United States 

 Environmental Protection 

 Agency 

Development of Estimated Quantitation Levels 

for the Second Six-Year Review of 

 National Primary Drinking Water Regulations 
Office of Water (4607M) 

EPA 815-B-09-005 

October 2009 

www.epa.gov/safewater 
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 MRL Data .................................................................................................................... 2-1 

2.2 MDL Data .................................................................................................................... 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 Polychlorinated Biphenyls (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 1,2-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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Second Six-Year Review of National Primary Drinking Water Regulations October 2009 

Exhibit 4-37. Summary of MRL Data for Chlordane................................................................ 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 2,4-dichlorophenoxyacetic acid 

BAT best available technology 

DBCP 1,2-dibromo-3-chloropropane 

DEHP di(2-ethylhexyl)phthalate 

EDB ethylene dibromide 

EPA U.S. Environmental Protection Agency 

EQL estimated quantitation level 

ICR Information Collection Request 

MCL maximum contaminant level 

MCLG maximum contaminant level goal 

MDL method detection limit 

mg/L milligrams per liter 

μg/L micrograms per liter 

MRL minimum reporting level 

NPDWR National Primary Drinking Water Regulation 

PCBs polychlorinated biphenyls 

PQL practical quantitation level 

PE performance evaluation 

PT proficiency testing 

SDWA Safe Drinking Water Act 

SOC synthetic organic compound 

VOC volatile organic compounds 

WS water supply 

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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
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
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 μg/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 of ten. 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
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 

MRL Distribution 

Contaminant Type 

Current 

PQL 

Threshold

(EQL or 

healthbased) 

Basis for 

Threshold % = mode % ≤ mode 

Ongoing health effects assessment, MCL limited by PQL 

Benzo[a]pyrene SOC 0.2 μg/L na na 53.7 55.4 

Carbon tetrachloride VOC 5 μg/L 0.5 μg/L Modal MRL 86.1 97.7 

DEHP SOC 6 μg/L na na 45.5 76.9 

1,2-Dichloroethane VOC 5 μg/L 0.5 μg/L Modal MRL 83.8 96.9 

Dichloromethane VOC 5 μg/L 0.5 μg/L Modal MRL 87.8 96.0 

Pentachlorophenol SOC 1 μg/L na na 44.7 48.7 

PCBs SOC 0.5 μg/L na na 67.6 74.3 

Dioxin SOC 3×10-5 μg/L na na 75.4 90.3 

Tetrachloroethylene VOC 5 μg/L 0.5 μg/L Modal MRL 84.6 96.1 

Thallium IOC 2 μg/L na na 63.0 78.0 

Trichloroethylene VOC 5 μg/L 0.5 μg/L Modal MRL 85.4 97.7 

No new health effects assessment or new health effects assessment indicates no MCLG change, 

MCL is limited by PQL 

Benzene VOC 5 μg/L 0.5 μg/L Modal MRL 86.4 99.3 

Chlordane SOC 2 μg/L 1 μg/L 10×MDL 46.8 63.8 

DBCP SOC 0.2 μg/L 0.1 μg/L 10×MDL 35.6 84.7 

1,2-Dichloropropane VOC 5 μg/L 0.5 μg/L Modal MRL 86.1 99.3 

EDB SOC 0.05 μg/L na na 32.4 32.9 

Heptachlor SOC 0.4 μg/L 0.1 μg/L 10×MDL 42.4 56.0 

Heptachlor epoxide SOC 0.2 μg/L 0.1 μg/L 10×MDL 45.0 55.2 

Hexachlorobenzene SOC 1 μg/L 0.1 μg/L Modal MRL 69.5 82.1 

Toxaphene SOC 3 μg/L 1 μg/L Modal MRL 67.4 83.0 

1,1,2-Trichloroethane VOC 5 μg/L 3 μg/L1 Current MCLG 99.9% below current MCLG 

Vinyl chloride VOC 2 μg/L 0.5 μg/L Modal MRL 75.6 94.0 

New health effects assessment indicates possible MCLG below PQL 

Endothall SOC 90 μg/L 50 μg/L1 Possible MCLG 98.4% below possible MCLG 

Oxamyl SOC 50 μg/L 2 μg/L1 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
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 SixYear 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. 

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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. 

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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 0 0.2 μg/L 0.2 μg/L 03 No 

Carbon Tetrachloride 0 5 μg/L 5 μg/L 03 Yes 

Di (2-ethylhexyl)phthalate (DEHP) 0 6 μg/L 6 μg/L 03 No 

1,2-Dichloroethane 0 5 μg/L 5 μg/L 03 Yes 

Dichloromethane 0 5 μg/L 5 μg/L 03 No 

Pentachlorophenol 0 1 μg/L 1 μg/L 03 No 

Polychlorinated biphenyls (PCBs) 0 0.5 μg/L 0.5 μg/L 03 No 

2,3,7,8-TCDD (Dioxin) 0 3×10-5 μg/L 3×10-5 μg/L 03 No 

Tetrachloroethylene 0 5 μg/L 5 μg/L 03 Yes 

Thallium 0.5 μg/L 2 μg/L 2 μg/L Not determined4 No 

Trichloroethylene 0 5 μg/L 5 μg/L 03 Yes 

No new health effects assessment or new health effects assessment indicates no MCLG change, 

MCL is limited by PQL 

Benzene 0 5 μg/L 5 μg/L No new HEA Yes 

Chlordane 0 2 μg/L 2 μg/L No new HEA Yes 

1,2-Dibromo-3-Chloropropane 

(DBCP) 

0 

0.2 μg/L 0.2 μg/L No new HEA Possibly 

1,2-Dichloropropane 0 5 μg/L 5 μg/L No new HEA Yes 

Ethylene Dibromide (EDB) 

0 

0.05 μg/L 0.05 μg/L 

No MCLG 

change No 

Heptachlor 0 0.4 μg/L 0.4 μg/L No new HEA Possibly 

Heptachlor Epoxide 0 0.2 μg/L 0.2 μg/L No new HEA Possibly 

Hexachlorobenzene 0 1 μg/L 1 μg/L No new HEA Yes 

Toxaphene 0 3 μg/L 3 μg/L No new HEA Possibly 

1,1,2-Trichloroethane 3 μg/L 5 μg/L 5 μg/L No new HEA Yes 

Vinyl Chloride 0 2 μg/L 2 μg/L No new HEA Possibly 

New health effects assessment indicates possible MCLG below PQL 

Endothall 100 μg/L 90 μg/L 100 μg/L 50 μg/L5 No 

Oxamyl 200 μg/L 20 μg/L 200 μg/L 2 μg/L5 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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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 contaminantspecific information and results.

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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 

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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 “<MRL” (i.e., less than the MRL) in the result field of the 

dataset and a numeric MRL value in the MRL field of the dataset. Because of inconsistencies in 

data entry or reporting across laboratories or states, EPA performed a variety of data quality 

checks and data transformations on the MRL data in consultation with state data management 

staff. USEPA (2009a) describes the data management process, including measures taken to 

address data quality concerns that affect the occurrence and exposure analysis; the Appendix 

provides summary information from this process. 

2.2 MDL Data 

The MDL multiplier approach for estimating an EQL applies a multiplier of five or ten to an 

MDL. An MDL is a measure of analytical method sensitivity (USEPA, 2009b), defined in 40 

CFR Part 136 Appendix B as “the minimum concentration of a substance that can be reported 

with 99% confidence that the analyte concentration is greater than zero” for a given method. 

Although EPA has used this method to establish PQLs in the past, EPA is not using MDLs for 

this purpose during Six-Year Review 2. Instead, EPA is using the MDL approach to help identify 

EQLs below current PQLs for the occurrence and exposure analysis. 

MDLs can vary by analytical method and contaminant. USEPA (2009b) provides complete 

information for MDLs by contaminant and analytical method. The MDL values or ranges of 

values in USEPA (2009b) are for the approved analytical methods developed by EPA for 

drinking water compliance monitoring. 

Summary data by contaminant and method in Section 4 of this document includes only upper 

bound values for any MDL ranges reported in USEPA (2009b).2

 EPA used only upper bound 

values when there was an MDL range for a particular method and contaminant in an effort to 

derive an EQL value that would represent a level where most laboratories should be able to 

quantitate; the lower bound value for each method’s MDL range is likely to result in an EQL that 

would represent analytical capabilities of fewer laboratories. 

2.3 Data Summary 

Exhibit 2-1 provides a summary of the information used to develop EQL values: the modal 

MRL values from the Six-Year Review 2 MRL data and a range of upper bound MDL values 

across the analytical methods developed by EPA. The exhibit also reports the current PQL for 

comparisons purposes. The modal MRL values for Six-Year Review 2 are generally lower than 

the PQL values. The MDL ranges for several contaminants, however, are not low enough to 

 

2

 For some methods in USEPA (2009b), the reported MDL is range; for others the MDL is a single value. For 

example, the MDL range for analysis of carbon tetrachloride using EPA Method 502.2 is 0.01 – 0.02 μg/L. In the 

EQL analysis, EPA used only the upper bound values whenever USEPA (2009b) reported an MDL range; i.e., 0.02 

μg/L for EPA Method 502.2 for carbon tetrachloride. 

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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 PQL (μg/L)

Six Year 2 Modal MRL 

(μg/L) 

MDL Range for EPA Methods 

(μg/L) 

Ongoing health effects assessment, MCL limited by PQL 

Benzo[a]pyrene 0.2 0.02 0.016 - 0.23 

Carbon Tetrachloride 5 0.5 0.02 - 0.21 

DEHP 6 1.0 1.3 - 2.25 

1,2-Dichloroethane 5 0.5 0.03 - 0.06 

Dichloromethane 5 0.5 0.02 - 0.09 

Pentachlorophenol 1 0.04 0.032 - 1.6 

PCBs 0.5 0.1 0.08 

Dioxin 0.00003 0.000005 0.00001 (ML) 

Tetrachloroethylene 5 0.5 0.008 - 0.14 

Thallium 2 1 0.3 - 0.7 

Trichloroethylene 5 0.5 0.042 - 0.19 

No new health effects assessment or new health effects assessment indicates no MCLG change, 

MCL is limited by PQL 

Benzene 5 0.5 0.01 - 0.04 

Chlordane 2 0.2 0.004 - 0.22

DBCP 0.2 0.5 0.009 - 0.01 

1,2-Dichloropropane 5 0.5 0.03 - 0.04 

EDB 0.05 0.01 0.01 - 0.032 

Heptachlor 0.4 0.04 0.0015 - 0.15 

Heptachlor Epoxide 0.2 0.02 0.001 - 0.202 

Hexachlorobenzene 1 0.1 0.001 - 0.13 

Toxaphene 3 1 0.13 - 1.7 

1,1,2-Trichloroethane 5 0.5 0.017 - 0.1 

Vinyl chloride 2 0.5 0.17 - 0.18 

New health effects assessment indicates possible MCLG below PQL 

Endothall 90 9 1.79 

Oxamyl 50 2 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 μg/L, which is higher than the PQL of 

0.2 μg/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 μg/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. 

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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 

Review PQL

Analysis

Findings

No potential

to revise

Potential

to revise

Review MRL

data for

feasible EQL

Review MRL

data for

feasible EQL

EQL

identified

No EQL

identified

Review MDL

data to

confirm EQL

Review MDL

data to

identify EQL

EQL

Identified

No EQL 

identified

Review MDL

data to

Identify EQL

Review MDL

data to

confirm EQL

Carbon tetrachloride

1,2-Dichloroethane

Tetrachloroethylene

Trichloroethylene

Benzene

1,2-Dichloropropane

Hexachlorobenzene

Toxaphene

1,1,2-Trichloroethane*

Vinyl chloride

Chlordane

DBCP

Heptachlor

Heptachlor epoxide

Dichloromethane

Endothall*

Oxamyl*

Benzo[a]pyrene

DEHP

Pentachlorophenol

PCBs

Dioxin

Thallium*

* Health-based threshold instead of EQL EDB

Review PQL

Analysis

Findings

No potential

to revise

Potential

to revise

Review MRL

data for

feasible EQL

Review MRL

data for

feasible EQL

EQL

identified

No EQL

identified

Review MDL

data to

confirm EQL

Review MDL

data to

identify EQL

EQL

Identified

No EQL 

identified

Review MDL

data to

Identify EQL

Review MDL

data to

confirm EQL

Carbon tetrachloride

1,2-Dichloroethane

Tetrachloroethylene

Trichloroethylene

Benzene

1,2-Dichloropropane

Hexachlorobenzene

Toxaphene

1,1,2-Trichloroethane*

Vinyl chloride

Chlordane

DBCP

Heptachlor

Heptachlor epoxide

Dichloromethane

Endothall*

Oxamyl*

Benzo[a]pyrene

DEHP

Pentachlorophenol

PCBs

Dioxin

Thallium*

EDB

Benzo[a]pyrene

DEHP

Pentachlorophenol

PCBs

Dioxin

Thallium*

* Health-based threshold instead of EQL EDB

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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 μg/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 of ten. 

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. 

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Exhibit 3-2. Summary of Threshold Determination 

MRL Distribution1

Contaminant Type 

Current 

PQL 

Threshold 

(EQL or 

healthbased) 

Basis for 

Threshold % = mode % ≤ mode 

Ongoing health effects assessment, MCL limited by PQL 

Benzo[a]pyrene SOC 0.2 μg/L na na 53.7 55.4 

Carbon tetrachloride VOC 5 μg/L 0.5 μg/L Modal MRL 86.1 97.7 

DEHP SOC 6 μg/L na na 45.5 76.9 

1,2-Dichloroethane VOC 5 μg/L 0.5 μg/L Modal MRL 83.8 96.9 

Dichloromethane VOC 5 μg/L 0.5 μg/L Modal MRL 87.8 96.0 

Pentachlorophenol SOC 1 μg/L na na 44.7 48.7 

PCBs SOC 0.5 μg/L na na 67.6 74.3 

Dioxin SOC 3×10-5 μg/L na na 75.4 90.3 

Tetrachloroethylene VOC 5 μg/L 0.5 μg/L Modal MRL 84.6 96.1 

Thallium IOC 2 μg/L na na 63.0 78.0 

Trichloroethylene VOC 5 μg/L 0.5 μg/L Modal MRL 85.4 97.7 

No new health effects assessment or new health effects assessment indicates no MCLG change, 

MCL is limited by PQL 

Benzene VOC 5 μg/L 0.5 μg/L Modal MRL 86.4 99.3 

Chlordane SOC 2 μg/L 1 μg/L 10×MDL 46.8 63.8 

DBCP SOC 0.2 μg/L 0.1 μg/L 10×MDL 35.6 84.7 

1,2-Dichloropropane VOC 5 μg/L 0.5 μg/L Modal MRL 86.1 99.3 

EDB SOC 0.05 μg/L na na 32.4 32.9 

Heptachlor SOC 0.4 μg/L 0.1 μg/L 10×MDL 42.4 56.0 

Heptachlor epoxide SOC 0.2 μg/L 0.1 μg/L 10×MDL 45.0 65.2 

Hexachlorobenzene SOC 1 μg/L 0.1 μg/L Modal MRL 69.5 82.1 

Toxaphene SOC 3 μg/L 1 μg/L Modal MRL 67.4 83.0 

1,1,2-Trichloroethane VOC 5 μg/L 3 μg/L1 Current MCLG 99.9% below current MCLG 

Vinyl chloride VOC 2 μg/L 0.5 μg/L Modal MRL 75.6 94.0 

New health effects assessment indicates possible MCLG below PQL 

Endothall SOC 90 μg/L 50 μg/L1 Possible MCLG 98.4% below possible MCLG 

Oxamyl SOC 50 μg/L 2 μg/L1 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. 

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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 μg/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 μg/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 Number of Records Percentage of Records 

All MRL Values 55,487 100% 

Value < Modal MRL 970 1.7% 

Value = Modal MRL (0.02 μg/L) 29,769 53.7% 

Value > Modal MRL and ≤ 0.1 μg/L 21,198 38.2% 

Value > 0.1 μg/L 3,550 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 

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Exhibit 4-2. MRL Distribution for Benzo[a]pyrene 

1.7%

53.7%

0.7% 2.0% 1.2%

8.8%

0.0% 0.1% 0.7%

24.7%

6.2%

0.2%

0%

20%

40%

60%

80%

≤0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 MCL =

0.2

≥0.3

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L, which is higher 

than the current PQL of 0.2 μg/L. The MDL data do not support an EQL below the PQL. 

Exhibit 4-3. Analytical Methods for 

Benzo[a]pyrene 

Method MDL (μg/L) 

525.2 0.23 

550 0.029 

550.1 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 μg/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 μg/L, and 97.7% of the MRL values are equal to or less than the modal 

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value. Because more than 80% of the MRLs are equal to or less than 0.5 μg/L, EPA based the 

EQL on the modal MRL. 

Exhibit 4-4. Summary of MRL Data for Carbon Tetrachloride 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 139,221 100% 

Value < Modal MRL 16,195 11.6% 

Value = Modal MRL (0.5 μg/L) 119,849 86.1% 

Value > Modal MRL 3,177 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 

0.6% 2.1% 3.1% 3.6% 2.2%

86.1%

2.2% 0.0% 0.0% 0.0% 0.1%

0%

20%

40%

60%

80%

≤0.09 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ≥1

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L. This 

range contains the modal MRL. Therefore, EPA retained 0.5 μg/L as the EQL value. 

Exhibit 4-6. Analytical Methods for Carbon 

Tetrachloride 

Method MDL (μg/L) 

502.2 0.02 

524.2 0.21 

551.1 0.050 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.5 μg/L is ten times lower than the current PQL. EPA also performed the 

occurrence analysis with intermediate values of 1 μg/L (2 x EQL) and 2.5 μg/L (1/2 MCL). 

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4.1.3 Di(2-ethylhexyl)phthalate (DEHP) 

The MCL for DEHP is based on a PQL of 6 μg/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 μg/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 Number of Records Percentage of Records 

All MRL Values 50,490 100% 

Value < Modal MRL 15,842 31.4% 

Value = Modal MRL (1 μg/L) 22,980 45.5% 

Value > Modal MRL 11,668 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 

3.7%

0.6%

26.9%

0.0% 0.0% 0.1%

45.5%

16.4%

1.8% 2.9% 1.6% 0.4% 0.0%

0%

20%

40%

60%

80%

≤0.4 0.5 0.6 0.7 0.8 0.9 1 2 3 4 5 MCL = 6 ≥7

MRL (ug/L)

Percent of Samples with Reported MRL

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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 μg/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 MDL (μg/L) 

506 2.25 

525.2 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 μg/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 μg/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 

μg/L, EPA based the EQL on the modal MRL. 

Exhibit 4-10. Summary of MRL Data for 1,2-Dichloroethane 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 139,085 100% 

Value < Modal MRL 18,160 13.1% 

Value = Modal MRL (0.5 μg/L) 116,533 83.8% 

Value > Modal MRL 4,392 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 

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Exhibit 4-11. MRL Distribution for 1,2-Dichloroethane 

0.7% 0.5% 0.0% 2.8% 3.8% 3.8% 1.6%

83.8%

2.3% 0.0% 0.7% 0.1%

0%

20%

40%

60%

80%

≤0.07 0.08 0.09 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 ≥0.9

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L. This 

range contains the modal MRL. Therefore, EPA retained 0.5 μg/L as the EQL value. 

Exhibit 4-12. Analytical Methods for 1,2-

Dichloroethane 

Method MDL (μg/L) 

502.2 0.03 

524.2 0.06 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.5 μg/L is ten times lower than the current PQL. EPA also performed the 

occurrence analysis with intermediate values of 1 μg/L (2 x EQL) and 2.5 μg/L (1/2 MCL). 

4.1.5 Dichloromethane 

The MCL for dichloromethane is based on a PQL of 5 μg/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 μg/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 

μg/L, EPA based the EQL on the modal MRL. 

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Exhibit 4-13. Summary of MRL Data for Dichloromethane 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 138,625 100% 

Value < Modal MRL 11,294 8.2% 

Value = Modal MRL (0.5 μg/L) 121,532 87.8% 

Value > Modal MRL 5.619 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 

1.1% 0.5% 2.2% 2.5% 1.8%

87.8%

1.4% 0.0% 0.0% 0.0% 1.3% 1.3%

0%

20%

40%

60%

80%

≤0.09 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ≥2

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L. This 

range contains the modal MRL. Therefore, EPA retained 0.5 μg/L as the EQL value. 

Exhibit 4-15. Analytical Methods for 

Dichloromethane 

Method MDL (μg/L) 

502.2 0.02 

524.2 0.09 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.5 μg/L is ten times lower than the current PQL. EPA also performed the 

occurrence analysis with intermediate values of 1 μg/L (2 x EQL) and 2.5 μg/L (1/2 MCL). 

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4.1.6 Pentachlorophenol 

The MCL for pentachlorophenol is based on a PQL of 1 μg/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 μg/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 Number of Records Percentage of Records 

All MRL Values 59,594 100% 

Value < Modal MRL 2,399 4.0% 

Value = Modal MRL (0.04 μg/L) 26,666 44.7% 

Value > Modal MRL 30,529 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 

4.0%

44.7%

0.9% 0.0% 0.0% 0.6% 1.6%

19.4%

9.4%

0.2% 0.9%

5.6%

12.5%

0%

20%

40%

60%

80%

≤0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.2 0.3 0.4 0.5 ≥0.6

MRL (ug/L)

Percent of Samples with Reported MRL

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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 μg/L with a median of 

1.225 μg/L. This range does not contain the modal MRL and most of the 10 × 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 MDL (μg/L) 

515.1 0.032 

515.2 0.16 

515.3 0.085 

515.4 0.084 

525.2 1.0 

555 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 μg/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 μg/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 Number of Records Percentage of Records 

All MRL Values 35,178 100% 

Value < Modal MRL 2,355 6.7% 

Value = Modal MRL (0.1 μg/L) 23,785 67.6% 

Value > Modal MRL 9,038 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 

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Exhibit 4-20. MRL Distribution for PCBs 

2.1% 0.1% 2.3% 2.0% 0.2% 0.0%

67.6%

4.9%

0.1%

10.4%

5.9% 4.4%

0%

20%

40%

60%

80%

≤0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.2 0.3 0.4 MCL =

0.5

≥0.6

MRL (ug/L)

Percent of Samples with Reported MRL

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 0.8 

μg/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 MDL (μg/L) 

508A 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 3×10-5 μg/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 5×10-6 μg/L. 

Summary data show that 75.4% of the MRLs are equal to this value, and 90.3% of the MRL 

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values are equal to or less than it. Because the majority of MRLs equal 5×10-6 μg/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 Number of Records Percentage of Records 

All MRL Values 1,278 100% 

Value < Modal MRL 191 14.9% 

Value = Modal MRL (0.000005 μg/L) 964 75.4% 

Value > Modal MRL 123 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 

13.2%

1.7% 0.3% 0.5% 0.5% 0.5% 0.2% 0.9% 0.4%

6.5%

75.4%

0%

20%

40%

60%

80%

≤0.000003 0.000004 0.000005 0.000006 0.000007 0.000008 0.000009 0.00001 0.00002 MCL =

0.00003

≥0.00004

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/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 MDL (μg/L) 

1613 1×10-5 (ML)

ML = minimum level 

Source: USEPA, 2009b and 40 CFR 141.24 (Organic chemicals, sampling 

and analytical requirements). 

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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 μg/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 μg/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 

μg/L, EPA based the EQL on the modal MRL. 

Exhibit 4-25. Summary of MRL Data for Tetrachloroethylene 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 138,348 100% 

Value < Modal MRL 15,848 11.5% 

Value = Modal MRL (0.5 μg/L) 117,033 84.6% 

Value > Modal MRL 5,467 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 

0.4% 0.2% 0.4% 1.7%

4.7% 3.1% 1.0%

84.6%

2.9% 0.9% 0.2%

0%

20%

40%

60%

80%

≤0.07 0.08 0.09 0.1 0.2 0.3 0.4 0.5 0.6 0.7 ≥0.8

MRL (ug/L)

Percent of Samples with Reported MRL

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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 μg/L. 

This range contains the modal MRL. Therefore, EPA retained 0.5 μg/L as the EQL value. 

Exhibit 4-27. Analytical Methods for 

Tetrachloroethylene 

Method MDL (μg/L) 

502.2 0.05 

524.2 0.14 

551.1 0.008 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.5 μg/L is ten times lower than the current PQL. EPA also performed the 

occurrence analysis with intermediate values of 1 μg/L (2 x EQL) and 2.5 μg/L (1/2 MCL). 

4.1.10 Thallium 

The MCL for thallium is based on a PQL of 2 μg/L. Its MCLG is 0.5 μg/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 μg/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 μg/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 Number of Records Percentage of Records 

All MRL Values 73,409 100% 

Value < Modal MRL 11,032 15.0% 

Value = Modal MRL (1 μg/L) 46,273 63.0% 

Value > Modal MRL 16,104 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 

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Exhibit 4-29. MRL Distribution for Thallium 

1.7% 2.0% 1.5%

9.5%

0.0% 0.1% 0.0% 0.1%

63.0%

21.7%

0.2%

0%

20%

40%

60%

80%

≤0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 MCL = 2 ≥3

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/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 MDL (μg/L) 

200.8 0.3 

200.9 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 μg/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 μg/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 

μg/L, EPA based the EQL on the modal MRL. 

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Exhibit 4-31. Summary of MRL Data for Trichloroethylene 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 138,439 100% 

Value < Modal MRL 17,057 12.3% 

Value = Modal MRL (0.5 μg/L) 118,193 85.4% 

Value > Modal MRL 3,189 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 

0.5% 0.1% 0.4% 0.2% 2.1% 4.4% 2.6% 2.0%

85.4%

2.2% 0.1%

0%

20%

40%

60%

80%

≤0.06 0.07 0.08 0.09 0.1 0.2 0.3 0.4 0.5 0.6 ≥0.7

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L. This 

range contains the modal MRL. Therefore, EPA retained 0.5 μg/L as the EQL value. 

Exhibit 4-33. Analytical Methods for 

Trichloroethylene 

Method MDL (μg/L) 

502.2 0.06 

524.2 0.19 

551.1 0.042 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.5 μg/L is ten times lower than the current PQL. EPA also performed the 

occurrence analysis with intermediate values of 1 μg/L (2 x EQL) and 2.5 μg/L (1/2 MCL). 

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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 μg/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 μg/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 

μg/L, EPA based the EQL on the modal MRL. 

Exhibit 4-34. Summary of MRL Data for Benzene 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 139,190 100% 

Value < Modal MRL 17,964 12.9% 

Value = Modal MRL (0.5 μg/L) 120,328 86.4% 

Value > Modal MRL 898 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 

1.5% 3.0% 3.9% 3.5% 1.0%

86.4%

0.5% 0.0% 0.0% 0.0% 0.1% 0.1%

0%

20%

40%

60%

80%

≤0.09 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 ≥2

MRL (ug/L)

Percent of Samples with Reported MRL

4-16 
EPA-OGWDW Development of Estimated Quantitation Levels for the EPA 815-B-09-005 

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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 μg/L. This range is 

lower than the modal MRL. Therefore, EPA retained 0.5 μg/L as the EQL value. 

Exhibit 4-36. Analytical Methods for Benzene 

Method MDL (μg/L) 

502.2 0.01 

524.2 0.04 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.5 μg/L is ten times lower than the current PQL. EPA also performed the 

occurrence analysis with intermediate values of 1 μg/L (2 x EQL) and 2.5 μg/L (1/2 MCL). 

4.2.2 Chlordane 

The MCL for chlordane is based on a PQL of 2 μg/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 μg/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 Number of Records Percentage of Records 

All MRL Values 57,506 100% 

Value < Modal MRL 9,764 17.0% 

Value = Modal MRL 26,893 46.8% 

Value > Modal MRL (0.2 μg/L) and ≤ 1.0 μg/L 15,453 26.9% 

Value > 1.0 μg/L 5,396 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 
EPA-OGWDW Development of Estimated Quantitation Levels for the EPA 815-B-09-005 

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Exhibit 4-38. MRL Distribution for Chlordane 

5.5%

11.5%

46.8%

7.0% 9.4%

1.7% 3.9%

0.0% 0.1% 0.0%

4.7%

9.4%

0.0%

0%

20%

40%

60%

80%

≤0.09 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 MCL = 2 ≥3

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/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 μg/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 MDL (μg/L) 

505 0.14 

508 0.0041 

508.1 0.004 

525.2 0.22 

Source: USEPA, 2009b 

The EQL of 1.0 μg/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 μg/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). 

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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 μg/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 Number of Records Percentage of Records 

All MRL Values 85,801 100% 

Value ≤ MCL (0.2 μg/L) 37,844 44.1% 

Value > MCL (0.2 μg/L) and ≤ Modal MRL (0.5 

μg/L) 34,813 40.6% 

Value > Modal MRL 13,144 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 

7.0%

30.0%

0.4% 2.7% 0.8% 0.1% 0.0% 0.0% 0.0% 0.6% 2.6% 2.6% 2.4%

15.3%

35.6%

0%

20%

40%

60%

80%

≤0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 MCL =

0.2

0.3 0.4 0.5 ≥0.6

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L. Averaging 

these values and rounding to one significant figure gives an EQL of 0.1 μg/L. Almost 56% of the 

MRL values are equal to or greater than the current MCL (0.2 μg/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 μg/L. 

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Exhibit 4-42. Analytical Methods for DBCP 

Method MDL (μg/L) 

504.1 0.01 

551.1 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 μg/L based on the MDLs. The EQL of 0.1 

μg/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 μg/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 μg/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 

μg/L, EPA based the EQL on the modal MRL. 

Exhibit 4-43. Summary of MRL Data for 1,2-Dichloropropane 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 139,237 100% 

Value < Modal MRL 18,311 13.2% 

Value = Modal MRL (0.5 μg/L) 119,831 86.1% 

Value > Modal MRL 1,095 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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Exhibit 4-44. MRL Distribution for 1,2-Dichloropropane 

0.9% 0.0% 0.0% 0.0% 0.0% 1.7% 4.4% 5.6%

0.5%

86.1%

0.7% 0.1%

0%

20%

40%

60%

80%

≤0.05 0.06 0.07 0.08 0.09 0.1 0.2 0.3 0.4 0.5 0.6 ≥0.7

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L. 

This range is lower than the modal MRL. Therefore, EPA retained 0.5 μg/L as the EQL value. 

Exhibit 4-45. Analytical Methods for 1,2-

Dichloropropane 

Method MDL (μg/L) 

502.2 0.03 

524.2 0.04 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.5 μg/L is ten times lower than the current PQL. EPA also performed the 

occurrence analysis with intermediate values of 1 μg/L (2 x EQL) and 2.5 μg/L (1/2 MCL). 

4.2.5 Ethylene Dibromide (EDB) 

The MCL for EDB is based on a PQL of 0.05 μg/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 μg/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.

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Exhibit 4-46. Summary of MRL Data for EDB 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 83,063 100% 

Value < Modal MRL 454 0.5% 

Value = Modal MRL 26,926 32.4% 

Value > Modal MRL (0.01 μg/L) and ≤ MCL (0.05 μg/L) 8,416 10.1% 

Value > MCL (0.05 μg/L) 47,267 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 

0.5%

32.4%

4.4%

0.5%

3.8% 1.4% 0.0% 0.1% 0.0% 0.0%

11.1%

5.0% 6.6%

25.8%

6.0%

2.3%

0%

20%

40%

60%

80%

≤0.009 0.01 0.02 0.03 0.04 MCL =

0.05

0.06 0.07 0.08 0.09 0.1 0.2 0.3 0.4 0.5 ≥0.6

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/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 MDL (μg/L) 

504.1 0.01 

551.1 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. 

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4.2.6 Heptachlor 

The MCL for heptachlor is based on a PQL of 0.4 μg/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 μg/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 Number of Records Percentage of Records 

All MRL Values 58,758 100% 

Value < Modal MRL 7,966 13.6% 

Value = Modal MRL 24,918 42.4% 

Value > Modal MRL (0.04 μg/L) and ≤ 0.1 μg/L 24,752 42.1% 

Value > 0.1 μg/L 1,122 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 

2.6%

6.8%

0.5%

3.7%

42.4%

1.5% 0.7% 0.0%

9.1%

4.5%

26.3%

1.9%

0%

20%

40%

60%

80%

≤0.009 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 ≥0.2

MRL (ug/L)

Percent of Samples with Reported MRL

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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 μg/L and rounded up to 0.1 μg/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 μg/L. 

Exhibit 4-51. Analytical Methods for Heptachlor 

Method MDL (μg/L) 

505 0.003 

508 0.0015 

508.1 0.005 

525.2 0.15 

551.1 0.081 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.1 μg/L is one quarter of the current PQL. Therefore, EPA also performed the 

occurrence analysis with an intermediate threshold value of 0.2 μg/L. 

4.2.7 Heptachlor Epoxide 

The MCL for heptachlor epoxide is based on a PQL of 0.2 μg/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 μg/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 Number of Records Percentage of Records 

All MRL Values 58,731 100% 

Value < Modal MRL 5,969 10.2% 

Value = Modal MRL 26,424 45.0% 

Value > Modal MRL (0.02 μg/L) and ≤ 0.1 μg/L 25,888 44.1% 

Value > 0.1 μg/L 450 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 

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

Exhibit 4-53. MRL Distribution for Heptachlor Epoxide 

2.6%

7.5%

45.0%

0.1%

10.2%

2.3% 0.0%

3.8%

0.0% 0.0%

27.6%

0.7% 0.0%

0%

20%

40%

60%

80%

≤0.009 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 MCL =

0.2

≥0.3

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L and rounded up 

to 0.1 μg/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 μg/L.. 

Exhibit 4-54. Analytical Methods for Heptachlor 

Epoxide 

Method MDL (μg/L) 

505 0.004 

508 0.0059 

508.1 0.001 

525.2 0.13 

551.1 0.202 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.1 μg/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 μg/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 

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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 μg/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 

μg/L, EPA based the EQL on the modal MRL. 

Exhibit 4-55. Summary of MRL Data for Hexachlorobenzene 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 58,713 100% 

Value < Modal MRL 7,380 12.6% 

Value = Modal MRL (0.1 μg/L) 40,791 69.5% 

Value > Modal MRL 10,542 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 

1.5% 0.6%

4.0%

0.6% 0.0%

4.4%

0.6% 0.8% 0.0% 0.0%

69.5%

10.8%

7.1%

0%

20%

40%

60%

80%

≤.009 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.2 ≥0.3

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L. 

This range contains the modal MRL of 0.1 μg/L. 

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Exhibit 4-57. Analytical Methods for 

Hexachlorobenzene 

Method MDL (μg/L) 

505 0.002 

508 0.0077 

508.1 0.001 

525.2 0.13 

551.1 0.003 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.1 μg/L is one-tenth the current PQL. Therefore, EPA also performed the 

occurrence analysis with an intermediate threshold value of 0.5 μg/L. 

4.2.9 Toxaphene 

The MCL for toxaphene is based on a PQL of 3 μg/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 μg/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 μg/L, 

EPA based the EQL on the modal MRL. 

Exhibit 4-58. Summary of MRL Data for Toxaphene 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 54,529 100% 

Value < Modal MRL 8,525 15.6% 

Value = Modal MRL (1 μg/L) 36,763 67.4% 

Value > Modal MRL 9,241 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 

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Exhibit 4-59. MRL Distribution for Toxaphene 

6.0%

0.2% 0.3% 0.7%

7.3%

0.3% 0.0% 0.7% 0.0%

67.4%

16.9%

0.0% 0.0%

0%

20%

40%

60%

80%

≤0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2 MCL = 3 ≥4

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L. This range 

exceeds the modal MRL of 1.0 μg/L. 

Exhibit 4-60. Analytical Methods for Toxaphene 

Method MDL (μg/L) 

505 1.0 

508 No MDL reported 

508.1 0.13 

525.2 1.7 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 1 μg/L is one-third the current PQL. Therefore, EPA also performed the occurrence 

analysis with an intermediate value of 1.5 μg/L. 

4.2.10 1,1,2-Trichloroethane 

The MCL for 1,1,2-trichloroethane is based on a PQL of 5 μg/L. Its MCLG is 3 μg/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 μg/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). 

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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 μg/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 Number of Records Percentage of Records 

All MRL Values 139,672 100% 

Value ≤ MCLG (3 μg/L) 139,616 >99.9% 

Value < Modal MRL 17,142 12.3% 

Value = Modal MRL (0.5 μg/L) 117,788 84.3% 

Value > Modal MRL 4,742 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 

2.7% 4.7% 3.8% 1.1%

84.3%

2.2% 0.0% 0.9% 0.0% 0.3% 0.0% 0.04%

0%

20%

40%

60%

80%

≤0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2 ≥3

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L. 

This range is below the current MCLG, which further supports use of the MCLG as the threshold 

in the occurrence analysis. 

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

Exhibit 4-63. Analytical Methods for 1,1,2-

Trichloroethane 

Method MDL (μg/L) 

502.2 0.04 

524.2 0.10 

551.1 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 μg/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 μg/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 μg/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 

μg/L, EPA based the EQL on the modal MRL. 

Exhibit 4-64. Summary of MRL Data for Vinyl Chloride 

MRL Value Category Number of Records Percentage of Records 

All MRL Values 139,494 100% 

Value < Modal MRL 25,723 18.4% 

Value = Modal MRL (0.5 μg/L) 105,410 75.6% 

Value > Modal MRL 8,361 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 

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Exhibit 4-65. MRL Distribution for Vinyl Chloride 

1.5%

5.4% 5.4% 6.2%

3.0%

0.0% 0.0% 0.9% 0.1% 1.9% 0.0%

75.6%

0%

20%

40%

60%

80%

≤0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 MCL = 2 ≥3

MRL (ug/L)

Percent of Samples with Reported MRL

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 μg/L, 

which is slightly below the MCL of 2.0 μg/L. This range is also higher than the EQL of 0.5 μg/L 

based on MRL data. 

Exhibit 4-66. Analytical Methods for Vinyl 

Chloride 

Method MDL (μg/L) 

502.2 0.18 

524.2 0.17 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

The EQL of 0.5 μg/L is one-fourth the current PQL. EPA also performed the occurrence analysis 

with an intermediate value of 1 μg/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 μg/L. A new health effects assessment 

indicates that there is potential to reduce the MCLG to 50 μg/L. The current PQL of 90 μg/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). 

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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 

of 50 μg/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 Number of Records Percentage of Records 

All MRL Values 21,792 100%

Value ≤ Possible MCLG (50 μg/L) 21,445 98.4%

Value < Modal MRL 5,695 26.1%

Value = Modal MRL (9 μg/L) 6,884 31.6%

Value > Modal MRL 9,213 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 

10.5%

15.2%

0.1% 0.0% 0.3%

31.6%

17.5% 19.9%

0.5% 1.8% 1.1% 1.6%

0%

20%

40%

60%

80%

≤4 5 6 7 8 9 10 20 30 40 Possible

MCLG =

50

≥60

MRL (ug/L)

Percent of Samples with Reported MRL

Exhibit 4-69 shows the EPA approved method for the detection of endothall, and its MDL. 

Because this MDL is more than a factor of ten lower than the possible MCLG, the MDL data 

also support use of the possible MCLG as a threshold in the occurrence analysis. 

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Exhibit 4-69. Analytical Methods for Endothall 

Method MDL (μg/L) 

548.1 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 μg/L. A new health effects assessment 

indicates that there is potential to reduce the MCLG to 2 μg/L. The present PQL of 20 μg/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 μg/L. 

As shown in Exhibit 4-70 and Exhibit 4-71, the modal MRL for oxamyl is 2 μg/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 Number of Records Percentage of Records 

All MRL Values 52,201 100% 

Value < Modal MRL/Possible MCLG 13,335 25.5% 

Value = Modal MRL/Possible MCLG (2 μg/L) 31,955 61.2% 

Value > Modal MRL/Possible MCLG 6,911 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 

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Exhibit 4-71. MRL Distribution for Oxamyl 

13.0%

0.0%

12.1%

61.2%

0.0% 1.1% 3.9%0.0% 0.0% 0.0% 0.0%

8.2%

0%

20%

40%

60%

80%

≤0.8 0.9 1 Possible

MCLG = 2

3456789 ≥10

MRL (ug/L)

Percent of Samples with Reported MRL

Exhibit 4-72 shows the EPA approved methods for the detection of oxamyl, and their MDLs. 

Applying a multiplier of 10 would give a possible PQL range from 0.65 to 8.6 μg/L. This range 

contains the modal MRL and possible MCLG of 2.0 μg/L. Therefore, EPA used the possible 

MCLG as a health-based threshold for the occurrence analysis. 

Exhibit 4-72. Analytical Methods for Oxamyl 

Method MDL (μg/L) 

531.1 0.86 

531.2 0.065 (DL) 

DL = detection limit 

Source: USEPA, 2009b (upper bound values when ranges are reported) 

EPA did not use any intermediate values in the occurrence analysis because the analysis 

threshold is based on a possible MCLG. 

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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. 

EPA 815-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. 

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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 μg/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 μg/L rounds to 0.1 μg/L and a result of 132 μg/L 

rounds to 100 μg/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