vvEPA
    United States
    Environmental Protection
    Agency
 Development of Estimated Quantitation Levels
       for the Second Six-Year Review of
  National Primary Drinking Water Regulations

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Office of Water (4607M)
EPA815-B-09-005
October 2009
www.epa.gov/safewater

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009


                               Table of Contents

Executive Summary	ES-1
1    Introduction	1-1
2    Data Sources	2-1
   2.1    MRLData	2-1
   2.2    MDLData	2-2
   2.3    Data Summary	2-2
3    Threshold Development Method	3-1
4    Development of Individual EQLs	4-1
   4.1    Contaminants with Ongoing Health Effects Review and MCL Limited by PQL	4-1
     4.1.1    Benzo[a]pyrene	4-1
     4.1.2    Carbon Tetrachloride	4-2
     4.1.3    Di(2-ethylhexyl)phthalate (DEHP)	4-4
     4.1.4    1,2-Dichloroethane	4-5
     4.1.5    Dichloromethane	4-6
     4.1.6    Pentachlorophenol	4-8
     4.1.7    PolychlorinatedBiphenyls(PCBs)	4-9
     4.1.8    2,3,7,8-TCDD (Dioxin)	4-10
     4.1.9    Tetrachloroethylene	4-12
     4.1.10   Thallium	4-13
     4.1.11   Trichloroethylene	4-14
   4.2    Contaminants with MCL Limited by PQL and No Health-Based Changes	4-16
     4.2.1    Benzene	4-16
     4.2.2    Chlordane	4-17
     4.2.3    l,2-Dibromo-3-Chloropropane(DBCP)	4-18
     4.2.4    1,2-Dichloropropane	4-20
     4.2.5    Ethylene Dibromide (EDB)	4-21
     4.2.6    Heptachlor	4-23
     4.2.7    Heptachlor Epoxide	4-24
     4.2.8    Hexachlorobenzene	4-25
     4.2.9    Toxaphene	4-27
     4.2.10   1,1,2-Trichloroethane	4-28
     4.2.11   Vinyl Chloride	4-30
   4.3    Contaminants with New Health Effects Information and Possible MCLG Decrease4-31
     4.3.1    Endothall	4-31
     4.3.2    Oxamyl	4-33
5    References	5-1

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
             Second Six-Year Review of National Primary Drinking Water Regulations  October 2009
                               Table of Exhibits
Exhibit ES-1. Summary of Threshold Determination	ES-4
Exhibit 1-1. Contaminants Requiring Methods or Occurrence Analyses	1-3
Exhibit 2-1. U.S. EPA Drinking Water MDLs, MRLs and Related Information for Contaminants
in the EQL Report	2-3
Exhibit 3-1. EQL Development Steps	3-1
Exhibit 3-2. Summary of Threshold Determination	3-3
Exhibit 4-1. Summary of MRL Data for Benzo[a]pyrene	4-1
Exhibit 4-2. MRL Distribution for Benzo[a]pyrene	4-2
Exhibit 4-3. Analytical Methods for Benzo[a]pyrene	4-2
Exhibit 4-4. Summary of MRL Data for Carbon Tetrachloride	4-3
Exhibit 4-5. MRL Distribution for Carbon Tetrachloride	4-3
Exhibit 4-6. Analytical Methods for Carbon Tetrachloride	4-3
Exhibit 4-7. Summary of MRL Data for DEHP	4-4
Exhibit 4-8. MRL Distribution for DEHP	4-4
Exhibit 4-9. Analytical Methods for DEHP	4-5
Exhibit 4-10. Summary of MRL Data for 1,2-Dichloroethane	4-5
Exhibit 4-11. MRL Distribution for 1,2-Dichloroethane	4-6
Exhibit 4-12. Analytical Methods for 1,2-Dichloroethane	4-6
Exhibit 4-13. Summary of MRL Data for Dichloromethane	4-7
Exhibit 4-14. MRL Distribution for Dichloromethane	4-7
Exhibit 4-15. Analytical Methods for Dichloromethane	4-7
Exhibit 4-16. Summary of MRL Data for Pentachlorophenol	4-8
Exhibit 4-17. MRL Distribution for Pentachlorophenol	4-8
Exhibit 4-18. Analytical Methods for Pentachlorophenol	4-9
Exhibit 4-19. Summary of MRL Data for PCBs	4-9
Exhibit 4-20. MRL Distribution for PCBs	4-10
Exhibit 4-21. Analytical Methods for PCBs	4-10
Exhibit 4-22. Summary of MRL Data for Dioxin	4-11
Exhibit 4-23. MRL Distribution for Dioxin	4-11
Exhibit 4-24. Analytical Methods for Dioxin	4-11
Exhibit 4-25. Summary of MRL Data for Tetrachloroethylene	4-12
Exhibit 4-26. MRL Distribution for Tetrachloroethylene	4-12
Exhibit 4-27. Analytical Methods for Tetrachloroethylene	4-13
Exhibit 4-28. Summary of MRL Data for Thallium	4-13
Exhibit 4-29. MRL Distribution for Thallium	4-14
Exhibit 4-30. Analytical Methods for Thallium	4-14
Exhibit 4-31. Summary of MRL Data for Trichloroethylene	4-15
Exhibit 4-32. MRL Distribution for Trichloroethylene	4-15
Exhibit 4-33. Analytical Methods for Trichloroethylene	4-15
Exhibit 4-34. Summary of MRL Data for Benzene	4-16
Exhibit 4-35. MRL Distribution for Benzene	4-16
Exhibit 4-36. Analytical Methods for Benzene	4-17

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

Exhibit 4-37. Summary of MRL Data for Chiordane	4-17
Exhibit 4-38. MRL Distribution for Chlordane	4-18
Exhibit 4-39. Analytical Methods for Chlordane	4-18
Exhibit 4-40. Summary of MRL Data for DBCP	4-19
Exhibit 4-41. MRL Distribution for DBCP	4-19
Exhibit 4-42. Analytical Methods for DBCP	4-20
Exhibit 4-43. Summary of MRL Data for 1,2-Dichloropropane	4-20
Exhibit 4-44. MRL Distribution for 1,2-Dichloropropane	4-21
Exhibit 4-45. Analytical Methods for 1,2-Dichloropropane	4-21
Exhibit 4-46. Summary of MRL Data for EDB	4-22
Exhibit 4-47. MRL Distribution for EDB	4-22
Exhibit 4-48. Analytical Methods for EDB	4-22
Exhibit 4-49. Summary of MRL Data for Heptachlor	4-23
Exhibit 4-50. MRL Distribution for Heptachlor	4-23
Exhibit 4-51. Analytical Methods for Heptachlor	4-24
Exhibit 4-52. Summary of MRL Data for Heptachlor Epoxide	4-24
Exhibit 4-53. MRL Distribution for Heptachlor Epoxide	4-25
Exhibit 4-54. Analytical Methods for Heptachlor Epoxide	4-25
Exhibit 4-55. Summary of MRL Data for Hexachlorobenzene	4-26
Exhibit 4-56. MRL Distribution for Hexachlorobenzene	4-26
Exhibit 4-57. Analytical Methods for Hexachlorobenzene	4-27
Exhibit 4-58. Summary of MRL Data for Toxaphene	4-27
Exhibit 4-59. MRL Distribution for Toxaphene	4-28
Exhibit 4-60. Analytical Methods for Toxaphene	4-28
Exhibit 4-61. Summary of MRL Data for 1,1,2-Trichloroethane	4-29
Exhibit 4-62. MRL Distribution for 1,1,2-Trichloroethane	4-29
Exhibit 4-63. Analytical Methods for 1,1,2-Trichloroethane	4-30
Exhibit 4-64. Summary of MRL Data for Vinyl Chloride	4-30
Exhibit 4-65. MRL Distribution for Vinyl Chloride	4-31
Exhibit 4-66. Analytical Methods for Vinyl Chloride	4-31
Exhibit 4-67. Summary of MRL Data for Endothall	4-32
Exhibit 4-68. MRL Distribution for Endothall	4-32
Exhibit 4-69. Analytical Methods for Endothall	4-33
Exhibit 4-70. Summary of MRL Data for Oxamyl	4-33
Exhibit 4-71. MRL Distribution for Oxamyl	4-34
Exhibit 4-72. Analytical Methods for Oxamyl	4-34

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EPA-OGWDW        Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
             Second Six-Year Review of National Primary Drinking Water Regulations  October 2009
                      Abbreviations and Acronyms
2,4-D
BAT
DBCP
DEHP
EDB
EPA
EQL
ICR
MCL
MCLG
MDL
mg/L
Hg/L
MRL
NPDWR
PCBs
PQL
PE
PT
SDWA
SOC
VOC
WS
2,4-dichlorophenoxyacetic acid
best available technology
l,2-dibromo-3-chloropropane
di(2-ethylhexyl)phthalate
ethylene dibromide
U.S. Environmental Protection Agency
estimated quantitation level
Information Collection Request
maximum contaminant level
maximum contaminant level goal
method detection limit
milligrams per liter
micrograms per liter
minimum reporting level
National Primary Drinking Water Regulation
polychlorinated biphenyls
practical quantitation level
performance evaluation
proficiency testing
Safe Drinking Water Act
synthetic organic compound
volatile organic compounds
water supply
                                         IV

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EPA-OGWDW          Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009


Executive Summary

The U.S. Environmental Protection Agency (EPA) has completed its second Six-Year Review
(Six-Year Review 2) of national primary drinking water regulations (NPDWRs). The 1996 Safe
Drinking Water Act (SDWA) Amendments require the U.S. Environmental Protection Agency
(EPA or the Agency) to periodically review existing National Primary Drinking Water
Regulations (NPDWRs). Section 1412(b)(9) of SDWA reads:

        ...[t]he Administrator shall, not less than every 6 years, review and revise, as
       appropriate, each primary drinking water regulation promulgated under this title.
       Any revision of a national primary drinking water regulation shall be promulgated
       in accordance with this section, except that each revision shall maintain, or
       provide for greater, protection of the health of persons.

The primary goal of the Six-Year Review process is to identify NPDWRs for possible regulatory
revision. Although the statute does not define when a  revision is "appropriate," as a general
benchmark, EPA considered a possible revision to be  "appropriate" if, at a minimum, it presents
a meaningful opportunity to:

   improve the level of public health protection, and/or
   achieve cost savings while maintaining or improving the level of public health protection.

For Six-Year Review 2, EPA obtained and evaluated new information that could affect a
NPDWR, including information on health effects (USEPA, 2009e), analytical feasibility
(USEPA, 2009b), treatment feasibility (USEPA, 2009f), and occurrence (USEPA, 2009a and
2009d). EPA identified new health effects or analytical methods information that indicated it
may be possible to revise NPDWRs for several contaminants. Consequently, EPA conducted
occurrence and exposure analyses at threshold concentrations that are below current maximum
contaminant levels (MCLs) to determine if there is a meaningful opportunity to improve the level
of public health protection by reducing MCLs. This document describes the method EPA used to
establish the threshold values that it used for the occurrence analyses.

For most contaminants, EPA established an estimated quantitation level (EQL), which is an
estimate of the possible lower bound for a practical quantitation level (PQL).  Current PQLs are
based on historical analytical capabilities, generally the quantitation capabilities at the time EPA
promulgated the existing NPDWRs. Current MCLs may be limited by historical PQLs. Thus,
improvements in analytical detection could present an opportunity to lower the MCL  closer to
MCLG. For a few contaminants, EPA established new threshold values based on new health
effects information that indicates adverse health effects might occur at concentrations below the
current MCLG, and evaluated whether these levels would also be feasible with respect to
quantitation capabilities.
                                          ES-1

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
             Second Six-Year Review of National Primary Drinking Water Regulations  October 2009
The EQLs do not represent the Agency's intent to promulgate new PQLs at this time. Any
revisions to PQLs will be made as part of future rule making efforts. For Six-Year Review 2,
EPA derived the new thresholds and conducted occurrence and exposure analyses only if an
MCL revision would be feasible: the current MCL is limited by analytical capability (i.e., the
MCL equals a PQL), and there is new information indicating improved analytical capability; or
the current MCL is set equal to the MCLG, and a new health effects assessment indicates it is
possible to revise the MCLG.

As a lower bound estimate of analytical capability for a given contaminant, ideally an EQL
would be based on the same type of data used to drive PQLs. Current PQLs are based on two
approaches: the lowest value for which 75% of laboratories can quantitate within prescribed
accuracy limits based on actual performance data, and an MDL-based method that involves
multiplying an MDL by five or ten to compute a PQL. EPA prefers laboratory performance data
over the MDL multiplier method.

However, the PT data available during Six-Year Review 2 were not sufficient to derive PQLs.
For example, there are no laboratory performance studies for several contaminants at
concentrations below the current PQL. For other contaminants, the range of concentrations is not
sufficient to determine what a lower bound on analytical capability might be (i.e., where at least
75% of laboratories can reliably and consistently quantitate).

Because of insufficient PT data, EPA used two other sources to derive EQLs: MRLs from the
Six-Year 2 ICR dataset, and MDLs associated with approved analytical methods developed by
EPA. The MRL data are the primary data source for most of the EQL estimates because these
data represent laboratory analytical limits nationwide. EPA used MDL data to estimate EQLs
when MRL data were inconclusive, and to confirm the analytical feasibility of the MRL-based
estimates.

EPA's method for developing an EQL has essentially three steps - one for each of the three
information sources (PT data, MRL data, and MDL values). The first step is to review the
conclusion of the PT analysis. If the PT data indicate potential to revise the PQL, then the
objective of the next steps is to identify an EQL (or verify the use of a health-based threshold)
for the occurrence analysis. The second step is to determine whether the modal MRL is a feasible
EQL and, if so, the third step is to determine whether the MDL multiplier approach supports that
EQL value. If the modal MRL is not a feasible EQL, then EPA uses the MDL multiplier
approach to establish an EQL.

If the PT data do not indicate potential to revise the PQL, then the objective of the next steps is
to determine whether the MRL and MDL data concur with this finding. When the MRL and
MDL data confirm the finding, there is no basis for an EQL that is less than the PQL. When
these data contradict the finding, however, EPA used these secondary data sources to derive an
EQL (or verify the use of a health-based threshold) for the occurrence analysis.
                                         ES-2

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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

For most contaminants, the MRL and MDL data supported EPA's conclusion based on PT data.
EPA relied primarily on the MRL data to calculate EQLs. The MRLs provide information on the
analytical capabilities of a large pool of laboratories. The MRL data also provide a threshold for
estimating occurrence. Setting an EQL below a substantial number of MRL values will limit the
monitoring data available for the occurrence analysis.

For all of the volatile organic compounds (VOCs) (except for 1,1,2-trichloroethane, for which
the threshold is based on health effects), more than 90% of MRL values are equal to or less than
the feasible EQL, with a clear majority  equal to the modal MRL. Proportions at the modal MRL
range from a low of 75.6% (vinyl chloride) to a high of 87.8% (dichloromethane). Proportions
that equal or are less than the modal MRL exceed 94%. The modal MRL is 0.5 ug/L in each
case.

EQLs for the synthetic organic compounds (SOCs) hexachlorobenzene and toxaphene are based
on modal MRLs. EQLs for DBCP, chlordane, heptachlor, and heptachlor epoxide are based on
the MDL multiplier method. This method involves taking the median MDL from the EPA
approved methods for analysis of a given contaminant (or average if there are only two MDL
values) and multiplying it by a factor often. Thresholds for endothall and oxamyl are based on
new health effects information.

There are several contaminants for which the MCLs are limited by analytical capability, and
EPA determined that there was no potential for PQL revision. In each case, the PQL assessment
did not identify potential to revise the PQL, the modal MRL - although sometimes lower than
the PQL - did not account for a large majority of MRL values, and the MDL multiplier result
generally concurred with the current PQL. One exception is dioxin, for which slightly more than
90% of the MRL values are equal to or less than the modal MRL. The MRL result, however, is
based on relatively few samples. Furthermore, EPA does not have PT data for dioxin, and the
MDL multiplier approach generates a result that is higher than the current PQL.  Therefore, EPA
categorized dioxin as not having potential for a PQL revision. Exhibit ES-1 provides a summary
of results.
                                         ES-3

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EPA-OGWDW          Development of Estimated Quantitation Levels for the       EPA 815-B-09-005
              Second Six-Year Review of National Primary Drinking Water Regulations   October 2009
                  Exhibit ES-1. Summary of Threshold Determination
Contaminant
Type
Current
PQL
Threshold
(EQL or
health-
based)
Basis for
Threshold
MRL Distribution
% = mode
% < mode
Ongoing health effects assessment, MCL limited by PQL
Benzo[a]pyrene
Carbon tetrachloride
DEHP
1,2-Dichloroethane
Dichloromethane
Pentachlorophenol
RGBs
Dioxin
Tetrachloroethylene
Thallium
Trichloroethylene
soc
voc
soc
voc
voc
soc
soc
soc
voc
IOC
voc
0.2 ng/L
5ng/L
6ng/L
5ng/L
5ng/L
1ng/L
0.5 ng/L
3x10-5n9/L
5ng/L
2ng/L
5ng/L
na
0.5 ng/L
na
0.5 ng/L
0.5 ng/L
na
na
na
0.5 ng/L
na
0.5 ng/L
na
Modal MRL
na
Modal MRL
Modal MRL
na
na
na
Modal MRL
na
Modal MRL
53.7
86.1
45.5
83.8
87.8
44.7
67.6
75.4
84.6
63.0
85.4
55.4
97.7
76.9
96.9
96.0
48.7
74.3
90.3
96.1
78.0
97.7
No new health effects assessment or new health effects assessment indicates no MCLG change,
MCL is limited by PQL
Benzene
Chlordane
DBCP
1,2-Dichloropropane
EDB
Heptachlor
Heptachlor epoxide
Hexachlorobenzene
Toxaphene
1,1,2-Trichloroethane
Vinyl chloride
voc
soc
soc
voc
soc
soc
soc
soc
soc
voc
voc
5ng/L
2ng/L
0.2 ng/L
5ng/L
0.05 ng/L
0.4 ng/L
0.2 ng/L
1M0/L
3ng/L
5ng/L
2ng/L
0.5 ng/L
1na/L
0.1 ng/L
0.5 ng/L
na
0.1 ng/L
0.1 ng/L
0.1 ng/L
1ng/L
3 ng/U
0.5 ng/L
Modal MRL
10xMDL
10xMDL
Modal MRL
na
10xMDL
10xMDL
Modal MRL
Modal MRL
Current MCLG
Modal MRL
86.4
46.8
35.6
86.1
32.4
42.4
45.0
69.5
67.4
99.3
63.8
84.7
99.3
32.9
56.0
55.2
82.1
83.0
99.9% below current MCLG
75.6
94.0
New health effects assessment indicates possible MCLG below PQL
Endothall
Oxamyl
soc
soc
90 ng/L
50 ng/L
50 ng/L1
2 ng/L1
Possible MCLG
Possible MCLG
98.4% below possible MCLG
96.7% below possible MCLG
 na = not applicable (PQL assessment does not support PQL revision, and MDL range and MRL data do not indicate potential
 for PQL revision).
 1. This threshold is based on health effects information instead of an EQL. The MRL results show the percent of MRL values
 below the health-based threshold.
                                              ES-4

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

1    Introduction

The U.S. Environmental Protection Agency (EPA or the Agency) has conducted its second Six-
Year Review ("Six-Year Review 2") of national primary drinking water regulations (NPDWRs).
The 1996 Safe Drinking Water Act (SDWA) Amendments require that the Agency periodically
review existing NPDWRs. Section 1412(b)(9) of SDWA reads:

        ...[t]he Administrator shall, not less than every 6 years, review and revise, as
       appropriate, each primary drinking water regulation promulgated under this title.
       Any revision of a national primary drinking water regulation shall be promulgated
       in accordance with this section, except that each revision shall maintain, or
       provide for greater, protection of the health of persons.

The primary goal of the Six-Year Review process is to identify possible regulatory revisions.
Although the statute does not define when a revision is "appropriate," as a general benchmark,
EPA considered a possible revision to be "appropriate" if, at a minimum, it presents a
meaningful opportunity to:

   improve the level of public health protection, and/or
   achieve cost savings while maintaining or improving the level of public health protection.

For Six-Year Review 2, EPA implemented the protocol that it developed for the first  Six-Year
Review (USEPA, 2003a), including minor revisions developed during the current review process
(USEPA, 2009c). EPA obtained and evaluated new information including information on health
effects (USEPA, 2009e), analytical feasibility (USEPA, 2009b), treatment feasibility  (USEPA,
2009f), and occurrence (USEPA, 2009a and 2009d). Some of the new health effects or analytical
methods information that indicated it may be possible to revise NPDWRs for several
contaminants. Consequently, EPA conducted occurrence and exposure analyses at threshold
concentrations that are below current maximum contaminant levels (MCLs) to determine if there
is a meaningful opportunity to improve the level of public health protection by reducing MCLs.1
This document describes the method EPA used to establish the threshold values that it used for
the occurrence analyses.

For most contaminants, EPA established an estimated quantitation level (EQL), which is an
estimate of the possible lower bound for a practical quantitation level (PQL). The current PQL
for a contaminant is based on historical analytical capabilities, generally the quantitation
capabilities at the time EPA promulgated the existing NPDWR for the contaminant. When a
contaminant has a PQL that is higher than its maximum contaminant level goal (MCLG), the
MCL cannot be lower than the PQL. Thus, improvements in analytical feasibility identified in
USEPA (2009b) indicate potential opportunity to lower the PQL for some contaminants that
1 EPA used these thresholds when it estimated possible system and population impacts in the occurrence and
exposure analysis conducted for the Six-Year Review 2 (USEPA, 2009a). EPA compared contaminant occurrence
estimates for these thresholds (i.e., the number of systems with water quality exceeding a threshold) with baseline
occurrence estimates at current MCLs. The difference between these two occurrence estimates indicates potential
incremental exposure and human health risks. EPA based its determinations about whether a potential reduction in
the MCL for a contaminant would provide a meaningful opportunity to improve the level of public health protection
on the incremental occurrence and exposure estimates for that contaminant.
                                            1-1

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

have MCLs limited by PQLs, and, therefore, lower the MCL closer to MCLG. The EQLs do not
represent the Agency's intent to promulgate new PQLs at this time. Any revisions to PQLs will
be made as part of future rule making efforts.

For two contaminants, EPA established new threshold values based on new health effects
information that indicates adverse health effects might occur at concentrations below the current
MCLG. These contaminants are included in this report because they have PQLs that are higher
than the new possible MCLG values, prompting EPA to evaluate the potential to estimate a
quantitation level below the PQL and perhaps as low as the possible MCLG.

Exhibit 1-1 shows the contaminants for both groups. In the first case, which applies to the 22
contaminants in Exhibit 1-1 with PQLs limiting their MCLs, any MCL revision depends on
whether the PQL can be lower. In the second case, which applies to the final two contaminants in
the exhibit, the new estimate of a possible MCLG is below the current PQL and it is necessary to
examine whether the PQL can be lowered to the possible MCLG.

Analyzing the feasibility of reducing a contaminant's current PQL was one of the review tasks of
the Six-Year Review 2. For the PQL assessment, EPA obtained and evaluated new information
regarding the potential to revise PQL values. The primary sources of information for the PQL
assessment were laboratory proficiency testing  (PT) study results obtained during Six-Year
Review 2 and laboratory performance evaluation (PE) data obtained during the first Six-Year
Review. The PT and PE studies involve the use of spiked samples to evaluate laboratory
quantitation capabilities. USEPA (2009b) describes the review method, PT and PE data, and
findings for the PQL analysis. For Six-Year Review 2, EPA did not have sufficient PT data
below current PQLs to actually recalculate any  PQL or derive EQLs for the occurrence and
exposure analysis. Instead,  EPA used the PT and PE study passing rate results (i.e., the percent
of laboratories passing a performance test for a  given study) at and below the current PQL and
the result of a linear regression analysis to indicate whether the PT and PE data support a
reduction in the PQL. Exhibit  1-1  provides summary information for whether the data indicate
there is potential to revise the PQL.

Because the PT and PE results were either not available below the PQL or did not provide
conclusive indications regarding a potential to revise a PQL or how far below the PQL
quantitation might be feasible, EPA relied on two alternate approaches to estimate EQLs: an
approach based on the minimum reporting levels (MRLs) obtained as part of the Six-Year
Review Information Collection Request (ICR),  and an approach based on method detection
limits (MDL). While EPA prefers to use laboratory performance data to calculate the PQL, the
MRL and MDL information can be valuable to  indicate whether it is possible to quantitate at
levels below the current PQL.

An MRL is the lowest level or contaminant concentration that a laboratory can reliably achieve
within specified limits of precision and accuracy under routine laboratory operating conditions
using a given method (USEPA, 2009a). The MRL values provide direct evidence from actual
monitoring results about whether quantitation below the PQL using current analytical methods is
feasible.
                                          1-2

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EPA-OGWDW          Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
              Second Six-Year Review of National Primary Drinking Water Regulations  October 2009
       Exhibit 1-1. Contaminants Requiring Methods or Occurrence Analyses
Contaminant
Current
MCLG1
Current
PQL1
Current MCL
Possible
MCLG
Do PT Data
Support PQL
Revision?2
Ongoing health effects assessment, MCL limited by PQL
Benzo[a]pyrene
Carbon Tetrachloride
Di (2-ethylhexyl)phthalate (DEHP)
1,2-Dichloroethane
Dichloromethane
Pentachlorophenol
Polychlorinated biphenyls (RGBs)
2,3,7,8-TCDD (Dioxin)
Tetrachloroethylene
Thallium
Trichloroethylene
0
0
0
0
0
0
0
0
0
0.5 ng/L
0
0.2 jig/L
5jig/L
6jig/L
5jig/L
5jig/L
1H9/L
0.5 jig/L
3x1 05 jig/L
5jig/L
2ng/L
5ng/L
0.2 ng/L
5ng/L
6ng/L
5ng/L
5ng/L
1ng/L
0.5 ng/L
3xio-5ng/L
5ng/L
2ng/L
5ng/L
O3
O3
O3
O3
O3
O3
O3
O3
O3
Not determined4
O3
No
Yes
No
Yes
No
No
No
No
Yes
No
Yes
No new health effects assessment or new health effects assessment indicates no MCLG change,
MCL is limited by PQL
Benzene
Chlordane
1 ,2-Dibromo-3-Chloropropane
(DBCP)
1,2-Dichloropropane
Ethylene Dibromide (EDB)
Heptachlor
Heptachlor Epoxide
Hexachlorobenzene
Toxaphene
1,1,2-Trichloroethane
Vinyl Chloride
0
0
0
0
0
0
0
0
0
3ng/L
0
5jig/L
2ng/L
0.2 ng/L
5ng/L
0.05 jig/L
0.4 jig/L
0.2 ng/L
1nq/L
3ng/L
5ua/L
2ng/L
5ng/L
2ng/L
0.2 ng/L
5ng/L
0.05 ng/L
0.4 ng/L
0.2 ng/L
1ng/L
3ng/L
5ng/L
2ng/L
No new HEA
No new HEA
No new HEA
No new HEA
No MCLG
change
No new HEA
No new HEA
No new HEA
No new HEA
No new HEA
No new HEA
Yes
Yes
Possibly
Yes
No
Possibly
Possibly
Yes
Possibly
Yes
Possibly
New health effects assessment indicates possible MCLG below PQL
Endothall
Oxamyl
100jig/L
200 jig/L
90 ng/L
20 ng/L
100^g/L
200 ng/L
50 ng/L5
2 ng/L5
No
No
 1. Boldface in the Current MCLG or Current PQL column indicates that the MCL is based on the MCLG or PQL, respectively.
 2. Results of PQL assessment based on analysis of PT and PE data.
 3. Although a health effects assessment is in progress, the current MCLG is zero. When the MCLG < MCL, the protocol
 includes a review of whether the MCL can be lowered even when a health effects assessment is ongoing.
 4. There is a health effects assessment in progress that may change the reference dose, which is the basis for the current
 MCLG. Because the MCL is based on the PQL, EPA reviewed the potential to revise the PQL.
 5. Possible MCLG based on a recently completed health effects assessment.
An MDL is a measure of analytical method sensitivity (USEPA, 2009b). MDLs have been used
in the past to derive PQLs for regulated contaminants. In addition, EPA used MDLs to help
identify possible analytical feasibility levels for Six-Year Review 1 (USEPA, 2003b).
Consequently, EPA used the MDLs as a second input to the EQL development process.
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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
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Both sources of data provide additional information on the feasibility of revising PQLs.
Therefore, the Agency also evaluated whether MRL and MDL data confirmed or contradicted
the conclusions of the PT and PE data review. For most contaminants, the MRL and MDL data
supported EPA's conclusion based on PT and PE data.

Section 2 provides a description of the MRL and MDL data that EPA used to derive the EQLs or
evaluate analytical feasibility or the health-based thresholds. The method EPA used to derive the
EQLs is in Section 3 along with an overview of the results. Section 4 contains contaminant-
specific information and results.
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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

2  Data Sources

As a lower bound estimate of analytical capability for a given contaminant, ideally an EQL
would be based on the same type of data used to drive PQLs. Current PQLs are based on two
approaches. The first comprises laboratory performance data; a PQL based on laboratory
performance data is the lowest value for which 75% of laboratories can quantitate within
prescribed accuracy limits. The second is an MDL-based method that involves multiplying an
MDL by five or ten to compute a PQL. EPA prefers laboratory performance data over the MDL
multiplier method. USEPA (2009b) provides detailed descriptions of these two PQL derivation
methods.

The PT and PE study results available during Six-Year Review 2 are not sufficient to derive
PQLs, however. For example, several contaminants have no laboratory performance studies for
concentrations below the contaminant's current PQL. For other contaminants that have some
performance results below the PQL, generally the range of concentrations  are not sufficient to
determine where a lower bound on analytical capability might be (i.e., where at least 75% of
laboratories can reliably and consistently quantitate).

Because of insufficient PT or PE data, EPA used two other sources to derive EQLs: MRLs from
the Six-Year 2 ICR dataset (Section 2.1), and MDLs associated with approved analytical
methods developed by EPA (Section 2.2). The MRL data are the primary data source for most of
the EQL estimates because these data represent laboratory analytical limits nationwide. EPA
used MDL data to estimate EQLs when MRL data were inconclusive, and  to confirm the
analytical  feasibility of the MRL-based estimates.

2.1    MRL Data
For the Six-Year Review 2 occurrence analysis, EPA obtained compliance monitoring data for
1998 through 2005. USEPA (2009a) provides a description of the data collection, data
management, and quality assurance methods the Agency used to establish  a high quality,
national contaminant occurrence database consisting of data from 47 States/Primacy Entities.
This dataset - the Second Six-Year Review National Compliance Monitoring Information
Collection Rule dataset (Six-Year Review 2 ICR dataset) - is the largest and most
comprehensive drinking water compliance monitoring dataset ever compiled and analyzed by
EPA. It contains over 17 million records.

The Six-Year Review 2 ICR dataset also contains a substantial number of MRL values. An MRL
is the lowest level or contaminant concentration that a laboratory can reliably achieve within
specified limits of precision and accuracy under routine laboratory operating conditions using a
given method (USEPA, 2009a). In other words, the MRL is the lowest contaminant
concentration that can be reliably quantified in the laboratory and reported with the contaminant
occurrence data to primacy agencies (e.g., States).

The MRL values provide EPA with valuable insights into current analytical capabilities across
laboratories and States. MRLs can vary across laboratories because of differences in the
analytical  method used as well as differences in instrumentation, implementation, and reporting.
By examining the distribution of MRL values for a contaminant, EPA can  identify whether
                                          2-1

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	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

laboratory performance is relatively uniform (e.g., most MRLs are the same) or highly variable
(e.g., MRLs that vary by one or more orders of magnitude). In particular, the mode or most
frequently occurring value is a potential candidate for EQL when a substantial share of the MRL
values for a contaminant equal the modal MRL.

When compliance monitoring data are recorded for a contaminant concentration that does not
exceed the MRL for that contaminant at a laboratory using an applicable analytical method, then
the compliance record should specify "
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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
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indicate potential for a PQL reduction (i.e., lower than the current PQL by more than a factor of
ten). Section 4 addresses each contaminant in detail.

   Exhibit 2-1. U.S. EPA Drinking Water MDLs, MRLs and Related Information for
                          Contaminants in the EQL Report
Contaminant
Ongoing
Benzo[a]pyrene
Carbon Tetrachloride
DEHP
1,2-Dichloroethane
Dichloromethane
Pentachlorophenol
RGBs
Dioxin
Tetrachloroethylene
Thallium
Trichloroethylene
PQL (ng/L)
Six Year 2 Modal MRL
(nq/L)
MDL Range for EPA Methods
(nq/L)
health effects assessment, MCL limited by PQL
0.2
5
6
5
5
1
0.5
0.00003
5
2
5
0.02
0.5
1.0
0.5
0.5
0.04
0.1
0.000005
0.5
1
0.5
0.016-0.23
0.02-0.21
1.3-2.25
0.03-0.06
0.02 - 0.09
0.032-1.6
0.08
0.00001 (ML)
0.008-0.14
0.3-0.7
0.042-0.19
No new health effects assessment or new health effects assessment indicates no MCLG change,
MCL is limited by PQL
Benzene
Chlordane
DBCP
1,2-Dichloropropane
EDB
Heptachlor
Heptachlor Epoxide
Hexachlorobenzene
Toxaphene
1,1,2-Trichloroethane
Vinyl chloride
5
2
0.2
5
0.05
0.4
0.2
1
3
5
2
0.5
0.2
0.5
0.5
0.01
0.04
0.02
0.1
1
0.5
0.5
0.01-0.04
0.004-0.22
0.009-0.01
0.03-0.04
0.01-0.032
0.0015-0.15
0.001-0.202
0.001-0.13
0.13-1.7
0.017-0.1
0.17-0.18
New health effects assessment indicates possible MCLG below PQL
Endothall
Oxamyl
90
50
9
2
1.79
0.065 - 0.86
 Sources: USEPA (2009a and 2009b)
 ML=minimum level

For DBCP, the modal MRL for Six-Year Review 2 is 0.5 ug/L, which is higher than the PQL of
0.2 ug/L, which is also the MCL. Given the range of upper bound MDL values for EPA methods
shown in Exhibit 2-1 (0.009 to 0.01 ug/L), MRL values greater than the MCL are generally not
expected. The MRL data for this contaminant indicate a data quality issue. As the discussion in
Section 4 indicates, EPA disregarded MRL data in setting an EQL for DBCP.
                                         2-3

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
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3   Threshold Development Method

EPA's method for developing an EQL has essentially three steps - one for each of the three
information sources (PT and PE data, MRL data, and MDL values). The first step is to review
the conclusion of the PQL analysis (Exhibit 3-1). The next two steps depend somewhat on
whether the PT and PE data indicate potential to revise the PQL. If they do, then the objective of
the next two steps is to identify an EQL (or verify the use of a health-based threshold) for the
occurrence analysis. The second step is to determine whether the modal MRL is a feasible EQL
and, if so, the third step is to determine whether the MDL multiplier approach supports that EQL
value. If EPA determines in the second step that the modal MRL is not a feasible EQL, then EPA
uses the MDL multiplier approach to establish an EQL in the third step. If the PT data do not
indicate potential to revise, then EPA determines whether the MRL and MDL data concur with
this finding or indicate an EQL value. As Exhibit 3-1 shows, there were two types of outcomes
for this case. For the first type, the MRL and MDL data contradict the PQL analysis finding, and
EPA used these secondary data sources to derive an EQL (or verify the use of a health-based
threshold) for the occurrence analysis. For the second type, the MRL and MDL data confirm the
PQL analysis finding and there is no basis for an EQL that is less than the PQL. Exhibit 3-1
provides an overview of the three steps and outcomes; Exhibit 3-2 provides a summary of the
EQL results.
                        Exhibit 3-1. EQL Development Steps
                                                                  Carbon tetrachloride
                                                                  1,2-Dichloro ethane
                                                                  Tetrachloroethylene
                                                                  Trichloroethylene
                                                                  Benzene
                                                                  1,2-Dichloro propane
                                                                  Hexachlorobenzene
                                                                  Toxaphene
                                                                  1,1,2-Trichloroethane*
                                                                  Vinyl chloride

                                                                   Chlordane
                                                                   DBCP
                                                                   Heptachlor
                                                                   Heptachlor epoxide
        No potential
        to revise
EQL
Identifiecl^^*
Review MDL
data to
confirm EQL
J

Dichloro methane
Endothall*
Oxamyl*
    * Health-based threshold instead of EQL
                                                                  Benzo[a]pyrene
                                                                  DEHP
                                                                  Pentachlorophenol
                                                                  PCBs
                                                                  Dioxin
                                                                  Thallium*
                                                                  EDB
                                          3-1

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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
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As Exhibit 3-2. Summary of Threshold DeterminationExhibit 3-2 indicates, EPA relied primarily
on the MRL data to calculate EQLs. The MRLs provide information on the analytical
capabilities of a large pool of laboratories. The MRL data also provide a threshold for estimating
occurrence. Setting an EQL below a substantial number of MRL values will limit the monitoring
data available for the occurrence analysis.

EPA set the EQL equal to the modal MRL when at least 80% of the MRL values were equal to
or less than the modal MRL. EPA determined that the contaminants with more than 80% of
MRL values equal to or less than the modal MRL also have MDL values that support an EQL
below the current PQL and often  as low as the modal MRL. Thus, an MRL distribution threshold
of 80% enables EPA to derive EQLs that tend to be supported by multiple  data sources.

For all of the volatile organic compounds (VOCs) in Exhibit 3-2 (except for 1,1,2-
trichloroethane, which has a health-based threshold) more than 90% of each contaminant's MRL
values are equal to or less than the modal MRL, with a clear majority equal to the modal MRL.
Proportions at the modal MRL range from a low of 75.6% (vinyl chloride) to a high of 87.8%
(dichloromethane). Proportions that equal or are less than the modal MRL  exceed 94%. In each
case, the modal MRL is 0.5 ug/L. Therefore, EQLs for the VOCs are based on modal MRL
values.

The synthetic organic compounds (SOCs) have EQLs based on different methods. Two have
with EQLs based on modal MRLs - hexachlorobenzene and toxaphene. Four SOCs have EQLs
based on the MDL multiplier method - DBCP, chlordane, heptachlor, and  heptachlor epoxide.
This method takes the median MDL from the EPA approved methods for analysis of a given
contaminant (or average if there are only two MDL values), and multiplies it by  a factor often.
Two have health-based thresholds - endothall and oxamyl.

As Exhibit 3-2 indicates, there are several contaminants with MCLs that are limited by analytical
capability, for which EPA determined there was no potential for PQL revision. In each case, the
PT analysis did not identify potential to revise the PQL, the modal MRL - although sometimes
lower than the PQL - did not meet the 80% distribution threshold, and the  MDL multiplier result
generally concurred with the current PQL. One exception is dioxin, which  has slightly more than
90% of the MRL values equal to or less than the modal MRL. The MRL result, however, is
based on relatively few samples. Furthermore, EPA does not have PT data for dioxin and the
MDL multiplier approach generates a  result that is higher than the current PQL.  Therefore, EPA
categorized dioxin as not having potential for a PQL revision.
                                         3-2

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EPA-OGWDW          Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
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                   Exhibit 3-2. Summary of Threshold Determination
Contaminant
Type
Current
PQL
Threshold
(EQL or
health-
based)
Basis for
Threshold
MRL Distribution1
% = mode
% < mode
Ongoing health effects assessment, MCL limited by PQL
Benzo[a]pyrene
Carbon tetrachloride
DEHP
1,2-Dichloroethane
Dichloromethane
Pentachlorophenol
RGBs
Dioxin
Tetrachloroethylene
Thallium
Trichloroethylene
soc
voc
soc
voc
voc
soc
soc
soc
voc
IOC
voc
0.2 ng/L
5ng/L
6ng/L
5ng/L
5ng/L
1ng/L
0.5 ng/L
3x10-5n9/L
5ng/L
2ng/L
5ng/L
na
0.5 ng/L
na
0.5 ng/L
0.5 ng/L
na
na
na
0.5 ng/L
na
0.5 ng/L
na
Modal MRL
na
Modal MRL
Modal MRL
na
na
na
Modal MRL
na
Modal MRL
53.7
86.1
45.5
83.8
87.8
44.7
67.6
75.4
84.6
63.0
85.4
55.4
97.7
76.9
96.9
96.0
48.7
74.3
90.3
96.1
78.0
97.7
No new health effects assessment or new health effects assessment indicates no MCLG change,
MCL is limited by PQL
Benzene
Chlordane
DBCP
1,2-Dichloropropane
EDB
Heptachlor
Heptachlor epoxide
Hexachlorobenzene
Toxaphene
1,1,2-Trichloroethane
Vinyl chloride
voc
soc
soc
voc
soc
soc
soc
soc
soc
voc
voc
5ng/L
2ng/L
0.2 ng/L
5ng/L
0.05 ng/L
0.4 ng/L
0.2 ng/L
1M0/L
3ng/L
5ng/L
2ng/L
0.5 ng/L
1na/L
0.1 ng/L
0.5 ng/L
na
0.1 ng/L
0.1 ng/L
0.1 ng/L
1M0/L
3 ng/U
0.5 ng/L
Modal MRL
10xMDL
10xMDL
Modal MRL
na
10xMDL
10xMDL
Modal MRL
Modal MRL
Current MCLG
Modal MRL
86.4
46.8
35.6
86.1
32.4
42.4
45.0
69.5
67.4
99.3
63.8
84.7
99.3
32.9
56.0
65.2
82.1
83.0
99.9% below current MCLG
75.6
94.0
New health effects assessment indicates possible MCLG below PQL
Endothall
Oxamyl
soc
soc
90 ng/L
50 ng/L
50 ng/L1
2 ng/L1
Possible MCLG
Possible MCLG
98.4% below possible MCLG
96.7% below possible MCLG
 na = not applicable (PQL assessment does not support PQL revision, and MDL range and MRL data do not indicate potential
 for PQL revision).
 1. This threshold is based on health effects information instead of an EQL. The MRL results show the percent of MRL values
 below the health-based threshold.
                                               3-3

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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
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4  Development of Individual  EQLs

This section provides a discussion of the EQL or health-based threshold determination for each
contaminant addressed in this report. The discussion for each contaminant contains an overview
of the PQL review in USEPA (2009b), followed by MRL summary data and MDLs. There are
three subsections - one for each of the three contaminant groups shown in previous exhibits.

4.1    Contaminants with Ongoing  Health Effects Review and MCL
Limited by PQL
4.1.1   Benzo[a]pyrene
The MCL for benzo[a]pyrene is based on a PQL of 0.2 ug/L. Its MCLG is zero, and although a
health effects assessment is in progress, there is no new health effects information that suggests a
change in the MCLG. Consequently, the threshold for the occurrence analysis will be based on
analytical feasibility. There are no PT or PE study results at spiked concentrations below the
current PQL.  Several passing rates for the available PT studies are below 75%, although none of
the PE data passing rates are  below 75%. Because of the variability in passing rates and lack of
data below the PQL,  EPA determined that a PQL revision is not appropriate at this time
(USEPA, 2009b).

As shown in Exhibit 4-1 and Exhibit 4-2, the modal MRL  for benzo[a]pyrene is 0.02 ug/L.
Summary data show  that 53.7% of the MRLs are equal to this value, and 55.4% of the MRL
values are equal to or less than it. There is, however, a second cluster of MRLs at or just below
the current PQL. Unlike the PT data, the MRL data indicate that there is potential to lower the
PQL because most of the MRL values are below the current PQL. Less than 80% of the MRL
values are equal to or less than the modal MRL and, therefore, EPA did not base the EQL on the
modal MRL. Consequently, EPA reviewed MDL values to  determine whether they support an
EQL below the PQL.

               Exhibit 4-1. Summary of MRL Data for Benzo[a]pyrene
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.02 ng/L)
Value > Modal MRL and < 0.1 ng/L
Value > 0.1 ng/L
Number of Records
55,487
970
29,769
21,198
3,550
Percentage of Records
100%
1.7%
53.7%
38.2%
6.4%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                         4-1

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
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                  Exhibit 4-2. MRL Distribution for Benzo[a]pyrene
   O
   CL
   0.3
Exhibit 4-3 shows the EPA approved methods for the detection of benzo[a]pyrene, and their
MDLs. Multiplying the median MDL by ten would give a value of 0.29 ug/L, which is higher
than the current PQL of 0.2 ug/L. The MDL data do not support an EQL below the PQL.

                          Exhibit 4-3. Analytical Methods for
                                   Benzo[a]pyrene
Method
525.2
550
550.1
MDL (nq/L)
0.23
0.029
0.016
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
EPA concluded that although MRL values are generally below the current PQL, the combination
of PT and MDL data do not support revision of the PQL for benzo[a]pyrene. Therefore, EPA did
not develop an EQL.

4.1.2   Carbon Tetrachloride
The MCL for carbon tetrachloride is based on a PQL of 5 ug/L. Its MCLG is zero. It has an
ongoing health effects assessment, but there is no new health information that suggests a change
in the MCLG. Consequently, the threshold for the occurrence analysis will be based on analytical
feasibility. The PT and PE studies show greater than 90% passing rates for most of the studies
around the PQL, which include ten studies with spiked concentrations below the current PQL.
EPA determined that the PQL assessment supports reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-4 and Exhibit 4-5, 86.1% of the MRL values in the ICR dataset are equal
to the modal value of 0.5 ug/L, and 97.7% of the MRL values are equal to or less than the modal
                                          4-2

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
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value. Because more than 80% of the MRLs are equal to or less than 0.5 ug/L, EPA based the
EQL on the modal MRL.

             Exhibit 4-4.  Summary of MRL Data for Carbon Tetrachloride
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.5 ng/L)
Value > Modal MRL
Number of Records
139,221
16,195
119,849
3,177
Percentage of Records
100%
11.6%
86.1%
2.3%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset

               Exhibit 4-5. MRL Distribution for Carbon Tetrachloride
   T3
   1
Exhibit 4-6 shows the EPA approved methods for the detection of carbon tetrachloride, and their
MDLs. Applying a multiplier of 10 would give a possible PQL range from 0.2 to 2.1 ug/L. This
range contains the modal MRL. Therefore, EPA retained 0.5 ug/L as the EQL value.

                      Exhibit 4-6. Analytical Methods for Carbon
                                      Tetrachloride
Method
502.2
524.2
551.1
MDL (ug/L)
0.02
0.21
0.050
                  Source: USEPA, 2009b (upper bound values when ranges are reported)


The EQL of 0.5 ug/L is ten times lower than the current PQL. EPA also performed the
occurrence analysis with intermediate values of 1 ug/L (2 x EQL) and 2.5 ug/L (1/2 MCL).
                                            4-3

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
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4.1.3    Di(2-ethylhexyl)phthalate (DEHP)
The MCL for DEHP is based on a PQL of 6 ug/L. Its MCLG is zero, and although a health
effects assessment is in progress, there is no new health effects information that suggests a
change in the MCLG. Consequently, the threshold for the occurrence analysis will be based on
analytical feasibility. Several PT and PE studies had passing rates below 75%, including two
studies with spiked concentrations below the PQL. Because of the low passing rates, EPA
determined that the PT and PE results do not support revision of the PQL (USEPA, 2009b).

As shown in Exhibit 4-7 and Exhibit 4-8, the modal MRL for DEHP is 1 ug/L. Summary data
show that 45.5% of the MRLs are equal to this value, and 76.9% of the MRL values are equal to
or less than it. Unlike the PT data, the MRL data appear to indicate that there is potential to
lower the PQL because most of the MRL values are below the current PQL. Less than 80% of
the MRL values are equal to or less than the modal MRL and, therefore, EPA did not base the
EQL on the  modal MRL. Consequently, EPA reviewed MDL values to determine whether they
support an EQL below the PQL.

                     Exhibit 4-7. Summary of MRL Data for DEHP
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (1 ^g/L)
Value > Modal MRL
Number of Records
50,490
15,842
22,980
11,668
Percentage of Records
100%
31.4%
45.5%
23.1%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                        Exhibit 4-8. MRL Distribution for DEHP
T3

O
Q.
7
                                           4-4

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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

Exhibit 4-9 shows the EPA approved methods for the detection of DEHP, and their MDLs.
Applying a multiplier of 10 would give a possible PQL range from 13.0 to 22.5 ug/L. This range
is greater than the current PQL. The MDL data do not support an EQL below the PQL.

                      Exhibit 4-9. Analytical Methods for DEHP
Method
506
525.2
MDL (nq/L)
2.25
1.3
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
EPA concluded that although MRL values are generally below the current PQL, the combination
of PT and MDL data do not support revision of the PQL for DEHP. Therefore, EPA did not
develop an EQL.

4.1.4   1,2-Dichloroethane
The MCL for 1,2-dichloroethane is based on a PQL of 5 ug/L. Its MCLG is zero, and there is no
new health effects information that suggests a change in the MCLG. Consequently, the threshold
for the occurrence analysis will be based on analytical feasibility. The PT and PE studies show
greater than 90% passing rates for most of the spiked concentrations around the PQL, which
include eight studies with concentrations below the current PQL. EPA determined that the PQL
assessment supports reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-10 and Exhibit 4-11, 83.8% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 0.5 ug/L, and 96.9% of the MRL values are equal to
or less than the modal value. Because more than 80% of the MRLs are equal to or less than 0.5
ug/L, EPA based the EQL on the modal MRL.

             Exhibit 4-10. Summary of MRL Data for 1,2-Dichloroethane
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.5 ng/L)
Value > Modal MRL
Number of Records
139,085
18,160
116,533
4,392
Percentage of Records
100%
13.1%
83.8%
3.2%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                          4-5

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                Exhibit 4-11. MRL Distribution for 1,2-Dichloroethane
o
Q.
0.9
Exhibit 4-12 shows the EPA approved methods for the detection of 1,2-dichloroethane, and their
MDLs. Applying a multiplier of 10 would give a possible PQL range from 0.3 to 0.6 ug/L. This
range contains the modal MRL. Therefore, EPA retained 0.5 ug/L as the EQL value.

                       Exhibit 4-12. Analytical Methods for 1,2-
                                    Dichloroethane
Method
502.2
524.2
MDL (nq/L)
0.03
0.06
                 Source: USEPA, 2009b (upper bound values when ranges are reported)
The EQL of 0.5 ug/L is ten times lower than the current PQL. EPA also performed the
occurrence analysis with intermediate values of 1 ug/L (2 x EQL) and 2.5 ug/L (1/2 MCL).

4.1.5    Dichloromethane
The MCL for dichloromethane is based on a PQL of 5 ug/L. Its MCLG is zero. It has an ongoing
health effects assessment, but there is no new health effects information that suggests a change in
the MCLG. Consequently, the threshold for the occurrence analysis will be based on analytical
feasibility. Although passing rates for the PT and PE studies are generally above 85%, there are
no studies with spiked concentrations below the current PQL. Therefore, EPA determined that
the PT and PE results do not support revision of the PQL (USEPA, 2009b).

As  shown in Exhibit 4-13 and Exhibit 4-14, 87.8% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 0.5 ug/L, and 96.0% of the MRL values are equal to
or less than the modal value. Because more than 80% of the MRLs are equal to or less than 0.5
    , EPA based the EQL on the modal MRL.
                                          4-6

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

              Exhibit 4-13. Summary of MRL Data for Dichloromethane
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.5 ng/L)
Value > Modal MRL
Number of Records
138,625
11,294
121,532
5.619
Percentage of Records
100%
8.2%
87.8%
4.1%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                 Exhibit 4-14. MRL Distribution for Dichloromethane
                                            87.8%
   O
   Q.
   3
   CO
   2
Exhibit 4-15 shows the EPA approved methods for the detection of dichloromethane, and their
MDLs. Applying a multiplier of 10 would give a possible PQL range from 0.2 to 0.9 ug/L. This
range contains the modal MRL. Therefore, EPA retained 0.5 ug/L as the EQL value.
                          Exhibit 4-15. Analytical Methods for
                                    Dichloromethane
Method
502.2
524.2
MDL (ug/L)
0.02
0.09
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
The EQL of 0.5 ug/L is ten times lower than the current PQL. EPA also performed the
occurrence analysis with intermediate values of 1 ug/L (2 x EQL) and 2.5 ug/L (1/2 MCL).
                                            4-7

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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

4.1.6    Pentachlorophenol
The MCL for pentachlorophenol is based on a PQL of 1 ug/L. Its MCLG is zero, and although a
health effects assessment is in progress, there is no new health effects information that suggests a
change in the MCLG. Consequently, the threshold for the occurrence analysis will be based on
analytical feasibility. Several PT and PE studies had passing rates below 75%, and only one PE
study had a spiked concentration below the current PQL. Therefore, EPA determined that the PT
and PE results do not support revision of the PQL (USEPA, 2009b).

As shown in  Exhibit 4-16  and Exhibit 4-17, the modal MRL for pentachlorophenol is 0.4 ug/L.
Summary data show that 44.7% of the MRLs are equal to this value, and 48.7%  of the MRL
values are equal to or less than it. Unlike the PT data, the MRL data appear to indicate that there
is potential to lower the PQL because most of the MRL values are below the current PQL. Less
than 80% of the MRL values are equal to or less than the modal MRL and, therefore, EPA did
not base the EQL on the modal MRL. Consequently, EPA reviewed MDL values to determine
whether they support an EQL below the PQL.

             Exhibit 4-16. Summary of MRL Data for Pentachlorophenol
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.04 pg/L)
Value > Modal MRL
Number of Records
59,594
2,399
26,666
30,529
Percentage of Records
100%
4.0%
44.7%
51.2%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                Exhibit 4-17. MRL Distribution for Pentachlorophenol
   o
   CL
   0.6
                                           4-8

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EPA-OGWDW
Development of Estimated Quantitation Levels for the
EPA815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

Exhibit 4-18 shows the EPA approved methods for the detection of pentachlorophenol, and their
MDLs. Applying a multiplier of 10 would give a range from 0.32 to 16.0 ug/L with a median of
1.225 ug/L. This range does not contain the modal MRL and most of the 10 x MDL values
exceed or nearly equal the current PQL. The MDL data do not support an EQL below the PQL.

                         Exhibit 4-18. Analytical Methods for
                                 Pentachlorophenol
Method
515.1
515.2
515.3
515.4
525.2
555
MDL (ng/L)
0
.032
0.16
0
0
.085
.084
1.0
1.6
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
EPA concluded that although MRL values are generally below the current PQL, the combination
of PT and MDL data do not support revision of the PQL for pentachlorophenol. Therefore, EPA
did not develop an EQL.

4.1.7   Polychlorinated Biphenyls (PCBs)
The MCL for PCBs is based on a PQL of 0.5  ug/L. The MCLG is zero, and although a health
effects assessment is in progress, there is no new health effects information that suggests a
change in the MCLG. Consequently, the threshold for the occurrence analysis will be based on
analytical feasibility. The only PE study with a spiked concentration below the current PQL had
a passing rate below 75%. The PT data had few data points with spiked concentrations near the
PQL and none below it. Therefore, EPA determined that the PT data do not support revision of
the PQL (USEPA, 2009b).

As shown in Exhibit 4-19 and Exhibit 4-20, the modal MRL for PCBs is 0.1 ug/L. Summary
data show that 67.6% of the MRLs are equal to this value, and 74.3% of the MRL values are
equal to or less than it. Unlike the PT data, the MRL data appear to indicate that there is potential
to lower the PQL because most of the MRL values are below the current PQL. Less than 80% of
the MRL values are equal to or less than the modal MRL and, therefore, EPA did not base the
EQL on the modal MRL. Consequently, EPA reviewed MDL values to determine whether they
support an EQL below the PQL.

                    Exhibit 4-19. Summary of MRL Data for PCBs
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.1 ng/L)
Value > Modal MRL
Number of Records
35,178
2,355
23,785
9,038
Percentage of Records
100%
6.7%
67.6%
25.7%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                          4-9

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
             Second Six-Year Review of National Primary Drinking Water Regulations  October 2009
                      Exhibit 4-20. MRL Distribution for PCBs
      80%
   o
   Q.
   0.6
Exhibit 4-21 shows the EPA approved method for the detection of PCBs (as
decachlorobiphenyl), and its MDL. Applying a multiplier of 10 would give a possible PQL of 0.
ug/L, which is greater than the current PQL. The MDL data do not support an EQL below the
PQL.

                     Exhibit 4-21. Analytical Methods for PCBs
Method
508A
MDL (nq/L)
0.08
                 Source: USEPA, 2009b
EPA concluded that although MRL values are generally below the current PQL, the combination
of PT and MDL data do not support revision of the PQL for PCBs. Therefore, EPA did not
develop an EQL.

4.1.8   2,3,7,8-TCDD (Dioxin)
The MCL for dioxin is based on a PQL of 3 x 10"5 ug/L. Its MCLG is zero, and although a health
effects assessment is in progress, there is no new health effects information that suggests a
change in the MCLG. Consequently, the threshold for the occurrence analysis will be based on
analytical feasibility. There is only one PT study result available, and its spiked value is above
the PQL; there are no PE studies. Therefore, EPA determined that the PT data do not support
revision of the PQL (USEPA, 2009b).

As shown in Exhibit 4-22 and Exhibit 4-23, the modal MRL for dioxin is 5x 10"6 ug/L.
Summary data show that 75.4% of the MRLs are equal to this value, and 90.3% of the MRL
                                         4-10

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

values are equal to or less than it. Because the majority of MRLs equal  5* 10"6 ug/L, the MRL
data suggest that value as the EQL. Unlike the PT data, the MRL data appear to indicate that
there is potential to lower the PQL because most of the MRL values are below the current PQL.
The small sample  size, however, limits its usefulness for EQL development. Therefore, EPA
reviewed MDL values to determine whether they support an EQL below the PQL.

                    Exhibit 4-22. Summary of MRL Data for Dioxin
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.000005 ^g/L)
Value > Modal MRL
Number of Records
1,278
191
964
123
Percentage of Records
100%
14.9%
75.4%
9.6%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                       Exhibit 4-23. MRL Distribution for Dioxin
   o
   Q.
   0.00004
                                                                            0.00003
Exhibit 4-24 shows the EPA approved method for the detection of dioxin, and its minimum level
(ML). Applying a multiplier of 5 would give a possible PQL of 5* 10"5 ug/L, which is higher
than the current PQL. The MDL data do not support an EQL below the PQL.

                      Exhibit 4-24. Analytical Methods for Dioxin
Method
1613
MDL (ug/L)
1x10-5 (ML)
                   ML = minimum level
                   Source: USEPA, 2009b and 40 CFR 141.24 (Organic chemicals, sampling
                   and analytical requirements).
                                            4-11

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations   October 2009

EPA concluded that although MRL values are generally below the current PQL, the combination
of PT and MDL data do not support revision of the PQL for dioxin. Therefore, EPA did not
develop an EQL.

4.1.9   Tetrachloroethylene
The MCL for tetrachloroethylene is based on a PQL of 5 ug/L. Its MCLG is zero. It has an
ongoing health effects assessment, but there is no new health effects information that suggests a
change in the MCLG. Consequently, the threshold for the occurrence analysis will be based on
analytical feasibility. The passing rates for the PT and PE studies with spiked concentrations near
the PQL are above 90%, including 13 studies with spiked concentrations below the current PQL.
EPA determined that the PQL assessment supports reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-25 and Exhibit 4-26, 84.6%  of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 0.5 ug/L, and 96.1% of the MRL values are equal to
or less than the modal value. Because more than 80% of the MRLs are equal to or less than 0.5
ug/L, EPA based the EQL on the modal MRL.

             Exhibit 4-25. Summary of MRL Data for Tetrachloroethylene
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.5 ng/L)
Value > Modal MRL
Number of Records
138,348
15,848
117,033
5,467
Percentage of Records
100%
11.5%
84.6%
4.0%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
               Exhibit 4-26. MRL Distribution for Tetrachloroethylene
a! 80%
a
c
o
1" 60%

3
E 40%
01
CO
'o
'E
0.8
                                          4-12

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

Exhibit 4-27 shows the EPA approved methods for the detection of tetrachloroethylene, and
their MDLs. Applying a multiplier of 10 would give a possible PQL range from 0.08 to 1.4 ug/L.
This range contains the modal MRL. Therefore, EPA retained 0.5 ug/L as the EQL value.

                         Exhibit 4-27. Analytical Methods for
                                 Tetrachloroethylene
Method
502.2
524.2
551.1
MDL (^g/L)
0.05
0.14
0.008
                 Source: USEPA, 2009b (upper bound values when ranges are reported)
The EQL of 0.5 ug/L is ten times lower than the current PQL. EPA also performed the
occurrence analysis with intermediate values of 1  ug/L (2 x EQL) and 2.5 ug/L (1/2 MCL).

4.1.10   Thallium
The MCL for thallium is based on a PQL of 2 ug/L. Its MCLG is 0.5 ug/L, and a health effects
assessment is ongoing. Since it is not presently  known how the health effects assessment may
change the MCLG, the threshold for an occurrence analysis would be based on a potential
change in analytical feasibility. A few PE studies with spiked concentrations slightly below the
PQL have passing rates close to 80%. Two PT studies with concentrations slightly above the
PQL have passing rates of 75% and there is a downward trend in the passing rate as the spiked
value approaches the  PQL. Therefore, EPA determined that the PT and PE studies do not support
revision of the PQL (USEPA, 2009b).

As shown in Exhibit  4-28 and Exhibit 4-29, the modal MRL for thallium is 1 ug/L.  Summary
data show that 63.0% of the MRLs are equal to this value, and 78.0% of the MRL values are
equal to or less than it. Unlike the PT data, the MRL data appear to indicate that there is potential
to lower the PQL because most of the MRL values are below the current PQL. The data do not,
however, support an EQL as low as the current  MCLG of 0.5 ug/L. Furthermore, less than 80%
of the MRL values are equal to or less than the modal MRL and, therefore, EPA did not base the
EQL on the modal MRL. Consequently, EPA reviewed MDL values to determine whether they
support an EQL below the PQL.

                  Exhibit 4-28. Summary  of MRL Data for Thallium
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (1 ^g/L)
Value > Modal MRL
Number of Records
73,409
11,032
46,273
16,104
Percentage of Records
100%
15.0%
63.0%
21.9%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                          4-13

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                     Exhibit 4-29. MRL Distribution for Thallium
      80%
   o
   Q.
   Si  60%
   
   3
   CO
                                                                  63.0%
   .
   CL
   E
      40%
o
'E
OJ
"  20%
3
Exhibit 4-30 shows the EPA approved methods for the detection of thallium, and their MDLs.
Applying a multiplier of 10 would give a possible PQL rage of 3.0 to 7.0 ug/L. The current PQL
is below this range. The MDL data do not support an EQL below the PQL.

                    Exhibit 4-30. Analytical Methods for Thallium
Method
200.8
200.9
MDL (ug/L)
0.3
0.7
                  Source: USEPA, 2009b (upper bound values when ranges are reported)

EPA concluded that although MRL values are generally below the current PQL, the combination
of PT and MDL data do not support revision of the PQL for thallium. Therefore, EPA did not
develop an EQL.

4.1.11  Trichloroethylene
The MCL for trichloroethylene is based on a PQL of 5 ug/L. Its MCLG is zero. It has an ongoing
health effects assessment, but there is no new health effects information that suggests a change in
the MCLG. Consequently, the threshold for the occurrence analysis will be based on analytical
feasibility. The PT studies show greater than 95% passing rates for most of the studies around
the PQL, including six studies with spiked concentrations below the current PQL. Passing rates
for PE studies are also high near the PQL, but there are no studies with concentrations below the
PQL. EPA determined that the PQL assessment supports reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-31 and Exhibit 4-32, 85.4% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the  modal value of 0.5 ug/L, and 97.7% of the MRL values are equal to
or less than the modal value. Because more than 80% of the MRLs are equal to or less than 0.5
    , EPA based the EQL on the modal MRL.
                                          4-14

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

              Exhibit 4-31. Summary of MRL Data for Trichloroethylene
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.5 ng/L)
Value > Modal MRL
Number of Records
138,439
17,057
118,193
3,189
Percentage of Records
100%
12.3%
85.4%
2.3%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                 Exhibit 4-32. MRL Distribution for Trichloroethylene
   0.7
Exhibit 4-33 shows the EPA approved methods for the detection of trichloroethylene, and their
MDLs. Applying a multiplier of 10 would give a possible PQL range from 0.42 to 1.9 ug/L. This
range contains the modal MRL. Therefore, EPA retained 0.5 ug/L as the EQL value.

                          Exhibit 4-33. Analytical Methods for
                                    Trichloroethylene
Method
502.2
524.2
551.1
MDL (ug/L)
0.06
0.19
0.042
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
The EQL of 0.5 ug/L is ten times lower than the current PQL. EPA also performed the
occurrence analysis with intermediate values of 1 ug/L (2 x EQL) and 2.5 ug/L (1/2 MCL).
                                            4-15

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

4.2   Contaminants with MCL Limited by PQL and No Health-Based
Changes
4.2.1   Benzene
The MCL for benzene is based on a PQL of 5 ug/L. Its MCLG is zero. It has a new health effects
assessment, but there is no new health effects information that suggests a change in the MCLG.
Consequently, the threshold for the occurrence analysis will be based on analytical feasibility.
The PT studies show greater than 90% passing rates for most of the spiked concentrations around
the PQL, which include eight studies with concentrations below the current PQL. PE studies with
spiked concentrations near the PQL also have passing rates above 90% including two studies
with spiked concentrations below the PQL. EPA determined that the PQL assessment supports
reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-34 and Exhibit 4-35, 86.4% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 0.5 ug/L, and  99.3% of the MRL values are equal to
or less than the modal value. Because more than 80% of the MRLs are equal to or less than 0.5
     , EPA based the EQL on the modal MRL.

                  Exhibit 4-34. Summary of MRL Data for Benzene
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.5 ng/L)
Value > Modal MRL
Number of Records
139,190
17,964
120,328
898
Percentage of Records
100%
12.9%
86.4%
0.6%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset

                     Exhibit 4-35. MRL Distribution for Benzene
      80%
      60%
   E"  40%
   o
   'E
   2
                                         4-16

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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

Exhibit 4-36 shows the EPA approved methods for the detection of benzene, and their MDLs.
Applying a multiplier of 10 would give a possible PQL range from 0.1 to 0.4 ug/L. This range is
lower than the modal MRL. Therefore, EPA retained 0.5 ug/L as the EQL value.

                    Exhibit 4-36. Analytical  Methods for Benzene
Method
502.2
524.2
MDL (ng/L)
0.01
0.04
                 Source: USEPA, 2009b (upper bound values when ranges are reported)

The EQL of 0.5 ug/L is ten times lower than the current PQL. EPA also performed the
occurrence analysis with intermediate values of 1  ug/L (2 x EQL) and 2.5 ug/L (1/2 MCL).

4.2.2    Chlordane
The MCL for chlordane is based on a PQL of 2 ug/L. Its MCLG is zero, and there is no new
health effects information that suggests a change in the MCLG. Consequently, the threshold for
the occurrence analysis will be based on analytical feasibility. The PT data show greater than
80% passing rates for most of the studies including all studies near the PQL. There are no PT
studies with spiked values below the  current PQL, but three PE studies have spiked values below
the PQL and passing rates above 85%. EPA determined that the PQL assessment supports
reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-37 and Exhibit 4-38, 46.8% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 0.2 ug/L, and 63.8% of the MRL values are equal to
or less than the modal value. Less than 80% of the MRL values are equal to or less than the
modal MRL and, therefore, EPA did  not base the EQL on the modal MRL. Consequently, EPA
reviewed MDL values to determine whether they  support an EQL below the PQL.

                 Exhibit 4-37. Summary  of  MRL Data for Chlordane
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL
Value > Modal MRL (0.2 jig/L) and < 1.0 ng/L
Value >1.0(ig/L
Number of Records
57,506
9,764
26,893
15,453
5,396
Percentage of Records
100%
17.0%
46.8%
26.9%
9.4%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                          4-17

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                    Exhibit 4-38. MRL Distribution for Chlordane
   T3

   
   O
   Q.
   
CL
E
ra
W
      80%
      60%
      40%
                      46.8%
   "  20%
   3
Exhibit 4-39 shows the EPA approved methods for the detection of chlordane, and their MDLs.
Applying a multiplier of 10 would give a range of 0.04 to 2.2 ug/L. For an EQL, EPA multiplied
the median of the MDLs by ten and rounded to one significant digit, for a value of 1.0 ug/L.
Approximately 90% of the MRLs in the Six-Year Review 2 ICR dataset are less than or equal to
this value.

                   Exhibit 4-39. Analytical  Methods for Chlordane
Method
505
508
508.1
525.2
MDL (nq/L)
0.14
0.0041
0.004
0.22
                  Source: USEPA, 2009b


The EQL of 1.0 ug/L is half of the current PQL. Therefore, EPA performed the occurrence
analysis only with this EQL value and the current MCL (which is equal to the PQL).

4.2.3   1,2-Dibromo-3-Chloropropane (DBCP)
The MCL for DBCP is based on a PQL of 0.2 ug/L. Its MCLG is zero, and there is no new
health effects information that suggests a change in the MCLG. Consequently, the threshold for
the occurrence analysis will be based on analytical feasibility. The PT data show greater than
80% passing rates for most of the studies around the PQL, which include three studies with
spiked values below the current PQL. PE studies with spiked concentrations near the PQL also
have passing rates above 80% including one study with a spiked concentration below the PQL
EPA determined that PQL assessment may support reduction of the PQL (USEPA, 2009b).
                                         4-18

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations   October 2009

As shown in Exhibit 4-40 and Exhibit 4-41, only 44.1% of the MRL values in the Six-Year
Review 2 ICR dataset are less than or equal to MCL of 0.2 ug/L, and the modal MRL is greater
than the MCL. Because the modal MRL is greater than the PQL, EPA did not use the MRL data
to derive an EQL value. EPA reviewed MDL values to determine whether they support an EQL
below the PQL.

                    Exhibit 4-40. Summary of MRL Data for DBCP
MRL Value Category
All MRL Values
Value < MCL (0.2 ^g/L)
Value > MCL (0.2 ng/L) and < Modal MRL (0.5
MS/L)
Value > Modal MRL
Number of Records
85,801
37,844
34,813
13,144
Percentage of Records
100%
44.1%
40.6%
15.3%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                       Exhibit 4-41. MRL Distribution for DBCP
   O
   CL
   0.6
                                                               0.2
                                             MRL (ug/L)

Exhibit 4-42 shows the EPA approved methods for the detection of DBCP and their MDLs.
Applying a multiplier of 10 would give a possible PQL range from 0.09 to 0.1 ug/L. Averaging
these values and rounding to one significant figure gives an EQL of 0.1 ug/L. Almost 56% of the
MRL values are equal to or greater than the current MCL (0.2 ug/L), indicating that the
occurrence analysis of any EQL value below the current PQL could be biased by missing
occurrence values. Most of the MRL values below the current MCL are, however, also below the
EQL of 0.1 ug/L.
                                           4-19

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                     Exhibit 4-42. Analytical Methods for DBCP
Method
504.1
551.1
MDL (ng/L)
0.01
0.009
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
Because the PT data for DBCP suggests that there may be potential to lower the PQL, EPA
disregarded the MRL data and adopted an EQL of 0.1 ug/L based on the MDLs. The EQL of 0.1
ug/L is half of the current PQL. Therefore, EPA performed the occurrence analysis only with this
EQL value and the current MCL.

4.2.4   1,2-Dichloropropane
The MCL for 1,2-dichloropropane is based on a PQL of 5 ug/L. Its MCLG is zero, and there is
no new health effects information that suggests a change in the MCLG. Consequently, the
threshold for the occurrence analysis will be based on analytical feasibility. The PT data show
greater than 90% passing rates for most of the studies around the PQL, which include nine
studies with spiked concentrations below the current PQL. PE studies with spiked concentrations
near the PQL also have passing rates above 90%, but there are no PE studies with spiked
concentrations below the PQL. EPA determined that the PQL assessment supports reduction of
the PQL (USEPA, 2009b).

As  shown in Exhibit 4-43 and Exhibit 4-44, 86.1% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 0.5 ug/L, and 99.3% of the MRL values are equal to
or less than the modal value. Because more than 80% of the MRLs are equal to or less than 0.5
     , EPA based the EQL on the modal MRL.

            Exhibit 4-43. Summary of  MRL Data for 1,2-Dichloropropane
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.5 ng/L)
Value > Modal MRL
Number of Records
139,237
18,311
119,831
1,095
Percentage of Records
100%
13.2%
86.1%
0.8%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                          4-20

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

               Exhibit 4-44. MRL Distribution for 1,2-Dichloropropane
o
Q.
0.7
MRL (ug/L)
Exhibit 4-45 shows the EPA approved methods for the detection of 1,2-dichloropropane, and
their MDLs. Applying a multiplier of 10 would give a possible PQL range from 0.3 to 0.4 ug/L.
This range is lower than the modal MRL. Therefore, EPA retained 0.5 ug/L as the EQL value.

                      Exhibit 4-45. Analytical Methods for 1,2-
                                  Dichloropropane
Method
502.2
524.2
MDL (nq/L)
0.03
0.04
                 Source: USEPA, 2009b (upper bound values when ranges are reported)

The EQL of 0.5 ug/L is ten times lower than the current PQL. EPA also performed the
occurrence analysis with intermediate values of 1 ug/L (2 x EQL) and 2.5 ug/L (1/2 MCL).

4.2.5   Ethylene Dibromide (EDB)
The MCL for EDB is based on a PQL of 0.05 ug/L. Its MCLG is zero. A new health effects
assessment completed during the Six-Year Review 2 cycle did not change the MCLG. Therefore,
the threshold for an occurrence analysis would be based on analytical feasibility. There are no PT
or PE study results with spiked concentrations below the current PQL. Although most passing
rates are above 80%, a few studies have passing rates equal to 75%. Therefore, EPA determined
that the PT and PE data do not support revision of the PQL (USEPA, 2009b).

As shown in Exhibit 4-46 and Exhibit 4-47, the modal MRL for EDB is 0.01 ug/L.  Summary
data show that 32.9% of the MRL values are equal to or less than the modal MRL; 56.9% of the
MRL values are greater than the MCL. Less than 80% of the  MRL values are equal to or less
than the modal MRL. Therefore, EPA did not use the MRL data to derive an EQL value. EPA
reviewed MDL values to determine whether they support an EQL below the PQL.
                                         4-21

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EPA-OGWDW
                Development of Estimated Quantitation Levels for the
                                      EPA815-B-09-005
              Second Six-Year Review of National Primary Drinking Water Regulations  October 2009



                     Exhibit 4-46.  Summary of MRL Data for EDB
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL
Value > Modal MRL (0.01 ug/L) and < MCL (0.05 ug/L)
Value > MCL (0.05 ug/L)
Number of Records
83,063
454
26,926
8,416
47,267
Percentage of Records
100%
0.5%
32.4%
10.1%
56.9%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail

 in the accompanying chart because of independent rounding.

 Source: Six-Year Review 2 ICR dataset
                        Exhibit 4-47. MRL Distribution for EDB
   T3
   0.6

                                    0.05
                                             MRL (ug/L)




EPA approved methods for the detection of EDB, and their MDLs, are summarized in Exhibit

4-48. Applying a multiplier of 5 would give a possible PQL range from 0.05 to 0.16 ug/L. The

lower bound of this range is the current PQL. The MDL data do not support an EQL below the

PQL.



                       Exhibit 4-48. Analytical Methods for EDB
Method
504.1
551.1
MDL (ug/L)
0.01
0.032
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
EPA concluded that the combination of PT, MRL, and MDL data do not support revision of the

PQL for EDB. Therefore, EPA did not develop an EQL.
                                           4-22

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

4.2.6    Heptachlor
The MCL for heptachlor is based on a PQL of 0.4 ug/L. Its MCLG is zero, and there is no new
health effects information that suggests  a change in the MCLG. Consequently, the threshold for
the occurrence analysis will be based on analytical feasibility. The PT data have passing rates
above 75% for all but three of the studies with spiked values close to the PQL, but there are no
PT studies with spiked values below the current PQL.  Three PE studies had spiked
concentrations below the PQL and passing rates above 90%. EPA determined that the PQL
assessment may support reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-49 and Exhibit 4-50, 42.4% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 0.04 ug/L, and 56.0% of the MRL values are equal to
or less than the modal value. Less than 80% of the MRL values are equal to or less than the
modal MRL and, therefore, EPA did not base the EQL on the modal MRL. EPA reviewed MDL
values to determine whether they support an EQL below the PQL.

                 Exhibit 4-49. Summary of MRL Data for Heptachlor
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL
Value > Modal MRL (0.04 pg/L) and < 0.1 ^g/L
Value > 0.1 ng/L
Number of Records
58,758
7,966
24,918
24,752
1,122
Percentage of Records
100%
13.6%
42.4%
42.1%
1.9%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset

                    Exhibit 4-50. MRL Distribution for Heptachlor
   ^  80%

   T3
   0.2
                                          4-23

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

Exhibit 4-51 shows the EPA approved methods for the detection of heptachlor, and their MDLs.
EPA applied a multiplier of 10 to the median value of 0.005 ug/L and rounded up to 0.1 ug/L to
obtain an EQL. More than 98% of the MRLs in the  Six-Year Review 2 ICR dataset are less than
or equal to this value, indicating little potential for bias in occurrence estimates because of
missing occurrence data above an EQL of 0.1 ug/L.

                   Exhibit 4-51. Analytical Methods for Heptachlor
Method
505
508
508.1
525.2
551.1
MDL (nq/L)
0.003
0.0015
0.005
0.15
0.081
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
The EQL of 0.1 ug/L is one quarter of the current PQL. Therefore, EPA also performed the
occurrence analysis with an intermediate threshold value of 0.2 ug/L.

4.2.7    Heptachlor Epoxide
The MCL for heptachlor epoxide is based on a PQL of 0.2 ug/L. Its MCLG is zero, and there is
no new health effects information that suggests a change in the MCLG. Consequently, the
threshold for the occurrence analysis will be based on analytical feasibility. The PT data show
greater than 75% passing rates for all but two of the studies near the PQL, but there are no PT
studies with spiked values below the current PQL. Three PE studies had spiked values below the
PQL and passing rates above 85%. EPA determined that the PQL assessment may support
reduction of the PQL (USEPA, 2009b).

As  shown in Exhibit 4-52 and Exhibit 4-53, 45.0% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 0.02 ug/L, and 55.2% of the MRL values are equal to
or less than the modal value. Less than 80% of the MRL values are equal to or less than the
modal MRL and, therefore, EPA did not base the EQL on the modal MRL. EPA reviewed MDL
values to determine whether they support an EQL below the PQL.

             Exhibit 4-52. Summary of MRL Data for Heptachlor Epoxide
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL
Value > Modal MRL (0.02 pg/L) and < 0.1 ^g/L
Value > 0.1 ng/L
Number of Records
58,731
5,969
26,424
25,888
450
Percentage of Records
100%
10.2%
45.0%
44.1%
0.8%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                          4-24

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                Exhibit 4-53. MRL Distribution for Heptachlor Epoxide
ce
2
T3

O
CL
0.3
                                                                         0.2
Exhibit 4-54 shows the EPA approved methods for the detection of heptachlor epoxide, and
their MDLs. EPA applied a multiplier of 10 to the median value of 0.0059 ug/L and rounded up
to 0.1 ug/L to obtain an EQL. More than 99% of the MRLs in the Six-Year Review 2 ICR
dataset are less than or equal to this value, indicating little potential for bias in occurrence
estimates because of missing occurrence data above an EQL of 0.1 ug/L..

                   Exhibit 4-54. Analytical Methods for Heptachlor
                                        Epoxide
Method
505
508
508.1
525.2
551.1
MDL (nq/L)
0.004
0.0059
0.001
0.13
0.202
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
The EQL of 0.1 ug/L is half of the current PQL. Therefore, EPA performed the occurrence
analysis only with this EQL value and the current MCL.

4.2.8    Hexachlorobenzene
The MCL for hexachlorobenzene is based on a PQL of 1 ug/L. Its MCLG is zero, and there is no
new health effects information that suggests a change in the MCLG. Consequently, the threshold
for the occurrence analysis will be based on analytical feasibility. The PT data show greater than
75% passing rates for most of the studies around the PQL, which include nine studies with
spiked values below the current PQL. PE study results, including eight with spiked
concentrations below the PQL, have passing rates above 80% Although one PT study with a
                                          4-25

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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

spiked value below the PQL has a passing rate below 75%, EPA determined that the PQL
assessment supports reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-55 and Exhibit 4-56, 69.5% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 0.1 ug/L,  and 82.1% of the MRL values are equal to
or less than the modal value. Because more than 80% of the MRLs are equal to or less than 0.1
     , EPA based the EQL on the modal MRL.

             Exhibit 4-55. Summary of MRL Data for Hexachlorobenzene
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.1 ng/L)
Value > Modal MRL
Number of Records
58,713
7,380
40,791
10,542
Percentage of Records
100%
12.6%
69.5%
18.0%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                Exhibit 4-56. MRL Distribution for Hexachlorobenzene
   T3
   0.3
Exhibit 4-57 shows the EPA approved methods for the detection of hexachlorobenzene, and
their MDLs. Applying a multiplier of 10 would give a possible PQL range from 0.01 to 1.3 ug/L.
This range contains the modal MRL of 0.1 ug/L.
                                          4-26

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                         Exhibit 4-57. Analytical Methods for
                                 Hexachlorobenzene
Method
505
508
508.1
525.2
551.1
MDL (nq/L)
0.002
0.0077
0.001
0.13
0.003
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
The EQL of 0.1 ug/L is one-tenth the current PQL. Therefore, EPA also performed the
occurrence analysis with an intermediate threshold value of 0.5 ug/L.

4.2.9    Toxaphene
The MCL for toxaphene is based on a PQL of 3 ug/L. Its MCLG is zero, and there is no new
health effects information that suggests a change in the MCLG. Consequently, the threshold for
the occurrence analysis will be based on analytical feasibility. The PT studies have passing rates
above 85% for most of the studies around the PQL, but there are no PT studies with spiked
values below the current PQL. Two PT studies have passing rates below 75%. Three PE studies
have spiked concentrations below the PQL and passing rates above 90%. EPA determined that
the PQL assessment may support reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-58 and Exhibit 4-59, 67.4% of the MRL values in the Six-Year Review 2
ICR dataset are equal to the modal value of 1 ug/L, and 83.0% of the MRL values are equal to or
less than the modal value. Because more than 80% of the MRLs are equal to or less than 1 ug/L,
EPA based the EQL on the modal MRL.

                 Exhibit 4-58.  Summary of MRL Data for Toxaphene
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (1 ^g/L)
Value > Modal MRL
Number of Records
54,529
8,525
36,763
9,241
Percentage of Records
100%
15.6%
67.4%
16.9%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                          4-27

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 81 5-B-09-005
_ Second Six- Year Review of National Primary Drinking Water Regulations  October 2009
                    Exhibit 4-59. MRL Distribution for Toxaphene
1 80%
c
Q.
 60%

3
CO
4
Exhibit 4-60 shows the EPA approved methods for the detection of toxaphene, and their MDLs.
Applying a multiplier of 10 would give a possible PQL range from 1.3 to 17.0 ug/L. This range
exceeds the modal MRL of 1.0 ug/L.

                  Exhibit 4-60. Analytical Methods for Toxaphene
Method
505
508
508.1
525.2
MDL (ug/L)
1.0
No MDL reported
0.13
1.7
                  Source: USEPA, 2009b (upper bound values when ranges are reported)


The EQL of 1 ug/L is one-third the current PQL. Therefore, EPA also performed the occurrence
analysis with an intermediate value of 1.5 ug/L.

4.2.10  1,1,2-Trichloroethane
The MCL for 1,1,2-trichloroethane is based on a PQL of 5 ug/L. Its MCLG is 3 ug/L, and there
is no new health effects information that suggests a change in the MCLG. Because the MCLG is
lower than the current PQL, the lowest threshold of interest for the occurrence analysis is the
current MCLG of 3 ug/L. The PT results have passing rates above 90% for most of the studies
with spiked concentrations around the PQL, which include 12  studies with  spiked values below
the current PQL.  PE studies also have passing rates above 90% for studies with spiked
conentrations close to the PQL, although none are below the PQL. EPA determined that the PQL
assessment supports reduction of the PQL (USEPA, 2009b).
                                         4-28

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EPA-OGWDW          Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations   October 2009

As shown in Exhibit 4-61 and Exhibit 4-62, the MRL data for 1,1,2-trichloroethane share
general characteristics with other VOCs, e.g., a modal MRL of 0.5 ug/L. The exhibit also shows
that more than 99.9% of MRLs are at or below the MCLG. The MRL data thus indicate that it is
appropriate to use the MCLG as the threshold in the occurrence analysis.

             Exhibit 4-61. Summary of MRL Data for 1,1,2-Trichloroethane
MRL Value Category
All MRL Values
Value < MCLG (3 ng/L)
Value < Modal MRL
Value = Modal MRL (0.5 ng/L)
Value > Modal MRL
Number of Records
139,672
139,616
17,142
117,788
4,742
Percentage of Records
100%
>99.9%
12.3%
84.3%
3.4%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
               Exhibit 4-62. MRL Distribution for 1,1,2-Trichloroethane
   I  80%
   T3
   3
Exhibit 4-63 shows the EPA approved methods for the detection of 1,1,2-trichloroethane, and
their MDLs. Applying a multiplier of 10 would give a possible PQL range from 0.17 to 1.0 ug/L.
This range is below the current MCLG, which further supports use of the MCLG as the threshold
in the occurrence analysis.
                                            4-29

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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                      Exhibit 4-63. Analytical Methods for 1,1,2-
                                    Trichloroethane
Method
502.2
524.2
551.1
MDL (nq/L)
0.04
0.10
0.017
                  Source: USEPA, 2009b (upper bound values when ranges are reported)
An EQL based on the MRL data would be lower than the current MCLG for 1,1,2-
trichloroethane, and there is no information that suggests a potential to lower the MCLG.
Therefore, EPA used the current MCLG of 3 ug/L as the threshold in the occurrence analysis.
Since the current MCLG differs from the MCL by less than a factor of two, EPA did not use any
intermediate thresholds in the analysis.

4.2.11  Vinyl Chloride
The MCL for vinyl chloride is based on a PQL of 2 ug/L. Its MCLG is zero, and there is no new
health effects information that suggests a change in the MCLG. Consequently, the threshold for
the occurrence analysis will be based on analytical feasibility. The PT studies have passing rates
greater than 80% for most of the studies with spiked concentrations near the PQL, including two
studies with spiked values below the current PQL. PE data also have passing rates above 75% for
studies with spiked concentrations near the PQL, although none are below the PQL. EPA
determined that the PQL assessment may support reduction of the PQL (USEPA, 2009b).

As shown in Exhibit 4-64 and Exhibit 4-65, 75.6% of the MRL values in the  Six-Year Review 2
ICR dataset are equal to the modal value of 0.5 ug/L, and 94.0% of the MRL values are  equal to
or less than the modal value. Because more than 80% of the MRLs are equal to or less than 0.5
ug/L, EPA based the EQL on the modal MRL.

               Exhibit 4-64. Summary of MRL Data for Vinyl Chloride
MRL Value Category
All MRL Values
Value < Modal MRL
Value = Modal MRL (0.5 ng/L)
Value > Modal MRL
Number of Records
139,494
25,723
105,410
8,361
Percentage of Records
100%
18.4%
75.6%
6.0%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                          4-30

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                  Exhibit 4-65. MRL Distribution for Vinyl Chloride
1 80%
T3
O
Q.
0)
K 60%

3
in
3
MRL (ug/L)
Exhibit 4-66 shows the EPA approved methods for the detection of vinyl chloride, and their
MDLs. Applying a multiplier of 10 would give a possible PQL range from 1.8 to 1.7 ug/L,
which is slightly below the MCL of 2.0 ug/L. This range is also higher than the EQL of 0.5  ug/L
based on MRL data.

                     Exhibit 4-66. Analytical Methods for Vinyl
                                     Chloride
Method
502.2
524.2
MDL (nq/L)
0.18
0.17
                 Source: USEPA, 2009b (upper bound values when ranges are reported)
The EQL of 0.5 ug/L is one-fourth the current PQL. EPA also performed the occurrence analysis
with an intermediate value of 1 ug/L (2 x EQL and 1/2 MCL).

4.3   Contaminants with New Health Effects Information and Possible
MCLG Decrease
4.3.1   Endothall
The MCL for endothall is based on its MCLG of 100 ug/L. A new health effects assessment
indicates that there is potential to reduce the MCLG to 50 ug/L. The current PQL of 90 ug/L
would limit an MCL revision. There are no PT or PE study results with spiked values below the
current PQL. Furthermore, some passing rates for PT studies are below 75%. Because of the lack
of data below the PQL, and variability in the PT data,  EPA concluded that the available PT data
do not support PQL revision (USEPA, 2009b).
                                        4-31

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

In light of the new information about health effects of endothall, however, EPA needed to
determine whether it could examine occurrence at levels below the current PQL. EPA used MRL
and MDL data to evaluate the feasibility of a quantitation threshold equal to the possible MCLG
of50ug/L.

As shown in Exhibit 4-67 and Exhibit 4-68, 98.4% of the MRLs are at or below the possible
MCLG. The MRL data thus indicate that it is appropriate to use the possible MCLG as the
threshold in the occurrence analysis.

                   Exhibit 4-67. Summary of MRL Data for Endothall
MRL Value Category
All MRL Values
Value < Possible MCLG (50 ng/L)
Value < Modal MRL
Value = Modal MRL (9 ^g/L)
Value > Modal MRL
Number of Records
21,792
21,445
5,695
6,884
9,213
Percentage of Records
100%
98.4%
26.1%
31.6%
42.3%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                      Exhibit 4-68. MRL Distribution for Endothall
   O
   CL
   ssible   >60
                                                                            MCLG =
                                                                              50
Exhibit 4-69 shows the EPA approved method for the detection of endothall, and its MDL.
Because this MDL is more than a factor often lower than the possible MCLG, the MDL data
also support use of the possible MCLG as a threshold in the occurrence analysis.
                                           4-32

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EPA-OGWDW          Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                    Exhibit 4-69. Analytical Methods for Endothall
Method
548.1
MDL (ng/L)
1.79
                  Source: USEPA, 2009b (upper bound value when a range is reported)


EPA did not use any intermediate values in the occurrence analysis because the analysis
threshold is based on a possible MCLG.

4.3.2   Oxamyl
The MCL for oxamyl is based on its MCLG of 200 ug/L. A new health effects assessment
indicates that there is potential to reduce the MCLG to 2 ug/L. The present PQL of 20 ug/L
would limit an MCL revision. There are no PT study results below the current PQL, and some
passing rates in the available data are below 75%. Seven PE studies with spiked concentrations
below the PQL had passing rates at or below 75%, and none had passing rates above 75%. A
new method (EPA Method 531.2) for the analysis of oxamyl in drinking water was approved in
2002, however, and it may have improved laboratory performance at low concentrations in the
more recent PT data. Nevertheless, because of the lack of data below the PQL, and variability in
the data that is available, EPA concluded that the available PT data do not support PQL revision
(USEPA, 2009b).

In light of the new information about health effects of oxamyl, however, EPA needed to
determine whether it could examine occurrence at levels below the current PQL. EPA used MRL
and MDL data to evaluate the feasibility of a quantitation threshold equal to the possible MCLG
of 2
As shown in Exhibit 4-70 and Exhibit 4-71, the modal MRL for oxamyl is 2 ug/L, which is
equal to the possible MCLG. Summary data show that 61.2% of the MRLs are equal to this
value, and 86.7% of the MRL values are equal to or less than it. The MRL data thus indicate that
it is appropriate to use the possible MCLG as the threshold in the occurrence analysis.

                   Exhibit 4-70. Summary of MRL Data for Oxamyl
MRL Value Category
All MRL Values
Value < Modal MRL/Possible MCLG
Value = Modal MRL/Possible MCLG (2 ug/L)
Value > Modal MRL/Possible MCLG
Number of Records
52,201
13,335
31,955
6,911
Percentage of Records
100%
25.5%
61.2%
13.2%
 Percentages may not sum to 100% because of independent rounding. Aggregate percentages in the table may differ from detail
 in the accompanying chart because of independent rounding.
 Source: Six-Year Review 2 ICR dataset
                                          4-33

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                      Exhibit 4-71.  MRL Distribution for Oxamyl
      80%
      60%
                               61.2%
40%
   E"
   ra
   W
   o
   'E

   S  20%
   
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EPA-OGWDW         Development of Estimated Quantitation Levels for the     EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations   October 2009

5  References

Glaser, J.A, et al. 1981. "Trace analyses for wastewaters." Env. Sci. Technol. Vol. 15(12).

USEPA. 2003a. EPA Protocol for Review of Existing National Primary Drinking Water
Regulations. EPA 815-R-03-002. June.

USEPA. 2003b. Occurrence Estimation Methodology and Occurrence Findings Report for the
Six-Year Review of Existing National Primary Drinking Water Regulations. EPA-815-R-03-006.
June 2003.

USEPA. 2009a. Analysis of Occurrence Data from the Second Six-Year Review of Existing
National Primary Drinking Water Regulations. EPA 815-B-09-006.

USEPA. 2009b. Analytical Feasibility Support Document for the Second Six-Year Review of
Existing National Primary Drinking Water Regulations. EPA 815-B-09-003.

USEPA. 2009c. EPA Protocol for the Second Review of Existing National Primary Drinking
Water Regulations (Updated). EPA 815-B-09-002.

USEPA. 2009d. Occurrence Analysis for Potential Source Waters for the Second Six-Year
Review of National Primary Drinking Water Regulations. EPA 815-B-09-004.

USEPA. 2009e Six-Year Review 2 - Health Effects Assessment - Summary Report. EPA 822-
R-09-006.

USEPA. 2009f Water Treatment Technology Feasibility Support Document for Chemical
Contaminants for the Second Six-Year Review of National Primary Drinking Water Regulations.
EPA815-B-09-007.

USEPA. 2009f. Consideration of Other Regulatory Revisions in Support of the Second Six-Year
Review of the National Primary Drinking Water Regulations. EPA 815-B-09-008.
                                         5-1

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EPA-OGWDW         Development of Estimated Quantitation Levels for the      EPA 815-B-09-005
	Second Six-Year Review of National Primary Drinking Water Regulations  October 2009

                     Appendix: MRL Data Description
The MRL data used for the EQL analysis include some edits and transformations to address the
following data quality issues that directly affect EPA's efforts to derive EQL values using MRL
data:

   minimum detection level (MDL) values reported instead of MRL values3
   value not identified as either an MRL or MDL (i.e., only "<" reported in the result field)
   MRL or MDL units not identified (i.e., unclear whether value is reported in mg/L or ug/L)
   MRL value of zero reported
   no MRL value reported (i.e., the MRL field is blank).

To address the first two issues, EPA excluded observations that did not clearly identify a
nondetect value as an MRL. This led EPA to exclude data from 14 of the 47 states/territories in
the Second Six-Year Review Dataset.4 Because MDL values should be lower than MRL values,
excluding MDL observations should remove the lower values from the distribution of actual
MRL values used in the EQL analysis. Removing the non-identified values could, however,
reduce the number or MRL values, thereby  introducing uncertainty into the use of MRL data to
derive EQL values. Because the EQL development process depends on the mode or most
frequently occurring MRL value, deleting a substantial number of values could affect the result.

To address the next two data quality issues, EPA excluded approximately 1% of the remaining
MRL values because their units of measure were not reported and another 0.7% with reported
MRL values of zero.5 Both of these revisions affect relatively few values and, therefore, they
have no effect on the EQL development process.

Finally, when an MRL value was missing for a record, EPA replaced the missing value with the
most common MRL (i.e., the mode of the MRL distribution) reported in the given state for that
chemical. These values vary by state and contaminant.

To account for variations in reporting precision, EPA rounded all of the MRL values to one
significant digit. For example, a result of 0.132 ug/L rounds to 0.1 ug/L and a result of 132 ug/L
rounds to 100 ug/L. EPA used the resulting distribution for each contaminant to identify a modal
MRL as well as the percentage of MRLs below and above the mode.
3 An MDL is defined in Glaser et al. (1981) as "the minimum concentration of a substance that can be identified,
measured, and reported with 99% confidence that the analyte concentration is greater than zero and is determined
from replicate analyses of a sample from a given matrix containing the analyte." The lowest concentration at which
one can be almost certain the contaminant is present (i.e., MDL) is below the lowest concentration that can be
measured with some degree of reliability (i.e., MRL).
4 The original Six-Year Review 2 ICR dataset did not contain data from DC, MS, LA, KS, PA, and WA. In addition,
the following states did not provide usable MRL data: AR, CA, FL, HI, MA, MD, MI, MN, NH, SD, TN, TX, and
WI.
5 The ability of any instrument or method to measure contaminant levels within specific precision bounds will be
exhausted above zero, so no MRL can equal zero.
                                           A-1

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