targeted sampling efforts, including California, Maryland, and
Pennsylvania, demonstrate results consistent with the non-targeted
state monitoring. For example, in Pennsylvania, 26.3 and 24.9 percent
of monitored systems found PFOA and PFOS, respectively, with reported
concentrations of PFOA ranging from 1.7 to 59.6 ppt and PFOS ranging
from 1.8 to 94 ppt. California reported 26.2 and 29.9 percent of
monitored systems found PFOA and PFOS, respectively, including reported
concentrations of PFOA ranging from 0.9 to 120 ppt and reported
concentrations of PFOS from 0.4 to 250 ppt. In Maryland, PFOA and PFOS
were found in 57.6 and 39.4 percent of systems monitored, respectively,
with reported concentrations of PFOA ranging from 1.02 to 23.98 ppt and
reported concentrations of PFOS ranging from 2.05 to 235 ppt.
    As discussed above in section VI of this preamble, EPA is proposing
individual MCLs of 4.0 ppt for PFOA and PFOS, and an HI level of 1.0
for PFHxS, PFNA, PFBS, and HFPO-DA. EPA also evaluated occurrence for
the regulatory alternatives discussed in section VI of this preamble
including alternative MCLs for PFOA and PFOS of 5.0 ppt and 10.0 ppt.
Table 7, Table 8, and Table 9 demonstrate, based on available state
data, the total state reported number and percentages of monitored
systems that exceed these proposed and alternative MCL values across
the non-targeted state finished water monitoring data.

 Table 7--Non-Targeted State PFOS and PFOA Finished Water Data--Summary of Monitored Systems With State Reported
                                            Detections \1\ >=4.0 ppt
----------------------------------------------------------------------------------------------------------------
                                                               PFOS state                          PFOA state
                                           PFOS  monitored      reported       PFOA  monitored      reported
                  State                     systems with        monitored       systems with        monitored
                                           state reported    systems percent   state reported    systems percent
                                             detections         detection        detections         detection
----------------------------------------------------------------------------------------------------------------
Alabama \2\.............................                37               N/A                19               N/A
Colorado................................                22               5.5                18               4.5
Illinois................................                17               2.6                16               2.5
Kentucky................................                 4               4.9                 9              11.1

Massachusetts...........................                72              31.9                90              39.6
Michigan................................                15               0.7                24               1.1
New Hampshire...........................               107              19.1               210              37.5
New Jersey..............................               315              29.3               411              38.2
North Dakota............................                 0               0.0                 0               0.0
Ohio....................................                29               2.0                32               2.2
South Carolina..........................                27              52.9                30              58.8
Vermont.................................                16               2.9                24               4.3
----------------------------------------------------------------------------------------------------------------
Notes:
\1\ Detections determined by individual state reported limits which are not defined consistently across all
  states.
\2\ Only reported detections.

 Table 8--Non-Targeted State PFOS and PFOA Finished Water Data--Summary of Monitored Systems With State Reported
                                            Detections \1\ >=5.0 ppt
----------------------------------------------------------------------------------------------------------------
                                                               PFOS state                          PFOA state
                                           PFOS  monitored      reported       PFOA  monitored      reported
                  State                     systems with        monitored       systems with        monitored
                                           state reported    systems percent   state reported    systems percent
                                             detections         detection        detections         detection
----------------------------------------------------------------------------------------------------------------
Alabama \2\.............................                31               N/A                15               N/A
Colorado................................                16               4.0                14               3.5
Illinois................................                12               1.8                11               1.7
Kentucky................................                 3               3.7                 4               4.9
Massachusetts...........................                64              28.3                83              36.6
Michigan................................                12               0.6                17               0.8
New Hampshire...........................                86              15.4               186              33.2
New Jersey..............................               272              25.3               363              33.7
North Dakota............................                 0               0.0                 0               0.0
Ohio....................................                29               2.0                32               2.2
South Carolina..........................                25              49.0                25              49.0
Vermont.................................                13              2.33                16               2.9
----------------------------------------------------------------------------------------------------------------
Notes:
\1\ Detections determined by individual state reported limits which are not defined consistently across all
  states.
\2\ Only reported detections.

 Table 9--Non-Targeted State PFOS and PFOA Finished Water Data--Summary of Monitored Systems With State Reported
                                            Detections \1\ >=10.0 ppt
----------------------------------------------------------------------------------------------------------------
                                                               PFOS state                          PFOA state
                                           PFOS  monitored      reported       PFOA  monitored      reported
                  State                     systems with        monitored       systems with        monitored
                                           state reported    systems percent   state reported    systems percent
                                             detections         detection        detections         detection
----------------------------------------------------------------------------------------------------------------
Alabama \2\.............................                23               N/A                 8               N/A
Colorado................................                 3               0.8                 2               0.5
Illinois................................                 3               0.5                 6               0.9
Kentucky................................                 1               1.2                 1               1.2
Massachusetts...........................                32              14.2                32              14.1
Michigan................................                 6               0.3                 7               0.3
New Hampshire...........................                39               7.0                83              14.8
New Jersey..............................               133              12.4               189              17.6
North Dakota............................                 0               0.0                 0               0.0
Ohio....................................                20               1.4                15               1.0
South Carolina..........................                 3               5.9                 3               5.9
Vermont.................................                 4               0.7                 7               1.3
----------------------------------------------------------------------------------------------------------------
Notes:
\1\ Detections determined by individual state reported limits which are not defined consistently across all
  states.
\2\ Only reported detections.

    Based on the available state data evaluated and presented in Table
7, Table 8, and Table 9, within 12 states that conducted non-targeted
monitoring there are 661 systems that show exceedances of the proposed
PFOS MCL

of 4.0 ppt and 883 systems with exceedances of the proposed PFOA MCL of
4.0 ppt. These systems serve populations of approximately 8.8 and 10.5
million people, respectively. As expected, the number of systems
exceeding either of the proposed alternative MCLs decreases as the
values are higher, however, even at the highest alternative PFOS and
PFOA MCL values of 10.0 ppt, would still be 267 and 353 systems with
exceedances, serving populations of approximately 3.7 and 4.4 million
people, respectively.
    Monitoring data for PFOA and PFOS from states that conducted
targeted sampling efforts shows additional systems that would exceed
the proposed and alternative MCLs. For example, in California, Maine,
Maryland, and Pennsylvania, 23.4 percent (25 PWSs), 30.4 percent (7
PWSs), 22.7 percent (15 PWSs), and 19.3 percent (66 PWSs) of monitored
systems exceeded the proposed PFOS MCL of 4.0 ppt, respectively, and
20.6 percent (22 PWSs), 21.7 percent (5 PWSs), 25.8 percent (17 PWSs),
and 21.1 percent (72 PWSs) of monitored systems exceeded the proposed
PFOA MCL of 4.0 ppt, respectively. While these frequencies may be
anticipated given the sampling locations, within only these four states
that conducted limited, targeted monitoring, the monitored systems
exceeding the proposed PFOS MCL and proposed PFOA MCL serve significant
populations of approximately 4.6 million people and approximately 4.4
million people, respectively.

C. Co-Occurrence

    While the discussions in sections III.B, VII.A. and VII.B of this
preamble describe how PFOA, PFOS, PFHxS, HFPO-DA, PFNA, and PFBS occur
individually, PFAS have been documented to co-occur in finished
drinking water (Adamson et al., 2017; Cadwallader et al., 2022; Guelfo
and Adamson, 2018). As discussed in section VI of this preamble, EPA is
proposing regulation of four PFAS including PFHxS, HFPO-DA, PFNA, and
PFBS (collectively referred to as ``HI PFAS'') as part of an HI
approach. Sampling results in the aggregated state dataset were
examined to determine the extent to which the HI PFAS occurred with
each other as well as with PFOA and/or PFOS. This involved considering
the observed occurrence in terms of grouping (i.e., groups of HI PFAS
and ``PFOS or PFOA'') as well as pairwise by means of odds ratios. For
the group assessment, the aggregated state dataset was limited to
samples from non-targeted monitoring efforts where at least one HI PFAS
was analyzed and PFOS and PFOA were analyzed sufficiently to determine
whether one was present.
1. Groupwise Chemical Co-Occurrence
    Table 10 shows the distribution of systems and samples according to
whether states report detections for any HI PFAS (PFHxS, HFPO-DA, PFNA
and PFBS) and whether they also reported detections of PFOS or PFOA.
USEPA (2023e) provides additional information for this analysis.

 Table 10--Non-Targeted State PFAS Finished Water Data--Samples and Systems Binned According to Whether PFOS or
                    PFOA Were Reported by States and Whether Additional HI PFAS Were Reported
----------------------------------------------------------------------------------------------------------------
                                        No PFOS or PFOA reported            PFOS or PFOA reported
                                  ----------------------------------------------------------------------
               Type                                    At least one                       At least one    Total
                                      No HI PFAS         HI PFAS         No HI PFAS         HI PFAS       count
                                       reported          reported         reported          reported
----------------------------------------------------------------------------------------------------------------
Samples..........................    12,704 (65.2%)       357 (1.8%)     3,380 (17.3%)    3,041 (15.6%)   19,482
Systems..........................     5,560 (78.8%)       196 (2.8%)        516 (7.3%)      784 (11.1%)    7,056
----------------------------------------------------------------------------------------------------------------

    Considering eligible samples and systems within the aggregated
state dataset, states reported detections of either PFOS, PFOA, or one
or more HI PFAS in 34.8 percent (6,778 of 19,482) of samples and 21.2
percent (1,496 of 7,056) of systems. When any PFAS (among PFOA, PFOS,
and the HI PFAS) were reported detected, at least one HI PFAS was also
reported in 50.1 percent (3,398 of 6,778) of samples and at 65.5
percent (980 of 1,496) of systems. Further, among samples and systems
that reported detections of PFOS or PFOA, at least one HI PFAS was
detected in 47.4 percent (3,041 of 6,421) of samples and at 60.3
percent (784 of 1,300) of systems. This demonstrated strong co-
occurrence of HI PFAS with PFOA and PFOS and a substantial likelihood
(over 50 percent) of at least one HI PFAS being present at systems with
reported detections of PFOS or PFOA. Overall, one or more HI PFAS were
reported at about 13.9 percent (980 of 7,056) of systems included in
the aggregated state dataset of non-targeted monitoring. If this
percentage were extrapolated to the nation, one or more HI PFAS would
be at detectable levels in over 9,000 systems. Table 11 shows the
distribution of systems in a similar manner but provides a breakdown by
state and includes only systems that monitored for either three or four
of the HI PFAS.

 Table 11--Non-Targeted State PFAS Finished Water Data--Systems That Sampled for 3 or 4 HI PFAS Binned According
         to Whether PFOS or PFOA Were Reported and Whether Any Additional HI PFAS Were Reported by State
----------------------------------------------------------------------------------------------------------------
                                        No PFOA/S detected                    PFOA/S detected             Total
             State             ------------------------------------------------------------------------  system
                                  No HI detected     HI detected      No HI detected     HI detected      count
----------------------------------------------------------------------------------------------------------------
CO............................        270 (68.0%)        26 (6.5%)          11 (2.8%)       90 (22.7%)       397
IL............................        582 (89.7%)        22 (3.4%)          15 (2.3%)        30 (4.6%)       649
KY............................         37 (52.9%)         2 (2.9%)         16 (22.9%)       15 (21.4%)        70
MA............................         60 (35.5%)         2 (1.2%)          12 (7.1%)       95 (56.2%)       169
MI............................      1,969 (91.5%)        82 (3.8%)          43 (2.0%)        58 (2.7%)     2,152
ND............................           49 (98%)         1 (2.0%)           0 (0.0%)         0 (0.0%)        50
NH............................         60 (43.2%)         2 (1.4%)         34 (24.5%)       43 (30.9%)       139
NJ............................        225 (36.3%)         7 (1.1%)        127 (20.5%)      261 (42.1%)       620

OH............................      1,397 (94.5%)        31 (2.1%)          25 (1.7%)        26 (1.8%)     1,479
SC............................         10 (22.2%)         1 (2.2%)         10 (22.2%)       24 (53.3%)        45
VT............................        488 (87.6%)        15 (2.7%)          31 (5.6%)        23 (4.1%)       557
----------------------------------------------------------------------------------------------------------------

    The percentage of systems included in Table 11 that reported
detections of any HI PFAS ranged from 2.0 to 57.4 percent of systems
when broken down by state, with six states exceeding 20 percent of
systems. The percentage of systems that reported detections of any PFAS
ranged from 2.0 to 77.8 percent. Many systems and/or samples that were
included in the aggregated state dataset did not monitor for all four
HI PFAS. It is possible that more systems would have detected HI PFAS
if they had monitored for all four HI PFAS. Additionally, as
demonstrated in Table 11, when PFOA and/or PFOS were reported, at least
one of the HI PFAS chemicals were also frequently reported. Table 12
presents system counts for systems where PFOS or PFOA were detected
according to (a) how many HI PFAS were monitored and (b) how many HI
PFAS were reported to be detected.

 Table 12--Non-Targeted State PFAS Finished Water Data--System Counts According to HI PFAS Analyzed and Reported Present for Systems Where PFOS and PFOA
                                                                      Were Reported
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                HI reported present
                          HI analyzed                           --------------------------------------------------------------------------------  Total
                                                                        0               1               2               3               4
--------------------------------------------------------------------------------------------------------------------------------------------------------
1..............................................................     143 (70.1%)      61 (29.9%)  ..............  ..............  ..............      204
2..............................................................      49 (45.8%)      41 (38.3%)      17 (15.9%)  ..............  ..............      107
3..............................................................     153 (34.7%)      95 (21.5%)     137 (31.1%)      56 (12.7%)  ..............      441
4..............................................................     171 (31.2%)     135 (24.6%)     179 (32.7%)      61 (11.1%)        2 (0.4%)      548
                                                                ----------------------------------------------------------------------------------------
    Total......................................................             516             332             333             117               2  .......
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Among systems that reported detections of PFOS and/or PFOA, the
fraction of systems that also reported detections of any HI PFAS tended
to increase as systems monitored for more of the HI PFAS. At systems
monitoring for a single HI PFAS, 29.9 percent reported a detection at
some point during sampling. This increased to 68.8 percent of systems
reporting detections of at least one HI PFAS when monitoring for all
four HI PFAS. Not only did the fraction of systems reporting detections
of any HI PFAS increase as the number of HI PFAS increased, so did the
number of HI PFAS that were reported. When three or four HI PFAS were
monitored, over 40 percent of systems reported detections of two to
three of the HI PFAS. Thus, if PFOS or PFOA are reported, there is a
reasonable likelihood that multiple HI PFAS would be present as well.
2. Pairwise Chemical Co-Occurrence
    In addition to considering the co-occurrence of six PFAS as two
groups, EPA conducted a pairwise analysis to further explore co-
occurrence relationships. Table 13 shows the calculated system-level
odds ratios for every unique pair of PFAS chemicals evaluated. The
equation for calculating odds ratios is symmetrical. Because of this,
in a given row it does not matter which chemical is ``Chemical A'' and
which is ``Chemical B.'' Additional information on odds ratios may be
found in USEPA (2023e) and a brief explanation is described following
Table 13.

  Table 13--Non-Targeted State PFAS Finished Water Data--System-Level Counts of Pairwise Chemical Occurrence and Odds Ratios Calculated From Aggregated
                                                 State Dataset PFAS Samples for PFOS, PFOA, and HI PFAS
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                   Chems A  and B    Only Chem B     Only Chem A    Neither Chem
                Chem A                            Chem B              reported        reported        reported        reported      Odds ratio  [95% CI]
--------------------------------------------------------------------------------------------------------------------------------------------------------
HFPO-DA...............................  PFBS.....................              10             452              10           5,116        11.3 [4.8-26.7]
HFPO-DA...............................  PFHxS....................               2             339              18           5,229          1.7 [0.4-6.7]
HFPO-DA...............................  PFNA.....................               2              77              18           5,491         7.9 [2.0-31.4]
HFPO-DA...............................  PFOA.....................              16             438               4           5,129      46.8 [16.3-134.1]
HFPO-DA...............................  PFOS.....................              14             399               6           5,168       30.2 [11.9-76.5]
PFBS..................................  PFHxS....................             433             133             261           5,501       68.6 [54.5-86.5]
PFBS..................................  PFNA.....................             135              33             560           5,601       40.9 [27.7-60.4]
PFBS..................................  PFOA.....................             517             360             178           5,273       42.5 [34.8-52.0]
PFBS..................................  PFOS.....................             503             278             192           5,355       50.5 [41.1-62.0]
PFHxS.................................  PFNA.....................             150              38             473           5,939       49.6 [34.3-71.6]
PFHxS.................................  PFOA.....................             510             466             113           5,510       53.4 [42.6-66.9]
PFHxS.................................  PFOS.....................             507             353             116           5,623       69.6 [55.4-87.6]

PFNA..................................  PFOA.....................             236             934              15           5,871      98.9 [58.7-166.5]
PFNA..................................  PFOS.....................             234             789              17           6,016     105.0 [64.1-171.9]
PFOA..................................  PFOS.....................             893             130             277           5,756    142.7 [114.5-177.9]
--------------------------------------------------------------------------------------------------------------------------------------------------------

    Odds ratios reflect the change in the odds of detecting one
chemical (e.g., Chemical A) given that the second chemical (e.g.,
Chemical B) is known to be present compared to the odds of detecting if
the second chemical is not present. For example, as shown in Table 13,
the point estimate of 142.7 for the odds ratio between PFOA and PFOS
indicates that the odds of detecting PFOA after knowing that PFOS has
been observed are 142.7 times what the odds would have been if PFOS was
not observed, and vice versa. For every pair of chemicals, except for
HFPO-DA and PFHxS, both the point estimate and 95 percent CI were above
1, indicating significant increases in the likelihood of detecting one
chemical if the other is present. For HFPO-DA and PFHxS, 1 fell within
the 95 percent CI, and thus the odds ratio was not determined to be
statistically significantly different from 1.
    Both as a group and as individual chemicals, the HI PFAS had a
higher likelihood of being reported if PFOS or PFOA were present.
PFHxS, HFPO-DA, PFNA and PFBS (the individual HI PFAS) are demonstrated
to generally co-occur with each other, as well. As such, these data
support that there is a substantial likelihood PFHxS, HFPO-DA, PFNA,
and PFBS co-occur with a frequency of public health concern in drinking
water systems.

D. Occurrence Relative to the Hazard Index

    EPA analyzed the available state data in comparison to the proposed
HI MCL of 1.0 to evaluate the co-occurrence of PFHxS, HFPO-DA, PFNA,
and PFBS. Table 14 presents the total number and percentage of
monitored systems that exceeded the proposed HI MCL based on state
reported HI PFAS detections for the states that conducted non-targeted
monitoring and that sampled all four HI PFAS as a part of their overall
monitoring efforts. EPA notes that for equivalent comparison purposes
Table 14 only accounts for samples that included reported values
(including non-detects) of all four HI PFAS. As shown within the table,
the majority of states evaluated had monitored systems exceed the
proposed HI MCL, ranging from 0.72 to 7.41 percent of total monitored
systems.

 Table 14--Non-Targeted State PFAS Finished Water Data--Summary of Total
  Number and Percent of Monitored Systems Exceeding the HI With Samples
                Containing Reported Values of All HI PFAS
------------------------------------------------------------------------
                                 Total monitored
            State              systems > proposed    Percent  systems >
                                    HI of 1.0        proposed  HI of 1.0
------------------------------------------------------------------------
Colorado....................                     5                  1.26
Illinois....................                    10                  1.54
Kentucky....................                     6                  7.41
Massachusetts...............                     8                  6.40
Michigan....................                    14                  0.65
New Hampshire...............                     4                  2.99
North Dakota................                     0                  0.00
Ohio........................                    25                  1.69
South Carolina..............                     0                  0.00
Vermont.....................                     4                  0.72
------------------------------------------------------------------------

    Further evaluating the available state data related to the proposed
HI MCL of 1.0, Table 15 presents the total number of systems and
associated populations served that exceed the proposed HI of 1.0 based
on state reported HI PFAS detections for the same states shown in Table
15. However, in this case, EPA also analyzed the same non-targeted
state data adding in additional samples even if those samples did not
contain reported values (including non-detects) for all four HI PFAS
(i.e., exceeding the HI based on only one to three HI PFAS with
reported values included within a sample). Moreover, while these states
did monitor for all four HI PFAS as a part of their overall monitoring,
in a subset of those states some samples did not include reported data
on all four HI PFAS (i.e., values of one or more of the HI PFAS were
not reported as non-detect, rather no value was reported). This
analysis, presented in Table 15, shows an increase in the number of
monitored systems exceeding the proposed HI of 1.0 and demonstrates
prevalence of these PFAS at levels of concern, even when all four PFAS
may not be included within a sample.

 Table 15--Non-Targeted State PFAS Finished Water Data--Summary of Total
   Monitored Systems Exceeding the HI With Samples Containing Reported
                     Values of Any Number of HI PFAS
------------------------------------------------------------------------
                                 Total monitored
            State              systems > proposed     Population served
                                    HI of 1.0
------------------------------------------------------------------------
Colorado....................                     5                 5,429
Illinois....................                    10               107,461
Kentucky....................                     6               103,315
Massachusetts...............                    19               302,482
Michigan....................                    14               221,484
New Hampshire...............                    25                36,463
North Dakota................                     0                     0
Ohio........................                    25               234,834
South Carolina..............                     0                     0
Vermont.....................                     4                   410
------------------------------------------------------------------------

    Combining the non-targeted monitoring results shown previously with
targeted state monitoring conducted for all four HI PFAS showed at
least 917 samples from 157 PWSs in 15 states that exceed the proposed
HI of 1.0 for PFHxS, HFPO-DA, PFNA, and PFBS. These systems serve
approximately 3.08 million people. Additionally, data from New Jersey,
which conducted non-targeted monitoring but did not conduct any
monitoring that included all four HI PFAS, showed an additional 243
samples within 57 systems serving a total population of approximately
1.43 million people exceeding the proposed HI of 1.0 based solely upon
the reported detections of three of the four HI PFAS (i.e., PFHxS,
PFNA, and PFBS). USEPA (2023e) presents a detailed discussion on state
PFAS monitoring information. More information on occurrence in state
monitoring is available in section III.B. of this preamble.
    In summary, the finished water data collected under both non-
targeted and targeted state monitoring efforts from 22 states showed
there are at least 1,007 PWSs serving a total population of
approximately 15.3 million people that have at least one result
exceeding the proposed PFOA MCL of 4.0 ppt. In those same 22 states,
there are also at least 805 PWSs serving a total population of
approximately 13.6 million people that have at least one result
exceeding the proposed PFOS MCL of 4.0 ppt. Related to the proposed HI,
finished water data collected under both non-targeted and targeted
state monitoring efforts in 16 states showed there are at least 214
systems serving a total population of approximately 4.5 million people
that exceed the proposed HI value of 1.0 for PFHxS, HFPO-DA, PFNA, and
PFBS. USEPA (2023e) presents a detailed discussion on state PFAS
monitoring information. Additionally, EPA is aware that since the data
were collected some of these states may have updated data available and
that additional states have or intend to conduct monitoring of finished
drinking water, such as New York and Virginia. EPA will consider, and
as appropriate, analyze additional data submitted in response to this
proposal to inform future regulatory decision making.

E. Occurrence Model

    A Bayesian hierarchical occurrence model was developed to explore
national occurrence of the four PFAS that were most frequently detected
in the UCMR 3: PFOS, PFOA, PFHxS, and PFHpA. While PFNA and PFBS were
included in the UCMR 3 as well, they lacked sufficient reported values
above the UCMR 3 MRLs to be incorporated into the model. The model has
been peer reviewed and is described extensively in Cadwallader et al.
(2022). Briefly, inputs to the model include the UCMR 3 dataset as well
as subsequent data in publicly available state datasets that were
collected at PWSs that took part in the UCMR 3. 23,130 analytical
results from state datasets were used to supplement the UCMR 3. These
results were derived from 17 state datasets. The objective of the model
was to enable national estimates of PFAS occurrence by using available
UCMR 3 and state data to inform occurrence distributions both within
and across PWSs. Note that while PFHpA was included in the model
because of its UCMR 3 occurrence data availability, EPA is not
proposing to regulate it in this document.
    The model uses Markov chain Monte Carlo (MCMC) and the assumption
of lognormality in PFAS chemical occurrence. After log-transforming all
available data, system-level means (where each system has a mean
concentration for each chemical) were assumed to be distributed
multivariate normally. Further, within-system occurrence was assumed to
be distributed normally for each chemical. Since system-level means are
distributed multivariate normally, correlation between estimated
system-level means across chemicals could also be assessed. The
assumption of lognormality as well as the incorporation of state data
with lower reporting limits allowed the model to generate reasonable
estimates for PFAS occurrence at levels below the UCMR 3 MRLs. EPA has
used similar hierarchical statistical models to inform regulatory
decision making in the past, such as for development of the NPDWR for
Arsenic and Cryptosporidium parvum (USEPA, 2006b; USEPA, 2000e).
    After the model was fit with available data from PWSs that were
included in the UCMR 3, it was used to simulate occurrence at an
inventory of active CWS and NTNCWS extracted from the Safe Drinking
Water Information System (SDWIS). System-level means for non-UCMR 3
systems were simulated by sampling from the multivariate normal
distribution of system-level means that was produced during the model
fitting process. For systems that were included in the UCMR 3, the
fitted system-level mean was used directly. Using population data
retrieved from SDWIS, the total number of systems with system-level
mean concentrations of each chemical, as well as their associated
population served, could be estimated. The median estimate and the 90
percent credible interval are shown for the systems with system-level
means at or above various PFAS concentrations in Table 16 and the
population served by those systems in Table 17.

  Table 16--National Occurrence Model Estimate--Estimated Number of Systems With System-Level Means at or Above
                                             Various Concentrations
----------------------------------------------------------------------------------------------------------------
            Concentration  (ppt)                PFHxS  [90% CI]         PFOA  [90% CI]         PFOS  [90% CI]
----------------------------------------------------------------------------------------------------------------
4.0........................................    1,697 [1,053-2,702]    1,987 [1,338-3,016]    3,427 [2,326-4,900]
5.0........................................      1,232 [745-2,009]      1,351 [903-2,083]    2,593 [1,737-3,770]
10.0.......................................          417 [241-730]          349 [223-577]        986 [627-1,531]
----------------------------------------------------------------------------------------------------------------

 Table 17--National Occurrence Model Estimate--Estimated Population Served by Systems With System-Level Means at
                                         or Above Various Concentrations
----------------------------------------------------------------------------------------------------------------
    Concentration  (ppt)            PFHxS  [90% CI]             PFOA  [90% CI]              PFOS  [90% CI]
----------------------------------------------------------------------------------------------------------------
4.0.........................                  18,641,000                  28,051,000                  30,627,000
                                 [15,669,000-21,693,000]     [24,966,000-33,071,000]     [27,407,000-35,665,000]
5.0.........................                  14,092,000                  20,844,000                  24,405,000
                                 [11,129,000-16,887,000]     [18,193,000-24,239,000]     [21,611,000-28,440,000]
10.0........................                   4,608,000                   7,111,000                  10,561,000
                                   [3,432,000-7,262,000]       [5,566,000-9,335,000]      [7,858,000-12,866,000]
----------------------------------------------------------------------------------------------------------------

    For PFOA, PFOS, and PFHxS, thousands of systems were estimated to
have mean concentrations over the lowest thresholds (i.e., 4.0 and 5.0
ppt) presented in Tables 16 and 17 with the total population served
estimated to be in the tens of millions. The populations shown here
represent the entire populations served by systems estimated to have
system-level means over the various thresholds. It is likely that
different subpopulations would be exposed to different mean PFAS
concentrations if multiple source waters are used.
    In addition to the estimates of individual chemical occurrence, the
multivariate normal distribution of system-level means allowed the
model to provide insight on estimated co-occurrence. Untransformed
estimates of system-level means were assessed for correlation across
each unique pair of the four modeled chemicals included in the model.
Estimates of the Pearson correlation coefficient are shown in Table 18.
The Pearson correlation coefficient serves as an indicator of the
strength of the linear relationship between two variables and may range
from -1 to 1. Positive values indicate a positive relationship (i.e.,
as one variable increases, so does the other).

 Table 18--National Occurrence Model Estimate--Median Estimated Pearson
  Correlation Coefficient and 90% Credible Interval Among System-Level
                                  Means
------------------------------------------------------------------------
                                                    Pearson correlation
                  Chemical pair                    coefficient  [90% CI]
------------------------------------------------------------------------
PFOS-PFOA........................................       0.71 [0.60-0.79]
PFOS-PFHpA.......................................       0.69 [0.57-0.78]
PFOS-PFHxS.......................................       0.85 [0.74-0.92]
PFOA-PFHpA.......................................       0.85 [0.80-0.89]
PFOA-PFHxS.......................................       0.55 [0.41-0.65]
PFHpA-PFHxS......................................       0.62 [0.47-0.72]
------------------------------------------------------------------------

    EPA considered a moderate strength correlation as greater than 0.5
and a strong correlation as greater than 0.7. Each point estimate of
correlation coefficients between two chemicals was above the threshold
for a moderate strength correlation. The carboxylic acids (PFOA-PFHpA)
and sulfonic acids (PFOS-PFHxS) had the highest estimated correlation
strengths, with both the point estimate and the 90% credible interval
above 0.7. PFOS-PFOA and PFOS-PFHpA had similar point estimates and 90%
credible interval ranges, spanning the moderate-to-strong correlation
range. Both PFOA-PFHxS and PFHpA-PFHxS had the bulk of their posterior
distributions fall in the range of a moderate strength correlation.
Thus, the model predicted significant positive relationships among
system-level means of all four chemicals that were included. These
results support the co-occurrence discussion presented in section VII.C
of this preamble that indicated extensive co-occurrence of PFOA, PFOS,
and the HI PFAS observed in state datasets from both groupwise and
pairwise chemical perspectives.

F. Combining State Data With Model Output To Estimate National
Exceedance of Either MCLs or Hazard Index

    In order to broadly estimate the number of systems that would be
impacted by the proposed regulation, including MCLs of 4.0 ppt for PFOA
and PFOS alongside an HI of 1.0 for PFHxS, HFPO-DA, PFNA, and PFBS,
findings from non-targeted monitoring in state datasets were combined
with model estimates. Specific details on the methodology can be found
in USEPA (2023e). Briefly, information collected from non-targeted
state datasets included the fractions of systems that reported a
measurement at or above the UCMR 5 MRL for a given analyte and an
empirical cumulative distribution function (eCDF) consisting of system-
level maximum observed concentrations of that chemical at these
systems. The UCMR 5 MRLs for HFPO-DA, PFNA, and PFBS are equivalent to
5.0 ppt, 4.0 ppt, and 3.0 ppt, respectively (USEPA, 2021e). This
applies the assumption that

the fraction of systems that observed HFPO-DA, PFNA, and PFBS at or
above UCMR 5 MRLs and the maximum concentrations observed at those
systems are reasonably representative of the nation.
    The model was used to simulate entry point-level concentrations of
the four modeled PFAS (PFOA, PFOS, PFHpA, and PFHxS) under the
assumption that within-system concentrations are lognormally
distributed (a common assumption for drinking water contaminants, see
(Cadwallader et al. (2022)) and that variability in concentrations is
entirely across entry points (thus a given entry point is assumed to
have a constant concentration) For each system, the maximum estimated
entry point PFOA or PFOS concentration was selected to determine
whether the system exceeded either of the proposed MCLs of 4.0 ppt. The
entry point with the maximum concentration is the point that determines
whether a system has an entry point that is above an MCL. Estimates of
the system-level maximum for PFHxS were also selected for the HI
calculation. The maximum value of the sum of the four modeled PFAS at
each system was selected and used as a basis for determining which
systems would receive superimposed concentrations of the three
remaining HI chemicals (HFPO-DA, PFNA, and PFBS). This approach was
selected due to the extensive observed co-occurrence of PFAS in the
UCMR 3, state data, and modeled estimates.
    Multiple methods of system selection were used that reflected
different degrees of co-occurrence. The chemical concentration that was
applied to selected systems were randomly sampled from the eCDF for
each chemical. Based on the model output, this assumes that system-
level maximums for HFPO-DA, PFNA, and PFBS would occur at the same
location within a system. Substantial co-occurrence among PFAS was
observed in the model output, state datasets, and the UCMR 3 dataset.
Combination of system-level maximums independently pulled from chemical
eCDFs is a reasonable simplifying assumption given this co-occurrence.
This is particularly true given that the systems selected for each
chemical are not necessarily the same and in most cases were
probability-weighted. Estimates of the range of systems impacted were
developed by taking Q5 and Q95 estimates for each method. The low end
of the range was taken as the lowest Q5 estimate across methods,
rounded down, while the high end of the range was taken as the highest
Q95 estimate across methods, rounded up. This was also done for the
total population served by these systems.
    The resulting range of systems estimated to be impacted by the
proposed regulation of an MCL of 4.0 ppt for PFOA and PFOS and an HI of
1.0 for a mixture of PFHxS, HFPO-DA, PFNA, and PFBS was 3,400-6,300
systems serving a total population of 70-94 million people. Among these
systems, 100-500 were estimated to be systems exceeding the HI for
PFHxS, HFPO-DA, PFNA, and PFBS that had not already exceeded the MCLs
for PFOA and/or PFOS. The total population served by these systems was
estimated to be 0.6 to 6.3 million people.
    In summary, using the MCMC occurrence model, EPA estimated baseline
occurrence to derive occurrence and exposure estimates for the proposed
MCLs for PFOA and PFOS, as well as alternative MCLs. EPA then used
these modeled estimates to inform the costs and benefits determination
as described in section XIII of this preamble. Here and in section XIII
of this preamble, EPA requests comment on the number of systems
estimated to solely exceed the HI (but not the PFOA or PFOS MCLs)
according to the approach outlined in USEPA (2023e).

VIII. Analytical Methods

    EPA developed the following liquid chromatography/tandem mass
spectrometry (LC/MS/MS) analytical methods to quantitatively monitor
drinking water for targeted PFAS: EPA Method 533 (USEPA, 2019b) and EPA
Method 537.1, Version 2.0 (USEPA, 2009b; USEPA, 2020a). All six PFAS
proposed for regulation can be measured by both EPA Methods 533 and
537.1 and both methods are acceptable for meeting the monitoring
requirements of this regulation.
    EPA Method 533 monitors for 25 select PFAS, including PFOA, PFOS,
PFHxS, HFPO-DA, PFNA, and PFBS, with published measurement accuracy and
precision data for PFOA in reagent water, finished ground water, and
finished surface water. For further details about the procedures for
this analytical method, please see Method 533: Determination of Per-
and Polyfluoroalkyl Substances in Drinking Water by Isotope Dilution
Anion Exchange Solid Phase Extraction and Liquid Chromatography/Tandem
Mass Spectrometry (USEPA, 2019b).
    EPA Method 537.1 (an update to EPA Method 537), monitors for 18
select PFAS, including PFOA, PFOS, PFHxS, HFPO-DA, PFNA, and PFBS, with
published measurement accuracy and precision data for PFOA in reagent
water, finished ground water, and finished surface water. For further
details about the procedures for this analytical method, please see
Method 537.1, Version 2.0, Determination of Selected Per- and
Polyfluorinated Alkyl Substances in Drinking Water by Solid Phase
Extraction and Liquid Chromatography/Tandem Mass Spectrometry (LC/MS/
MS) (USEPA, 2020a).

A. Practical Quantitation Levels (PQLs) for Regulated PFAS

    As described in section VI of this preamble, a PQL is defined as
the ``lowest concentration of an analyte that can be reliably measured
within specified limits of precision and accuracy during routine
laboratory operating conditions'' (USEPA, 1985). EPA uses the PQL to
estimate or evaluate the minimum, reliable quantitation level that most
laboratories can be expected to meet during day-to-day operations. The
basis for setting PQLs is (1) quantitation, (2) precision and accuracy,
(3) normal operations of a laboratory, and (4) the fundamental need (in
the compliance monitoring program) to have a sufficient number of
laboratories available to conduct the analyses. For the PFAS regulated
in this proposal, EPA is proposing the following PQLs outlined in Table
19:

                    Table 19--PQLs for Regulated PFAS
------------------------------------------------------------------------
                                                                  PQL
                         Contaminant                             (ppt)
------------------------------------------------------------------------
PFOA.........................................................        4.0
PFOS.........................................................        4.0
HFPO-DA......................................................        5.0
PFHxS........................................................        3.0
PFNA.........................................................        4.0
PFBS.........................................................        3.0
------------------------------------------------------------------------

    Drinking water analytical laboratories have different performance
capabilities dependent upon their instrumentation (manufacturer, age,
usage, routine maintenance, operating configuration, etc.) and analyst
experience. Some laboratories will effectively generate accurate,
precise, quantifiable results at lower concentrations than others.
Organizations that collect data need to establish data quality
objectives (DQOs) to meet the needs of their program. These DQOs should
consider establishing reasonable quantitation levels that laboratories
can routinely meet. Establishing a quantitation level that is too low
may result in recurring QC failures that will necessitate repeating
sample analyses, increase

costs, and potentially reduce laboratory capacity. Establishing a
quantitation level that is too high may result in important lower-
concentration results not being quantitated.
    EPA's approach to establishing DQOs within the UCMR program serves
as an example. EPA established MRLs for UCMR 5, finalized in December
2021, and requires laboratories approved to analyze UCMR samples to
demonstrate that they can make quality measurements at or below the
established MRLs. EPA calculated the UCMR 5 MRLs using quantitation-
limit data from multiple laboratories participating in an MRL-setting
study. An MRL is set after a statistical determination that 75% of
laboratories will be able to meet that level with a 95% CI (USEPA,
2022g). The UCMR 5 MRLs are not intended to represent the lowest
achievable measurement level an individual laboratory may achieve. As
noted above, these MRLs are derived using the quantitation level
results from multiple laboratories participating in an analytical study
and account for differences in the capability of laboratories across
the country.
    For UCMR 5, EPA calculated and published the following multi-
laboratory MRLs for the PFAS addressed in this proposed rule: PFOA:
0.004 [micro]g/L (4.0 ppt); PFOS: 0.004 [micro]g/L (4.0 ppt); PFHxS:
0.003 [micro]g/L (3.0 ppt); HFPO-DA: 0.005 [micro]g/L (5.0 ppt); PFNA:
0.004 [micro]g/L (4.0 ppt); PFBS: 0.003 [micro]g/L (3.0 ppt). Based on
the multi-laboratory data acquired for the UCMR 5 rule, EPA has defined
the PQL for PFAS addressed in this proposed rule to be equal to the
UCMR 5 MRL (see Table 19, above).
    Some laboratories are capable of measuring the PFAS addressed in
this proposed rule at lower concentrations. Indeed, EPA received some
public comments prior to developing the final UCMR 5 recommending lower
MRLs than those that were ultimately promulgated (USEPA, 2022g).
However, after reviewing the data from laboratories that participated
in the MRL-setting study for UCMR 5, EPA concluded that the MRLs set in
that rule represented ``lowest feasible'' levels for a national
measurement program. Based on laboratory performance in EPA's UCMR 5
Laboratory Approval Program, during 2021-2022, EPA believes that the
UCMR 5 MRLs are appropriate for using as PQL for this proposed
rulemaking. EPA recognizes that as more laboratories upgrade their
instrumentation and gain more experience analyzing drinking water
samples for PFAS, more laboratories may become capable of
quantitatively measuring PFAS at lower concentrations.
    While the values below the PQL will not be used to calculate
compliance with the proposed MCLs under this proposed rule (see
discussion above in Section VI of this preamble), values lower than the
PQL are achievable by individual laboratories, and therefore lower
levels can be used for purposes of screening and to determine
compliance monitoring frequency. EPA is proposing the use of a rule
trigger level for less frequent compliance monitoring under certain
circumstances in which systems can demonstrate PFAS concentrations in
finished drinking water are below:
     one-third of the MCLs for PFOA and PFOS, i.e., 1.3 ppt;
and
     one-third of the HI MCL for the HI PFAS (PFHxS, HFPO-DA,
PFNA, and PFBS), i.e., 0.33.
    Based on laboratory calibration standard data submitted as part of
the UCMR 5 Laboratory Approval Program, described in more detail in
section VI.A. of this preamble, EPA maintains that laboratories are
capable of screening to this level. For additional discussion on this
rule trigger level and monitoring requirements for this proposal,
please see sections VI.A. and IX of this preamble.

IX. Monitoring and Compliance Requirements

A. What are the monitoring requirements?

    EPA is proposing requirements for CWS and NTNCWSs to monitor for
certain PFAS. The Agency is proposing to amend 40 CFR part 141 by
adding a new subpart to incorporate the regulated PFAS discussed in
this preamble. Under this new subpart, PWSs must sample entry points to
the distribution system using a monitoring regime based on EPA's SMF
for SOCs. Under the SMF for SOCs, the monitoring frequency for a PWS is
dependent on previous monitoring results, among other things (USEPA,
2004). EPA is proposing that, consistent with the SMF for SOCs,
groundwater systems serving greater than 10,000 and all surface water
systems are initially required to monitor quarterly within a 12-month
period for regulated PFAS. To provide additional flexibilities for
small groundwater systems, EPA is also proposing and taking comment on
a modification to the SMF for SOCs in that groundwater systems serving
10,000 or fewer are initially required to only monitor twice for
regulated PFAS within a 12-month period, each sample at least 90 days
apart. In this proposal, all systems would be allowed to use previously
acquired monitoring data to satisfy the initial monitoring requirements
(see subsection (C) of this preamble below for additional details about
using previously acquired monitoring data to satisfy initial monitoring
requirements). Based on the SMF, EPA is also proposing that based upon
the initial monitoring results, primacy agencies would be able to
reduce compliance monitoring frequency for a system to once or twice
every three years (depending on system size) if the monitoring results
are below the rule trigger level (defined below).
    EPA is proposing that water systems will conduct compliance
monitoring to demonstrate that finished drinking water does not exceed
the MCLs for regulated PFAS. Water systems must show the primacy agency
that the contaminant is not present in the drinking water supply or, if
present, it does not exceed the proposed MCLs for regulated PFAS. For
compliance monitoring frequency purposes only, EPA is proposing a rule
trigger level of one-third the MCLs (1.3 ppt for PFOA and PFOS and 0.33
for HI PFAS (PFHxS, HFPO-DA, PFNA, and PFBS)). As such, EPA is
proposing amendments for a new subpart to include the following term to
describe the circumstances in which water systems may be eligible for
reduced monitoring for PFOA and PFOS and the HI PFAS if below this:
     Rule Trigger Level: One-third of the MCLs for regulated
PFAS, i.e., 1.3 ppt for PFOA and PFOS and 0.33 for PFAS regulated by
the HI (PFHxS, HFPO-DA, PFNA, and PFBS).
    For more information, including the basis of the rule trigger
level, please see sections VI.A. and VIII.A. of this preamble.
    EPA notes that for some proposed regulated PFAS, the values used to
determine reduced monitoring may be below their PQLs (e.g., PFOA and
PFOS at 1.3 ppt when the PQL is 4.0 ppt). For purposes of screening to
determine monitoring frequency, however, EPA has sufficient confidence
that while measurements below the PQL may be slightly less precise and
accurate, they are achievable by individual laboratories and
appropriate for this intended purpose. EPA requests comment on this
finding regarding feasibility of the proposed MCLs and more generally
on laboratory capacity. As noted earlier, EPA anticipates laboratories
will be able to adjust to demand (including possible price effects),
which the Agency anticipates will be distributed across the
implementation period. Further, at the proposed rule trigger level, the
measurement is primarily useful in determining whether the contaminant
is

present in a sample and for evaluating monitoring flexibilities, rather
than to determine its specific concentration. EPA has set these values
below the MCLs to allow systems the opportunity to reduce their
monitoring schedule and burden, while minimizing the chance of random
normal variation resulting in a single sample close to, but below the
MCLs, when the ``true'' annual average value would be above the MCL.
For additional discussion on PQL, please see section VII of this
preamble. Systems below the rule trigger level would be required to
conduct compliance monitoring according to the following schedule:
     Systems that do not detect regulated PFAS in their systems
at or above the rule trigger level (1.3 ppt for PFOA and PFOS and 0.33
for the HI PFAS (PFHxS, HFPO-DA, PFNA, and PFBS)), and that serve 3,300
or fewer customers will be required to analyze one sample for all
regulated PFAS per three-year compliance period at each entry point to
the distribution system (EPTDS) that does not meet or exceed the rule
trigger level.
     Systems that do not detect regulated PFAS in their systems
at or above the rule trigger level (1.3 ppt for PFOA and PFOS and 0.33
for the HI PFAS (PFHxS, HFPO-DA, PFNA, and PFBS), and that serve a
population of greater than 3,300 will be required to analyze two
samples for all regulated PFAS at least 90 days apart in one calendar
year per three-year compliance period at each EPTDS that does not meet
or exceed the rule trigger level.
    If a water system is not below the rule trigger level for regulated
PFAS at a given EPTDS, it will be required to monitor for all regulated
PFAS quarterly at that EPTDS. Systems monitoring less frequently than
quarterly whose sample result is at or exceeds the rule trigger level
must also begin quarterly sampling at the EPTDS where regulated PFAS
were observed at or above the trigger level. In either case, the
primacy agency may allow a system to move to a reduced monitoring
frequency when the primacy agency determines that the system is below
the rule trigger level and reliably and consistently below the MCL.
However, primacy agencies cannot determine that the system is below the
rule trigger level and reliably and consistently below the MCL until at
least four consecutive quarters of quarterly monitoring have occurred.
EPA notes that, as described above, systems may have EPTDS within a
system on different compliance monitoring schedules depending on
monitoring results.
    In this document, EPA requests comment on the reduced monitoring
approach the Agency is proposing which will save resources for many
lower-risk water systems. First, EPA is requesting comment on the
allowance of a water system to potentially have each EPTDS on a
different compliance monitoring schedule based on specific entry point
sampling results (i.e., some EPTDS being sampled quarterly and other
EPTDS sampled only once or twice during each three-year compliance
period), and if instead, compliance monitoring frequency should be
consistent across all of the system's sampling points. EPA is also
requesting comment on establishing the proposed rule trigger level
values of 1.3 ppt for PFOA and PFOS and 0.33 for the PFAS regulated by
the HI (PFHxS, HFPO-DA, PFNA, and PFBS). EPA is seeking comment on
establishing the trigger level at other levels, specifically
alternative values of 2.0 ppt for PFOA and PFOS and 0.50 for the HI
PFAS. EPA notes that adjusting the trigger levels to 2.0 ppt for PFOA
and PFOS and 0.50 for the HI PFAS would result in a considerable number
of additional water systems significantly reducing their monitoring
frequency from at least four times each year to once or twice every
three years. EPA also notes that the higher trigger may provide
slightly less assurance of the water systems' current regulated PFAS
levels as a result of the more intermittent monitoring. EPA is seeking
comment on the merits and drawbacks of these higher trigger levels
compared to those proposed in this document.

B. How are PWS compliance and violations determined?

    Consistent with existing rules for determining compliance with
NPDWRs, EPA is proposing that compliance with this rule will be
determined based on the analytical results obtained at each sampling
point. For systems monitoring quarterly, compliance with the proposed
MCLs for regulated PFAS will be determined by running annual averages
at the sampling point. Systems monitoring less frequently whose sample
result(s) are at or exceed the rule trigger level must revert to
quarterly sampling at each EPTDS where the trigger level is met or
exceeded for all regulated PFAS in the next quarter, with the triggered
sample result being used for the first quarter of monitoring in
calculating the running annual average.
    A running annual average is an average of sample analytical results
for samples taken at a particular monitoring location during the
previous four consecutive quarters. If a system takes more than one
compliance sample during each quarter at a particular monitoring
location, the system must average all samples taken in the quarter at
that location to determine the quarterly averages to be used in
calculating the running annual averages. Conversely, if a system does
not collect required samples for a quarter, the running annual average
will be based on the total number of samples collected for the quarters
in which sampling was conducted. A system will not be considered in
violation of an MCL until it has completed one year of quarterly
sampling, except in the case where, if a quarterly sampling result will
cause the running annual averages to exceed an MCL at any sampling
point (i.e., the analytical result is greater than four times the MCL).
In that case, the system is out of compliance with the MCL immediately.
    When calculating the running annual averages, if a sample result is
less than the PQL for the monitored PFAS, EPA is proposing to use zero
to calculate the average for compliance purposes. For example, if a
system has sample results for PFOA that are 2.0, 1.5, 5.0, and 1.5 ppt
for their last four quarters at a sample location, the values used to
calculate the running annual average would be 0.0, 0.0, 5.0, and 0.0
with a resulting PFOA running annual average of 1.3 ppt. As described
in sections VI and VIII of this preamble, EPA is proposing that values
below the PQL will not be used to determine compliance with the
proposed MCLs as these PQLs are the lowest concentration of analyte
that can be reliably measured within specified limits of precision and
accuracy during routine laboratory conditions. As such, quantifying
concentrations below the PQL for compliance purposes may decrease the
precision and accuracy of the measured value and may not be achievable
for some individual laboratories. In this document, EPA is requesting
comment on whether EPA should consider an alternative approach when
calculating the running annual averages for compliance. Specifically,
in the case where a regulated PFAS is detected but below its proposed
PQL, that the proposed rule trigger level (1.3 ppt for PFOA and PFOS
and 0.33 of each of the HI PFAS PQLs (i.e., PFHxS=1.0, HFPO-DA=1.7,
PFNA=1.3, and PFBS=1.0)) be used as the value in calculating the
running annual average for compliance purposes. While this approach may
be more complicated to implement than using zero when below the PQL, it
is largely consistent with EPA's NPDWRs related to other SOCs and has
the

potential to slightly increase the public health protection provided by
this proposed regulation.

C. Can systems use previously collected data to satisfy the initial
monitoring requirement?

    As proposed, systems would be allowed to use previously collected
monitoring data to satisfy the initial monitoring requirements. In
general, a system with appropriate historical monitoring data for each
distribution system entry point, collected using EPA Methods 533 or
537.1 as part of UCMR 5 or a state-level or other appropriate
monitoring campaign, could use that monitoring data to satisfy initial
monitoring requirements.
    EPA is proposing that systems with previously acquired monitoring
data from UCMR 5 will not be required to conduct separate initial
monitoring for regulated PFAS. To satisfy the initial monitoring
requirements for these systems using UCMR 5 data, data collected after
January 1st, 2023, can be used for entry point samples.
    While EPA expects most systems serving 3,300 or greater will have
UCMR 5 data, EPA is also proposing that systems with previously
acquired monitoring data from outside UCMR 5, including State-led or
other appropriate occurrence monitoring using EPA methods 533 or 537.1
will also not be required to conduct separate initial monitoring for
regulated PFAS. This addition may allow systems serving fewer than
3,300 to satisfy the initial monitoring requirements. Data collected
after January 1st, 2023, can be used for entry point samples. Data
collected between January 1st, 2019, and December 31, 2022, may also be
used if it is below the proposed rule trigger level of 1.3 ppt for PFOA
and PFOS and an HI of 0.33 for PFHxS, HFPO-DA, PFNA, and PFBS. The
additional analytical requirement for older data is to ensure the use
of these data is adequately representative of current water quality
conditions. If systems have multiple years of data, the most recent
data must be used.

D. Can systems composite samples?

    40 CFR 141.24 subpart C describes instances where primacy agencies
may reduce the samples a system must analyze by allowing samples to be
composited. Composite sampling is an approach in which equal volumes of
water from multiple entry points are combined into a single container
and analyzed as a mixture. The reported concentration from the analysis
of the composite sample therefore reflects the average of the analyte
concentrations from the contributing entry points. Composite sampling
can potentially reduce analytical costs because the number of required
analyses is reduced by combining multiple samples into one and
analyzing the composited sample. However, based on comments EPA
received in consulting with state regulators and small business
entities (operators of small PWSs), PFAS are ubiquitous in the
environment at low concentrations which necessitates robust laboratory
analytical precision at these low concentrations. For example,
incidental contamination from or adherence to surface laboratory
equipment may artificially lower contaminant concentrations or result
in false negatives. Additionally, PFAS are demonstrated to be
ubiquitous in the environment such that the risk for false positives
may increase when combining samples for composite analysis. Based on
these potential implementation issues, EPA is proposing a deviation
from the SMF for SOCs by not allowing samples to be composited.

E. Can primacy agencies grant monitoring waivers?

    40 CFR 141.24 Subpart C describes instances where the primacy
agency may grant waivers predicated on proximity of the system to
contaminant sources (i.e., susceptibility to contamination) and
previous uses of the contaminant within the watershed (including
transport, storage, or disposal). Based on EPA's consultation with
state regulators and operators of small PWSs, the Agency believes that
due to the ubiquity, environmental persistence, and transport abilities
of PFAS, granting waivers based on these conditions would be
challenging, therefore EPA is not incorporating this flexibility as a
part of these proposed monitoring requirements. However, in this
proposal, EPA is considering and taking comment on waivers based on
sampling results. Specifically, EPA is requesting comment on whether
water systems should be permitted to apply to the primacy agency for a
monitoring waiver of up to 9-years (one full compliance cycle) for
these proposed PFAS if after at least one year of quarterly sampling
the results are below the rule trigger level of one-third of the MCLs,
or for systems that may be monitoring less frequently than quarterly if
at least two consecutive three year-compliance period sample results
are below the rule trigger level. Additionally, EPA is requesting
comment on allowing similar monitoring waivers to be granted based on
previously acquired monitoring data as described above in subsection
(C) of this preamble. In either case, systems with a monitoring waiver
would be required to take at least one sample per nine-year compliance
cycle in order to maintain or renew an existing waiver. Furthermore,
EPA is seeking comment on the identification of possible alternatives
to traditional vulnerability assessments that should be considered to
identify systems as low risk and potential eligibility for monitoring
waivers.

F. When must systems complete initial monitoring?

    Pursuant to Section 1412(b)(10), this proposed rule would require
compliance three years after promulgation. To satisfy initial
monitoring requirements and demonstrate rule compliance, within the
three years following rule promulgation, groundwater systems serving a
population greater than 10,000 and all surface water systems will be
required to demonstrate their baseline concentrations using data from
four quarterly samples collected over a one-year period. Groundwater
systems serving a population 10,000 or fewer may collect two quarterly
samples at least 90 days apart over a one-year period for the purpose
of initial monitoring, rather than collecting four quarterly samples.
Additionally, as described earlier in this section (subsection C of
this preamble), EPA is proposing that systems with appropriate,
previously acquired monitoring data from UCMR 5, state-led, or other
applicable monitoring programs using EPA Methods 533 or 537.1, will not
be required to conduct separate initial monitoring for regulated PFAS.
As such, given the advantageous timing of UCMR 5 monitoring data for
all systems serving greater than 3,300 and the availability of
historical monitoring data that many small systems serving 3,300 or
fewer may utilize from state-level monitoring programs, EPA notes this
proposed allowance will offer significant burden reduction for these
systems and sufficient timing to take necessary actions and ensure rule
compliance. For systems that may not have available data and/or choose
to conduct additional monitoring, as proposed in this document, EPA
would encourage those systems to conduct their initial monitoring as
soon as practicable following rule promulgation to allow for actions
that may need to be taken based on monitoring results and to certify
rule compliance. The Agency seeks comment on EPA's proposed initial
monitoring timeframe, particularly for NTNCWS or all systems serving
3,300 or fewer.

G. What are the laboratory certification requirements?

    EPA is proposing that laboratories demonstrate their ability to
achieve the precision and detection limits necessary to meet the
objectives of this regulation. The proposal would require laboratories
to analyze performance evaluation (PE) samples every year in order to
achieve and maintain certification.

X. Safe Drinking Water Act (SDWA) Right To Know Requirements

A. What are the Consumer Confidence Report requirements?

    A CWS must prepare and deliver to its customers an annual Consumer
Confidence Report (CCR) in accordance with requirements in 40 CFR 141
Subpart O. A CCR provides customers with information about their local
drinking water quality as well as information regarding the water
system compliance with drinking water regulations. Under this proposal
CWSs would be required to report detected PFAS in their CCR;
specifically, PFOA, PFOS, PFHxS, HFPO-DA, PFNA, and PFBS, and the HI
for the mixtures of PFHxS, HFPO-DA, PFNA, and PFBS.

B. What are the public notification (PN) requirements?

    As part of SDWA, the Public Notification (PN) rule ensures that
consumers will know if there is a problem with their drinking water.
Notices alert consumers if there is risk to public health. They also
notify customers: If the water does not meet drinking water standards;
if the water system fails to test its water; if the system has been
granted a variance (use of less costly technology); or if the system
has been granted an exemption (more time to comply with a new
regulation).
    All PWSs must give the public notice for all violations of NPDWRs
and for other situations. Under this proposal, EPA is proposing that
violations of the three MCLs in the proposal would be designated as
Tier 2 and as such, PWSs would be required to comply with 40 CFR
141.203. Per 40 CFR 141.203(b)(1), notification of an MCL violation
should be provided as soon as practicable but no later than 30 days
after the system learns of the violation.

XI. Treatment Technologies

    Water systems with PFAS levels that exceed the MCLs proposed would
need to take action to provide drinking water which meets the NPDWR by
the compliance dates established in the rule when final. For example,
systems may install water treatment or consider other options such as
source remediation or connecting to an uncontaminated water system.
While conventional treatment technologies are unable to remove PFOS,
PFOA, PFNA, PFHxS, PFBS, or HFPO-DA to levels protective of public
health (McCleaf et al., 2017), there are technologies currently
available that effectively remove these and other PFAS.
    Section 1412(b)(4)(E) of SDWA requires that the Agency ``list the
technology, treatment techniques, and other means which the
Administrator finds to be feasible for purposes of meeting [the MCL],''
which are referred to as BATs. These BATs are used by states to
establish conditions for source water variances under Section 1415(a).
Section 1412(b)(4)(E)(ii) also requires that the Agency identify small
system compliance technologies (SSCTs), which are affordable treatment
technologies, or other means that can achieve compliance with the MCL
(or treatment technique [TT], where applicable).

A. What are the best available technologies?

    The Agency identifies the BATs as those meeting the following
criteria: (1) The capability of a high removal efficiency; (2) a
history of full-scale operation; (3) general geographic applicability;
(4) reasonable cost based on large and metropolitan water systems; (5)
reasonable service life; (6) compatibility with other water treatment
processes; and (7) the ability to bring all the water in a system into
compliance. The Agency is proposing the following technologies as BAT
for PFAS removal from drinking water based its review of the treatment
and cost literature (USEPA, 2023g):

 GAC
 AIX
 High pressure membranes (RO and NF)

    Operationally, GAC and AIX are sorptive processes meaning a process
where one substance becomes attached to another. Sorption is typically
composed of absorption where one substance is incorporated into
another, adsorption where one substance is incorporated onto another,
or ion exchange (IX) where an aqueous ion (the contaminant) is traded
for a different less dangerous ion (typically chloride in AIX) on an
insoluble matrix. Sorptive processes pour feed water through a vessel
filled with a sorbent known as a contactor. The operation continues
until the sorbent no longer effectively removes the target contaminant;
this is when the contaminant ``breaks through'' the treatment process.
At this point, the sorbent must be disposed then replaced or
regenerated. The length of time until the sorbent must be replaced or
regenerated is known as bed life and is a critical factor in the cost
effectiveness of sorptive technology. One bed life measurement is the
water volume that can be treated before breakthrough and is measured in
bed volumes (BV). BVs are how many times the sorbent (i.e., media) can
be filled in the bed in which the sorbent resides before contaminant
breakthrough. EPA estimates GAC treatment will be sufficiently
available to support cost-effective compliance with this proposed
regulation, and requests comment on whether additional guidance on
applicable circumstances for GAC treatment is needed.
    High pressure membranes are a separation process where feed water
is split into two streams across a membrane. One stream has few
contaminants or other solutes left in it and is known as permeate or
produced water. The other stream contains the concentrated contaminant
and other solutes which is known as concentrate, brine, retentate, or
reject water. Membrane flux is how much permeate is produced for a
given surface area and time; different system configurations operating
at the same flux produce differing quantities of finished water. This
means that membrane systems with differing configurations cannot be
directly compared based on flux. Flux can be reduced during membrane
fouling which is where things accumulate on or in the membrane. Fouling
can require membrane cleaning and replacement or operational changes.
    There are also non-treatment options which may be used for
compliance such as replacing a PFAS-contaminated drinking water source
with a new uncontaminated source (e.g., a new well), or purchasing
compliant water from another system. Conventional and most advanced
water treatment methods are ineffective at removing PFAS (Rahman et
al., 2014). Further information on the proposed BATs is provided below.
1. Granular Activated Carbon
    GAC is a separation process where contaminants become attached to
specially treated carbon with a high surface area. The GAC
manufacturing process can accept any highly carbonaceous material as an
input such as bituminous coal, lignite coal, peat, wood, coconut
shells, and peach pits. Activation is predominantly a thermal process,
although it may also be a chemical process, that creates as well as

enlarges pores generating a porous structure with a large surface area
per unit mass. Literature suggests that the primary mechanisms of
adsorption include both hydrophobic and electrostatic interactions
(Ateia et al., 2019). In addition to removing PFAS, GAC can remove
contaminants including taste and odor compounds, natural organic matter
(NOM), VOCs, SOCs, DBP precursors, and radon. Organic compounds with
high molecular weights are also readily adsorbable.
    Demonstrated PFAS removal efficiencies can exceed >99 percent and
can achieve concentrations less than 4 ng/L (Forrester and Bostardi,
2019; Zeng et al., 2020; Westreich et al., 2018; Belkouteb et al.,
2020; Woodard et al., 2017; and Hopkins et al., 2018). During the
operation, carbon is removed from the system periodically, for disposal
or regeneration, based on treatment objectives. Several factors affect
bed life, including the presence of competing contaminants such as
nitrate and the carbon type used. Most studies found that natural or
dissolved organic matter (NOM/DOM) interferes with PFAS sorption, in
general, and its presence dramatically lowers treatment efficacy
(McNamara et al., 2018; Pramanik et al., 2015; Yu et al., 2012). The
lowered treatment effectiveness was found to be less pronounced for
HFPO-DA than for perfluoroalkyl carboxylic acid (PFCA) C7 and above for
GAC (Park et al., 2020).
    Reactivation is a process that removes organic compounds from
adsorption sites on GAC enabling reuse. Although different methods are
available for GAC reactivation, the process most commonly involves high
temperature thermal treatment in a specialized facility such as a
multiple hearth furnace or rotary kiln (Matthis and Carr, 2018; USEPA,
2023g). Reactivated carbon can become totally exhausted with other
contaminants not removed during reactivation and must be replaced.
However, for GAC, the loss of approximately 10 percent of the media due
to abrasion within the reactivation process can result in a somewhat
steady state for performance as new GAC is added each time to replace
the lost GAC. Systems may decide to dispose of GAC (i.e., operate on a
`throw-away' basis) instead of reactivating the media. GAC can be a
cost-effective treatment option despite needing to dispose of
contaminated carbon.
2. Anion Exchange
    AIX is a separation process where an anion in the aqueous phase is
exchanged for an ion attached to an exchange resin. Similar to GAC, AIX
uses contactors. These contactors, however, are filled with a bed of
beads or gel known as resin instead of carbon. As feed water moves
through the resin, an anionic contaminant, such as PFAS exchanges, for
an anion, typically chloride, on the resin. For PFAS compounds, vendors
generally recommend using PFAS-selective resins (Boodoo, 2018; Boodoo
et al., 2019; Lombardo et al., 2018; Woodard et al., 2017). AIX may
also have a beneficial effect by removing other undesirable anions from
the treated water such as nitrate or sulfate.
    Demonstrated PFAS removal efficiencies may be >99 percent and can
achieve concentrations less than 4 ng/L (Dixit et al., 2021; Dixit et
al., 2020; Zeng et al., 2020; Liu, 2017; Kumarasamy et al., 2020;
Arevalo et al., 2014; and Yan et al., 2020). The operation continues
until enough of the resin's available IX sites have ions from the feed
water and the resin no longer effectively removes the target
contaminant, also known as ``breaks through.'' At this point, the resin
must be disposed and replaced or regenerated. The length of time until
resin must be replaced or regenerated is known as bed life and is a
critical factor in the cost effectiveness of IX as a treatment
technology. Several factors affect bed life, including the presence of
competing ions such as nitrate and the resin type used.
    Conventional regeneration solutions are not generally effective for
restoring the capacity of PFAS-selective resins (Liu and Sun, 2021).
Regeneration may be possible using organic solvents (Boodoo, 2018;
Zaggia et al., 2016) or proprietary methods (Woodard et al., 2017).
These alternative regeneration practices are generally practical or
cost-effective only with very high influent concentrations, such as in
remediation settings. Therefore, in drinking water applications using
PFAS-selective resin, vendors recommend a single-use approach where the
spent resin is disposed and replaced with fresh resin (Boodoo, 2018;
Lombardo et al., 2018). Exhausted resin must be disposed; due to the
difficulties mentioned earlier and vendor recommendation, resins are
often operated on a `throw-away' basis. This operational mode avoids
generating spent regenerant liquid residuals. AIX can be a cost-
effective treatment option.
3. High Pressure Membranes (RO and NF)
    RO and NF are membrane separation processes where water is forced
through a membrane at greater than osmotic pressure. The water that
transverses the membrane is known as permeate or produce water, and has
few solutes left in it; the remaining water is known as concentrate,
brine, retentate, or reject water and forms a waste stream with
concentrated solutes. NF has a less dense active layer than RO, which
enables lower operating pressures but also makes it less effective at
removing contaminants. In drinking water treatment, these membranes are
most often used in a spiral-wound configuration that consists of
several membrane envelopes, layered with feed spacers, and rolled
together in and around a central collection tube. Feed pressures for NF
membranes are typically in the range of 50 to 150 pounds per square
inch (psi). Feed pressures for RO membranes are in the range of 125 to
300 psi in low pressure applications (such as PFAS removal) but can be
as high as 1,200 psi in applications such as seawater desalination
(USEPA, 2023d). RO may remove other contaminants including arsenic and
chromium-VI.
    RO and NF may achieve PFAS removal >99 percent (Lipp et al., 2010;
Horst et al., 2018; Liu et al., 2021; Dickenson and Higgins, 2016;
Steinle-Darling et al., 2008; Boonya-Atichart et al., 2016; Appleman et
al., 2014; Thompson et al., 2011; CDM Smith, 2018; Dickenson and
Higgins, 2016; and Dowbiggin et al., 2021). While water quality affects
process design (e.g., recovery rate, cleaning frequency, and
antiscalant selection), it has relatively little effect on PFAS removal
percent. High pressure membranes generate a relatively large
concentrate stream, which will contain PFAS as well as other rejected
dissolved species, which will require disposal or additional treatment.
The large concentrate stream also means less treated water is available
for distribution (e.g., 70 to 85 percent of source water), which is a
disadvantage for systems with limited water supply.

B. PFAS Co-Removal

    AIX and GAC are effective at removing PFAS and there is generally a
linear relationship between PFAS chain length and removal efficiency
shifted by functional group (McCleaf et al., 2017; S[ouml]reng[aring]rd
et al., 2020). Perfluoroalkyl sulfonates (PFSA), such as PFOS, are
removed with greater efficiency than the corresponding PFCA, such as
PFOA, of the same carbon backbone length (Appleman et al., 2014; Du et
al., 2014; Eschauzier et al., 2012; Ochoa-Herrera and Sierra-Alvarez,
2008; Zaggia et al., 2016). Generally, for a given water type and
concentration, a PFSA is removed

about as well as a PFCA which has two more fully perfluorinated carbons
in its backbone. For example, PFHxS (six carbon backbone and a sulfonic
acid functional group) is removed about as well as PFOA (eight carbon
backbone and a carboxylate head) and perfluorohexanoic acid (PFHxA)
(six carbon backbone with a carboxylate head) is removed approximately
as well as PFBS (four carbon backbone and a sulfonic acid functional
group). Additionally, the compounds with longer carbon chain displayed
a smaller percentage decrease in average removal efficiency over time
(McCleaf et al., 2017).
    The three technologies discussed above have all been demonstrated
to be effective in removing all six PFAS proposed for regulation as
part of this rulemaking. As discussed in section VII.C. of this
preamble, PFAS have been shown to co-occur. Hence, where the six PFAS
being regulated today occur in concentrations above their respective
regulatory standards there is also an increased probability of other
unregulated PFAS being present. Further, since these same technologies
also remove other long-chain and higher carbon/higher molecular weight
PFAS EPA expects this rulemaking will provide additional public health
benefits and protection by removing unregulated PFAS that may have
adverse health effects. While EPA has not quantified those benefits as
part of this rulemaking, the Agency believes these important secondary
benefits further enhance public protection offered by this proposed
regulation.

C. Management of Treatment Residuals

    As part of EPA's BAT evaluation, the Agency assesses the
availability of studies of full-scale treatment of residuals that fully
characterize residual waste streams and disposal options. At present,
the most likely management option for spent material containing PFAS is
reactivation for GAC and incineration for spent IX resin. For disposal
of RO/NF membrane concentrate, most systems use surface water discharge
or discharge to sanitary sewer. The large volume of residuals is a
well-known obstacle to adoption of membrane separation technology in
general. For more information on current residuals management
practices, see Best Available Technologies and Small System Compliance
Technologies for Per- and Polyfluoroalkyl Substances (PFAS) in Drinking
Water (USEPA, 2023g) or Managing and Treating Per- and Polyfluoroalkyl
Substances (PFAS) in Membrane Concentrates (Tow et al., 2021).
    EPA recognizes that future actions through several statutory
authorities other than SDWA may have direct or indirect implications
for drinking water treatment facilities and some actions may prevent or
reduce PFAS entering drinking water sources. EPA is addressing PFAS
through statutory authorities including the CERCLA, Resource
Conservation and Recovery Act (RCRA), Toxic Substances Control Act
(TSCA), Clean Water Act, Clean Air Act, and Emergency Planning and
Community Right-to-Know Act (EPCRA). For example, as part of EPA's PFAS
Strategic Roadmap, EPA proposed certain PFAS be designated as CERCLA
hazardous substances to require reporting of PFOA and PFOS releases,
enhance the availability of data, and ensure agencies can recover
cleanup costs (USEPA, 2022c). In the Strategic Roadmap, EPA has also
committed to expanding research on and accelerating the deployment of
emerging PFAS treatment, remediation, destruction, disposal, and
control technologies (USEPA, 2022c). EPA's 2020 Interim Guidance on the
Destruction and Disposal of Perfluoroalkyl and Polyfluoroalkyl
Substances and Materials Containing Perfluoroalkyl and Polyfluoroalkyl
Substances outlines the current state of the science on techniques and
treatments that may be used to destroy or dispose of PFAS (USEPA,
2020b). In accordance with EPA's PFAS Strategic Roadmap, EPA
anticipates releasing an updated version of the Guidance in 2023. As
part of this rulemaking, EPA considered that in drinking water
treatment, large volumes of spent GAC and ion exchange resin must be
removed which does not lend itself to on-site storage over time. The
disposal options identified in the Interim Guidance (USEPA, 2020b) are
landfill disposal and thermal treatment.
    Stakeholders have expressed concern to EPA that a hazardous
substance designation for certain PFAS may limit their disposal options
for drinking water treatment residuals (e.g., spent media, concentrated
waste streams) and/or potentially increase costs. Although EPA
anticipates that designating chemicals as hazardous substances under
CERCLA generally should not result in limits on for disposal of PFAS
drinking water treatment residuals, EPA has estimated the treatment
costs for systems both with the use of hazardous waste disposal and
non-hazardous disposal options to assess the effects of potentially
increased disposal costs. Specifically, EPA assessed the potential
impact on PWS treatment costs associated with hazardous residual
management requirements in a sensitivity analysis on the proposed
option. Relative to the national analysis for the proposed option
assuming non-hazardous disposal, the hazardous waste disposal
assumption would increase PWS costs by 4% ($30 million annually) at the
3% discount rate and 5% ($61 million annually) at the 7% discount rate
should spent media need to be disposed of as hazardous waste in the
future because of separate EPA or State regulatory action. EPA's
sensitivity analysis demonstrates that potential hazardous waste
disposal requirements may increase PWS treatment costs marginally,
however the increase in PWS costs are not significant enough to change
the determination that benefits of the rulemaking justify the costs.
These estimates are discussed in greater detail in the HRRCA section of
this proposed rulemaking and in Appendix N of the Economic Analysis
(USEPA, 2023i). These costs are limited to the disposal of the PFAS
contaminated residuals and wastes. Results for small systems are
presented in Section D of this preamble below. EPA is seeking public
input related to PFAS treatment residual disposal in Section XIV of
this preamble.

D. What are small system compliance technologies (SSCTs)?

    EPA is proposing the SSCTs shown in Table 20. The table shows which
of the BATs listed above are also affordable for each small system size
category listed in Section 1412(b)(4)(E)(ii) of SDWA. The Agency
identified these technologies based on an analysis of treatment
effectiveness and affordability.

                                    Table 20--Proposed SSCTs for PFAS Removal
----------------------------------------------------------------------------------------------------------------
   System size  (population                                                                      Point of use
           served)                     GAC                   IX                 RO/NF           (POU) RO/NF \1\
----------------------------------------------------------------------------------------------------------------
25-500.......................  Yes................  Yes................  No.................  Yes.
501-3,300....................  Yes................  Yes................  No.................  Yes.

3,301-10,000.................  Yes................  Yes................  Yes................  not applicable.\2\
----------------------------------------------------------------------------------------------------------------
Notes:
\1\ POU RO is not currently listed as a compliance option because the regulatory options under consideration
  require treatment to concentrations below the current NSF International/American National Standards Institute
  (NSF/ANSI) certification standard for POU device removal of PFAS. However, POU treatment is reasonably
  anticipated to become a compliance option for small systems in the future if NSF/ANSI or other independent
  third-party certification organizations develop a new certification standard that mirrors EPA's proposed
  regulatory standard. The affordability conclusions presented here reflect the costs of devices certified under
  the current standard, not a future standard, which may change dependent on future device design.
\2\ EPA's work breakdown structure (WBS) model for POU treatment does not cover systems larger than 3,300 people
  (greater than 1 million gallons per day [MGD] design flow), because implementing and maintaining a large-scale
  POU program is likely to be impractical.

    The operating principle for POU RO devices is the same as
centralized RO: Steric exclusion and electrostatic repulsion of ions
from the charged membrane surface. In addition to a RO membrane for
dissolved ion removal, POU RO devices often have a sediment pre-filter
and a carbon filter in front of the RO membrane, a 3- to 5-gallon
treated water storage tank, and a carbon filter between the tank and
the tap.
    EPA identified SSCTs using the affordability criteria methodology
developed for drinking water rules (USEPA, 1998b). The analysis method
is a comparison of estimated incremental household costs for PFAS
treatment to an expenditure margin, which is the difference between
baseline household water costs and a threshold equal to 2.5% of median
household income (MHI). Table 21 shows the expenditure margins derived
for the analysis. These margins show the cap on affordable incremental
annual expenditures.

                          Table 21--Expenditure Margins for SSCT Affordability Analysis
----------------------------------------------------------------------------------------------------------------
                                                              Affordability    Baseline water      Expenditure
     System size  (population served)           MHI \1\       threshold \2\       cost \3\           margin
                                                         A      B = 2.5% x A                 C           D = B-C
----------------------------------------------------------------------------------------------------------------
25-500....................................         $55,377            $1,384              $507              $877
501-3,300.................................          53,596             1,340               587               753
3,301-10,000..............................          58,717             1,468               613               855
----------------------------------------------------------------------------------------------------------------
Notes:
\1\ MHI based on U.S. Census Bureau's American Community Survey five-year estimates (United States Census
  Bureau, 2010) stated in 2010 dollars, adjusted to 2020 dollars using the Consumer Price Index (CPI) (for all
  items) for areas under 2.5 million persons.
\2\ Affordability threshold equals 2.5 percent of MHI.
\3\ Household water costs derived from 2006 Community Water System Survey (USEPA, 2009c), based on residential
  revenue per connection within each size category, adjusted to 2020 dollars based on the CPI for All Urban
  Consumers: Water and Sewer and Trash Collection Services in U.S. City Average.

    Table 21 shows the estimates of per-household costs by treatment
technology and size category generated using the treatment cost method
described in section XII.B of this preamble as well as Best Available
Technologies and Small System Compliance Technologies for Perchlorate
in Drinking Water (USEPA, 2019c) and Technologies and Costs for
Treating Perchlorate-Contaminated Waters (USEPA, 2018c). Based on the
results presented in Table 22, EPA identified candidate technologies
available for which costs do not exceed the corresponding expenditure
margin and, therefore, meet the SSCT affordability criterion. As such,
EPA has determined that affordable SSCTs are available, and the Agency
is not proposing any variance technologies.

                      Table 22--Total Annual Cost per Household for Candidate Technologies
----------------------------------------------------------------------------------------------------------------
   System size  (population
            served)                    GAC                 IX               RO/NF             POU RO/NF \1\
----------------------------------------------------------------------------------------------------------------
25-500........................  $395 to $727.....  $376 to $645.....  $3,711 to $4,676.  $317 to $326.
501-3,300.....................  $139 to $332.....  $133 to $235.....  $608 to $1,169...  $299 to $300.
3,301-10,000..................  $136 to $329.....  $121 to $218.....  $326 to $462.....  not applicable.\2\
----------------------------------------------------------------------------------------------------------------
Notes:
\1\ POU RO is not currently a compliance option because the regulatory options under consideration require
  treatment to concentrations below the current NSF/ANSI certification standard for POU device removal of PFAS.
  However, POU treatment is reasonably anticipated to become a compliance option for small systems in the future
  if NSF/ANSI or other independent third-party certification organizations develop a new certification standard
  that mirrors EPA's proposed regulatory standard. Costs presented here reflect the costs of devices certified
  under the current testing standard, not a future standard, which may change dependent on future device design.
\2\ EPA's WBS model for POU treatment does not cover systems larger than 3,300 people (greater than 1 MGD design
  flow), because implementing and maintaining a large-scale POU program is likely to be impractical.

    The results discussed above assume management of spent GAC and
spent IX resin using current typical management practices (reactivation
for GAC and incineration for resin). EPA is in the process of proposing
some PFAS be designated as hazardous substances under CERCLA and listed
as hazardous constituents under RCRA. If finalized, neither of these
actions should result in limiting disposal options and how PFAS
containing waste, including spent GAC or resin, is required to be
managed. However, waste management facilities may, at their own
discretion, refuse to accept PFAS-containing materials or drinking
water treatment operations may choose to send spent GAC and resin
containing PFAS to facilities permitted to treat and/or dispose of
hazardous wastes. To consider the implications of this possibility, EPA
has developed an assessment of the current unit costs for disposing
spent treatment materials and the costs associated with their disposal
as hazardous waste. Table 23 shows the resulting cost per household if
systems dispose of these residuals as hazardous waste. Although costs
would increase somewhat compared to if they do not treat the spent
media as hazardous waste, those increases are not significant enough to
change the conclusions about affordability.

   Table 23--Total Annual Cost per Household Assuming Hazardous Waste
                    Disposal for Spent GAC and Resin
------------------------------------------------------------------------
   System size  (population
           served)                    GAC                   IX
------------------------------------------------------------------------
25-500.......................  $417 to $827....  $397 to $678.
501-3,300....................  $149 to $368....  $138 to $243.
3,301-10,000.................  $146 to $360....  $124 to $222.
------------------------------------------------------------------------

    In addition to the required analysis for small system
affordability, EPA having received a number of recommendations from the
SAB, the NDWAC, and other stakeholders, is exploring the use of
alternative expenditure margins and other potential changes to the
national level affordability methodology to better understand the cost
impacts of new standards on low income and disadvantaged households
served by small drinking water systems. The Agency conducted
supplemental affordability analyses using alternative metrics suggested
to EPA by stakeholders to demonstrate the potential affordability
implications of the proposed NPDWR on the determination of affordable
technologies for small systems at the national level of analysis.
    As required under the 1996 amendments to SDWA, EPA lists treatment
technologies for small systems that are affordable and that achieve
compliance with the regulatory standard. As part of its affordability
analysis for the proposed PFAS rule, EPA determined that there are
several affordable treatment technologies for small systems, including
GAC, IX, RO, and POU RO.\5\ EPA is seeking public comment on the
national level analysis of affordability of SSCTs and specifically on
the potential methodologies presented. EPA's national small system
affordability determination can be found in Section 9.12.1 of the EA.
EPA's supplementary affordability analyses can be found in Section
9.12.2 of the EA. EPA is also seeking comment on whether there are
additional technologies which are viable for PFAS removal to the
proposed MCLs as well as any additional costs which may be associated
with non-treatment options such as water rights procurement. Finally,
EPA is seeking comment on the benefits from using treatment
technologies (such as reverse osmosis and GAC) that have been
demonstrated to co-remove other types of contaminants found in drinking
water and whether employing these treatment technologies are sound
strategies to address PFAS and other regulated or unregulated
contaminants that may co-occur in drinking water.
---------------------------------------------------------------------------

    \5\ POU RO is not currently a compliance option because the
regulatory options under consideration require treatment to
concentrations below 70 ppt total of PFOA and PFOS, the current
certification standard for POU devices. However, POU treatment is
anticipated to become a compliance option for small systems in the
future should NSF/ANSI or another accredited third-party
certification entity develop a new certification standard that
mirrors (or is demonstrated to treat to concentrations lower than)
EPA's proposed regulatory standard. The affordability conclusions
for POU RO should be considered preliminary because they reflect the
costs of devices certified under the current standard, not a future
standard.
---------------------------------------------------------------------------

    Following finalization of the PFAS NPDWR, EPA will work with
primacy agencies to provide assistance to support implementation of the
rule. EPA requests comment on the type of assistance that would help
small public water systems identify laboratories that can perform the
required monitoring, evaluate treatment technologies and determine the
most appropriate way to dispose of PFAS contaminated residuals and
waste the systems may generate when implementing the rule.

XII. Rule Implementation and Enforcement

A. What are the requirements for primacy?

    This section describes the regulations, procedures, and policies
primacy entities must adopt, or have in place, to implement the PFAS
rule, when it is final. States, Territories, and Tribes must continue
to meet all other conditions of primacy in 40 CFR part 142. Section
1413 of SDWA establishes requirements that primacy entities (States or
Indian Tribes) must meet to maintain primary enforcement responsibility
(primacy) for its PWSs. These include:
     Adopting drinking water regulations that are no less
stringent than Federal NPDWRs in effect under sections 1412(a) and
1412(b) of the Act;
     Adopting and implementing adequate procedures for
enforcement;
     Keeping records and making reports available on activities
that EPA requires by regulations;
     Issuing variances and exemptions (if allowed by the State)
under conditions no less stringent than allowed by SDWA Sections 1415
and 1416; and
     Adopting and being capable of implementing an adequate
plan for the provision of safe drinking water under emergency
situations.
    40 CFR part 142 sets out the specific program implementation
requirements for States to obtain primacy for the Public Water System
Supervision (PWSS) Program, as authorized under 1413 of the Act.
    Under 40 CFR 142.12(b), all primacy States/territories/tribes would
be required to submit a revised program to

EPA for approval within two years of promulgation of any final PFAS
NPDWR or could request an extension of up to two years in certain
circumstances. To be approved for a program revision, primacy States/
territories/tribes would be required to adopt revisions at least as
stringent as the revised PFAS-related provisions in 40 CFR 141.6
(Effective Dates); 40 CFR 141.900 subpart Z (Control of Per- and
Polyfluoroalkyl Substances); 40 CFR 141.50 (Maximum Contaminant Level
Goals for organic contaminants); 40 CFR 141.60 (Maximum Contaminant
Levels for organic contaminants); appendix A to subpart O ([Consumer
Confidence Report] Regulated contaminants); Appendix A to Subpart Q
((NPDWR violations and other situations requiring public notice);
Appendix B to Subpart Q (Standard health effects language for public
notification); 40 CFR 142.62 (Variances and exemptions from the MCLs
for organic and inorganic contaminants); and 40 CFR 142.16 (Primary
Enforcement Responsibility).

B. What are the primacy agency record keeping requirements?

    The current regulations in 40 CFR 142.14 require primacy agencies
to keep records of analytical results to determine compliance, system
inventories, sanitary surveys, state approvals, vulnerability and
waiver determinations, monitoring requirements, monitoring frequency
decisions, enforcement actions, and the issuance of variances and
exemptions. If primacy agencies grant monitoring waivers, they must
record monitoring results that are below the rule trigger level in
order to ensure systems are eligible for reduced monitoring schedules
(for additional discussion on the rule trigger level and monitoring
waivers, please see sections VIII and IX of this preamble). The primacy
agency record keeping requirements remain unchanged and would apply to
PFAS as with any other regulated contaminant.

C. What are the primacy agency reporting requirements?

    Currently, primacy agencies must report to EPA information under 40
CFR 142.15 regarding violations, variances and exemptions, enforcement
actions, and general operations of State PWS programs. These reporting
requirements remain unchanged and would apply to PFAS as with any other
regulated contaminant. However, the proposed PFAS MCLs, when final,
could result in a greater frequency of reporting by certain primacy
agencies. See discussion of PRA compliance in Section XV of this
preamble for more information.

D. Exemptions and Extensions

    In accordance with SDWA Sec.  1412(b)(10), a state or EPA may grant
an extension of up to two additional years to comply with an NPDWR's
MCL(s) if the state or EPA determines an individual system needs
additional time for capital improvements. At this time, EPA does not
intend to provide a two-year extension nationwide. However, States may
provide such an extension on an individual system basis. Where a State
or EPA chooses to provide such an extension, the system would have up
to five years from the rule's promulgation date to meet the MCLs. In
addition, under SDWA Sec.  1416, EPA or primacy Agencies may grant an
exemption for systems meeting specified criteria that provides an
additional period for compliance not to exceed 3 years beyond the time
period provided by Section 1412(b)(10). Under SDWA Sec.  1416(a), a
State which has primary enforcement responsibility may exempt any
public water system within the State's jurisdiction from any
requirement respecting a MCL of any applicable NPDWR upon a finding
that:
     Due to compelling factors (which may include economic
factors, including qualification of the public water system as a system
serving a disadvantaged community pursuant to section 300j-12(d) of
this title), the public water system is unable to comply with such
contaminant level or treatment technique requirement, or to implement
measures to develop an alternative source of water supply,
     The public water system was in operation on the effective
date of such contaminant level or treatment technique requirement, or,
for a system that was not in operation by that date, only if no
reasonable alternative source of drinking water is available to such
new system,
     The granting of the exemption will not result in an
unreasonable risk to health; and
     Management or restructuring changes (or both) cannot
reasonably be made that will result in compliance with this subchapter,
or if compliance cannot be achieved, improve the quality of the
drinking water.
    In addition, SDWA Sec.  1416(b)(2)(C) also allows for a small
system that does not serve a population of more than 3,300 and which
needs financial assistance for the necessary improvements to receive up
to three additional two-year exemptions, not to exceed a total of six
years provided that the system establishes that it is taking all
practicable steps to meet the requirements. In total, this means that
some systems could potentially exceed the MCLs' numerical standards for
up to 14 years after the rule promulgation date (or approximately 2037/
2038). EPA is seeking comment as to whether there are specific
conditions that should be mandated for systems to be eligible for
exemptions under 1416 to ensure that they are only used in rare
circumstances where there are no other viable alternatives and what
those conditions would be. EPA has established requirements for EPA
issuance of these exemptions in 40 CFR 142 subpart F but could consider
amending these requirements or establishing requirements for State
exemptions.

XIII. Health Risk Reduction and Cost Analysis

    This section summarizes the HRRCA for the proposed NPDWR for PFAS,
which is written in compliance with SDWA section 1412(b)(3)(C). Section
1412(b)(3)(C)(i) lists the analytical elements required in a HRRCA
applicable to a NPDWR that includes an MCL. The prescribed HRRCA
elements include:
    (1) Quantifiable and nonquantifiable health risk reduction
benefits;
    (2) quantifiable and nonquantifiable health risk reduction benefits
from reductions in co-occurring contaminants;
    (3) quantifiable and nonquantifiable costs that are likely to occur
solely as a result of compliance;
    (4) incremental costs and benefits of rule options;
    (5) effects of the contaminant on the general population and
sensitive subpopulations including infants, children, pregnant women,
the elderly, and individuals with a history of serious illness;
    (6) any increased health risks that may occur as a result of
compliance, including risks associated with co-occurring contaminants;
and
    (7) other relevant factors such as uncertainties in the analysis
and factors with respect to the degree and nature of the risk.
    Based on this analysis and pursuant to Section 1412(b)(4)(C) of
SDWA, the Administrator has determined that the quantified and
nonquantifiable benefits of the proposed regulation justify the costs.
The complete HRRCA for the proposed NPDWR, Economic Analysis for the
Proposed PFAS Rule, is hereafter referred to as the ``Economic
Analysis,'' and can be found in the docket at USEPA (2023j).
    For purposes of this Economic Analysis, EPA assumes that the NPDWR

will be promulgated by the end of 2023. This analysis follows the
standard NPDWR compliance schedule with regulatory requirements taking
effect three years after the date on which the regulation is
promulgated. If EPA issues a final NPDWR for PFAS by the end of 2023,
EPA assumes actions to comply with the rule, including installation of
treatment technologies, will occur by 2026. Based on an assumed mean
human lifespan of 80 years, EPA evaluates costs and benefits under the
proposed rule through the year 2104. EPA selected this period of
analysis to capture health effects from chronic illnesses that are
typically experienced later in life (i.e., cardiovascular disease [CVD]
and cancer). EPA annualized the future estimated streams of costs and
benefits symmetrically over this same period of analysis. Capital costs
for installation of treatment technologies are spread over the useful
life of the technologies. EPA does not capture effects of compliance
with the proposed rule after the end of the period of analysis. Costs
and benefits discussed in this section are presented as annualized
present values in 2021 dollars. EPA determined the present value of
these costs using discount rates of 3 and 7 percent, which are discount
rates prescribed by the (OMB Circular A-4, 2003).
    Estimates of PFAS occurrence used for cost-benefit modeling rely on
a Bayesian hierarchical estimation model of national PFAS occurrence in
drinking water (Cadwallader et al., 2022) discussed in Section VII.E.
of this preamble above. The model was fitted using sample data from
systems participating in PFAS sampling under UCMR 3 and included
systems serving over 10,000 customers, as well as a subset of 800
smaller systems. A best-fit model was selected using sample data to
define occurrence and co-occurrence of PFOA, PFOS, PFHpA, and PFHxS in
water systems stratified by system size and incorporating variations
within and among systems. Sample data were derived from state-level
datasets as well as from UCMR 3. For more information on EPA's
occurrence model, please see Section VII.E. of this preamble and USEPA
(2023e).
    In the Economic Analysis, EPA analyzes the costs and benefits of
the proposed rule, as well as several regulatory alternatives. EPA
analyzed the costs and benefits of setting individual MCLs for PFOA and
PFOS at 4.0 ppt, 5.0 ppt, and 10.0 ppt, referred to as Option 1a,
Option 1b, and Option1c, respectively. EPA assessed these options in
the Economic Analysis to understand the impact of less stringent PFOA
and PFOS MCLs, and the Agency is asking for comment on these
assessments in the Economic Analysis. The Agency is also inviting
comment on whether establishing a traditional MCLG and MCL for PFHxS,
HFPO-DA, PFNA, and PFBS instead of or in addition to the HI approach
would change public health protection, improve clarity of the rule, or
change costs. EPA has not separately presented changes in quantified
costs and benefits for these approaches. If EPA adds individual MCLs in
addition to using the HI approach, EPA anticipates there will be no
change in costs and benefits relative to the proposed rule (i.e., the
same number of systems will incur identical costs to the proposed
option and the same benefits will be realized). EPA has not separately
quantified the benefits and costs for the alternative approach to
regulate PFHxS, PFNA, PFBS, and HFPO-DA with individual MCLs instead of
the HI. However, EPA expects both the costs and benefits would be
reduced under this approach as fewer systems may be triggered into
treatment and its associated costs. Additionally, systems that exceed
one or more of the individual MCLs will treat to a less stringent and
public health-protective standard. Furthermore, under the proposed
option, PWSs are required to treat based on the combined occurrence of
PFAS included in the HI which considers the known and additive toxic
effects and occurrence and likely co-occurrence of PFAS compounds in
the HI, providing more public health protection compared to an
individual MCL approach.
    Section A summarizes the entities which would be affected by the
rule and provides a list of key data sources used to develop EPA's
baseline water system characterization. Section B provides an overview
of the cost-benefit model used to estimate the national costs and
benefits of the proposed rule. Section C summarizes the methods EPA
used to estimate costs associated with the proposed rule. Section D
summarizes the methods EPA used to estimate quantified benefits
associated with the proposed rule. Section E provides a summary of the
nonquantifiable benefits associated with reductions in exposure to both
PFOA and PFOS. Section F provides a qualitative summary of benefits
expected to result from the removal of PFAS included in the HI
component of the proposed regulation and additional co-removed PFAS
contaminants. Section G summarizes benefits expected to result from
DBPs co-removal. Section H provides a comparison of cost and benefit
estimates. Section I summarizes and discusses key uncertainties in the
cost and benefit analyses. Quantified costs and benefits for the
proposed option and alternative options considered are summarized in
section H, specifically Tables 66-69. Tables 70-71 summarizes the non-
quantified B-Cs and assess the potential impact of non-quantifiable
benefits and costs on the overall B-C estimate. Finally, Section J
presents the Administrator's cost-benefit determination for the
proposed rule.

A. Affected Entities and Major Data Sources Used To Develop the
Baseline Water System Characterization

    The entities potentially affected by the proposed PFAS regulation
are primacy agencies and PWSs. PWSs subject to the proposed rule
requirements are either CWSs or NTNCWSs. These water systems can be
publicly or privately owned. PWSs subject to the rule would be required
to meet the MCL and comply with monitoring and reporting requirements.
Primacy agencies would be required to adopt and enforce the drinking
water standard as well as the monitoring and reporting requirements.
    Both PWSs and primacy agencies are expected to incur costs,
including administrative costs, monitoring and reporting costs, and--in
a limited number of cases--anticipated costs to reduce PFAS levels in
drinking water to meet this proposed NPDWR using treatment or
nontreatment options. Section C of this preamble below summarizes the
method EPA used to estimate these costs.
    The systems that reduce PFAS concentrations will reduce associated
health risks. EPA developed methods to estimate the potential benefits
of reduced PFAS exposure among the service populations of systems with
PFAS levels exceeding the proposed drinking water standard. Section B
of this preamble below summarizes this method used to estimate these
benefits.
    In its Guidelines for Preparing Economic Analyses, EPA
characterizes the ``baseline'' as a reference point that reflects the
world without the proposed regulation (USEPA, 2010). It is the starting
point for estimating the potential benefits and costs of the proposed
PFAS NPDWR. EPA used a variety of data sources to develop the baseline
drinking water system characterization for the regulatory analysis.
Table 24 lists the major data sources and the baseline data derived
from them. Additional detailed descriptions of these data sources and
how they were used in the characterization of baseline conditions

can be found in the Chapter 4 of USEPA (2023j).

      Table 24--Data Sources Used To Develop Baseline Water System
                            Characterization
------------------------------------------------------------------------
         Data source             Baseline data derived from the source
------------------------------------------------------------------------
SDWIS/Federal version fourth    Water System Inventory: PWS
 quarter 2021 Q4 ``frozen''     inventory, including system unique
 dataset \1\.                   identifier, population served, number of
                                service connections, source water type,
                                and system type.
                                Population and Households
                                Served: PWS population served.
                                Treatment Plant
                                Characterization: Number of unique
                                treatment plant facilities per system,
                                which are used as a proxy for entry
                                points when UCMR 3 sampling site data
                                are not available.
UCMR 3 (USEPA, 2017).........   Treatment Plant
                                Characterization: Number of unique entry
                                point sampling sites, which are used as
                                a proxy for entry points.
                                Treatment Plant
                                Characterization: PFAS concentration
                                data collected as part of UCMR 3.
Independent state sampling      Treatment Plant
 programs.                      Characterization: PFAS concentration
                                data collected by states. These data
                                supplemented the occurrence modeling for
                                systems included in UCMR 3.
Six-Year Review 4 Information   Treatment Plant
 Collection Request (SYR4       Characterization: Total organic carbon
 ICR) Occurrence Dataset        (TOC).
 (2012-2019).
Geometries and                  Treatment Plant
 Characteristics of Public      Characterization: Design and average
 Water Systems (USEPA, 2000f).  daily flow per system.
2006 Community Water System     Public Water System Labor Rates:
 Survey (CWSS; USEPA, 2009c).   PWS labor rates.
------------------------------------------------------------------------
Notes:
\1\ Contains information extracted on January 14, 2022.

B. Overview of the Cost-Benefit Model

    EPA's existing SafeWater Cost Benefit Model (CBX) was designed to
calculate the costs and benefits associated with setting a new or
revised MCL. Since the proposed PFAS rule simultaneously regulates
multiple PFAS contaminants, EPA developed a new model version called
the SafeWater Multi-Contaminant Benefit Cost Model (MCBC) to
efficiently handle more than one contaminant. SafeWater MCBC, allows
for inputs that include differing mixtures of contaminants based on
available occurrence data as well as multiple regulatory thresholds.
The model structure allows for assignment of compliance technology or
technologies that achieve all regulatory requirements and estimates
costs and benefits associated with multiple PFAS contaminant
reductions. SafeWater MCBC is designed to model co-occurrence,
sampling, treatment, and administrative costs, and simultaneous
contaminants reductions and resultant benefits. The modifications to
the SafeWater model are consistent with the methodology that was
developed in the single MCL SafeWater CBX Beta version that was peer
reviewed. More detail on the modifications to the SafeWater model can
be found in Section 5.2 of EPA's economic analysis.
    The costs incurred by a PWS depend on water system characteristics;
SDWIS/Fed provides information on PWS characteristics that typically
define PWS categories, or strata, for which EPA has develops cost
estimates in rulemakings, including system type (CWS, NTNCWS), number
of people served by the PWS, the PWS's primary raw water source (ground
water or surface water), the PWS's ownership type (public or private),
and PWS state.
    Because EPA does not have complete PWS-specific data across the
approximately 49,000 CWSs and 17,000 NTNCWSs in SDWIS/Fed for many of
the baseline and compliance characteristics necessary to estimate costs
and benefits, such as design and average daily flow rates, water
quality characteristics, treatment in-place, and labor rates, EPA
adopted a ``model PWS'' approach. SafeWater MCBC creates model PWSs by
combining the PWS-specific data available in SDWIS/Fed with data on
baseline and compliance characteristics available at the PWS category
level. In some cases, the categorical data are simple point estimates.
In this case, every model PWS in a category is assigned the same value.
In other cases, where more robust data representing system variability
are available, the category-level data include a distribution of
potential values. In the case of distributional information, SafeWater
MCBC assigns each model PWS a value sampled from the distribution.
These distributions are assumed to be independent.
    For a list of PWS characteristics that impact model PWS compliance
costs, please see Chapter 5 of USEPA (2023j). These data include
inventory data specific to each system and categorical data for which