### United States Environmental Protection Agency



# Development of Estimated Quantitation Levels for the Second Six-Year Review of National Primary Drinking Water RegulationsOffice of Water (4607M) EPA 815-B-09-005 October 2009 www.epa.gov/safewaterEPA-OGWDW Development of Estimated Quant itation Levels for the EPA 815-B-09-005 Second Six-Year Review of National Primar y 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 Eff ects 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.............................................................................................................4-1 3 4.1.11 Trichloroethylene...............................................................................................4-14 4.2 Contaminants with MCL Limited by PQ L 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

iEPA-OGWDW Development of Estimated Quant itation Levels for the EPA 815-B-09-005 Second Six-Year Review of National Primar y 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 Thre shold Determination .................................................................... 3-3 Exhibit 4-1. Summary of MRL Da ta for Benzo[a]pyrene ...........................................................4-1 Exhibit 4-2. MRL Distributi on for Benzo[a]pyrene .................................................................... 4-2 Exhibit 4-3. Analytical Met hods for Benzo[a]pyrene.................................................................. 4-2 Exhibit 4-4. Summary of MRL Data for Carbon Tetrachloride .................................................. 4-3 Exhibit 4-5. MRL Distribution fo r 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 Da ta for 1,2-Dichloroethane.................................................... 4-5 Exhibit 4-11. MRL Distribution for 1,2-Dichloroethane............................................................. 4-6 Exhibit 4-12. Analytical Met hods for 1,2-Dichloroethane .......................................................... 4-6 Exhibit 4-13. Summary of MRL Da ta for Dichloromethane....................................................... 4-7 Exhibit 4-14. MRL Distributi on for Dichloromethane................................................................ 4-7 Exhibit 4-15. Analytical Met hods for Dichloromethane ............................................................. 4-7 Exhibit 4-16. Summary of MRL Da ta for Pentachlorophenol..................................................... 4-8 Exhibit 4-17. MRL Distribution for Pentachlorophenol.............................................................. 4-8 Exhibit 4-18. Analytical Met hods 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 Distri bution 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 Da ta 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

iiEPA-OGWDW Development of Estimated Quant itation Levels for the EPA 815-B-09-005 Second Six-Year Review of National Primar y 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 Da ta for 1,2-Dichloropropane ............................................... 4-20 Exhibit 4-44. MRL Distribution for 1,2-Dichloropropane ........................................................ 4-21 Exhibit 4-45. Analytical Met hods 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 MR L Data for Heptachlor............................................................... 4-23 Exhibit 4-50. MRL Distribu tion for Heptachlor........................................................................4-23 Exhibit 4-51. Analytical Me thods for Heptachlor ..................................................................... 4-24 Exhibit 4-52. Summary of MRL Da ta for Heptachlor Epoxide................................................. 4-24 Exhibit 4-53. MRL Distributi on for Heptachlor Epoxide..........................................................4-25 Exhibit 4-54. Analytical Met hods for Heptachlor Epoxide....................................................... 4-25 Exhibit 4-55. Summary of MRL Da ta 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 MR L 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 Da ta 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 Distributi on for Vinyl Chloride..................................................................4-31 Exhibit 4-66. Analytical Met hods 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

iiiEPA-OGWDW Development of Estimated Quant itation Levels for the EPA 815-B-09-005 Second Six-Year Review of National Primar y 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 μ g/L micrograms per liter MRL minimum reporting level NPDWR National Primary Dr inking 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

ivEPA-OGWDW Development of Estimated Quant itation Levels for the EPA 815-B-09-005 Second Six-Year Review of National Primar y 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 dri nking water regulations (NPDWRs). The 1996 Safe Drinking Water Act (SDWA) Ame ndments require the U.S. Environmental Protection Agency (EPA or the Agency) to peri odically review existing Na tional Primary Drinking Water Regulations (NPDWRs). Section 1412(b)(9) of SDWA reads:
...[t]he Administrator shall, not less thanevery 6 years, review and revise, as appropriate, each primary drinking water regulation promulgated under this title. Any revision of a national primary drinki ng water regulation sh all 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 proces s 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 im proving the level of public health protection.
For Six-Year Review 2, EPA obtained and eval uated new information that could affect a NPDWR, including information on health effect s (USEPA, 2009e), anal ytical feasibility (USEPA, 2009b), treatment feasibility (USEPA , 2009f), and occurrence (USEPA, 2009a and 2009d). EPA identified new health effects or anal ytical methods information that indicated it may be possible to revise NPDWRs for several contaminants. Consequently, EPA conducted occurrence and exposure analyses at threshold co ncentrations 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. Th is document describes the method EPA used to establish the threshold values that it used for the occurrence analyses.
For most contaminants, EPA established an es timated 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 quantit ation capabilities at the time EPA promulgated the existing NPDWRs. Current MCLs may be limited by historical PQLs. Thus, improvements in analytical det ection could present anopportunity to lower the MCL closer to MCLG. For a few contaminants, EPA establishe d 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.

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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 futu re rule making efforts. For Six-Year Review 2, EPA derived the new thresholds and conducted occurrence and e xposure analyses only if an MCL revision would be feasible: the current MC L is limited by analytical capability (i.e., the MCL equals a PQL), and there is new informati on indicating improved anal ytical capability; or the current MCL is set equal to the MCLG, and a new health e ffects assessment indicates it is possible to revise the MCLG.
As a lower bound estimate of analytical capabil ity 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 perfor mance studies for several contaminants at concentrations below the current PQL. For other c ontaminants, the range of concentrations is not sufficient to determine what a lower bound on analyt ical 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 MD Ls associated with approved an alytical 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 na tionwide. 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 essentia lly 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 us e 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 mu ltiplier approach supports that EQL value. If the modal MRL is not a feas ible 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 objectiv e 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 fo r 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.

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For most contaminants, the MRL and MDL data supported EPA’s conclusion based on PT data. EPA relied primarily on the MRL data to calcul ate EQLs. The MRLs provide information on the analytical capabilities of a larg e pool of laboratories. The MRL da ta also provide a threshold for estimating occurrence. Setting an EQL below a s ubstantial number of MRL values will limit the monitoring data available fo r 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 th an 90% of MRL values ar e equal to or less than the feasible EQL, with a clear majority equal to the moda l 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 moda l MRL exceed 94%. The modal MRL is 0.5 μ g/L in each

case. EQLs for the synthetic organic compounds (SOCs) hexachlorobenzene and toxaphene are based on modal MRLs. EQLs for DBCP, chlordane, he ptachlor, 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 cont aminant (or average if there are only two MDL values) and multiplying it by a factor of ten. Th resholds 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 curr ent 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.

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Exhibit ES-1. Summary of Threshold Determination

MRL Distribution

Contaminant Type Current PQL Threshold (EQL orhealthbased) Basis for Threshold % = mode % ≤ mode Ongoing health effects assessment, MCL limited by PQL Benzo[a]pyrene SOC 0.2 μ g/L na na 53.7 55.4 Carbon tetrachloride VOC 5 μ g/L 0.5 μ g/L Modal MRL 86.1 97.7 DEHP SOC 6 μ g/L na na 45.5 76.9 1,2-Dichloroethane VOC 5 μ g/L 0.5 μ g/L Modal MRL 83.8 96.9 Dichloromethane VOC 5 μ g/L 0.5 μ g/L Modal MRL 87.8 96.0 Pentachlorophenol SOC 1 μ g/L na na 44.7 48.7 PCBs SOC 0.5 μ g/L na na 67.6 74.3 Dioxin SOC 3×10 -5 μ g/L na na 75.4 90.3 Tetrachloroethylene VOC 5 μ g/L 0.5 μ g/L Modal MRL 84.6 96.1 Thallium IOC 2 μ g/L na na 63.0 78.0 Trichloroethylene VOC 5 μ g/L 0.5 μ g/L Modal MRL 85.4 97.7

No new health effects assessment or new health effects assessment indi cates no MCLG change, MCL is limited by PQL Benzene VOC 5 μ g/L 0.5 μ g/L Modal MRL 86.4 99.3 Chlordane SOC 2 μ g/L 1 μ g/L 10×MDL 46.8 63.8 DBCP SOC 0.2 μ g/L 0.1 μ g/L 10×MDL 35.6 84.7 1,2-Dichloropropane VOC 5 μ g/L 0.5 μ g/L Modal MRL 86.1 99.3 EDB SOC 0.05 μ g/L na na 32.4 32.9 Heptachlor SOC 0.4 μ g/L 0.1 μ g/L 10×MDL 42.4 56.0 Heptachlor epoxide SOC 0.2 μ g/L 0.1 μ g/L 10×MDL 45.0 55.2 Hexachlorobenzene SOC 1 μ g/L 0.1 μ g/L Modal MRL 69.5 82.1 Toxaphene SOC 3 μ g/L 1 μ g/L Modal MRL 67.4 83.0 1,1,2-Trichloroethane VOC 5 μ g/L 3 μ g/L 1 Current MCLG 99.9% below current MCLG Vinyl chloride VOC 2 μ g/L 0.5 μ g/L Modal MRL 75.6 94.0 New health effects assessment indi cates possible MCLG below PQL Endothall SOC 90 μ g/L 50 μ g/L 1 Possible MCLG 98.4% below possible MCLG Oxamyl SOC 50 μ g/L 2 μ g/L 1 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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## 1 Introduction

The U.S. Environmental Protection Agency (EPA or the Agency) has conducted its second SixYear Review (“Six-Year Review 2”) of nationa l primary drinking water regulations (NPDWRs). The 1996 Safe Drinking Water Act (SDWA) Amendm ents require that th e Agency periodically review existing NPDWRs. Secti on 1412(b)(9) of SDWA reads:
...[t]he Administrator shall, not less thanevery 6 years, review and revise, as appropriate, each primary drinking water regulation promulgated under this title. Any revision of a national primary drinki ng water regulation sh all 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 proces s is to identify possible regulatory revisions. Although the statute does not define when a revi sion is “appropriate,” as a general benchmark, EPA considered a possible revision to be “ap propriate” 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 im proving the level of public health protection.
For Six-Year Review 2, EPA implemented the prot ocol that it developed for the first Six-Year Review (USEPA, 2003a), including minor revisions developed during the current review process (USEPA, 2009c). EPA obtained a nd evaluated new information including information on health effects (USEPA, 2009e), analytic al feasibility (USEPA, 2009b), treatment feasibility (USEPA, 2009f), and occurrence (USEPA, 2009a and 2009d). Some of the new health e ffects or analytical methods information that indicated it may be possible to revise NPDWRs for several contaminants. Consequently, EPA conducted occu rrence and exposure analyses at threshold concentrations that are below current maximum c ontaminant levels (MCLs) to determine if there is a meaningful opportunity to improve the leve l of public health prot ection by reducing MCLs. 1 This document describes the method EPA used to es tablish the threshold valu es that it used for the occurrence analyses.
For most contaminants, EPA established an es timated quantitation level (EQL), which is an estimate of the possible lower bound for a practic al quantitation level (P QL). The current PQL for a contaminant is based on hi storical 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, improvem ents 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 possibl e system and population impacts in the occurrence and exposure analysis conducted for the Six-Year Review 2 (USEPA, 2009a). EPA compar ed contaminant occurrence estimates for these thresholds (i.e., the number of syst ems with water quality exceedin g a threshold) with baseline occurrence estimates at current MCLs. The difference between these two occu rrence 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 expo sure estimates for that contaminant.

1-1EPA-OGWDW Development of Estimated Quant itation Levels for the EPA 815-B-09-005 Second Six-Year Review of National Primar y Drinking Water Regulations October 2009
have MCLs limited by PQLs, and, therefore, lowe r 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 th reshold 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 pe rhaps 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 cu rrent PQL and it is necessary to examine whether the PQL can be lowered to the possible MCLG.
Analyzing the feasibility of reducing a contaminan t’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 testi ng (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 us e of spiked samples to evaluate laboratory quantitation capabilities. USEP A (2009b) describes the review method, PT and PE data, and findings for the PQL analysis. For Six-Year Re view 2, EPA did not have sufficient PT data below current PQLs to actually recalculate a ny PQL or derive EQLs for the occurrence and exposure analysis. Instead, EPA used the PT and PE study passing rate resu lts (i.e., the percent of laboratories passing a performa nce test for a given study) at and below the current PQL and the result of a linear regression analysis to i ndicate whether the PT and PE data support a reduction in the PQL. Exhibit 1-1 provides summary information fo r 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 tw o 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 laborator y performance data to calculate the PQL, the MRL and MDL information can be valuable to in dicate whether it is possible to quantitate at levels below the current PQL.

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

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Exhibit 1-1. Contaminants Requiri ng Methods or Occurrence Analyses

Contaminant Current MCLG 1 Current PQL 1 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 μ g/L 0.2 μ g/L 0 3 No Carbon Tetrachloride 0 5 μ g/L 5 μ g/L 0 3 Yes Di (2-ethylhexyl)p hthalate (DEHP) 0 6 μ g/L 6 μ g/L 0 3 No 1,2-Dichloroethane 0 5 μ g/L 5 μ g/L 0 3 Yes Dichloromethane 0 5 μ g/L 5 μ g/L 0 3 No Pentachlorophenol 0 1 μ g/L 1 μ g/L 0 3 No Polychlorinated biphenyls (PCBs) 0 0.5 μ g/L 0.5 μ g/L 0 3 No 2,3,7,8-TCDD (Dioxin) 0 3×10 -5 μ g/L 3×10 -5 μ g/L 0 3 No Tetrachloroethylene 0 5 μ g/L 5 μ g/L 0 3 Yes Thallium 0.5 μ g/L 2 μ g/L 2 μ g/L Not determined 4 No Trichloroethylene 0 5 μ g/L 5 μ g/L 0 3 Yes

No new health effects assessment or new health effects assessment indi cates no MCLG change, MCL is limited by PQL Benzene 0 5 μ g/L 5 μ g/L No new HEA Yes Chlordane 0 2 μ g/L 2 μ g/L No new HEA Yes 1,2-Dibromo-3-Chloropropane (DBCP) 0 0.2 μ g/L 0.2 μ g/L No new HEA Possibly 1,2-Dichloropropane 0 5 μ g/L 5 μ g/L No new HEA Yes
Ethylene Dibromide (EDB) 0 0.05 μ g/L 0.05 μ g/L No MCLG change No Heptachlor 0 0.4 μ g/L 0.4 μ g/L No new HEA Possibly Heptachlor Epoxide 0 0.2 μ g/L 0.2 μ g/L No new HEA Possibly Hexachlorobenzene 0 1 μ g/L 1 μ g/L No new HEA Yes Toxaphene 0 3 μ g/L 3 μ g/L No new HEA Possibly 1,1,2-Trichloroethane 3 μ g/L 5 μ g/L 5 μ g/L No new HEA Yes Vinyl Chloride 0 2 μ g/L 2 μ g/L No new HEA Possibly New health effects assessment indi cates possible MCLG below PQL Endothall 100 μ g/L 90 μ g/L 100 μ g/L 50 μ g/L 5 No Oxamyl 200 μ g/L 20 μ g/L 200 μ g/L 2 μ g/L 5 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 ba sed 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 se nsitivity (USEPA, 2009b). MDLs have been used in the past to derive PQLs for regulated cont aminants. In addition, EP A used MDLs to help identify possible analytical feasibility levels for Six-Year Review 1 (USEPA, 2003b). Consequently, EPA used the MDLs as a sec ond input to the EQL de velopment process.

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Both sources of data provide additional info rmation 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 MD L data that EPA used to derive the EQLs or evaluate analytical feasibility or the health-based thresholds. Th e 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 capabil ity 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 whic h 75% of laboratories ca n 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 pref ers 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 contaminan ts 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 ar e not sufficient to determine where a lower bound on analytical capabil ity 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 represen t 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 desc ription of the data collection, data management, and quality assurance methods the Agency used to establish a high quality, national contaminant occurrence da tabase 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 monito ring 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 concentrati on 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 word s, the MRL is the lowest contaminant concentration that can be reliab ly quantified in the laboratory a nd reported with the contaminant occurrence data to primacy agencies (e.g., States).
The MRL values provide EPA with valuable insight s 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 sa me) 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 fo r a contaminant concentration that does not exceed the MRL for that contaminant at a laborator y using an applicable analytical method, then the compliance record should specif y “<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 th e 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 me thod sensitivity (USEPA, 2009b), defined in 40 CFR Part 136 Appendix B as “the minimum concen tration 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 PQ Ls in the past, EPA is not using MDLs for this purpose during Six-Year Revi ew 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 c ontaminant. USEPA (2009b) provides complete information for MDLs by contaminant and anal ytical method. The MDL values or ranges of values in USEPA (2009b) are for the approve d 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 partic ular method and contaminant in an effort to derive an EQL value that would represent a level where most la boratories 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 capab ilities 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 de veloped 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 μ g/L. In the EQL analysis, EPA used only the upper bound values when ever USEPA (2009b) reported an MDL range; i.e., 0.02 μ g/L for EPA Method 502.2 for carbon tetrachloride.

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indicate potential for a PQL reducti on (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 Dr inking Water MDLs, MRLs a nd Related Information for Contaminants in the EQL Report

Contaminant PQL ( μ g/L) Six Year 2 Modal MRL ( μ g/L) MDL Range for EPA Methods ( μ g/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 indi cates 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 indi cates 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 μ g/L, which is higher than the PQL of 0.2 μ g/L, which is also the MCL. Given the ra nge of upper bound MDL values for EPA methods shown in Exhibit 2-1 (0.009 to 0.01 μ g/L), MRL values greater than the MCL are generally not expected. The MRL data for this contaminant indi cate a data quality issue. As the discussion in Section 4 indicates, EPA disregarded MR L data in setting an EQL for DBCP.

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## 3 Threshold Development Method

EPA’s method for developing an EQL has essentia lly 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 indica te potential to revise the PQL. If they do, then the objective of the next two steps is to identify an EQL (or veri fy the use of a health-based threshold) for the occurrence analysis. The second st ep is to determine whether th e modal MRL is a feasible EQL and, if so, the third step is to determine whether the MDL multiplier appr oach 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 determin es 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 MD L 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* EDB * Health-based threshold instead of EQL 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* EDB * Health-based threshold instead of EQL

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As Exhibit 3-2. Summary of Threshold Determination Exhibit 3-2 indicates, EPA relied primarily on the MRL data to calculate EQLs. The MR Ls provide information on the analytical capabilities of a large po ol of laboratories. The MRL data al so 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 th at the contaminants with more than 80% of MRL values equal to or less than the modal MR L 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 thres hold) more than 90% of each contaminant’s MRL values are equal to or less than the modal MRL, with a clear majority e qual 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 th at equal or are less than the m odal MRL exceed 94%. In each case, the modal MRL is 0.5 μ g/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 – hexachlorobe nzene and toxaphene. Four SOCs have EQLs based on the MDL multiplier method – DBCP, chlo rdane, heptachlor, and heptachlor epoxide. This method takes the median MDL from the EP A approved methods for analysis of a given contaminant (or average if there are only two MD L values), and multiplies it by a factor of ten. Two have health-based threshol ds – 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 revisi on. 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% dist ribution threshold, and the MDL multiplier result generally concurred with the curr ent PQL. One exception is dioxin, which has slightly more than 90% of the MRL values equal to or less thanthe modal MRL. The MR L 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 Distribution 1

Contaminant Type Current PQL Threshold (EQL orhealthbased) Basis for Threshold % = mode % ≤ mode Ongoing health effects assessment, MCL limited by PQL Benzo[a]pyrene SOC 0.2 μ g/L na na 53.7 55.4 Carbon tetrachloride VOC 5 μ g/L 0.5 μ g/L Modal MRL 86.1 97.7 DEHP SOC 6 μ g/L na na 45.5 76.9 1,2-Dichloroethane VOC 5 μ g/L 0.5 μ g/L Modal MRL 83.8 96.9 Dichloromethane VOC 5 μ g/L 0.5 μ g/L Modal MRL 87.8 96.0 Pentachlorophenol SOC 1 μ g/L na na 44.7 48.7 PCBs SOC 0.5 μ g/L na na 67.6 74.3 Dioxin SOC 3×10 -5 μ g/L na na 75.4 90.3 Tetrachloroethylene VOC 5 μ g/L 0.5 μ g/L Modal MRL 84.6 96.1 Thallium IOC 2 μ g/L na na 63.0 78.0 Trichloroethylene VOC 5 μ g/L 0.5 μ g/L Modal MRL 85.4 97.7

No new health effects assessment or new health effects assessment indi cates no MCLG change, MCL is limited by PQL Benzene VOC 5 μ g/L 0.5 μ g/L Modal MRL 86.4 99.3 Chlordane SOC 2 μ g/L 1 μ g/L 10×MDL 46.8 63.8 DBCP SOC 0.2 μ g/L 0.1 μ g/L 10×MDL 35.6 84.7 1,2-Dichloropropane VOC 5 μ g/L 0.5 μ g/L Modal MRL 86.1 99.3 EDB SOC 0.05 μ g/L na na 32.4 32.9 Heptachlor SOC 0.4 μ g/L 0.1 μ g/L 10×MDL 42.4 56.0 Heptachlor epoxide SOC 0.2 μ g/L 0.1 μ g/L 10×MDL 45.0 65.2 Hexachlorobenzene SOC 1 μ g/L 0.1 μ g/L Modal MRL 69.5 82.1 Toxaphene SOC 3 μ g/L 1 μ g/L Modal MRL 67.4 83.0 1,1,2-Trichloroethane VOC 5 μ g/L 3 μ g/L 1 Current MCLG 99.9% below current MCLG Vinyl chloride VOC 2 μ g/L 0.5 μ g/L Modal MRL 75.6 94.0 New health effects assessment indi cates possible MCLG below PQL Endothall SOC 90 μ g/L 50 μ g/L 1 Possible MCLG 98.4% below possible MCLG Oxamyl SOC 50 μ g/L 2 μ g/L 1 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 disc ussion 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 c ontaminant groups shown in previous exhibits. 

### 4.1 Contaminants with Ongoing H ealth 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 μ g/L. Its MCLG is zero, and although a health effects assessment is in pr ogress, there is no new health e ffects information that suggests a change in the MCLG. Consequent ly, the threshold for the occurrence analysis will be based on analytical feasibility. There are no PT or PE st udy results at spiked concentrations below the current PQL. Several passing rates for the avai lable 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 PQ L revision is not appropr iate 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 μ g/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, how ever, 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 belo w the current PQL. Less than 80% of the MRL values are equal to or less than the modal MRL and, th erefore, EPA did not base the EQL on the modal MRL. Consequently, EPA reviewed MDL va lues 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 μ g/L) 29,769 53.7% Value > Modal MRL and ≤ 0.1 μ g/L 21,198 38.2% Value > 0.1 μ g/L 3,550 6.4% Percentages may not sum to 100% because of independent rounding. A ggregate 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 Distribut ion 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 μ g/L, which is higher than the current PQL of 0.2 μ g/L. The MDL data do not support an EQL below the PQL.
Exhibit 4-3. Analytical Methods for Benzo[a]pyrene
Method MDL ( μ g/L) 525.2 0.23 550 0.029 550.1 0.016 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

EPA concluded that although MRL values are gene rally below the current PQL, the combination of PT and MDL data do not support revision of th e 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 μ g/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 thre shold 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 suppor ts 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 μ g/L, and 97.7% of the MRL values ar e equal to or less than the modal

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value. Because more than 80% of th e MRLs are equal to or less than 0.5 μ g/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 μ g/L) 119,849 86.1% Value > Modal MRL 3,177 2.3% Percentages may not sum to 100% because of independent rounding. A ggregate 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 Distributi on 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 de tection of carbon tetrachloride, and their MDLs. Applying a multiplier of 10 would gi ve a possible PQL range from 0.2 to 2.1 μ g/L. This range contains the modal MRL. Therefore, EPA retained 0.5 μ g/L as the EQL value.
Exhibit 4-6. Analytical Methods for Carbon Tetrachloride
Method MDL ( μ g/L) 502.2 0.02 524.2 0.21 551.1 0.050 Source: USEPA, 2009b (upper bound val ues when ranges are reported)
The EQL of 0.5 μ g/L is ten times lower than the current PQL. EPA also performed the occurrence analysis with intermediate values of 1 μ g/L (2 x EQL) and 2.5 μ g/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 μ g/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. Consequent ly, 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 th e 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 μ g/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 review ed 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 μ g/L) 22,980 45.5% Value > Modal MRL 11,668 23.1% Percentages may not sum to 100% because of independent rounding. A ggregate 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 th e detection of DEHP , and their MDLs. Applying a multiplier of 10 would give a possible PQL range from 13.0 to 22.5 μ g/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 ( μ g/L) 506 2.25 525.2 1.3 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

EPA concluded that although MRL values are gene rally 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 μ g/L. Its MCLG is ze ro, and there is no new health effects information that suggests a ch ange in the MCLG. Consequently, the threshold for the occurrence analysis will be based on analy tical 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 reducti on 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 μ g/L, and 96.9% of the MRL values are equal to or less than the modal value. B ecause more than 80% of the MRLs are equal to or less than 0.5 μ g/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 μ g/L) 116,533 83.8% Value > Modal MRL 4,392 3.2% Percentages may not sum to 100% because of independent rounding. A ggregate 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 Distributi on 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 de tection of 1,2-dichloroethane, and their MDLs. Applying a multiplier of 10 would gi ve a possible PQL range from 0.3 to 0.6 μ g/L. This range contains the modal MRL. Therefore, EPA retained 0.5 μ g/L as the EQL value.
Exhibit 4-12. Analytical Methods for 1,2- Dichloroethane
Method MDL ( μ g/L) 502.2 0.03 524.2 0.06 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

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

4.1.5 Dichloromethane The MCL for dichloromethane is based on a PQL of 5 μ g/L. Its MCLG is zero. It has an ongoing health effects assessment, but there is no new heal th effects information that suggests a change in the MCLG. Consequently, the threshold for the o ccurrence 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 suppor t 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 μ g/L, and 96.0% of the MRL values are equal to or less than the modal value. B ecause more than 80% of the MRLs are equal to or less than 0.5 μ g/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 μ g/L) 121,532 87.8% Value > Modal MRL 5.619 4.1% Percentages may not sum to 100% because of independent rounding. A ggregate 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 de tection of dichloromethane, and their MDLs. Applying a multiplier of 10 would gi ve a possible PQL range from 0.2 to 0.9 μ g/L. This range contains the modal MRL. Therefore, EPA retained 0.5 μ g/L as the EQL value.

Exhibit 4-15. Analytical Methods for Dichloromethane
Method MDL ( μ g/L) 502.2 0.02 524.2 0.09 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

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

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4.1.6 Pentachlorophenol The MCL for pentachlorophenol is based on a PQL of 1 μ g/L. Its MCLG is zero, and although a health effects assessment is in pr ogress, there is no new health e ffects information that suggests a change in the MCLG. Consequent ly, the threshold for the occurrence analysis will be based on analytical feasibility. Several PT and PE studi es had passing rates belo w 75%, and only one PE study had a spiked concentration be low the current PQL. Therefore, EPA determined that the PT and PE results do not support revi sion of the PQL (USEPA, 2009b).

As shown in Exhibit 4-16 and Exhibit 4-17 , the modal MRL for pentachlorophenol is 0.4 μ g/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. Consequen tly, 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 μ g/L) 26,666 44.7% Value > Modal MRL 30,529 51.2% Percentages may not sum to 100% because of independent rounding. A ggregate 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 de tection of pentachlorophenol, and their MDLs. Applying a multiplier of 10 w ould give a range from 0.32 to 16.0 μ g/L with a median of 1.225 μ g/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 ( μ g/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 val ues when ranges are reported)

EPA concluded that although MRL values are gene rally 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 μ g/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. Consequent ly, the threshold for the occurrence analysis will be based on analytical feasibility. The only PE study with a spiked concentration be low the current PQL had a passing rate below 75%. The PT data had few data points with spiked c oncentrations near the PQL and none below it. Therefore, EPA determin ed 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 μ g/L. Summary data show that 67.6% of the MR Ls are equal to this value, and 74.3% of the MRL values are equal to or less than it. Unlike the PT data, the MR L 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 review ed 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 μ g/L) 23,785 67.6% Value > Modal MRL 9,038 25.7% Percentages may not sum to 100% because of independent rounding. A ggregate 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 multip lier of 10 would give a possible PQL of 0.8 μ g/L, which is greater than th e current PQL. The MDL data do not support an EQL below the PQL.
Exhibit 4-21. Analytical Methods for PCBs
Method MDL ( μ g/L) 508A 0.08 Source: USEPA, 2009b

EPA concluded that although MRL values are gene rally 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 μ g/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. Consequent ly, 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, EP A 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 μ g/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. B ecause the majority of MRLs equal 5×10 -6 μ g/L, the MRL data suggest that value as the EQL. Unlike the PT data, the MRL data app ear to indicate that there is potential to lower the PQ L because most of the MRL values are below the current PQL. The small sample size, however, limits its us efulness for EQL development. Therefore, EPA reviewed MDL values to determine whet her 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 μ g/L) 964 75.4% Value > Modal MRL 123 9.6% Percentages may not sum to 100% because of independent rounding. A ggregate 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 de tection of dioxin, and its minimum level (ML). Applying a multiplier of 5 would give a possible PQL of 5×10 -5 μ g/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 ( μ g/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 gene rally 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 tetrachloroethylen e is based on a PQL of 5 μ g/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. Consequent ly, 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 concentr ations below the current PQL. EPA determined that the PQL assessment s upports 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 μ g/L, and 96.1% of the MRL values are equal to or less than the modal value. B ecause more than 80% of the MRLs are equal to or less than 0.5 μ g/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 μ g/L) 117,033 84.6% Value > Modal MRL 5,467 4.0% Percentages may not sum to 100% because of independent rounding. A ggregate 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 woul d give a possible PQL range from 0.08 to 1.4 μ g/L. This range contains the modal MR L. Therefore, EPA retained 0.5 μ g/L as the EQL value.
Exhibit 4-27. Analytical Methods for Tetrachloroethylene
Method MDL ( μ g/L) 502.2 0.05 524.2 0.14 551.1 0.008 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

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

4.1.10 Thallium The MCL for thallium is based on a PQL of 2 μ g/L. Its MCLG is 0.5 μ g/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 occu rrence 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 con centrations 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. Theref ore, 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 μ g/L. Summary data show that 63.0% of the MR Ls are equal to this value, and 78.0% of the MRL values are equal to or less than it. Unlike the PT data, the MR L data appear to indicate that there is potential to lower the PQL because most of the MRL valu es are below the current PQL. The data do not, however, support an EQL as low as the current MCLG of 0.5 μ g/L. Furthermore, less than 80% of the MRL values are equal to or less than th e modal MRL and, therefore, EPA did not base the EQL on the modal MRL. Consequently, EPA review ed 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 μ g/L) 46,273 63.0% Value > Modal MRL 16,104 21.9% Percentages may not sum to 100% because of independent rounding. A ggregate 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 de tection of thallium, and their MDLs. Applying a multiplier of 10 would give a possible PQL rage of 3.0 to 7.0 μ g/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 ( μ g/L) 200.8 0.3 200.9 0.7 Source: USEPA, 2009b (upper bound val ues when ranges are reported)
EPA concluded that although MRL values are gene rally 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 μ g/L. Its MCLG is zero. It has an ongoing health effects assessment, but there is no new heal th effects information that suggests a change in the MCLG. Consequently, the threshold for the o ccurrence 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 μ g/L, and 97.7% of the MRL values are equal to or less than the modal value. B ecause more than 80% of the MRLs are equal to or less than 0.5 μ g/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 μ g/L) 118,193 85.4% Value > Modal MRL 3,189 2.3% Percentages may not sum to 100% because of independent rounding. A ggregate 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 Distribut ion 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 de tection of trichloroe thylene, and their MDLs. Applying a multiplier of 10 would gi ve a possible PQL range from 0.42 to 1.9 μ g/L. This range contains the modal MRL. Therefore, EPA retained 0.5 μ g/L as the EQL value.
Exhibit 4-33. Analytical Methods for Trichloroethylene
Method MDL ( μ g/L) 502.2 0.06 524.2 0.19 551.1 0.042 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

The EQL of 0.5 μ g/L is ten times lower than the current PQL. EPA also performed the occurrence analysis with intermediate values of 1 μ g/L (2 x EQL) and 2.5 μ g/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 μ g/L. Its MCLG is zero. It has a new health effects assessment, but there is no new health effects in formation 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 stud ies with concentrations below the current PQL. PE studies with spiked concentrations near th e PQL also have passing rates above 90% including two studies with spiked concentrations below the PQL. EP A 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 μ g/L, and 99.3% of the MRL values are equal to or less than the modal value. B ecause more than 80% of the MRLs are equal to or less than 0.5 μ g/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 μ g/L) 120,328 86.4% Value > Modal MRL 898 0.6% Percentages may not sum to 100% because of independent rounding. A ggregate 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

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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 μ g/L. This range is lower than the modal MRL. Therefore, EPA retained 0.5 μ g/L as the EQL value.
Exhibit 4-36. Analytical Methods for Benzene
Method MDL ( μ g/L) 502.2 0.01 524.2 0.04 Source: USEPA, 2009b (upper bound val ues when ranges are reported)
The EQL of 0.5 μ g/L is ten times lower than the current PQL. EPA also performed the occurrence analysis with intermediate values of 1 μ g/L (2 x EQL) and 2.5 μ g/L (1/2 MCL).

4.2.2 Chlordane The MCL for chlordane is based on a PQL of 2 μ g/L. Its MCLG is zero, and there is no new health effects information that suggests a chan ge 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 incl uding 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 μ g/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 whet her 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 μ g/L) and ≤ 1.0 μ g/L 15,453 26.9% Value > 1.0 μ g/L 5,396 9.4% Percentages may not sum to 100% because of independent rounding. A ggregate 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 4-38. 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 de tection of chlordane, and their MDLs. Applying a multiplier of 10 would give a range of 0.04 to 2.2 μ g/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 μ g/L. Approximately 90% of the MRLs in the Six-Year Re view 2 ICR dataset are less than or equal to this value.
Exhibit 4-39. Analytical Methods for Chlordane
Method MDL ( μ g/L) 505 0.14 508 0.0041 508.1 0.004 525.2 0.22 Source: USEPA, 2009b

The EQL of 1.0 μ g/L is half of the current PQL. Th erefore, 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 μ g/L. Its MCLG is zero, and there is no new health effects information that suggests a chan ge 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 ar ound 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% in cluding one study with a spiked concentration below the PQL EPA determined that PQL assessment may s upport reduction of th e 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 μ g/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 valu es 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 μ g/L) 37,844 44.1% Value > MCL (0.2 μ g/L) and ≤ Modal MRL (0.5 μ g/L) 34,813 40.6%
Value > Modal MRL 13,144 15.3% Percentages may not sum to 100% because of independent rounding. A ggregate 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 th e detection of DBCP and their MDLs. Applying a multiplier of 10 would give a possible PQL range from 0.09 to 0.1 μ g/L. Averaging these values and rounding to one sign ificant figure gives an EQL of 0.1 μ g/L. Almost 56% of the MRL values are equal to or grea ter than the current MCL (0.2 μ g/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 MC L are, however, also below the EQL of 0.1 μ g/L.

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Exhibit 4-42. Analytical Methods for DBCP
Method MDL ( μ g/L) 504.1 0.01 551.1 0.009 Source: USEPA, 2009b (upper bound va lues 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 μ g/L based on the MDLs. The EQL of 0.1 μ g/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 μ g/L. Its MCLG is zero, and there is no new health effects information that suggest s a change in the MCLG. Consequently, the threshold for the occurrence analysis will be base d 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 curr ent 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 μ g/L, and 99.3% of the MRL values are equal to or less than the modal value. B ecause more than 80% of the MRLs are equal to or less than 0.5 μ g/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 μ g/L) 119,831 86.1% Value > Modal MRL 1,095 0.8% Percentages may not sum to 100% because of independent rounding. A ggregate 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 4-44. MRL Distributi on 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 th e detection of 1,2-dichloropropane, and their MDLs. Applying a multiplier of 10 woul d give a possible PQL range from 0.3 to 0.4 μ g/L. This range is lower than the modal MRL. Therefore, EPA retained 0.5 μ g/L as the EQL value.
Exhibit 4-45. Analytical Methods for 1,2- Dichloropropane
Method MDL ( μ g/L) 502.2 0.03 524.2 0.04 Source: USEPA, 2009b (upper bound val ues when ranges are reported)
The EQL of 0.5 μ g/L is ten times lower than the current PQL. EPA also performed the occurrence analysis with intermediate values of 1 μ g/L (2 x EQL) and 2.5 μ g/L (1/2 MCL).
4.2.5 Ethylene Dibromide (EDB) The MCL for EDB is based on a PQL of 0.05 μ g/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 s upport revision of the PQL (USEPA, 2009b).

As shown in Exhibit 4-46 and Exhibit 4-47 , the modal MRL for EDB is 0.01 μ g/L. Summary data show that 32.9% of the MRL values are equa l to or less than the mo dal 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 whet her 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 μ g/L) and ≤ MCL (0.05 μ g/L) 8,416 10.1% Value > MCL (0.05 μ g/L) 47,267 56.9% Percentages may not sum to 100% because of independent rounding. A ggregate 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 μ g/L. The lower bound of this range is th e current PQL. The MDL data do not support an EQL below the PQL.
Exhibit 4-48. Analytical Methods for EDB
Method MDL ( μ g/L) 504.1 0.01 551.1 0.032 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

EPA concluded that the combination of PT, MR L, 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 μ g/L. Its MCLG is zero, and there is no new health effects information that suggests a chan ge 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 sp iked 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 rate s 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 μ g/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 μ g/L) and ≤ 0.1 μ g/L 24,752 42.1% Value > 0.1 μ g/L 1,122 1.9% Percentages may not sum to 100% because of independent rounding. A ggregate 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 de tection of heptachl or, and their MDLs. EPA applied a multiplier of 10 to the median value of 0.005 μ g/L and rounded up to 0.1 μ g/L to obtain an EQL. More than 98% of the MRLs in th e 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 μ g/L.
Exhibit 4-51. Analytical Methods for Heptachlor
Method MDL ( μ g/L) 505 0.003 508 0.0015 508.1 0.005 525.2 0.15 551.1 0.081 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

The EQL of 0.1 μ g/L is one quarter of the current PQL. Therefore, EPA also performed the occurrence analysis with an inte rmediate threshold value of 0.2 μ g/L.
4.2.7 Heptachlor Epoxide The MCL for heptachlor epoxi de is based on a PQL of 0.2 μ g/L. Its MCLG is zero, and there is no new health effects information that suggest s a change in the MCLG. Consequently, the threshold for the occurrence analysis will be base d on analytical feasibility. The PT data show greater than 75% passing rates fo r 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 determ ined 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 μ g/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 μ g/L) and ≤ 0.1 μ g/L 25,888 44.1% Value > 0.1 μ g/L 450 0.8% Percentages may not sum to 100% because of independent rounding. A ggregate 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 4-53. 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 μ g/L and rounded up to 0.1 μ g/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, in dicating little potential for bias in occurrence estimates because of missing occurrence data above an EQL of 0.1 μ g/L..
Exhibit 4-54. Analytical Methods for Heptachlor Epoxide
Method MDL ( μ g/L) 505 0.004 508 0.0059 508.1 0.001 525.2 0.13 551.1 0.202 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

The EQL of 0.1 μ g/L is half of the current PQL. Th erefore, 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 μ g/L. Its MCLG is zer o, and there is no new health effects information that suggests a ch ange in the MCLG. Consequently, the threshold for the occurrence analysis will be based on analytic al feasibility. The PT data show greater than 75% passing rates for most of the studies ar ound 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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spiked value below the PQL has a passing rate below 75%, EPA determined that the PQL assessment supports reducti on 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 μ g/L, and 82.1% of the MRL values are equal to or less than the modal value. B ecause more than 80% of the MRLs are equal to or less than 0.1 μ g/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 μ g/L) 40,791 69.5% Value > Modal MRL 10,542 18.0% Percentages may not sum to 100% because of independent rounding. A ggregate 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 Distribut ion 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 woul d give a possible PQL range from 0.01 to 1.3 μ g/L. This range contains the modal MRL of 0.1 μ g/L.

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Exhibit 4-57. Analytical Methods for Hexachlorobenzene
Method MDL ( μ g/L) 505 0.002 508 0.0077 508.1 0.001 525.2 0.13 551.1 0.003 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

The EQL of 0.1 μ g/L is one-tenth the current PQL. Therefore, EPA also performed the occurrence analysis with an inte rmediate threshold value of 0.5 μ g/L.
4.2.9 Toxaphene The MCL for toxaphene is based on a PQL of 3 μ g/L. Its MCLG is zero, and there is no new health effects information that suggests a chan ge 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 PQ L, but there are no PT studies with spiked values below the current PQL. Two PT studies have passing rate s below 75%. Three PE studies have spiked concentrations below the PQL a nd passing rates above 90%. EPA determined that the PQL assessment may support re duction 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 μ g/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 μ g/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 μ g/L) 36,763 67.4% Value > Modal MRL 9,241 16.9% Percentages may not sum to 100% because of independent rounding. A ggregate 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 de tection of toxaphene, and their MDLs. Applying a multiplier of 10 would give a possible PQL range from 1.3 to 17.0 μ g/L. This range exceeds the modal MRL of 1.0 μ g/L.
Exhibit 4-60. Analytical Methods for Toxaphene
Method MDL ( μ g/L) 505 1.0 508 No MDL reported 508.1 0.13 525.2 1.7 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

The EQL of 1 μ g/L is one-third the current PQL. Therefore, EPA also performed the occurrence analysis with an intermediate value of 1.5 μ g/L.
4.2.10 1,1,2-Trichloroethane The MCL for 1,1,2-trichloroethan e is based on a PQL of 5 μ g/L. Its MCLG is 3 μ g/L, and there is no new health effects information that suggest s 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 μ g/L. The PT results have passing rates above 90% for most of the studies with spiked concentrations around the PQL, which include 12 studi es with spiked values below the current PQL. PE studies also have pa ssing rates above 90% for studies with spiked conentrations close to the PQL, although none ar e below the PQL. EPA determined that the PQL assessment supports reducti on 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 othe r VOCs, e.g., a modal MRL of 0.5 μ g/L. The exhibit also shows that more than 99.9% of MRLs are at or below th e 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 μ g/L) 139,616 >99.9% Value < Modal MRL 17,142 12.3% Value = Modal MRL (0.5 μ g/L) 117,788 84.3% Value > Modal MRL 4,742 3.4% Percentages may not sum to 100% because of independent rounding. A ggregate 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 Distributi on 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 woul d give a possible PQL range from 0.17 to 1.0 μ g/L. This range is below the current MCLG, which furt her supports use of the MCLG as the threshold in the occurrence analysis.

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Exhibit 4-63. Analyti cal Methods for 1,1,2- Trichloroethane
Method MDL ( μ g/L) 502.2 0.04 524.2 0.10 551.1 0.017 Source: USEPA, 2009b (upper bound val ues 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 μ g/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 μ g/L. Its MCLG is zero, and there is no new health effects information that suggests a chan ge 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 th e studies with spiked concentrat ions 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 concentr ations 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 μ g/L, and 94.0% of the MRL values are equal to or less than the modal value. B ecause more than 80% of the MRLs are equal to or less than 0.5 μ g/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 μ g/L) 105,410 75.6% Value > Modal MRL 8,361 6.0% Percentages may not sum to 100% because of independent rounding. A ggregate 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 gi ve a possible PQL range from 1.8 to 1.7 μ g/L, which is slightly below the MCL of 2.0 μ g/L. This range is also higher than the EQL of 0.5 μ g/L based on MRL data.
Exhibit 4-66. Analytical Methods for Vinyl Chloride
Method MDL ( μ g/L) 502.2 0.18 524.2 0.17 Source: USEPA, 2009b (upper bound val ues when ranges are reported)

The EQL of 0.5 μ g/L is one-fourth the current PQL. EPA also performed the occurrence analysis with an intermediate value of 1 μ g/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 μ g/L. A new health effects assessment indicates that there is potential to reduce the MCLG to 50 μ g/L. The current PQL of 90 μ g/L would limit an MCL revision. There are no PT or PE study results with spik ed 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 variab ility in the PT data, EPA concl uded 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 f easibility of a quantitation thres hold equal to the possible MCLG of 50 μ g/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 indica te 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 μ g/L) 21,445 98.4% Value < Modal MRL 5,695 26.1% Value = Modal MRL (9 μ g/L) 6,884 31.6% Value > Modal MRL 9,213 42.3% Percentages may not sum to 100% because of independent rounding. A ggregate 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 endotha ll, 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 inthe occurrence analysis.

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Exhibit 4-69. Analytical Methods for Endothall
Method MDL ( μ g/L) 548.1 1.79 Source: USEPA, 2009b (upper bound va lue 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 μ g/L. A new health effects assessment indicates that there is potential to reduce the MCLG to 2 μ g/L. The present PQL of 20 μ g/L would limit an MCL revision. There are no PT st udy 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 passi ng 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 laboratoryperformance 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 f easibility of a quantitation thres hold equal to the possible MCLG of 2 μ g/L.

As shown in Exhibit 4-70 and Exhibit 4-71 , the modal MRL for oxamyl is 2 μ g/L, which is equal to the possible MCLG. Summa ry 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 μ g/L) 31,955 61.2% Value > Modal MRL/Possible MCLG 6,911 13.2% Percentages may not sum to 100% because of independent rounding. A ggregate 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 μ g/L. This range contains the modal MRL and possible MCLG of 2.0 μ g/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 ( μ g/L) 531.1 0.86 531.2 0.065 (DL) DL = detection limit Source: USEPA, 2009b (upper bound val ues 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 Methodol ogy and Occurrence Findings Report for the Six-Year Review of Existing National Prim ary Drinking Water Regulations. EPA-815-R-03-006. June 2003.
USEPA. 2009a. Analysis of Occurrence Data fr om the Second Six-Year Review of Existing National Primary Drinking Wate r Regulations. EPA 815-B-09-006.
USEPA. 2009b. Analytical Feasib ility 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 Re view of Existing National Primary Drinking Water Regulations (Upda ted). EPA 815-B-09-002.
USEPA. 2009d. Occurrence Analysis for Potent ial Source Waters for the Second Six-Year Review of National Primary Drinki ng 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 F easibility 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 Regulator y Revisions in Support of the Second Six-Year Review of the National Primary Dri nking 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) valu es reported instead of MRL values 3

• value not identified as either an MRL or MD L (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 μ g/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 ob servations 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 lo wer values from the distribution of actual MRL values used in the EQL analysis. Rem oving the non-identified values could, however, reduce the number or MRL values, thereby introduc ing uncertainty into the use of MRL data to derive EQL values. Because the EQL developm ent process depends on the mode or most frequently occurring MRL value, deleting a substa ntial number of values could affect the result.
To address the next two data quality issues, EP A excluded approximately 1% of the remaining MRL values because their units of measure we re not reported and anot her 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 r ecord, EPA replaced the missing value with the most common MRL (i.e., the mode of the MRL dist ribution) reported in th e given state for that chemical. These values vary by state and contaminant.
To account for variations in reporting precisi on, EPA rounded all of the MRL values to one significant digit. For ex ample, a result of 0.132 μ g/L rounds to 0.1 μ g/L and a result of 132 μ g/L rounds to 100 μ g/L. EPA used the resulting distribution fo r 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% confid ence that the analyte con centration is greater than zero and is determined from replicate analyses of a sample from a given matrix containing the analyte.” The lo west 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 cont aminant levels with in specific precision bounds will be exhausted above zero, so no MRL can equal zero.

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