedule required under the LT2ESWTR promulgated elsewhere in
today's Federal Register.
3. Summary of Major Comments
    EPA received significant public comment on the compliance schedule
in the August 18, 2003 proposal. Major issues raised by commenters
include providing more time for PWSs to prepare for monitoring, giving
States or primacy agencies more time to oversee monitoring, and
establishing consistent schedules for consecutive PWSs. A summary of
these comments and EPA's responses follows.
    Standard monitoring plan and system specific study plan
preparation. Many commenters were concerned about the proposed
requirement to develop and execute an IDSE monitoring plan without any
primacy agency review. PWSs specifically expressed concern about the
financial commitment without prior State approval and noted that some
PWSs would need more than the time allowed under the proposed rule to
develop and implement an IDSE monitoring plan, especially without an
opportunity for State or primacy agency review and approval. Smaller
PWSs may require substantial time and planning to budget for IDSE
expenses, especially for systems that have not previously complied with
DBP MCLs.
    EPA recognizes these concerns and today's final rule provides time
for PWSs to submit IDSE plans (monitoring plans, study plans, or 40/30
certifications) for State or primacy agency review and more time before
having to begin monitoring. Specifically, PWSs serving 50,000 to 99,999
people and those serving 10,000 to 49,999 people must submit IDSE plans
about 12 months and 18 months after the effective date, respectively,
and complete standard monitoring or a system specific study within two
years after submitting their IDSE plan. This is significantly more time
than was specified under the proposal, where these systems would have
had to conduct their IDSE and submit their IDSE report 24 months after
the effective date. PWSs serving at least 100,000 people must submit
IDSE plans about six months after the effective date and complete
standard monitoring or a system specific study about 30 months after
the effective date, which also provides more time than was specified
under the proposal. PWSs serving fewer than 10,000 people, not
associated with a larger system in their combined distribution system,
do not begin monitoring until more than 36 months after the effective
date.
    EPA believes that the final compliance schedule allows PWSs
sufficient time to develop IDSE plans with these compliance dates. The
schedule also allows 12 months for State or primacy agency review of
IDSE plans, which allows additional time for review and for
coordination with systems and provides more time to address
deficiencies in IDSE plans. This is especially important for smaller
PWSs, which are likely to need the most assistance from States. By
staggering monitoring start dates, today's rule also eases
implementation by reducing the number of PWSs that will submit plans at
any one time, when the most assistance from regulatory agencies will be
required.
    In summary, today's schedule has been modified so that systems are
required to submit IDSE plans for primacy agency review and approval
prior to conducting their IDSE. Systems can consider that their plan
has been approved if they have not heard back from the State by the end
of the State review period. Systems are also required to conduct the
approved monitoring and submit their IDSE report (including the
system's recommended Stage 2 compliance monitoring) for State or
primacy agency review on a schedule that allows for systems to still
have a minimum of full three years to comply with Stage 2 following
State or primacy agency review of the system's Stage 2 recommended
monitoring. As with the review of plans, systems can consider that
their IDSE report has been approved if they have not heard back from
the State by the end of the State review period.
    State/primacy agency oversight. EPA is preparing to support
implementation of IDSE requirements that must be completed prior to
States achieving primacy. Several States have expressed concern about
EPA providing guidance and reviewing reports from systems that the
State has permitted, inspected, and worked with for a long time. These
States believe that their familiarity with

the systems enables them to make the best decisions to implement the
rule and protect public health and that the rule requirement should be
delayed until States receive primacy. Commenters were concerned that
some States will not participate in early implementation activities and
indicated that States would prefer monitoring to begin 24 months after
rule promulgation. Commenters also noted that States need sufficient
time to become familiar with the rule, train their staff, prepare
primacy packages, and train PWSs.
    EPA agrees that State familiarity is an important component of the
review and approval process, looks forward to working closely with the
State drinking water program representatives during IDSE
implementation, and welcomes proactive State involvement. However, the
Agency believes that delaying implementation of risk-based IDSE
targeting activities until States receive primacy is an unacceptable
delay in public health protection and also inconsistent with the
Advisory Committee's recommendations. EPA remains committed to working
with States to the greatest extent feasible to implement today's rule,
consistent with the schedule promulgated today. For States unable to
actively participate in IDSE implementation, however, EPA believes it
has an obligation to provide support and guidance to PWSs who are
covered and independently responsible for complying with the IDSE
requirements of today's rule and is prepared to oversee implementation.
Moreover, EPA believes that the staggered compliance schedule in
today's final rule will enhance States' ability to help implement the
rule.
    Consecutive systems. Most commenters supported consecutive systems
being on the same IDSE schedule as wholesale systems, recognizing the
benefits of treatment plant capital and operational improvements by the
wholesale system as the preferred method of DBP compliance, with the
timely collection of DBP data throughout the combined distribution
system a key component. Several commenters preferred that consecutive
systems have a later Stage 2 compliance date to allow for evaluation of
whether wholesale system treatment changes are adequate to ensure
compliance and to consider changes to water delivery specifications.
    EPA disagrees with those commenters recommending a different Stage
2 compliance date and thus has maintained the approach in the proposal,
which keeps all systems that are part of a combined distribution system
(the interconnected distribution system consisting of the distribution
systems of wholesale systems and of the consecutive systems that
receive finished water) on the same Stage 2 compliance schedule.
Extending the Stage 2 compliance dates would unnecessarily delay the
public health protection afforded by this rule. Consecutive systems
must be able to evaluate whether wholesale system changes are
sufficient to ensure compliance and, if they are not, to make cost-
effective changes to ensure compliance where wholesale system efforts
address some, but not all, of the concerns with compliance. Public
health protection through compliance with Stage 2 MCLs will occur on
the schedule of the largest system for all systems in the combined
distribution system (regardless of size). If a consecutive system must
make capital improvements to comply with this rule, the State may use
its existing authority to grant up to an additional 24 months to that
system. In addition, implementation and data tracking will be
simplified because all systems in a combined distribution system will
be on the same IDSE and Stage 2 compliance schedule. EPA believes that
this is a better approach from both a public health standpoint and an
implementation standpoint.
    EPA agrees with many commenters that a high level of coordination
among wholesaler, consecutive system, and States will be necessary to
ensure compliance. The schedule in today's rule provides more time for
planning, reviewing, and conducting the IDSE than the schedule in the
proposed rule, which will allow more time for necessary coordination,
including small consecutive systems that need help in negotiations with
their wholesale system. EPA will work with ASDWA and States to develop
guidance to facilitate wholesale/consecutive system cooperation. This
additional time and the staggered schedule discussed in this section
also lessens the laboratory burden associated with IDSE monitoring.
    The staggered schedule also helps address commenter concerns about
evaluating combined distribution systems. Other commenters' concerns
about time needed for developing contracts between systems and for
planning, funding, and implementing treatment changes are addressed by
not requiring Stage 2 compliance until at least six years following
rule promulgation.

F. Initial Distribution System Evaluation (IDSE)

1. Today's Rule
    Today's rule establishes requirements for systems to perform an
Initial Distribution System Evaluation (IDSE). The IDSE is intended to
identify sample locations for Stage 2 compliance monitoring that
represent distribution system sites with high DBP concentrations.
Systems will develop an IDSE plan, collect data on DBP levels
throughout their distribution system, evaluate these data to determine
which sampling locations are most representative of high DBP levels,
and compile this information into a report for submission to the State
or primacy agency. Systems must complete one IDSE to meet the
requirements of today's rule.
    a. Applicability. This requirement applies to all community water
systems, and to large nontransient noncommunity water systems (those
serving at least 10,000 people) that use a primary or residual
disinfectant other than ultraviolet light, or that deliver water that
has been treated with a primary or residual disinfectant other than
ultraviolet light. Systems serving fewer than 500 people are covered by
the very small system waiver provisions of today's rule and are not
required to complete an IDSE if they have TTHM and HAA5 data collected
under Subpart L. Consecutive systems are subject to the IDSE
requirements of today's rule. Consecutive systems must comply with IDSE
requirements on the same schedule as the system serving the largest
population in the combined distribution system, as described in section
IV.E.
    b. Data collection. For those systems not receiving a very small
system waiver, there are three possible approaches by which a system
can meet the IDSE requirement.
    i. Standard monitoring. Standard monitoring requires one year of
DBP monitoring throughout the distribution system on a specified
schedule. Prior to commencing standard monitoring, systems must prepare
a monitoring plan and submit it to the primacy agency for review. The
frequency and number of samples required under standard monitoring is
determined by source water type and system size. The number of samples
does not depend on the number of plants per system. Section IV.G
provides a detailed discussion of the specific population-based
monitoring requirements for IDSE standard monitoring. Although standard
monitoring results are not to be used for determining compliance with
MCLs,

systems are required to include individual sample results for the IDSE
results when determining the range of TTHM and HAA5 levels to be
reported in their Consumer Confidence Report (see section IV.J).
    ii. System specific study. Under this approach, systems may choose
to perform a system specific study based on earlier monitoring studies
or distribution system hydraulic models in lieu of standard monitoring.
Prior to commencing a system specific study, systems must prepare a
study plan and submit it to the primacy agency for approval. The two
options for system specific studies are: (1) TTHM and HAA5 monitoring
data that encompass a wide range of sample sites representative of the
entire distribution system, including those judged to represent high
TTHM and HAA5 concentrations, and (2) extended period simulation
hydraulic models that simulate water age in the distribution system, in
conjunction with one round of TTHM and HAA5 sampling.
    iii. 40/30 certification. Under this approach, systems must certify
to their State or primacy agency that every individual compliance
sample taken under subpart L during the period specified in Table IV.F-
2 were less than or equal to 0.040 mg/L for TTHM and less than or equal
to 0.030 mg/L for HAA5, and that there were no TTHM or HAA5 monitoring
violations during the same period. The State or primacy agency may
require systems to submit compliance monitoring results, distribution
system schematics, or recommend subpart V compliance monitoring
locations as part of the certification. This certification must be kept
on file and submitted to the State or primacy agency for review.
Systems that qualify for reduced monitoring for the Stage 1 DBPR during
the two years prior to the start of the IDSE may use results of reduced
Stage 1 DBPR monitoring to prepare the 40/30 certification. The
requirements for the 40/30 certification are listed in Table IV.F-1.

             Table IV.F-1.--40/30 Certification Requirements
------------------------------------------------------------------------

------------------------------------------------------------------------
40/30 Certification Requirements..   A certification that every
                                     individual compliance sample taken
                                     under subpart L during the period
                                     specified in Table IV.F-2 were less
                                     than or equal to 0.040 mg/L for
                                     TTHM and less than or equal to
                                     0.030 mg/L for HAA5, and that there
                                     were no TTHM or HAA5 monitoring
                                     violations during the same period.
                                     Compliance monitoring
                                     results, distribution system
                                     schematics, and/or recommended
                                     subpart V compliance monitoring
                                     locations as required by the State
                                     or primacy agency.
------------------------------------------------------------------------

                 Table IV.F-2.--40/30 Eligibility Dates
------------------------------------------------------------------------
                                         Then your eligibility for 40/30
                                         certification is based on eight
                                          consecutive calendar quarters
   If your 40/30 Certification Is Due        of subpart L compliance
                                         monitoring results beginning no
                                                 earlier than\1\
------------------------------------------------------------------------
(1) October 1, 2006....................  January 2004.
(2) April 1, 2007......................  January 2004.
(3) October 1, 2007....................  January 2005.
(4) April 1, 2008......................  January 2005.
------------------------------------------------------------------------
\1\ Unless you are on reduced monitoring under subpart L and were not
  required to monitor during the specified period. If you did not
  monitor during the specified period, you must base your eligibility on
  compliance samples taken during the 12 months preceding the specified
  period.

    c. Implementation. All systems subject to the IDSE requirement
under this final rule (except those covered by the very small system
waiver) must prepare and submit an IDSE plan (monitoring plan for
standard monitoring, study plan for system specific study) or 40/30
certification to the State or primacy agency. IDSE plans and 40/30
certifications must be submitted according to the schedule described in
section IV.E and IV.M. The requirements for the IDSE plan depend on the
IDSE approach that the system selects and are listed in Tables IV.F-1
and IV.F-3.

            TABLE IV.F-3.--IDSE Monitoring Plan Requirements
------------------------------------------------------------------------
 IDSE data collection alternative          IDSE plan requirements
------------------------------------------------------------------------
Standard Monitoring...............   Schematic of the
                                     distribution system (including
                                     distribution system entry points
                                     and their sources, and storage
                                     facilities), with notes indicating
                                     locations and dates of all
                                     projected standard monitoring, and
                                     all projected subpart L compliance
                                     monitoring.
                                     Justification for all
                                     standard monitoring locations
                                     selected and a summary of data
                                     relied on to select those
                                     locations.
                                     Population served and
                                     system type (subpart H or ground
                                     water).
System Specific Study:
Hydraulic Model...................  Hydraulic models must meet the
                                     following criteria:
                                     Extended period simulation
                                     hydraulic model.
                                     Simulate 24 hour variation
                                     in demand and show a consistently
                                     repeating 24 hour pattern of
                                     residence time.
                                     Represent 75% of pipe
                                     volume; 50% of pipe length; all
                                     pressure zones; all 12-inch
                                     diameter and larger pipes; all 8-
                                     inch and larger pipes that connect
                                     pressure zones, influence zones
                                     from different sources, storage
                                     facilities, major demand areas,
                                     pumps, and control valves, or are
                                     known or expected to be significant
                                     conveyors of water; all pipes 6
                                     inches and larger that connect
                                     remote areas of a distribution
                                     system to the main portion of the
                                     system; all storage facilities with
                                     standard operations represented in
                                     the model; all active pump stations
                                     with controls represented in the
                                     model; and all active control
                                     valves.

                                     The model must be
                                     calibrated, or have calibration
                                     plans, for the current
                                     configuration of the distribution
                                     system during the period of high
                                     TTHM formation potential. All
                                     storage facilities must be
                                     evaluated as part of the
                                     calibration process.
                                     All required calibration
                                     must be completed no later than 12
                                     months after plan submission.
                                    Submission must include:
                                     Tabular or spreadsheet data
                                     demonstrating percent of total pipe
                                     volume and pipe length represented
                                     in the model, broken out by pipe
                                     diameter, and all required model
                                     elements.
                                     A description of all
                                     calibration activities undertaken,
                                     and if calibration is complete, a
                                     graph of predicted tank levels
                                     versus measured tank levels for the
                                     storage facility with the highest
                                     residence time in each pressure
                                     zone, and a time series graph of
                                     the residence time at the longest
                                     residence time storage facility in
                                     the distribution system showing the
                                     predictions for the entire
                                     simulation period (i.e., from time
                                     zero until the time it takes for
                                     the model to reach a consistently
                                     repeating pattern of residence
                                     time).
                                     Model output showing
                                     preliminary 24 hour average
                                     residence time predictions
                                     throughout the distribution system.
                                     Timing and number of
                                     samples planned for at least one
                                     round of TTHM and HAA5 monitoring
                                     at a number of locations no less
                                     than would be required for the
                                     system under standard monitoring in
                                     Sec.   141.601 during the
                                     historical month of high TTHM.
                                     These samples must be taken at
                                     locations other than existing
                                     subpart L compliance monitoring
                                     locations.
                                     Description of how all
                                     requirements will be completed no
                                     later than 12 months after
                                     submission of the system specific
                                     study plan.
                                     Schematic of the
                                     distribution system (including
                                     distribution system entry points
                                     and their sources, and storage
                                     facilities), with notes indicating
                                     the locations and dates of all
                                     completed system specific study
                                     monitoring (if calibration is
                                     complete) and all subpart L
                                     compliance monitoring.
                                     Population served and
                                     system type (subpart H or ground
                                     water).
                                     If the model submitted does
                                     not fully meet the requirements,
                                     the system must correct the
                                     deficiencies and respond to State
                                     inquiries on a schedule the State
                                     approves, or conduct standard
                                     monitoring.
System Specific Study:
Existing Monitoring Results.......  Existing monitoring results must
                                     meet the following criteria:
                                     TTHM and HAA5 results must
                                     be based on samples collected and
                                     analyzed in accordance with Sec.
                                     141.131. Samples must be collected
                                     within five years of the study plan
                                     submission date.
                                     The sampling locations and
                                     frequency must meet the
                                     requirements identified in Table
                                     IV.F-4. Each location must be
                                     sampled once during the peak
                                     historical month for TTHM levels or
                                     HAA5 levels or the month of warmest
                                     water temperature for every 12
                                     months of data submitted for that
                                     location. Monitoring results must
                                     include all subpart L compliance
                                     monitoring results plus additional
                                     monitoring results as necessary to
                                     meet minimum sample requirements.
                                    Submission must include:
                                     Previously collected
                                     monitoring results
                                     Certification that the
                                     reported monitoring results include
                                     all compliance and non-compliance
                                     results generated during the time
                                     period beginning with the first
                                     reported result and ending with the
                                     most recent subpart L results.
                                     Certification that the
                                     samples were representative of the
                                     entire distribution system and that
                                     treatment and distribution system
                                     have not changed significantly
                                     since the samples were collected.
                                     Schematic of the
                                     distribution system (including
                                     distribution system entry points
                                     and their sources, and storage
                                     facilities), with notes indicating
                                     the locations and dates of all
                                     completed or planned system
                                     specific study monitoring.
                                     Population served and
                                     system type (subpart H or ground
                                     water).
                                     If a system submits
                                     previously collected data that
                                     fully meet the number of samples
                                     required for IDSE monitoring in
                                     Table IV.F-4 and some of the data
                                     are rejected due to not meeting the
                                     additional requirements, the system
                                     must either conduct additional
                                     monitoring to replace rejected data
                                     on a schedule the State approves,
                                     or conduct standard monitoring.
------------------------------------------------------------------------

                        Table IV.F-4.--SSS Existing Monitoring Data Sample Requirements.
----------------------------------------------------------------------------------------------------------------
                                                                     Number of           Number of samples
              System type                    Population size        monitoring   -------------------------------
                                                 category            locations         TTHM            HAA5
----------------------------------------------------------------------------------------------------------------

 Subpart H:

                                                            < 500               3               3               3

                                                       500-3,300               3               9               9

                                                     3,301-9,999               6              36              36

                                                   10,000-49,999              12              72              72

                                                  50,000-249,999              24             144             144

                                                 250,000-999,999              36             216             216

                                             1,000,000-4,999,999              48             288             288

                                                    >= 5,000,000              60             360             360

 Ground Water:                                              < 500               3               3               3

                                                       500-9,999               3               9               9

                                                   10,000-99,999              12              48              48

                                                 100,000-499,999              18              72              72

                                                      >= 500,000              24              96              96
----------------------------------------------------------------------------------------------------------------

    The State or primacy agency will approve the IDSE plan or 40/30
certification, or request modifications. If the State or primacy agency
has not taken action by the date specified in section IV.E or has not
notified the system that review is not yet complete, systems may
consider their submissions to be approved. Systems must implement the
IDSE option described in the IDSE plan approved by the State or primacy
agency according to the schedule described in section IV.E.
    All systems completing standard monitoring or a system specific
study must submit a report to the State or primacy agency according to
the schedule described in section IV.E. Systems that have completed
their system specific study at the time of monitoring plan submission
may submit a combined monitoring plan and report on the required
schedule for IDSE plan submissions. The requirements for the IDSE
report are listed in Table IV.F-5. Some of these reporting requirements
have changed from the proposal to reduce reporting and paperwork burden
on systems.

                 TABLE IV.F-5.--IDSE Report Requirements
------------------------------------------------------------------------
 IDSE data collection alternative         IDSE report requirements
------------------------------------------------------------------------
Standard Monitoring...............   All subpart L compliance
                                     monitoring and standard monitoring
                                     TTHM and HAA5 analytical results in
                                     a tabular format acceptable to the
                                     State.
                                     If changed from the
                                     monitoring plan, a schematic of the
                                     distribution system, population
                                     served, and system type.
                                     An explanation of any
                                     deviations from the approved
                                     monitoring plan.
                                     Recommendations and
                                     justifications for subpart V
                                     compliance monitoring locations and
                                     timing.
System Specific Study.............   All subpart L compliance
                                     monitoring and all system specific
                                     study monitoring TTHM and HAA5
                                     analytical results conducted during
                                     the period of the system specific
                                     study in a tabular or spreadsheet
                                     form acceptable to the State.
                                     If changed from the study
                                     plan, a schematic of the
                                     distribution system, population
                                     served, and system type.
                                     If using the modeling
                                     provision, include final
                                     information for required plan
                                     submissions and a 24-hour time
                                     series graph of residence time for
                                     each subpart V compliance
                                     monitoring location selected.
                                     An explanation of any
                                     deviations from the original study
                                     plan.
                                     All analytical and modeling
                                     results used to select subpart V
                                     compliance monitoring locations
                                     that show that the system specific
                                     study characterized TTHM and HAA5
                                     levels throughout the entire
                                     distribution system.
                                     Recommendations and
                                     justifications for subpart V
                                     compliance monitoring locations and
                                     timing.
------------------------------------------------------------------------

    All systems must prepare Stage 2 compliance monitoring
recommendations. All IDSE reports must include recommendations for
Stage 2 compliance monitoring locations and sampling schedule. Systems
submitting a 40/30 certification must include their Stage 2 compliance
monitoring recommendations in their Stage 2 (Subpart V) monitoring plan
unless the State requests Subpart V site recommendations as part of the
40/30 certification. The number of sampling locations and the criteria
for their selection are described in Sec.  141.605 of today's final
rule, and in section IV.G. Generally, a system must recommend locations
with the highest LRAAs unless it provides a rationale (such as ensuring
geographical coverage of the distribution system instead of clustering
all sites in a particular section of the distribution system) for
selecting other locations. In evaluating possible Stage 2 compliance
monitoring locations, systems must consider both Stage 1 DBPR
compliance data and IDSE data.
    The State or primacy agency will approve the IDSE report or request
modifications. If the State or primacy agency has not taken action by
the date specified in section IV.E or has not notified the system that
review is not yet complete, systems may consider their submission to be
approved and prepare to begin Stage 2 compliance monitoring.
    EPA has developed the Initial Distribution System Evaluation
Guidance Manual for the Final Stage 2 Disinfectants and Disinfection
Byproducts Rule (USEPA 2006) to assist systems with implementing each
of these requirements. This guidance may be requested from EPA's Safe
Drinking

Water Hotline, which may be contacted as described under FOR FURTHER
INFORMATION CONTACT in the beginning of this notice. This guidance
manual is also available on the EPA Web site at http://www.epa.gov/safewater/stage2/index.html
.

2. Background and Analysis
    In the Stage 2 DBPR proposal (USEPA, 2003a), EPA proposed
requirements for systems to complete an IDSE. The Agency based its
proposal upon the Stage 2 M-DBP Advisory Committee recommendations in
the Agreement in Principle. The Advisory Committee believed and EPA
concurs that maintaining Stage 1 DBPR monitoring sites for the Stage 2
DBPR would not accomplish the risk-targeting objective of minimizing
high DBP levels and providing consistent and equitable protection
across the distribution system. Most of these requirements have not
changed from the proposed rule.
    The data collection requirements of the IDSE are designed to find
both high TTHM and high HAA5 sites (see section IV.G for IDSE
monitoring requirements). High TTHM and HAA5 concentrations often occur
at different locations in the distribution system. The Stage 1 DBPR
monitoring sites identified as the maximum location are selected
according to residence time. HAAs can degrade in the distribution
system in the absence of sufficient disinfectant residual (Baribeau et
al. 2000). Consequently, residence time is not an ideal criterion for
identifying high HAA5 sites. In addition, maximum residence time
locations that are associated with high TTHM levels may not be constant
due to daily or seasonal changes in demand. The analysis of maximum
residence time completed for the selection of Stage 1 monitoring sites
may not have been capable of detecting these variations. The
Information Collection Rule data show that over 60 percent of the
highest HAA5 LRAAs and 50 percent of the highest TTHM LRAAs were found
at sampling locations in the distribution system other than the maximum
residence time compliance monitoring location (USEPA 2003a). Therefore,
the method and assumptions used to select the Information Collection
Rule monitoring sites and the Stage 1 DBPR compliance monitoring sites
may not reliably capture high DBP levels for Stage 2 DBPR compliance
monitoring sites.
    a. Standard monitoring. The Advisory Committee recommended that
systems sample throughout the distribution system at twice the number
of locations as required under Stage 1 and, using these results in
addition to Stage 1 compliance data, identify high DBP locations.
Monitoring at additional sites increases the chance of finding sites
with high DBP levels and targets both DBPs that degrade and DBPs that
form as residence time increases in the distribution system. EPA
believes that the required number of standard monitoring locations plus
Stage 1 monitoring results will provide an adequate characterization of
DBP levels throughout the distribution system at a reasonable cost. By
revising Stage 2 compliance monitoring plans to target locations with
high DBPs, systems will be required to take steps to address high DBP
levels at locations that might otherwise have gone undetected.
    The Advisory Committee recommended that an IDSE be performed by all
community water systems, unless the system had sufficiently low DBP
levels or is a very small system with a simple distribution system. EPA
believes that large nontransient noncommunity water systems (NTNCWS)
(those serving at least 10,000 people) also have distribution systems
that require further evaluation to determine the locations most
representative of high DBP levels and proposed that they be required to
conduct an IDSE. Therefore, large NTNCWS and all community water
systems are required to comply with IDSE requirements under today's
final rule, unless they submit a 40/30 certification or they are
covered by the very small system waiver provisions.
    b. Very small system waivers. Systems serving fewer than 500 people
that have taken samples under the Stage 1 DBPR will receive a very
small system waiver. EPA proposed and the Advisory Committee
recommended a very small system waiver following a State determination
that the existing Stage 1 compliance monitoring location adequately
characterizes both high TTHM and high HAA5 for the distribution system
because many very small systems have small or simple distribution
systems. The final rule grants the very small system waiver to all
systems serving fewer than 500 that have Stage 1 DBPR data. This
provision was changed from the proposal to reflect that most very small
systems that sample under the Stage 1 DBPR have sampling locations that
are representative of both high TTHM and high HAA5 because most very
small systems have small and simple distribution systems. In addition,
many very small systems are ground water systems that typically have
stable DBP levels that tend to be lower than surface water DBP levels.
NRWA survey data show that free chlorine residual in very small systems
(serving < 500) at both average residence time and maximum residence
time locations are lower than levels at both of those locations in
larger systems, and the change in residual concentration between those
two locations is smaller in very small systems compared to larger sized
systems. The magnitude of the reduction in residual concentration gives
an indication of how much disinfectant has reacted to form DBPs,
including TTHM and HAA5. The smaller reduction in disinfectant
concentration between average residence time and maximum residence time
in very small systems compared to larger systems indicates that DBP
formation potential is probably lower in very small systems compared to
larger systems, and the likelihood for significant DBP variation within
the distribution system of very small systems is low if the
distribution system is small and not complex. However, there may be
some small systems with extended or complex distribution systems that
should be studied further to determine new sampling locations. For this
reason, States or primacy agencies can require any particular very
small system to conduct an IDSE. Very small systems subject to the
Stage 2 DBPR that do not have a Stage 1 compliance monitoring location
may monitor in accordance with the Stage 1 DBPR provisions to be
eligible for this waiver.
    c. 40/30 certifications. Systems that certify to their State or
primacy agency that all compliance samples taken during eight
consecutive calendar quarters prior to the start of the IDSE were
< =0.040 mg/L TTHM and < =0.030 mg/L HAA5 are not required to collect
additional DBP monitoring data under the IDSE requirements as long as
the system has no TTHM or HAA5 monitoring violations. These criteria
were developed because both EPA and the AdvisoryCommittee determined
that these systems most likely would not have DBP levels that exceed
the MCLs. Systems must have qualifying TTHM and HAA5 data for eight
consecutive calendar quarters according to the schedule in Table IV.F-2
to be eligible for this option. Systems on reduced monitoring that did
not monitor during the specified time period may use data from the
prior year to meet the 40/30 certification criteria. Systems that have
not previously conducted Stage 1 DBPR compliance monitoring may begin
such monitoring to collect the data necessary to qualify for 40/30
certification. The certification and data supporting it must be
available to the public upon request.

    The qualifying time period for the 40/30 certification has changed
from the proposed rule.
    Under the proposed rule, the rule language identified a specific
two year window with start and end dates. In today's final rule, the
qualifying time period has been changed to ``eight consecutive calendar
quarters of subpart L compliance monitoring results beginning no
earlier than * * *'' (see Table IV.F-2). This change was made so that
systems that have made a treatment change within the two years prior to
rule promulgation and have collected initial data that meet the 40/30
criteria might have the opportunity to collect eight consecutive
quarters of qualifying data and apply for a 40/30 certification. This
schedule change also allows systems that have not previously monitored
under Stage 1 an opportunity to qualify for a 40/30 certification.
    Under the proposed Stage 2 DBPR, systems that missed the deadline
for submitting a 40/30 certification would be required to conduct
either standard monitoring or a system specific study even if the
system otherwise qualified for the 40/30 certification. Under today's
final rule, systems that do not make any submission by the IDSE plan
submission deadline will still receive a violation, but may submit a
late 40/30 certification if their data meet the requirements. This
change was made so that systems and primacy agencies do not spend time
preparing and reviewing standard monitoring plans and IDSE reports for
systems with a low likelihood of finding high TTHM and HAA5 levels.
    The reporting requirements for this provision have been reduced
from the requirements in the proposed rulemaking. In the proposal,
systems qualifying for the 40/30 certification were required to submit
all qualifying data and provide recommendations for Stage 2 compliance
monitoring locations. The final rule requires systems to submit a
certification that their data meet all the requirements of the 40/30
certification and to include their Stage 2 compliance monitoring
recommendations in their Stage 2 monitoring plan. These changes were
made to reduce the reporting burden on systems that qualify for the 40/
30 certification and to maintain consistency with monitoring plan
requirements under the Stage 1 DBPR. This approach also gives systems
more time to select appropriate monitoring sites for Stage 2 compliance
monitoring. The State or primacy agency may request systems to submit
the data, a distribution system schematic, and/or recommendations for
Stage 2 compliance monitoring as part of the 40/30 certification. This
provision was included to facilitate primacy agency review of 40/30
certifications; the additional information is only required if
requested by the primacy agency.
    d. System specific studies. Advisory Committee members recognized
that some systems have detailed knowledge of their distribution systems
by way of ongoing hydraulic modeling and/or existing widespread
monitoring plans (beyond that required for compliance monitoring) that
would provide equivalent or superior monitoring site selection
information compared to standard monitoring. Therefore, the Advisory
Committee recommended that such systems be allowed to determine new
monitoring sites using system-specific data such as hydraulic model
results or existing monitoring data; this provision remains in the
final rule. In the proposed rule, the only specification for SSSs was
to identify monitoring sites that would be equivalent or superior to
those identified under Standard Monitoring. The final rule includes
more specific requirements on how these studies should be completed.
The requirements in the final rule were developed to be consistent with
the proposal, yet more specific to help systems better understand
expectations under this provision and lessen the chances of a study
plan not being approved.
    The new modeling requirements were developed to reflect that
hydraulic models can identify representative high TTHM monitoring
locations by predicting hydraulic residence time in the distribution
system. Water age has been found to correlate with TTHM formation in
the distribution system. Consequently, for this system specific study
approach, hydraulic residence time predicted by the model is used as a
surrogate for TTHM formation to locate appropriate Stage 2 compliance
monitoring locations. To predict hydraulic residence time in the
distribution system, the model must represent most of the distribution
system and must have been calibrated recently and appropriately to
reflect water age in the distribution system. Requirements to reflect
this are in today's rule. All storage facilities must be evaluated for
the calibration, and systems using this option must submit a graph of
predicted tank levels versus measured tank levels for the storage
facility with the highest residence time in each pressure zone. These
calibration requirements are focused on storage facilities because they
are the largest controlling factor for water age in the distribution
system. The calibration requirements reflect the fact that the purpose
of the model is to predict water age. ICR data show that HAA5 data do
not necessarily correlate well with water age (USEPA 2003a). Because
the purpose of the IDSE is to locate representative high locations for
both TTHM and HAA5, one round of monitoring must be completed at
potential Stage 2 compliance monitoring locations to determine
appropriate HAA5 monitoring locations during the historical high month
of TTHM concentrations. The number of locations must be no less than
would be required under standard monitoring.
    Preliminary average residence time data are required as a part of
the study plan for systems to demonstrate that their distribution
system hydraulic model is able to produce results for water age
throughout the distribution system, even though calibration may not be
complete. Systems also need to describe their plans to complete the
modeling requirements within 12 months of submitting the study plan.
These last two requirements were developed so that States can be
assured that systems have the technical capacity to complete their
modeling requirements by the IDSE report deadline. If systems cannot
demonstrate that they are in a position to complete the modeling
requirements according to the required schedule, they will be required
to complete standard monitoring.
    All new modeling requirements were added to help systems
demonstrate how their model will fulfill the purpose and requirements
of the IDSE and to assist primacy agencies with approval
determinations. The associated reporting requirements were developed to
balance the needs of systems to demonstrate that they have fulfilled
the requirements and the needs of primacy agency reviewers to be able
to understand the work completed by the system.
    EPA has specified new requirements for systems complete an SSS
using existing monitoring data to help systems understand the extent of
historical data that would meet the requirements of the IDSE. The
number of required sample locations and samples are consistent with
sampling requirements under standard monitoring and the recommendations
made by the Advisory Committee. The Advisory Committee recommended that
systems complete an IDSE sample at twice the number of sites required
by the Stage 1 DBPR in addition to Stage 1 DBPR sampling. Because the
number of required Stage 1 DBPR monitoring locations varies within each
population category under

the Stage 1 plant-based monitoring approach (since systems have
different numbers of plants), EPA used the number of required Standard
Monitoring locations plus the number of Stage 2 compliance monitoring
locations to develop minimum requirements for the use of existing
monitoring data for the SSS. The number of required locations and
samples are shown in Table IV.F-4. Systems will use their Stage 1
monitoring results plus additional non-compliance or operational
samples to fulfill these requirements. Small systems with many plants
may have been collecting a disproportionate number of samples under the
Stage 1 DBPR compared to the population based monitoring requirements
presented in today's rule and may have sufficient historical data to
characterize the entire distribution system. These requirements allow
those systems to submit an SSS based on existing Stage 1 monitoring
results, and they also accommodate systems that have been completing
additional monitoring throughout the distribution system.
    The requirement to sample during the historical month of high TTHM,
high HAA5, or warmest water temperature during each year for which data
were collected was added to maintain consistency with the standard
monitoring requirements where each location must be sampled one time
during the peak historical month. Samples that qualify for this SSS
must have been collected within five years of the study plan submission
date and must reflect the current configuration of treatment and the
distribution system. Five years was selected as a cut off for eligible
data so that all data submitted would be reasonably representative of
current source water conditions and DBP formation within the
distribution system. Data that are older may not reflect current DBP
formation potential in the distribution system. Five years prior to the
submission of the study plan also correlates with the signing of the
Agreement in Principle where the Advisory Committee made the
recommendation for this provision. Systems interested in using this
provision would have started eligible monitoring after the agreement
was signed.
    Systems that submit existing monitoring data must submit all Stage
1 sample results from the beginning of the SSS to the time when the SSS
plan is submitted. The purpose of this requirement is to demonstrate
that there have been no significant changes in source water quality
since the first samples were collected, especially if all existing
monitoring results were taken during the earliest eligible dates.
Again, these clarifications were made so that systems could better
understand the extent of data necessary for a monitoring plan to be
deemed acceptable and be confident that efforts to complete an SSS
would be found acceptable to the State or primacy agency.
    e. Distribution System Schematics. EPA has considered security
concerns that may result from the requirement for systems to submit a
distribution system schematic as part of their IDSE plan. EPA believes
that the final rule strikes an appropriate balance between security
concerns and the need for States and primacy agencies to be able to
review IDSE plans. EPA has developed guidance for systems on how to
submit a distribution system schematic that does not include sensitive
information.
3. Summary of Major Comments
    The Agency received significant comments on the following issues
related to the proposed IDSE requirements: Waiver limitations, and
State or primacy agency review of IDSE plans.
    In the proposed rule, EPA requested comment on what the appropriate
criteria should be for States or primacy agencies to grant very small
system waivers. Commenters responded with a wide range of suggestions
including support for the proposal as written, different population
cut-offs, State or primacy agency discretion on what system size should
qualify for the waiver, and alternative waiver criteria such as pipe
length or number of booster stations. There was no consensus among the
commenters on what changes should be made to the proposal for the very
small system waiver requirements. EPA did not change the population
cutoff for the very small system waiver because analysis of NRWA survey
data also showed that systems serving fewer than 500 had different
residence times and lower free chlorine residual concentrations
compared to other population categories, indicating that larger systems
have different DBP formation characteristics compared to very small
systems. Some of the suggested changes for very small system waiver
criteria may require data that are not readily available to systems
(such as pipe length in service) and for which there were no specific
criteria proposed or recommended by the commenters. Implementation of
subjective very small system waiver criteria would result in reduced
public health protection from the rule by allowing higher DBP levels to
go undetected.
    In addition to addressing the very small system waivers, commenters
suggested that different criteria should be used for the 40/30
certification, such as higher minimum DBP levels, cut-offs of 40/30 as
LRAAs or RAAs rather than single sample maximums, or State or primacy
agency discretion on which systems should qualify for 40/30
certification. There was no consensus among the commenters on what
changes should be made to the proposal for the 40/30 certification
requirements. EPA did not change the requirements for the 40/30
certification eligibility because the recommended alternatives were not
technically superior to the requirements of the proposed rule.
Implementation of 40/30 criteria using an LRAA or RAA would result in
reduced public health protection from the rule by allowing higher DBP
levels to go undetected. EPA did change the eligibility dates and
reporting requirements for the 40/30 certification to reduce the burden
on the system. Under today's final rule, States or primacy agencies can
request TTHM and HAA5 data as desired for a more in-depth review of a
system's qualifications.
    Many commenters expressed concern over the implementation schedule
for the IDSE. Commenters were especially concerned that IDSE plans
would be developed and implemented prior to State primacy, and once
States receive primacy, they might not support the IDSE plan and would
reject the results of the completed IDSE. To address this issue,
commenters requested the opportunity for States to review the IDSE
plans prior to systems completing their IDSEs. In today's rule EPA has
modified the compliance schedule for the Stage 2 DBPR so that systems
have the opportunity to complete their IDSE plan and have it reviewed
by the primacy agency prior to completing the IDSE to address the
concern that States or primacy agencies may reject the results of the
completed IDSE. The changes to the compliance schedule are discussed
further in section IV.E.

G. Monitoring Requirements and Compliance Determination for TTHM and
HAA5 MCLs

    EPA is finalizing monitoring requirements under a population-based
approach described in this section. EPA believes the population-based
approach will provide more representative high DBP concentrations
throughout distribution systems than would plant-based monitoring, is
equitable, and will simplify implementation for both States and
systems. For these reasons, EPA believes this approach is more
appropriate than the proposed plant-

based monitoring. Detailed discussion of the two approaches is
presented in the preamble of the proposed rule (USEPA 2003a) and EA for
today's rule (USEPA 2005a).
1. Today's Rule
    Today's rule establishes TTHM and HAA5 monitoring requirements for
all systems based on a population-based monitoring approach instead of
a plant-based approach. Under the population-based approach, monitoring
requirements are based solely on the retail population served and the
type of source water used and not influenced by the number of treatment
plants or entry points in the distribution system as in previous rules
(i.e., TTHM Rule (USEPA 1979) and Stage 1 DBPR (USEPA 1998a)).
    a. IDSE Monitoring. All systems conducting IDSE standard monitoring
must collect samples during the peak historical month for DBP levels or
water temperature; this will determine their monitoring schedule. Table
IV.G-1 contains the IDSE monitoring frequencies and locations for all
source water and size category systems. Section IV.F identifies other
approaches by which systems can meet IDSE requirements.

                                                Table IV.G-1.--IDSE Monitoring Frequencies and Locations
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Distribution system monitoring locations \1\
                                                                                ----------------------------------------------------------------
          Source water type                Population size       Monitoring periods and   Total per                  Average
                                               category          frequency of sampling    monitoring   Near entry   residence    High TTHM    High HAA5
                                                                                period       points        time      locations    locations
--------------------------------------------------------------------------------------------------------------------------------------------------------
Subpart H
                                       < 500 consecutive         one (during peak                   2            1  ...........            1
                                        systems.                 historical month) \2\.
                                       < 500 non-consecutive     .......................            2  ...........  ...........            1            1
                                        systems.
--------------------------------------
                                       500-3,300 non-           four (every 90 days)...            2            1  ...........            1  ...........
                                        consecutive systems.
                                       500-3,300 consecutive    .......................            2  ...........  ...........            1            1
                                        systems.
--------------------------------------
                                       3,301-9,999............  .......................            4  ...........            1            2            1
                                       10,000-49,999..........  six (every 60 days)....            8            1            2            3            2
                                       50,000-249,999.........  .......................           16            3            4            5            4
                                       250,000-999,999........  .......................           24            4            6            8            6
                                       1,000,000-4,999,999....  .......................           32            6            8           10            8
                                       >=5,000,000............  .......................           40            8           10           12           10
======================================
Ground Water
                                       < 500 consecutive         one (during peak                   2            1  ...........            1  ...........
                                        systems.                 historical month) \2\.
--------------------------------------
                                       < 500 non-consecutive     .......................            2  ...........  ...........            1            1
                                        systems.
                                       500-9,999..............  four (every 90 days)...            2  ...........  ...........            1            1
                                       10,000-99,999..........  .......................            6            1            1            2            2
                                       100,000-499,999........  .......................            8            1            1            3            3
                                       >=500,000..............  .......................           12            2            2            4           4
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ A dual sample set (i.e., a TTHM and an HAA5 sample) must be taken at each monitoring location during each monitoring period.
\2\ The peak historical month is the month with the highest TTHM or HAA5 levels or the warmest water temperature.

    b. Routine Stage 2 Compliance Monitoring. For all systems
conducting either standard monitoring or a system specific study,
initial Stage 2 compliance monitoring locations are based on the
system's IDSE results, together with an analysis of a system's Stage 1
DBPR compliance monitoring results. Systems receiving 40/30
certification or a very small system waiver, and nontransient
noncommunity water systems serving < 10,000 not required to conduct an
IDSE, base Stage 2 initial compliance monitoring locations on the
system's Stage 1 DBPR compliance monitoring results. Some of these
systems may also need an evaluation of distribution system
characteristics to identify additional monitoring locations, if
required by the transition from plant-based monitoring to population-
based monitoring.
    Systems recommend Stage 2 monitoring locations generally by
arraying results of IDSE standard monitoring (or system specific study
results) and Stage 1 compliance monitoring by monitoring location (from
highest to lowest LRAA for both TTHM and HAA5). Using the protocol in
Sec.  141.605(c) of today's rule, systems then select the required
number of locations. Larger systems include existing Stage 1 monitoring
locations in order to be able to have historical continuity for
evaluating how changes in operations or treatment affect DBP levels.
Systems may also recommend locations with lower levels of DBPs that
would not be picked up by the protocol if they provide a rationale for
the recommendation. Examples of rationales include ensuring better
distribution system or population coverage (not having all locations in
the same area) or maintaining existing locations with DBP levels that
are nearly as high as those that would otherwise be selected. The State
or primacy agency will review these recommendations as part of the
review of the IDSE report submitted by systems that conducted standard
monitoring or a system specific study.
    Table IV.G-2 contains the routine Stage 2 TTHM and HAA5 compliance

monitoring requirements for all systems (both non-consecutive and
consecutive systems), as well as the protocol for Stage 2 compliance
monitoring location selection in the IDSE report. Systems that do not
have to submit an IDSE report (those receiving a 40/30 certification or
very small system waiver and nontransient noncommunity water systems
serving < 10,000) must conduct Stage 2 compliance monitoring as
indicated in the ``Total per monitoring period'' column at current
Stage 1 compliance monitoring locations, unless the State or primacy
agency specifically directs otherwise. All systems are then required to
maintain and follow a Stage 2 compliance monitoring plan.

                                          Table IV.G-2. Routine Compliance Monitoring Frequencies and Locations
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Distribution system monitoring location
                                                                                ---------------------------------------------------
                                                                                Existing
          Source water type            Population size category   Monitoring frequency\1\   Total per     Highest      Highest     Subpart L
                                                                                monitoring      TTHM         HAA5      compliance
                                                                                period\2\    locations    locations    locations
---------------------------------------------------------------------------------------------------------------------------------------------
Subpart H:
                                       < 500....................  per year................            2            1            1  ...........
                                       500-3,300...............  per quarter.............            2            1            1  ...........
                                       3,301-9,999.............  per quarter.............            2            1            1  ...........
                                       10,000-49,999...........  per quarter.............            4            2            1            1
                                       50,000-249,999..........  per quarter.............            8            3            3            2
                                       250,000-999,999.........  per quarter.............           12            5            4            3
                                       1,000,000-4,999,999.....  per quarter.............           16            6            6            4
                                       >= 5,000,000............  per quarter.............           20            8            7            5
Ground water:
                                       < 500....................  per year................            2            1            1  ...........
                                       500-9,999...............  per year................            2            1            1  ...........
                                       10,000-99,999...........  per quarter.............            4            2            1            1
                                       100,000-499,999.........  per quarter.............            6            3            2            1
                                       >= 500,000..............  per quarter.............            8            3            3           2
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ All systems must monitor during month of highest DBP concentrations.
\2\ Systems on quarterly monitoring must take dual sample sets every 90 days at each monitoring location, except for subpart H systems serving 500-
  3,300. Systems on annual monitoring and subpart H systems serving 500-3,300 are required to take individual TTHM and HAA5 samples (instead of a dual
  sample set) at the locations with the highest TTHM and HAA5 concentrations, respectively. Only one location with a dual sample set per monitoring
  period is needed if highest TTHM and HAA5 concentrations occur at the same location, and month, if monitored annually).

    Today's rule provides States the flexibility to specify alternative
Stage 2 compliance monitoring requirements (but not alternative IDSE
monitoring requirements) for multiple consecutive systems in a combined
distribution system. As a minimum under such an approach, each
consecutive system must collect at least one sample among the total
number of samples required for the combined distribution system and
will base compliance on samples collected within its distribution
system. The consecutive system is responsible for ensuring that
required monitoring is completed and the system is in compliance. It
also must document its monitoring strategy as part of its subpart V
monitoring plan.
    Consecutive systems not already conducting disinfectant residual
monitoring under the Stage 1 DBPR must comply with the monitoring
requirements and MRDLs for chlorine and chloramines. States may use the
provisions of Sec.  141.134(c) to modify reporting requirements. For
example, the State may require that only the consecutive system
distribution system point-of-entry disinfectant concentration be
reported to demonstrate MRDL compliance, although monitoring
requirements may not be reduced.
    i. Reduced monitoring. Systems can qualify for reduced monitoring,
as specified in Table IV.G-3, if the LRAA at each location is < =0.040
mg/L for TTHM and < =0.030 mg/L for HAA5 based on at least one year of
monitoring at routine compliance monitoring locations. Systems may
remain on reduced monitoring as long as the TTHM LRAA is < =0.040 mg/L
and the HAA5 LRAA is < =0.030 mg/L at each monitoring location for
systems with quarterly reduced monitoring. If the LRAA at any location
exceeds either 0.040 mg/L for TTHM or 0.030 mg/L for HAA5 or if the
source water annual average TOC level, before any treatment, exceeds
4.0 mg/L at any of the system's treatment plants treating surface water
or ground water under the direct influence of surface water, the system
must resume routine monitoring. For systems with annual or less
frequent reduced monitoring, systems may remain on reduced monitoring
as long as each TTHM sample is < =0.060 mg/L and each HAA5 sample is
< =0.045 mg/L. If the annual (or less frequent) sample at any location
exceeds either 0.060 mg/L for TTHM or 0.045 mg/L for HAA5, or if the
source water annual average TOC level, before any treatment, exceeds
4.0 mg/L at any treatment plant treating surface water or ground water
under the direct influence of surface water, the system must resume
routine monitoring.

                                   Table IV.G-3.--Reduced Monitoring Frequency
----------------------------------------------------------------------------------------------------------------
                                                                                Distribution system
         Source water type               Population size     Monitoring frequency 1    monitoring location per
                                            category                                      monitoring period
----------------------------------------------------------------------------------------------------------------
Subpart H:
                                     < 500..................  ......................  Monitoring may not be
                                                                                reduced.

                                     500-3,300.............  per year..............  1 TTHM and 1 HAA5 sample:
                                                                                one at the location and
                                                                                during the quarter with
                                                                                the highest TTHM single
                                                                                measurement, one at the
                                                                                location and during the
                                                                                quarter with the highest
                                                                                HAA5 single measurement; 1
                                                                                dual sample set per year
                                                                                if the highest TTHM and
                                                                                HAA5 measurements occurred
                                                                                at the same location and
                                                                                quarter.
                                     3,301-9,999...........  per year..............  2 dual sample sets: one at
                                                                                the location and during
                                                                                the quarter with the
                                                                                highest TTHM single
                                                                                measurement, one at the
                                                                                location and during the
                                                                                quarter with the highest
                                                                                HAA5 single measurement.
                                     10,000-49,999.........  per quarter...........  2 dual sample sets at the
                                                                                locations with the highest
                                                                                TTHM and highest HAA5
                                                                                LRAAs.
                                     50,000-249,999........  per quarter...........  4 dual sample sets--at the
                                                                                locations with the two
                                                                                highest TTHM and two
                                                                                highest HAA5 LRAAs.
                                     250,000-999,999.......  per quarter...........  6 dual sample sets--at the
                                                                                locations with the three
                                                                                highest TTHM and three
                                                                                highest HAA5 LRAAs
                                     1,000,000-4,999,999...  per quarter...........  8 dual sample sets--at the
                                                                                locations with the four
                                                                                highest TTHM and four
                                                                                highest HAA5 LRAAs.
                                     >=5,000,000...........  per quarter...........  10 dual sample sets--at the
                                                                                locations with the five
                                                                                highest TTHM and five
                                                                                highest HAA5 LRAAs.
Ground Water:
                                     < 500..................  every third year......  1 TTHM and 1 HAA5 sample:
                                                                                one at the location and
                                                                                during the quarter with
                                                                                the highest TTHM single
                                                                                measurement, one at the
                                                                                location and during the
                                                                                quarter with the highest
                                                                                HAA5 single measurement; 1
                                                                                dual sample set per year
                                                                                if the highest TTHM and
                                                                                HAA5 measurements occurred
                                                                                at the same location and
                                                                                quarter.
                                     500-9,999.............  per year..............  1 TTHM and 1 HAA5 sample:
                                                                                one at the location and
                                                                                during the quarter with
                                                                                the highest TTHM single
                                                                                measurement, one at the
                                                                                location and during the
                                                                                quarter with the highest
                                                                                HAA5 single measurement; 1
                                                                                dual sample set per year
                                                                                if the highest TTHM and
                                                                                HAA5 measurements occurred
                                                                                at the same location and
                                                                                quarter.
                                     10,000-99,999.........  per year..............  2 dual sample sets: one at
                                                                                the location and during
                                                                                the quarter with the
                                                                                highest TTHM single
                                                                                measurement, one at the
                                                                                location and during the
                                                                                quarter with the highest
                                                                                HAA5 single measurement.
                                     100,000-499,999.......  per quarter...........  2 dual sample sets; at the
                                                                                locations with the highest
                                                                                TTHM and highest HAA5
                                                                                LRAAs.
                                     >=500,000.............  per quarter...........  4 dual sample sets at the
                                                                                locations with the two
                                                                                highest TTHM and two
                                                                                highest HAA5 LRAAs.
----------------------------------------------------------------------------------------------------------------
1 Systems on quarterly monitoring must take dual sample sets every 90 days.

    ii. Compliance determination. A PWS is in compliance when the
annual sample or LRAA of quarterly samples is less than or equal to the
MCLs. If an annual sample exceeds the MCL, the system must conduct
increased (quarterly) monitoring but is not immediately in violation of
the MCL. The system is out of compliance if the LRAA of the quarterly
samples for the past four quarters exceeds the MCL.
    Monitoring and MCL violations are assigned to the PWS where the
violation occurred. Several examples are as follows:
     If monitoring results in a consecutive system indicate an
MCL violation, the consecutive system is in violation because it has
the legal responsibility for complying with the MCL under State/EPA
regulations. The consecutive system may set up a contract with its
wholesale system that details water quality delivery specifications.
     If a consecutive system has hired its wholesale system
under contract to monitor in the consecutive system and the wholesale
system fails to monitor, the consecutive system is in violation because
it has the legal responsibility for monitoring under State/EPA
regulations.
     If a wholesale system has a violation and provides that
water to a consecutive system, the wholesale system is in violation.
Whether the consecutive system is in violation will depend on the
situation. The consecutive system will also be in violation unless it
conducted monitoring that showed that the violation was not present in
the consecutive system.
2. Background and Analysis
    EPA proposed the plant-based approach for all systems that produce
some or all of their finished water and the population-based monitoring
approach for systems purchasing all of their finished water year-round.
As part of the proposal, EPA presented a monitoring cost analysis for
applying this approach to all systems in the Economic Analysis to
better understand the impacts of using the population-based approach.
    The plant-based approach was adopted from the 1979 TTHM rule and
the Stage 1 DBPR and was derived from the generally valid assumption
that, as systems increase in size, they tend to have more plants and
increased complexity. During the development of the Stage 2 proposal,
EPA identified a number of issues associated with the use of the plant-
based monitoring approach. These included: (1) Plant-based monitoring
is not as effective as population-based monitoring in targeting
locations with the highest risk; (2) a plant-based approach can result
in disproportionate monitoring requirements for systems serving the
same number of people (due to widely varying numbers of plants per
system); (3) it cannot be adequately applied to plants or consecutive
system entry points that are operated seasonally or intermittently if
an LRAA is used for compliance due to complex implementation and a need
for repeated transactions between the State and

system to determine whether and how compliance monitoring requirements
may need to be changed; (4) State determinations of monitoring
requirements for consecutive systems would be complicated, especially
in large combined distribution systems with many connections between
systems; and (5) systems with multiple disinfecting wells would have to
conduct evaluation of common aquifers in order to avoid taking
unnecessary samples for compliance (if they did not conduct such
evaluations under Stage 1). EPA requested comment on two approaches to
address these issues: (1) keep the plant-based monitoring approach and
add new provisions to address specific concerns; and (2) base
monitoring requirements on source water type and population served, in
lieu of plant-based monitoring.
    The final rule's requirements of population-based monitoring for
all systems are based on improved public health protection,
flexibility, and simplified implementation. For determining monitoring
requirements, EPA's objective was to maintain monitoring loads
consistent with Stage 1 and similar to monitoring loads proposed for
Stage 2 under a plant-based approach, using a population-based approach
to facilitate implementation, better target high DBP levels, and
protect human health. This leads to a more cost-effective
characterization of where high levels occur. For the proposed rule, EPA
used 1995 CWSS data to derive the number of plants per system for
calculating the number of proposed monitoring sites per system. During
the comment period, 2000 CWSS data became available. Compared to the
1995 CWSS, the 2000 CWSS contained questions more relevant for
determining the number of plants in each system. Based on 2000 CWSS
data, EPA has modified the number of monitoring sites per system for
several categories (particularly for the larger subpart H systems) to
align the median population-based monitoring requirements with the
median monitoring requirements under plant-based monitoring, as was
proposed.
    EPA also believes that more samples are necessary to characterize
larger systems (as defined by population) than for smaller systems.
This progressive approach is included in Table IV.G-4. As system size
increases, the number of samples increases to better reflect the
hydraulic complexity of these systems. While the national monitoring
burden under the population-based approach is slightly less than under
a plant-based approach, some larger systems with few plants relative to
system population will take more samples per system than they had under
plant-based monitoring. However, EPA believes that many of these large
systems with few plants have traditionally been undermonitored (as
noted in the proposal). Systems with more plants will see a reduction
in monitoring (e.g., small ground water systems with multiple wells).
    While population-based monitoring requirements for ground water
systems in today's rule remain the same as those in the proposed rule,
the final rule consolidates ten population categories for subpart H
systems into eight categories for ease of implementation. As indicated
in Table IV.G-4, EPA has gone from four to three population size
categories for smaller subpart H systems (serving fewer than 10,000
people) and the ranges have been modified to be consistent with those
for other existing rules (such as the Lead and Copper Rule). This
change will reduce implementation transactional costs. For medium and
large subpart H systems (serving at least 10,000 people), EPA has gone
from seven categories in the proposal to five categories in final rule.
The population groups are sized so that the ratio of maximum population
to minimum population for each of the categories is consistent. EPA
believes that this will allow most systems to remain in one population
size category and maintain the same monitoring requirements within a
reasonable range of population variation over time. In addition, it
assures that systems within a size category will not have disparate
monitoring burdens as could occur if there were too few categories.
Overall, EPA believes that the population-based monitoring approach
allows systems to have more flexibility to designate their monitoring
sites within the distribution system to better target high DBP levels
and is more equitable.
    To derive the number of monitoring sites for IDSE standard
monitoring, EPA doubled the number of routine compliance monitoring
sites per system for each size category. This is consistent with the
advice and recommendations of the M-DBP Advisory Committee for the
IDSE. EPA has developed the Initial Distribution System Evaluation
Guidance Manual for the Final Stage 2 Disinfectants and Disinfection
Byproducts Rule (USEPA 2006) to assist systems in choosing IDSE
monitoring locations, including criteria for selecting monitoring.

 Table IV.G-4.--Comparison of Monitoring Locations per System for Stage 2 Routine Compliance Monitoring with Plant-Based and Population-Based Approaches
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                            Plant-based    Number of plants per      Calculated number of
                                                                             approach*     system (Based on 2000     sites per system for
                                                    Ratio of               -------------        CWSS data)           plant-based approach     Number of
                                                    maximum     Number of               ----------------------------------------------------  monitoring
               Population category                 population    sampling                                            Based on     Based on    sites per
                                                   to minimum  periods per                                   median    mean     i> plants    pop-based
                                                                               plant                                plants per   per system    approach
                                                                                system
                                                  ...........            A            B            C            D        E=B*C        F=B*D            G
-------------------------------------------------
< 500............................................  ...........            1          **1            1         1.21            1          1.2          **1
500-3,300.......................................          6.6            4          **1            1         1.22            1          1.2          **1
3,301-9,999.....................................            3            4            2            1         1.56            2          3.1            2
10,000-49,999...................................            5            4            4            1         1.37            4          5.5            4
50,000-249,999..................................            5            4            4            1         1.83            4          7.3            8
250,000-< 1 million..............................            4            4            4            2         2.53            8         10.1           12
1 million-< 5 million............................            5            4            4            4         3.62           16         14.5           16
>=5 million.....................................  ...........            4            4            4         4.33           16         17.3          20
--------------------------------------------------------------------------------------------------------------------------------------------------------
* As in the proposal.
** System is required to take individual TTHM and HAA5 samples at the locations with the highest TTHM and HAA5 concentrations, respectively, if highest
  TTHM and HAA5 concentrations do not occur at the same location.

Note: To determine the number of routine compliance monitoring sites per population category, EPA took these steps: (1) Maintaining about the same
  sampling loads in the nation as required under the plant-based approach, but basing on population rather than number of plants to better target high
  DBP levels in distribution systems and facilitate implementation; (2) The number of monitoring sites per plant under the plant-based approach (Column
  B) were multiplied by the number of plants per system (Columns C and D) to calculate the number of monitoring sites per system under the plant-based
  approach (Columns E and F in terms of median and mean, respectively); and (3) The number of monitoring sites per system under the population-based
  approach were derived with adjustments to keep categories consistent and to maintain an even incremental trend as the population size category
  increases (Column G).

3. Summary of Major Comments
    EPA received significant support for applying the population-based
approach to all systems. EPA also received comments concerning the
specific requirements in a population-based approach.
    Excessive Sampling Requirements. Several commenters believed that
the proposed sampling requirements were excessive (especially in the
larger population categories for subpart H systems) and that some
individual systems would be required to sample more under the
population-based approach than the plant-based approach. EPA recognizes
that a small fraction of systems in some categories will have to take
more samples under the population-based approach than the plant-based
approach because their number of plants is substantially less than the
national median or mean. However, the number of samples required under
the Stage 1 DBPR for these systems may not have been sufficient to
determine the concentrations of DBPs throughout the distribution system
of these systems. On the other hand, systems with many plants may have
taken excessive samples under the Stage 1 DBPR that were not necessary
to appropriately determine DBP levels throughout the distribution
system. Consequently, the total number of samples taken nationally will
be comparable to the Stage 1 DBPR, but will better target DBP risks in
individual distribution systems.
    Consecutive systems. Some commenters noted that a consecutive
system may need to take more samples than its associated wholesale
system. Under today's rule, all systems, including consecutive systems,
must monitor based on retail population served. Thus, large consecutive
systems will take more samples than a smaller wholesale system. The
population-based monitoring approach will allow the samples to better
represent the DBP concentrations consumed by the population associated
with the sampling locations and to understand the DBP concentrations
reaching consumers. There is also a provision that allows States to
specify alternative monitoring requirements for a consecutive system in
a combined distribution system (40 CFR 142.16(m)(3)). This special
primacy condition allows the State to establish monitoring requirements
that account for complicated distribution system relationships, such as
where neighboring systems buy from and sell to each other regularly
throughout the year. In this case, water may pass through multiple
consecutive systems before it reaches a user. Another example would be
a large group of interconnected systems that have a complicated
combined distribution system. This approach also allows the combined
distribution system to concentrate IDSE and Stage 2 monitoring sites in
the system with the highest known DBP concentrations, while assigning
fewer sample sites to systems with low DBP concentrations.
    Population Size Categories. Some commenters recommended fewer
population categories for subpart H systems (those using surface water
or ground water under the direct influence of surface water as a
source) than proposed while others recommended more. Today's rule has
fewer categories than proposed. However, EPA believes that further
reduction of the number of population size categories will not reflect
the fact that the number of plants and complexity of distribution
systems (and DBP exposure) tend to increase as the population served
increases. As a result, the population served by a large system in one
particular category would receive much less protection from the DBP
risks than a smaller system in the same size category. On the other
hand, too many categories with smaller population ranges would result
in frequent category and requirement shifts as population fluctuates.
Much greater implementation effort would be needed for those systems
without much benefit in DBP exposure knowledge.
    Population Definition. Some commenters supported use of the
population of a combined distribution system (i.e., the wholesale and
consecutive systems should be considered a single system for monitoring
purposes) while others preferred use of the retail population for each
individual system (i.e., wholesale systems and consecutive systems are
considered separately). Today's final rule uses the retail population
for each individual system. EPA chose this approach for today's rule
because of the complexity involved in making implementation decisions
for consecutive systems. Using the retail population to determine
requirements eases the complexity by specifying minimum system-level
requirements; simplicity is essential for meeting the implementation
schedule in today's rule. If monitoring requirements were determined by
the combined distribution system population, many implementation
problems would occur. Some of these problems would have the potential
to impact public health protection. For example, States or primacy
agencies would have to decide how to allocate IDSE distribution system
samples (where and how much to monitor in individual PWSs) in a
complicated combined distribution system with many systems, multiple
sources, multiple treatment plants, and varying water demand and with
limited understanding of DBP levels throughout the combined
distribution system. This would have to happen shortly after rule
promulgation in order to meet the schedule. For example, some
consecutive systems buy water seasonally (in times of high water
demand) or buy from more than one wholesale system (with the volume
purchased based on many factors). The State or primacy agency would
find it difficult to properly assign a limited number of IDSE
monitoring locations (especially since there are States where many
consecutive systems have no DBP data) to adequately reflect DBP levels
in such a system, as well as throughout the combined distribution
system.
    EPA believes that assigning compliance monitoring requirements
appropriately throughout the combined distribution system requires a
case-by-case determination based on factors such as amount and
percentage of finished water provided; whether finished water is
provided seasonally, intermittently, or full-time; and improved DBP
occurrence information. Since the IDSE will provide improved DBP
occurrence information throughout the combined distribution system,
States may consider modifications to Stage 2 compliance monitoring
requirements for consecutive systems on a case-by-case basis as allowed
by Sec.  141.29 or under the special primacy condition at Sec.
142.16(m)(3) by taking all these factors into consideration. In making
these case-by-case determinations, the State will be able to use its
system-specific knowledge, along with the IDSE results, to develop an
appropriate monitoring plan for each

system within the combined distribution system.
    Changes to monitoring plans. Commenters requested more specific
language regarding how IDSE and Stage 2 monitoring plans should be
updated as a result of treatment or population changes in the
distribution system. Changes to IDSE plans should not be necessary
since the State or primacy agency will have reviewed those plans
shortly before the system must conduct the IDSE and the reviewed plan
should identify such issues. EPA provided a process in the Stage 2 DBPR
proposal for updating monitoring plans for systems that have
significant changes to treatment or in the distribution system after
they complete their IDSE. This process remains in today's rule, with an
added requirement that systems must consult with the State or primacy
agency to determine whether the changes are necessary and appropriate
prior to implementing changes to their Stage 2 monitoring plan.
    In addition, the State or primacy agency may require a system to
revise its IDSE plan, IDSE report, or Stage 2 monitoring plan at any
time. This change was made so that systems could receive system-
specific guidance from the State or primacy agency on the appropriate
revisions to the Stage 2 monitoring plan. Regulatory language regarding
changes that might occur is not appropriate because any modifications
would be system-specific and a national requirement is not capable of
addressing these system-specific issues.

H. Operational Evaluation Requirements Initiated by TTHM and HAA5
Levels

    A system that is in full compliance with the Stage 2 DBPR LRAA MCL
may still have individual DBP measurements that exceed the Stage 2 DBPR
MCLs, since compliance is based on individual DBP measurements at a
location averaged over a four-quarter period. EPA and the Advisory
Committee were concerned about these higher levels of DBPs. This
concern was clearly reflected in the Agreement in Principle, which
states, ``. . . significant excursions of DBP levels will sometimes
occur, even when systems are in full compliance with the enforceable
MCL. . .''.
    Today's final rule addresses this concern by requiring systems to
conduct operational evaluations that are initiated by operational
evaluation levels identified in Stage 2 DBPR compliance monitoring and
to submit an operational evaluation report to the State.
1. Today's Rule
    Today's rule defines the Stage 2 DBP operational evaluation levels
that require systems to conduct operational evaluations. The Stage 2
DBP operational evaluation levels are identified using the system's
Stage 2 DBPR compliance monitoring results. The operational evaluation
levels for each monitoring location are determined by the sum of the
two previous quarters' TTHM results plus twice the current quarter's
TTHM result, at that location, divided by 4 to determine an average and
the sum of the two previous quarters' HAA5 results plus twice the
current quarter's HAA5 result, at that location, divided by 4 to
determine an average. If the average TTHM exceeds 0.080 mg/L at any
monitoring location or the average HAA5 exceeds 0.060 mg/L at any
monitoring location, the system must conduct an operational evaluation
and submit a written report of the operational evaluation to the State.
    Operational evaluation levels (calculated at each monitoring
location)
    IF (Q1 + Q2 + 2Q3)/4> MCL, then
the system must conduct an operational evaluation

where:

    Q3 = current quarter measurement
    Q2 = previoius quarter measurement
    Q1 = quarter before previous quarter measurement

    MCL = Stage 2 MCL for TTHM (0.080 mg/l) or Stage 2 MCL for HAA5
(0.060 mg/L)
    The operational evaluation includes an examination of system
treatment and distribution operational practices, including changes in
sources or source water quality, storage tank operations, and excess
storage capacity, that may contribute to high TTHM and HAA5 formation.
Systems must also identify what steps could be considered to minimize
future operational evaluation level exceedences. In cases where the
system can identify the cause of DBP levels that resulted in the
operational evaluation, based on factors such as water quality data,
plant performance data, and distribution system configuration the
system may request and the State may allow limiting the evaluation to
the identified cause. The State must issue a written determination
approving limiting the scope of the operational evaluation. The system
must submit their operational evaluation report to the State for review
within 90 days after being notified of the analytical result that
initiates the operational evaluation. Requesting approval to limit the