treatment feasibility. Since there is no new information regarding
analytical or treatment feasibility that suggests changes to the MCL,
EPA does not believe it is necessary to conduct an occurrence analysis
at this time.
    c. Review Result. The Agency does not believe a revision to the
NPDWR for gross alpha particles is appropriate at this time because a
reassessment of the health risks resulting from exposure to alpha
particles is in progress (USEPA, 2009b). Furthermore, there is no new
information regarding analytical or treatment feasibility that would
warrant reconsideration of the MCL.
4. Antimony
    a. Background. EPA published the current NPDWR for antimony on July
17, 1992 (57 FR 31776 (USEPA, 1992)). The NPDWR established an MCLG and
an MCL of 0.006 mg/L. EPA based the MCLG on a reference dose of 0.0004
mg/kg-day and a cancer classification of D, not classifiable as to
human carcinogenicity.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to antimony. The revised health
effects assessment will consider relevant studies on the toxicity of
antimony, including its potential developmental and reproductive
toxicity. The Agency does not expect the new health effects assessment
to be completed in the time frame of the current Six-Year Review cycle
(USEPA, 2009b). On December 21, 2007 (72 FR 72715 (USEPA, 2007c)), the
Agency noted that the health effects assessment for antimony is in
process.
    c. Review Result. Since the MCL for antimony is set at its MCLG and
a reassessment of the health risks resulting from exposure to antimony
is in progress, the Agency does not believe a revision to the NPDWR is
appropriate at this time.
5. Arsenic
    a. Background. EPA published the current NPDWR for arsenic on
January 22, 2001 (66 FR 6976 (USEPA, 2001c)). The NPDWR established an
MCLG of zero based on a cancer classification of A, known human
carcinogen. The NPDWR also established an MCL of 0.010 mg/L, which is
higher than the feasible analytical level of 0.003 mg/L. EPA exercised
its discretionary authority to set an MCL at a level higher than
feasible (SDWA Section 1412(b)(6)), based on the finding that a final
MCL of 0.010 mg/L represents the level that best maximizes health risk
reduction benefits at a cost that is justified by the benefits (66 FR
6976 at 7020 (USEPA, 2001c)).
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to arsenic. In June 2007, EPA's
Science Advisory Board (SAB) issued its evaluation of the Agency's 2005
draft toxicological review for inorganic arsenic (USEPA, 2007a). In its
2007 report, SAB supports the continued use of a linear cancer risk
model for inorganic arsenic, noting that the available data do not
describe the shape of the dose-response curve at low doses. The new
health effects assessment (both cancer and noncancer) were not
completed by March 1, 2009, the review cutoff date for this notice. The
revised health effects assessments will consider relevant studies on
the toxicity of arsenic, including its potential developmental and
reproductive toxicity. The IRIS Substance Assessment Tracking System
Web site (http://cfpub.epa.gov/iristrac/index.cfm) has the most up-to-
date information on the status of the health effects assessments.
    c. Review Result. The Agency does not believe a revision to the
NPDWR for arsenic is appropriate at this time because a reassessment of
the health risks resulting from exposure to arsenic is ongoing (USEPA,
2009b). As noted previously, the arsenic MCL is based on the SDWA cost
benefit provision (Section 1412(b)(6)) and the health effects
assessment is important for reviewing the benefits associated with the
basis of the MCL.
6. Asbestos
    a. Background. EPA published the current NPDWR for asbestos on
January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The NPDWR established an
MCLG and an MCL of 7 million fibers/L. EPA evaluated asbestos as a
Category II \12\ contaminant (equivalent to Group C, possible human
carcinogen) by the oral route of exposure.
---------------------------------------------------------------------------

    \12\ Category II contaminants include those contaminants for
which EPA has determined there is limited evidence of
carcinogenicity from drinking water considering weight of evidence,
pharmacokinetics, potency, and exposure. For Category II
contaminants, EPA has used two approaches to set the MCLG: Either
(1) setting the MCLG based upon noncarcinogenic endpoints of
toxicity (the RfD) then applying an additional risk management
factor of 1 to 10; or (2) setting the MCLG based upon a theoretical
lifetime excess cancer risk range of 10-5 to
10-6 using a conservative mathematical extrapolation
model.
---------------------------------------------------------------------------

    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to asbestos. The revised health
effects assessment will consider relevant studies on the toxicity of
asbestos, including its potential developmental and reproductive
toxicity. The Agency does not expect the new health effects assessment
to be completed in the time frame of the current Six-Year Review cycle
(USEPA, 2009b). The IRIS Substance Assessment Tracking System Web site
(http://cfpub.epa.gov/iristrac/index.cfm) has the most up-to-date
information on the status of the health effects assessment.
    c. Review Result. Since the MCL for asbestos is set at its MCLG and
a reassessment of the health risks resulting from exposure to asbestos
is in

progress, the Agency does not believe a revision to the NPDWR is
appropriate at this time.
7. Atrazine
    a. Background. EPA published the current NPDWR for atrazine on
January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The NPDWR established an
MCLG and an MCL of 0.003 mg/L. EPA based the MCLG on a reference dose
of 0.005 mg/kg-day and a cancer classification of C, possible human
carcinogen.
    b. Technical Reviews. In 2006, the Agency finalized a health
effects assessment for the reregistration of atrazine as a pesticide
(USEPA, 2006c). This assessment examined an extensive toxicology
database and included investigation of atrazine's neuroendocrine mode
of action and related reproductive and developmental effects. The
assessment established a new RfD of 0.018 mg/kg-day, based on
attenuation of pre-ovulatory luteinizing hormone (LH) surge, a key
event indicative of hypothalamic function disruption. In accordance
with the 1999 Interim Guidelines for Carcinogen Risk Assessment, EPA's
Cancer Assessment Review Committee (CARC) classified atrazine as ``not
likely to be carcinogenic to humans'' because the tumor response in the
Sprague-Dawley rats was determined to be a strain specific mechanism
which is not relevant to humans.
    c. Review Result. The Agency believes it is not appropriate to
consider revisions to the NPDWR for atrazine at this time and has place
atrazine in the emerging information/data gap category because of an
impending re-evaluation of the Agency's risk assessment for atrazine.
On October 7, 2009,\13\ the Agency announced its intent to launch a
comprehensive new evaluation of the atrazine to determine its effects
on humans. At the end of this process, the Agency will decide whether
to revise its current risk assessment for atrazine and whether new
restrictions are necessary to better protect public health. EPA will
evaluate the pesticide's potential cancer and non-cancer effects on
humans. Included in this new evaluation will be the most recent studies
on atrazine and its potential association with birth defects, low birth
weight, and premature births. Our examination of atrazine will be based
on transparency and sound science, including independent scientific
peer review and will help determine whether a change in EPA's
regulatory position on this pesticide is appropriate.
---------------------------------------------------------------------------

    \13\ Additional information is available at http://www.epa.gov/
pesticides/reregistration/atrazine/atrazine_update.htm.
---------------------------------------------------------------------------

8. Barium
    a. Background. EPA published the current NPDWR for barium on July
1, 1991 (56 FR 30266 (USEPA, 1991b)). The NPDWR established an MCLG and
an MCL of 2 mg/L. EPA based the MCLG on a reference dose of 0.07 mg/kg-
day and a cancer classification of D, not classifiable as to human
carcinogenicity via the oral route.
    b. Technical Reviews. In 2005, the Agency updated the health
effects assessment of barium and revised the RfD from 0.07 mg/kg-day to
0.2 mg/kg-day (USEPA, 2005a). The change in the RfD could lead to a
change in the MCLG. This assessment considered relevant studies on the
toxicity of barium including developmental and reproductive toxicity.
The assessment concluded that barium is not likely to be carcinogenic
to humans (USEPA, 2005a). Based on the new IRIS assessment and RfD of
0.2 mg/kg-day, and assuming 70 kg body weight and 2 liters water intake
per day, the DWEL could be 7.0 mg/L. An RSC of 80 percent \14\ results
in a possible MCLG of 6.0 mg/L.
---------------------------------------------------------------------------

    \14\ The present MCLG for barium does not include an RSC because
the dose used in the calculation applied to only the dose from the
drinking water. If a new MCLG were to be developed from the animal
data that support the 2005 IRIS RfD, an RSC would be required.
Regulations or guidelines pertaining to barium from media other than
water were not identified. Barium metaborate is a registered
pesticide but it does not have any food uses and does not have a
human health ambient water quality guideline value. EPA used the
subtraction calculation method to determine the possible RSC of 80
percent for drinking water (the ceiling on RSC specified by the
methodology).
---------------------------------------------------------------------------

    Analytical feasibility does not pose any limitations for the
current MCL and would not be a limiting factor if EPA were to raise the
MCLG. EPA evaluated the results of the occurrence and exposure analyses
for barium to determine whether a revised MCLG/MCL would be likely to
result in a meaningful opportunity to achieve cost savings for PWSs and
their customers while maintaining, or improving, the level of public
health protection (USEPA, 2009f). Although the Agency obtained and
evaluated the finished water occurrence data for barium, its usefulness
is limited for determining potential cost savings to PWSs and their
customers because the Agency does not know which systems are treating
for this contaminant. As an alternative, the Agency evaluated available
data on source water quality and conducted a qualitative assessment of
treatment cost savings.
    Table VI-3 provides summary data for contaminant occurrence based
on maximum sample values for the locations included in the STORET and
NAWQA data. Although the degree to which these occurrence rates
represent national drinking water source occurrence is uncertain, the
information shows no to low occurrence at threshold levels of interest.
This information indicates that any resulting NPDWR change would affect
systems that rely on source water at less than 0.1 percent of the NAWQA
locations and less than 1.4 percent of the STORET locations.

   Table VI-3--Ambient Water Quality Monitoring Occurrence Summary for
                                 Barium
------------------------------------------------------------------------
                                   Number of locations (% of locations)
     Maximum concentration      ----------------------------------------
                                    STORET \1\           NAWQA \2\
------------------------------------------------------------------------
Total..........................  16,595 (100.0%).  4,864 (100.0%)
Nondetect......................  2,299 (13.9%)...  43 (0.9%)
Detected.......................  14,296 (86.1%)..  4,821 (99.1%)
Exceeds current MCL/MCLG of 2.0  234 (1.4%)......  3 (0.1%)
 mg/L.
Exceeds alternative value of     163 (1.0%)......  0 (0.0%)
 6.0 mg/L.
------------------------------------------------------------------------
\1\ STORET database 2002-2006.
\2\ NAWQA database 1992-2008.
Source: USEPA, 2009d.

    The BATs and small system compliance technologies for barium have
other beneficial effects, e.g., reduction of other co-occurring
contaminants or other common impurities. Therefore, if EPA were to
consider a higher level, the Agency does not know how many PWSs that
are currently treating to comply with the existing MCL of 2 mg/L would
be likely to discontinue treatment that is already in place (USEPA,
2009d). Also, the Agency does not know to what extent affected systems
might be able to reduce costs given that capital costs are not
recoverable. However, the Agency recognizes that there may be
opportunities to achieve operational cost savings if these systems are
able to re-optimize current treatment.
    Given these considerations, the Agency believes that any resulting
revision is not likely to provide a meaningful opportunity for cost
savings. In view of this, any revision would be a low priority activity
and not appropriate at this time.
    c. Review Result. Although there are new data that support
consideration of whether to revise the MCLG/MCL for barium, EPA does
not believe a revision to the NPDWR for barium is appropriate at this
time. In making this decision, the Agency considered whether any
possible revision to the NPDWR for barium is likely to provide a
meaningful opportunity for cost savings to public water systems and
their customers. Taking into consideration the low occurrence of this
contaminant in source waters, EPA has decided that any revision to the
NPDWR would be a low priority activity for the Agency, and, thus, is
not appropriate to revise at this time because of:
     Competing workload priorities;
     The administrative costs associated with rulemaking; and
     The burden on States and the regulated community to
implement any regulatory change that resulted.
9. Benzene
    a. Background. EPA published the current NPDWR for benzene on July
8, 1987 (52 FR 25690 (USEPA, 1987)). The NPDWR established an MCLG of
zero based on a cancer classification of A, known human carcinogen. The
NPDWR also established an MCL of 0.005 mg/L, based on analytical
feasibility.
    b. Technical Reviews. In 2000 and 2003, the Agency updated the IRIS
assessment of benzene. The cancer assessment was completed first and
characterized benzene as a known human carcinogen by all routes of
exposure; the one-in-a million risk estimates for cancer by the oral
route of exposure ranged from 1 [mu]g/L to 10 [mu]g/L (USEPA, 2000b).
This cancer assessment was also noted in the first Six-Year Review (67
FR 19030, April 17, 2002 (USEPA, 2002c)). As part of the Six-Year
Review process, the Agency's Office of Water (OW) conducted a
literature search through June 2007 for relevant data on the
carcinogenicity of benzene as well as its potential developmental and
reproductive toxicity (USEPA, 2009b). While the literature search did
identify several new studies that evaluated the cancer and noncancer
effects of benzene, none of the new studies would affect the cancer
classification, which serves as the basis for the MCLG of zero. A
recent occupational study (Lan et al., 2004) of the noncancer effects
of benzene identified hematological effects in workers at levels below
those previously reported. The Agency for Toxic Substances and Disease
Registry (ATSDR) (2007) chronic minimum risk level based on the Lan et
al. (2004) data of 0.0005 mg/kg/day is lower than the IRIS RfD of 0.004
mg/kg/day. If the ATSDR minimum risk level were used as the basis for a
noncancer health reference level, the value would be 0.004 mg/l, a
value that is slightly below the current MCL. Because the MCLG remains
at zero, the Agency believes that a further review of the health
effects of benzene is not warranted at this time.
    The current MCL for benzene is based on a PQL of 0.005 mg/L. For
the Six-Year Review, the Agency considered whether changes in the
analytical feasibility of benzene might lead to a lower MCL. EPA
reviewed PE data from the first Six-Year Review cycle and then analyzed
more recent PT data to determine if the PQL can be revised (i.e.,
analytical feasibility). Passing rates for PE data available through
late 1999 for benzene are above 95 percent around the current PQL of
0.005 mg/L, including two studies with true values below the current
PQL. All passing rates in the PE data exceeded 75 percent. More recent
PT data from late 1999 through 2004, supplied by a PT provider, also
show greater than 90 percent passing rates for studies around the PQL,
including eight with true values below the current PQL. Because most of
the laboratory passing rates from PE and PT studies exceeded the 75
percent criterion typically used to derive a PQL, a lowering of the PQL
for benzene might be possible. These results, however, are insufficient
to recalculate a revised PQL for benzene because not enough data points
are available below the current PQL to derive a value at the 75 percent
passing rate (USEPA, 2009c).
    EPA evaluated two alternative sources of information to determine
whether an EQL below the current PQL could be estimated: Laboratory
MRLs in the Six-Year Review ICR dataset, and the MDLs for approved
methods for the detection of benzene (Methods 502.2 and 524.2). While
EPA prefers to use laboratory performance data to calculate the PQL,
the MRL and MDL information can be valuable for this review to indicate
whether it is possible to quantitate at levels below the current PQL.
The Six-Year Review ICR dataset contains MRL values for 139,190
samples. More than 80 percent of these values are less than or equal
the modal MRL, 120,308 (86 percent) equal the modal MRL of 0.0005 mg/L,
and an additional 17,964 (13 percent) are lower than 0.0005 mg/L.
Therefore, EPA selected the modal MRL as the EQL (USEPA, 2009e). The
MDLs of approved methods range from 0.00001 to 0.0004 mg/L. Applying a
multiplier of 10 would give a possible PQL range from 0.0001 to 0.004
mg/L, which contains the EQL (USEPA, 2009e).
    Based on these varied and unrelated approaches/sources of
information, EPA believes that there is potential to lower the PQL for
benzene. To determine whether any MCL revision is likely to provide a
meaningful opportunity to improve public health protection, EPA
evaluated the occurrence of benzene at the EQL of 0.0005 mg/L and
additional thresholds of 0.001, and 0.0025 mg/L (USEPA, 2009f). Table
VI-4 shows the results of the occurrence and exposure analysis for the
current MCL and these thresholds. The Six-Year Review ICR occurrence
data have a modal MRL of 0.0005 mg/L, which limits reliable contaminant
detection to 0.0005 mg/L. As indicated, average concentrations exceed
the current MCL for 10 of 50,435 systems (0.020 percent) serving 14,000
people (or 0.006 percent of 227 million people). Note that these
results are based on the subset of monitoring data provided in response
to the Six-Year Review ICR and do not necessarily reflect MCL
violations, which are based on annual average concentrations at entry
points; Safe Drinking Water Information System/Federal version (SDWIS/
FED) indicates 41 MCL violations for benzene between 1998 and 2005,
with annual violations ranging from 1 to 12 (USEPA, 2007g). The
occurrence and exposure analysis shows that average concentrations at
95 to 123 of 50,435 systems (0.188 to 0.244 percent), serving 304,000
to 485,000 people (or 0.134 to 0.214 percent of 227

million people), exceed the EQL of 0.0005 mg/L.

       Table VI-4--Number and Percent of Systems With Mean Concentrations Exceeding Benzene Thresholds and
                                  Corresponding Estimates of Population Served
----------------------------------------------------------------------------------------------------------------
                                      Systems with mean concentrations that are greater than the regulatory or
                                       feasibility-based threshold  (Percentages based on 50,435 systems with
 Regulatory or feasibility-based            benzene data in the  Six-Year Review ICR occurrence dataset)
            threshold             ------------------------------------------------------------------------------
                                    Nondetect values =  MRL    Nondetect values =  \1/    Nondetect values =  0
                                              \1\                    2\ MRL \2\                    \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.005 mg/L).................  10 (0.020%)..............  10 (0.020%).............  10 (0.020%)
\1/2\ MCL (0.0025 mg/L)..........  16 (0.032%)..............  14 (0.028%).............  14 (0.028%)
2xEQL (0.001 mg/L)...............  70 (0.139%)..............  58 (0.115%).............  52 (0.103%)
EQL (0.0005 mg/L)................  not applicable...........  123 (0.244%)............  95 (0.188%)
----------------------------------------------------------------------------------------------------------------
                                      Corresponding population served (Percentages based on 226,947,000 people
                                         served by the systems with benzene data in the Six-Year Review ICR
                                                                 occurrence dataset)
                                  ------------------------------------------------------------------------------
 Regulatory or feasibility-based       Nondetect values =        Nondetect values =        Nondetect values =
            threshold              MRL \1\..................  \1/2\ MRL \2\...........  0 \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.005 mg/L).................  14,000 (0.006%)..........  14,000 (0.006%).........  14,000 (0.006%)
\1/2\ MCL (0.0025 mg/L)..........  111,000 (0.049%).........  110,000 (0.048%)........  110,000 (0.048%)
2xEQL (0.001 mg/L)...............  180,000 (0.079%).........  159,000 (0.070%)........  158,000 (0.070%)
EQL (0.0005 mg/L)................  not applicable...........  485,000 (0.214%)........  304,000 (0.134%)
----------------------------------------------------------------------------------------------------------------
\1\ Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset.
  Results are not reported at the EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the
  results equal to this value results in an upwardly biased estimate of the number of systems with mean
  concentrations that exceed this value.
\2\ Results are based on setting all nondetect results equal to \1/2\ MRL values in the Six-Year Review ICR
  dataset.
\3\ Results are based on setting all nondetect results equal to zero.
Source: USEPA, 2009f.

    Since the occurrence analysis indicates that any revision to the
MCL is unlikely to provide a meaningful opportunity to improve the
level of public health protection, it was not necessary to perform any
additional reviews on treatment feasibility or economic considerations.
    c. Review Result. Although there are new data that support
consideration of a possibly lower PQL (and therefore a possibly lower
MCL), EPA does not believe a revision to the NPDWR for benzene is
appropriate at this time. The occurrence and exposure analysis based on
possible changes in analytical feasibility indicates that any revision
to the MCL is unlikely to provide a meaningful opportunity to improve
public health protection. Taking into consideration the low occurrence
of this contaminant, EPA has decided that any revision to the NPDWR
would be a low priority activity for the Agency, and, thus, is not
appropriate to revise at this time because of:
     Competing workload priorities;
     The administrative costs associated with rulemaking; and
     The burden on States and the regulated community to
implement any regulatory change that resulted.
10. Benzo(a)pyrene
    a. Background. EPA published the current NPDWR for benzo(a)pyrene
on July 17, 1992 (57 FR 31776 (USEPA, 1992)). The NPDWR established an
MCLG of zero based on a cancer classification of B2, probable human
carcinogen. The NPDWR also established an MCL of 0.0002 mg/L, based on
analytical feasibility.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to benzo(a)pyrene. The revised
health effects assessment will consider relevant studies on the
toxicity of benzo(a)pyrene, including its potential developmental and
reproductive toxicity. The new health effects assessment was not
completed by March 1, 2009, the review cutoff date for this notice
(USEPA, 2009b). The IRIS Substance Assessment Tracking System Web site
(http://cfpub.epa.gov/iristrac/index.cfm) has the most up-to-date
information on the status of the health effects assessment.
    Although a risk assessment is in process for benzo(a)pyrene, the
existing MCLG is zero and the current MCL of 0.0002 mg/L is based on
the PQL. Therefore, EPA reviewed whether there is potential to revise
the PQL. EPA reviewed PE data from the first Six-Year Review cycle and
then analyzed more recent PT data to determine if the PQL can be
revised (i.e., analytical feasibility). Passing rates for PE data
available through late 1999 for benzo(a)pyrene are all above 75
percent. However, the true concentrations were all higher than the
current PQL of 0.0002 mg/L. More recent PT data from late 1999 through
2004, supplied by a PT provider, show several true concentrations with
passing rates less than the 75 percent criterion typically used to
derive a PQL. All of the true concentrations in the PT data were higher
than the current PQL. Given the variability in passing rates and the
lack of data points below the current PQL, a lowering of the PQL for
benzo(a)pyrene is not appropriate at this time (USEPA, 2009c).
    EPA evaluated two alternative sources of information to determine
whether an EQL below the current PQL could be estimated: laboratory
MRLs in the Six-Year Review ICR dataset, and the MDLs for approved
methods for the detection of benzo(a)pyrene (Methods 550, 550.1, and
525.2). While EPA prefers to use laboratory performance data to
calculate the PQL, the MRL and MDL information can be valuable for this
review to indicate whether it is possible to quantitate at levels below
the current PQL. The Six-Year Review ICR dataset contains MRL values
for 55,487 samples. Fewer than 80 percent of these values are less than
or equal the modal MRL, 29,769 (54 percent) equal the modal MRL of
0.00002 mg/L and an additional 970 (2 percent) are lower

than 0.00002 mg/L. Therefore, EPA did not set the EQL equal to the
modal MRL (USEPA, 2009e). The MDLs of approved methods are 0.000016,
0.000029, and 0.00023 mg/L. EPA selected the median value, applied a
multiplier of 10, and rounded up to 0.0003 mg/L. The result is higher
than the current PQL and, therefore, EPA did not estimate an EQL
(USEPA, 2009e). Based on these varied and unrelated approaches/sources
of information, EPA believes that there is no potential to lower the
PQL for benzo(a)pyrene. Since the MCL is constrained by the PQL, and
the PQL is unchanged, EPA does not believe it is necessary to conduct
an occurrence analysis at this time.
    c. Review Result. The Agency does not believe a revision to the
NPDWR for benzo(a)pyrene is appropriate at this time because a
reassessment of the health risks resulting from exposure to
benzo(a)pyrene is in progress (USEPA, 2009b). Furthermore, a review of
analytical feasibility did not identify a potential to revise the MCL,
which is limited by feasibility.
11. Beryllium
    a. Background. EPA published the current NPDWR for beryllium on
July 17, 1992 (57 FR 31776 (USEPA, 1992)). The NPDWR established an
MCLG and an MCL of 0.004 mg/L. EPA classified beryllium in Group B2,
probable human carcinogen, based on clear evidence of its
carcinogenicity via inhalation or injection in several animal species.
However, EPA also placed beryllium in drinking water Category II for
regulation, based on the weight of evidence for carcinogenicity via
ingestion, and the potency, exposure and pharmacokinetics of this
chemical. EPA derived the MCLG by applying an additional risk
management factor of 10 to the RfD of 0.005 mg/kg-day (57 FR 31776 at
31785, July 17, 1992 (USEPA, 1992)).
    b. Technical Reviews. As noted in Six Year Review 1 (68 FR 42908,
USEPA, 2003e), EPA updated its assessment of the health risks resulting
from exposure to beryllium in 1998 (USEPA, 1998c). The 1998 IRIS
assessment uses the 1986 EPA cancer guidelines (USEPA, 1986b) and
classifies beryllium as Group B1, probable human carcinogen, via
inhalation route. However, the 1998 IRIS assessment states that the
database is inadequate for assessing the carcinogenicity of ingested
beryllium and concluded that the human carcinogenic potential of
ingested beryllium cannot be determined. The Agency considered the 1998
assessessment in Six Year Review 1 and decided that it was not
appropriate to revise the NPDWR at that time. EPA has initiated a
reassessment of the health risks resulting from exposure to beryllium.
The new assessment was not completed by March 1, 2009, the review
cutoff date for this notice (USEPA, 2009b). The IRIS Substance
Assessment Tracking System Web site (http://cfpub.epa.gov/iristrac/
index.cfm) has the most up-to-date information on the status of the
health effects assessment.
    c. Review Result. Since the MCL for beryllium is set at its MCLG
and a reassessment of the health risks resulting from exposure to
beryllium is in progress, the Agency does not believe a revision to the
NPDWR is appropriate at this time.
12. Beta Particle and Photon Emitters
    a. Background. EPA published an interim NPDWR and set an MCL of 4
millirems/yr (mrem/yr) for beta particle and photon emitters on July 9,
1976 (41 FR 28402 (USEPA, 1976)). As noted in the August 14, 1975
proposal (40 FR 34324 (USEPA, 1975)) and a subsequent September 30,
1986 FR (51 FR 34836 (USEPA, 1986a) advanced notice of proposed
rulemaking, EPA considered the feasibility of treatment techniques,
analytical methods and monitoring when establishing the MCL of 4 mrem/
yr. EPA also considered the risks associated with beta particle and
photon emitters, which generally fell within the Agency's acceptable
risk range of 10-4 to 10-6 at the MCL of 4 mrem/
yr. On December 7, 2000 (65 FR 76708 (USEPA, 2000c)), EPA established
an MCLG of zero based on a cancer classification of A (known human
carcinogen) and finalized the NPDWR by retaining the MCL of 4 mrem/yr.
EPA noted in the December 7, 2000, FR notice that new risk estimates
from Federal Guidance Report 13 reaffirmed that the 4 mrem/yr MCL was
appropriate and protective\15\.
---------------------------------------------------------------------------

    \15\ After the December 7, 2000, final regulation, two trade
associations and several municipal water systems challenged EPA's
standard for the beta photon emitters by claiming that the Agency
did not use the best available science when finalizing the standard.
In February of 2003, the District of Columbia (DC) Circuit Court of
Appeals upheld EPA's regulation for beta and photon emitters (as
well as radium 226 and 228 and uranium). In July, 2004, the DC
Circuit Court of Appeals also upheld the policy and scientific basis
of EPA's application of the beta particle and photon (man-made)
drinking water standards to the ground water protection standards
used for Yucca Mountain under 40 CFR part 197 (66 FR 32073, June 13,
2001 (USEPA, 2001d)).
---------------------------------------------------------------------------

    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to beta particles. The revised
health effects assessment will consider relevant studies on the
toxicity of beta particles, including its potential developmental and
reproductive toxicity. The new health effects assessment was not
completed by March 1, 2009, the review cutoff date for this notice
(USEPA, 2009b).
    Although there is an ongoing health effects assessment, the MCLG is
zero and the current MCL is higher than the MCLG. Therefore, EPA
reviewed whether there is potential to revise the MCL based on new
information available regarding the analytical and treatment
feasibility for beta particle and photon emitters. EPA promulgated the
MCL of 4 mrem/yr for man-made beta particle and photon emitters
(present in any combination) in 1976 (41 FR 28402 (USEPA, 1976)) and
retained the use of the detection limit as the required measure of
sensitivity in the December 2000 final rule (65 FR 76708 (USEPA,
2000c)). The original rule estimated a risk ceiling of
5.6x10-5 for whole body doses. Limits were set in picoCurie
units for each nuclide equivalent to a 4 mrem dose. The newer dosimetry
found in Federal Guidance13 and reported in the December 2000 final
rule reveals more exact risks that are still within the Agency's
acceptable limits. While individual dose estimates changed over time,
the overall limit of 4 mrem was retained along with a two-tiered
screening level to avoid analyzing each possible nuclide below the
screen, and still be protective. EPA did not identify new analytical
methods during the current review that would feasibly lower the
detection limits for beta particle and photon emitters. In addition,
since the December 7, 2000 regulation, there is no new information
regarding treatment feasibility. Since there is no new information
regarding analytical or treatment feasibility that suggests changes to
the MCL, EPA does not believe it is necessary to conduct an occurrence
analysis at this time.
    c. Review Result. The Agency does not believe a revision to the
NPDWR for beta particles is appropriate at this time because a
reassessment of the health risks resulting from exposure to beta
particles is in progress (USEPA, 2009b). Furthermore, there is no new
information regarding analytical or treatment feasibility that would
warrant reconsideration of the MCL.
13. Cadmium
    a. Background. EPA published the current NPDWR for cadmium on
January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The NPDWR established an
MCLG and an MCL of 0.005 mg/L. Because of inadequate dose-response data
to characterize the presence or lack of a carcinogenic hazard from oral

exposure, the Agency classified cadmium as a Group D carcinogen, not
classifiable as to human carcinogenicity by the oral route of exposure.
Therefore, EPA developed the MCLG for cadmium based on the RfD of
0.0005 mg/kg-day.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to cadmium. The revised health
effects assessment will consider relevant studies on the toxicity of
cadmium, including its potential developmental and reproductive
toxicity. The new health effects assessment was not completed by March
1, 2009, the review cutoff date for this notice (USEPA, 2009b). The
IRIS Substance Assessment Tracking System Web site (http://
cfpub.epa.gov/iristrac/index.cfm) has the most up-to-date information
on the status of the health effects assessment.
    c. Review Result. Since the MCL for cadmium is set at its MCLG and
a reassessment of the health risks resulting from exposure to cadmium
is in progress, the Agency does not believe a revision to the NPDWR is
appropriate at this time.
14. Carbofuran
    a. Background. EPA published the current NPDWR for carbofuran on
January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The NPDWR established an
MCLG and an MCL of 0.04 mg/L. EPA based the MCLG on a reference dose of
0.005 mg/kg-day and a cancer classification of E, evidence of non-
carcinogenicity for humans.
    b. Technical Reviews. In 2006, the Agency updated health effects
assessment of carbofuran. The Agency identified a change in this
assessment that could lead to a change in the MCLG (73 FR 44864, July
31, 2008 (USEPA, 2008a)). This assessment considered relevant studies
on the toxicity of carbofuran including developmental and reproductive
toxicity. The assessment revised the RfD from 0.005 mg/kg-day to an
acute RfD of 0.00006 mg/kg-day and concluded that carbofuran is not
likely to be carcinogenic to humans (USEPA, 2006d). Based on the
revised acute RfD of 0.00006 mg/kg-day, and assuming 10 kg body weight
and 1 liter water intake per day for a child, the resulting DWEL would
be 0.0006 mg/L. Using an RSC of 20 percent, a possible new MCLG would
be 0.00012 mg/L. The default RSC value of 20 percent was selected
because of the significant exposures resulting from actual food dietary
exposure for children from 1 to 6 years old, which approaches 100
percent of the updated RfD (USEPA, 2006d).
    Two recent Agency actions may affect carbofuran presence in food
and water sources. In May 2009, EPA revoked all tolerances (maximum
residue limits) for carbofuran, which could prohibit all carbofuran
residues on food, effective December 31, 2009 (74 FR 23046, May 15,
2009 (USEPA, 2009i)). The registrant and interested parties raised
objections and requested a hearing on the tolerance revocations. EPA
has reviewed the submissions and determined that a hearing was not
warranted. Revoking carbofuran tolerances is part of a broader series
of Agency actions to cancel all uses of carbofuran in the United States
due to dietary, occupational, and ecological risks of concern.
Following resolution of the current ongoing administrative process for
resolving the safety of the tolerances, EPA will proceed to cancel the
remaining uses of carbofuran.
    In addition, prior to the tolerance revocation, the registrant, FMC
Corporation, voluntarily cancelled 22 uses of carbofuran (74 FR 11551,
March 18, 2009 (USEPA, 2009j)). Existing stocks of carbofuran can be
applied to food crops until December 31, 2009, and to non-food crops
according to the label until supplies are depleted. These decisions are
expected to reduce exposure to carbofuran and its metabolite (3-
hydroxycarbofuran) in food products and in water, which would affect
the RSC used to derive a possible MCLG. Therefore, EPA believes that it
should factor in the effect of these actions, once completed, before
the Agency determines the potential for an NPDWR revision.
    The occurrence of carbofuran in drinking water is an additional
source of uncertainty in the review process that is compounded by the
recent voluntary cancellations and tolerance revocations. The Six-Year
Review ICR occurrence data are based on the Standardized Monitoring
Framework for synthetic organic compounds, which is designed to
evaluate long-term exposure to contaminants with chronic exposure
health endpoints. As a result, short-term seasonal peaks, which
correspond to carbofuran application as a pesticide, cannot be readily
detected in this dataset. The cancellation will reduce carbofuran
application and the potential for seasonal peaks to occur. Reductions
in overall carbofuran use is expected to reduce the potential
occurrence of carbofuran in drinking water sources.
    c. Review Result. Although there are new health data that support
consideration of whether to revise the MCLG/MCL for carbofuran, the
ongoing regulatory actions could affect the possible MCLG. Therefore,
EPA is placing carbofuran in the information gap category due to the
uncertainty of how the cancellation impacts the MCLG. In addition, EPA
notes that the decision to cancel the reregistration of carbofuran
would reduce the presence of this compound in the environment and the
likelihood of exposure to carbofuran in food and drinking water
sources. Consequently, EPA believes it is not appropriate to consider
any revisions to the NPDWR for carbofuran at this time.
15. Carbon Tetrachloride
    a. Background. EPA published the current NPDWR for carbon
tetrachloride on July 8, 1987 (52 FR 25690 (USEPA, 1987)). The NPDWR
established an MCLG of zero based on a cancer classification of B2,
probable human carcinogen. The NPDWR also established an MCL of 0.005
mg/L, based on analytical feasibility.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to carbon tetrachloride. The
revised health effects assessment will consider relevant studies on the
toxicity of carbon tetrachloride, including its potential developmental
and reproductive toxicity. The new health effects assessment was not
completed by March 1, 2009, the review cutoff date for this notice
(USEPA, 2009b). The IRIS Substance Assessment Tracking System Web site
(http://cfpub.epa.gov/iristrac/index.cfm) has the most up-to-date
information on the status of the health effects assessment.
    Although a risk assessment is in process for carbon tetrachloride,
the existing MCLG is zero and the current MCL of 0.005 mg/L is based on
the PQL. Therefore, EPA reviewed whether there is potential to revise
the PQL. EPA reviewed PE data from the first Six-Year Review cycle and
then analyzed more recent PT data to determine if the PQL can be
revised (i.e., analytical feasibility). Passing rates for PE data
available through late 1999 for carbon tetrachloride are at or above 95
percent around the current PQL of 0.005 mg/L, including one study with
a true value below the current PQL. More recent PT data from late 1999
through 2004, supplied by a PT provider, also show greater than 90
percent passing rates for studies around the PQL, except for one study
with a passing rate of 85 percent. Nine PT studies had true values
below the current PQL. Because most of the laboratory passing rates
from PE and PT studies exceeded the 75 percent criterion typically used
to derive a PQL, a lowering of the PQL for carbon tetrachloride might
be possible. These

results, however, are insufficient to recalculate a revised PQL for
carbon tetrachloride because not enough data points are available below
the current PQL to derive a value at the 75 percent passing rate
(USEPA, 2009c).
    EPA evaluated two alternative sources of information to determine
whether an EQL below the current PQL could be estimated: laboratory
MRLs in the Six-Year Review ICR dataset, and the MDLs for approved
methods for the detection of carbon tetrachloride (Methods 502.2,
524.2, and 551.1). While EPA prefers to use laboratory performance data
to calculate the PQL, the MRL and MDL information can be valuable for
this review to indicate whether it is possible to quantitate at levels
below the current PQL. The Six-Year Review ICR dataset contains MRL
values for 139,221 samples. More than 80 percent of these values are
less than or equal the modal MRL: 119,849 (86 percent) equal the modal
MRL of 0.0005 mg/L and an additional 16,195 (12 percent) are lower than
0.0005 mg/L. Therefore, EPA selected the modal MRL as the EQL (USEPA,
2009e). The MDLs of approved methods range from 0.000002 to 0.00021 mg/
L. Applying a multiplier of 10 would give a possible PQL range from
0.00002 to 0.0021 mg/L, which contains the EQL (USEPA, 2009e).
    Based on these varied and unrelated approaches/sources of
information, EPA believes that there is potential to lower the PQL for
carbon tetrachloride. To determine whether any MCL revision is likely
to provide a meaningful opportunity to improve public health
protection, EPA evaluated the occurrence of carbon tetrachloride at the
EQL of 0.0005 mg/L and additional thresholds of 0.001 and 0.0025 mg/L
(USEPA, 2009f). Table VI-5 shows the results of the occurrence and
exposure analysis for the current MCL and these thresholds. The
occurrence and exposure analysis shows that average concentrations
exceed the current MCL for five of 50,446 systems (0.010 percent),
serving fewer than 2,000 people (or 0.001 percent of 227 million
people). Note that these results are based on the subset of monitoring
data provided in response to the Six-Year Review ICR and do not
necessarily reflect MCL violations, which are based on annual average
concentrations at entry points; SDWIS/FED indicates 19 MCL violations
for carbon tetrachloride between 1998 and 2005 with annual violations
ranging from 1 to 4 (USEPA, 2007g). Average concentrations for 84 to
118 of 50,446 systems (0.167 to 0.234 percent), serving 368,000 to
750,000 people (or 0.162 to 0.330 percent of 227 million people),
exceed the EQL of 0.0005 mg/L.

Table VI-5--Number and Percent of Systems With Mean Concentrations Exceeding Carbon Tetrachloride Thresholds and
                                  Corresponding Estimates of Population Served
----------------------------------------------------------------------------------------------------------------
                                      Systems with mean Concentrations that are greater than the regulatory or
                                    feasibility-based threshold (Percentages based on 50,446 systems with carbon
 Regulatory or feasibility-based         tetrachloride data in the Six-Year Review ICR occurrence dataset)
            threshold             ------------------------------------------------------------------------------
                                                              Nondetect values = \1/2\
                                   Nondetect values = MRL\1\           MRL \2\          Nondetect values = 0 \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.005 mg/L).................  5 (0.010%)...............  5 (0.010%)..............  5 (0.010%)
\1/2\ MCL (0.0025 mg/L)..........  13 (0.026%)..............  12 (0.024%).............  12 (0.024%)
2xEQL (0.001 mg/L)...............  59 (0.117%)..............  50 (0.099%).............  40 (0.079%)
EQL (0.0005 mg/L)................  not applicable...........  118 (0.234%)............  84 (0.167%)
----------------------------------------------------------------------------------------------------------------
                                      Corresponding population served (Percentages based on 226,935,000 people
                                     served by the systems with carbon tetrachloride data in the Six-Year Review
                                                               ICR occurrence dataset)
                                  ------------------------------------------------------------------------------
 Regulatory or feasibility-based     Nondetect values = MRL   Nondetect values = \1/2\  Nondetect values = 0 \3\
            threshold                          \1\                     MRL \2\
----------------------------------------------------------------------------------------------------------------
MCL (0.005 mg/L).................  1,800 (0.001%)...........  1,700 (0.001%)..........  1,700 (0.001%)
\1/2\ MCL (0.0025 mg/L)..........  5,800 (0.003%)...........  5,500 (0.002%)..........  5,500 (0.002%)
2xEQL (0.001 mg/L)...............  265,000 (0.117%).........  212,000 (0.093%)........  190,000 (0.084%)
EQL (0.0005 mg/L)................  not applicable...........  750,000 (0.330%)........  368,000(0.162%)
----------------------------------------------------------------------------------------------------------------
\1\ Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset.
  Results are not reported at the EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the
  results equal to this value results in an upwardly biased estimate of the number of systems with mean
  concentrations that exceed this value.
\2\ Results are based on setting all nondetect results equal to \1/2\ MRL values in the Six-Year Review ICR
  dataset.
\3\ Results are based on setting all nondetect results equal to zero.
Source: USEPA, 2009f.

    Since the occurrence analysis indicates that any revision to the
MCL is unlikely to provide a meaningful opportunity to improve the
level of public health protection, it was not necessary to perform any
additional reviews on treatment feasibility or economic considerations.
    c. Review Result. The Agency does not believe a revision to the
NPDWR for carbon tetrachloride is appropriate at this time because a
reassessment of the health risks resulting from exposure to carbon
tetrachloride is in progress (USEPA, 2009b). Furthermore, the
occurrence and exposure analysis based on possible changes in
analytical feasibility indicates that any revision to the MCL is
unlikely to provide a meaningful opportunity to improve public health
protection. After consideration of the low occurrence of this
contaminant, EPA has decided that any revision to the NPDWR would be a
low priority activity for the Agency, and, thus, is not appropriate to
revise at this time because of:
     Competing workload priorities;
     The administrative costs associated with rulemaking; and
     The burden on States and the regulated community to
implement any regulatory change that resulted.
16. Chlordane
    a. Background. EPA published the current NPDWR for chlordane on
January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The NPDWR established an
MCLG of zero based on a cancer classification of B2, probable human
carcinogen. The NPDWR also established an MCL of 0.002 mg/L, based on
analytical feasibility.

    b. Technical Reviews. As part of the Six-Year Review process, EPA
conducted a literature search for relevant data on the carcinogenicity
of chlordane as well as its potential developmental and reproductive
toxicity. EPA has not identified any new information that indicates
that it is appropriate to consider revisions to the cancer
classification for chlordane at this time (USEPA, 2009b). Because the
MCLG remains at zero, the Agency believes that a further review of the
health effects of chlordane is not warranted at this time.
    The current MCL for chlordane is based on a PQL of 0.002 mg/L. For
the Six-Year Review, the Agency considered whether changes in the
analytical feasibility of chlordane might lead to a lower MCL. EPA
reviewed PE data from the first Six-Year Review cycle and then analyzed
more recent PT data to determine if the PQL can be revised (i.e.,
analytical feasibility). Passing rates for PE data available through
late 1999 for chlordane are above 80 percent around the current PQL of
0.002 mg/L, including three studies with true values below the current
PQL. More recent PT data from late 1999 through 2004, supplied by a PT
provider, also show greater than 80 percent passing rates, except for
two studies with passing rates equal to or below 75 percent. There are
no PT studies with true values below the PQL. Because most of the
laboratory passing rates from PE and PT studies--including three below
the PQL--exceeded the 75 percent criterion typically used to derive a
PQL, a lowering of the PQL for chlordane might be possible. These
results, however, are insufficient to recalculate a revised PQL for
chlordane because not enough data points are available below the
current PQL to derive a value at the 75 percent passing rate (USEPA,
2009c).
    EPA evaluated two alternative sources of information to determine
whether an EQL below the current PQL could be estimated: laboratory
MRLs in the Six-Year Review ICR dataset, and the MDLs for approved
methods for the detection of chlordane (Methods 505 and 508). While EPA
prefers to use laboratory performance data to calculate the PQL, the
MRL and MDL information can be valuable for this review to indicate
whether it is possible to quantitate at levels below the current PQL.
The Six-Year Review ICR dataset contains MRL values for 57,506 samples.
Fewer than 80 percent of these values are less than or equal the modal
MRL: 26,893 (47 percent) equal the modal MRL of 0.0002 mg/L and an
additional 9,764 (17 percent) are lower than 0.0002 mg/L. Therefore,
EPA did not set the EQL equal to the modal MRL (USEPA, 2009e). The MDLs
of approved methods are 0.0000041 and 0.00014 mg/L. Applying a
multiplier of 10 would give possible PQLs of 0.000041 and 0.0014 mg/L.
EPA took the mean of the two values and, rounded up to 0.001 mg/L for
the EQL (USEPA, 2009e).
    Based on these varied and unrelated approaches/sources of
information, EPA believes that there is potential to lower the PQL for
chlordane. To determine whether any MCL revision is likely to provide a
meaningful opportunity to improve public health protection, EPA
evaluated the occurrence of chlordane at the EQL of 0.001 mg/L (USEPA,
2009f). Table VI-6 shows the results of the occurrence and exposure
analysis for the current MCL and an EQL. The occurrence and exposure
analysis shows that average concentrations exceed the current MCL for
one of 31,841 systems (0.003 percent) serving 80 people (or 0.00004
percent of 182 million people). Note that these results are based on
the subset of monitoring data provided in response to the Six-Year
Review ICR do not necessarily reflect MCL violations, which are based
on annual average concentrations at entry points; SDWIS/FED indicates
no MCL violations for chlordane between 1998 and 2005 (USEPA, 2007g).
Average concentrations at one to two of 31,841 systems (0.003 to 0.006
percent), still serving approximately 80 to 120 people (or 0.00004 to
0.00007 percent of 182 million people), exceed the EQL of 0.001 mg/L.

      Table VI-6--Number and Percent of Systems With Mean Concentrations Exceeding Chlordane Thresholds and
                                  Corresponding Estimates of Population Served
----------------------------------------------------------------------------------------------------------------
                                        Systems with mean concentrations that are greater than the regulatory or
                                         feasibility-based threshold (Percentages based on 31,841 systems with
   Regulatory or feasibility-based           chlordane data in the Six-Year Review ICR occurrence dataset)
              threshold               --------------------------------------------------------------------------
                                       Nondetect Values =  MRL  Nondetect Values =  \1/   Nondetect Values =  0
                                                 \1\                   2\ MRL \2\                  \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.002 mg/L).....................  1 (0.003%).............  1 (0.003%).............  1 (0.003%)
EQL (0.001 mg/L).....................  2 (0.006%).............  2 (0.006%).............  1 (0.003%)
----------------------------------------------------------------------------------------------------------------
                                        Corresponding population served (Percentages based on 182,037,000 people
                                          served by the systems with chlordane data in the Six-Year Review ICR
                                                                   occurrence dataset)
                                      --------------------------------------------------------------------------
   Regulatory or feasibility-based        Nondetect Values =       Nondetect Values =       Nondetect Values =
              threshold                MRL \1\................  \1/2\ MRL \2\..........  0 \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.002 mg/L).....................  80 (0.00004%)..........  80 (0.00004%)..........  80 (0.00004%)
EQL (0.001 mg/L).....................  120 (0.00007%).........  120 (0.00007%).........  80 (0.00004%)
----------------------------------------------------------------------------------------------------------------
\1\ Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset.
\2\ Results are based on setting all nondetect results equal to 1/2 MRL values in the Six-Year Review ICR
  dataset.
\3\ Results are based on setting all nondetect results equal to zero.
Source: USEPA, 2009f.

    Since the occurrence analysis indicates that any revision to the
MCL is unlikely to provide a meaningful opportunity to improve the
level of public health protection, it was not necessary to perform any
additional reviews on treatment feasibility or economic considerations.
    c. Review Result. Although there are new data that support
consideration of a possibly lower PQL (and therefore a possibly lower
MCL), EPA does not believe a revision to the NPDWR for chlordane is
appropriate at this time. The occurrence and exposure analysis based on
possible changes in analytical feasibility indicates that any revision
to the MCL is unlikely to provide a

meaningful opportunity to improve public health protection. Taking into
consideration the low occurrence of this contaminant, EPA has decided
that any revision to the NPDWR would be a low priority activity for the
Agency, and, thus, is not appropriate to revise at this time because
of:
     Competing workload priorities;
     The administrative costs associated with rulemaking; and
     The burden on States and the regulated community to
implement any regulatory change that resulted.
17. Chromium
    a. Background. EPA published the current NPDWR for total chromium
on January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The NPDWR established
an MCLG and an MCL of 0.1 mg/L. Although the NPDWR regulates total
chromium, the adverse health effects associated with hexavalent
chromium (Cr VI) are the basis of the current MCLG because that is the
more toxic species (56 FR 3526, January 31, 1991 (USEPA, 1991a)). EPA
based the MCLG on an RfD of 0.005 mg/kg-day and an assumed RSC from
water of 70 percent for total chromium. EPA regulated chromium as a
Group D carcinogen, not classifiable as to human carcinogenicity by the
oral route of exposure.
    b. Technical Reviews. The health effects technical review
identified some information regarding the carcinogenicity of chromium
that may indicate the need to update the Agency's health effects
assessment (USEPA, 2009b). In 1998, the Agency (USEPA, 1998d) updated
the IRIS assessment for Cr VI, which revised the RfD from 0.0048 mg/kg-
day (rounded to 0.005) to 0.003 mg/kg-day. While both RfDs are based on
the same one-year drinking water rat study (MacKenzie et al., 1958),
the change in the RfD in 1998 was due to the following factors: (a) A
slight change in the no-observed-adverse-effect level (NOAEL), (b) a
modification to the original uncertainty factor, and (c) the addition
of a modifying factor of three because of data on the potential for
gastrointestinal effects in humans as a result of oral exposure. There
is no current RfD for soluble trivalent chromium (soluble Cr III); the
Cr III RfD of 1.5 mg/kg-day on IRIS (USEPA, 1998e) is for insoluble Cr
III salts.
    In 2002 and as part of the first Six Year Review (67 FR 19030
(USEPA, 2002c)), EPA noted that the National Toxicology Program (NTP)
had agreed to study the chronic toxicity and carcinogenicity of oral
exposure to Cr VI. The NTP study, conducted with sodium dichromate
dehydrate (i.e., Cr VI) in rats and mice, is now available (NTP, 2008),
as is a pre-peer review draft of a similar study with chromium
picolinate (Cr III) (NTP, 2007). The Cr VI study found clear evidence
of carcinogenic activity of sodium dichromate dihydrate in male and
female F344 rats based on increased incidences of squamous cell
neoplasms of the oral cavity, specifically the squamous epithelium that
lines the oral mucosa and tongue (NTP, 2008). NTP also concluded that
there was clear evidence of carcinogenic activity of sodium dichromate
dihydrate in male and female B6C3F1 mice based on increased incidences
of neoplasms in the small intestine (adenomas and/or carcinomas of the
duodenum, jejunum, or ileum). The observed noncancer effects in the Cr
VI study included histiocytic cellular infiltration in the liver, small
intestine, and pancreatic and mesenteric lymph nodes of rats and mice,
and diffuse epithelial hyperplasia in the small intestine of male and
female mice. A peer-reviewed report for the study of chromium
picolinate (Cr III) is not yet available. Zhang and Li (1987) evaluated
the effects of human exposure to Cr VI in drinking water in Chinese
villages. In a recent analysis of the human data originally reported in
these Chinese villages, Sedman et al. (2006) further support a
statistically significant increase in stomach cancer in the population
exposed to Cr VI in their drinking water, thus suggesting a potential
for carcinogenicity of Cr VI in drinking water.
    An assessment for chromium VI currently exists on IRIS but does not
include an evaluation of carcinogenicity via oral ingestion. As a
result, on December 21, 2007 (72 FR 72715 (USEPA, 2007c)), the Agency
nominated and included Cr VI on its 2008 IRIS agenda. The Agency is
currently working with California EPA, New Jersey Department of
Environmental Protection, and the Centers for Disease Control ATSDR
(since they have recently developed draft assessments for chromium VI)
and has posted a schedule for completion and the most up-to-date
information on the status of the health effects assessment on the IRIS
Substance Assessment Tracking System Web site (http://cfpub.epa.gov/
iristrac/index.cfm).
    A review of analytical or treatment feasibility is not necessary
for total chromium because changes to the MCLG are not warranted at
this time and the current MCL is set at the MCLG. Since EPA did not
identify a health or technology basis for revising the total chromium
NPDWR, the Agency did not conduct a detailed occurrence and exposure
analysis.
    c. Review Result. The Agency does not believe a revision to the
NPDWR for total chromium is appropriate at this time. A reassessment of
the health risks associated with chromium exposure is being initiated
and the Agency does not believe it is appropriate to revise the NPDWR
while that effort is in process.
18. Cyanide
    a. Background. EPA published the current NPDWR for cyanide on July
17, 1992 (57 FR 31776 (USEPA, 1992)). The NPDWR established an MCLG and
an MCL of 0.2 mg/L. EPA based the MCLG on a reference dose of 0.02 mg/
kg-day and a cancer classification of D, not classifiable as to human
carcinogenicity. During the first Six-Year Review cycle, EPA
recommended a revision to the BATs for cyanide to clarify that
``chlorine'' should be ``alkaline chlorine'' to avoid potential for the
formation of harmful cyanogen chloride. EPA promulgated that revision
in 69 FR 38850, June 29, 2004 (USEPA, 2004b).
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to cyanide. The revised health
effects assessment will consider relevant studies on the toxicity of
cyanide, including its potential developmental and reproductive
toxicity. The new health effects assessment was not completed by March
1, 2009, the review cutoff date for this notice (USEPA, 2009b). The
IRIS Substance Assessment Tracking System Web site (http://
cfpub.epa.gov/iristrac/index.cfm) has the most up-to-date information
on the status of the health effects assessment.\16\
---------------------------------------------------------------------------

    \16\ Note that cyanide is listed as hydrogen cyanide in the IRIS
tracking system.
---------------------------------------------------------------------------

    c. Review Result. Since the MCL for cyanide is set at its MCLG and
a reassessment of the health risks resulting from exposure to cyanide
is in progress, the Agency does not believe a revision to the NPDWR is
appropriate at this time.
19. 2,4-D (2,4-Dichlorophenoxyacetic acid)
    a. Background. EPA published the current NPDWR for 2,4-D on January
30, 1991 (56 FR 3526 (USEPA, 1991c)). The NPDWR established an MCLG and
an MCL of 0.07 mg/L. EPA based the MCLG on a reference dose of 0.01 mg/
kg-day and a cancer classification of D, not classifiable as to human
carcinogenicity.
    b. Technical Reviews. In 2005, the Agency updated its health
effects assessment of 2,4-D (USEPA, 2005c). The Agency identified a
change in this

assessment that could lead to a change in the MCLG. This assessment
considered relevant studies on the toxicity of 2,4-D including
developmental and reproductive toxicity. The assessment revised the RfD
from 0.01 mg/kg-day to 0.005 mg/kg-day and concluded that 2,4-D is not
classifiable as to its carcinogenicity (USEPA, 2005c). Based on the new
Office of Pesticide Programs (OPP) assessment and RfD of 0.005 mg/kg-
day, and assuming a 70-kg adult body weight and 2 liters water intake
per day, the DWEL could be 0.2 mg/L. An RSC of 20 percent results in a
possible MCLG of 0.04 mg/L (USEPA, 2009b).
    Analytical feasibility does not pose any limitations for the
current MCL and would not be a limiting factor for the possible MCLG
decrease under consideration. EPA evaluated the results of the
occurrence and exposure analyses for 2,4-D to determine whether a
revised MCLG/MCL would be likely to result in a meaningful opportunity
to improve the level of public health protection (USEPA, 2009f). Table
VI-7 shows the results of the occurrence and exposure analysis for the
current MCL and the possible MCLG set equal to 0.04 mg/L based on the
new health effects information. The occurrence and exposure analysis
shows that average concentrations do not exceed the current MCL for any
system in the analysis. Note that these results are based on the subset
of monitoring data provided in response to the Six-Year Review ICR and
do not necessarily reflect MCL violations, which are based on annual
average concentrations at entry points; SDWIS/FED indicates no MCL
violations for 2,4-D between 1998 and 2005 (USEPA, 2007g). The
occurrence and exposure analysis shows that average concentrations do
not exceed the possible MCLG based on new health effects information
(0.04 mg/L).

 Table VI-7--Number and Percent of Systems With Mean Concentrations Exceeding 2,4-D Thresholds and Corresponding
                                         Estimates of Population Served
----------------------------------------------------------------------------------------------------------------
                                      Systems with mean concentrations that are greater than the regulatory or
                                    health-based threshold (Percentages based on 33,187 systems with 2,4-D data
    Regulatory or health-based                     in the Six-Year Review ICR occurrence dataset)
            threshold             ------------------------------------------------------------------------------
                                    Nondetect values =  MRL    Nondetect values =  \1/    Nondetect values =  0
                                              \1\                    2\ MRL \2\                    \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.07 mg/L)..................  0 (0.000%)...............  0 (0.000%)..............  0 (0.000%)
Possible MCLG (0.04 mg/L)........  0 (0.000%)...............  0 (0.000%)..............  0 (0.000%)
----------------------------------------------------------------------------------------------------------------
                                      Corresponding population served (Percentages based on 187,451,200 people
                                     served by the systems with 2,4-D data in the Six-Year Review ICR occurrence
                                                                      dataset)
                                  ------------------------------------------------------------------------------
    Regulatory or health-based         Nondetect values =        Nondetect values =        Nondetect values =
            threshold              MRL \1\..................  \1/2\ MRL \2\...........  0 \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.07 mg/L)..................  0 (0.000%)...............  0 (0.000%)..............  0 (0.000%)
Possible MCLG (0.04 mg/L)........  0 (0.000%)...............  0 (0.000%)..............  0 (0.000%)
----------------------------------------------------------------------------------------------------------------
\1\ Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset.
\2\ Results are based on setting all nondetect results equal to \1/2\ MRL values in the Six-Year Review ICR
  dataset.
\3\ Results are based on setting all nondetect results equal to zero.
Source: USEPA, 2009f.

    Since the occurrence analysis indicates that any revision to the
MCL is unlikely to provide a meaningful opportunity to improve the
level of public health protection, it was not necessary to perform any
additional reviews on treatment feasibility or economic considerations.
    c. Review Result. Although there are new data that support
consideration of whether to revise the MCLG/MCL for 2,4-D, EPA does not
believe a revision to the NPDWR for 2,4-D is appropriate at this time.
In making this decision, the Agency considered whether any possible
revision to the NPDWR for 2,4-D is likely to provide a meaningful
opportunity for health risk reductions. Taking into consideration the
low occurrence of this contaminant, EPA has decided that any revision
to the NPDWR would be a low priority activity for the Agency, and,
thus, is not appropriate to revise at this time because of:
     Competing workload priorities;
     The administrative costs associated with rulemaking; and
     The burden on States and the regulated community to
implement any regulatory change that resulted.
20. Dalapon (2,2-Dichloropropionic Acid)
    a. Background. EPA published the current NPDWR for dalapon on July
17, 1992 (57 FR 31776 (USEPA, 1992)). The NPDWR established an MCLG and
an MCL of 0.2 mg/L. EPA based the MCLG on a reference dose of 0.03 mg/
kg-day and a cancer classification of D, not classifiable as to human
carcinogenicity.
    b. Technical Reviews. As part of the Six-Year Review process, EPA
conducted a literature search for relevant data on the toxicology of
dalapon, including its potential developmental and reproductive
toxicity. The literature search did not identify any studies that
warrant a review of the RfD or the cancer classification (USEPA,
2009b).
    A review of analytical or treatment feasibility is not necessary
for dalapon because changes to the MCLG are not warranted at this time
and the current MCL is set at the MCLG. Since EPA did not identify a
health or technology basis for revising the dalapon NPDWR, the Agency
did not conduct a detailed occurrence and exposure analysis.
    c. Review Result. EPA's review shows that there are no data
supporting a change to the dalapon NPDWR. As a result, a revision to
the NPDWR would not be appropriate at this time.
21. Di(2-ethylhexyl)adipate (DEHA)
    a. Background. EPA published the current NPDWR for DEHA on July 17,
1992 (57 FR 31776 (USEPA, 1992)). The NPDWR established an MCLG and an
MCL of 0.4 mg/L. EPA based the MCLG on a reference dose of 0.6 mg/kg-
day and a cancer classification of C, possible human carcinogen.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to DEHA. The revised health
effects assessment will consider relevant studies on the toxicity of
DEHA, including its potential

developmental and reproductive toxicity. The new health effects
assessment was not completed by March 1, 2009, the review cutoff date
for this notice (USEPA, 2009b). The IRIS Substance Assessment Tracking
System Web site (http://cfpub.epa.gov/iristrac/index.cfm) has the most
up-to-date information on the status of the health effects assessment.
    c. Review Result. Since the MCL for DEHA is set at its MCLG and a
reassessment of the health risks resulting from exposure to DEHA is in
progress, the Agency does not believe a revision to the NPDWR is
appropriate at this time.
22. Di(2-ethylhexyl)phthalate (DEHP)
    a. Background. EPA published the current NPDWR for DEHP on July 17,
1992 (57 FR 31776 (USEPA, 1992)). The NPDWR established an MCLG of zero
based on a cancer classification of B2, probable human carcinogen. The
NPDWR also established an MCL of 0.006 mg/L, based on analytical
feasibility.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to DEHP. The revised health
effects assessment will consider relevant studies on the toxicity of
DEHP, including its potential developmental and reproductive toxicity.
The new health effects assessment was not completed by March 1, 2009,
the review cutoff date for this notice (USEPA, 2009b). The IRIS
Substance Assessment Tracking System Web site (http://cfpub.epa.gov/
iristrac/index.cfm) has the most up-to-date information on the status
of the health effects assessment.
    Although a risk assessment is in process for DEHP, the existing
MCLG is zero and the current MCL of 0.006 mg/L is based on the PQL.
Therefore, EPA reviewed whether there is potential to revise the PQL.
EPA reviewed PE data from the first Six-Year Review cycle and then
analyzed more recent PT data to determine if the PQL can be revised
(i.e., analytical feasibility). Passing rates for PE data available
through late 1999 for DEHP are below 75 percent for several
concentrations around the current PQL, including two studies with true
values below the current PQL. More recent PT data from late 1999
through 2004, supplied by a PT provider, show passing rates below the
75 percent criterion for three studies, and all of the true
concentrations in the PT data were higher than the current PQL. Given
the passing rates around the current PQL, a lowering of the PQL for
DEHP is not appropriate at this time (USEPA, 2009c).
    EPA evaluated two alternative sources of information to determine
whether an EQL below the current PQL could be estimated: laboratory
MRLs in the Six-Year Review ICR dataset, and the MDLs for approved
methods for the detection of DEHP (Methods 525.2 and 506). While EPA
prefers to use laboratory performance data to calculate the PQL, the
MRL and MDL information can be valuable for this review to indicate
whether it is possible to quantitate at levels below the current PQL.
The Six-Year Review ICR dataset contains MRL values for 50,490 samples.
Fewer than 80 percent of these values are less than or equal the modal
MRL: 22,980 (45 percent) equal the modal MRL of 0.001 mg/L and an
additional 15,842 (31 percent) are lower than 0.001 mg/L. Therefore,
EPA did not set the EQL equal to the modal MRL (USEPA, 2009e). The MDLs
of approved methods are 0.0013 and 0.00225 mg/L. Applying a multiplier
of 10 would give a possible PQL range from 0.013 to 0.0225 mg/L. The
range is higher than the current PQL and, therefore, EPA did not
estimate an EQL (USEPA, 2009e). Based on these varied and unrelated
approaches/sources of information, EPA believes that there is no
potential to lower the PQL for DEHP. Since the MCL is constrained by
the PQL, and the PQL is unchanged, EPA does not believe it is necessary
to conduct an occurrence analysis at this time.
    c. Review Result. The Agency does not believe a revision to the
NPDWR for DEHP is appropriate at this time because a reassessment of
the health risks resulting from exposure to DEHP is in progress (USEPA,
2009b). Furthermore, a review of analytical feasibility did not
identify a potential to revise the MCL, which is limited by
feasibility.
23. 1,2-Dibromo-3-chloropropane (DBCP)
    a. Background. EPA published the current NPDWR for DBCP on January
30, 1991 (56 FR 3526 (USEPA, 1991c)). The NPDWR established an MCLG of
zero based on a cancer classification of B2, probable human carcinogen.
The NPDWR also established an MCL of 0.0002 mg/L, based on analytical
feasibility.
    b. Technical Reviews. As part of the Six-Year Review process, EPA
conducted a literature search for relevant data on the carcinogenicity
of DBCP as well as its potential developmental and reproductive
toxicity. EPA has not identified any new information that indicates
that it is appropriate to consider revisions to the cancer
classification for DBCP at this time (USEPA, 2009b). Because the MCLG
remains at zero, the Agency believes that a further review of the
health effects of DBCP is not warranted at this time.
    The current MCL for DBCP is based on a PQL of 0.0002 mg/L. For the
Six-Year Review, the Agency considered whether changes in the
analytical feasibility of DBCP might lead to a lower MCL. EPA reviewed
PE data from the first Six-Year Review cycle and then analyzed more
recent PT data to determine if the PQL can be revised (i.e., analytical
feasibility). Passing rates for PE data available through late 1999 for
DBCP are above 85 percent, including one study with a true value below
the current PQL. More recent PT data from late 1999 through 2004,
supplied by a PT provider, also show greater than 75 percent passing
rates, including three with a true value below the current PQL. Because
all of the laboratory passing rates from PE and PT studies, including
four with true values slightly below the PQL, exceeded the 75 percent
criterion typically used to derive a PQL, a lowering of the PQL for
DBCP might be possible. These results, however, are insufficient to
recalculate a revised PQL for DBCP because not enough data points are
available below the current PQL to derive a value at the 75 percent
passing rate (USEPA, 2009c).
    EPA examined two alternative sources of information to determine
whether an EQL below the current PQL could be estimated: laboratory
MRLs in the Six-Year Review ICR dataset, and the MDLs for approved
methods for the detection of DBCP (Methods 504.1 and 551.1). While EPA
prefers to use laboratory performance data to calculate the PQL, the
MRL and MDL information can be valuable for this review to indicate
whether it is possible to quantitate at levels below the current PQL.
However, there are substantial uncertainties in interpreting the MRLs
(USEPA, 2009e). For example, some States have reported modal MRLs that
are higher than the MCL. EPA therefore considered only MDL data to
verify the potential to revise the PQL, and to establish a threshold
for the occurrence and exposure analysis. The MDLs of approved methods
are 0.000009 and 0.00001 mg/L. Applying a multiplier of 10 would give a
possible PQLs of 0.00009 and 0.0001 mg/L. EPA took the mean and rounded
up to 0.0001 mg/L for the EQL (USEPA, 2009e).
    Based on the PT data and the MDLs for approved methods, EPA
believes that there may be potential to lower the PQL for DBCP. To
determine whether any MCL revision is likely to provide a meaningful
opportunity to improve

public health protection, EPA evaluated the occurrence of DBCP at the
EQL of 0.0001 mg/L (USEPA, 2009f). Table VI-8 shows the results of the
occurrence and exposure analysis for the current MCL and an EQL. The
occurrence and exposure analysis shows that average concentrations
exceed the current MCL for 42 of 37,618 systems (0.112 percent) serving
25,000 people (or 0.013 percent of 194 million people). Note that these
results are based on the subset of monitoring data provided in response
to the Six-Year Review ICR and do not necessarily reflect MCL
violations, which are based on annual average concentrations at entry
points; SDWIS/FED indicates only nine MCL violations for DBCP between
1998 and 2005 (USEPA, 2007g). Average concentrations at 92 to 97 of
37,618 systems (0.245 to 0.258 percent), serving approximately 1.2 to
1.4 million people (0.610 to 0.713 percent of 194 million people),
exceed the EQL of 0.0001 mg/L.

 Table VI-8--Number and Percent of Systems With Mean Concentrations Exceeding DBCP Thresholds and Corresponding
                                         Estimates of Population Served
----------------------------------------------------------------------------------------------------------------
                                      Systems with mean concentrations that are greater than the regulatory or
                                     feasibility-based threshold (Percentages based on 37,618 systems with DBCP
 Regulatory or feasibility-based                data in the Six-Year Review ICR occurrence dataset)
            threshold             ------------------------------------------------------------------------------
                                    Nondetect values =  MRL    Nondetect values =  \1/    Nondetect values =  0
                                              \1\                    2\ MRL \2\                    \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.0002 mg/L)................  42 (0.112%)..............  42 (0.112%).............  42 (0.112%)
EQL (0.0001 mg/L)................  97 (0.258%)..............  93 (0.247%).............  92 (0.245%)
----------------------------------------------------------------------------------------------------------------
                                   Corresponding population served (Percentages based on 193,749,000 people
                                    served by the systems with DBCP data in the Six-Year Review ICR occurrence
                                    dataset)
                                  ------------------------------------------------------------------------------
 Regulatory or feasibility-based       Nondetect values =        Nondetect values =        Nondetect values =
            threshold              MRL \1\..................  \1/2\ MRL \2\...........  0 \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.0002 mg/L)................  25,000 (0.013%)..........  25,000 (0.013%).........  25,000 (0.013%)
EQL (0.0001 mg/L)................  1,382,000 (0.713%).......  1,371,000 (0.707%)......  1,181,000 (0.610%)
----------------------------------------------------------------------------------------------------------------
\1\ Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset.
\2\ Results are based on setting all nondetect results equal to \1/2\ MRL values in the Six-Year Review ICR
  dataset.
\3\ Results are based on setting all nondetect results equal to zero.

Source: USEPA, 2009f.

    Since the occurrence analysis indicates that any revision to the
MCL is unlikely to provide a meaningful opportunity to improve the
level of public health protection, it was not necessary to perform any
additional reviews on treatment feasibility or economic considerations.
    c. Review Result. Although there are new data that support
consideration of a possibly lower PQL (and therefore a possibly lower
MCL), EPA does not believe a revision to the NPDWR for DBCP is
appropriate at this time. The occurrence and exposure analysis based on
possible changes in analytical feasibility indicates that any revision
to the MCL is unlikely to provide a meaningful opportunity to improve
public health protection. Taking into consideration the low occurrence
of this contaminant, EPA has decided that any revision to the NPDWR
would be a low priority activity for the Agency, and, thus, is not
appropriate to revise at this time because of:
     Competing workload priorities;
     The administrative costs associated with rulemaking; and
     The burden on States and the regulated community to
implement any regulatory change that resulted.
24. 1,2-Dichlorobenzene (o-Dichlorobenzene)
    a. Background. EPA published the current NPDWR for 1,2-
dichlorobenzene on January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The
NPDWR established an MCLG and an MCL of 0.6 mg/L. EPA based the MCLG on
a reference dose of 0.09 mg/kg-day and a cancer classification of D,
not classifiable as to human carcinogenicity.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to 1,2-dichlorobenzene. The
revised health effects assessment will consider relevant studies on the
toxicity of 1,2-dichlorobenzene, including its potential developmental
and reproductive toxicity. The new health effects assessment was not
completed by March 1, 2009, the review cutoff date for this notice
(USEPA, 2009b). The IRIS Substance Assessment Tracking System Web site
(http://cfpub.epa.gov/iristrac/index.cfm) has the most up-to-date
information on the status of the health effects assessment.
    c. Review Result. Since the MCL for 1,2-dichlorobenzene is set at
its MCLG and a reassessment of the health risks resulting from exposure
to 1,2-dichlorobenzene is in progress, the Agency does not believe a
revision to the NPDWR is appropriate at this time.
25. 1,4-Dichlorobenzene (p-Dichlorobenzene)
    a. Background. EPA published the current NPDWR for 1,4-
dichlorobenzene on July 8, 1987 (52 FR 25690 (USEPA, 1987)). The NPDWR
established an MCLG and an MCL of 0.075 mg/L. EPA based the MCLG on a
reference dose of 0.1 mg/kg-day and a cancer classification of C,
possible human carcinogen.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to 1,4-dichlorobenzene. The
revised health effects assessment will consider relevant studies on the
toxicity of 1,4-dichlorobenzene, including its potential developmental
and reproductive toxicity. The new health effects assessment was not
completed by March 1, 2009, the review cutoff date for this notice
(USEPA, 2009b). The IRIS Substance Assessment Tracking System Web site
(http://cfpub.epa.gov/iristrac/index.cfm) has the most up-to-date
information on the status of the health effects assessment.
    c. Review Result. Since the MCL for 1,4-dichlorobenzene is set at
its MCLG and a reassessment of the health risks resulting from exposure
to 1,4-

dichlorobenzene is in progress, the Agency does not believe a revision
to the NPDWR is appropriate at this time.
26. 1,2-Dichloroethane (Ethylene Dichloride)
    a. Background. EPA published the current NPDWR for 1,2-
dichloroethane on July 8, 1987 (52 FR 25690 (USEPA, 1987)). The NPDWR
established an MCLG of zero based on a cancer classification of B2,
probable human carcinogen. The NPDWR also established an MCL of 0.005
mg/L, based on analytical feasibility.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to 1,2-dichloroethane. The revised
health effects assessment will consider relevant studies on the
toxicity of 1,2-dichloroethane, including its potential developmental
and reproductive toxicity. The new health effects assessment was not
completed by March 1, 2009, the review cutoff date for this notice
(USEPA, 2009b). The IRIS Substance Assessment Tracking System Web site
(http://cfpub.epa.gov/iristrac/index.cfm) has the most up-to-date
information on the status of the health effects assessment.\17\
---------------------------------------------------------------------------

    \17\ Note that 1,2-dichloroethane is listed as ethylene
dichloride in the IRIS tracking system.
---------------------------------------------------------------------------

    Although a risk assessment is in process for 1,2-dichloroethane,
the existing MCLG is zero and the current MCL of 0.005 mg/L is based on
the PQL. Therefore, EPA reviewed whether there is potential to revise
the PQL EPA reviewed PE data from the first Six-Year Review cycle and
then analyzed more recent PT data to determine if the PQL can be
revised (i.e., analytical feasibility). Passing rates for PE data
available through late 1999 for 1,2-dichloroethane are above 95 percent
around the current PQL of 0.005 mg/L, including one study with a true
value below the current PQL. More recent PT data from late 1999 through
2004, supplied by a PT provider, also show greater than 90 percent
passing rates for studies around the current PQL, including seven with
true values below the current PQL. Because all of the laboratory
passing rates from PE and PT studies--including several with true
concentrations below the PQL--exceeded the 75 percent criterion
typically used to derive a PQL, a lowering of the PQL for 1,2-
dichloroethane might be possible. These results, however, are
insufficient to recalculate a revised PQL for 1,2-dichloroethane
because not enough data points are available below the current PQL to
derive a value at the 75 percent passing rate (USEPA, 2009c).
    EPA evaluated two alternative sources of information to determine
whether an EQL below the PQL could be estimated: laboratory MRLs in the
Six-Year Review ICR dataset, and the MDLs for approved methods for the
detection of 1,2-dichloroethane (Methods 502.2 and 524.2). While EPA
prefers to use laboratory performance data to calculate the PQL, the
MRL and MDL information can be valuable for this review to indicate
whether it is possible to quantitate at levels below the current PQL.
The Six-Year Review ICR dataset contains MRL values for 139,085
samples. More than 80 percent of these values are less than or equal
the modal MRL: 116,533 (84 percent) equal the modal MRL of 0.0005 mg/L
and an additional 18,160 (13 percent) are lower than 0.0005 mg/L.
Therefore, EPA selected the modal MRL as the EQL (USEPA, 2009e). The
MDLs of approved methods range from 0.00003 to 0.00006 mg/L. Applying a
multiplier of 10 would give a possible PQL range from 0.0003 to 0.0006
mg/L, which contains the EQL (USEPA, 2009e).
    Based on these varied and unrelated approaches/sources of
information, EPA believes that there is potential to lower the PQL for
1,2-dichloroethane. To determine whether any MCL revision is likely to
provide a meaningful opportunity to improve public health protection,
EPA evaluated the occurrence of 1,2-dichloroethane at the EQL of 0.0005
mg/L and additional thresholds of 0.001 and 0.0025 mg/L (USEPA, 2009f).
Table VI-9 shows the results of the occurrence and exposure analysis
for the current MCL and these thresholds. The occurrence and exposure
analysis shows that average concentrations exceed the current MCL for
three of 50,442 systems (0.006 percent) serving 150 people (or 0.00007
percent of 227 million people). Note that these results are based on
the subset of monitoring data provided in response to the Six-Year
Review ICR and do not necessarily reflect MCL violations, which are
based on annual average concentrations at entry points; SDWIS/FED
indicates 27 MCL violations for 1,2-dichloroethane between 1998 and
2005 (USEPA, 2007g). Average concentrations at 63 to 82 of 50,442
systems (0.125 to 0.163 percent), serving 210,000 to 277,000 people (or
0.092 to 0.122 percent of 227 million people), exceed the EQL of 0.0005
mg/L.

 Table VI-9--Number and Percent of Systems With Mean Concentrations Exceeding 1,2-Dichloroethane Thresholds and
                                  Corresponding Estimates of Population Served
----------------------------------------------------------------------------------------------------------------
                                      Systems with mean concentrations that are greater than the regulatory or
                                     feasibility-based threshold (Percentages based on 50,442 systems with 1,2-
 Regulatory or feasibility-based         dichloroethane data in the Six-Year Review ICR occurrence dataset)
            threshold             ------------------------------------------------------------------------------
                                     Nondetect values = MRL   Nondetect values = \1/2\
                                              \1\                      MRL \2\          Nondetect values = 0 \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.005 mg/L).................  3 (0.006%)...............  3 (0.006%)..............  3 (0.006%)
\1/2\ MCL (0.0025 mg/L)..........  9 (0.018%)...............  9 (0.018%)..............  8 (0.016%)
2xEQL (0.001 mg/L)...............  46 (0.091%)..............  37 (0.073%).............  30 (0.059%)
EQL (0.0005 mg/L)................  not applicable...........  82 (0.163%).............  63 (0.125%)
----------------------------------------------------------------------------------------------------------------
                                      Corresponding population served (percentages based on 226,934,000 people
                                      served by the systems with 1,2-dichloroethane data in the Six-Year Review
                                                               ICR occurrence dataset)
                                  ------------------------------------------------------------------------------
 Regulatory or feasibility-based     Nondetect values = MRL   Nondetect values = \1/2\  Nondetect values = 0 \3\
            threshold                          \1\                     MRL \2\
----------------------------------------------------------------------------------------------------------------
MCL (0.005 mg/L).................  150 (0.00007%)...........  150 (0.00007%)..........  150 (0.00007%)
\1/2\ MCL (0.0025 mg/L)..........  870 (0.0004%)............  870 (0.0004%)...........  830 (0.0004%)
2xEQL (0.001 mg/L)...............  190,000 (0.084%).........  145,200 (0.064%)........  87,150 (0.038%)

EQL (0.0005 mg/L)................  not applicable...........  277,000 (0.122%)........  210,000 (0.092%)
----------------------------------------------------------------------------------------------------------------
\1\ Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset.
  Results are not reported at the EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the
  results equal to this value results in an upwardly biased estimate of the number of systems with mean
  concentrations that exceed this value.
\2\ Results are based on setting all nondetect results equal to \1/2\ MRL values in the Six-Year Review ICR
  dataset.
\3\ Results are based on setting all nondetect results equal to zero.
Source: USEPA, 2009f.

    Since the occurrence analysis indicates that any revision to the
MCL is unlikely to provide a meaningful opportunity to improve the
level of public health protection, it was not necessary to perform any
additional reviews on treatment feasibility or economic considerations.
    c. Review Result. The Agency does not believe a revision to the
NPDWR for 1,2-dichloroethane is appropriate at this time because a
reassessment of the health risks resulting from exposure to 1,2-
dichloroethane is in progress (USEPA, 2009b). Furthermore, the
occurrence and exposure analysis based on possible changes in
analytical feasibility indicates that any revision to the MCL is
unlikely to provide a meaningful opportunity to improve public health
protection. After consideration of the low occurrence of this
contaminant, EPA has decided that any revision to the NPDWR would be a
low priority activity for the Agency, and, thus, is not appropriate to
revise at this time because of:
     Competing workload priorities;
     The administrative costs associated with rulemaking; and
     The burden on States and the regulated community to
implement any regulatory change that resulted.
27. 1,1-Dichloroethylene
    a. Background. EPA published the current NPDWR for 1,1-
dichloroethylene on July 8, 1987 (52 FR 25690 (USEPA, 1987)). The NPDWR
established an MCLG and an MCL of 0.007 mg/L. EPA based the MCLG on a
reference dose of 0.01 mg/kg-day and a cancer classification of C,
possible human carcinogen.
    b. Technical Reviews. In the first Six-Year Review cycle, EPA
evaluated new information from a health effects assessment completed in
2002 (USEPA, 2002b). At that time, the Agency could not determine that
a revision to the NPDWR would provide a meaningful opportunity for cost
savings to public water systems or their customers, and decided that
any revision would be a low priority activity for the Agency because of
competing workload priorities, the administrative costs associated with
rulemaking, and the burden on States and the regulated community to
implement any regulatory change (68 FR 42908 (USEPA, 2003e)). The 2002
assessment considered relevant studies on the toxicity of 1,1-
dichloroethylene including developmental and reproductive toxicity. The
assessment revised the RfD from 0.01 mg/kg-day to 0.05 mg/kg-day and
concluded that there is inadequate information to assess carcinogenic
potential via the oral route (USEPA, 2002b). In the current review
cycle, EPA conducted a literature search through June 2007 for relevant
data on the toxicology of 1,1-dichloroethylene, including its potential
developmental and reproductive toxicity. The literature search did not
identify any additional new data that would affect the RfD or cancer
classification (USEPA, 2009b). Based on the 2002 IRIS assessment and
RfD of 0.05 mg/kg-day, and assuming a 70-kg adult body weight and 2
liters water intake per day, the DWEL could be 1.75 mg/L. The 2002
cancer assessment indicates that the risk management factor of 10,
applied to the current MCLG, may no longer be needed. An RSC of 20
percent results in a possible MCLG of 0.35 mg/L (USEPA, 2009b).
    Analytical feasibility does not pose any limitations for the
current MCL and would not be a limiting factor if EPA were to raise the
MCLG. EPA evaluated the results of the occurrence and exposure analyses
for 1,1-dichloroethylene to determine whether a revised MCLG/MCL would
be likely to result in a meaningful opportunity to achieve cost savings
for PWSs and their customers while maintaining, or improving, the level
of public health protection (USEPA, 2009f). Although the Agency
obtained and evaluated the finished water occurrence data for 1,1-
dichloroethylene, its usefulness is limited for potential cost savings
to PWSs and their customers because the Agency does not know which
systems are treating for this contaminant. As an alternative, the
Agency evaluated available data on source water quality and conducted a
qualitative assessment of treatment cost savings.
    Table VI-10 provides summary data for contaminant occurrence based
on maximum sample values for the locations included in the STORET and
NAWQA data. Although the degree to which these occurrence rates
represent national drinking water source occurrence is uncertain, the
information shows no to low occurrence at threshold levels of interest.
This information indicates that any resulting NPDWR change would affect
systems that rely on source water at less than 0.02 percent of the
NAWQA locations. The STORET results are driven by the 157 sampling
locations in Phoenix, Arizona, that have a maximum sample above the MCL
of 0.007 mg/L. Five of these locations also account for those having a
maximum sample that exceeds 0.35 mg/L.

            Table VI-10--Ambient Water Quality Monitoring Occurrence Summary for 1,1-Dichloroethylene
----------------------------------------------------------------------------------------------------------------
                                                            Number of locations (% of locations)
          Maximum concentration           ----------------------------------------------------------------------
                                                       STORET \1\                          NAWQA \2\
----------------------------------------------------------------------------------------------------------------
Total....................................  2,448 (100.0%)....................  5,788 (100.0%)
Nondetect................................  1,498 (61.2%).....................  5,636 (97.37%)
Detected.................................  950 (38.8%).......................  152 (2.63%)
Exceeds current MCLG of 0.007 mg/L.......  165 (6.7%)........................  1 (0.02%)
Exceeds alternative value of 0.35 mg/L...  5 (0.2%)..........................  0 (0.0%)
----------------------------------------------------------------------------------------------------------------
\1\ STORET database 2002-2007.
\2\ NAWQA database 1992-2008.
Source: USEPA, 2009d.

    The BATs and small system compliance technologies for 1,1-
dichloroethylene have other beneficial effects, e.g., reduction of
other co-occurring contaminants, precursors for DBPs, or other common
impurities. Therefore, if EPA were to consider a higher level, the
Agency does not know how many PWSs that are currently treating to
comply with the existing MCL of 0.007 mg/L would be likely to
discontinue treatment that is already in place (USEPA, 2009d). Also,
the Agency does not know to what extent affected systems might be able
to reduce costs given that capital costs are not recoverable. However,
the Agency recognizes that there may be opportunities to achieve
operational cost savings if these systems are able to re-optimize
current treatment.
    Given these considerations, the Agency believes that any resulting
revision is not likely to provide a meaningful opportunity for cost
savings. In view of this, any revision would be a low priority activity
and not appropriate at this time.
    c. Review Result. Although there are new data that support
consideration of whether to revise the MCLG/MCL for 1,1-
dichloroethylene, EPA does not believe a revision to the NPDWR for 1,1-
dichloroethylene is appropriate at this time. In making this decision,
the Agency considered whether any possible revision to the NPDWR for
1,1-dichloroethylene is likely to provide a meaningful opportunity for
cost savings to public water systems and their customers. Taking into
consideration the low occurrence of this contaminant in source waters,
EPA has decided that any revision to the NPDWR would be a low priority
activity for the Agency, and, thus, is not appropriate to revise at
this time because of:
     Competing workload priorities;
     The administrative costs associated with rulemaking; and
     The burden on States and the regulated community to
implement any regulatory change that resulted.
28. cis-1,2-Dichloroethylene
    a. Background. EPA published the current NPDWR for cis-1,2-
dichloroethylene on January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The
NPDWR established an MCLG and an MCL of 0.07 mg/L. EPA based the MCLG
on a reference dose of 0.01 mg/kg-day and a cancer classification of D,
not classifiable as to human carcinogenicity.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to cis-1,2-dichloroethylene. The
revised health effects assessment will consider relevant studies on the
toxicity of cis-1,2-dichloroethylene, including its potential
developmental and reproductive toxicity. The new health effects
assessment was not completed by March 1, 2009, the review cutoff date
for this notice (USEPA, 2009b). The IRIS Substance Assessment Tracking
System Web site (http://cfpub.epa.gov/iristrac/index.cfm) has the most
up-to-date information on the status of the health effects assessment.
    c. Review Result. Since the MCL for cis-1,2-dichloroethylene is set
at its MCLG and a reassessment of the health risks resulting from
exposure to cis-1,2-dichloroethylene is in progress, the Agency does
not believe a revision to the NPDWR is appropriate at this time.
29. trans-1,2-Dichloroethylene
    a. Background. EPA published the current NPDWR for trans-1,2-
dichloroethylene on January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The
NPDWR established an MCLG and an MCL of 0.1 mg/L. EPA based the MCLG on
a reference dose of 0.02 mg/kg-day and a cancer classification of D,
not classifiable as to human carcinogenicity.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to trans-1,2-dichloroethylene. The
revised health effects assessment will consider relevant studies on the
toxicity of trans-1,2-dichloroethylene, including its potential
developmental and reproductive toxicity. The new health effects
assessment was not completed by March 1, 2009, the review cutoff date
for this notice (USEPA, 2009b). The IRIS Substance Assessment Tracking
System Web site (http://cfpub.epa.gov/iristrac/index.cfm) has the most
up-to-date information on the status of the health effects assessment.
    c. Review Result. Since the MCL for trans-1,2-dichloroethylene is
set at its MCLG and a reassessment of the health risks resulting from
exposure to trans-1,2-dichloroethylene is in progress, the Agency does
not believe a revision to the NPDWR is appropriate at this time.
30. Dichloromethane (Methylene Chloride)
    a. Background. EPA published the current NPDWR for dichloromethane
on July 17, 1992 (57 FR 31776 (USEPA, 1992)). The NPDWR established an
MCLG of zero based on a cancer classification of B2, probable human
carcinogen. The NPDWR also established an MCL of 0.005 mg/L, based on
analytical feasibility.
    b. Technical Reviews. EPA has initiated a reassessment of the
health risks resulting from exposure to dichloromethane. The revised
health effects assessment will consider relevant studies on the
toxicity of dichloromethane, including its potential developmental and
reproductive toxicity. The new health effects assessment was not
completed by March 1, 2009, the review cutoff date for this notice
(USEPA, 2009b). The IRIS Substance Assessment Tracking System Web site
(http://cfpub.epa.gov/iristrac/index.cfm) has the most up-to-date
information on the status of the health effects assessment.\18\
---------------------------------------------------------------------------

    \18\ Note that dichloromethane is listed as methylene chloride
in the IRIS tracking system.
---------------------------------------------------------------------------

    Although a risk assessment is in process for dichloromethane, the
existing MCLG is zero and the current MCL of 0.005 mg/L is based on the
PQL. Therefore, EPA reviewed whether there is potential to revise the
PQL. EPA reviewed PE data from the first Six-Year

Review cycle and then analyzed more recent PT data to determine if the
PQL can be revised (i.e., analytical feasibility). Passing rates for PE
data available through late 1999 for dichloromethane are all above 90
percent for studies near the PQL. More recent PT data from late 1999
through 2004, supplied by a PT provider, also show greater than 85
percent passing rates for studies around the PQL, except for one study
with a passing rate of 76 percent. However, all of the true
concentrations in the PE and PT data were higher than the current PQL
of 0.005 mg/L. Given the lack of PE and PT study results below the
current PQL to derive a value at the 75 percent passing rate, PE and PT
data are insufficient to support a PQL reduction (USEPA, 2009c).
    EPA evaluated two alternative sources of information to determine
whether an EQL below the current PQL could be estimated: Laboratory
MRLs in the Six-Year Review ICR dataset, and the MDLs for approved
methods for the detection of dichloromethane (Methods 502.2 and 524.2).
While EPA prefers to use laboratory performance data to calculate the
PQL, the MRL and MDL information can be valuable for this review to
indicate whether it is possible to quantitate at levels below the
current PQL. The Six-Year Review ICR dataset contains MRL values for
138,445 samples. More than 80 percent of these values are less than or
equal the modal MRL: 121,532 (88 percent) equal the modal MRL of 0.0005
mg/L and an additional 11,294 (8 percent) are lower than 0.0005 mg/L.
Therefore, EPA selected the modal MRL as the EQL (USEPA, 2009e). The
MDLs of approved methods range from 0.00002 to 0.00009 mg/L. Applying a
multiplier of 10 would give a possible PQL range from 0.0002 to 0.0009
mg/L, which includes the EQL (USEPA, 2009e).
    Based on these varied and unrelated approaches/sources of
information, there is evidence of a potential to lower the PQL for
dichloromethane even though the PE and PT data are insufficient to
support a PQL reduction. To determine whether any MCL revision is
likely to provide a meaningful opportunity to improve public health
protection, EPA evaluated the occurrence of dichloromethane at the EQL
of 0.0005 mg/L and additional thresholds of 0.001 and 0.0025 mg/L
(USEPA, 2009f). Table VI-11 shows the results of the occurrence and
exposure analysis for the current MCL and these thresholds. The
occurrence and exposure analysis shows that average concentrations
exceed the current MCL for 13 to 17 of 50,169 systems (0.026 to 0.034
percent) serving 11,000 to 12,000 people (or 0.005 percent of 227
million people). Note that these results are based on the subset of
monitoring data provided in response to the Six-Year Review ICR and do
not necessarily reflect MCL violations, which are based on annual
average concentrations at entry points; SDWIS/FED indicates 67 MCL
violations for dichloromethane between 1998 and 2005 with annual
violations ranging from 4 to 14 (USEPA, 2007g). Average concentrations
at 383 to 579 of 50,169 systems (0.763 to 1.154 percent), serving
approximately 1.8 to 3.5 million people (or 0.813 to 1.542 percent of
227 million people), exceed the EQL of 0.0005 mg/L.

  Table VI-11--Number and Percent of Systems With Mean Concentrations Exceeding Dichloromethane Thresholds and
                                  Corresponding Estimates of Population Served
----------------------------------------------------------------------------------------------------------------
                                      Systems with mean concentrations that are greater than the regulatory or
                                       feasibility-based threshold (percentages based on 50,169 systems with
 Regulatory or feasibility-based        dichloromethane data in the Six-Year Review ICR occurrence dataset)
            threshold             ------------------------------------------------------------------------------
                                     Nondetect values = MRL   Nondetect values = \1/2\
                                              \1\                      MRL \2\          Nondetect values = 0 \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.005 mg/L).................  17 (0.034%)..............  16 (0.032%).............  13 (0.026%)
EQL (0.0025 mg/L)................  53 (0.106%)..............  51 (0.102%).............  46 (0.092%)
EQL (0.001 mg/L).................  276 (0.550%).............  208 (0.415%)............  169 (0.337%)
EQL (0.0005 mg/L)................  not applicable...........  579 (1.154%)............  383 (0.763%)
----------------------------------------------------------------------------------------------------------------
                                      Corresponding population served (percentages based on 226,844,000 people
                                     served by the systems with dichloromethane data in the Six-Year Review ICR
                                                                 occurrence dataset)
                                  ------------------------------------------------------------------------------
 Regulatory or feasibility-based     Nondetect values = MRL   Nondetect values = \1/2\    Nondetect values = >0
            threshold                          \1\                     MRL \2\                     \3\
----------------------------------------------------------------------------------------------------------------
MCL (0.005 mg/L).................  12,000 (0.005%)..........  12,000 (0.005%).........  11,000 (0.005%)
EQL (0.0025 mg/L)................  44,000 (0.019%)..........  40,000 (0.018%).........  39,000 (0.017%)
EQL (0.001 mg/L).................  1,517,000 (0.669%).......  1,386,000 (0.611%)......  946,000 (0.417%)
EQL (0.0005 mg/L)................  not applicable...........  3,497,000 (1.542%)......  1,844,000 (0.813%)
----------------------------------------------------------------------------------------------------------------
\1\ Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset.
  Results are not reported at the EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the
  results equal to this value results in an upwardly biased estimate of the number of systems with mean
  concentrations that exceed this value.
\2\ Results are based on setting all nondetect results equal to \1/2\ MRL values in the Six-Year Review ICR
  dataset.
\3\ Results are based on setting all nondetect results equal to zero.
Source: USEPA, 2009f.

    During Six-Year Review 1, a stakeholder questioned the feasibility
of lowering the PQL for dichloromethane below 0.001 mg/L because its
use in EPA analytical methods makes it a common laboratory contaminant
(68 FR 42908 (USEPA, 2003e)). EPA responded that the high passing rates
among PE studies at concentrations close to the current PQL of 0.005
mg/L would not be expected if this were the case and that EPA had no
data to suggest that the occurrence estimates reflected monitoring
sample contamination (68 FR 42908 (USEPA, 2003e)). For Six-Year Review
2, EPA notes that it does not have PE or PT study results at either
0.001 mg/L or 0.0005 mg/L and, therefore, cannot assess the potential
for laboratory contamination of dichloromethane to affect passing rates
at this level. A USGS study of volatile organic compound (VOC)
occurrence (Moran, 2006) indicates this potential exists at low
concentrations. The study presented dichloromethane laboratory
reporting levels for newer low-level

analytical methods (i.e., defined as the level that limits the
frequency of false positives and false negatives to 1 percent of test
results) that ranged from 0.00006 mg/L to 0.00757 mg/L, with a median
value of 0.00038 mg/L. The report noted that the laboratory reporting
levels for dichloromethane tend to be higher than levels for other VOCs
such as PCE (levels ranging from 0.000027 mg/L to 0.0005 mg/L with a
median of 0.0001 mg/L) and TCE (ranging from 0.000038 mg/L to 0.0005
mg/L with a median of 0.000038 mg/L) because it was a frequent
laboratory contaminant.
    A USGS study of ground water, source water, and drinking water
quality indicated consistently lower dichloromethane (methylene
chloride) occurrence frequencies compared to either PCE or TCE, which
are among the most frequently occurring VOCs included in the study
(Moran, 2006). Table VI-12 provides a summary of the occurrence results
reported in the USGS study. This study also determined that population
density was the strongest predictor of dichloromethane occurrence.

       Table VI-12--Summary of USGS VOC Occurrence Study Findings for Dichloromethane (Methylene Chloride)
----------------------------------------------------------------------------------------------------------------
                                         Ground water samples     Source water samples    Drinking water samples
----------------------------------------------------------------------------------------------------------------
Number...............................  5,054..................  577....................  1,680
Type.................................  3,877 NAWQA 1,177 Other  Ground water sources     Ground water community
                                        sources.                 for community water      water systems.
                                                                 systems.
Location.............................  National...............  National...............  New England and Mid-
                                                                                Atlantic States.
Dichloromethane Results..............   3% exceed        0.2% (1          3% exceed
                                        0.00002 mg/L.            sample) exceed 0.0002    0.0002 mg/L.
                                        <1% exceed       mg/L.                    Ranked 11th of
                                        0.0002 mg/L.             Ranked 8th of    51 VOCs in detection
                                        Ranked 30th of   52 VOCs based on         frequency.
                                        55 VOCs based on         median concentration     Ranked 31st of
                                        median concentration     (0.0017 mg/L--1          55 solvents in median
                                        (0.00005 mg/L).          sample).                 concentration (0.001
                                                                                mg/L).
PCE..................................   11% exceed       4% exceed        4% exceed
                                        0.00002 mg/L.            0.0002 mg/L.             0.0002 mg/L.
                                        4% exceed        Ranked 16th of   Ranked 7th of
                                        0.0002 mg/L.             52 VOCs based on         51 VOCs in detection
                                        Ranked 12th of   median concentration     frequency.
                                        55 VOCs based on         (0.0009 mg/L).           Ranked 11th of
                                        median concentration                              55 solvents in median
                                        (0.00007 mg/L).                                   concentration (0.0014
                                                                                mg/L).
TCE..................................   5% exceed        3% exceed        4% exceed
                                        0.00002 mg/L.            0.0002 mg/L.             0.0002 mg/L.
                                        2.5% exceed      Ranked 10th of   Ranked 8th of
                                        0.0002 mg/L.             52 VOCs based on         51 VOCs in detection
                                        Ranked 20th of   median concentration     frequency.
                                        55 VOCs based on         (0.0015 mg/L).           Ranked 8th of
                                        median concentration                              55 solvents in median
                                        (0.00012 mg/L).                                   concentration (0.0015
                                                                                mg/L).
----------------------------------------------------------------------------------------------------------------
Source: Moran, 2006.

    EPA compared Six-Year Review ICR occurrence patterns for
dichloromethane with contaminant release information to determine if
drinking water occurrence corresponds with potential contaminant
sources reported in the Toxics Release Inventory (TRI) and found that
the states with the majority of systems with mean concentrations that
exceed 0.0005 mg/L did not tend to be the States with the highest
dichloromethane releases (Moran, 2006). Table VI-13 provides summary
information from that comparison. In particular, the numbers of system
means exceeding 0.0005 mg/L in Montana and Alaska seem inconsistent
with TRI release information and the USGS study finding that population
density is the strongest predictor of dichloromethane occurrence.
    Because of data gaps regarding the feasibility of PQL reduction and
potential occurrence data accuracy at the lowest EQL, EPA concluded
that revising the MCL may not constitute a meaningful opportunity to
improve the level of public health protection.

                                               Table VI-13--Stage 2 Occurrence Summary for Dichloromethane
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                       Systems with mean > 0.0005 mg/   Total reported TRI on-site or off-site    Total reported TRI on-site or off-site
                                           L Nondetect = \1/2\ MRL     disposal or release of dichloromethane--  disposal or release of dichloromethane--
                                      --------------------------------         all industries, 2006 \1\                  all industries, 2004 \1\

                State                                                 ----------------------------------------------------------------------------------
                                           Number      Percent of 579                            Percent of 6.8                           Percent of 7.9
                                                        total systems           Pounds            Million Total           Pounds           Million Total
                                                                                Pounds                                   Pounds
--------------------------------------------------------------------------------------------------------------------------------------------------------
MT...................................              67              12  22,700..................               0  30,600.................               0
TX...................................              45               8  314,120.................               5  410,103................               5
FL...................................              40               7  31,451..................               0  246,775................               3
AK...................................              37               6  No data.................               0  No data................               0
IN...................................              29               5  509,303.................               7  699,783................               9
WI...................................              28               5  111,403.................               2  98,113.................               1
MO...................................              27               5  51,002..................               1  32,860.................               0
CA...................................              26               4  149,423.................               2  86,554.................               1
OH...................................              24               4  192,237.................               3  203,269................               3
NM...................................              21               4  No data.................               0  No data................               0

IL...................................              19               3  279,024.................               4  285,101................               4
AL...................................              18               3  319,529.................               5  375,650................               5
MN...................................              17               3  39,851..................               1  81,309.................               1
CO...................................              15               3  18,475..................               0  17,003.................               0
MI...................................              13               2  75,141..................               1  129,959................               2
WY...................................              13               2  No data.................               0  No data................               0
IA...................................              12               2  2,348...................               0  1,657..................               0
MD...................................              12               2  36,990..................               1  31,347.................               0
NC...................................              12               2  49,800..................               1  600,032................               8
NY...................................              11               2  322,382.................               5  712,197................               9
--------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Source: TRI Explorer Chemical Report Summary on-line state summaries for 2006 and 2004.

    c. Review Result. The Agency does not believe a revision to the
NPDWR for dichloromethane is appropriate at this time because a
reassessment of the health risks resulting from exposure to
dichloromethane is in progress (USEPA, 2009b). In view of the fact that
dichloromethane is a common laboratory contaminant, there is
uncertainty regarding the extent to which a PQL revision is feasible or
whether the Six-Year Review ICR data are reliable at concentrations
well below the current PQL. Furthermore, the occurrence and exposure
analysis based on possible changes in analytical feasibility indicates
that any revision to the MCL is unlikely to provide a meaningful
opportunity to improve public health protection. After consideration of
these factors, EPA has decided that any revision to the NPDWR would be
a low priority activity for the Agency, and, thus, is not appropriate
to revise at this time because of:
     Competing workload priorities;
     The administrative costs associated with rulemaking; and
     The burden on States and the regulated community to
implement any regulatory change that resulted.
31. 1,2-Dichloropropane
    a. Background. EPA published the current NPDWR for 1,2-
dichloropropane on January 30, 1991 (56 FR 3526 (USEPA, 1991c)). The
NPDWR established an MCLG of zero based on a cancer classification of
B2, probable human carcinogen. The NPDWR also established an MCL of
0.005 mg/L, based on analytical feasibility.
    b. Technical Reviews. As part of the Six-Year Review process, EPA
conducted a literature search for relevant data on the carcinogenicity
of 1,2-dichloropropane as well as its potential developmental and