Monday, 

March 29, 2010 

Part II 

Environmental 

Protection Agency 

National Primary Drinking Water 

Regulations; Announcement of the Results 

of EPA’s Review of Existing Drinking 

Water Standards and Request for Public 

Comment and/or Information on Related 

Issues; Notice 


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ENVIRONMENTAL PROTECTION 

AGENCY 

[EPA–HQ–OW–2008–0747; FRL–9130–3] 

RIN 2040–AE90 

National Primary Drinking Water 

Regulations; Announcement of the 

Results of EPA’s Review of Existing 

Drinking Water Standards and Request 

for Public Comment and/or Information 

on Related Issues 

AGENCY: Environmental Protection 

Agency (EPA). 

ACTION: Notice; request for comments. 

SUMMARY: The Safe Drinking Water Act 

(SDWA) requires the United States 

Environmental Protection Agency (EPA) 

to conduct a periodic review of existing 

National Primary Drinking Water 

Regulations (NPDWRs) and determine 

which, if any, need to be revised. The 

purpose of the review, called the SixYear Review, is to identify those 

NPDWRs for which current health 

effects assessments, changes in 

technology, and/or other factors provide 

a health or technical basis to support a 

regulatory revision that will improve or 

strengthen public health protection. 

EPA has completed its detailed review 

of 71 NPDWRs and at this time believes 

that four NPDWRs are candidates for 

regulatory revision. These four NPDWRs 

are acrylamide, epichlorohydrin, 

tetrachloroethylene, and 

trichloroethylene. EPA requests public 

comment and/or relevant information 

that will assist the Agency as we move 

forward with regulatory action to revise 

these four NPDWRs. In addition to the 

71 NPDWRs discussed in detail in 

today’s action, this review also includes 

14 other NPDWRs that need no detailed 

review because of recent or ongoing 

revision actions. 

DATES: Comments must be received on 

or before May 28, 2010, 60 days after 

publication in the Federal Register. 

ADDRESSES: Submit your comments, 

identified by Docket ID No. EPA–HQ– 

OW–2008–0747, by one of the following 

methods: 

• http://www.regulations.gov: Follow 

the online instructions for submitting 

comments. 

• Mail: Water Docket, Environmental 

Protection Agency, Mailcode: 2822T, 

1200 Pennsylvania Ave., NW., 

Washington, DC 20460. 

• Hand Delivery: EPA Docket Center 

Public Reading Room, EPA 

Headquarters West, Room 3334, 1301 

Constitution Ave., NW., Washington, 

DC. Such deliveries are only accepted 

during the Docket’s normal hours of 

operation, and special arrangements 

should be made for deliveries of boxed 

information. 

Instructions: Direct your comments to 

Docket ID No. EPA–HQ–OW–2008– 

0747. EPA’s policy is that all comments 

received will be included in the public 

docket without change and may be 

made available online at http:// 

www.regulations.gov, including any 

personal information provided, unless 

the comment includes information 

claimed to be Confidential Business 

Information (CBI) or other information 

whose disclosure is restricted by statute. 

Do not submit information that you 

consider to be CBI or otherwise 

protected using http:// 

www.regulations.gov. Please contact 

EPA prior to submitting CBI. 

The http://www.regulations.gov Web 

site is an ‘‘anonymous access’’ system, 

which means EPA will not know your 

identity or contact information unless 

you provide it in the body of your 

comment. If you send an e-mail 

comment directly to EPA without going 

through http://www.regulations.gov 

your e-mail address will be 

automatically captured and included as 

part of the comment that is placed in the 

public docket and made available on the 

Internet. If you submit an electronic 

comment, EPA recommends that you 

include your name and other contact 

information in the body of your 

comment and with any disk or CD–ROM 

you submit. If EPA cannot read your 

comment due to technical difficulties 

and cannot contact you for clarification, 

EPA may not be able to consider your 

comment. Electronic files should avoid 

the use of special characters, any form 

of encryption, and be free of any defects 

or viruses. For additional instructions 

on submitting comments, go to section 

I.B of this document. 

Docket: All documents in the docket 

are listed in the http:// 

www.regulations.gov index. Although 

listed in the index, some information is 

not publicly available, e.g., CBI or other 

information whose disclosure is 

restricted by statute. Certain other 

material, such as copyrighted material, 

will be publicly available only in hard 

copy. Publicly available docket 

materials are available either 

electronically in http:// 

www.regulations.gov or in hard copy at 

the Water Docket, EPA/DC, EPA West, 

Room 3334, 1301 Constitution Ave., 

NW., Washington, DC. The Public 

Reading Room is open from 8:30 a.m. to 

4:30 p.m., Monday through Friday, 

excluding legal holidays. The telephone 

number for the Public Reading Room is 

(202) 566–1744, and the telephone 

number for the EPA Docket Center is 

(202) 566–2426. 

FOR FURTHER INFORMATION CONTACT: For 

technical inquiries contact: Rajiv Khera, 

(202) 564–4881, or Karen Wirth, (202) 

564–5246, Office of Ground Water and 

Drinking Water, Environmental 

Protection Agency. For general 

information about, and copies of, this 

document or information about the 

existing NPDWRs discussed in this 

action, contact the Safe Drinking Water 

Hotline. Callers within the United States 

may reach the Hotline at (800) 426– 

4791. The Hotline is open Monday 

through Friday, excluding Federal 

holidays, from 9 a.m. to 5:30 p.m. 

Eastern Time. 

Abbreviations and Acronyms Used in 

This Action 

>—greater than 

2,4-D—2,4-dichlorophenoxyacetic acid 

μg/L—microgram per liter 

AMG—Alternative Monitoring Guidelines 

ASDWA—Association of State Drinking 

Water Administrators 

ATSDR—Agency for Toxic Substances and 

Disease Registry 

AWWA—American Water Works Association 

BAT—best available technology 

CARC—Cancer Assessment Review 

Committee 

CBI—Confidential Business Information 

CCL—Contaminant Candidate List 

CFR—Code of Federal Regulations 

Cr III—trivalent chromium 

Cr VI—hexavalent chromium 

CWS—community water system 

DBPs—disinfection byproducts 

DBCP—1,2-dibromo-3-chloropropane 

DBPR—Disinfectants and Disinfection 

Byproducts Rule 

DEHA—di(2-ethylhexyl)adipate 

DEHP—di(2-ethylhexyl)phthalate 

DWEL—drinking water equivalent level 

EDB—ethylene dibromide 

EPA—U.S. Environmental Protection Agency 

EQL—estimated quantitation level 

ESA—ethanesulfonic acid 

FR—Federal Register 

FQPA—Food Quality Protection Act 

GAC—granular activated carbon 

GWR—Ground Water Rule 

HAA5—haloacetic acids 

IARC—International Agency for Research on 

Cancer 

ICR—Information Collection Request 

IRED—Interim Reregistration Eligibility 

Decision 

IRIS—Integrated Risk Information System 

LCR—Lead and Copper Rule 

LH—lutenizing hormone 

LOAEL—lowest-observed-adverse-effect level 

LT2ESWTR—Long-Term 2 Enhanced Surface 

Water Treatment Rule 

MCL—maximum contaminant level 

MCLG—maximum contaminant level goal 

MDL—method detection limit 

mg/kg-day—milligrams per kilogram of body 

weight per day 

mg/L—milligrams per liter 

MOA—mode of action 

MRL—minimum reporting level 

N—nitrogen 

NAS—National Academy of Sciences 


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NAWQA—National Water Quality 

Assessment 

NCFAP—National Center for Food and 

Agricultural Policy 

NCOD—National Drinking Water 

Contaminant Occurrence Database 

NDWAC—National Drinking Water Advisory 

Council 

NELAC—National Environmental Laboratory 

Accreditation Conference 

NOAEL—no-observed-adverse-effect level 

NPDWR—National Primary Drinking Water 

Regulation 

NRC—National Research Council 

NTNCWS—non-transient, non-community 

water system 

NTP—National Toxicology Program 

OPP—Office of Pesticide Programs 

ORD—Office of Research and Development 

OW—Office of Water 

PCBs—polychlorinated biphenyls 

PCE—tetrachloroethylene 

PE—Performance Evaluation 

pCi/L—picoCurie per liter 

PN—public notification 

ppb—part per billion (e.g., microgram per 

liter) 

ppm—part per million (e.g., milligram per 

liter) 

PQL—practical quantitation limit 

PT—Performance Testing 

PTA—packed tower aeration 

PWS—public water system 

R2S2—Regulatory Review Support 

Spreadsheet 

RED—Reregistration Eligibility Decision 

RfD—reference dose 

RSC—relative source contribution 

SAB—Science Advisory Board 

SSCT—Small System Compliance 

Technology 

SDWA—Safe Drinking Water Act 

SDWIS/FED—Safe Drinking Water 

Information System/Federal version 

SMCL—secondary maximum contaminant 

level 

SOC—synthetic organic chemical 

STORET—STOrage and RETrieval data 

system 

SWTR—Surface Water Treatment Rule 

T3—triiodothyronine (thyroid hormone) 

T4—levothyroxine (thyroid hormone) 

TCDD—tetrachlorodibenzo-p-dioxin 

TCE—trichloroethylene 

TNCWS—transient, non-community water 

system 

TP—trichlorophenoxypropionic acid 

TRED—Interim Tolerance Reassessment and 

Risk Management Decisions 

TRI—Toxics Release Inventory 

TSC—Technical Support Center 

TT—treatment technique 

TTHM—total trihalomethanes 

USDA—U.S. Department of Agriculture 

UCMR 2—second Unregulated Contaminant 

Monitoring Rule 

USGS—U.S. Geological Survey 

VOC—volatile organic compound 

WS—water supply 

Table of Contents 

I. General Information 

A. Does This Action Apply to My Public 

Water System? 

B. How Should I Submit Comments on 

This Action? 

C. What Should I Consider as I Prepare My 

Comments for EPA? 

II. Statutory Requirements for the Six-Year 

Review 

III. Stakeholder Involvement in the Six-Year 

Review Process 

A. How Have Stakeholders Been Involved 

in the Review Process? 

B. How Did EPA Incorporate Feedback 

from the Science Advisory Board’s 2002 

Comments on the Six-Year Review 

Protocol? 

IV. Regulations Included in the Six-Year 

Review 

V. EPA’s Protocol for Reviewing the NPDWRs 

Included in This Action 

A. What Was EPA’s Review Process? 

B. How Did EPA Conduct the Initial 

Review and Evaluate Key Technical 

Elements of the NPDWRs? 

1. Initial Review 

2. Health Effects 

3. Analytical Feasibility 

4. Occurrence and Exposure Analysis 

5. Treatment Feasibility 

6. Other Regulatory Revisions 

C. How Did EPA Factor Children’s Health 

Concerns Into the Review? 

VI. Results of EPA’s Review of NPDWRs 

A. What Are the Review Result Categories? 

1. No Action at This Time and the NPDWR 

is Still Appropriate 

2. Candidate for Revision 

B. What Are the Details of EPA’s Review 

of Each NPDWR? 

1. Acrylamide 

2. Alachlor 

3. Alpha Particle Emitters 

4. Antimony 

5. Arsenic 

6. Asbestos 

7. Atrazine 

8. Barium 

9. Benzene 

10. Benzo(a)pyrene 

11. Beryllium 

12. Beta Particle and Photon Emitters 

13. Cadmium 

14. Carbofuran 

15. Carbon Tetrachloride 

16. Chlordane 

17. Chromium 

18. Cyanide 

19. 2,4-D (2,4-Dichlorophenoxyacetic acid) 

20. Dalapon (2,2-Dichloropropionic Acid) 

21. Di(2-ethylhexyl)adipate (DEHA) 

22. Di(2-ethylhexyl)phthalate (DEHP) 

23. 1,2-Dibromo-3-chloropropane (DBCP) 

24. 1,2-Dichlorobenzene (oDichlorobenzene) 

25. 1,4-Dichlorobenzene (pDichlorobenzene) 

26. 1,2-Dichloroethane (Ethylene 

Dichloride) 

27. 1,1-Dichloroethylene 

28. cis-1,2-Dichloroethylene 

29. trans-1,2-Dichloroethylene 

30. Dichloromethane (Methylene Chloride) 

31. 1,2-Dichloropropane 

32. Dinoseb 

33. Diquat 

34. Endothall 

35. Endrin 

36. Epichlorohydrin 

37. Ethylbenzene 

38. Ethylene Dibromide (EDB; 1,2- 

Dibromoethane) 

39. Fluoride 

40. Glyphosate 

41. Heptachlor 

42. Heptachlor Epoxide 

43. Hexachlorobenzene 

44. Hexachlorocyclopentadiene 

45. Lindane (gammaHexachlorocyclohexane) 

46. Mercury (Inorganic) 

47. Methoxychlor 

48. Monochlorobenzene (Chlorobenzene) 

49. Nitrate (as N) 

50. Nitrite (as N) 

51. Oxamyl (Vydate) 

52. Pentachlorophenol 

53. Picloram 

54. Polychlorinated Biphenyls (PCBs) 

55. Combined Radiums (226 and 228) 

56. Selenium 

57. Simazine 

58. Styrene 

59. 2,3,7,8–TCDD (Dioxin) 

60. Tetrachloroethylene 

61. Thallium 

62. Toluene 

63. Toxaphene 

64. 2,4,5-TP (Silvex; 2,4,5- 

Trichlorophenoxypropionic Acid) 

65. 1,2,4-Trichlorobenzene 

66. 1,1,1-Trichloroethane 

67. 1,1,2-Trichloroethane 

68. Trichloroethylene 

69. Uranium 

70. Vinyl chloride 

71. Xylenes (Total) 

VII. EPA’s Request for Comments 

A. Request for Comment and/or 

Information on the Candidates for 

Revision 

B. Request for Information/Data on Other 

Review Topics 

C. Requests for Information on the Impacts 

of Climate Change on Water Quality 

VIII. EPA’s Next Steps 

IX. References 

SUPPLEMENTARY INFORMATION: 

I. General Information 

A. Does This Action Apply to My Public 

Water System? 

This action itself does not impose any 

requirements on anyone. Instead, it 

notifies interested parties of EPA’s 

review of existing NPDWRs and its 

conclusions about which of these 

warrants new regulatory action at this 

time. EPA requests public comment on 

the four NPDWRs identified as 

candidates for revision, with a specific 

focus on comments and/or relevant 

information that will inform the 

regulatory revisions. 

B. How Should I Submit Comments on 

This Action? 

Please see Section VII for the issues 

related to this notice for which EPA 

requests comment and/or information. 

EPA will accept written or electronic 

comments (please do not send both). 

Instructions for submitting comments 

are in the preceding section. EPA 

prefers electronic comments. No 


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1Under limited circumstances, SDWA Section 

1412(b)(6)(A) also gives the Administrator the 

discretion to promulgate an MCL that is less 

stringent than the feasible level and that 

‘‘maximizes health risk reduction benefits at a cost 

that is justified by the benefits.’’ 

facsimiles (faxes) will be accepted. 

Commenters who want EPA to 

acknowledge receipt of their comments 

should also send a self-addressed, 

stamped envelope. 

The Agency intends to address the 

comments received on the four 

NPDWRs identified as candidates for 

revision in subsequent Federal Register 

notices proposing and finalizing the 

regulatory revisions, and in documents 

that will be made available in the docket 

for those notices. 

C. What Should I Consider as I Prepare 

My Comments for EPA? 

You may find the following 

suggestions helpful for preparing your 

comments: 

• Explain your views as clearly as 

possible. 

• Describe any assumptions that you 

used. 

• Provide any technical information 

and/or data you used that support your 

views. 

• If you estimate potential burden or 

costs, explain how you arrived at your 

estimate. 

• Provide specific examples to 

illustrate your concerns. 

• Offer alternatives. 

• Make sure to submit your 

comments by the comment period 

deadline. 

• To ensure proper receipt by EPA, 

identify the appropriate docket 

identification number in the subject line 

on the first page of your response. It 

would also be helpful if you provided 

the name, date, and Federal Register 

citation related to your comments. 

II. Statutory Requirements for the SixYear Review 

Under the SDWA, as amended in 

1996, EPA must periodically review 

existing national primary drinking water 

regulations (NPDWRs) and, if 

appropriate, revise them. Section 

1412(b)(9) of SDWA states: 

The Administrator shall, not less often 

than every 6 years, review and revise, as 

appropriate, each national primary drinking 

water regulation promulgated under this title. 

Any revision of a national primary drinking 

water regulation shall be promulgated in 

accordance with this section, except that 

each revision shall maintain, or provide for 

greater, protection of the health of persons. 

Pursuant to the 1996 SDWA 

Amendments, EPA completed and 

published the results of its first Six-Year 

Review (Six-Year Review 1) July 18, 

2003 (68 FR 42908, USEPA, 2003e) after 

developing a systematic approach, or 

protocol, for the review of NPDWRs. 

EPA has applied the same protocol with 

minor refinements (revised protocol) to 

the second Six-Year Review of NPDWRs 

(Six-Year Review 2). Section V of 

today’s action describes the protocol 

and the minor refinements used for the 

Six-Year Review 2 and section VI 

describes the review findings for each of 

the NPDWRs covered by the current 

effort (see Table IV–1). 

III. Stakeholder Involvement in the SixYear Review Process 

A. How Have Stakeholders Been 

Involved in the Review Process? 

The Agency developed a Six-Year 

Review protocol during the first review 

cycle with extensive stakeholder inputs, 

including a stakeholder meeting, 

Agency presentations at a variety of 

meetings, and consultation with the 

National Drinking Water Advisory 

Council (NDWAC). NDWAC formed a 

working group to develop 

recommendations regarding the process 

the Agency should apply to conduct a 

periodic and systematic review of 

existing NPDWRs. The Working Group 

held two meetings and a conference call 

during June through September 2000 (67 

FR 19030, April 17, 2002, USEPA, 

2002c). The NDWAC approved the 

Working Group’s recommendations in 

November 2000, and formally provided 

them to EPA in December 2000 

(NDWAC, 2000). The NDWAC 

recommended that EPA’s review 

include consideration of five key 

elements, as appropriate: health effects, 

analytical and treatment feasibility, 

implementation-related issues, 

occurrence and exposure, and economic 

impacts. As discussed in more detail in 

section V of today’s action, EPA 

continues to follow the general protocol 

recommended by the NDWAC. 

B. How Did EPA Incorporate Feedback 

From the Science Advisory Board’s 2002 

Comments on the Six-Year Review 

Protocol? 

In June 2002 and during the Six-Year 

Review 1, EPA consulted with the 

Science Advisory Board (SAB) Drinking 

Water Committee and requested their 

review and comment on whether the 

protocol that EPA developed based on 

the NDWAC’s recommendations was 

consistently applied and appropriately 

documented. The SAB provided verbal 

feedback regarding the transparency and 

clarity of EPA’s criteria for making its 

Six-Year Review 1 decisions. At that 

time, EPA revised the protocol to better 

explain how the decision criteria were 

applied. For the Six-Year Review 2 and 

to increase transparency and clarity, 

EPA also developed a more detailed 

decision tree and an automated tool, 

called the Regulatory Review Support 

Spreadsheet (R2S2). The more detailed 

decision tree incorporates the sequential 

relationships between the various 

NPDWR review elements and R2S2 

tracks each contaminant through the 

decision making process. The Agency 

has documented the decision tree and 

the automated tool in the document, 

‘‘EPA Protocol for the Second Review of 

Existing National Primary Drinking 

Water Regulations (Updated)’’ (USEPA, 

2009a). 

IV. Regulations Included in the SixYear Review 

Table IV–1 lists all the NPDWRs 

established to date. The table also 

reports the maximum contaminant level 

goal (MCLG), which is ‘‘set at the level 

at which no known or anticipated 

adverse effects on the health of persons 

occur and which allows an adequate 

margin of safety’’ (SDWA section 

1412(b)(4)), and the maximum 

contaminant level (MCL), which is the 

maximum permissible level of a 

contaminant in water delivered to any 

user of a public water system and ‘‘is as 

close to the maximum contaminant 

level goal as is feasible’’ (SDWA section 

1412(b)(4)(B)), except for contaminants 

that have a treatment technique (TT) in 

lieu of an MCL because it is not 

‘‘economically or technically feasible’’ to 

set an MCL (SDWA section 

1412(b)(7)(A)).1 Of these 85 NPDWRs, 

EPA has reviewed 14 as part of recent 

or ongoing regulatory actions and, as a 

result, they are not subject to a detailed 

review in today’s notice. The review for 

the remaining 71 is discussed in detail 

in today’s action. 


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TABLE IV–1—CONTAMINANTS WITH NPDWRS INCLUDED IN SIX-YEAR REVIEW 2 

Contaminants MCLG 

(mg/L) 1

MCL 

(mg/L) 1 Contaminants MCLG (mg/L) 1

MCL 

(mg/L) 1

Acrylamide ............................. 0 .................................. TT ................................ Epichlorohydrin ..................... 0 .................. TT 

Alachlor .................................. 0 .................................. 0.002 ........................... Ethylbenzene ........................ 0.7 ............... 0.7 

Alpha particles ....................... 0 (pCi/L) ...................... 15 (pCi/L) .................... Ethylene dibromide (EDB) .... 0 .................. 0.00005 

Antimony ................................ 0.006 ........................... 0.006 ........................... Fluoride ................................. 4 .................. 4 

Arsenic ................................... 0 .................................. 0.01 ............................. Giardia lamblia ...................... 0 .................. TT 

Asbestos ................................ 7 (million fibers/L) ........ 7 (million fibers/L) ........ Glyphosate ............................ 0.7 ............... 0.7 

Atrazine ................................. 0.003 ........................... 0.003 ........................... Haloacetic acids (HAA5) ...... n/a 2 ............. 0.06 

Barium ................................... 2 .................................. 2 .................................. Heptachlor ............................ 0 .................. 0.0004 

Benzene ................................ 0 .................................. 0.005 ........................... Heptachlor Epoxide .............. 0 .................. 0.0002 

Benzo(a)pyrene ..................... 0 .................................. 0.0002 ......................... Hexachlorobenzene .............. 0 .................. 0.001 

Beryllium ................................ 0.004 ........................... 0.004 ........................... Hexachlorocyclopentadiene .. 0.05 ............. 0.05 

Beta particles ......................... 0 (millirems/yr) ............. 4 (millirems/yr) ............. Lead ...................................... 0 .................. TT 

Bromate ................................. 0 .................................. 0.01 ............................. Legionella ............................. 0 .................. TT 

Cadmium ............................... 0.005 ........................... 0.005 ........................... Lindane ................................. 0.0002 ......... 0.0002 

Carbofuran ............................. 0.04 ............................. 0.04 ............................. Mercury (Inorganic) .............. 0.002 ........... 0.002 

Carbon tetrachloride .............. 0 .................................. 0.005 ........................... Methoxychlor ........................ 0.04 ............. 0.04 

Chloramines .......................... 4 .................................. 4 .................................. Monochlorobenzene (Chlorobenzene).0.1 ............... 0.1 

Chlordane .............................. 0 .................................. 0.002 ........................... Nitrate (as nitrogen, N) ......... 10 ................ 10 

Chlorine ................................. 4 .................................. 4 .................................. Nitrite (as N) ......................... 1 .................. 1 

Chlorine dioxide ..................... 0.8 ............................... 0.8 ............................... Oxamyl (Vydate) ................... 0.2 ............... 0.2 

Chlorite .................................. 0.8 ............................... 1 .................................. Pentachlorophenol ................ 0 .................. 0.001 

Chromium (total) .................... 0.1 ............................... 0.1 ............................... Picloram ................................ 0.5 ............... 0.5 

Coliform ................................. 0% 3 ............................. 5% 3 ............................. Polychlorinated biphenyls 

(PCBs).

0 .................. 0.0005 

Copper ................................... 1.3 ............................... TT ................................ Radium ................................. 0 (pCi/L) ...... 5 (pCi/L) 

Cryptosporidium .................... 0 .................................. TT ................................ Selenium ............................... 0.05 ............. 0.05 

Cyanide ................................. 0.2 ............................... 0.2 ............................... Simazine ............................... 0.004 ........... 0.004 

2,4-Dichlorophenoxyacetic 

acid (2,4-D).

0.07 ............................. 0.07 ............................. Styrene ................................. 0.1 ............... 0.1

Dalapon ................................. 0.2 ............................... 0.2 ............................... 2,3,7,8-Tetrachlorodibenzo-pdioxin (2,3,7,8-TCDD or 

dioxin).

0 .................. 3.00E-08 

Di(2-ethylhexyl)adipate 

(DEHA).

0.4 ............................... 0.4 ............................... Tetrachloroethylene (PCE) ... 0 .................. 0.005 

Di(2-ethylhexyl)phthalate 

(DEHP).

0 .................................. 0.006 ........................... Thallium ................................ 0.0005 ......... 0.002

1,2-Dibromo-3-chloropropane 

(DBCP).

0 .................................. 0.0002 ......................... Toluene ................................. 1 .................. 1 

1,2-Dichlorobenzene (oDichlorobenzene).0.6 ............................... 0.6 ............................... Total trihalomethanes 

(TTHM).

n/a 4 ............. 0.08 

1,4-Dichlorobenzene (pDichlorobenzene).0.075 ........................... 0.075 ........................... Toxaphene ............................ 0 .................. 0.003 

1,2-Dichloroethane (Ethylene 

dichloride).

0 .................................. 0.005 ........................... 2,4,5-Trichlorophenoxypropionic acid (2,4,5-TP or 

Silvex).

0.05 ............. 0.05 

1,1-Dichloroethylene .............. 0.007 ........................... 0.007 ........................... 1,2,4-Trichlorobenzene ......... 0.07 ............. 0.07 

cis-1,2-Dichloroethylene ........ 0.07 ............................. 0.07 ............................. 1,1,1-Trichloroethane ............ 0.2 ............... 0.2 

trans-1,2-Dichloroethylene .... 0.1 ............................... 0.1 ............................... 1,1,2-Trichloroethane ............ 0.003 ........... 0.005 

Dichloromethane (Methylene 

chloride).

0 .................................. 0.005 ........................... Trichloroethylene (TCE) ....... 0 .................. 0.005 

1,2-Dichloropropane .............. 0 .................................. 0.005 ........................... Uranium ................................ 0 (μg/L) ....... 30 (μg/L) 

Dinoseb ................................. 0.007 ........................... 0.007 ........................... Vinyl chloride ........................ 0 .................. 0.002 

Diquat .................................... 0.02 ............................. 0.02 ............................. Viruses .................................. 0 .................. TT 

Endothall ................................ 0.1 ............................... 0.1 ............................... Xylenes (total) ....................... 10 ................ 10 

Endrin .................................... 0.002 ........................... 0.002.

1. Units are in milligrams per liter (mg/L) unless otherwise noted, e.g., micrograms per liter (μg/L) and picoCuries per liter (pCi/L). Milligrams 

per liter are equivalent to parts per million (ppm) and micrograms per liter are equivalent to parts per billion (ppb). 

2. There is no MCLG for all five haloacetic acids. MCLGs for some of the individual contaminants are: dichloroacetic acid (zero), trichloroacetic 

acid (0.02 mg/L), and monochloroacetic acid (0.07 mg/L). Bromoacetic acid and dibromoacetic acid are regulated with this group but have no 

MCLGs. 

3. No more than 5.0% samples total coliform-positive in a month. 

4. There is no MCLG for total trihalomethanes. MCLGs for some of the individual contaminants are: bromodichloromethane (zero), bromoform 

(zero), dibromochloromethane (0.06 mg/L), and chloroform (0.07mg/L). 


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2Note that the legislative history of the 1996 

SDWA Amendments indicate that Congress 

envisioned the possibility that a relaxed standard 

might be appropriate under circumstances that 

would not result in a lessening of the level of public 

health protection (see Senate Report Number 104– 

169, 104th Congress, 1st Session, 1995 at 38). In 

other words, an MCL could be relaxed (i.e., 

increased) in cases where a revised health risk 

assessment leads to a less stringent (higher) MCLG 

than the existing MCL so that the level of health 

protection is maintained. There have been several 

instances in which revised health assessments have 

suggested higher MCLGs and the Agency could 

have considered relaxing the MCLs. In these 

instances and because SDWA allows EPA to 

determine when revisions are appropriate, the 

Agency decided that there would be a negligible 

gain in public health protection and/or cost savings 

and any revision would be a low priority activity 

because of competing workload priorities, the 

administrative costs associated with rulemaking, 

and the burden on States and the regulated 

community to implement any regulatory changes. 

3The following Federal Register notices describe 

the process the Agency has used to determine 

analytical feasibility for drinking water 

contaminants: 50 FR 46880, November 13, 1985 

(USEPA, 1985); 52 FR 25690, July 8, 1987 (USEPA, 

1987); 54 FR 22062, May 22, 1989 (USEPA, 1989b). 

For this Six Year Review effort and to supplement 

the analytical feasibility evaluation, the Agency also 

reviewed extensive minimum reporting level (MRL) 

data obtained from States and primacy entities as 

part of the Six-Year Review information collection 

request (ICR) for SDWA compliance monitoring 

data. 

V. EPA’s Protocol for Reviewing the 

NPDWRs Included in This Action 

A. What Was EPA’s Review Process? 

The protocol document, ‘‘EPA 

Protocol for the Review of Existing 

National Primary Drinking Water 

Regulations (Updated)’’ (USEPA, 2009a), 

contains a detailed description of the 

process the Agency used to review the 

NPDWRs discussed in today’s action. 

EPA’s primary goal was to identify and 

prioritize candidates for regulatory 

revision to target those revisions that are 

most likely to result in an increased 

level of public health protection and/or 

result in substantial cost savings for 

systems and their customers while 

maintaining the level of public health 

protection.2 This section provides an 

overview of the review process and 

section V.B provides a more detailed 

description of how EPA applied the 

process to the review of the NPDWRs 

discussed in today’s action. 

EPA applied the following basic 

principles to the review process: 

• The Agency sought to avoid 

redundant review efforts. Because EPA 

has reviewed information for 14 

contaminants as part of recent or 

ongoing regulatory actions, they are not 

subject to the detailed review in today’s 

notice. 

• EPA evaluated the potential for new 

information to affect NPDWRs in a 

manner consistent with existing policies 

and procedures for developing 

NPDWRs. For example, in determining 

whether a possible change in analytical 

feasibility existed, the Agency 

considered the current policy and 

procedures for calculating the practical 

quantitation level for drinking water 

contaminants.3

• Because any possible change in an 

MCLG affects other NPDWR elements, 

EPA will not generally consider 

potential revisions to any contaminant 

with a health effects assessment in 

process that would not be completed 

during the review period, where either 

the contaminant’s MCL is equal to its 

MCLG or the MCL is based on the 1996 

SDWA Amendments’ cost-benefit 

provision. The rationale for this 

outcome is that any new information 

from the health effects assessment could 

affect the MCL or the assessment of the 

benefits associated with the MCL for 

these contaminants. Therefore, the 

Agency does not believe it is 

appropriate to consider revisions to 

these NPDWRs while a health effects 

assessment is ongoing. 

• For those contaminants with 

ongoing health assessments that have 

MCLGs equal to or greater than zero and 

MCLs limited by analytical feasibility or 

the standard is based on a Treatment 

Technique, EPA conducted a further 

review of the potential to revise the 

MCL or TT. The rationale for this 

approach is that the MCL or TT is based 

on technology limitations and therefore, 

EPA should consider whether there 

have been improvements in technology 

and whether any revision might provide 

a meaningful opportunity to improve or 

at least maintain public health 

protection. If EPA found that there were 

no changes in technology (i.e., 

analytical feasibility or a TT) or if 

changes were possible but there was no 

meaningful opportunity to improve 

public health protection or reduce costs 

(while maintaining public health 

protection), these contaminants 

remained in the ongoing health effects 

assessment category. 

• For this review, EPA considered 

new information from health effects 

assessments that were completed by a 

March 1, 2009 cutoff date. If an updated 

assessment is completed after the March 

1, 2009 information cutoff date, then 

EPA will review the update and any 

new conclusions or additional 

information associated with the 

contaminant during the next review 

cycle or during the revision of an 

NPDWR (e.g., acrylamide, PCE and 

TCE). If the health effects assessments 

are not completed in time for the 

regulatory revisions for acrylamide, PCE 

and TCE, EPA does not plan to change 

the existing MCLG of zero. EPA is 

currently considering how best to 

evaluate the benefits for these regulatory 

revisions if the EPA health effects 

assessments are not complete. One 

option would be to use the same health 

effects information that was used for 

promulgating the original regulation. 

Another option is to consider using 

other best available, peer-reviewed 

health risk assessments that are 

complete as the Agency is proceeding 

with the regulatory revisions. EPA 

requests comment on these options and 

any other options that the public 

considers appropriate to evaluate the 

benefits. 

• The Agency may consider 

accelerating a review and potential 

revision for a particular NPDWR before 

the next review cycle when justified by 

new public health risk information. 

• During the review, EPA identified 

areas where information is inadequate 

or unavailable (data gaps) or emerging 

and is needed to determine whether 

revision to an NPDWR is appropriate. 

When the Agency is able to fill such 

gaps or fully evaluate the emerging 

information, the Agency will consider it 

as part of the next review cycle. The 

Agency may consider accelerating a 

review and potential revision for a 

particular NPDWR if the information 

becomes available before the next 

review cycle and if review and a 

potential revision are justified by new 

public health risk information. 

• EPA applied the Agency’s peer 

review policy (USEPA, 2000d), where 

appropriate, to any new analyses. 

During Six-Year Review 1, the Agency 

developed a systematic approach or 

protocol (USEPA, 2003b). The Agency 

based this protocol on the 

recommendations of the NDWAC, 

through internal Agency deliberations, 

and discussions with the diverse group 

of stakeholders involved in drinking 

water and its protection. The overview 

of the protocol in Figure V–1 shows the 

sequence of key decisions that led to 

EPA assigning each NPDWR to one of 

two major categories of outcomes in the 

Six-Year Review 2. The two major 

outcomes of the review are either: 1) 

The NPDWR is still appropriate and no 

action is necessary at this time, or 2) the 

NPDWR is a candidate for revision. The 

reasons for a Six-Year Review outcome 

of no further action at this time include 

at least one or more of the following 

reasons: 

• The NPDWR has been reviewed or 

is being reviewed in a recent or ongoing 

action; 

• The NPDWR has an ongoing health 

effects assessment (i.e., for those 


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NPDWRs with an MCL set at the MCLG 

or the MCL is based on the SDWA cost 

benefit provision); 

• EPA is considering whether a new 

health effects assessment is needed; 

• EPA did not identify any new, 

relevant information that indicate 

changes to the NPDWR; 

• New information indicate a possible 

change to the MCLG and/or MCL but 

changes to the NPDWR are a low 

priority activity due to negligible gains 

in public health protection and/or cost 

savings; or 

• There are data gaps or emerging 

information that needs to be evaluated. 

During the current Six-Year Review, 

the Agency assessed the protocol and 

determined it remained appropriate and 

suitable for the second review. The 

research requirements and decisionmaking process of the Six-Year Review 

2 protocol are essentially the same as 

those implemented during Six-Year 

Review 1. The Agency made some 

minor refinements to enhance the 

Agency’s effectiveness in applying the 

protocol to the review of NPDWRs. The 


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refinements that address SAB’s 

comments about the clarity and the 

transparency of the protocol’s decision 

making process are described in the 

next two paragraphs. Section V.B 

describes the key technical elements 

and any refinements in the data and/or 

the analysis methods used during SixYear Review 2. 

The primary refinement to the 

protocol during Six-Year Review 2 is the 

implementation of a more detailed 

‘‘decision tree’’ than either the one used 

during Six-Year Review 1 (USEPA, 

2003b) or the overview shown in Figure 

V–1. The protocol is broken down into 

a series of questions about whether 

there is new information for a 

contaminant that suggests potential to 

revise each of the NPDWR elements. 

These questions are logically ordered 

into a decision tree that incorporates the 

sequential relationships between the 

different NPDWR elements. For 

example, when EPA establishes an 

MCL, it must generally set the MCL as 

close to the MCLG as feasible. 

Consequently, for a contaminant that 

has an MCL equal to its MCLG, EPA 

must make decisions about the 

availability and adequacy of new 

information regarding the possibility to 

revise the MCLG before decisions 

regarding the possibility to revise the 

MCL. It also means that if there is no 

possibility to revise a contaminant’s 

MCLG and the MCL is already equal to 

the MCLG, then there is no basis for 

revising the MCL. In this instance, the 

MCL branch of the decision tree is not 

reached, and it is not necessary to make 

related decisions such as whether the 

practical quantitation limit (PQL) can be 

revised. This approach results in a more 

efficient review process. EPA also 

developed an automated tool called the 

R2S2 that tracks each contaminant’s 

movement through the decision tree, 

including the revise/take no action 

outcomes. This tool enhances 

transparency throughout the decision 

process. The automation also 

streamlines the decision process and 

facilitates the Agency’s reporting of its 

review results. The Agency has 

documented the decision tree and the 

automated tool in the document 

entitled, ‘‘EPA Protocol for the Second 

Review of Existing National Primary 

Drinking Water Regulations (Updated)’’ 

(USEPA, 2009a). 

B. How Did EPA Conduct the Initial 

Review and Evaluate Key Technical 

Elements of the NPDWRs? 

This section describes the specific 

technical reviews that EPA conducted, 

including the initial review, health 

effects, analytical methods, occurrence 

and exposure, treatment feasibility, and 

economic analysis. 

1. Initial Review 

EPA’s initial review of all the 

contaminants included in the Six-Year 

Review 2 involved a simple 

identification of the NPDWRs that were 

being reviewed under concurrent EPA 

actions or had been reviewed and 

revised in EPA actions completed since 

2002. Table V–1 provides a list of the 14 

contaminants that met one of these 

criteria and identifies the recent or 

ongoing action in which the 

contaminant has been reviewed or is 

undergoing review. While these 14 

contaminants are part of the Six-Year 

Review 2, they were not subject to any 

detailed analysis given that new 

information on these contaminants has 

been recently reviewed under separate 

actions. However, EPA requests 

comments on these contaminants along 

with the other contaminants discussed 

in detail in this notice. 

The remaining 71 contaminants pass 

through this step to the review of the 

technical NPDWR elements, which are 

described in the following sections. 

TABLE V–1—NPDWRS THAT HAVE BEEN REVIEWED OR ARE BEING REVIEWED UNDER RECENT OR ONGOING ACTIONS

Contaminant/indicator Recent or ongoing action 

Disinfection Byproducts 

Bromate .................................................................................................... Stage 2 DBPR. 

Chlorite1 .................................................................................................... Stage 2 DBPR. 

HAA5: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, 

monobromoacetic acid, dibromoacetic acid.

Stage 2 DBPR. 

TTHMs: chloroform, bromodichloromethane, ...........................................

dibromochloromethane, bromoform .........................................................

Stage 2 DBPR. 

Disinfectant Residuals 

Chloramines1 ............................................................................................ Stage 2 DBPR. 

Chlorine1 ................................................................................................... Stage 2 DBPR. 

Chlorine dioxide ........................................................................................ Stage 2 DBPR. 

Inorganics 

Copper ...................................................................................................... Under consideration for long-term revisions. 

Lead .......................................................................................................... LCR Short-Term Revisions 

Under consideration for long-term revisions. 

Microorganisms 

Coliform .................................................................................................... Total Coliform Rule-making currently underway. 

Cryptosporidium ........................................................................................ LT2ESWTR. 

Giardia lamblia .......................................................................................... LT2ESWTR. 

Legionella 2 ............................................................................................... LT2ESWTR, 

CCL3 3. 

Viruses 2 .................................................................................................... LT2ESWTR, GWR, CCL3 3. 

DBPR—Disinfectants and Disinfection Byproducts Rule. 

LT2ESWTR—Long-Term 2 Enhanced Surface Water Treatment Rule. 

LCR—Lead and Copper Rule. 

GWR—Ground Water Rule. 


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Dates of promulgation are as follows: 

Stage 2 DBPR: 71 FR 388, January 4, 2006 (USEPA, 2006h). 

LT2ESWTR: 71 FR 654, January 5, 2006 (USEPA, 2006g). 

LCR Short-Term Regulatory Revisions: 72 FR 57782, October 10, 2007 (USEPA, 2007f). 

GWR: 71 FR 65574, November 8, 2006 (USEPA, 2006f). 

1Although the standard for this disinfectant was not revised as part of the Stage 2 DBPR, regulatory revisions need to be considered in conjunction with other disinfectant residuals and disinfection byproducts. 2 LT2ESWTR and GWR promulgated treatment techniques that built upon and enhanced the existing regulations (Surface Water Treatment 

Rule, Interim Enhanced Surface Water Treatment Rule, and Long-Term 1 Enhanced Surface Water Treatment Rule) that address broad categories of microorganisms in treated water. 3 Listed on the third Drinking Water Contaminant Candidate List or CCL3 (74 FR 51850, October 8, 2009 (USEPA, 2009l) in order to capture 

health and treatment information that may not be addressed by the current regulations. 

2. Health Effects 

The document, ‘‘Six-Year Review 2 

—Health Effects Assessment—Summary 

Report’’ (USEPA, 2009b), describes how 

EPA reviewed the contaminants 

discussed in today’s action and provides 

the results of the health effects technical 

review. The principal objectives of the 

health effects review are to identify: (1) 

Contaminants for which a new health 

effects assessment indicates that a 

change in MCLG might be appropriate 

(e.g., because of a change in cancer 

classification or a reference dose (RfD)), 

and (2) contaminants for which the 

Agency identifies new health effects 

information suggesting a need to initiate 

a new health effects assessment. 

To meet the first objective, the Agency 

reviewed the results of health effects 

assessments completed under the 

following programs and identified, 

where feasible, possible MCLG values. 

• EPA Integrated Risk Information 

System (IRIS). 

• EPA Office of Pesticide Programs 

(OPP). 

• National Academy of Sciences 

(NAS; when commissioned by EPA). 

To meet the second objective, the 

Agency first conducted an extensive 

literature review to identify peerreviewed studies. Then the Agency 

reviewed the studies to determine 

whether there was new health effects 

information such as reproductive and 

developmental toxicity that potentially 

affects the MCLG of any of the 

remaining contaminants that do not 

have an ongoing health effects 

assessment, including those with 

recently completed health effects 

assessments. 

Table V–2 reflects the outcome of the 

health effects review for the NPDWRs 

discussed in today’s action. EPA placed 

each contaminant into one of the 

following 13 categories. 

• Agency health effects assessment in 

process and not completed as of March 

1, 2009. The Agency currently is 

conducting a health effects assessment 

for the contaminant. That assessment 

will consider all available, relevant 

studies on the toxicology of the 

contaminant, including developmental 

and reproductive toxicity. This outcome 

contains three categories of 

contaminants. 

• Category 1 contains 15 

contaminants with MCLGs equal to or 

greater than zero and either MCLs that 

are limited by analytical feasibility or 

TT standards. For this category, EPA 

conducted further review of the 

potential for revisions to the MCL due 

to possible changes in analytical 

feasibility. The Agency’s review of new 

information that might affect the MCL 

for one of these contaminants is a 

refinement of the protocol. During SixYear Review 1, EPA took no further 

action on any contaminants with 

ongoing health effects assessments. EPA 

generally sets each MCL as close to the 

MCLG as is feasible, and a common 

limitation is the availability of 

analytical methods to reliably measure 

the contaminant. 

• Category 2 contains two 

contaminants (arsenic and uranium) 

that have MCLGs equal to zero and 

MCLs that are based on the costs and 

benefits balancing provision in SDWA 

1412(b)(6)(A). Any changes in the 

ongoing health effects assessment could 

impact the evaluation of benefits for 

these contaminants. Therefore, EPA has 

decided to take no further action to 

evaluate these two contaminants until 

completion of the health effects 

assessment. 

• Category 3 contains 13 

contaminants with non-zero MCLGs and 

MCLs generally equal to their respective 

MCLGs. Because EPA cannot determine 

whether there is potential to revise 

either the MCLG or the MCL until after 

the health effects assessment is 

completed, EPA plans to take no further 

action on these contaminants at this 

time. 

• New health effects assessment 

completed since Six-Year Review 1. An 

IRIS or OPP assessment has been 

completed since 2002. EPA also 

conducted a follow-up literature search 

to confirm that no new information 

became available following the 

completion of the new health effects 

assessment. Table V–2 shows four 

categories of contaminants with new 

health effects assessments: four with 

results indicating potential for lower 

MCLG (Category 4), five with results 

indicating potential for higher MCLG 

(Category 5), two with results indicating 

the MCLG remains appropriate 

(Category 6), and three contaminants for 

which emerging information following 

the completion of a health effects 

assessment or a pending pesticide 

cancellation decision may affect EPA’s 

review (Category 7). 

• Literature review only conducted 

during Six-Year Review 2. For the 

contaminants that did not have an 

ongoing health effects assessment or a 

new one completed during the current 

review period, EPA conducted a review 

of the health effects literature to identify 

whether there was new information 

with potential to revise the MCLG. 

There are six categories of 

contaminants. 

• Three categories pertain to 

contaminants that had a health effects 

assessment completed during Six-Year 

Review 1, including two with possible 

lower MCLGs (Category 8), three with 

possible higher MCLGs (Category 9), 

and three with no potential to revise 

their MCLGs (Category 10). During SixYear Review 1, the Agency determined 

that possible changes to these 

contaminants’ NPDWRs were a low 

priority activity for the Agency because 

of: competing workload priorities, the 

administrative costs associated with 

rulemaking, and the burden on States 

and the regulated community to 

implement any regulatory changes. As 

part of Six-Year Review 2, EPA is 

assessing whether there is new 

information that affects this 

determination. 

• Category 11 contains five 

contaminants for which the Agency 

identified new information, described in 

section VI, that could impact the MCLG 

and, therefore, these contaminants are 

considered potential nominees for a 

new health assessment. 

• Category 12 contains seven 

carcinogens for which the literature 

review sought new information on 

whether there might be a nonlinear 

mode of action or other reproductive 

and developmental health effects. 


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• Category 13 contains seven 

contaminants with non-zero MCLGs, for 

which EPA conducted a full literature 

search, including developmental and 

reproductive toxicity. 


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In addition to identifying for which 

contaminants there is information that 

potentially affects the MCLG, the health 

effects review indicates which 

contaminants proceed to other review 

steps under the protocol. Several 


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contaminants proceed to the analytical 

methods review to determine whether 

improvements in analytical methods 

indicate potential to revise the practical 

quantitation limit (PQL) in the 

NPDWRs. As Table V–3 shows, 14 

contaminants from Category 1 proceed 

to the analytical methods review— 

despite an ongoing health effects 

assessment—because their MCLs are 

limited by their respective PQLs. These 

14 include alpha particles; 

benzo(a)pyrene; beta particles; carbon 

tetrachloride; DEHP; 1,2-dichloroethane; 

dichloromethane; pentachlorophenol; 

PCBs; radium; dioxin; 

tetrachloroethylene; thallium; 

trichloroethylene. In addition, two 

contaminants in Category 6 (benzene 

and EDB) and two in Category 10 

(chlordane and vinyl chloride) have 

MCLs that are limited by PQLs and, 

therefore, these contaminants proceed to 

the analytical methods review even 

though their health effects assessments 

indicated no change to their respective 

MCLG values. Similarly, six 

contaminants in Category 12 (DBCP; 1,2- 

dichloropropane; heptachlor; heptachlor 

epoxide; hexachlorobenzene; 

toxaphene) and one in Category 13 

(1,1,2-trichloroethane) have MCLs that 

are limited by their respective PQL and, 

therefore, proceed to the analytical 

methods review despite there being no 

new information on health effects. 

Among the contaminants having new 

health effects information during either 

Six-Year Review 2 or the previous 

review that potentially affects their 

respective MCLG values (i.e., potentially 

lower MCLGs), four in Category 4 (2,4- 

D; endothall; toluene; total xylenes) and 

two in Category 8 

(hexachlorocyclopentadiene and 

oxamyl) proceed to the analytical 

methods review. For each of these 

contaminants, EPA evaluated whether 

analytical feasibility might become a 

limiting factor if EPA were to consider 

a lower MCLG and whether new 

information indicates there is a 

potential to revise the PQL. 

Two contaminants (acrylamide from 

Category 1 and epichlorohydrin from 

Category 12) bypass the analytical 

methods review because they have TT 

standards and PQLs are not a limiting 

factor for the standards. Five 

contaminants from Category 5 (alachlor; 

barium; diquat; glyphosate; 1,1,1- 

trichloroethane) and three from 

Category 9 (1,1-dichloroethylene; 

lindane; picloram) bypass the analytical 

methods review because the new health 

effects information identified either 

during Six-Year Review 2 or Six-Year 

Review 1 indicated possible increases in 

their respective MCLGs. Each of these 

contaminants has a PQL that is lower 

than its MCLG and, therefore, a review 

of whether the PQL could be lower is 

inconsequential. 

TABLE V–3—CONTAMINANTS PROCEEDING TO ANALYTICAL FEASIBILITY REVIEW FROM HEALTH EFFECTS REVIEW

Health effects review 

category1 Contaminants proceeding to analytical feasibility review 

Health Effects Assessment 

in Process During Information Review Period for 

the Notice (and not available by the March 1, 

2009 cutoff date): 

Category 1 ................... 14 of 15 proceeding because PQL limits MCL: alpha particles; benzo(a)pyrene; beta particles; carbon tetrachloride; DEHP; 1,2-dichloroethane; dichloromethane; pentachlorophenol; PCBs; radium; dioxin; 

tetrachloroethylene; thallium; trichloroethylene. Acrylamide bypasses the analytical review because it does not 

have a PQL. 

Category 2 ................... 0 of 2 proceeding because there is no potential to revise MCL unless completed health effects assessment indicates change to benefits analysis (arsenic and uranium). 

Category 3 ................... 0 of 13 did not proceed because MCL set at MCLG and health assessment still in process. 

Health Effects Assessment 

Completed Since SixYear Review 1: 

Category 4 ................... 4 of 4 proceeding to evaluate whether PQL is or could be below possible MCLG: 2,4-D; endothall; toluene; total 

xylenes. 

Category 5 ................... 0 of 5 proceeding; all 5 bypass analytical review because PQL not a factor in review. 

Category 6 ................... 2 of 2 proceeding because PQL limits MCL: benzene and EDB. 

Category 7 ................... 0 of 3 proceeding because there is no potential to revise an MCL that is based on the MCLG under review. 

Literature Review Only: 

Category 8 ................... 2 of 2 proceeding to evaluate whether PQL is or could be below possible MCLG: hexachlorocyclopentadiene; 

oxamyl. 

Category 9 ................... 0 of 3 proceeding; all 3 bypass analytical review because PQL not a factor in review. 

Category 10 ................. 2 of 3 proceeding because PQL limits MCL: chlordane and vinyl chloride. 

Category 11 ................. 0 of 3 proceeding because there is no potential to revise an MCL that is based on the MCLG that may be further 

reviewed. 

Category 12 ................. 6 of 7 proceeding because PQL limits MCL: DBCP; 1,2-dichloropropane; heptachlor; heptachlor epoxide; 

hexachlorobenzene; toxaphene epichlorohydrin bypasses the analytical review because it does not have a PQL. 

Category 13 ................. 1 of 7 proceeding because PQL limits MCL: 1,1,2-trichloroethane. 

1These categories correspond to the categories in Table V–2. 

3. Analytical Feasibility 

EPA has a process in place to approve 

new analytical methods for drinking 

water contaminants; therefore, the 

review and approval of potential new 

methods are outside the scope of the 

Six-Year Review protocol. EPA 

recognizes, however, that the approval 

and addition of new and/or improved 

analytical methods (since the 

promulgation of the NPDWRs 

considered under this section of the 

review) may enhance the ability of 

laboratories to quantify contaminants at 

lower levels. This ability of laboratories 

to measure a contaminant at lower 

levels could affect its PQL, the value at 

which an MCL is set when it is limited 

by analytical feasibility. Therefore, the 

Six-Year Review process includes a 


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4As noted in Table V–4 and sections VI.38 and 

VI.59, EPA found that there was no potential to 

lower the PQL for dioxin and EDB. Even if EPA had 

used a 10 × MDL multiplier for these two 

contaminant instead of the 5 x MDL multiplier, this 

would not have changed the outcome of the 

analytical feasibility assessments. 

review of whether there have been 

changes in analytical feasibility for the 

subset of the NPDWRs that reached this 

stage of the decision tree. These include 

contaminants with or without ongoing 

health effects assessments that have 

MCLs limited by analytical feasibility 

and contaminants with possible MCLGs 

that are lower than their current PQLs. 

The document, ‘‘Analytical Feasibility 

Support Document for the Second SixYear Review of Existing National 

Primary Drinking Water Regulations’’ 

(USEPA, 2009c), describes the process 

EPA used to evaluate whether changes 

in PQL are possible in those instances 

where the MCL is limited, or might be 

limited, by analytical feasibility. EPA 

uses the PQL to estimate the level at 

which laboratories can routinely 

measure a chemical contaminant in 

drinking water. Historically, EPA has 

used two main approaches to determine 

a PQL for SDWA analytes: (1) 

Performance Evaluation (PE) data from 

Water Supply (WS) studies, which is the 

preferred alternative when sufficient 

data are available; or (2) a multiplier 

method, in which the PQL is calculated 

by multiplying the EPA-derived method 

detection limit (MDL) by a factor of 5 or 

10 (50 FR 46880, November 13, 1985 

(USEPA, 1985); 52 FR 25690 July 8, 

1987 (USEPA, 1987); 54 FR 22062 May 

22, 1989 (USEPA, 1989b)). 

The review protocol for Six-Year 

Review 1 utilized data from PE studies, 

which were laboratory accreditation 

studies conducted under EPA oversight 

until 1999, when the program was 

privatized. Now, the National 

Environmental Laboratory Accreditation 

Conference (NELAC) conducts the 

accreditation program via Performance 

Testing (PT) studies. PQL reassessments 

discussed in this notice are based on the 

Six-Year 1 PE data collected through 

late 1999 and laboratory passing rate PT 

data collected from late 1999 through 

2004. One PT provider made pass/fail 

rates from PT studies available to EPA. 

This major provider accounts for a large 

portion of the PT results nationwide 

(USEPA, 2009c). 

Using PE or PT data to derive the PQL 

for chemical NPDWRs involves 

determining the concentration of an 

analyte at which 75 percent of EPA 

Regional and State laboratories achieve 

results within a specified acceptance 

range (see 54 FR 22062 at 22100, May 

22, 1989 (USEPA, 1989b)). For Six-Year 

Review 2, EPA did not have sufficient 

PT and PE data to recalculate any PQL 

values, in part because the spiked 

concentrations were rarely far enough 

below current PQLs. Instead, EPA used 

the PT and PE passing rate results (i.e., 

the percent of laboratories passing a 

performance test for a given study) at 

and below the current PQL to determine 

whether data may support a lower PQL. 

When PT results were not available 

below the PQL or when the results did 

not provide conclusive indications 

regarding a potential to revise a PQL, 

EPA used two alternate approaches to 

estimate possible PQLs: an approach 

based on the minimum reporting levels 

(MRLs) obtained as part of the Six-Year 

Review Information Collection Request 

(ICR) (see section V.B.4), and an 

approach based on method detection 

limits (MDL). While EPA prefers to use 

laboratory performance data to calculate 

a PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. 

A laboratory reports an MRL when it 

does not detect a particular contaminant 

in a sample of water. The MRL is the 

lowest concentration level of a 

contaminant that a laboratory can 

reliably measure or quantitate within 

specified limits of precision and 

accuracy under routine laboratory 

operating conditions using a given 

method (USEPA, 2009c). MRL values 

were included with the data provided 

by the States in response to the Six-Year 

Review ICR. EPA evaluated the 

distribution of MRL values for each 

contaminant to identify the mode or 

value occurring most frequently for that 

contaminant (i.e., the modal MRL) and 

estimated the percentage of MRL values 

that are equal to or less than the modal 

MRL. When this percentage was at least 

80 percent and the modal MRL was 

below the PQL, EPA chose to use this 

modal MRL value as an estimated 

quantitation limit (also referred to as an 

EQL throughout this document). The 

use of modal MRLs is a refinement of 

the protocol, necessitated by limited 

availability of PT and PE data below the 

current PQL and made possible by the 

extensive amount of information 

included in the Six-Year Review ICR 

dataset (see section V.B.4). 

When the MRL data did not meet the 

80 percent threshold used for deriving 

an EQL via this approach, EPA used an 

MDL approach to derive an EQL. As 

noted previously, this approach has 

been used in the past to derive PQLs for 

regulated contaminants. In addition, 

this same approach was used to identify 

possible analytical feasibility levels for 

Six-Year Review 1 (USEPA, 2003a). In 

deriving these levels, the Agency used 

the MDLs associated with the analytical 

methods approved by EPA for drinking 

water analysis. EPA obtained MDL 

values from individual analytical 

methods developed and approved by 

EPA for use on drinking water. EPA 

applied a multiplier to these MDL 

values and based the EQL on the 

midpoint of the resulting range (i.e., the 

mean if there are two MDLs or a median 

if there are more than two MDLs). The 

multiplier is 10 for most contaminants 

except dioxin and EDB, which have 

PQLs that were historically based on an 

MDL multiplier of 5.4 EPA also used the 

MDL multiplier approach to confirm 

whether EQLs based on MRL data are 

consistent with the range of values 

based on an MDL multiplier approach. 

EPA used the EQL thresholds derived 

via the modal MRL or MDL-multiplier 

approaches for the occurrence analysis 

(see section V.B.4) to help the Agency 

determine if there may be a meaningful 

opportunity to improve public health 

protection. It should be noted, however, 

that the EQL does not represent the 

Agency’s intent to promulgate new 

PQLs with this notice. Any revisions to 

PQLs will be part of future rule making 

efforts. 

EPA performed analytical feasibility 

analyses for the contaminants identified 

in Table V–3 as proceeding to this 

portion of the review. Table V–4 shows 

the contaminants gathered into three 

more general categories and the 

outcomes of the Agency’s review. 

• A health effects assessment 

indicates potential for lower MCLG. This 

category includes the six contaminants 

identified in the health effects review as 

having information indicating the 

potential for a lower MCLG—four with 

new health effects assessments 

completed during Six-Year Review 2 

and two with health effects assessments 

completed during Six-Year Review 1. 

Although their current MCLs are not 

limited by a PQL, EPA reviewed 

analytical feasibility to determine if 

analytical feasibility might limit the 

potential for MCL revisions. For two 

contaminants (endothall and oxamyl), 

the current PQL is higher than the 

possible MCLG identified in the health 

effects review. For these contaminants, 

the potential to lower their PQLs based 

on PE and PT data is inconclusive, but 

MRL and MDL data indicate the 

potential to revise the PQL. EPA thus 

proceeded to evaluate occurrence data 

to determine whether a lower PQL, and 

thus the MCL, may provide a 

meaningful opportunity to improve 

public health protection. The current 

PQL is not a limiting factor for the 


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5 If EPA found that there was no meaningful 

opportunity to revise the MCL (i.e., carbon 

tetrachloride, 1,2-dichloroethane and dichloromethane), these contaminants remained in 

the health effects assessment in process category. 

remaining four contaminants identified 

by the health effects review as having 

possible changes in their MCLG (i.e., 

2,4–D, hexachlorocyclopentadiene, 

toluene, and xylenes). 

• Contaminants with ongoing health 

effects assessments and existing MCLs 

are based on analytical feasibility. This 

category includes 14 contaminants with 

ongoing health assessments with 

existing MCLs that are greater than their 

MCLGs because they are limited by 

analytical feasibility. One contaminant 

has a non-zero MCLG (thallium) and the 

remaining 13 contaminants have MCLGs 

equal to zero. Although a risk 

assessment is in process for these 

contaminants, because SDWA requires 

the Agency to set the MCL as close to 

the MCLG as feasible, EPA evaluated 

whether the PQL is likely to be lower for 

these contaminants. For four of these 

contaminants (carbon tetrachloride, 1,2- 

dichloroethane, tetrachloroethylene, 

and trichloroethylene), EPA concluded 

that new information from PT studies, 

along with MRL and MDL data, indicate 

the potential to revise the PQL. For one 

contaminant (dichloromethane), data 

from PT studies are inconclusive, but 

MRL and MDL data indicate the 

potential to revise the PQL. For these 

five contaminants, EPA proceeded to 

evaluate occurrence data to determine 

whether lowering the PQL, and thus the 

MCL, may provide a meaningful 

opportunity to improve public health 

protection.5 For the remaining nine 

contaminants, either EPA did not have 

sufficient new information to evaluate 

analytical feasibility or EPA concluded 

that new information does not indicate 

the potential for a PQL revision. 

Consequently, the outcome of the 

review for these nine contaminants is to 

take no action at this time. 

• Contaminants without ongoing 

health effects assessments or for which 

no new health risk information was 

identified and for which existing MCLs 

are based on analytical feasibility and 

greater than their MCLGs. For the 11 

contaminants in this category, EPA 

evaluated available PT and PE data as 

well as MRL and MDL data to determine 

whether there is potential to lower the 

PQL and thereby set the MCL closer to 

the MCLG. For five of these 

contaminants (benzene chlordane, 1,2- 

dichloropropane, hexachlorobenzene, 

and 1,1,2-trichloroethane) EPA 

concluded that new information from 

PT studies, along with MRL and MDL 

data, indicates that while it might be 

possible to set a lower PQL, the data are 

insufficient to support an actual PQL 

recalculation at this time. Consequently, 

the outcome of the review for these 

contaminants is to take no action at this 

time. For five additional contaminants 

(DBCP, heptachlor, heptachlor epoxide, 

toxaphene, and vinyl chloride), the data 

from PT studies are inconclusive, but 

MRL and/or MDL data indicate 

potential for a lower PQL, as indicated 

in Table V–4. For these five 

contaminants, EPA proceeded to 

evaluate occurrence data to determine 

whether lowering the PQL, and thus the 

MCL, may provide a meaningful 

opportunity to improve public health 

protection. For the final contaminant, 

ethylene dibromide (EDB), none of the 

data sources indicate potential to revise 

and the outcome of the review for this 

contaminant is to take no action at this 

time. 

Table V–4 lists the type of data that 

indicate potential for a PQL reduction. 

The list includes ‘‘PT’’ when the PQL 

reassessment based on PT and PE data 

(USEPA, 2009c) reports that a reduction 

is supported. The list also includes 

‘‘MRL’’ and ‘‘MDL’’ when either of these 

approaches indicates potential for PQL 

reduction. A result of ‘‘PQL reduction 

supported’’ without a ‘‘PT’’ in the list 

indicate that the PQL reassessment 

outcome is uncertain, but other data 

(i.e., MRL and/or MDL) indicate 

potential for PQL reduction. When the 

PQL reassessment outcome is that the 

current PQL remains appropriate, Table 

V–4 shows the result ‘‘Data do not 

support PQL reduction.’’ The 

contaminant specific discussions in 

section VI of today’s action provide the 

results of the analytical feasibility 

review for all the contaminants in Table 

V–4. 

TABLE V–4—NPDWRS INCLUDED IN THE ANALYTICAL FEASIBILITY REASSESSMENT AND THE RESULT OF THAT

ASSESSMENT

Contaminant Current PQL Analytical feasibility reassessment result 

6 Contaminants Identified Under the Health Effects Review as Having Potential for Lower MCLG 

2,4-D (possible MCLG: 0.04 mg/L) ......................................................... 0.005 mg/L .................. PQL not limiting. 

Endothall (possible MCLG: 0.05 mg/L) ................................................... 0.09 mg/L .................... PQL reduction supported (MRL, MDL). 

Hexachlorocyclopentadiene (possible MCLG: 0.04 mg/L) ...................... 0.001 mg/L .................. PQL not limiting. 

Oxamyl (possible MCLG: 0.002 mg/L) .................................................... 0.02 mg/L .................... PQL reduction supported (MRL, MDL). 

Toluene (possible MCLG: 0.6 mg/L) ....................................................... 0.005 mg/L .................. PQL not limiting. 

Total xylenes (possible MCLG: 1 mg/L) .................................................. 0.005 mg/L .................. PQL not limiting. 

14 Contaminants with Ongoing Health Effects Assessments (as of March 1, 2009) and MCLs Are Based on Analytical Feasibility and 

Higher than MCLGs 

Alpha particles ......................................................................................... No PQL and no new information. 

Benzo(a)pyrene ....................................................................................... 0.0002 mg/L ................ Data do not support PQL reduction. 

Beta particles ........................................................................................... No PQL and no new information. 

Carbon Tetrachloride ............................................................................... 0.005 mg/L .................. PQL reduction supported (PT, MRL, MDL). 

DEHP ....................................................................................................... 0.006 mg/L .................. Data do not support PQL reduction. 

1,2-dichloroethane ................................................................................... 0.005 mg/L .................. PQL reduction supported (PT, MRL, MDL). 

Dichloromethane ...................................................................................... 0.005 mg/L .................. PQL reduction supported (MRL, MDL). 

Pentachlorophenol ................................................................................... 0.001 mg/L .................. Data do not support PQL reduction. 

PCBs ........................................................................................................ 0.0005 mg/L ................ Data do not support PQL reduction. 

Radium ..................................................................................................... No PQL and no new information. 


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TABLE V–4—NPDWRS INCLUDED IN THE ANALYTICAL FEASIBILITY REASSESSMENT AND THE RESULT OF THAT

ASSESSMENT—Continued 

Contaminant Current PQL Analytical feasibility reassessment result 

Dioxin ....................................................................................................... 3E–08 mg/L ................. Data do not support PQL reduction. 

Tetrachloroethylene ................................................................................. 0.005 mg/L .................. PQL reduction supported (PT, MRL, MDL). 

Thallium ................................................................................................... 0.002 mg/L .................. Data do not support PQL reduction. 

Trichloroethylene ..................................................................................... 0.005 mg/L .................. PQL reduction supported (PT, MRL, MDL). 

11 Contaminants without Ongoing Health Effects Assessments and MCLs Are Based on Analytical Feasibility and Higher than MCLGs 

Benzene ................................................................................................... 0.005 mg/L .................. PQL reduction supported (PT, MRL, MDL). 

Chlordane ................................................................................................ 0.002 mg/L .................. PQL reduction supported (PT, MRL, MDL). 

DBCP ....................................................................................................... 0.0002 mg/L ................ PQL reduction supported (MDL). 

1,2-dichloropropane ................................................................................. 0.005 mg/L .................. PQL reduction supported (PT, MRL, MDL). 

EDB .......................................................................................................... 0.0005 mg/L ................ Data do not support PQL reduction. 

Heptachlor ................................................................................................ 0.0004 mg/L ................ PQL reduction supported (MRL, MDL). 

Heptachlor epoxide .................................................................................. 0.0002 mg/L ................ PQL reduction supported (MRL, MDL). 

Hexachlorobenzene ................................................................................. 0.001 mg/L .................. PQL reduction supported (PT, MRL, MDL). 

Toxaphene ............................................................................................... 0.003 mg/L .................. PQL reduction supported (MRL, MDL). 

1,1,2-trichloroethane ................................................................................ 0.005 mg/L .................. PQL reduction supported (PT, MRL, MDL). 

Vinyl chloride ........................................................................................... 0.002 mg/L .................. PQL reduction supported (MRL). 

mg/L—milligrams per liter 

EPA conducted occurrence and 

exposure analyses for the contaminants 

in Table V–4 for which a PQL reduction 

is supported or the PQL is not limiting. 

This includes the 6 contaminants with 

new health effects assessments that 

indicate potentially lower MCLGs, 5 of 

the 14 contaminants with ongoing 

health effects assessments and MCLs 

limited by PQLs, and 10 of the 11 

contaminants without ongoing health 

effects assessments and MCLs limited 

by PQLs. 

4. Occurrence and Exposure Analysis 

To support the national contaminant 

occurrence assessments under Six-Year 

Review 2, EPA conducted an 

Information Collection Request. 

Through this process EPA requested 

that all States and primacy entities 

voluntarily submit their SDWA 

compliance monitoring data. This 

request was for the submission of 

compliance monitoring data collected 

between January 1998 and December 

2005 for 79 regulated contaminants. A 

total of 51 States and entities provided 

compliance monitoring data that 

included all analytical detection and 

non-detection records. These data 

represent the national occurrence of 

regulated contaminants in public 

drinking water systems. Through 

extensive data management efforts, 

quality assurance evaluations, and 

communications with State data 

management staff, EPA established a 

high quality dependable contaminant 

occurrence database consisting of data 

from 45 States and two Indian Tribes 

(see map in Figure V–2). Details of the 

data management and data quality 

assurance evaluations are available in 

the support document entitled, 

‘‘Analysis of Occurrence Data from the 

Second Six-Year Review of Existing 

National Primary Drinking Water 

Regulations’’ (USEPA, 2009f). 


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6The compliance monitoring data for the 

microbiological contaminants were collected to 

support ongoing rule development so these data 

have not been analyzed separately in this action. 

7The use of the stage 1 and stage 2 terminology 

should not be confused with the Stage 1 and Stage 

2 Disinfectants and Disinfection By Products 

Rulemakings. Instead, this terminology has been 

used to describe the two stages of the occurrence 

analyses performed for Six-Year Review 2, as well 

as Six-Year Review 1. 

8These analyses are conservative in the sense that 

they are protective of human health (i.e., they are 

more likely to overestimate risks to human health 

than underestimate them). 

The contaminant occurrence data 

from the 45 States and two Indian 

Tribes comprise more than 15 million 

analytical records from approximately 

132,000 public water systems. 

Approximately 254 million people are 

served by these public water systems 

nationally. Records were submitted for 

16 inorganic chemicals, 32 synthetic 

organic chemicals, 21 volatile organic 

chemicals, 7 radiological contaminants, 

and 3 microbiological 6 contaminants. 

The number of States and public water 

systems represented in the dataset 

varies across contaminants because of 

variability in voluntary State data 

submissions and contaminant 

monitoring schedules. This is the 

largest, most comprehensive set of 

drinking water compliance monitoring 

data ever compiled and analyzed by 

EPA. 

EPA used a two-stage analytical 

approach to analyze these data and 

characterize the national occurrence of 

contaminants.7 The first stage of 

analysis provides a straightforward 

evaluation of contaminant occurrence. 

This stage 1 occurrence analysis is a 

simple, non-parametric count of 

occurrence of regulated contaminants in 

public water systems.8 A typical stage 1 

occurrence analysis generates a count of 

the number (or percentage) of systems 

with at least one analytical detection 

having a concentration greater than a 

concentration threshold of interest, i.e., 

a possible MCLG or EQL. It provides a 

health protective approach that may be 

more appropriate for contaminants that 

produce health effects after shorter than 

lifetime exposure periods (e.g., several 

months or less). This approach also 

generates a conservative (i.e., upwardly 

biased) estimate of the number of 

potential systems having contaminant 

occurrence at levels of interest for 

contaminants having health risks that 

are only related to chronic or long-term 

exposure over many years. 

The stage 2 occurrence analysis 

estimates national contaminant 

occurrence by generating estimated 

long-term mean concentrations of a 

specific contaminant at systems 

nationally. This provides occurrence 

analyses that are less conservative than 

the stage 1 occurrence analysis (because 

the stage 2 occurrence analysis is based 

on estimated mean concentrations 

rather than on single maximum 

concentrations), and also provides 

occurrence analyses that may be more 

reflective of potential chronic exposure. 

Generally, the stage 1 occurrence 

analysis reflects a rough approximation 

of peak occurrence while the stage 2 

occurrence analysis is based on 

estimated average occurrence. A 

complete description of the two-stage 

analytical approach and a detailed 

presentation of occurrence estimates are 

available in the support document 

entitled, ‘‘Analysis of Occurrence Data 

from the Second Six-Year Review of 

Existing National Primary Drinking 

Water Regulations’’ (USEPA, 2009f). 

EPA calculated the system means for 

the stage 2 occurrence analysis using a 

simple arithmetic average of all 

detection and non-detection data for 

each public water system. Because the 

contaminant concentrations associated 

with the non-detection data are 

unknown, EPA assigned three different 

values to the non-detection results to 

estimate a range of system-level means, 

which then allowed EPA to estimate 

number and percent of systems with 

estimated means exceeding selected 

threshold values. Two of the three 

values are based on the MRL values that 

accompany the non-detection results in 

the Six-Year Review ICR dataset. The 

MRL is the lowest level that can be 

reliably achieved within specified limits 

of precision and accuracy under routine 

laboratory operating conditions using a 

given method. The three values that 

EPA substituted for non-detection 

results were MRL, 1⁄2 MRL, and zero. 

The most conservative approach was 

to assume that all non-detection results 


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were equal to the MRL. This approach 

yields an upper-bound estimate of each 

system’s level of exposure. EPA also 

explored the less conservative 

assumption that concentrations of the 

non-detection results were uniformly 

distributed between the MRL and zero, 

thereby substituting one-half the MRL 

for all non-detection results. Finally, 

EPA considered the assumption that the 

actual concentration for each nondetection result was typically much 

smaller than the MRL, supporting the 

use of zero to represent each nondetection. This method yielded a lowerbound estimate of the system’s mean. 

This simplified approach differs from 

the stage 2 occurrence analysis 

approach in the Six-Year Review 1, 

which used more sophisticated 

modeling methods to address the nondetection results. That analysis, 

however, was based on a substantially 

smaller dataset (i.e., data from 16 States 

instead of 45 States). (Note that many 

States substitute zero for all nondetections when determining 

compliance with the NPDWRs.) EPA 

uses each of the assumptions in the 

stage 2 occurrence analyses in order to 

obtain reasonable bounds on the actual 

system mean concentrations. Once the 

system means were calculated for each 

of the three substitution methods, the 

results means were then compared to 

the various thresholds of interest (e.g., 

the number and percent of systems with 

a mean concentration above a health 

threshold of concern). 

The two-stage analytical approach 

was previously developed for Six-Year 

Review 1. The data management and 

general occurrence analytical approach 

were peer-reviewed for use under the 

Six-Year Review 1. 

EPA conducted the stage 2 occurrence 

analysis for 5 of the 14 NPDWRs in 

Table V–4 with ongoing health effects 

assessment and MCLs that are limited 

by PQLs for which EPA identified 

analytical feasibility data supporting 

possible PQL revision: carbon 

tetrachloride; dichloromethane; 1,2- 

dichloroethane; tetrachloroethylene; 

and trichloroethylene. EPA also 

conducted the stage 2 occurrence 

analysis for the five contaminants with 

health effects assessment changes that 

indicate potential to reduce the MCLG 

and the ten contaminants that do not 

have ongoing health effects assessments, 

but do have MCLs limited by PQLs and 

new data indicate potential to reduce 

the PQLs (see Table V–4). Note that EPA 

conducted the Stage 1 analysis for one 

contaminant with health effects 

assessment changes that indicate a 

potential to reduce the MCLG (i.e., 

oxamyl) because the health endpoint is 

associated with acute exposure. EPA 

used the results of these analyses to 

identify which possible NPDWR 

revisions present a meaningful 

opportunity to improve the level of 

health protection. Section VI contains 

the occurrence estimates for each of the 

21 contaminants (shown in Table V–4) 

having either new information 

suggesting potentially lower MCLGs or 

MCLs based on PQLs that might be 

lower based on new information. 

Because the Six-Year Review ICR data 

reflect water quality at entry points to 

the distribution system, the occurrence 

analysis method described above is not 

adequate to evaluate the cost savings 

potential for the nine contaminants that 

have health effects assessment changes 

that indicate potential for higher MCLG 

values (see Table V–2). EPA lacks the 

comprehensive information on source 

water quality and existing treatment 

needed to determine how many systems 

would be able to alter treatment 

practices were an MCLG to increase. To 

review the potential for cost savings, 

EPA conducted a qualitative assessment 

of the potential for treatment cost 

savings based on three factors: the 

magnitude of the difference between the 

current MCLG and the possible MCLG; 

available source water occurrence 

information; and the potential for 

systems having best available 

technologies (BATs) or small system 

compliance technologies (SSCTs) to 

realize operational cost savings (USEPA, 

2009g). 

There is no comprehensive database 

of water quality in drinking water 

sources. Therefore, EPA used source 

water quality information from two 

national data sources, the National 

Water Quality Assessment (NAWQA) 

program conducted by the U.S. 

Geological Survey (USGS), and EPA’s 

STORET (short for STOrage and 

RETrieval) data system, which are part 

of EPA’s Office of Ground Water and 

Drinking Water’s National Contaminant 

Occurrence Database (NCOD). The 

STORET data come from a variety of 

monitoring programs and the NAWQA 

data come from watershed or ‘‘study 

units’’ that USGS selected to reflect 

important hydrologic and ecological 

resources; critical sources of 

contaminants, including agricultural, 

urban, and natural sources; and a high 

percentage of population served by 

municipal water supply and irrigated 

agriculture. The original 51 study units 

account for more than 70 percent of 

total water use (excluding 

thermoelectric and hydropower) and 

more than 50 percent of the 

population’s supply of drinking water 

(Gilliom et al., 2006). For each dataset, 

EPA estimated the number and percent 

of monitoring locations with at least one 

sample result above each contaminant’s 

current MCL, and above a possible 

MCLG based on the new information 

from the contaminant’s health effects 

assessment. Although these results do 

not indicate how many systems may be 

treating for each contaminant, they 

provide the best available information 

regarding the frequency of contaminant 

occurrence at levels of interest. Section 

VI reports the results by contaminant. 

5. Treatment Feasibility 

An NPDWR either identifies the BATs 

for meeting an MCL, or establishes 

enforceable treatment technique 

requirements. For the NPDWRs 

addressed in section VI of today’s 

action, two have TT requirements and 

the rest have an MCL. All of the MCLs 

are set equal to the MCLG or the PQL 

or by benefit-cost analysis; none are 

currently limited by treatment 

feasibility. As a refinement for Six-Year 

Review 2, EPA considered treatment 

feasibility after identifying 

contaminants with potential to lower an 

MCL or change a TT that constituted a 

meaningful opportunity to improve the 

level of health protection. The EPA 

document, ‘‘Water Treatment 

Technology Feasibility Support 

Document for Chemical Contaminants 

for the Second Six-Year Review of 

National Primary Drinking Water 

Regulations’’ (USEPA, 2009g), describes 

the process EPA used to evaluate 

treatment feasibility, where appropriate, 

and provides the results of these 

analyses. As a part of this review, EPA 

utilized the same sources that have been 

the primary resources in development of 

EPA regulations and guidance, 

including published EPA treatment 

reports, peer-reviewed journals, and 

other technology sources, as well as 

information received from EPA 

stakeholders. 

a. MCL-Type Rules 

EPA evaluated existing treatment 

technology information for two MCLtype NPDWRs (tetrachloroethylene and 

trichloroethylene) where EPA 

determined that lowering the PQL and 

thus the MCL could lead to a 

meaningful opportunity to improve 

public health protection, to determine 

whether treatment feasibility would be 

a limiting factor. 

Based on this evaluation, the Agency 

believes that treatment capabilities 

would be adequate to support a lower 

MCL value for these contaminants for 

which a lower MCL may be appropriate 

(USEPA, 2009g). EPA’s assessment of 

the treatment technologies for these 


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9The subject of the Six-Year-Review, as specified 

in section 1412(b)(9) of the SDWA, is ‘‘each national 

primary drinking water regulation,’’ as defined 

under section 1401 of the SDWA. 10Currently, PWSs that exceed the fluoride MCL 

of 4.0 mg/L are required to notify their customers 

within 30 days of the exceedance. If a PWS exceeds 

the fluoride SMCL of 2.0 mg/L, they are required 

to notify their customers within 12 months of the 

exceedance. The States voiced concerns about (1) 

the confusion that occurs between the different PN 

requirements for the MCL and the SMCL, and (2) 

the timeliness of the PN requirement for the SMCL. 

The workgroup indicated that waiting 12 months to 

notify customers of an exceedance of the SMCL 

does not adequately protect young children from 

dental fluorosis during a critical stage of tooth 

enamel development. The participating States 

requested that EPA consider regulatory revisions to 

clarify the PN requirements and better reflect the 

health and aesthetic implications of each. EPA 

noted that PN requirements are not within the 

scope of this NPDWR review. However the Agency 

agreed that a fact or information sheet may be 

useful to clarify any confusion. 

contaminants that are specified as BAT 

in the current NPDWR and some of the 

small system compliance technologies 

specified by EPA in 1998 (USEPA, 

1998b), shows that they are effective 

enough to achieve concentrations as low 

as the EQL. If EPA were to determine 

that it is appropriate to revise these 

NPDWRs, it would undertake a more 

thorough review of treatment feasibility, 

including a consideration of costs, to 

determine whether treatment feasibility 

would be a constraint or not. 

b. Treatment Technique-Type Rules 

EPA reviewed two chemical 

NPDWRs—acrylamide and 

epichlorohydrin (both classified B2 

carcinogens)—for which a TT is set in 

lieu of an MCL. The TT requirement 

limits the allowable acrylamide and 

epichlorohydrin monomer levels in 

polymeric coagulant aids and their 

dosages for drinking water treatment, 

storage, and distribution. Although a 

health effects assessment for acrylamide 

is ongoing, it is a carcinogen with an 

MCLG of zero and the draft health 

effects assessment indicates that the 

cancer classification remains the same. 

As a refinement in Six-Year Review 2, 

EPA considered new information to 

determine if the TTs for these 

contaminants may need to be revised. 

This information indicates that 

improvements in manufacturing 

capabilities have reduced the residual 

monomer content in acrylamide and 

epichlorohydrin-based polymeric 

coagulants aids and these changes 

would support revisions to the TTs for 

acrylamide and epichlorohydrin. 

Sections VI.B.1 and VI.B.36 of today’s 

action summarize these issues for 

acrylamide and epichlorohydrin, 

respectively. 

6. Other Regulatory Revisions 

In addition to possible revisions to 

MCLGs, MCLs, and TTs, EPA 

considered whether other regulatory 

revisions are needed, such as 

monitoring and system reporting 

requirements, as a part of the Six-Year 

Review 2. EPA utilized the protocol 

established during the Six-Year Review 

1 to evaluate which implementation 

issues to consider (USEPA, 2003b). 

EPA’s protocol focused on items that 

were not already being addressed, or 

had not been addressed, through 

alternative mechanisms (e.g., as a part of 

a recent or ongoing rulemaking). EPA 

considered potential implementationrelated revisions in these cases if the 

revisions: 

• Represented a change to an 

NPDWR, as defined under section 1401 

of SDWA; 9

• Were ‘‘ready’’ for rulemaking—that 

is, the problem to be resolved had been 

clearly defined, and specific options to 

address the problem had been 

formulated; and 

• Would clearly improve the level of 

public health protection and/or provide 

a meaningful opportunity for cost 

savings (either monetary or burden 

reduction) while not lessening public 

health protection. 

a. Issues Identified by the EPA/State 

Workgroup 

To gather input regarding 

implementation-related concerns and 

help the Agency identify the top one or 

two issues for Six-Year Review 2 

(USEPA, 2009h), EPA requested that the 

Association of State Drinking Water 

Administrators (ASDWA) form a 

workgroup of member States and 

primacy agencies. In the fall of 2007, ten 

member States agreed to participate and 

confer with EPA on a joint EPA/State 

workgroup. The State/EPA workgroup 

initially identified 22 issues, but 

narrowed the list to 4 items. Of these 

four items, three appeared to be within 

the scope of this NPDWR review, and 

EPA agreed that an information or fact 

sheet might be appropriate for the fourth 

item regarding public notification (PN) 

requirements for fluoride.10 The EPA/ 

State workgroup agreed that public 

input via the Federal Register would 

provide additional insight on the 

national scope of these three issues (i.e., 

Are the issues isolated to a few States 

or more widespread?), the importance of 

these issues to other States as well as 

water systems, and ideas on potential 

resolutions. Table V–5 provides a brief 

description of the remaining three 

issues and some of the potential 

solutions discussed in the workgroup 

meetings. 

EPA is requesting public input and 

further information on these three 

implementation issues to better inform 

future State/EPA workgroup 

discussions. More specifically, EPA 

would like to gauge how many States 

and/or public water utilities may be 

affected by these issues, and which one 

or two issues are most important to 

States. EPA also requests input and 

suggestions from commenters regarding 

any other potential solutions to the 

issues. As part of the public comment 

process, EPA also welcomes any data on 

the occurrence of nitrates and/or nitrites 

in the distribution system, especially as 

it may relate to nitrification associated 

with the use of chloramines for 

disinfection. 

TABLE V–5—ISSUES IDENTIFIED BY THE EPA/STATE WORKGROUP THAT FALL WITHIN THE SCOPE OF THIS NPDWR 

REVIEW

Implementation issue Examples of potential solutions discussed by the workgroup 

Change the location of nitrate-nitrite monitoring to address possible nitrification within the distribution system for water systems using chloramines1• Location of Monitoring 

—Move sampling location from the entry point to the distribution to within the distribution system. 

—Or, maintain entry point sampling and also sample in the distribution system. 


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TABLE V–5—ISSUES IDENTIFIED BY THE EPA/STATE WORKGROUP THAT FALL WITHIN THE SCOPE OF THIS NPDWR 

REVIEW—Continued 

Implementation issue Examples of potential solutions discussed by the workgroup 

• Frequency of Monitoring 

—Consider sampling in conjunction with DBPs, TCR or some other scheme. 

• EPA notes that 40 Code of Federal Regulations (CFR) 141.23(a)(2) may allow surface water systems discretion to locate the sampling point in the distribution system 

if that is more representative of the source after treatment.2

Reduce the monitoring for ground water systems with 

historically low levels of nitrate-nitrite.

• Consider revisions to change the frequency of monitoring, the trigger level and the 

duration of time for systems to qualify for reduced monitoring. Examples included: 

—A monitoring frequency of 3, 6, or 9 years (consistent with the existing standardized monitoring framework) or some other frequency. 

—A new trigger level set at either 1⁄2 the MCL (or some other fraction), the PQL/ 

MDL (or some other level of detection), or another appropriate level. 

—As for the duration of how long a system would need to meet the trigger level in 

order to be allowed to begin reduced monitoring, some options included a 3-, 6-, 

or 9-year period (consistent with the standardized monitoring framework) or a 5-, 

10-, or 15-year period. 

• Or consider providing a waiver option to give States discretion to reduce monitoring. 

• Or consider a non-regulatory option such as the Alternative Monitoring Guidelines 

(which some considered too burdensome). 

Revise the monitoring requirements for Non Community 

Water Systems (NCWS) to better target the potential 

health risks associated with chronic contaminants. In 

light of the probability and magnitude of health 

threats, some monitoring requirements for these systems may be insufficient, and others may be excessive.

• Revise all contaminant rules to include additional monitoring requirements for Transient Non Community Water Systems (TNCWS), as well as radionuclide monitoring 

requirements for Non Transient Non Community Water Systems (NTNCWS). 

• Or review existing regulated contaminants and include TNCWS monitoring requirements based on the relative health risk from chronic exposure. 

• Or develop general language that would apply to all contaminant rules, giving States 

the discretion to require additional monitoring for contaminants that pose chronic exposure risks and can have acute health effects at elevated levels potentially found at 

TNCWSs (the preferred option from States). 

• Note: For some of these options, EPA would need to evaluate whether sufficient occurrence and exposure information is available for TNCWS and NTNCWS to assess 

the need for revised monitoring strategies. 

1The health effects technical review identified new information on developmental effects of nitrate and nitrite, as well as data regarding its carcinogenicity, that may indicate the need to update the Agency’s risk assessment (see section VI.B.49 and VI.B.50 of today’s action). In light of 

this information, EPA is considering nitrate and nitrite as potential candidates for new health effects assessments. If new assessments are initiated and completed, EPA will be able to determine the potential impacts on the MCLG, MCL, and/or monitoring requirements, and what future 

actions may or may not be appropriate. 2 40 CFR 141.23(a)(2) states: Surface water systems shall take a minimum of one sample at every entry point to the distribution system after 

any application of treatment or in the distribution system at a point which is representative of each source after treatment (hereafter called a 

sampling point) beginning in the initial compliance period. The system shall take each sample at the same sampling point unless conditions 

make another sampling point more representative of each source or treatment plant. 

b. Other Issues (Synthetic Organic 

Chemicals Trigger Levels) 

40 CFR 141.24(h)18 of the national 

primary drinking water regulations lists 

detection limits for the synthetic organic 

chemicals (SOCs), including pesticides. 

These detection limits serve as triggers 

for determining whether the compliance 

monitoring frequency for SOCs may be 

reduced; public water systems detecting 

SOCs at or below trigger concentration 

can qualify for reduced monitoring. 

Several Regions and States have 

requested guidance and clarification on 

the use of detection limits in monitoring 

of drinking water samples for SOCs. The 

primary concern is that some 

laboratories have reported difficulty in 

achieving the detection limits for some 

SOCs on a regular basis and, in those 

cases, the water systems that they 

support are not able to qualify for 

reduced monitoring. 

EPA is seeking information about the 

extent and magnitude of any issues 

related to the ability of laboratories to 

achieve the SOC trigger levels specified 

in section 141.24(h)(18). EPA wishes to 

determine if this issue is widespread or 

limited to specific SOCs and/or specific 

laboratories. EPA is requesting that 

stakeholders provide information/data 

to support their concerns related to SOC 

triggers. 

C. How Did EPA Factor Children’s 

Health Concerns Into the Review? 

The 1996 amendments to SDWA 

require special consideration of all 

sensitive populations (e.g., infants, 

children, pregnant women, elderly, and 

individuals with a history of serious 

illness) in the development of drinking 

water regulations (section 

1412(b)(3)(C)(V) of SDWA, as amended 

in 1996). As a part of the Six-Year 

Review 2, EPA completed a literature 

search covering developmental and 

reproductive endpoints (e.g., fertility, 

embryo survival, developmental delays, 

birth defects, and endocrine effects) for 

regulated chemicals that have not been 

the subject of a health effects assessment 

during this review period (see section 

V.B.1 of today’s action). EPA reviewed 

the output from the literature searches 

to identify any studies that might have 

an influence on the present MCLG. 

Three chemicals were identified with 

potential developmental/reproductive 

endpoints of concern that might not be 

addressed by the current NPDWR: 

Nitrate, nitrite, and selenium. In each 

case, where the literature search 

indicated a need to consider recent 

studies of developmental or 

reproductive toxicity, EPA is 

considering whether to nominate the 

contaminant for a new health effects 

assessment. 

VI. Results of EPA’s Review of NPDWRs 

Table VI–1 lists EPA’s review results 

for each of the 71 NPDWRs discussed in 

this section of today’s action along with 

the principal rationale for the review 

outcomes. Table VI–1 also includes a 

list of the 14 NPDWRs that have been 

or are being reviewed/revised by recent 

or ongoing regulatory actions. 


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A. What Are the Review Result 

Categories? 

For each of the 71 NPDWRs discussed 

in detail in the following sections of 

today’s action, the review results in one 

of the following outcomes: 

1. No Action at This Time and the 

NPDWR Is Still Appropriate 

The NPDWR is appropriate and no 

action is necessary at this time for one 

of the following reasons: 

a. A health effects assessment is in process 

or the Agency is considering whether to 

initiate an assessment. The MCL remains 

appropriate because either, (1) it is equal to 

the MCLG, (2) the MCL is based on SDWA’s 

cost-benefit provision, (3) there is no 

potential to change the MCL based on 

changes in analytical feasibility, or (4) there 

may be a potential change to the MCL based 

on analytical feasibility, but any such change 

is unlikely to provide a meaningful 

opportunity to improve public health 

protection. This group includes both 

contaminants where an assessment is in 

process, and contaminants where EPA 

identified new health information that may 

warrant a new health effects assessment. 

b. NPDWR remains appropriate after data/ 

information review. There is no ongoing 

health assessment and the outcome of the 

review indicates that the current regulatory 

requirements remain appropriate and, 

therefore, no regulatory revisions are 

warranted at this time. Any new information 

available to the Agency either supports the 

current regulatory requirements or does not 

justify a revision. 

c. New information is available that 

indicates potential for a regulatory revision, 

but no revision recommended because: 

• Negligible gain in public health 

protection and/or cost savings: Any resulting 

changes to the NPDWR would not 

significantly improve the level of public 

health protection or result in a major cost 

savings for public water systems and their 

customers. 

• Information Gaps or Emerging 

Information: Either new information is 

emerging that could affect EPA’s evaluation 

of the NPDWR or the available data are 

insufficient to support a definitive regulatory 

recommendation at this time. 

2. Candidate for Revision 

The NPDWR is a candidate for 

revision based on the review of new 

information. 


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B. What Are the Details of EPA’s Review 

of Each NPDWR? 

1. Acrylamide 

a. Background. EPA published the 

current NPDWR for acrylamide on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR imposes a TT 

requirement that limits the allowable 

monomer levels in products used during 

drinking water treatment, storage, and 

distribution to 0.05 percent acrylamide 

in polyacrylamide coagulant aids, and 

limits the dosage of such products to a 

maximum of 1 mg/L (ppm). Each water 

system is required to certify, in writing, 

to the State (using third-party or 

manufacturer’s certification) that the 

product used meets these residual 

monomers and use-level specifications. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

acrylamide. The revised health effects 

assessment is considering relevant 

studies on the toxicity of acrylamide, 

including its potential developmental 

and reproductive toxicity. The draft 

assessment was published in the 

Federal Register on December 28, 2007 

(72 FR 73813 (USEPA, 2007b)). The 

Science Advisory Board (SAB) 

conducted a peer review of the 

document, which also included a 

review of public comments received on 

the draft assessment. The SAB panel 

concurred with the Agency’s rationale 

and justification for acrylamide being a 

‘‘likely human carcinogen’’ via 

mutagenic mechanism. At the present 

time, acrylamide is still under 

evaluation by the Agency, and the IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

Although there is an ongoing health 

effects assessment, the MCLG is zero 

and the current TT standard allows 

exposure at levels above the MCLG. 

Therefore, EPA reviewed whether there 

is potential to revise the TT for 

acrylamide. EPA has identified 

information that suggests that the 

residual acrylamide content in water 

treatment polymers has decreased 

significantly, likely due to 

improvements in manufacturing 

processes and technologies (USEPA, 

2009g). NSF International analyses 

conducted between January 2005 and 

June 2007 found that, in 66 

polyacrylamide products submitted for 

certification under NSF Standard 60, the 

median residual acrylamide content was 

0.006 percent, and the 90th percentile 

acrylamide content was 0.025 percent, 

half of the limit set in the treatment 

technique. 

Acrylamide standards in Europe and 

Australia are also stricter than the 

NPDWR. Based on the maximum 

allowable dosage and monomer level in 

the NPDWR, finished water could 

contain up to 0.5 μg/L (ppb) of 

acrylamide. By contrast, the European 

Union requires that finished water 

contain less than 0.1 μg/L (parts per 

billion or ppb) acrylamide, and 

Australia requires that the concentration 

in finished water be less than 0.2 μg/L 

(ppb). The United Kingdom requires 

that polyacrylamides used in drinking 

water contain less than 0.02 percent 

residual acrylamide, and that the 

polyacrylamide dose be less than 0.5 

mg/L (parts per million or ppm) at all 

times, for a maximum finished water 

concentration of 0.1 μg/L (ppb). 

To assess the occurrence of 

acrylamide in drinking water, EPA 

sought data on current usage practices 

for polyacrylamide coagulant aids. The 

Agency is not presently aware of any 

recent, large-scale studies of polymer 

usage in drinking water facilities, and 

therefore cannot fully characterize the 

occurrence of acrylamide in drinking 

water. However, the 1996 

WATER:\STATS database (described in 

Levine et al., 2004), based on an 

American Water Works Association 

(AWWA) survey, indicates that 13 

percent of ground water systems and 66 

percent of surface water systems 

surveyed use a polymer for water 

treatment. Many of these are anionic 

and nonionic polymers, particularly for 

ground water systems; anionic and 

nonionic polymers used to treat 

drinking water are most likely 

polyacrylamides. 

Additional information on the extent 

of use of polyacrylamide in drinking 

water and the impending health effects 

assessment will further assist the 

Agency in determining the potential 

public health benefits associated with a 

revision to the treatment technique for 

acrylamide. Because most 

polyacrylamides available today have a 

lower residual monomer content than 

that specified in the current treatment 

technique (USEPA, 2009g), EPA 

believes that the costs of a revision 

would be minimal and recognizes that 

the benefits may also be small. 

c. Review Result. The Agency believes 

it is appropriate to revise the NPDWR 

for acrylamide although a health effects 

assessment is currently in progress. The 

existing MCLG is still zero (based on the 

current B2 cancer classification) and 

NSF International data indicate that 

polyacrylamides are widely available 

with lower residual monomer levels 

than required by the existing NPDWR. 

Hence, revisions to the acrylamide 

NPDWR will provide a meaningful 

opportunity to maintain the health risk 

reductions achieved by technological 

advances in manufacturing. If the 

updated health effects assessment is 

completed in time to consider for the 

regulatory revision of acrylamide, the 

Agency will consider this final 

assessment in its evaluation of health 

benefits. As discussed in Section VII, 

the Agency solicits information from the 

public on the extent of use of 

polyacrylamide in drinking water 

facilities (since this may provide 

additional information on the 

occurrence of acrylamide in drinking 

water) to help inform the regulatory 

revision. EPA notes that any changes to 

the NPDWR for acrylamide may also 

include revisions to the closely related 

NPDWR for epichlorohydrin. 

2. Alachlor 

a. Background. EPA published the 

current NPDWR for alachlor on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG of 

zero based on a cancer classification of 

B2, probable human carcinogen. The 

NPDWR also established an MCL of 

0.002 mg/L, based on analytical 

feasibility. 

b. Technical Reviews. In 2006, the 

Agency updated its health effects 

assessment of alachlor (USEPA, 2006a). 

The Agency identified a change in this 

assessment that could lead to a change 

in the MCLG. This assessment 

considered relevant studies on the 

toxicity of alachlor including 

developmental and reproductive 

toxicity. For noncancer effects, the 

assessment confirmed the RfD of 0.01 

mg/kg-day (milligrams per kilogram of 

body weight per day). The assessment 

also concluded that alachlor is likely to 

be a human carcinogen at high doses; 

not likely to be a human carcinogen at 

low doses, and that a linear doseresponse extrapolation is no longer 

appropriate. It established a health 

reference value of 0.005 mg/kg-day for 

the nonlinear cancer assessment 

(USEPA, 2006a). Since the health 

reference value of 0.005 mg/kg-day is 

lower than the RfD of 0.01 mg/kg-day, 

the Agency used this value to calculate 

a possible MCLG. Based on the health 

reference value of 0.005 mg/kg-day, and 

assuming a 70-kg adult body weight and 

2 liters water intake per day, the 

drinking water equivalent level (DWEL) 

could be 0.2 mg/L. A relative source 

contribution (RSC) of 20 percent results 


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11Between 2004 and 2006, the United States 

Department Agriculture’s Pesticide Data Program 

(USDA PDP) collected data for alachlor and its ESA 

and OA degradates from finished and untreated 

water samples for a limited number of water 

systems (USDA, 2004, 2005, and 2006). While 

alachlor was rarely detected (i.e., 0 to 0.8 percent 

of the samples by year), the alachlor ESA and OA 

degradates were commonly detected (i.e., 19 to 51 

percent of the samples by year for the ESA 

degradate and 7 to 40 percent of the samples by 

year for the OA degradate). The detected values for 

the ESA and OA degradates ranged from 0.0028 to 

0.357 μg/L and 0.001 to 0.102 μg/L, respectively. 

The detected values for alachlor ranged from 0.0163 

to 0.043 μg/L. 

in a possible MCLG of 0.04 mg/L 

(USEPA, 2009b). 

Since the health review for alachlor 

indicates that the MCLG could possibly 

increase to 0.04 mg/L (from its current 

MCLG of zero) and because the current 

MCL is based on a PQL of 0.002 mg/L, 

neither analytical nor treatment 

feasibility would be a limiting factor for 

a possible higher level of 0.04 mg/L. 

EPA evaluated the results of the 

occurrence and exposure analyses for 

alachlor to determine whether a revised 

MCLG/MCL would be likely to result in 

a meaningful opportunity for cost 

savings to PWSs and their customers 

while maintaining or improving the 

level of public health protection 

(USEPA, 2009f). Review of health 

information for alachlor indicated that 

the MCLG could be increased to 0.04 

mg/L from its current MCLG of zero. 

Consequently, the MCL of alachlor 

possibly can also increase to 0.04 mg/L. 

Although the Agency obtained and 

evaluated the finished water occurrence 

data for alachlor, its usefulness is 

limited for determining potential cost 

savings to PWSs and their customers 

because the Agency does not know 

which systems are treating for this 

contaminant. As an alternative, the 

Agency evaluated available data on 

source water quality and conducted a 

qualitative assessment of treatment cost 

savings. 

Table VI–2 provides summary data for 

contaminant occurrence based on 

maximum sample values for the 

locations included in the STORET and 

NAWQA data. Although the degree to 

which these occurrence rates represent 

national drinking water source 

occurrence is uncertain, the information 

shows no to low occurrence at threshold 

levels of interest. This information 

indicates that any resulting NPDWR 

change would affect systems that rely on 

source water at less than 0.4 percent of 

the NAWQA locations and less than 1.8 

percent of the STORET locations. 

TABLE VI–2—AMBIENT WATER QUALITY MONITORING OCCURRENCE SUMMARY FOR ALACHLOR

Maximum concentration 

Number of locations (% of locations) 

STORET 1 NAWQA2

Total ............................................................................................................................ 2,252 (100.0%) ........................................ 9,236 (100.0%) 

Nondetect ................................................................................................................... 1,669 (74.1%) .......................................... 8,571 (92.8%) 

Detected ..................................................................................................................... 583 (25.9%) ............................................. 665 (7.2%) 

Exceeds current MCL of 0.002 mg/L ......................................................................... 40 (1.8%) ................................................. 35 (0.38%) 

Exceeds alternative value of 0.04 mg/L ..................................................................... 0 (0.0%) ................................................... 1 (0.01%) 

1STORET database 2002–2006. 

2 NAWQA database 1992–2008. 

Source: USEPA, 2009d. 

The BATs and small system 

compliance technologies for alachlor 

have other beneficial effects, e.g., 

reduction of other co-occurring 

contaminants, precursors for 

disinfection byproducts (DBPs) or other 

common impurities. Therefore, if EPA 

were to consider a higher level, the 

Agency does not know how many PWSs 

that are currently treating to comply 

with the existing MCL of 0.002 mg/L 

would be likely to discontinue 

treatment that is already in place 

(USEPA, 2009d). Also, the Agency does 

not know to what extent affected 

systems might be able to reduce costs 

given that capital costs are not 

recoverable. However, the Agency 

recognizes that there may be 

opportunities to achieve operational 

cost savings if these systems are able to 

re-optimize current treatment. 

Given these considerations, the 

Agency believes that any resulting 

revision is not likely to provide a 

meaningful opportunity for cost savings. 

In view of this, any revision would be 

a low priority activity and not 

appropriate at this time. 

The Agency notes that alachlor and 

two of its unregulated acid degradates 

(alachlor ethanesulfonic acid or ESA 

and alachlor oxanilic acid or OA11) are 

currently listed on the second 

Unregulated Contaminants Monitoring 

Rule (UCMR 2) (72 FR 367, January 4, 

2007 (USEPA, 2007e)). The Agency also 

listed alachlor ESA and OA on the CCL3 

(74 FR 51850, October 8, 2009 (USEPA, 

2009l)). Once the UCMR 2 monitoring 

results are available for alachlor and its 

degradates, the Agency will be able to 

more fully evaluate alachlor along with 

its degradates in determining how this 

information might impact the current 

regulation for alachlor and/or the need 

for any revised or new regulation to 

capture the impact from the ESA and 

OA degradates. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

alachlor, EPA does not believe a 

revision to the NPDWR for alachlor is 

appropriate at this time. In making this 

decision, the Agency considered 

whether any possible revision to the 

NPDWR for alachlor is likely to provide 

a meaningful opportunity for cost 

savings to public water systems and 

their customers. Taking into 

consideration the low occurrence of this 

contaminant in source waters, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

In addition, the Agency considers it 

premature to make any decision to 

revise the alachlor NPDWR pending the 

final UCMR 2 monitoring results. 

3. Alpha Particle Emitters 

a. Background. EPA published an 

interim NPDWR and set an MCL of 15 

pCi/L for gross alpha particle activity on 

July 9, 1976 (41 FR 28402 (USEPA, 

1976)). As noted in the August 14, 1975 

proposal (40 FR 34324 (USEPA, 1975)) 


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12Category II contaminants include those 

contaminants for which EPA has determined there 

is limited evidence of carcinogenicity from drinking 

water considering weight of evidence, 

pharmacokinetics, potency, and exposure. For 

Category II contaminants, EPA has used two 

approaches to set the MCLG: Either (1) setting the 

MCLG based upon noncarcinogenic endpoints of 

toxicity (the RfD) then applying an additional risk 

management factor of 1 to 10; or (2) setting the 

MCLG based upon a theoretical lifetime excess 

cancer risk range of 10¥5 to 10¥6 using a 

conservative mathematical extrapolation model. 

and a subsequent September 30, 1986 

FR notice (51 FR 34836 (USEPA, 1986a), 

EPA considered the feasibility of 

treatment techniques, analytical 

methods and monitoring when 

establishing the MCL of 15 pCi/L. EPA 

also considered the risks associated 

with other alpha particle emitters 

relative to radium-226, which generally 

fell within the Agency’s acceptable risk 

range of 10¥4 to 10¥6 at the MCL of 15 

pCi/L. On December 7, 2000 (65 FR 

76708 (USEPA, 2000c)), EPA 

established an MCLG of zero based on 

a cancer classification of A (known 

human carcinogen) and finalized the 

NPDWR by retaining the MCL of 15 pCi/ 

L. EPA noted in the December 7, 2000, 

FR notice that new risk estimates from 

Federal Guidance Report 13 reaffirmed 

that the 15 pCi/L gross alpha particle 

MCL (including radium 226 but 

excluding uranium and radon) was 

appropriate and protective. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to alpha 

particle emitters. The revised health 

effects assessment will consider relevant 

studies on the toxicity of alpha particle 

emitters, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). 

Although there is an ongoing health 

effects assessment, the MCLG is zero 

and the current MCL is higher than the 

MCLG. Therefore, EPA reviewed 

whether there is potential to revise the 

MCL based on new information 

regarding analytical and treatment 

feasibility for gross alpha particles. EPA 

promulgated a detection limit of 3 pCi/ 

L in 1976 (41 FR 28402 (USEPA, 1976)) 

and retained the use of a detection limit 

as the required measure of sensitivity 

for radiochemical analysis in lieu of an 

MDL or PQL in the final rule (65 FR 

76708 (USEPA, 2000c)). EPA did not 

identify new analytical methods during 

the current review that would feasibly 

lower the detection limit. In addition, 

since the December 7, 2000, regulation, 

there is no new information regarding 

treatment feasibility. Since there is no 

new information regarding analytical or 

treatment feasibility that suggests 

changes to the MCL, EPA does not 

believe it is necessary to conduct an 

occurrence analysis at this time. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

gross alpha particles is appropriate at 

this time because a reassessment of the 

health risks resulting from exposure to 

alpha particles is in progress (USEPA, 

2009b). Furthermore, there is no new 

information regarding analytical or 

treatment feasibility that would warrant 

reconsideration of the MCL. 

4. Antimony 

a. Background. EPA published the 

current NPDWR for antimony on July 

17, 1992 (57 FR 31776 (USEPA, 1992)). 

The NPDWR established an MCLG and 

an MCL of 0.006 mg/L. EPA based the 

MCLG on a reference dose of 0.0004 mg/ 

kg-day and a cancer classification of D, 

not classifiable as to human 

carcinogenicity. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

antimony. The revised health effects 

assessment will consider relevant 

studies on the toxicity of antimony, 

including its potential developmental 

and reproductive toxicity. The Agency 

does not expect the new health effects 

assessment to be completed in the time 

frame of the current Six-Year Review 

cycle (USEPA, 2009b). On December 21, 

2007 (72 FR 72715 (USEPA, 2007c)), the 

Agency noted that the health effects 

assessment for antimony is in process. 

c. Review Result. Since the MCL for 

antimony is set at its MCLG and a 

reassessment of the health risks 

resulting from exposure to antimony is 

in progress, the Agency does not believe 

a revision to the NPDWR is appropriate 

at this time. 

5. Arsenic 

a. Background. EPA published the 

current NPDWR for arsenic on January 

22, 2001 (66 FR 6976 (USEPA, 2001c)). 

The NPDWR established an MCLG of 

zero based on a cancer classification of 

A, known human carcinogen. The 

NPDWR also established an MCL of 

0.010 mg/L, which is higher than the 

feasible analytical level of 0.003 mg/L. 

EPA exercised its discretionary 

authority to set an MCL at a level higher 

than feasible (SDWA Section 

1412(b)(6)), based on the finding that a 

final MCL of 0.010 mg/L represents the 

level that best maximizes health risk 

reduction benefits at a cost that is 

justified by the benefits (66 FR 6976 at 

7020 (USEPA, 2001c)). 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to arsenic. 

In June 2007, EPA’s Science Advisory 

Board (SAB) issued its evaluation of the 

Agency’s 2005 draft toxicological review 

for inorganic arsenic (USEPA, 2007a). In 

its 2007 report, SAB supports the 

continued use of a linear cancer risk 

model for inorganic arsenic, noting that 

the available data do not describe the 

shape of the dose-response curve at low 

doses. The new health effects 

assessment (both cancer and noncancer) 

were not completed by March 1, 2009, 

the review cutoff date for this notice. 

The revised health effects assessments 

will consider relevant studies on the 

toxicity of arsenic, including its 

potential developmental and 

reproductive toxicity. The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessments. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

arsenic is appropriate at this time 

because a reassessment of the health 

risks resulting from exposure to arsenic 

is ongoing (USEPA, 2009b). As noted 

previously, the arsenic MCL is based on 

the SDWA cost benefit provision 

(Section 1412(b)(6)) and the health 

effects assessment is important for 

reviewing the benefits associated with 

the basis of the MCL. 

6. Asbestos 

a. Background. EPA published the 

current NPDWR for asbestos on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG and 

an MCL of 7 million fibers/L. EPA 

evaluated asbestos as a Category II 12

contaminant (equivalent to Group C, 

possible human carcinogen) by the oral 

route of exposure. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

asbestos. The revised health effects 

assessment will consider relevant 

studies on the toxicity of asbestos, 

including its potential developmental 

and reproductive toxicity. The Agency 

does not expect the new health effects 

assessment to be completed in the time 

frame of the current Six-Year Review 

cycle (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

c. Review Result. Since the MCL for 

asbestos is set at its MCLG and a 

reassessment of the health risks 

resulting from exposure to asbestos is in 


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13Additional information is available at http:// 

www.epa.gov/pesticides/reregistration/atrazine/ 

atrazine_update.htm. 14The present MCLG for barium does not include 

an RSC because the dose used in the calculation 

applied to only the dose from the drinking water. 

If a new MCLG were to be developed from the 

animal data that support the 2005 IRIS RfD, an RSC 

would be required. Regulations or guidelines 

pertaining to barium from media other than water 

were not identified. Barium metaborate is a 

registered pesticide but it does not have any food 

uses and does not have a human health ambient 

water quality guideline value. EPA used the 

subtraction calculation method to determine the 

possible RSC of 80 percent for drinking water (the 

ceiling on RSC specified by the methodology). 

progress, the Agency does not believe a 

revision to the NPDWR is appropriate at 

this time. 

7. Atrazine 

a. Background. EPA published the 

current NPDWR for atrazine on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG and 

an MCL of 0.003 mg/L. EPA based the 

MCLG on a reference dose of 0.005 mg/ 

kg-day and a cancer classification of C, 

possible human carcinogen. 

b. Technical Reviews. In 2006, the 

Agency finalized a health effects 

assessment for the reregistration of 

atrazine as a pesticide (USEPA, 2006c). 

This assessment examined an extensive 

toxicology database and included 

investigation of atrazine’s 

neuroendocrine mode of action and 

related reproductive and developmental 

effects. The assessment established a 

new RfD of 0.018 mg/kg-day, based on 

attenuation of pre-ovulatory luteinizing 

hormone (LH) surge, a key event 

indicative of hypothalamic function 

disruption. In accordance with the 1999 

Interim Guidelines for Carcinogen Risk 

Assessment, EPA’s Cancer Assessment 

Review Committee (CARC) classified 

atrazine as ‘‘not likely to be carcinogenic 

to humans’’ because the tumor response 

in the Sprague-Dawley rats was 

determined to be a strain specific 

mechanism which is not relevant to 

humans. 

c. Review Result. The Agency believes 

it is not appropriate to consider 

revisions to the NPDWR for atrazine at 

this time and has place atrazine in the 

emerging information/data gap category 

because of an impending re-evaluation 

of the Agency’s risk assessment for 

atrazine. On October 7, 2009,13 the 

Agency announced its intent to launch 

a comprehensive new evaluation of the 

atrazine to determine its effects on 

humans. At the end of this process, the 

Agency will decide whether to revise its 

current risk assessment for atrazine and 

whether new restrictions are necessary 

to better protect public health. EPA will 

evaluate the pesticide’s potential cancer 

and non-cancer effects on humans. 

Included in this new evaluation will be 

the most recent studies on atrazine and 

its potential association with birth 

defects, low birth weight, and premature 

births. Our examination of atrazine will 

be based on transparency and sound 

science, including independent 

scientific peer review and will help 

determine whether a change in EPA’s 

regulatory position on this pesticide is 

appropriate. 

8. Barium 

a. Background. EPA published the 

current NPDWR for barium on July 1, 

1991 (56 FR 30266 (USEPA, 1991b)). 

The NPDWR established an MCLG and 

an MCL of 2 mg/L. EPA based the MCLG 

on a reference dose of 0.07 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity 

via the oral route. 

b. Technical Reviews. In 2005, the 

Agency updated the health effects 

assessment of barium and revised the 

RfD from 0.07 mg/kg-day to 0.2 mg/kgday (USEPA, 2005a). The change in the 

RfD could lead to a change in the 

MCLG. This assessment considered 

relevant studies on the toxicity of 

barium including developmental and 

reproductive toxicity. The assessment 

concluded that barium is not likely to be 

carcinogenic to humans (USEPA, 

2005a). Based on the new IRIS 

assessment and RfD of 0.2 mg/kg-day, 

and assuming 70 kg body weight and 2 

liters water intake per day, the DWEL 

could be 7.0 mg/L. An RSC of 80 

percent 14 results in a possible MCLG of 

6.0 mg/L. 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor if EPA 

were to raise the MCLG. EPA evaluated 

the results of the occurrence and 

exposure analyses for barium to 

determine whether a revised MCLG/ 

MCL would be likely to result in a 

meaningful opportunity to achieve cost 

savings for PWSs and their customers 

while maintaining, or improving, the 

level of public health protection 

(USEPA, 2009f). Although the Agency 

obtained and evaluated the finished 

water occurrence data for barium, its 

usefulness is limited for determining 

potential cost savings to PWSs and their 

customers because the Agency does not 

know which systems are treating for this 

contaminant. As an alternative, the 

Agency evaluated available data on 

source water quality and conducted a 

qualitative assessment of treatment cost 

savings. 

Table VI–3 provides summary data for 

contaminant occurrence based on 

maximum sample values for the 

locations included in the STORET and 

NAWQA data. Although the degree to 

which these occurrence rates represent 

national drinking water source 

occurrence is uncertain, the information 

shows no to low occurrence at threshold 

levels of interest. This information 

indicates that any resulting NPDWR 

change would affect systems that rely on 

source water at less than 0.1 percent of 

the NAWQA locations and less than 1.4 

percent of the STORET locations. 

TABLE VI–3—AMBIENT WATER QUALITY MONITORING OCCURRENCE SUMMARY FOR BARIUM

Maximum concentration 

Number of locations (% of locations) 

STORET 1 NAWQA2

Total ............................................................................................................................ 16,595 (100.0%) ...................................... 4,864 (100.0%) 

Nondetect ................................................................................................................... 2,299 (13.9%) .......................................... 43 (0.9%) 

Detected ..................................................................................................................... 14,296 (86.1%) ........................................ 4,821 (99.1%) 

Exceeds current MCL/MCLG of 2.0 mg/L .................................................................. 234 (1.4%) ............................................... 3 (0.1%) 

Exceeds alternative value of 6.0 mg/L ....................................................................... 163 (1.0%) ............................................... 0 (0.0%) 

1STORET database 2002–2006. 

2 NAWQA database 1992–2008. 

Source: USEPA, 2009d. 


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The BATs and small system 

compliance technologies for barium 

have other beneficial effects, e.g., 

reduction of other co-occurring 

contaminants or other common 

impurities. Therefore, if EPA were to 

consider a higher level, the Agency does 

not know how many PWSs that are 

currently treating to comply with the 

existing MCL of 2 mg/L would be likely 

to discontinue treatment that is already 

in place (USEPA, 2009d). Also, the 

Agency does not know to what extent 

affected systems might be able to reduce 

costs given that capital costs are not 

recoverable. However, the Agency 

recognizes that there may be 

opportunities to achieve operational 

cost savings if these systems are able to 

re-optimize current treatment. 

Given these considerations, the 

Agency believes that any resulting 

revision is not likely to provide a 

meaningful opportunity for cost savings. 

In view of this, any revision would be 

a low priority activity and not 

appropriate at this time. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

barium, EPA does not believe a revision 

to the NPDWR for barium is appropriate 

at this time. In making this decision, the 

Agency considered whether any 

possible revision to the NPDWR for 

barium is likely to provide a meaningful 

opportunity for cost savings to public 

water systems and their customers. 

Taking into consideration the low 

occurrence of this contaminant in 

source waters, EPA has decided that any 

revision to the NPDWR would be a low 

priority activity for the Agency, and, 

thus, is not appropriate to revise at this 

time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

9. Benzene 

a. Background. EPA published the 

current NPDWR for benzene on July 8, 

1987 (52 FR 25690 (USEPA, 1987)). The 

NPDWR established an MCLG of zero 

based on a cancer classification of A, 

known human carcinogen. The NPDWR 

also established an MCL of 0.005 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. In 2000 and 

2003, the Agency updated the IRIS 

assessment of benzene. The cancer 

assessment was completed first and 

characterized benzene as a known 

human carcinogen by all routes of 

exposure; the one-in-a million risk 

estimates for cancer by the oral route of 

exposure ranged from 1 μg/L to 10 μg/ 

L (USEPA, 2000b). This cancer 

assessment was also noted in the first 

Six-Year Review (67 FR 19030, April 17, 

2002 (USEPA, 2002c)). As part of the 

Six-Year Review process, the Agency’s 

Office of Water (OW) conducted a 

literature search through June 2007 for 

relevant data on the carcinogenicity of 

benzene as well as its potential 

developmental and reproductive 

toxicity (USEPA, 2009b). While the 

literature search did identify several 

new studies that evaluated the cancer 

and noncancer effects of benzene, none 

of the new studies would affect the 

cancer classification, which serves as 

the basis for the MCLG of zero. A recent 

occupational study (Lan et al., 2004) of 

the noncancer effects of benzene 

identified hematological effects in 

workers at levels below those previously 

reported. The Agency for Toxic 

Substances and Disease Registry 

(ATSDR) (2007) chronic minimum risk 

level based on the Lan et al. (2004) data 

of 0.0005 mg/kg/day is lower than the 

IRIS RfD of 0.004 mg/kg/day. If the 

ATSDR minimum risk level were used 

as the basis for a noncancer health 

reference level, the value would be 

0.004 mg/l, a value that is slightly below 

the current MCL. Because the MCLG 

remains at zero, the Agency believes 

that a further review of the health effects 

of benzene is not warranted at this time. 

The current MCL for benzene is based 

on a PQL of 0.005 mg/L. For the SixYear Review, the Agency considered 

whether changes in the analytical 

feasibility of benzene might lead to a 

lower MCL. EPA reviewed PE data from 

the first Six-Year Review cycle and then 

analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

benzene are above 95 percent around 

the current PQL of 0.005 mg/L, 

including two studies with true values 

below the current PQL. All passing rates 

in the PE data exceeded 75 percent. 

More recent PT data from late 1999 

through 2004, supplied by a PT 

provider, also show greater than 90 

percent passing rates for studies around 

the PQL, including eight with true 

values below the current PQL. Because 

most of the laboratory passing rates from 

PE and PT studies exceeded the 75 

percent criterion typically used to 

derive a PQL, a lowering of the PQL for 

benzene might be possible. These 

results, however, are insufficient to 

recalculate a revised PQL for benzene 

because not enough data points are 

available below the current PQL to 

derive a value at the 75 percent passing 

rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: Laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of benzene (Methods 502.2 and 524.2). 

While EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. The SixYear Review ICR dataset contains MRL 

values for 139,190 samples. More than 

80 percent of these values are less than 

or equal the modal MRL, 120,308 (86 

percent) equal the modal MRL of 0.0005 

mg/L, and an additional 17,964 (13 

percent) are lower than 0.0005 mg/L. 

Therefore, EPA selected the modal MRL 

as the EQL (USEPA, 2009e). The MDLs 

of approved methods range from 

0.00001 to 0.0004 mg/L. Applying a 

multiplier of 10 would give a possible 

PQL range from 0.0001 to 0.004 mg/L, 

which contains the EQL (USEPA, 

2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is potential to lower 

the PQL for benzene. To determine 

whether any MCL revision is likely to 

provide a meaningful opportunity to 

improve public health protection, EPA 

evaluated the occurrence of benzene at 

the EQL of 0.0005 mg/L and additional 

thresholds of 0.001, and 0.0025 mg/L 

(USEPA, 2009f). Table VI–4 shows the 

results of the occurrence and exposure 

analysis for the current MCL and these 

thresholds. The Six-Year Review ICR 

occurrence data have a modal MRL of 

0.0005 mg/L, which limits reliable 

contaminant detection to 0.0005 mg/L. 

As indicated, average concentrations 

exceed the current MCL for 10 of 50,435 

systems (0.020 percent) serving 14,000 

people (or 0.006 percent of 227 million 

people). Note that these results are 

based on the subset of monitoring data 

provided in response to the Six-Year 

Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; Safe Drinking Water 

Information System/Federal version 

(SDWIS/FED) indicates 41 MCL 

violations for benzene between 1998 

and 2005, with annual violations 

ranging from 1 to 12 (USEPA, 2007g). 

The occurrence and exposure analysis 

shows that average concentrations at 95 

to 123 of 50,435 systems (0.188 to 0.244 

percent), serving 304,000 to 485,000 

people (or 0.134 to 0.214 percent of 227 


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million people), exceed the EQL of 

0.0005 mg/L. 

TABLE VI–4—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING BENZENE THRESHOLDS

AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold 

(Percentages based on 50,435 systems with benzene data in the 

Six-Year Review ICR occurrence dataset) 

Nondetect values = 

MRL 1

Nondetect values = 1⁄2 MRL 2Nondetect values = 

0 3

MCL (0.005 mg/L) ........................... 10 (0.020%) .................................. 10 (0.020%) .................................. 10 (0.020%) 1⁄2 MCL (0.0025 mg/L) .................... 16 (0.032%) .................................. 14 (0.028%) .................................. 14 (0.028%) 

2xEQL (0.001 mg/L) ........................ 70 (0.139%) .................................. 58 (0.115%) .................................. 52 (0.103%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 123 (0.244%) ................................ 95 (0.188%) 

Corresponding population served (Percentages based on 226,947,000 people served by the systems with 

benzene data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = 

MRL 1

Nondetect values = 1⁄2 MRL 2Nondetect values = 

0 3

MCL (0.005 mg/L) ........................... 14,000 (0.006%) ........................... 14,000 (0.006%) ........................... 14,000 (0.006%) 1⁄2 MCL (0.0025 mg/L) .................... 111,000 (0.049%) ......................... 110,000 (0.048%) ......................... 110,000 (0.048%) 

2xEQL (0.001 mg/L) ........................ 180,000 (0.079%) ......................... 159,000 (0.070%) ......................... 158,000 (0.070%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 485,000 (0.214%) ......................... 304,000 (0.134%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 

benzene is appropriate at this time. The 

occurrence and exposure analysis based 

on possible changes in analytical 

feasibility indicates that any revision to 

the MCL is unlikely to provide a 

meaningful opportunity to improve 

public health protection. Taking into 

consideration the low occurrence of this 

contaminant, EPA has decided that any 

revision to the NPDWR would be a low 

priority activity for the Agency, and, 

thus, is not appropriate to revise at this 

time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

10. Benzo(a)pyrene 

a. Background. EPA published the 

current NPDWR for benzo(a)pyrene on 

July 17, 1992 (57 FR 31776 (USEPA, 

1992)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.0002 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

benzo(a)pyrene. The revised health 

effects assessment will consider relevant 

studies on the toxicity of 

benzo(a)pyrene, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

Although a risk assessment is in 

process for benzo(a)pyrene, the existing 

MCLG is zero and the current MCL of 

0.0002 mg/L is based on the PQL. 

Therefore, EPA reviewed whether there 

is potential to revise the PQL. EPA 

reviewed PE data from the first Six-Year 

Review cycle and then analyzed more 

recent PT data to determine if the PQL 

can be revised (i.e., analytical 

feasibility). Passing rates for PE data 

available through late 1999 for 

benzo(a)pyrene are all above 75 percent. 

However, the true concentrations were 

all higher than the current PQL of 

0.0002 mg/L. More recent PT data from 

late 1999 through 2004, supplied by a 

PT provider, show several true 

concentrations with passing rates less 

than the 75 percent criterion typically 

used to derive a PQL. All of the true 

concentrations in the PT data were 

higher than the current PQL. Given the 

variability in passing rates and the lack 

of data points below the current PQL, a 

lowering of the PQL for benzo(a)pyrene 

is not appropriate at this time (USEPA, 

2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of benzo(a)pyrene (Methods 550, 550.1, 

and 525.2). While EPA prefers to use 

laboratory performance data to calculate 

the PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 55,487 

samples. Fewer than 80 percent of these 

values are less than or equal the modal 

MRL, 29,769 (54 percent) equal the 

modal MRL of 0.00002 mg/L and an 

additional 970 (2 percent) are lower 


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15After the December 7, 2000, final regulation, 

two trade associations and several municipal water 

systems challenged EPA’s standard for the beta 

photon emitters by claiming that the Agency did 

not use the best available science when finalizing 

the standard. In February of 2003, the District of 

Columbia (DC) Circuit Court of Appeals upheld 

EPA’s regulation for beta and photon emitters (as 

well as radium 226 and 228 and uranium). In July, 

2004, the DC Circuit Court of Appeals also upheld 

the policy and scientific basis of EPA’s application 

of the beta particle and photon (man-made) 

drinking water standards to the ground water 

protection standards used for Yucca Mountain 

under 40 CFR part 197 (66 FR 32073, June 13, 2001 

(USEPA, 2001d)). 

than 0.00002 mg/L. Therefore, EPA did 

not set the EQL equal to the modal MRL 

(USEPA, 2009e). The MDLs of approved 

methods are 0.000016, 0.000029, and 

0.00023 mg/L. EPA selected the median 

value, applied a multiplier of 10, and 

rounded up to 0.0003 mg/L. The result 

is higher than the current PQL and, 

therefore, EPA did not estimate an EQL 

(USEPA, 2009e). Based on these varied 

and unrelated approaches/sources of 

information, EPA believes that there is 

no potential to lower the PQL for 

benzo(a)pyrene. Since the MCL is 

constrained by the PQL, and the PQL is 

unchanged, EPA does not believe it is 

necessary to conduct an occurrence 

analysis at this time. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

benzo(a)pyrene is appropriate at this 

time because a reassessment of the 

health risks resulting from exposure to 

benzo(a)pyrene is in progress (USEPA, 

2009b). Furthermore, a review of 

analytical feasibility did not identify a 

potential to revise the MCL, which is 

limited by feasibility. 

11. Beryllium 

a. Background. EPA published the 

current NPDWR for beryllium on July 

17, 1992 (57 FR 31776 (USEPA, 1992)). 

The NPDWR established an MCLG and 

an MCL of 0.004 mg/L. EPA classified 

beryllium in Group B2, probable human 

carcinogen, based on clear evidence of 

its carcinogenicity via inhalation or 

injection in several animal species. 

However, EPA also placed beryllium in 

drinking water Category II for 

regulation, based on the weight of 

evidence for carcinogenicity via 

ingestion, and the potency, exposure 

and pharmacokinetics of this chemical. 

EPA derived the MCLG by applying an 

additional risk management factor of 10 

to the RfD of 0.005 mg/kg-day (57 FR 

31776 at 31785, July 17, 1992 (USEPA, 

1992)). 

b. Technical Reviews. As noted in Six 

Year Review 1 (68 FR 42908, USEPA, 

2003e), EPA updated its assessment of 

the health risks resulting from exposure 

to beryllium in 1998 (USEPA, 1998c). 

The 1998 IRIS assessment uses the 1986 

EPA cancer guidelines (USEPA, 1986b) 

and classifies beryllium as Group B1, 

probable human carcinogen, via 

inhalation route. However, the 1998 

IRIS assessment states that the database 

is inadequate for assessing the 

carcinogenicity of ingested beryllium 

and concluded that the human 

carcinogenic potential of ingested 

beryllium cannot be determined. The 

Agency considered the 1998 

assessessment in Six Year Review 1 and 

decided that it was not appropriate to 

revise the NPDWR at that time. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

beryllium. The new assessment was not 

completed by March 1, 2009, the review 

cutoff date for this notice (USEPA, 

2009b). The IRIS Substance Assessment 

Tracking System Web site (http:// 

cfpub.epa.gov/iristrac/index.cfm) has 

the most up-to-date information on the 

status of the health effects assessment. 

c. Review Result. Since the MCL for 

beryllium is set at its MCLG and a 

reassessment of the health risks 

resulting from exposure to beryllium is 

in progress, the Agency does not believe 

a revision to the NPDWR is appropriate 

at this time. 

12. Beta Particle and Photon Emitters 

a. Background. EPA published an 

interim NPDWR and set an MCL of 4 

millirems/yr (mrem/yr) for beta particle 

and photon emitters on July 9, 1976 (41 

FR 28402 (USEPA, 1976)). As noted in 

the August 14, 1975 proposal (40 FR 

34324 (USEPA, 1975)) and a subsequent 

September 30, 1986 FR (51 FR 34836 

(USEPA, 1986a) advanced notice of 

proposed rulemaking, EPA considered 

the feasibility of treatment techniques, 

analytical methods and monitoring 

when establishing the MCL of 4 mrem/ 

yr. EPA also considered the risks 

associated with beta particle and photon 

emitters, which generally fell within the 

Agency’s acceptable risk range of 10¥4

to 10¥6 at the MCL of 4 mrem/yr. On 

December 7, 2000 (65 FR 76708 

(USEPA, 2000c)), EPA established an 

MCLG of zero based on a cancer 

classification of A (known human 

carcinogen) and finalized the NPDWR 

by retaining the MCL of 4 mrem/yr. EPA 

noted in the December 7, 2000, FR 

notice that new risk estimates from 

Federal Guidance Report 13 reaffirmed 

that the 4 mrem/yr MCL was 

appropriate and protective15. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to beta 

particles. The revised health effects 

assessment will consider relevant 

studies on the toxicity of beta particles, 

including its potential developmental 

and reproductive toxicity. The new 

health effects assessment was not 

completed by March 1, 2009, the review 

cutoff date for this notice (USEPA, 

2009b). 

Although there is an ongoing health 

effects assessment, the MCLG is zero 

and the current MCL is higher than the 

MCLG. Therefore, EPA reviewed 

whether there is potential to revise the 

MCL based on new information 

available regarding the analytical and 

treatment feasibility for beta particle 

and photon emitters. EPA promulgated 

the MCL of 4 mrem/yr for man-made 

beta particle and photon emitters 

(present in any combination) in 1976 

(41 FR 28402 (USEPA, 1976)) and 

retained the use of the detection limit as 

the required measure of sensitivity in 

the December 2000 final rule (65 FR 

76708 (USEPA, 2000c)). The original 

rule estimated a risk ceiling of 5.6×10-5

for whole body doses. Limits were set in 

picoCurie units for each nuclide 

equivalent to a 4 mrem dose. The newer 

dosimetry found in Federal Guidance13 

and reported in the December 2000 final 

rule reveals more exact risks that are 

still within the Agency’s acceptable 

limits. While individual dose estimates 

changed over time, the overall limit of 

4 mrem was retained along with a twotiered screening level to avoid analyzing 

each possible nuclide below the screen, 

and still be protective. EPA did not 

identify new analytical methods during 

the current review that would feasibly 

lower the detection limits for beta 

particle and photon emitters. In 

addition, since the December 7, 2000 

regulation, there is no new information 

regarding treatment feasibility. Since 

there is no new information regarding 

analytical or treatment feasibility that 

suggests changes to the MCL, EPA does 

not believe it is necessary to conduct an 

occurrence analysis at this time. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

beta particles is appropriate at this time 

because a reassessment of the health 

risks resulting from exposure to beta 

particles is in progress (USEPA, 2009b). 

Furthermore, there is no new 

information regarding analytical or 

treatment feasibility that would warrant 

reconsideration of the MCL. 

13. Cadmium 

a. Background. EPA published the 

current NPDWR for cadmium on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG and an MCL of 0.005 mg/L. 

Because of inadequate dose-response 

data to characterize the presence or lack 

of a carcinogenic hazard from oral 


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exposure, the Agency classified 

cadmium as a Group D carcinogen, not 

classifiable as to human carcinogenicity 

by the oral route of exposure. Therefore, 

EPA developed the MCLG for cadmium 

based on the RfD of 0.0005 mg/kg-day. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

cadmium. The revised health effects 

assessment will consider relevant 

studies on the toxicity of cadmium, 

including its potential developmental 

and reproductive toxicity. The new 

health effects assessment was not 

completed by March 1, 2009, the review 

cutoff date for this notice (USEPA, 

2009b). The IRIS Substance Assessment 

Tracking System Web site (http:// 

cfpub.epa.gov/iristrac/index.cfm) has 

the most up-to-date information on the 

status of the health effects assessment. 

c. Review Result. Since the MCL for 

cadmium is set at its MCLG and a 

reassessment of the health risks 

resulting from exposure to cadmium is 

in progress, the Agency does not believe 

a revision to the NPDWR is appropriate 

at this time. 

14. Carbofuran 

a. Background. EPA published the 

current NPDWR for carbofuran on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG and an MCL of 0.04 mg/L. EPA 

based the MCLG on a reference dose of 

0.005 mg/kg-day and a cancer 

classification of E, evidence of noncarcinogenicity for humans. 

b. Technical Reviews. In 2006, the 

Agency updated health effects 

assessment of carbofuran. The Agency 

identified a change in this assessment 

that could lead to a change in the MCLG 

(73 FR 44864, July 31, 2008 (USEPA, 

2008a)). This assessment considered 

relevant studies on the toxicity of 

carbofuran including developmental 

and reproductive toxicity. The 

assessment revised the RfD from 0.005 

mg/kg-day to an acute RfD of 0.00006 

mg/kg-day and concluded that 

carbofuran is not likely to be 

carcinogenic to humans (USEPA, 

2006d). Based on the revised acute RfD 

of 0.00006 mg/kg-day, and assuming 10 

kg body weight and 1 liter water intake 

per day for a child, the resulting DWEL 

would be 0.0006 mg/L. Using an RSC of 

20 percent, a possible new MCLG would 

be 0.00012 mg/L. The default RSC value 

of 20 percent was selected because of 

the significant exposures resulting from 

actual food dietary exposure for 

children from 1 to 6 years old, which 

approaches 100 percent of the updated 

RfD (USEPA, 2006d). 

Two recent Agency actions may affect 

carbofuran presence in food and water 

sources. In May 2009, EPA revoked all 

tolerances (maximum residue limits) for 

carbofuran, which could prohibit all 

carbofuran residues on food, effective 

December 31, 2009 (74 FR 23046, May 

15, 2009 (USEPA, 2009i)). The registrant 

and interested parties raised objections 

and requested a hearing on the tolerance 

revocations. EPA has reviewed the 

submissions and determined that a 

hearing was not warranted. Revoking 

carbofuran tolerances is part of a 

broader series of Agency actions to 

cancel all uses of carbofuran in the 

United States due to dietary, 

occupational, and ecological risks of 

concern. Following resolution of the 

current ongoing administrative process 

for resolving the safety of the tolerances, 

EPA will proceed to cancel the 

remaining uses of carbofuran. 

In addition, prior to the tolerance 

revocation, the registrant, FMC 

Corporation, voluntarily cancelled 22 

uses of carbofuran (74 FR 11551, March 

18, 2009 (USEPA, 2009j)). Existing 

stocks of carbofuran can be applied to 

food crops until December 31, 2009, and 

to non-food crops according to the label 

until supplies are depleted. These 

decisions are expected to reduce 

exposure to carbofuran and its 

metabolite (3-hydroxycarbofuran) in 

food products and in water, which 

would affect the RSC used to derive a 

possible MCLG. Therefore, EPA believes 

that it should factor in the effect of these 

actions, once completed, before the 

Agency determines the potential for an 

NPDWR revision. 

The occurrence of carbofuran in 

drinking water is an additional source of 

uncertainty in the review process that is 

compounded by the recent voluntary 

cancellations and tolerance revocations. 

The Six-Year Review ICR occurrence 

data are based on the Standardized 

Monitoring Framework for synthetic 

organic compounds, which is designed 

to evaluate long-term exposure to 

contaminants with chronic exposure 

health endpoints. As a result, short-term 

seasonal peaks, which correspond to 

carbofuran application as a pesticide, 

cannot be readily detected in this 

dataset. The cancellation will reduce 

carbofuran application and the potential 

for seasonal peaks to occur. Reductions 

in overall carbofuran use is expected to 

reduce the potential occurrence of 

carbofuran in drinking water sources. 

c. Review Result. Although there are 

new health data that support 

consideration of whether to revise the 

MCLG/MCL for carbofuran, the ongoing 

regulatory actions could affect the 

possible MCLG. Therefore, EPA is 

placing carbofuran in the information 

gap category due to the uncertainty of 

how the cancellation impacts the 

MCLG. In addition, EPA notes that the 

decision to cancel the reregistration of 

carbofuran would reduce the presence 

of this compound in the environment 

and the likelihood of exposure to 

carbofuran in food and drinking water 

sources. Consequently, EPA believes it 

is not appropriate to consider any 

revisions to the NPDWR for carbofuran 

at this time. 

15. Carbon Tetrachloride 

a. Background. EPA published the 

current NPDWR for carbon tetrachloride 

on July 8, 1987 (52 FR 25690 (USEPA, 

1987)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.005 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to carbon 

tetrachloride. The revised health effects 

assessment will consider relevant 

studies on the toxicity of carbon 

tetrachloride, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

Although a risk assessment is in 

process for carbon tetrachloride, the 

existing MCLG is zero and the current 

MCL of 0.005 mg/L is based on the PQL. 

Therefore, EPA reviewed whether there 

is potential to revise the PQL. EPA 

reviewed PE data from the first Six-Year 

Review cycle and then analyzed more 

recent PT data to determine if the PQL 

can be revised (i.e., analytical 

feasibility). Passing rates for PE data 

available through late 1999 for carbon 

tetrachloride are at or above 95 percent 

around the current PQL of 0.005 mg/L, 

including one study with a true value 

below the current PQL. More recent PT 

data from late 1999 through 2004, 

supplied by a PT provider, also show 

greater than 90 percent passing rates for 

studies around the PQL, except for one 

study with a passing rate of 85 percent. 

Nine PT studies had true values below 

the current PQL. Because most of the 

laboratory passing rates from PE and PT 

studies exceeded the 75 percent 

criterion typically used to derive a PQL, 

a lowering of the PQL for carbon 

tetrachloride might be possible. These 


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results, however, are insufficient to 

recalculate a revised PQL for carbon 

tetrachloride because not enough data 

points are available below the current 

PQL to derive a value at the 75 percent 

passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of carbon tetrachloride (Methods 502.2, 

524.2, and 551.1). While EPA prefers to 

use laboratory performance data to 

calculate the PQL, the MRL and MDL 

information can be valuable for this 

review to indicate whether it is possible 

to quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 139,221 

samples. More than 80 percent of these 

values are less than or equal the modal 

MRL: 119,849 (86 percent) equal the 

modal MRL of 0.0005 mg/L and an 

additional 16,195 (12 percent) are lower 

than 0.0005 mg/L. Therefore, EPA 

selected the modal MRL as the EQL 

(USEPA, 2009e). The MDLs of approved 

methods range from 0.000002 to 0.00021 

mg/L. Applying a multiplier of 10 

would give a possible PQL range from 

0.00002 to 0.0021 mg/L, which contains 

the EQL (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is potential to lower 

the PQL for carbon tetrachloride. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of carbon tetrachloride at the 

EQL of 0.0005 mg/L and additional 

thresholds of 0.001 and 0.0025 mg/L 

(USEPA, 2009f). Table VI–5 shows the 

results of the occurrence and exposure 

analysis for the current MCL and these 

thresholds. The occurrence and 

exposure analysis shows that average 

concentrations exceed the current MCL 

for five of 50,446 systems (0.010 

percent), serving fewer than 2,000 

people (or 0.001 percent of 227 million 

people). Note that these results are 

based on the subset of monitoring data 

provided in response to the Six-Year 

Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; SDWIS/FED indicates 19 

MCL violations for carbon tetrachloride 

between 1998 and 2005 with annual 

violations ranging from 1 to 4 (USEPA, 

2007g). Average concentrations for 84 to 

118 of 50,446 systems (0.167 to 0.234 

percent), serving 368,000 to 750,000 

people (or 0.162 to 0.330 percent of 227 

million people), exceed the EQL of 

0.0005 mg/L. 

TABLE VI–5—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING CARBON TETRACHLORIDE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean Concentrations that are greater than the regulatory or feasibility-based threshold 

(Percentages based on 50,446 systems with carbon tetrachloride data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = MRL1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.005 mg/L) ........................... 5 (0.010%) .................................... 5 (0.010%) .................................... 5 (0.010%) 1⁄2 MCL (0.0025 mg/L) .................... 13 (0.026%) .................................. 12 (0.024%) .................................. 12 (0.024%) 

2xEQL (0.001 mg/L) ........................ 59 (0.117%) .................................. 50 (0.099%) .................................. 40 (0.079%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 118 (0.234%) ................................ 84 (0.167%) 

Corresponding population served (Percentages based on 226,935,000 people served by the systems with 

carbon tetrachloride data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.005 mg/L) ........................... 1,800 (0.001%) ............................. 1,700 (0.001%) ............................. 1,700 (0.001%) 1⁄2 MCL (0.0025 mg/L) .................... 5,800 (0.003%) ............................. 5,500 (0.002%) ............................. 5,500 (0.002%) 

2xEQL (0.001 mg/L) ........................ 265,000 (0.117%) ......................... 212,000 (0.093%) ......................... 190,000 (0.084%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 750,000 (0.330%) ......................... 368,000(0.162%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

carbon tetrachloride is appropriate at 

this time because a reassessment of the 

health risks resulting from exposure to 

carbon tetrachloride is in progress 

(USEPA, 2009b). Furthermore, the 

occurrence and exposure analysis based 

on possible changes in analytical 

feasibility indicates that any revision to 

the MCL is unlikely to provide a 

meaningful opportunity to improve 

public health protection. After 

consideration of the low occurrence of 

this contaminant, EPA has decided that 

any revision to the NPDWR would be a 

low priority activity for the Agency, 

and, thus, is not appropriate to revise at 

this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

16. Chlordane 

a. Background. EPA published the 

current NPDWR for chlordane on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.002 mg/L, 

based on analytical feasibility. 


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Federal Register / Vol. 75, No. 59 / Monday, March 29, 2010 / Notices 15529 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the carcinogenicity of 

chlordane as well as its potential 

developmental and reproductive 

toxicity. EPA has not identified any new 

information that indicates that it is 

appropriate to consider revisions to the 

cancer classification for chlordane at 

this time (USEPA, 2009b). Because the 

MCLG remains at zero, the Agency 

believes that a further review of the 

health effects of chlordane is not 

warranted at this time. 

The current MCL for chlordane is 

based on a PQL of 0.002 mg/L. For the 

Six-Year Review, the Agency considered 

whether changes in the analytical 

feasibility of chlordane might lead to a 

lower MCL. EPA reviewed PE data from 

the first Six-Year Review cycle and then 

analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

chlordane are above 80 percent around 

the current PQL of 0.002 mg/L, 

including three studies with true values 

below the current PQL. More recent PT 

data from late 1999 through 2004, 

supplied by a PT provider, also show 

greater than 80 percent passing rates, 

except for two studies with passing rates 

equal to or below 75 percent. There are 

no PT studies with true values below 

the PQL. Because most of the laboratory 

passing rates from PE and PT studies— 

including three below the PQL— 

exceeded the 75 percent criterion 

typically used to derive a PQL, a 

lowering of the PQL for chlordane might 

be possible. These results, however, are 

insufficient to recalculate a revised PQL 

for chlordane because not enough data 

points are available below the current 

PQL to derive a value at the 75 percent 

passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of chlordane (Methods 505 and 508). 

While EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. The SixYear Review ICR dataset contains MRL 

values for 57,506 samples. Fewer than 

80 percent of these values are less than 

or equal the modal MRL: 26,893 (47 

percent) equal the modal MRL of 0.0002 

mg/L and an additional 9,764 (17 

percent) are lower than 0.0002 mg/L. 

Therefore, EPA did not set the EQL 

equal to the modal MRL (USEPA, 

2009e). The MDLs of approved methods 

are 0.0000041 and 0.00014 mg/L. 

Applying a multiplier of 10 would give 

possible PQLs of 0.000041 and 0.0014 

mg/L. EPA took the mean of the two 

values and, rounded up to 0.001 mg/L 

for the EQL (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is potential to lower 

the PQL for chlordane. To determine 

whether any MCL revision is likely to 

provide a meaningful opportunity to 

improve public health protection, EPA 

evaluated the occurrence of chlordane at 

the EQL of 0.001 mg/L (USEPA, 2009f). 

Table VI–6 shows the results of the 

occurrence and exposure analysis for 

the current MCL and an EQL. The 

occurrence and exposure analysis shows 

that average concentrations exceed the 

current MCL for one of 31,841 systems 

(0.003 percent) serving 80 people (or 

0.00004 percent of 182 million people). 

Note that these results are based on the 

subset of monitoring data provided in 

response to the Six-Year Review ICR do 

not necessarily reflect MCL violations, 

which are based on annual average 

concentrations at entry points; SDWIS/ 

FED indicates no MCL violations for 

chlordane between 1998 and 2005 

(USEPA, 2007g). Average concentrations 

at one to two of 31,841 systems (0.003 

to 0.006 percent), still serving 

approximately 80 to 120 people (or 

0.00004 to 0.00007 percent of 182 

million people), exceed the EQL of 

0.001 mg/L. 

TABLE VI–6—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING CHLORDANE THRESHOLDS

AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold 

(Percentages based on 31,841 systems with chlordane data in the Six-Year Review ICR occurrence 

dataset) 

Nondetect Values = 

MRL 1

Nondetect Values = 1⁄2 MRL 2Nondetect Values = 

0 3

MCL (0.002 mg/L) .......................... 1 (0.003%) .................................... 1 (0.003%) .................................... 1 (0.003%) 

EQL (0.001 mg/L) .......................... 2 (0.006%) .................................... 2 (0.006%) .................................... 1 (0.003%) 

Regulatory or feasibility-based 

threshold 

Nondetect Values = 

MRL 1

Nondetect Values = 1⁄2 MRL 2Nondetect Values = 

0 3

MCL (0.002 mg/L) .......................... 80 (0.00004%) .............................. 80 (0.00004%) .............................. 80 (0.00004%) 

EQL (0.001 mg/L) .......................... 120 (0.00007%) ............................ 120 (0.00007%) ............................ 80 (0.00004%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. 

2 Results are based on setting all nondetect results equal to 1/2 MRL values in the Six-Year Review ICR dataset. 

3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 

chlordane is appropriate at this time. 

The occurrence and exposure analysis 

based on possible changes in analytical 

feasibility indicates that any revision to 

the MCL is unlikely to provide a 


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16Note that cyanide is listed as hydrogen cyanide 

in the IRIS tracking system. 

meaningful opportunity to improve 

public health protection. Taking into 

consideration the low occurrence of this 

contaminant, EPA has decided that any 

revision to the NPDWR would be a low 

priority activity for the Agency, and, 

thus, is not appropriate to revise at this 

time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

17. Chromium 

a. Background. EPA published the 

current NPDWR for total chromium on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG and an MCL of 0.1 mg/L. 

Although the NPDWR regulates total 

chromium, the adverse health effects 

associated with hexavalent chromium 

(Cr VI) are the basis of the current 

MCLG because that is the more toxic 

species (56 FR 3526, January 31, 1991 

(USEPA, 1991a)). EPA based the MCLG 

on an RfD of 0.005 mg/kg-day and an 

assumed RSC from water of 70 percent 

for total chromium. EPA regulated 

chromium as a Group D carcinogen, not 

classifiable as to human carcinogenicity 

by the oral route of exposure. 

b. Technical Reviews. The health 

effects technical review identified some 

information regarding the 

carcinogenicity of chromium that may 

indicate the need to update the 

Agency’s health effects assessment 

(USEPA, 2009b). In 1998, the Agency 

(USEPA, 1998d) updated the IRIS 

assessment for Cr VI, which revised the 

RfD from 0.0048 mg/kg-day (rounded to 

0.005) to 0.003 mg/kg-day. While both 

RfDs are based on the same one-year 

drinking water rat study (MacKenzie et 

al., 1958), the change in the RfD in 1998 

was due to the following factors: (a) A 

slight change in the no-observedadverse-effect level (NOAEL), (b) a 

modification to the original uncertainty 

factor, and (c) the addition of a 

modifying factor of three because of data 

on the potential for gastrointestinal 

effects in humans as a result of oral 

exposure. There is no current RfD for 

soluble trivalent chromium (soluble Cr 

III); the Cr III RfD of 1.5 mg/kg-day on 

IRIS (USEPA, 1998e) is for insoluble Cr 

III salts. 

In 2002 and as part of the first Six 

Year Review (67 FR 19030 (USEPA, 

2002c)), EPA noted that the National 

Toxicology Program (NTP) had agreed to 

study the chronic toxicity and 

carcinogenicity of oral exposure to Cr 

VI. The NTP study, conducted with 

sodium dichromate dehydrate (i.e., Cr 

VI) in rats and mice, is now available 

(NTP, 2008), as is a pre-peer review 

draft of a similar study with chromium 

picolinate (Cr III) (NTP, 2007). The Cr VI 

study found clear evidence of 

carcinogenic activity of sodium 

dichromate dihydrate in male and 

female F344 rats based on increased 

incidences of squamous cell neoplasms 

of the oral cavity, specifically the 

squamous epithelium that lines the oral 

mucosa and tongue (NTP, 2008). NTP 

also concluded that there was clear 

evidence of carcinogenic activity of 

sodium dichromate dihydrate in male 

and female B6C3F1 mice based on 

increased incidences of neoplasms in 

the small intestine (adenomas and/or 

carcinomas of the duodenum, jejunum, 

or ileum). The observed noncancer 

effects in the Cr VI study included 

histiocytic cellular infiltration in the 

liver, small intestine, and pancreatic 

and mesenteric lymph nodes of rats and 

mice, and diffuse epithelial hyperplasia 

in the small intestine of male and 

female mice. A peer-reviewed report for 

the study of chromium picolinate (Cr III) 

is not yet available. Zhang and Li (1987) 

evaluated the effects of human exposure 

to Cr VI in drinking water in Chinese 

villages. In a recent analysis of the 

human data originally reported in these 

Chinese villages, Sedman et al. (2006) 

further support a statistically significant 

increase in stomach cancer in the 

population exposed to Cr VI in their 

drinking water, thus suggesting a 

potential for carcinogenicity of Cr VI in 

drinking water. 

An assessment for chromium VI 

currently exists on IRIS but does not 

include an evaluation of carcinogenicity 

via oral ingestion. As a result, on 

December 21, 2007 (72 FR 72715 

(USEPA, 2007c)), the Agency nominated 

and included Cr VI on its 2008 IRIS 

agenda. The Agency is currently 

working with California EPA, New 

Jersey Department of Environmental 

Protection, and the Centers for Disease 

Control ATSDR (since they have 

recently developed draft assessments for 

chromium VI) and has posted a 

schedule for completion and the most 

up-to-date information on the status of 

the health effects assessment on the IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm). 

A review of analytical or treatment 

feasibility is not necessary for total 

chromium because changes to the 

MCLG are not warranted at this time 

and the current MCL is set at the MCLG. 

Since EPA did not identify a health or 

technology basis for revising the total 

chromium NPDWR, the Agency did not 

conduct a detailed occurrence and 

exposure analysis. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

total chromium is appropriate at this 

time. A reassessment of the health risks 

associated with chromium exposure is 

being initiated and the Agency does not 

believe it is appropriate to revise the 

NPDWR while that effort is in process. 

18. Cyanide 

a. Background. EPA published the 

current NPDWR for cyanide on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.2 mg/L. EPA based the MCLG 

on a reference dose of 0.02 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

During the first Six-Year Review cycle, 

EPA recommended a revision to the 

BATs for cyanide to clarify that 

‘‘chlorine’’ should be ‘‘alkaline chlorine’’ 

to avoid potential for the formation of 

harmful cyanogen chloride. EPA 

promulgated that revision in 69 FR 

38850, June 29, 2004 (USEPA, 2004b). 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

cyanide. The revised health effects 

assessment will consider relevant 

studies on the toxicity of cyanide, 

including its potential developmental 

and reproductive toxicity. The new 

health effects assessment was not 

completed by March 1, 2009, the review 

cutoff date for this notice (USEPA, 

2009b). The IRIS Substance Assessment 

Tracking System Web site (http:// 

cfpub.epa.gov/iristrac/index.cfm) has 

the most up-to-date information on the 

status of the health effects assessment.16

c. Review Result. Since the MCL for 

cyanide is set at its MCLG and a 

reassessment of the health risks 

resulting from exposure to cyanide is in 

progress, the Agency does not believe a 

revision to the NPDWR is appropriate at 

this time. 

19. 2,4-D (2,4-Dichlorophenoxyacetic 

acid) 

a. Background. EPA published the 

current NPDWR for 2,4-D on January 30, 

1991 (56 FR 3526 (USEPA, 1991c)). The 

NPDWR established an MCLG and an 

MCL of 0.07 mg/L. EPA based the MCLG 

on a reference dose of 0.01 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. In 2005, the 

Agency updated its health effects 

assessment of 2,4-D (USEPA, 2005c). 

The Agency identified a change in this 


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assessment that could lead to a change 

in the MCLG. This assessment 

considered relevant studies on the 

toxicity of 2,4-D including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.01 mg/kg-day to 0.005 mg/kg-day 

and concluded that 2,4-D is not 

classifiable as to its carcinogenicity 

(USEPA, 2005c). Based on the new 

Office of Pesticide Programs (OPP) 

assessment and RfD of 0.005 mg/kg-day, 

and assuming a 70-kg adult body weight 

and 2 liters water intake per day, the 

DWEL could be 0.2 mg/L. An RSC of 20 

percent results in a possible MCLG of 

0.04 mg/L (USEPA, 2009b). 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor for the 

possible MCLG decrease under 

consideration. EPA evaluated the results 

of the occurrence and exposure analyses 

for 2,4-D to determine whether a revised 

MCLG/MCL would be likely to result in 

a meaningful opportunity to improve 

the level of public health protection 

(USEPA, 2009f). Table VI–7 shows the 

results of the occurrence and exposure 

analysis for the current MCL and the 

possible MCLG set equal to 0.04 mg/L 

based on the new health effects 

information. The occurrence and 

exposure analysis shows that average 

concentrations do not exceed the 

current MCL for any system in the 

analysis. Note that these results are 

based on the subset of monitoring data 

provided in response to the Six-Year 

Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; SDWIS/FED indicates no 

MCL violations for 2,4-D between 1998 

and 2005 (USEPA, 2007g). The 

occurrence and exposure analysis shows 

that average concentrations do not 

exceed the possible MCLG based on 

new health effects information (0.04 mg/ 

L). 

TABLE VI–7—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING 2,4-D THRESHOLDS AND

CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or health-based threshold 

Systems with mean concentrations that are greater than the regulatory or health-based threshold 

(Percentages based on 33,187 systems with 2,4-D data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = 

MRL 1

Nondetect values = 1⁄2 MRL 2Nondetect values = 

0 3

MCL (0.07 mg/L) ............................. 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Possible MCLG (0.04 mg/L) ........... 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Corresponding population served (Percentages based on 187,451,200 people served by the systems with 

2,4–D data in the Six-Year Review ICR occurrence dataset) 

Regulatory or health-based 

threshold 

Nondetect values = 

MRL 1

Nondetect values = 1⁄2 MRL 2Nondetect values = 

0 3

MCL (0.07 mg/L) ............................. 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Possible MCLG (0.04 mg/L) ........... 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. 

2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

2,4-D, EPA does not believe a revision 

to the NPDWR for 2,4-D is appropriate 

at this time. In making this decision, the 

Agency considered whether any 

possible revision to the NPDWR for 2,4- 

D is likely to provide a meaningful 

opportunity for health risk reductions. 

Taking into consideration the low 

occurrence of this contaminant, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

20. Dalapon (2,2-Dichloropropionic 

Acid) 

a. Background. EPA published the 

current NPDWR for dalapon on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.2 mg/L. EPA based the MCLG 

on a reference dose of 0.03 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the toxicology of 

dalapon, including its potential 

developmental and reproductive 

toxicity. The literature search did not 

identify any studies that warrant a 

review of the RfD or the cancer 

classification (USEPA, 2009b). 

A review of analytical or treatment 

feasibility is not necessary for dalapon 

because changes to the MCLG are not 

warranted at this time and the current 

MCL is set at the MCLG. Since EPA did 

not identify a health or technology basis 

for revising the dalapon NPDWR, the 

Agency did not conduct a detailed 

occurrence and exposure analysis. 

c. Review Result. EPA’s review shows 

that there are no data supporting a 

change to the dalapon NPDWR. As a 

result, a revision to the NPDWR would 

not be appropriate at this time. 

21. Di(2-ethylhexyl)adipate (DEHA) 

a. Background. EPA published the 

current NPDWR for DEHA on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.4 mg/L. EPA based the MCLG 

on a reference dose of 0.6 mg/kg-day 

and a cancer classification of C, possible 

human carcinogen. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to DEHA. 

The revised health effects assessment 

will consider relevant studies on the 

toxicity of DEHA, including its potential 


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developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

c. Review Result. Since the MCL for 

DEHA is set at its MCLG and a 

reassessment of the health risks 

resulting from exposure to DEHA is in 

progress, the Agency does not believe a 

revision to the NPDWR is appropriate at 

this time. 

22. Di(2-ethylhexyl)phthalate (DEHP) 

a. Background. EPA published the 

current NPDWR for DEHP on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG of zero 

based on a cancer classification of B2, 

probable human carcinogen. The 

NPDWR also established an MCL of 

0.006 mg/L, based on analytical 

feasibility. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to DEHP. 

The revised health effects assessment 

will consider relevant studies on the 

toxicity of DEHP, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

Although a risk assessment is in 

process for DEHP, the existing MCLG is 

zero and the current MCL of 0.006 

mg/L is based on the PQL. Therefore, 

EPA reviewed whether there is potential 

to revise the PQL. EPA reviewed PE data 

from the first Six-Year Review cycle and 

then analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

DEHP are below 75 percent for several 

concentrations around the current PQL, 

including two studies with true values 

below the current PQL. More recent PT 

data from late 1999 through 2004, 

supplied by a PT provider, show 

passing rates below the 75 percent 

criterion for three studies, and all of the 

true concentrations in the PT data were 

higher than the current PQL. Given the 

passing rates around the current PQL, a 

lowering of the PQL for DEHP is not 

appropriate at this time (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of DEHP (Methods 525.2 and 506). 

While EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. The SixYear Review ICR dataset contains MRL 

values for 50,490 samples. Fewer than 

80 percent of these values are less than 

or equal the modal MRL: 22,980 (45 

percent) equal the modal MRL of 0.001 

mg/L and an additional 15,842 (31 

percent) are lower than 0.001 mg/L. 

Therefore, EPA did not set the EQL 

equal to the modal MRL (USEPA, 

2009e). The MDLs of approved methods 

are 0.0013 and 0.00225 mg/L. Applying 

a multiplier of 10 would give a possible 

PQL range from 0.013 to 0.0225 mg/L. 

The range is higher than the current 

PQL and, therefore, EPA did not 

estimate an EQL (USEPA, 2009e). Based 

on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is no potential to 

lower the PQL for DEHP. Since the MCL 

is constrained by the PQL, and the PQL 

is unchanged, EPA does not believe it is 

necessary to conduct an occurrence 

analysis at this time. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

DEHP is appropriate at this time 

because a reassessment of the health 

risks resulting from exposure to DEHP is 

in progress (USEPA, 2009b). 

Furthermore, a review of analytical 

feasibility did not identify a potential to 

revise the MCL, which is limited by 

feasibility. 

23. 1,2-Dibromo-3-chloropropane 

(DBCP) 

a. Background. EPA published the 

current NPDWR for DBCP on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG of 

zero based on a cancer classification of 

B2, probable human carcinogen. The 

NPDWR also established an MCL of 

0.0002 mg/L, based on analytical 

feasibility. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the carcinogenicity of 

DBCP as well as its potential 

developmental and reproductive 

toxicity. EPA has not identified any new 

information that indicates that it is 

appropriate to consider revisions to the 

cancer classification for DBCP at this 

time (USEPA, 2009b). Because the 

MCLG remains at zero, the Agency 

believes that a further review of the 

health effects of DBCP is not warranted 

at this time. 

The current MCL for DBCP is based 

on a PQL of 0.0002 mg/L. For the SixYear Review, the Agency considered 

whether changes in the analytical 

feasibility of DBCP might lead to a lower 

MCL. EPA reviewed PE data from the 

first Six-Year Review cycle and then 

analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

DBCP are above 85 percent, including 

one study with a true value below the 

current PQL. More recent PT data from 

late 1999 through 2004, supplied by a 

PT provider, also show greater than 75 

percent passing rates, including three 

with a true value below the current 

PQL. Because all of the laboratory 

passing rates from PE and PT studies, 

including four with true values slightly 

below the PQL, exceeded the 75 percent 

criterion typically used to derive a PQL, 

a lowering of the PQL for DBCP might 

be possible. These results, however, are 

insufficient to recalculate a revised PQL 

for DBCP because not enough data 

points are available below the current 

PQL to derive a value at the 75 percent 

passing rate (USEPA, 2009c). 

EPA examined two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of DBCP (Methods 504.1 and 551.1). 

While EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. However, 

there are substantial uncertainties in 

interpreting the MRLs (USEPA, 2009e). 

For example, some States have reported 

modal MRLs that are higher than the 

MCL. EPA therefore considered only 

MDL data to verify the potential to 

revise the PQL, and to establish a 

threshold for the occurrence and 

exposure analysis. The MDLs of 

approved methods are 0.000009 and 

0.00001 mg/L. Applying a multiplier of 

10 would give a possible PQLs of 

0.00009 and 0.0001 mg/L. EPA took the 

mean and rounded up to 0.0001 mg/L 

for the EQL (USEPA, 2009e). 

Based on the PT data and the MDLs 

for approved methods, EPA believes 

that there may be potential to lower the 

PQL for DBCP. To determine whether 

any MCL revision is likely to provide a 

meaningful opportunity to improve 


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public health protection, EPA evaluated 

the occurrence of DBCP at the EQL of 

0.0001 mg/L (USEPA, 2009f). Table VI– 

8 shows the results of the occurrence 

and exposure analysis for the current 

MCL and an EQL. The occurrence and 

exposure analysis shows that average 

concentrations exceed the current MCL 

for 42 of 37,618 systems (0.112 percent) 

serving 25,000 people (or 0.013 percent 

of 194 million people). Note that these 

results are based on the subset of 

monitoring data provided in response to 

the Six-Year Review ICR and do not 

necessarily reflect MCL violations, 

which are based on annual average 

concentrations at entry points; SDWIS/ 

FED indicates only nine MCL violations 

for DBCP between 1998 and 2005 

(USEPA, 2007g). Average concentrations 

at 92 to 97 of 37,618 systems (0.245 to 

0.258 percent), serving approximately 

1.2 to 1.4 million people (0.610 to 0.713 

percent of 194 million people), exceed 

the EQL of 0.0001 mg/L. 

TABLE VI–8—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING DBCP THRESHOLDS AND

CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold 

(Percentages based on 37,618 systems with DBCP data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = 

MRL 1

Nondetect values = 1⁄2 MRL 2Nondetect values = 

0 3

MCL (0.0002 mg/L) ......................... 42 (0.112%) .................................. 42 (0.112%) .................................. 42 (0.112%) 

EQL (0.0001 mg/L) ......................... 97 (0.258%) .................................. 93 (0.247%) .................................. 92 (0.245%) 

Corresponding population served (Percentages based on 193,749,000 people served by the systems with 

DBCP data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = 

MRL 1

Nondetect values = 1⁄2 MRL 2Nondetect values = 

0 3

MCL (0.0002 mg/L) ......................... 25,000 (0.013%) ........................... 25,000 (0.013%) ........................... 25,000 (0.013%) 

EQL (0.0001 mg/L) ......................... 1,382,000 (0.713%) ...................... 1,371,000 (0.707%) ...................... 1,181,000 (0.610%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. 

2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 

DBCP is appropriate at this time. The 

occurrence and exposure analysis based 

on possible changes in analytical 

feasibility indicates that any revision to 

the MCL is unlikely to provide a 

meaningful opportunity to improve 

public health protection. Taking into 

consideration the low occurrence of this 

contaminant, EPA has decided that any 

revision to the NPDWR would be a low 

priority activity for the Agency, and, 

thus, is not appropriate to revise at this 

time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

24. 1,2-Dichlorobenzene (oDichlorobenzene) 

a. Background. EPA published the 

current NPDWR for 1,2-dichlorobenzene 

on January 30, 1991 (56 FR 3526 

(USEPA, 1991c)). The NPDWR 

established an MCLG and an MCL of 0.6 

mg/L. EPA based the MCLG on a 

reference dose of 0.09 mg/kg-day and a 

cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 1,2- 

dichlorobenzene. The revised health 

effects assessment will consider relevant 

studies on the toxicity of 1,2- 

dichlorobenzene, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

c. Review Result. Since the MCL for 

1,2-dichlorobenzene is set at its MCLG 

and a reassessment of the health risks 

resulting from exposure to 1,2- 

dichlorobenzene is in progress, the 

Agency does not believe a revision to 

the NPDWR is appropriate at this time. 

25. 1,4-Dichlorobenzene (pDichlorobenzene) 

a. Background. EPA published the 

current NPDWR for 1,4-dichlorobenzene 

on July 8, 1987 (52 FR 25690 (USEPA, 

1987)). The NPDWR established an 

MCLG and an MCL of 0.075 mg/L. EPA 

based the MCLG on a reference dose of 

0.1 mg/kg-day and a cancer 

classification of C, possible human 

carcinogen. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 1,4- 

dichlorobenzene. The revised health 

effects assessment will consider relevant 

studies on the toxicity of 1,4- 

dichlorobenzene, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

c. Review Result. Since the MCL for 

1,4-dichlorobenzene is set at its MCLG 

and a reassessment of the health risks 

resulting from exposure to 1,4- 


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17Note that 1,2-dichloroethane is listed as 

ethylene dichloride in the IRIS tracking system. 

dichlorobenzene is in progress, the 

Agency does not believe a revision to 

the NPDWR is appropriate at this time. 

26. 1,2-Dichloroethane (Ethylene 

Dichloride) 

a. Background. EPA published the 

current NPDWR for 1,2-dichloroethane 

on July 8, 1987 (52 FR 25690 (USEPA, 

1987)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.005 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 1,2- 

dichloroethane. The revised health 

effects assessment will consider relevant 

studies on the toxicity of 1,2- 

dichloroethane, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment.17

Although a risk assessment is in 

process for 1,2-dichloroethane, the 

existing MCLG is zero and the current 

MCL of 0.005 mg/L is based on the PQL. 

Therefore, EPA reviewed whether there 

is potential to revise the PQL EPA 

reviewed PE data from the first Six-Year 

Review cycle and then analyzed more 

recent PT data to determine if the PQL 

can be revised (i.e., analytical 

feasibility). Passing rates for PE data 

available through late 1999 for 1,2- 

dichloroethane are above 95 percent 

around the current PQL of 0.005 mg/L, 

including one study with a true value 

below the current PQL. More recent PT 

data from late 1999 through 2004, 

supplied by a PT provider, also show 

greater than 90 percent passing rates for 

studies around the current PQL, 

including seven with true values below 

the current PQL. Because all of the 

laboratory passing rates from PE and PT 

studies—including several with true 

concentrations below the PQL— 

exceeded the 75 percent criterion 

typically used to derive a PQL, a 

lowering of the PQL for 1,2- 

dichloroethane might be possible. These 

results, however, are insufficient to 

recalculate a revised PQL for 1,2- 

dichloroethane because not enough data 

points are available below the current 

PQL to derive a value at the 75 percent 

passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the PQL could be estimated: 

laboratory MRLs in the Six-Year Review 

ICR dataset, and the MDLs for approved 

methods for the detection of 1,2- 

dichloroethane (Methods 502.2 and 

524.2). While EPA prefers to use 

laboratory performance data to calculate 

the PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 139,085 

samples. More than 80 percent of these 

values are less than or equal the modal 

MRL: 116,533 (84 percent) equal the 

modal MRL of 0.0005 mg/L and an 

additional 18,160 (13 percent) are lower 

than 0.0005 mg/L. Therefore, EPA 

selected the modal MRL as the EQL 

(USEPA, 2009e). The MDLs of approved 

methods range from 0.00003 to 0.00006 

mg/L. Applying a multiplier of 10 

would give a possible PQL range from 

0.0003 to 0.0006 mg/L, which contains 

the EQL (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is potential to lower 

the PQL for 1,2-dichloroethane. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of 1,2-dichloroethane at the 

EQL of 0.0005 mg/L and additional 

thresholds of 0.001 and 0.0025 mg/L 

(USEPA, 2009f). Table VI–9 shows the 

results of the occurrence and exposure 

analysis for the current MCL and these 

thresholds. The occurrence and 

exposure analysis shows that average 

concentrations exceed the current MCL 

for three of 50,442 systems (0.006 

percent) serving 150 people (or 0.00007 

percent of 227 million people). Note 

that these results are based on the subset 

of monitoring data provided in response 

to the Six-Year Review ICR and do not 

necessarily reflect MCL violations, 

which are based on annual average 

concentrations at entry points; SDWIS/ 

FED indicates 27 MCL violations for 1,2- 

dichloroethane between 1998 and 2005 

(USEPA, 2007g). Average concentrations 

at 63 to 82 of 50,442 systems (0.125 to 

0.163 percent), serving 210,000 to 

277,000 people (or 0.092 to 0.122 

percent of 227 million people), exceed 

the EQL of 0.0005 mg/L. 

TABLE VI–9—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING 1,2-DICHLOROETHANE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold 

(Percentages based on 50,442 systems with 1,2-dichloroethane data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.005 mg/L) ........................... 3 (0.006%) .................................... 3 (0.006%) .................................... 3 (0.006%) 1⁄2 MCL (0.0025 mg/L) .................... 9 (0.018%) .................................... 9 (0.018%) .................................... 8 (0.016%) 

2xEQL (0.001 mg/L) ........................ 46 (0.091%) .................................. 37 (0.073%) .................................. 30 (0.059%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 82 (0.163%) .................................. 63 (0.125%) 

Corresponding population served (percentages based on 226,934,000 people served by the systems with 

1,2-dichloroethane data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.005 mg/L) ........................... 150 (0.00007%) ............................ 150 (0.00007%) ............................ 150 (0.00007%) 1⁄2 MCL (0.0025 mg/L) .................... 870 (0.0004%) .............................. 870 (0.0004%) .............................. 830 (0.0004%) 

2xEQL (0.001 mg/L) ........................ 190,000 (0.084%) ......................... 145,200 (0.064%) ......................... 87,150 (0.038%) 


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TABLE VI–9—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING 1,2-DICHLOROETHANE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED—Continued 

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold 

(Percentages based on 50,442 systems with 1,2-dichloroethane data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

EQL (0.0005 mg/L) ......................... not applicable ............................... 277,000 (0.122%) ......................... 210,000 (0.092%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 1,2- 

dichloroethane is appropriate at this 

time because a reassessment of the 

health risks resulting from exposure to 

1,2-dichloroethane is in progress 

(USEPA, 2009b). Furthermore, the 

occurrence and exposure analysis based 

on possible changes in analytical 

feasibility indicates that any revision to 

the MCL is unlikely to provide a 

meaningful opportunity to improve 

public health protection. After 

consideration of the low occurrence of 

this contaminant, EPA has decided that 

any revision to the NPDWR would be a 

low priority activity for the Agency, 

and, thus, is not appropriate to revise at 

this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

27. 1,1-Dichloroethylene 

a. Background. EPA published the 

current NPDWR for 1,1- 

dichloroethylene on July 8, 1987 (52 FR 

25690 (USEPA, 1987)). The NPDWR 

established an MCLG and an MCL of 

0.007 mg/L. EPA based the MCLG on a 

reference dose of 0.01 mg/kg-day and a 

cancer classification of C, possible 

human carcinogen. 

b. Technical Reviews. In the first SixYear Review cycle, EPA evaluated new 

information from a health effects 

assessment completed in 2002 (USEPA, 

2002b). At that time, the Agency could 

not determine that a revision to the 

NPDWR would provide a meaningful 

opportunity for cost savings to public 

water systems or their customers, and 

decided that any revision would be a 

low priority activity for the Agency 

because of competing workload 

priorities, the administrative costs 

associated with rulemaking, and the 

burden on States and the regulated 

community to implement any regulatory 

change (68 FR 42908 (USEPA, 2003e)). 

The 2002 assessment considered 

relevant studies on the toxicity of 1,1- 

dichloroethylene including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.01 mg/kg-day to 0.05 mg/kg-day 

and concluded that there is inadequate 

information to assess carcinogenic 

potential via the oral route (USEPA, 

2002b). In the current review cycle, EPA 

conducted a literature search through 

June 2007 for relevant data on the 

toxicology of 1,1-dichloroethylene, 

including its potential developmental 

and reproductive toxicity. The literature 

search did not identify any additional 

new data that would affect the RfD or 

cancer classification (USEPA, 2009b). 

Based on the 2002 IRIS assessment and 

RfD of 0.05 mg/kg-day, and assuming a 

70-kg adult body weight and 2 liters 

water intake per day, the DWEL could 

be 1.75 mg/L. The 2002 cancer 

assessment indicates that the risk 

management factor of 10, applied to the 

current MCLG, may no longer be 

needed. An RSC of 20 percent results in 

a possible MCLG of 0.35 mg/L (USEPA, 

2009b). 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor if EPA 

were to raise the MCLG. EPA evaluated 

the results of the occurrence and 

exposure analyses for 1,1- 

dichloroethylene to determine whether 

a revised MCLG/MCL would be likely to 

result in a meaningful opportunity to 

achieve cost savings for PWSs and their 

customers while maintaining, or 

improving, the level of public health 

protection (USEPA, 2009f). Although 

the Agency obtained and evaluated the 

finished water occurrence data for 1,1- 

dichloroethylene, its usefulness is 

limited for potential cost savings to 

PWSs and their customers because the 

Agency does not know which systems 

are treating for this contaminant. As an 

alternative, the Agency evaluated 

available data on source water quality 

and conducted a qualitative assessment 

of treatment cost savings. 

Table VI–10 provides summary data 

for contaminant occurrence based on 

maximum sample values for the 

locations included in the STORET and 

NAWQA data. Although the degree to 

which these occurrence rates represent 

national drinking water source 

occurrence is uncertain, the information 

shows no to low occurrence at threshold 

levels of interest. This information 

indicates that any resulting NPDWR 

change would affect systems that rely on 

source water at less than 0.02 percent of 

the NAWQA locations. The STORET 

results are driven by the 157 sampling 

locations in Phoenix, Arizona, that have 

a maximum sample above the MCL of 

0.007 mg/L. Five of these locations also 

account for those having a maximum 

sample that exceeds 0.35 mg/L. 


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18Note that dichloromethane is listed as 

methylene chloride in the IRIS tracking system. 

TABLE VI–10—AMBIENT WATER QUALITY MONITORING OCCURRENCE SUMMARY FOR 1,1-DICHLOROETHYLENE

Maximum concentration 

Number of locations (% of locations) 

STORET 1 NAWQA2

Total ............................................................................................................................................................... 2,448 (100.0%) .. 5,788 (100.0%) 

Nondetect ....................................................................................................................................................... 1,498 (61.2%) ..... 5,636 (97.37%) 

Detected ......................................................................................................................................................... 950 (38.8%) ....... 152 (2.63%) 

Exceeds current MCLG of 0.007 mg/L .......................................................................................................... 165 (6.7%) .......... 1 (0.02%) 

Exceeds alternative value of 0.35 mg/L ........................................................................................................ 5 (0.2%) ............. 0 (0.0%) 

1STORET database 2002–2007. 

2 NAWQA database 1992–2008. 

Source: USEPA, 2009d. 

The BATs and small system 

compliance technologies for 1,1- 

dichloroethylene have other beneficial 

effects, e.g., reduction of other cooccurring contaminants, precursors for 

DBPs, or other common impurities. 

Therefore, if EPA were to consider a 

higher level, the Agency does not know 

how many PWSs that are currently 

treating to comply with the existing 

MCL of 0.007 mg/L would be likely to 

discontinue treatment that is already in 

place (USEPA, 2009d). Also, the Agency 

does not know to what extent affected 

systems might be able to reduce costs 

given that capital costs are not 

recoverable. However, the Agency 

recognizes that there may be 

opportunities to achieve operational 

cost savings if these systems are able to 

re-optimize current treatment. 

Given these considerations, the 

Agency believes that any resulting 

revision is not likely to provide a 

meaningful opportunity for cost savings. 

In view of this, any revision would be 

a low priority activity and not 

appropriate at this time. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

1,1-dichloroethylene, EPA does not 

believe a revision to the NPDWR for 1,1- 

dichloroethylene is appropriate at this 

time. In making this decision, the 

Agency considered whether any 

possible revision to the NPDWR for 1,1- 

dichloroethylene is likely to provide a 

meaningful opportunity for cost savings 

to public water systems and their 

customers. Taking into consideration 

the low occurrence of this contaminant 

in source waters, EPA has decided that 

any revision to the NPDWR would be a 

low priority activity for the Agency, 

and, thus, is not appropriate to revise at 

this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

28. cis-1,2-Dichloroethylene 

a. Background. EPA published the 

current NPDWR for cis-1,2- 

dichloroethylene on January 30, 1991 

(56 FR 3526 (USEPA, 1991c)). The 

NPDWR established an MCLG and an 

MCL of 0.07 mg/L. EPA based the MCLG 

on a reference dose of 0.01 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to cis-1,2- 

dichloroethylene. The revised health 

effects assessment will consider relevant 

studies on the toxicity of cis-1,2- 

dichloroethylene, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

c. Review Result. Since the MCL for 

cis-1,2-dichloroethylene is set at its 

MCLG and a reassessment of the health 

risks resulting from exposure to cis-1,2- 

dichloroethylene is in progress, the 

Agency does not believe a revision to 

the NPDWR is appropriate at this time. 

29. trans-1,2-Dichloroethylene 

a. Background. EPA published the 

current NPDWR for trans-1,2- 

dichloroethylene on January 30, 1991 

(56 FR 3526 (USEPA, 1991c)). The 

NPDWR established an MCLG and an 

MCL of 0.1 mg/L. EPA based the MCLG 

on a reference dose of 0.02 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to trans1,2-dichloroethylene. The revised health 

effects assessment will consider relevant 

studies on the toxicity of trans-1,2- 

dichloroethylene, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

c. Review Result. Since the MCL for 

trans-1,2-dichloroethylene is set at its 

MCLG and a reassessment of the health 

risks resulting from exposure to trans1,2-dichloroethylene is in progress, the 

Agency does not believe a revision to 

the NPDWR is appropriate at this time. 

30. Dichloromethane (Methylene 

Chloride) 

a. Background. EPA published the 

current NPDWR for dichloromethane on 

July 17, 1992 (57 FR 31776 (USEPA, 

1992)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.005 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

dichloromethane. The revised health 

effects assessment will consider relevant 

studies on the toxicity of 

dichloromethane, including its potential 

developmental and reproductive 

toxicity. The new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment.18

Although a risk assessment is in 

process for dichloromethane, the 

existing MCLG is zero and the current 

MCL of 0.005 mg/L is based on the PQL. 

Therefore, EPA reviewed whether there 

is potential to revise the PQL. EPA 

reviewed PE data from the first Six-Year 


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Review cycle and then analyzed more 

recent PT data to determine if the PQL 

can be revised (i.e., analytical 

feasibility). Passing rates for PE data 

available through late 1999 for 

dichloromethane are all above 90 

percent for studies near the PQL. More 

recent PT data from late 1999 through 

2004, supplied by a PT provider, also 

show greater than 85 percent passing 

rates for studies around the PQL, except 

for one study with a passing rate of 76 

percent. However, all of the true 

concentrations in the PE and PT data 

were higher than the current PQL of 

0.005 mg/L. Given the lack of PE and PT 

study results below the current PQL to 

derive a value at the 75 percent passing 

rate, PE and PT data are insufficient to 

support a PQL reduction (USEPA, 

2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: Laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of dichloromethane (Methods 502.2 and 

524.2). While EPA prefers to use 

laboratory performance data to calculate 

the PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 138,445 

samples. More than 80 percent of these 

values are less than or equal the modal 

MRL: 121,532 (88 percent) equal the 

modal MRL of 0.0005 mg/L and an 

additional 11,294 (8 percent) are lower 

than 0.0005 mg/L. Therefore, EPA 

selected the modal MRL as the EQL 

(USEPA, 2009e). The MDLs of approved 

methods range from 0.00002 to 0.00009 

mg/L. Applying a multiplier of 10 

would give a possible PQL range from 

0.0002 to 0.0009 mg/L, which includes 

the EQL (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, 

there is evidence of a potential to lower 

the PQL for dichloromethane even 

though the PE and PT data are 

insufficient to support a PQL reduction. 

To determine whether any MCL revision 

is likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of dichloromethane at the 

EQL of 0.0005 mg/L and additional 

thresholds of 0.001 and 0.0025 mg/L 

(USEPA, 2009f). Table VI–11 shows the 

results of the occurrence and exposure 

analysis for the current MCL and these 

thresholds. The occurrence and 

exposure analysis shows that average 

concentrations exceed the current MCL 

for 13 to 17 of 50,169 systems (0.026 to 

0.034 percent) serving 11,000 to 12,000 

people (or 0.005 percent of 227 million 

people). Note that these results are 

based on the subset of monitoring data 

provided in response to the Six-Year 

Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; SDWIS/FED indicates 67 

MCL violations for dichloromethane 

between 1998 and 2005 with annual 

violations ranging from 4 to 14 (USEPA, 

2007g). Average concentrations at 383 to 

579 of 50,169 systems (0.763 to 1.154 

percent), serving approximately 1.8 to 

3.5 million people (or 0.813 to 1.542 

percent of 227 million people), exceed 

the EQL of 0.0005 mg/L. 

TABLE VI–11—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING DICHLOROMETHANE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold (percentages based on 50,169 systems with dichloromethane data in the Six-Year Review ICR occurrence 

dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.005 mg/L) ........................... 17 (0.034%) .................................. 16 (0.032%) .................................. 13 (0.026%) 

EQL (0.0025 mg/L) ......................... 53 (0.106%) .................................. 51 (0.102%) .................................. 46 (0.092%) 

EQL (0.001 mg/L) ........................... 276 (0.550%) ................................ 208 (0.415%) ................................ 169 (0.337%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 579 (1.154%) ................................ 383 (0.763%) 

Corresponding population served (percentages based on 226,844,000 people served by the systems with 

dichloromethane data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = ≤0 3

MCL (0.005 mg/L) ........................... 12,000 (0.005%) ........................... 12,000 (0.005%) ........................... 11,000 (0.005%) 

EQL (0.0025 mg/L) ......................... 44,000 (0.019%) ........................... 40,000 (0.018%) ........................... 39,000 (0.017%) 

EQL (0.001 mg/L) ........................... 1,517,000 (0.669%) ...................... 1,386,000 (0.611%) ...................... 946,000 (0.417%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 3,497,000 (1.542%) ...................... 1,844,000 (0.813%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

During Six-Year Review 1, a 

stakeholder questioned the feasibility of 

lowering the PQL for dichloromethane 

below 0.001 mg/L because its use in 

EPA analytical methods makes it a 

common laboratory contaminant (68 FR 

42908 (USEPA, 2003e)). EPA responded 

that the high passing rates among PE 

studies at concentrations close to the 

current PQL of 0.005 mg/L would not be 

expected if this were the case and that 

EPA had no data to suggest that the 

occurrence estimates reflected 

monitoring sample contamination (68 

FR 42908 (USEPA, 2003e)). For Six-Year 

Review 2, EPA notes that it does not 

have PE or PT study results at either 

0.001 mg/L or 0.0005 mg/L and, 

therefore, cannot assess the potential for 

laboratory contamination of 

dichloromethane to affect passing rates 

at this level. A USGS study of volatile 

organic compound (VOC) occurrence 

(Moran, 2006) indicates this potential 

exists at low concentrations. The study 

presented dichloromethane laboratory 

reporting levels for newer low-level 


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analytical methods (i.e., defined as the 

level that limits the frequency of false 

positives and false negatives to 1 

percent of test results) that ranged from 

0.00006 mg/L to 0.00757 mg/L, with a 

median value of 0.00038 mg/L. The 

report noted that the laboratory 

reporting levels for dichloromethane 

tend to be higher than levels for other 

VOCs such as PCE (levels ranging from 

0.000027 mg/L to 0.0005 mg/L with a 

median of 0.0001 mg/L) and TCE 

(ranging from 0.000038 mg/L to 0.0005 

mg/L with a median of 0.000038 mg/L) 

because it was a frequent laboratory 

contaminant. 

A USGS study of ground water, 

source water, and drinking water quality 

indicated consistently lower 

dichloromethane (methylene chloride) 

occurrence frequencies compared to 

either PCE or TCE, which are among the 

most frequently occurring VOCs 

included in the study (Moran, 2006). 

Table VI–12 provides a summary of the 

occurrence results reported in the USGS 

study. This study also determined that 

population density was the strongest 

predictor of dichloromethane 

occurrence. 

TABLE VI–12—SUMMARY OF USGS VOC OCCURRENCE STUDY FINDINGS FOR DICHLOROMETHANE (METHYLENE

CHLORIDE) 

Ground water samples Source water samples Drinking water samples 

Number .......................................... 5,054 ............................................. 577 ................................................ 1,680 

Type ............................................... 3,877 NAWQA 1,177 Other 

sources.

Ground water sources for community water systems.Ground water community water 

systems. 

Location ......................................... National ......................................... National ......................................... New England and Mid-Atlantic 

States. 

Dichloromethane Results ............... • 3% exceed 0.00002 mg/L .........

• <1% exceed 0.0002 mg/L .........

• Ranked 30th of 55 VOCs based 

on median concentration 

(0.00005 mg/L).

• 0.2% (1 sample) exceed 0.0002 

mg/L.

• Ranked 8th of 52 VOCs based 

on median concentration 

(0.0017 mg/L—1 sample).

• 3% exceed 0.0002 mg/L. 

• Ranked 11th of 51 VOCs in detection frequency. 

• Ranked 31st of 55 solvents in 

median concentration (0.001 

mg/L). 

PCE ................................................ • 11% exceed 0.00002 mg/L .......

• 4% exceed 0.0002 mg/L ...........

• Ranked 12th of 55 VOCs based 

on median concentration 

(0.00007 mg/L).

• 4% exceed 0.0002 mg/L ...........

• Ranked 16th of 52 VOCs based 

on median concentration 

(0.0009 mg/L).

• 4% exceed 0.0002 mg/L. 

• Ranked 7th of 51 VOCs in detection frequency. 

• Ranked 11th of 55 solvents in 

median concentration (0.0014 

mg/L). 

TCE ................................................ • 5% exceed 0.00002 mg/L .........

• 2.5% exceed 0.0002 mg/L ........

• Ranked 20th of 55 VOCs based 

on median concentration 

(0.00012 mg/L).

• 3% exceed 0.0002 mg/L ...........

• Ranked 10th of 52 VOCs based 

on median concentration 

(0.0015 mg/L).

• 4% exceed 0.0002 mg/L. 

• Ranked 8th of 51 VOCs in detection frequency. 

• Ranked 8th of 55 solvents in 

median concentration (0.0015 

mg/L). 

Source: Moran, 2006. 

EPA compared Six-Year Review ICR 

occurrence patterns for 

dichloromethane with contaminant 

release information to determine if 

drinking water occurrence corresponds 

with potential contaminant sources 

reported in the Toxics Release Inventory 

(TRI) and found that the states with the 

majority of systems with mean 

concentrations that exceed 0.0005 mg/L 

did not tend to be the States with the 

highest dichloromethane releases 

(Moran, 2006). Table VI–13 provides 

summary information from that 

comparison. In particular, the numbers 

of system means exceeding 0.0005 mg/ 

L in Montana and Alaska seem 

inconsistent with TRI release 

information and the USGS study finding 

that population density is the strongest 

predictor of dichloromethane 

occurrence. 

Because of data gaps regarding the 

feasibility of PQL reduction and 

potential occurrence data accuracy at 

the lowest EQL, EPA concluded that 

revising the MCL may not constitute a 

meaningful opportunity to improve the 

level of public health protection. 

TABLE VI–13—STAGE 2 OCCURRENCE SUMMARY FOR DICHLOROMETHANE

State 

Systems with mean > 0.0005 

mg/L 

Nondetect = 1⁄2 MRL 

Total reported TRI on-site or off-site disposal or release of dichloromethane—all 

industries, 2006 1

Total reported TRI on-site or off-site 

disposal or release of 

dichloromethane—all industries, 2004 1

Number Percent of 579 

total systems Pounds 

Percent of 6.8 

Million Total 

Pounds 

Pounds 

Percent of 7.9 

Million Total 

Pounds 

MT ................................ 67 12 22,700 ......................... 0 30,600 ......................... 0 

TX ................................. 45 8 314,120 ....................... 5 410,103 ....................... 5 

FL ................................. 40 7 31,451 ......................... 0 246,775 ....................... 3 

AK ................................. 37 6 No data ....................... 0 No data ....................... 0 

IN .................................. 29 5 509,303 ....................... 7 699,783 ....................... 9 

WI ................................. 28 5 111,403 ....................... 2 98,113 ......................... 1 

MO ................................ 27 5 51,002 ......................... 1 32,860 ......................... 0 

CA ................................. 26 4 149,423 ....................... 2 86,554 ......................... 1 

OH ................................ 24 4 192,237 ....................... 3 203,269 ....................... 3 

NM ................................ 21 4 No data ....................... 0 No data ....................... 0 


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TABLE VI–13—STAGE 2 OCCURRENCE SUMMARY FOR DICHLOROMETHANE—Continued 

State 

Systems with mean > 0.0005 

mg/L 

Nondetect = 1⁄2 MRL 

Total reported TRI on-site or off-site disposal or release of dichloromethane—all 

industries, 2006 1

Total reported TRI on-site or off-site 

disposal or release of 

dichloromethane—all industries, 2004 1

Number Percent of 579 

total systems Pounds 

Percent of 6.8 

Million Total 

Pounds 

Pounds 

Percent of 7.9 

Million Total 

Pounds 

IL ................................... 19 3 279,024 ....................... 4 285,101 ....................... 4 

AL ................................. 18 3 319,529 ....................... 5 375,650 ....................... 5 

MN ................................ 17 3 39,851 ......................... 1 81,309 ......................... 1 

CO ................................ 15 3 18,475 ......................... 0 17,003 ......................... 0 

MI .................................. 13 2 75,141 ......................... 1 129,959 ....................... 2 

WY ................................ 13 2 No data ....................... 0 No data ....................... 0 

IA .................................. 12 2 2,348 ........................... 0 1,657 ........................... 0 

MD ................................ 12 2 36,990 ......................... 1 31,347 ......................... 0 

NC ................................ 12 2 49,800 ......................... 1 600,032 ....................... 8 

NY ................................. 11 2 322,382 ....................... 5 712,197 ....................... 9 

1Source: TRI Explorer Chemical Report Summary on-line state summaries for 2006 and 2004. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

dichloromethane is appropriate at this 

time because a reassessment of the 

health risks resulting from exposure to 

dichloromethane is in progress (USEPA, 

2009b). In view of the fact that 

dichloromethane is a common 

laboratory contaminant, there is 

uncertainty regarding the extent to 

which a PQL revision is feasible or 

whether the Six-Year Review ICR data 

are reliable at concentrations well below 

the current PQL. Furthermore, the 

occurrence and exposure analysis based 

on possible changes in analytical 

feasibility indicates that any revision to 

the MCL is unlikely to provide a 

meaningful opportunity to improve 

public health protection. After 

consideration of these factors, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

31. 1,2-Dichloropropane 

a. Background. EPA published the 

current NPDWR for 1,2-dichloropropane 

on January 30, 1991 (56 FR 3526 

(USEPA, 1991c)). The NPDWR 

established an MCLG of zero based on 

a cancer classification of B2, probable 

human carcinogen. The NPDWR also 

established an MCL of 0.005 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the carcinogenicity of 

1,2-dichloropropane as well as its 

potential developmental and 

reproductive toxicity. EPA has not 

identified any new information that 

indicates that it is appropriate to 

consider revisions to the cancer 

classification for 1,2-dichloropropane at 

this time (USEPA, 2009b). Because the 

MCLG remains at zero, the Agency 

believes that a further review of the 

health effects of 1,2-dichloropropane is 

not warranted at this time. 

The current MCL for 1,2- 

dichloropropane is based on a PQL of 

0.005 mg/L. For the Six-Year Review, 

the Agency considered whether changes 

in the analytical feasibility of 1,2- 

dichloropropane might lead to a lower 

MCL. EPA reviewed PE data from the 

first Six-Year Review cycle and then 

analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

1,2-dichloropropane are above 90 

percent near the current PQL of 0.005 

mg/L, but there were no results for PE 

studies with true values below the 

current PQL. More recent PT data from 

late 1999 through 2004, supplied by a 

PT provider, also show greater than 90 

percent passing rates around the PQL, 

including nine studies with true values 

below the current PQL. Because most of 

the laboratory passing rates from PE and 

PT studies—including several with true 

concentrations below the PQL— 

exceeded the 75 percent criterion 

typically used to derive a PQL, a 

lowering of the PQL for 1,2- 

dichloropropane might be possible. 

These results, however, are insufficient 

to recalculate a revised PQL for 1,2- 

dichloropropane because not enough 

data points are available below the 

current PQL to derive a value at the 75 

percent passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of 1,2-dichloropropane (Methods 502.2 

and 524.2). While EPA prefers to use 

laboratory performance data to calculate 

the PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 139,237 

samples. More than 80 percent of these 

values are less than or equal the modal 

MRL: 119,831 (86 percent) equal the 

modal MRL of 0.0005 mg/L and an 

additional 18,311 (13 percent) are lower 

than 0.0005 mg/L. Therefore, EPA 

selected the modal MRL as the EQL 

(USEPA, 2009e). The MDLs of approved 

methods range from 0.00003 to 0.00004 

mg/L. Applying a multiplier of 10 

would give a possible PQL range from 

0.0003 to 0.0004 mg/Lwhich supports 

the EQL (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is potential to lower 

the PQL for 1,2-dichloropropane. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of 1,2-dichloropropane at 

the EQL of 0.0005 mg/L and additional 

thresholds of 0.001 and 0.0025 mg/L 

(USEPA, 2009f). Table VI–14 shows the 

results of the occurrence and exposure 

analysis for the current MCL and these 

thresholds. The occurrence and 

exposure analysis shows that average 

concentrations do not exceed the 

current MCL for any system in the 

analysis. Note that these results are 

based on the subset of monitoring data 

provided in response to the Six-Year 


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Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; SDWIS/FED indicates 

three MCL violations for 1,2- 

dichloropropane between 1998 and 

2005 (USEPA, 2007g). Average 

concentrations at 47 to 61 of 50,437 

systems (0.093 to 0.121 percent), serving 

296,000 to 494,000 people (0.130 to 

0.218 percent of 227 million people), 

exceed the EQL of 0.0005 mg/L. 

TABLE VI–14—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING 1,2-DICHLOROPROPANE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold (percentages based on 50,437 systems with 1,2-dichloropropane data in the Six-Year Review ICR occurrence 

dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.005 mg/L) ........................... 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 1⁄2 MCL (0.0025 mg/L) .................... 2 (0.004%) .................................... 2 (0.004%) .................................... 2 (0.004%) 

2xEQL (0.001 mg/L) ........................ 27 (0.054%) .................................. 24 (0.048%) .................................. 21 (0.042%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 61 (0.121%) .................................. 47 (0.093%) 

Corresponding Population Served (percentages based on 226,912,000 people served by the systems with 

1,2-dichloropropane data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.005 mg/L) ........................... 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 1⁄2 MCL (0.0025 mg/L) .................... 120 (0.00005%) ............................ 120 (0.00005%) ............................ 120 (0.00005%) 

2xEQL (0.001 mg/L) ........................ 286,000 (0.126%) ......................... 286,000 (0.126%) ......................... 284,000 (0.125%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 494,000 (0.218%) ......................... 296,000 (0.130%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 1,2- 

dichloropropane is appropriate at this 

time. The occurrence and exposure 

analysis based on possible changes in 

analytical feasibility indicates that any 

revision to the MCL is unlikely to 

provide a meaningful opportunity to 

improve public health protection. 

Taking into consideration the low 

occurrence of this contaminant, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

32. Dinoseb 

a. Background. EPA published the 

current NPDWR for dinoseb on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.007 mg/L. EPA based the 

MCLG on a reference dose of 0.001 mg/ 

kg-day and a cancer classification of D, 

not classifiable as to human 

carcinogenicity. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the toxicology of 

dinoseb, including its potential 

developmental and reproductive 

toxicity. The literature search did not 

identify any studies that warrant a 

review of the RfD or the cancer 

classification (USEPA, 2009b). 

A review of analytical or treatment 

feasibility is not necessary for dinoseb 

because changes to the MCLG are not 

warranted at this time and the current 

MCL is set at the MCLG. Since EPA did 

not identify a health or technology basis 

for revising the dinoseb NPDWR, the 

Agency did not conduct a detailed 

occurrence and exposure analysis. 

c. Review Result. EPA’s review shows 

that there are no data supporting a 

change to the dinoseb NPDWR. As a 

result, a revision to the NPDWR would 

not be appropriate at this time. 

33. Diquat 

a. Background. EPA published the 

current NPDWR for diquat on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.02 mg/L. EPA based the MCLG 

on a reference dose of 0.0022 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. In 2001, the 

Agency updated its health effects 

assessment of diquat (USEPA, 2001a). A 

subsequent reassessment of tolerances 

for residues in or on raw agricultural 

products (USEPA, 2002d) did not 

identify any new health effects 

information and based the updated 

tolerances on health effects information 

in the 2001 assessment (USEPA, 2001a). 

The Agency identified a change in this 

assessment that could lead to a change 

in the MCLG. This assessment 

considered relevant studies on the 

toxicity of diquat including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.002 mg/kg-day to 0.005 mg/kgday and developed a cancer 

classification of E, evidence of 

noncarcinogenicity (USEPA, 2001a). 

Based on the new OPP assessment and 

RfD of 0.005 mg/kg-day, and assuming 

a 70-kg adult body weight and 2 liters 

water intake per day, the DWEL could 


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be 0.175 mg/L. An RSC of 20 percent 

results in a possible MCLG of 0.035 mg/ 

L, rounded to 0.04 mg/L. 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor if EPA 

were to raise the MCLG. EPA evaluated 

the available occurrence and exposure 

information for diquat to determine 

whether a revised MCLG/MCL would be 

likely to result in a meaningful 

opportunity to achieve cost savings for 

PWSs and their customers while 

maintaining, or improving, the level of 

public health protection (USEPA, 

2009f). Although the Agency obtained 

and evaluated the finished water 

occurrence data for diquat, its 

usefulness is limited for determining 

potential cost savings to PWS and their 

customers because the Agency does not 

know which systems are treating for this 

contaminant. As an alternative, the 

Agency evaluated available data on 

source water quality and conducted a 

qualitative assessment of treatment cost 

savings. Because the primary 

informations sources used to evaluate 

potential source water occurrence— 

STORET and NAWQA—do not report 

monitoring results for diquat, the 

Agency obtained available information 

on diquat use and fate and transport. 

Diquat’s primary uses are as an 

algaecide, defoliant, desiccant, and 

herbicide (USEPA, 1995a). The most 

recent pesticide application estimates in 

the Pesticide Use Database developed by 

the National Center for Food and 

Agricultural Policy (NCFAP) indicate 

overall cropland application of almost 

270,000 pounds in 1997, primarily on 

potato and alfalfa crops (NCFAP, 2000). 

The NCFAP based these estimates on 

State-level pesticide usage patterns for 

the period 1994–1998 and State-level 

crop acreage for 1997. These estimates 

reflect several limitations: they do not 

include noncropland applications, the 

data sources vary in quality, and Statelevel pesticide use data gaps are filled 

using data for nearby states. The USGS 

estimated county-level pesticide usage 

for 2002 based on crop acreage estimates 

in the 2002 Census of Agriculture and 

State-level application rates for the 

period 1999–2004 developed by the 

CropLife Foundation (USGS, no date), 

which implemented the NCFAP method 

for estimating pesticide usage (Gianessi 

and Regner, 2006) and, therefore, has 

similar limitations. The USGS estimates 

total diquat application to crops of 

approximately 200,000 pounds per year, 

with potatoes accounting for almost 90 

percent of these applications (USGS, no 

date). Diquat use on crops occurred 

primarily in regions of New England, 

the Great Lakes, North Dakota, the 

Pacific Northwest, California, and 

Florida. In comparison to other 

commonly used pesticides, diquat has 

the lowest national estimate for use on 

crops (Gianessi and Regner, 2006). 

The Reregistration Eligibility Decision 

(RED) for Diquat Dibromide (USEPA, 

1995a) notes that although diquat is 

persistent (i.e., it does not hydrolyze 

and is resistant to degradation), it 

becomes immobile when it adsorbs to 

soil particles and, therefore, is not 

expected to contaminate ground water. 

Furthermore, diquat dissipates quickly 

from surface water because it adsorbs to 

soil sediments, vegetation, and organic 

matter; the estimated half-life is 1 to 2 

days for diquat in surface water based 

on a study of two ponds in Florida 

(USEPA, 1995a). These factors indicate 

the possibility of low occurrence in 

drinking water sources. 

The BAT and small system 

compliance technologies for diquat have 

other beneficial effects, e.g., removing 

other co-occurring contaminants. 

Therefore, if EPA were to consider a 

higher level, the Agency does not know 

how many PWSs that are currently 

treating to comply with the existing 

MCL of 0.02 mg/L would be likely to 

discontinue treatment that is already in 

place (USEPA, 2009d). Also, the Agency 

does not know to what extent affected 

systems might be able to reduce costs 

given that capital costs are not 

recoverable. However, the Agency 

recognizes that there may be 

opportunities to achieve operational 

cost savings if these systems are able to 

re-optimize current treatment. 

Given these considerations, the 

Agency believes that any resulting 

revision is not likely to provide a 

meaningful opportunity for cost savings. 

In view of this, any revision would be 

a low priority activity and not 

appropriate at this time. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

diquat, EPA does not believe a revision 

to the NPDWR for diquat is appropriate 

at this time. In making this decision, the 

Agency considered whether any 

possible revision to the NPDWR for 

diquat is likely to provide a meaningful 

opportunity for cost savings to public 

water systems and their customers. 

After consideration of this factor, EPA 

has decided that any revision to the 

NPDWR would be a low priority activity 

for the Agency, and, thus, is not 

appropriate to revise at this time 

because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

34. Endothall 

a. Background. EPA published the 

current NPDWR for endothall on July 

17, 1992 (57 FR 31776 (USEPA, 1992)). 

The NPDWR established an MCLG and 

an MCL of 0.1 mg/L. EPA based the 

MCLG on a reference dose of 0.02 mg/ 

kg-day and a cancer classification of D, 

not classifiable as to human 

carcinogenicity. 

b. Technical Reviews. In 2005, the 

Agency updated its health effects 

assessment of endothall (USEPA, 

2005d). The Agency identified a change 

in this assessment that could lead to a 

change in the MCLG. This assessment 

considered relevant studies on the 

toxicity of endothall including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.02 mg/kg-day to 0.007 mg/kg-day 

and concluded that endothall is 

unlikely to be carcinogenic to humans 

(USEPA, 2005d). Based on the new OPP 

assessment and RfD of 0.007 mg/kg-day, 

and assuming a 70-kg adult body weight 

and 2 liters water intake per day, the 

DWEL could be 0.245 mg/L. An RSC of 

20 percent results in a possible MCLG 

of 0.05 mg/L. 

Because of a possible change in the 

MCLG for endothall, EPA considered 

whether analytical feasibility is likely to 

be a limitation if the Agency were to 

consider lowering the MCL to 0.05 mg/ 

L (the possible MCLG). EPA reviewed 

PE data from the first Six-Year Review 

cycle and then analyzed more recent PT 

data to determine if the PQL can be 

revised (i.e., analytical feasibility). 

Passing rates for PE data available 

through late 1999 for endothall are 

generally above 80 percent, but there 

were no results for PE studies with true 

values below the current PQL of 0.09 

mg/L. More recent PT data from late 

1999 through 2004, supplied by a PT 

provider, show passing rates above 75 

percent for most studies, but there are 

four studies with passing rates equal to 

or less than the 75 percent criterion, 

including two close to the current PQL. 

No PT studies had true values below the 

current PQL. Given the variable results 

from the PT studies and the lack of PE 

and PT study results below the current 

PQL, PE and PT data are insufficient to 

support a PQL reduction (USEPA, 

2009c). 

While the PT data are not sufficient to 

support a lowering of the PQL for 

endothall at this time, the current PQL 

of 0.09 mg/L is greater than the possible 

MCLG. It would therefore limit a 

possible revision to the MCL. EPA 


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evaluated two alternative sources of 

information to determine whether they 

indicate any potential to revise the PQL: 

laboratory minimum reporting levels in 

the Six-Year Review ICR dataset, and 

the MDLs for the approved method for 

the detection of endothall (Method 

548.1). While EPA prefers to use 

laboratory performance data to calculate 

the PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 21,792 

samples. Of these, 21,445 (98 percent) 

have an MRL value of 0.05 mg/L or 

lower. Because more than 80 percent of 

the MRL values are at or below the 

possible MCLG of 0.05 mg/L, EPA 

selected that value as the minimum 

threshold for the occurrence and 

exposure analysis (USEPA, 2009e). The 

MDL of the approved method is 0.00179 

mg/L. Applying a multiplier of 10 

would give a possible PQL of 0.0179 

mg/L,which is below the possible MCLG 

(USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, 

there is evidence of a potential to lower 

the PQL for endothall even though the 

PE and PT data are insufficient to 

support a PQL reduction. To determine 

whether any MCL revision is likely to 

provide a meaningful opportunity to 

improve public health protection, EPA 

evaluated the occurrence of endothall at 

the possible MCLG of 0.05 mg/L 

(USEPA, 2009f). Table VI–15 shows the 

results of the occurrence and exposure 

analysis for the current MCL and the 

possible MCLG set equal to 0.05 mg/L 

based on the new health effects 

information and the laboratory 

minimum reporting levels in the SixYear Review ICR dataset. The 

occurrence and exposure analysis shows 

that average concentrations do not 

exceed the current MCL for any system 

in the analysis. Note that these results 

are based on the subset of monitoring 

data provided in response to the SixYear Review ICR and do not necessarily 

reflect MCL violations, which are based 

on running annual average 

concentrations at entry points; 

nevertheless, SDWIS/FED indicates no 

MCL violations for endothall between 

1998 and 2005 (USEPA, 2007g). The 

average concentration at one of the 

14,156 systems (0.007 percent), serving 

10,000 people (or 0.008 percent of 119 

million people), exceeds the possible 

MCLG based on new health effects 

information (0.05 mg/L). 

TABLE VI–15—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING ENDOTHALL THRESHOLDS

AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or health-based threshold 

Systems with mean concentrations that are greater than the regulatory or health-based threshold (percentages based on 14,156 systems with endothall data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.1 mg/L) ............................... 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Possible MCLG (0.05 mg/L) ........... 1 (0.007%) .................................... 1 (0.007%) .................................... 1 (0.007%) 

Corresponding population served (percentages based on 118,536,800 people served by the systems with 

endothall data in the Six-Year Review ICR occurrence dataset) 

Regulatory or health-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.1 mg/L) ............................... 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Possible MCLG (0.05 mg/L) ........... 10,000 (0.008%) ........................... 10,000 (0.008%) ........................... 10,000 (0.008%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset 

2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

endothall, EPA does not believe a 

revision to the NPDWR for endothall is 

appropriate at this time. In making this 

decision, the Agency considered 

whether any possible revision to the 

NPDWR for endothall is likely to 

provide a meaningful opportunity for 

health risk reductions. Taking into 

consideration the low occurrence of this 

contaminant, EPA has decided that any 

revision to the NPDWR would be a low 

priority activity for the Agency, and, 

thus, is not appropriate to revise at this 

time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

35. Endrin 

a. Background. EPA published the 

current NPDWR for endrin on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.002 mg/L. EPA based the 

MCLG on a reference dose of 0.0003 mg/ 

kg-day and a cancer classification of D, 

not classifiable as to human 

carcinogenicity. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the toxicology of 

endrin, including its potential 

developmental and reproductive 

toxicity. The literature search did not 

identify any studies that warrant a 

review of the RfD or the cancer 

classification (USEPA, 2009b). 

A review of analytical or treatment 

feasibility is not necessary for endrin 

because changes to the MCLG are not 

warranted at this time and the current 

MCL is set at the MCLG. Since EPA did 

not identify a health or technology basis 

for revising the endrin NPDWR, the 

Agency did not conduct a detailed 

occurrence and exposure analysis. 

c. Review Result. EPA’s review shows 

that there are no data supporting a 

change to the endrin NPDWR. As a 

result, a revision to the NPDWR would 

not be appropriate at this time. 


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36. Epichlorohydrin 

a. Background. EPA published the 

current NPDWR for epichlorohydrin on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR imposes a TT 

requirement that limits the allowable 

level of epichlorohydrin monomer in 

the polymer that is added to water as a 

flocculent to remove particulates. Each 

water system is required to certify, in 

writing, to the State (using third-party or 

manufacturer’s certification) that the 

combination (or product) of dose and 

monomer level does not exceed the 

following level: 0.01 percent residual 

epichlorohydrin monomer in polymer 

products used during water treatment 

and dosed at 20 mg/L (ppm). 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the carcinogenicity of 

epichlorohydrin as well as its potential 

developmental and reproductive 

toxicity. EPA has not identified any new 

information that indicates that it is 

appropriate to consider revisions to the 

cancer classification for epichlorohydrin 

at this time (USEPA, 2009b). Because 

the MCLG remains at zero, the Agency 

believes that a further review of the 

health effects of epichlorohydrin is not 

warranted at this time. 

EPA has identified information that 

suggests that the residual 

epichlorohydrin content in water 

treatment polymers has decreased 

significantly, likely due to 

improvements in manufacturing 

processes and technologies (USEPA, 

2009g). NSF International analyses 

conducted between January 2005 and 

June 2007 found that, in 84 

epichlorohydrin-based polymers/copolymers submitted for certification 

under NSF Standard 60, the residual 

epichlorohydrin content was always 

below the detection limit of 0.002 

percent. 

Epichlorohydrin standards in Europe 

and Australia are also stricter than the 

NPDWR. Based on the concentration of 

dose and monomer level in the NPDWR, 

finished water could contain up to 2 μg/ 

L (ppb) of epichlorohydrin. By contrast, 

the European Union requires that 

finished water contain less than 0.1 μg/ 

L (ppb) epichlorohydrin, and Australia 

requires that the concentration in 

finished water be less than 0.5 μg/L 

(ppb). The United Kingdom requires 

that polymers used in drinking water 

contain less than 0.002 percent residual 

epichlorohydrin, and the dose of these 

polymers be less than 5 mg/L (ppm) at 

all times, for a maximum finished water 

concentration of 0.1 μg/L (ppb). 

To assess the occurrence of 

epichlorohydrin in drinking water, EPA 

sought data on current usage practices 

for polymers containing it. The Agency 

is not presently aware of any recent, 

large-scale studies of polymer usage in 

drinking water facilities, and therefore 

cannot fully characterize the occurrence 

of epichlorohydrin in drinking water. 

However, cationic polymers used in 

water treatment often contain 

epichlorohydrin. The 1996 

WATER:\STATS database (described in 

Levine et al., 2004), based on an AWWA 

survey, indicates that 13 percent of 

ground water systems and 66 percent of 

surface water systems surveyed use a 

polymer for water treatment. Many of 

these are cationic polymers, particularly 

for surface water systems; cationic 

polymers used to treat drinking water 

often use epichlorohydrin monomer. 

Additional information on the extent 

of use of epichlorohydrin based 

polymers/co-polymers in drinking water 

would further assist the Agency in 

evaluating the potential public health 

benefits associated with a revision to the 

treatment technique for 

epichlorohydrin. Because most 

epichlorohydrin-based polymers 

available today have a significantly 

lower residual monomer content than 

that specified in the treatment technique 

(2009g), EPA believes that the costs of 

a revision would be minimal and 

recognizes that benefits may also be 

small. 

c. Review Result. The Agency believes 

it is appropriate to revise the NPDWR 

for epichlorohydrin. The existing MCLG 

is zero (based on the current B2 cancer 

classification) and NSF International 

data indicate that epichlorohydrin based 

polymers/copolymers are widely 

available with lower monomer levels 

than required by the existing NPDWR. 

Hence, revisions to the epichlorohydrin 

NPDWR will provide a meaningful 

opportunity to maintain the health risk 

reductions achieved by technological 

advances in manufacturing. As 

discussed in Section VII, the Agency 

solicits public comment on the use of 

epichlorohydrin-based polymers/copolymers in drinking water facilities 

(since this may provide additional 

information on the occurrence of 

epichlorohydrin in drinking water) to 

help inform the regulatory revisions. 

EPA notes that any changes to the 

NPDWR for epichlorohydin may also 

include revisions to the closely related 

NPDWR for acrylamide. 

37. Ethylbenzene 

a. Background. EPA published the 

current NPDWR for ethylbenzene on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG and an MCL of 0.7 mg/L. EPA 

based the MCLG on a reference dose of 

0.1 mg/kg-day and a cancer 

classification of D, not classifiable as to 

human carcinogenicity. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

ethylbenzene. The revised health effects 

assessment will consider relevant 

studies on the toxicity of ethylbenzene, 

including its potential developmental 

and reproductive toxicity. The new 

health effects assessment was not 

completed by March 1, 2009, the review 

cutoff date for this notice (USEPA, 

2009b). The IRIS Substance Assessment 

Tracking System Web site (http:// 

cfpub.epa.gov/iristrac/index.cfm) has 

the most up-to-date information on the 

status of the health effects assessment. 

c. Review Result. Since the MCL for 

ethylbenzene is set at its MCLG and a 

reassessment of the health risks 

resulting from exposure to ethylbenzene 

is in progress, the Agency does not 

believe a revision to the NPDWR is 

appropriate at this time. 

38. Ethylene Dibromide (EDB; 1,2- 

Dibromoethane) 

a. Background. EPA published the 

current NPDWR for EDB on January 30, 

1991 (56 FR 3526 (USEPA, 1991c)). The 

NPDWR established an MCLG of zero 

based on a cancer classification of B2, 

probable human carcinogen. The 

NPDWR also established an MCL of 

0.00005 mg/L, based on analytical 

feasibility. 

b. Technical Reviews. The Agency 

updated the health effects assessment 

for EDB in 2004 and retained the cancer 

classification on which the 1991 MCLG 

is based (USEPA, 2004a). As a part of 

the 2004 assessment, EPA considered 

relevant studies on the toxicity of EDB, 

including its potential developmental 

and reproductive toxicity. 

The current MCL for EDB is based on 

a PQL of 0.00005 mg/L. For the Six-Year 

Review, the Agency considered whether 

changes in the analytical feasibility of 

EDB might lead to a lower MCL. EPA 

reviewed PE data from the first Six-Year 

Review cycle and then analyzed more 

recent PT data to determine if the PQL 

can be revised (i.e., analytical 

feasibility). Passing rates for PE data 

available through late 1999 for EDB are 

all 75 percent or higher. However, the 

true concentrations were all higher than 

the current PQL of 0.00005 mg/L. More 


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19At this time, the results of the osteosarcoma 

cancer study recommended by NAS have not been 

published. 

recent PT data from late 1999 through 

2004, supplied by a PT provider, 

likewise show passing rates of 75 

percent or higher, but again, all of the 

true concentrations in the PT data were 

higher than the current PQL. Because of 

the lack of data below the PQL, a 

lowering of the PQL for EDB is not 

appropriate at this time (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of EDB (Methods 504.1 and 551.1). 

While EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. The SixYear Review ICR dataset contains MRL 

values for 83,063 samples. Fewer than 

80 percent of these values are less than 

or equal the modal MRL: 26,926 (32 

percent) equal the modal MRL of 

0.00001 mg/L and an additional 454 (0.5 

percent) are lower than 0.00001 mg/L. 

Therefore, EPA did not set the EQL 

equal to the modal MRL (USEPA, 

2009e). The MDLs of approved methods 

are 0.00001 and 0.000032 mg/L. 

Applying a multiplier of 5, which was 

used to establish the PQL, would give a 

possible PQL range from 0.00005 to 

0.00016 mg/L. The result is higher than 

or equal to the current PQL and, 

therefore, EPA did not estimate an EQL 

(USEPA, 2009e). Based on these varied 

and unrelated approaches/sources of 

information, EPA believes that there is 

no potential to lower the PQL. Since the 

MCL is constrained by the PQL, and the 

PQL is unchanged, EPA does not believe 

it is necessary to conduct an occurrence 

analysis at this time. 

c. Review Result. EPA did not identify 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL). Therefore, EPA 

does not believe a revision to the 

NPDWR for EDB is appropriate at this 

time. 

39. Fluoride 

a. Background. EPA published the 

current NPDWR for fluoride on April 2, 

1986 (51 FR 11396 (USEPA, 1986c)). 

The NPDWR established an MCLG and 

an MCL of 4.0 mg/L. The MCLG was 

developed from a lowest effect level for 

crippling skeletal fluorosis of 20 mg/day 

with continuous exposures over a 20- 

year or longer period. The lowestobserved-adverse-effect level (LOAEL) 

was divided by an uncertainty factor of 

2.5 and a drinking water intake of 2 

liters/day (L/day) to obtain the MCLG. 

Drinking water was considered to be the 

only source of exposure for the 

calculation. At the same time, EPA 

published a secondary maximum 

contaminant level (SMCL) for fluoride 

of 2.0 mg/L to protect against dental 

fluorosis, which was considered to be 

an adverse cosmetic effect. PWSs 

exceeding the fluoride SMCL must 

provide public notification to their 

customers. 

Fluoride is unique because of its 

beneficial effects at low level exposures, 

and because it is voluntarily added to 

some drinking water systems as a public 

health measure for reducing the 

incidence of cavities among the treated 

population. The amount of fluoride 

added to drinking water for fluoridation 

ranges from 0.7 to 1.2 mg/L, depending 

on ambient air temperatures. The 

decision to fluoridate a water supply is 

made by the State or local municipality, 

and is not mandated by EPA or any 

other Federal entity. 

b. Technical Reviews. As a result of 

the first Six-Year Review of the fluoride 

NPDWR (67 FR 19030 (USEPA, 2002c) 

(preliminary); 68 FR 42908 (USEPA, 

2003e) (final)), EPA requested that the 

National Research Council (NRC) of the 

National Academies of Science (NAS) 

conduct a review of the recent health 

and exposure data on orally ingested 

fluoride. In 2006, the NRC published the 

results of their evaluation in a report 

entitled, Fluoride in Drinking Water: A 

Scientific Review of EPA’s Standards. 

Based on its review, NRC concluded 

that severe dental fluorosis is an adverse 

health effect when it causes confluent 

thinning and pitting of the enamel, a 

situation that compromises the function 

of the enamel in protecting the dentin 

and eventually the pulp from decay and 

infection. There was consensus among 

the committee that severe dental 

fluorosis is an effect that should be 

avoided and that ‘‘exposure at the MCLG 

clearly puts children at risk of 

developing severe enamel fluorosis.’’ In 

addition, the committee examined the 

scientific data on the impact of fluoride 

on the strength and structure of bone 

and the majority concluded that the 

MCLG ‘‘is not likely to be protective 

against bone fractures.’’ NRC 

recommended that EPA use the 

available dose-response data for the 

effects of fluoride on severe dental 

fluorosis and skeletal fractures in 

combination with data on the relative 

contribution of drinking water to total 

fluoride exposure to identify an MCLG 

that would be protective against these 

effects. 

The NRC also evaluated the impact of 

fluoride on reproduction and 

development, neurotoxicity and 

behavior, the endocrine system, 

genotoxicity, cancer and other effects. 

They concluded that the available data 

were inadequate to determine if a risk 

for effects on these endpoints exists at 

an MCLG of 4 mg/L and made 

recommendations for additional 

research. After considering the 

genotoxicity data, cancer studies in 

humans and animals, and studies of 

mode of action in cell systems, NRC 

determined that the evidence on the 

potential of fluoride to initiate or 

promote cancers, particularly of the 

bone, is tentative and mixed. They 

recommended that EPA await the 

results and publication of an in-process 

hospital-based, case-control study of 

osteosarcoma and fluoride exposure 

from the Harvard School of Dental 

Medicine before determining if an 

Agency update of the cancer risk 

assessment for fluoride is necessary.19

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

fluoride is appropriate at this time 

because the Agency’s Office of Water 

(OW) is in the process of developing its 

dose-response assessment of the 

noncancer impacts of fluoride on severe 

dental fluorosis and the skeletal system. 

In addition, the OW is updating its 

evaluation of the relative contribution of 

drinking water to total fluoride exposure 

considering the contributions from 

dental products, foods, pesticide 

residues, and other sources such as 

ambient air and medications. Once the 

Agency completes and publishes peer 

reviewed versions of these in-process 

assessments, it will be able to determine 

the potential impacts on the MCLG, 

MCL, and/or the SMCL and whether any 

revisions to these would be appropriate. 

40. Glyphosate 

a. Background. EPA published the 

current NPDWR for glyphosate on July 

17, 1992 (57 FR 31776 (USEPA, 1992)). 

The NPDWR established an MCLG and 

an MCL of 0.7 mg/L. EPA based the 

MCLG on a reference dose of 0.1 mg/kgday and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. In 2002, the 

Agency updated its health effects 

assessment of glyphosate (USEPA, 

2002a). The Agency identified a change 

in this assessment that could lead to a 

change in the MCLG. This assessment 

considered relevant studies on the 

toxicity of glyphosate including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.1 mg/kg-day to 2 mg/kg-day and 


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concluded that glyphosate has evidence 

of non-carcinogenicity in humans 

(USEPA, 2002a). Based on the new OPP 

assessment and RfD of 2 mg/kg-day, and 

assuming a 70-kg adult body weight and 

2 liters water intake per day, the DWEL 

could be 70 mg/L. An RSC of 20 percent 

results in a possible MCLG of 14 mg/L, 

(USEPA, 2009b). 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor if EPA 

were to raise the MCLG. EPA evaluated 

the results of the occurrence and 

exposure analyses for glyphosate to 

determine whether a revised MCLG/ 

MCL would be likely to result in a 

meaningful opportunity to achieve cost 

savings for PWSs and their customers 

while maintaining, or improving, the 

level of public health protection 

(USEPA, 2009f). Although the Agency 

obtained and evaluated the finished 

water occurrence data for glyphosate, its 

usefulness is limited for determining 

potential cost savings to PWSs and their 

customers because the Agency does not 

know which systems are treating for this 

contaminant. As an alternative, the 

Agency evaluated available data on 

source water quality and conducted a 

qualitative assessment of treatment cost 

savings. 

Table VI–16 provides summary data 

for contaminant occurrence based on 

maximum sample values for the 

locations included in the STORET and 

NAWQA data. Although the degree to 

which these occurrence rates represent 

national drinking water source 

occurrence is uncertain, the information 

shows no to low occurrence at 

thresholds levels of interest. This 

information indicates that any resulting 

NPDWR change would not affect 

systems that rely on source water at any 

of the NAWQA or STORET locations. 

TABLE VI–16—AMBIENT WATER QUALITY MONITORING OCCURRENCE SUMMARY FOR GLYPHOSATE

Maximum Concentration 

Number of locations (% of locations) 

STORET 1 NAWQA2

Total ................................................................... 241 (100.0%) .................................................... 41 (100.0%) 

Nondetect .......................................................... 180 (74.7%) ...................................................... 37 (90.2%) 

Detected ............................................................ 61 (25.3%) ........................................................ 4 (9.8%) 

Exceeds current MCLG of 0.7 mg/L ................. 0 (0.0%) ............................................................ 0 (0.0%) 

Exceeds alternative value of 14.0 mg/L ............ 0 (0.0%) ............................................................ 0 (0.0%) 

1STORET database 2002–2007. 

2 NAWQA database 1992–2005. 

Source: USEPA, 2009d. 

The BAT and small system 

compliance technologies for glyphosate 

have other beneficial effects, e.g., 

pretreatment for other co-occurring 

contaminants or disinfection. Therefore, 

if EPA were to consider a higher level, 

the Agency does not know how many 

PWSs that are currently treating to 

comply with the existing MCL of 0.7 

mg/L would be likely to discontinue 

treatment that is already in place 

(USEPA, 2009d). Also, the Agency does 

not know to what extent affected 

systems might be able to reduce costs 

given that capital costs are not 

recoverable. However, the Agency 

recognizes that there may be 

opportunities to achieve operational 

cost savings if these systems are able to 

re-optimize current treatment. 

Given these considerations, the 

Agency believes that any resulting 

revision is not likely to provide a 

meaningful opportunity for cost savings. 

In view of this, any revision would be 

a low priority activity and not 

appropriate at this time. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

glyphosate, EPA does not believe a 

revision to the NPDWR for glyphosate is 

appropriate at this time. In making this 

decision, the Agency considered 

whether any possible revision to the 

NPDWR for glyphosate is likely to 

provide a meaningful opportunity for 

cost savings to public water systems and 

their customers. Taking into 

consideration the low occurrence of this 

contaminant in source waters, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

41. Heptachlor 

a. Background. EPA published the 

current NPDWR for heptachlor on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.0004 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the carcinogenicity of 

heptachlor as well as its potential 

developmental and reproductive 

toxicity. EPA has not identified any new 

information that indicates that it is 

appropriate to consider revisions to the 

cancer classification for heptachlor at 

this time (USEPA, 2009b). Because the 

MCLG remains at zero, the Agency 

believes that a further review of the 

health effects of heptachlor is not 

warranted at this time. 

The current MCL for heptachlor is 

based on a PQL of 0.0004 mg/L. For the 

Six-Year Review, the Agency considered 

whether changes in the analytical 

feasibility of heptachlor might lead to a 

lower MCL. EPA reviewed PE data from 

the first Six-Year Review cycle and then 

analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

heptachlor are above 90 percent around 

the current PQL of 0.0004 mg/L, 

including three studies with true values 

below the current PQL. All passing rates 

in the PE data exceeded 80 percent. 

More recent PT data from late 1999 

through 2004, supplied by a PT 

provider, show greater than 75 percent 

passing rates for a majority of studies, 

but there are no studies with true values 

below the current PQL. There are three 

PT studies with passing rates below 75 

percent. Despite this variability, most of 

the laboratory passing rates from PE and 

PT studies, including three with true 

values below the PQL, exceeded the 75 

percent criterion typically used to 

derive a PQL. Therefore, a lowering of 

the PQL for heptachlor might be 

possible. These results, however, are 

insufficient to recalculate a revised PQL 

for heptachlor because not enough data 

points are available below the current 


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PQL to derive a value at the 75 percent 

passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of heptachlor (Methods 505, 508, 508.1, 

525.2, and 551.1). While EPA prefers to 

use laboratory performance data to 

calculate the PQL, the MRL and MDL 

information can be valuable for this 

review to indicate whether it is possible 

to quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 58,758 

samples. Fewer than 80 percent of these 

values are less than or equal the modal 

MRL: 24,918 (42 percent) equal the 

modal MRL of 0.00004 mg/L and an 

additional 7,966 (14 percent) are lower 

than 0.00004 mg/L. Therefore, EPA did 

not set the EQL equal to the modal MRL 

(USEPA, 2009e). The MDLs of approved 

methods are 0.000003, 0.0000015, 

0.000005, 0.00015, and 0.000081 mg/L. 

Applying a multiplier of 10 would give 

a possible PQL range from 0.000015 to 

0.0015 mg/L. EPA used the median 

10×MDL value of 0.00005 mg/L and 

rounded up to 0.0001 mg/L for the EQL 

(USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there may be potential to 

lower the PQL for heptachlor. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of heptachlor at the EQL of 

0.0001 mg/L and additional threshold of 

0.0002 mg/L (USEPA, 2009f). Table VI– 

17 shows the results of the occurrence 

and exposure analysis for the current 

MCL and these thresholds. The 

occurrence and exposure analysis shows 

that average concentrations exceed the 

current MCL for one of 33,020 systems 

(0.003 percent) serving 325 people (or 

0.0002 percent of 184 million people). 

Note that these results are based on the 

subset of monitoring data provided in 

response to the Six-Year Review ICR 

and do not necessarily reflect MCL 

violations, which are based on annual 

average concentrations at entry points; 

SDWIS/FED indicates no MCL 

violations for heptachlor between 1998 

and 2005 (USEPA, 2007g). Average 

concentrations at 42 of 33,020 systems 

(0.127 percent), serving 31,500 people 

(or 0.017 percent of 184 million people), 

exceed the EQL of 0.0001 mg/L. 

TABLE VI–17—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING HEPTACHLOR

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold (percentages based on 33,020 systems with heptachlor data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.0004 mg/L) ......................... 1 (0.003%) .................................... 1 (0.003%) .................................... 1 (0.003%) 1⁄2 MCL (0.0002 mg/L) .................... 1 (0.003%) .................................... 1 (0.003%) .................................... 1 (0.003%) 

EQL (0.0001 mg/L) ......................... 42 (0.127%) .................................. 42 (0.127%) .................................. 42 (0.127%) 

Corresponding population served (percentages based on 184,444,000 people served by the systems with 

heptachlor data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.0004 mg/L) ......................... 325 (0.0002%) .............................. 325 (0.0002%) .............................. 325 (0.0002%) 1⁄2 MCL (0.0002 mg/L) .................... 325 (0.0002%) .............................. 325 (0.0002%) .............................. 325 (0.0002%) 

EQL (0.0001 mg/L) ......................... 31,500 (0.017%) ........................... 31,500 (0.017%) ........................... 31,500 (0.019%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. 

2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 

heptachlor is appropriate at this time. 

The occurrence and exposure analysis 

based on possible changes in analytical 

feasibility indicates that any revision to 

the MCL is unlikely to provide a 

meaningful opportunity to improve 

public health protection. Taking into 

consideration the low occurrence of this 

contaminant, EPA has decided that any 

revision to the NPDWR would be a low 

priority activity for the Agency, and, 

thus, is not appropriate to revise at this 

time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

42. Heptachlor Epoxide 

a. Background. EPA published the 

current NPDWR for heptachlor epoxide 

on January 30, 1991 (56 FR 3526 

(USEPA, 1991c)). The NPDWR 

established an MCLG of zero based on 

a cancer classification of B2, probable 

human carcinogen. The NPDWR also 

established an MCL of 0.0002 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the carcinogenicity of 

heptachlor epoxide as well as its 

potential developmental and 

reproductive toxicity. EPA has not 

identified any new information that 

indicates that it is appropriate to 

consider revisions to the cancer 

classification for heptachlor epoxide at 

this time (USEPA, 2009b). Because the 

MCLG remains at zero, the Agency 

believes that a further review of the 

health effects of heptachlor epoxide is 

not warranted at this time. 

The current MCL for heptachlor 

epoxide is based on a PQL of 0.0002 mg/ 

L. For the Six-Year Review, the Agency 


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considered whether changes in the 

analytical feasibility of heptachlor 

epoxide might lead to a lower MCL. 

EPA reviewed PE data from the first SixYear Review cycle and then analyzed 

more recent PT data to determine if the 

PQL can be revised (i.e., analytical 

feasibility). Passing rates for PE data 

available through late 1999 for 

heptachlor epoxide are above 85 percent 

around the current PQL of 0.0002 mg/ 

L, including two studies with true 

values below the current PQL. All 

passing rates in the PE data exceeded 80 

percent. More recent PT data from late 

1999 through 2004, supplied by a PT 

provider, show greater than 75 percent 

passing rates for a majority of studies, 

but there are no studies with true values 

below the PQL. There are two PT 

studies with passing rates below 75 

percent. Despite this variability, most of 

the laboratory passing rates from PE and 

PT studies exceeded the 75 percent 

criterion typically used to derive a PQL. 

Therefore, a lowering of the PQL for 

heptachlor epoxide might be possible. 

These results, however, are insufficient 

to recalculate a revised PQL for 

heptachlor epoxide because not enough 

data points are available below the 

current PQL to derive a value at the 75 

percent passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: Laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of heptachlor epoxide (Methods 505, 

508, 508.1, 525.2, and 551.1). While 

EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. The SixYear Review ICR dataset contains MRL 

values for 58,731 samples. Fewer than 

80 percent of these values are less than 

or equal the modal MRL: 26,424 (45 

percent) equal the modal MRL of 

0.00002 mg/L and an additional 5,969 

(10 percent) are lower than 0.00002 mg/ 

L. Therefore, EPA did not set the EQL 

equal to the modal MRL (USEPA, 

2009e). The MDLs of approved methods 

are 0.000004, 0.0000059, 0.000001, 

0.00013, and 0.000202 mg/L. Applying 

a multiplier of 10 would give a possible 

PQL range from 0.00001 to 0.00202 mg/ 

L. EPA used the median 10 × MDL value 

of 0.000059 mg/L and rounded up to 

0.0001 mg/L for the EQL (USEPA, 

2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there may be potential to 

lower the PQL for heptachlor epoxide. 

To determine whether any MCL revision 

is likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of heptachlor epoxide at an 

EQL of 0.0001 mg/L (USEPA, 2009f). 

Table VI–18 shows the results of the 

occurrence and exposure analysis for 

the current MCL and an EQL. The 

occurrence and exposure analysis shows 

that average concentrations exceed the 

current MCL for one of 33,015 systems 

(0.003 percent) serving 325 people (or 

0.0002 percent of 184 million people). 

Note that these results are based on the 

subset of monitoring data provided in 

response to the Six-Year Review ICR 

and do not necessarily reflect MCL 

violations, which are based on annual 

average concentrations at entry points; 

SDWIS/FED indicates two MCL 

violations for heptachlor epoxide 

between 1998 and 2005 (USEPA, 

2007g). Average concentrations at three 

of 33,015 systems (0.009 percent), 

serving 14,400 people (or 0.008 percent 

of 184 million people), exceed the EQL 

of 0.0001 mg/L. 

TABLE VI–18—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING HEPTACHLOR EPOXIDE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold (percentages based on 33,015 systems with heptachlor epoxide data in the Six-Year Review ICR occurrence 

dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.0002 mg/L) ......................... 1 (0.003%) .................................... 1 (0.003%) .................................... 1 (0.003%) 

EQL (0.0001 mg/L) ......................... 3 (0.009%) .................................... 3 (0.009%) .................................... 3 (0.009%) 

Corresponding population served (percentages based on 184,478,000 people served by the systems with 

heptachlor epoxide data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL 1 Nondetect Values = 1⁄2 MRL 2 Nondetect Values = 0 3

MCL (0.0002 mg/L) ......................... 325 (0.0002%) .............................. 325 (0.0002%) .............................. 325 (0.002%) 

EQL (0.0001 mg/L) ......................... 14,400 (0.008%) ........................... 14,400 (0.008%) ........................... 14,400 (0.008%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. 

2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 

heptachlor epoxide is appropriate at this 

time. The occurrence and exposure 

analysis based on possible changes in 

analytical feasibility indicates that any 

revision to the MCL is unlikely to 

provide a meaningful opportunity to 

improve public health protection. 

Taking into consideration the low 

occurrence of this contaminant, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 


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43. Hexachlorobenzene 

a. Background. EPA published the 

current NPDWR for hexachlorobenzene 

on July 17, 1992 (57 FR 31776 (USEPA, 

1992)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.001 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the carcinogenicity of 

hexachlorobenzene as well as its 

potential developmental and 

reproductive toxicity. EPA has not 

identified any new information that 

indicates that it is appropriate to 

consider revisions to the cancer 

classification for hexachlorobenzene at 

this time (USEPA, 2009b). Because the 

MCLG remains at zero, the Agency 

believes that a further review of the 

health effects of hexachlorobenzene is 

not warranted at this time. 

The current MCL for 

hexachlorobenzene is based on a PQL of 

0.001 mg/L. For the Six-Year Review, 

the Agency considered whether changes 

in the analytical feasibility of 

hexachlorobenzene might lead to a 

lower MCL. EPA reviewed PE data from 

the first Six-Year Review cycle and then 

analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

hexachlorobenzene are above 80 percent 

around the current PQL of 0.001 mg/L, 

including eight studies with true values 

below the current PQL. More recent PT 

data from late 1999 through 2004, 

supplied by a PT provider, also show 

greater than 75 percent passing rates for 

a majority of studies, including eight out 

of nine studies with true values below 

the current PQL. There are two PT 

studies with passing rates equal to or 

less than 75 percent, including one with 

a true value below the PQL. Despite this 

variability, most of the laboratory 

passing rates from PE and PT studies— 

including several with true 

concentrations below the PQL— 

exceeded the 75 percent criterion 

typically used to derive a PQL. 

Therefore, a lowering of the PQL for 

hexachlorobenzene might be possible. 

These results, however, are insufficient 

to recalculate a revised PQL for 

hexachlorobenzene because not enough 

data points are available below the 

current PQL to derive a value at the 75 

percent passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: Laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of hexachlorobenzene (Methods 505, 

508, 508.1, 525.2, and 551.1). While 

EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. The SixYear Review ICR dataset contains MRL 

values for 58,713 samples. More than 80 

percent of these values are less than or 

equal the modal MRL: 40,791 (69 

percent) equal the modal MRL of 0.0001 

mg/L and an additional 7,380 (13 

percent) are lower than 0.0001 mg/L. 

Therefore, EPA selected the modal MRL 

as the EQL (USEPA, 2009e). The MDLs 

of approved methods are 0.000002, 

0.0000077, 0.000001, 0.00013, and 

0.000003 mg/L. Applying a multiplier of 

10 would give a possible PQL range 

from 0.00001 to 0.0013 mg/L, which 

contains the EQL (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is potential to lower 

the PQL for hexachlorobenzene. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of hexachlorobenzene at the 

EQL of 0.0001 mg/L and an additional 

threshold of 0.0005 mg/L (USEPA, 

2009f). Table VI–19 shows the results of 

the occurrence and exposure analysis 

for the current MCL and these 

thresholds. The occurrence and 

exposure analysis shows that average 

concentrations exceed the current MCL 

for three of 32,826 systems (0.009 

percent) serving 2,000 people (or 0.001 

percent of 184 million people). Note 

that these results are based on the subset 

of monitoring data provided in response 

to the Six-Year Review ICR and do not 

necessarily reflect MCL violations, 

which are based on annual average 

concentrations at entry points; SDWIS/ 

FED indicates two MCL violations for 

hexachlorobenzene between 1998 and 

2005 (USEPA, 2007g). Average 

concentrations at 9 to 16 of 32,826 

systems (0.027 to 0.049 percent), serving 

approximately 9,000 to 94,000 people 

(or 0.005 to 0.051 percent of 184 million 

people), exceed the EQL of 0.0001 mg/ 

L. 

TABLE VI–19—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING HEXACHLOROBENZENE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold (percentages based on 32,826 systems with hexachlorobenzene data in the Six-Year Review ICR occurrence 

dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.001 mg/L) ........................... 3 (0.009%) .................................... 3 (0.009%) .................................... 3 (0.009%) 1⁄2 MCL (0.0005 mg/L) .................... 4 (0.012%) .................................... 4 (0.012%) .................................... 4 (0.012%) 

EQL (0.0001 mg/L) ......................... not applicable ............................... 16 (0.049%) .................................. 9 (0.027%) 

Corresponding population served (percentages based on 184,124,800 people served by the systems with 

hexachlorobenzene data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.001 mg/L) ........................... 2,000 (0.001%) ............................. 2,000 (0.001%) ............................. 2,000 (0.001%) 1⁄2 MCL (0.0005 mg/L) .................... 5,000 (0.003%) ............................. 5,000 (0.003%) ............................. 5,000 (0.003%) 

EQL (0.0001 mg/L) ......................... not applicable ............................... 94,000 (0.051%) ........................... 9,000 (0.005%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.001 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate 

of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 


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Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 

hexachlorobenzene is appropriate at this 

time. The occurrence and exposure 

analysis based on possible changes in 

analytical feasibility indicates that any 

revision to the MCL is unlikely to 

provide a meaningful opportunity to 

improve public health protection. 

Taking into consideration the low 

occurrence of this contaminant, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

44. Hexachlorocyclopentadiene 

a. Background. EPA published the 

current NPDWR for 

hexachlorocyclopentadiene on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.05 mg/L. EPA based the MCLG 

on a reference dose of 0.007 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. In the first SixYear Review cycle, EPA evaluated new 

information from a health effects 

assessment completed in 2001 (USEPA, 

2001b). At that time, the Agency could 

not determine that a revision to the 

NPDWR would provide a meaningful 

opportunity for public health protection 

(67 FR 19030 (USEPA, 2002c)). The 

2001 assessment considered relevant 

studies on the toxicity of 

hexachlorocyclopentadiene including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.007 mg/kg-day to 0.006 mg/kgday (USEPA, 2001b). In the current 

review cycle, EPA conducted a 

literature search through June 2007 for 

relevant data on the toxicology of 

hexachlorocyclopentadiene, including 

its potential developmental and 

reproductive toxicity. The literature 

search did not identify any new data 

that would affect the RfD or cancer 

classification (USEPA, 2009b). Based on 

the 2001 IRIS assessment and RfD of 

0.006 mg/kg-day, and assuming a 70-kg 

adult body weight and 2 liters water 

intake per day, the DWEL could be 0.21 

mg/L. An RSC of 20 percent results in 

a possible MCLG of 0.04 mg/L (USEPA, 

2009b). 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor for the 

possible MCLG decrease under 

consideration. 

EPA evaluated the results of the 

occurrence and exposure analyses for 

hexachlorocyclopentadiene to 

determine whether a revised MCLG/ 

MCL would be likely to result in a 

meaningful opportunity to improve the 

level of public health protection 

(USEPA, 2009f). Table VI–20 shows the 

results of the occurrence and exposure 

analysis for the current MCL and the 

possible MCLG. The occurrence and 

exposure analysis shows that average 

concentrations do not exceed the 

current MCL for any systems in the 

analysis. Note that these results are 

based on the subset of monitoring data 

provided in response to the Six-Year 

Review ICR and do not necessarily 

reflect MCL violations, which are based 

on running annual average 

concentrations at entry points; SDWIS/ 

FED indicates no MCL violations for 

hexachlorocyclopentadiene between 

1998 and 2005 (USEPA, 2007g). The 

occurrence and exposure analysis shows 

that average concentration do not 

exceed the possible MCLG based on 

health effects information (0.04 mg/L). 

TABLE VI–20—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING

HEXACHLOROCYCLOPENTADIENE THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or health-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or health-based threshold (percentages based on 32,801 systems with hexachlorocyclopentadiene data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.05 mg/L) ............................. 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Possible MCLG (0.04 mg/L) ........... 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Corresponding population served (percentages based on 184,738,000 people served by the systems with 

hexachlorocyclopentadiene data in the Six-Year Review ICR occurrence dataset) 

Regulatory or health-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.05 mg/L) ............................. 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Possible MCLG (0.04 mg/L) ........... 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. 

2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

hexachlorocyclopentadiene, EPA does 

not believe a revision to the NPDWR for 

hexachlorocyclopentadiene is 

appropriate at this time. In making this 


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decision, the Agency considered 

whether any possible revision to the 

NPDWR for hexachlorocyclopentadiene 

is likely to provide a meaningful 

opportunity for health risk reductions. 

Taking into consideration the low 

occurrence of this contaminant, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

45. Lindane (gammaHexachlorocyclohexane) 

a. Background. EPA published the 

current NPDWR for lindane on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG and 

an MCL of 0.0002 mg/L. EPA based the 

MCLG on a reference dose of 0.0003 mg/ 

kg-day and a cancer classification of C, 

possible human carcinogen. 

b. Technical Reviews. In the first SixYear Review cycle, EPA evaluated new 

information from a health effects 

assessment completed in 2002 (USEPA, 

2006b). At that time, the Agency could 

not determine that a revision to the 

NPDWR would provide a meaningful 

opportunity for cost savings to public 

water systems or their customers, and 

decided that any revision would be a 

low priority activity for the Agency 

because of competing workload 

priorities, the administrative costs 

associated with rulemaking, and the 

burden on States and the regulated 

community to implement any regulatory 

change (68 FR 42908, July 18, 2003 

(USEPA, 2003e)). The 2002 assessment 

considered relevant studies on the 

toxicity of lindane including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.0003 mg/kg-day to 0.0047 mg/kgday and classified it as ‘‘Suggestive 

evidence of carcinogenicity, but not 

sufficient to assess human carcinogenic 

potential’’ (USEPA, 2006b). During the 

current review cycle, all uses of lindane 

were cancelled voluntarily (71 FR 

74905, December 13, 2006 (USEPA, 

2006e)), effective July 1, 2007. However, 

lindane is a persistent and 

bioaccumulative pesticide. Accordingly, 

EPA conducted a literature search for 

relevant data on the toxicology of 

lindane, including its potential 

developmental and reproductive 

toxicity. The literature search did not 

identify any additional new data that 

would affect the RfD or cancer 

classification (USEPA, 2009b).The 

possible revised MCLG is based on the 

2002 OPP assessment and RfD of 0.0047 

mg/kg-day, a body weight of 70 kg, 

water intake of 2 L/day, and an RSC of 

20 percent. Uncertainty factors related 

to reproductive and developmental 

effects, and/or a possible risk 

management factor based on the 

suggested evidence of carcinogenicity, 

could be used in developing a possible 

revised MCLG. Depending on the choice 

of uncertainty factors, the MCLG could 

range between 0.001 mg/L and 0.03 mg/ 

L. 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor if EPA 

were to raise the MCLG. EPA evaluated 

the results of the occurrence and 

exposure analyses for lindane to 

determine whether a revised MCLG/ 

MCL would be likely to result in a 

meaningful opportunity to achieve cost 

savings for PWSs and their customers 

while maintaining, or improving, the 

level of public health protection 

(USEPA, 2009f). Although the Agency 

obtained and evaluated the finished 

water occurrence data for lindane, its 

usefulness is limited for determining 

potential cost savings to PWSs and their 

customers because the Agency does not 

know which systems are treating for this 

contaminant. As an alternative, the 

Agency evaluated available data on 

source water quality and conducted a 

qualitative assessment of treatment cost 

savings. 

Table VI–21 provides summary data 

for contaminant occurrence based on 

maximum sample values for the 

locations included in the STORET and 

NAWQA. Although the degree to which 

these occurrence rates represent 

national drinking water source 

occurrence is uncertain, the information 

shows no to low occurrence at threshold 

levels of interest. In the upper bound 

analysis, an NPDWR change would 

affect systems that rely on source water 

at less than 0.01 percent of the NAWQA 

locations and less than 0.3 percent of 

the STORET locations. Any MCLG/MCL 

revision to a potentially higher level of 

0.001 mg/L (the lower bound) or 0.03 

mg/L (the upper bound) would likely 

affect fewer systems. 

TABLE VI–21—AMBIENT WATER QUALITY MONITORING OCCURRENCE SUMMARY FOR LINDANE

Maximum concentration 

Number of locations 

(% of locations) 

STORET 1 NAWQA2

Total ................................................................................................................................................................... 2,691 (100.0%) 8,195 (100.0%) 

Nondetect ........................................................................................................................................................... 2,017 (75%) ...... 8,058 (98.3%) 

Detected ............................................................................................................................................................. 674 (25%) ......... 137 (1.7%) 

Exceeds current MCLG of 0.0002 mg/L ............................................................................................................ 7 (0.26%) .......... 1 (0.01%) 

Exceeds upper bound alternative value of 0.03 mg/L ...................................................................................... 1 (0.04%) .......... 0 (0.0%) 

1STORET database 2002–2007. 

2 NAWQA database 1992–2005. 

Source: USEPA, 2009d. 

The BATs and small system 

compliance technologies for lindane 

have other beneficial effects, e.g., 

reduction of other co-occurring 

contaminants, precursors for DBPs, or 

other common impurities. Therefore, if 

EPA were to consider a higher level, the 

Agency does not know how many PWSs 

that are currently treating to comply 

with the existing MCL of 0.0002 mg/L 

would be likely to discontinue 

treatment that is already in place 

(USEPA, 2009d). Also, the Agency does 

not know to what extent affected 

systems might be able to reduce costs 

given that capital costs are not 

recoverable. However, the Agency 

recognizes that there may be 

opportunities to achieve operational 

cost savings if these systems are able to 

re-optimize current treatment. 

Given these considerations, the 

Agency believes that any resulting 

revision is not likely to provide a 

meaningful opportunity for cost savings. 

In view of this, any revision would be 


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20The DWEL was recommended by a panel of 

experts on mercury, and was derived using the 

weight of evidence from the entire inorganic 

mercury database. The DWEL was later backcalculated to an RfD of 0.0003 mg/kg-day (USEPA, 

1995). 

a low priority activity and not 

appropriate at this time. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

lindane, EPA does not believe a revision 

to the NPDWR for lindane is appropriate 

at this time. In making this decision, the 

Agency considered whether any 

possible revision to the NPDWR for 

lindane is likely to provide a 

meaningful opportunity for cost savings 

to public water systems and their 

customers. Taking into consideration 

the low occurrence of this contaminant 

in source waters, EPA has decided that 

any revision to the NPDWR would be a 

low priority activity for the Agency, 

and, thus, is not appropriate to revise at 

this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

46. Mercury (Inorganic) 

a. Background. EPA published the 

current NPDWR for inorganic mercury 

on January 30, 1991 (56 FR 3526 

(USEPA, 1991c)). The NPDWR 

established an MCLG and an MCL of 

0.002 mg/L. The Agency based the 

MCLG on a DWEL of 0.01 mg/L 20 and 

a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the toxicology of 

inorganic mercury, including its 

potential developmental and 

reproductive toxicity. The literature 

search did not identify any studies that 

warrant a review of the RfD or the 

cancer classification (USEPA, 2009b). 

A review of analytical or treatment 

feasibility is not necessary for inorganic 

mercury because changes to the MCLG 

are not warranted at this time and the 

current MCL is set at the MCLG. Since 

EPA did not identify a health or 

technology basis for revising the 

inorganic mercury NPDWR, the Agency 

did not conduct a detailed occurrence 

and exposure analysis. 

c. Review Result. EPA’s review shows 

that there are no data supporting a 

change to the inorganic mercury 

NPDWR. As a result, a revision to the 

NPDWR would not be appropriate at 

this time. 

47. Methoxychlor 

a. Background. EPA published the 

current NPDWR for methoxychlor on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG and an MCL of 0.04 mg/L. EPA 

based the MCLG on a reference dose of 

0.005 mg/kg-day and a cancer 

classification of D, not classifiable as to 

human carcinogenicity. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the toxicology of 

methoxychlor, including its potential 

developmental and reproductive 

toxicity. The literature search did not 

identify any studies that warrant a 

review of the RfD or the cancer 

classification (USEPA, 2009b). The SixYear Review 1 stated that the Agency 

had initiated a reassessment of the 

health risks posed by exposure to 

methoxychlor (67 FR 19030 (USEPA, 

2002c)). Since 2002, the Agency has 

cancelled all product uses and 

concluded that the database to complete 

the health effects assessment for 

methoxychlor was inadequate (USEPA, 

2004c). In its Reregistration Eligibility 

Decision, OPP noted substantive data 

gaps for methoxychlor, including lack of 

Guideline studies for chronic systemic 

toxicity as well as reproductive and 

developmental toxicity (USEPA, 2004c). 

A review of analytical or treatment 

feasibility is not necessary for 

methoxychlor because changes to the 

MCLG are not warranted at this time 

and the current MCL is set at the MCLG. 

Since EPA did not identify a health or 

technology basis for revising the 

methoxychlor NPDWR, the Agency did 

not conduct a detailed occurrence and 

exposure analysis. 

c. Review Result. EPA’s review shows 

that there are no data supporting a 

change to the methoxychlor NPDWR. As 

a result, a revision to the NPDWR would 

not be appropriate at this time. 

48. Monochlorobenzene 

(Chlorobenzene) 

a. Background. EPA published the 

current NPDWR for monochlorobenzene 

on January 30, 1991 (56 FR 3526 

(USEPA, 1991c)). The NPDWR 

established an MCLG and an MCL of 0.1 

mg/L. EPA based the MCLG on a 

reference dose of 0.02 mg/kg-day and a 

cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the toxicology of 

monochlorobenzene, including its 

potential developmental and 

reproductive toxicity. The literature 

search did not identify any studies that 

warrant a review of the RfD or the 

cancer classification (USEPA, 2009b). 

A review of analytical or treatment 

feasibility is not necessary for 

monochlorobenzene because changes to 

the MCLG are not warranted at this time 

and the current MCL is set at the MCLG. 

Since EPA did not identify a health or 

technology basis for revising the 

monochlorobenzene NPDWR, the 

Agency did not conduct a detailed 

occurrence and exposure analysis. 

c. Review Result. EPA’s review shows 

that there are no data supporting a 

change to the monochlorobenzene 

NPDWR. As a result, a revision to the 

NPDWR would not be appropriate at 

this time. 

49. Nitrate (as N) 

a. Background. EPA published the 

current NPDWR for nitrate on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG and 

an MCL of 10 mg/L (as N). EPA based 

the MCLG on a survey of epidemiologic 

studies of infant methemoglobinemia in 

populations exposed to nitrate 

contaminated water. No cancer 

classification is currently available for 

nitrate (USEPA, 2009b). 

b. Technical Reviews. The health 

effects technical review identified new 

information on developmental effects of 

nitrate, as well as data regarding its 

carcinogenicity, that may indicate the 

need to update the Agency’s health 

effects assessment (USEPA, 2009b). 

Several studies suggest that nitrate in 

drinking water can have adverse effects 

on the thyroid (Mukhopadhyay et al., 

2005; Tajtakova et al., 2006; Zaki et al., 

2004). Nitrate has long been known as 

a competitive inhibitor of iodide uptake 

in the thyroid (Wolff and Maury, 1963). 

Inhibition of iodide uptake can lead to 

alteration in thyroid hormone levels 

including decreases in levothyroxine 

(T4) levels. NAS (1995) stated that it is 

likely that the motor changes reported 

by Markel et al. (1989) when the 

animals were young were not a direct 

effect of nitrate, but were secondary to 

effects on learning behavior. Based on 

these considerations, a new assessment 

of the noncancer effects of nitrate may 

be warranted, including consideration 

of whether methemoglobinemia in 

infants, which is an acute effect, is still 

the most appropriate basis for the 

chronic exposure limit for nitrate. In 

addition, recent information may 

suggest the consideration of separate 

acute and chronic values for nitrate. 

The health effects review identified a 

number of relevant new studies that 

may warrant a review of the cancer 


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21A child’s body weight and drinking water 

intake were used to calculate the DWEL because 

children are the population with the highest risk 

from dietary exposure. 

classification for nitrate. These studies 

include a number of new epidemiology 

studies (Cocco et al., 2003; Coss et al., 

2004; de Roos et al., 2003; Mueller et al., 

2004; Volkmer et al., 2005; Ward et al., 

2003; Ward et al., 2005a; Ward et al., 

2005b; Ward et al., 2006; Yang et al., 

2007; Zeegers et al., 2006), as well as a 

recent report from an International 

Agency for Research on Cancer (IARC) 

Working group (Grosse et al., 2006). 

This latter report concluded that, under 

conditions that result in endogenous 

nitrosation, ingested nitrate or nitrite is 

probably carcinogenic to humans. 

In light of this information, EPA 

considers nitrate as a potential 

candidate for a new health effects 

assessment. The Agency solicits 

feedback on its plans to reassess health 

risks resulting from exposure to nitrate. 

The Agency also welcomes any 

scientific information related to nitrate 

health risks from the public. Because 

EPA considers nitrate as a candidate for 

a new assessment, EPA does not believe 

it is appropriate to consider any 

possible revisions to the MCLG (as well 

as the MCL) at this time. 

A review of analytical or treatment 

feasibility is not necessary for nitrate 

because changes to the MCLG are not 

warranted at this time and the current 

MCL is set at the MCLG. Since EPA did 

not identify a health or technology basis 

for revising the nitrate NPDWR, the 

Agency did not conduct a detailed 

occurrence and exposure analysis. 

c. Review Result. The Agency is 

considering whether to initiate a new 

health assessment for nitrate and 

therefore does not believe a revision to 

the NPDWR is appropriate at this time. 

As discussed in Section VII, the 

Agency is asking for input and 

information about several 

implementation issues related to nitrate 

(see section V.B.6). 

50. Nitrite (as N) 

a. Background. EPA published the 

current NPDWR for nitrite on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG and 

an MCL of 1 mg/L (as N). EPA based the 

MCLG on extrapolation from nitrate, 

assuming the conversion of 10 percent 

of nitrate-nitrogen to nitrite-nitrogen. No 

cancer classification is currently 

available for nitrite (USEPA, 2009b). 

b. Technical Reviews. The health 

effects technical review identified new 

information on developmental effects of 

nitrite, as well as data regarding its 

carcinogenicity, that may indicate the 

need to update the Agency’s health 

effects assessment (USEPA, 2009b). 

Several studies suggest that nitrate in 

drinking water can have adverse effects 

on the thyroid (Mukhopadhyay et al., 

2005; Tajtakova et al., 2006; Zaki et al., 

2004). Since nitrite is formed from 

nitrate, and the current nitrite RfD is 

based on nitrate data, the impact of 

these new data on a nitrite noncancer 

assessment should be evaluated. Nitrite 

has long been known as a competitive 

inhibitor of iodide uptake in the 

thyroid; although it is a weaker inhibitor 

than nitrate (Wolff and Maury, 1963). 

Inhibition of iodide uptake can lead to 

alteration in thyroid hormone levels 

including decreases in T4. A 

developmental toxicity study in rats 

(Vorhees et al., 1984) observed 

statistically significant delays in 

swimming development in addition to 

pup mortality and body weight changes. 

Based on these considerations, a new 

assessment of the noncancer effects of 

nitrite may be warranted, including 

consideration of whether 

methemoglobinemia in infants, which is 

an acute effect, is still the most 

appropriate basis for the chronic 

exposure limit for nitrite. In addition, 

recent information may suggest the 

consideration of separate acute and 

chronic values for nitrite. 

The health effects review identified a 

number of relevant new studies that 

may warrant a review of the cancer 

classification for nitrate. These studies 

include a number of new epidemiology 

studies (Cocco et al., 2003; Coss et al., 

2004; de Roos et al., 2003; Mueller et al., 

2004; Volkmer et al., 2005; Ward et al., 

2003; Ward et al., 2005a; Ward et al., 

2005b; Ward et al., 2006; Yang et al., 

2007; Zeegers et al., 2006). In addition, 

a recent report from an International 

Agency for Research on Cancer (IARC) 

Working group (Grosse et al., 2006) 

concluded that, under conditions that 

result in endogenous nitrosation, 

ingested nitrate or nitrite is probably 

carcinogenic to humans. 

In light of this information, EPA 

considers nitrite as a potential candidate 

for a new health effects assessment. The 

Agency solicits feedback on its plans to 

reassess health risks resulting from 

exposure to nitrite. The Agency also 

welcomes any scientific information 

related to nitrite health risks from the 

public. Because EPA considers nitrite as 

a candidate for a new assessment, EPA 

does not believe it is appropriate to 

consider any possible revisions to the 

MCLG (as well as the MCL) at this time. 

A review of analytical or treatment 

feasibility is not necessary for nitrite 

because changes to the MCLG are not 

warranted at this time and the current 

MCL is set at the MCLG. Since EPA did 

not identify a health or technology basis 

for revising the nitrite NPDWR, the 

Agency did not conduct a detailed 

occurrence and exposure analysis. 

c. Review Result. The Agency is 

considering whether to initiate a new 

health assessment for nitrite and 

therefore does not believe a revision to 

the NPDWR is appropriate at this time. 

As discussed in Section VII, the 

Agency is requesting input and 

information about several 

implementation issues related to nitrite 

(see section V.B.6). 

51. Oxamyl (Vydate) 

a. Background. EPA published the 

current NPDWR for oxamyl on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.2 mg/L. EPA based the MCLG 

on a reference dose of 0.025 mg/kg-day 

and a cancer classification of E, 

evidence of non-carcinogenicity for 

humans. 

b. Technical Reviews. In 2000, the 

Agency updated its health effects 

assessment of oxamyl (USEPA, 2000a). 

The Agency identified a change in this 

assessment that could lead to a change 

in the MCLG. This assessment 

considered relevant studies on the 

toxicity of oxamyl including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.025 mg/kg-day to 0.001 mg/kgday and concluded that there is 

evidence that oxamyl is 

noncarcinogenic to humans (USEPA, 

2000a). Based on the new OPP 

assessment and RfD of 0.001 mg/kg-day, 

and assuming a 10-kg child body weight 

and 1 liter water intake per day, the 

DWEL could be 0.01 mg/L.21 An RSC of 

20 percent was selected based on the 

actual food dietary exposure (81 

percent) for children who are 1 to 6 

years old (USEPA, 2000a); this RSC 

results in a possible MCLG of 0.002 mg/ 

L (USEPA, 2009b). 

Because of a possible change in the 

MCLG for oxamyl, EPA considered 

whether analytical feasibility is likely to 

be a limitation if the Agency were to 

consider lowering the MCL to 0.002 mg/ 

L (the possible MCLG). EPA reviewed 

PE data from the first Six-Year Review 

cycle and then analyzed more recent PT 

data to determine if it might be possible 

to recalculate the PQL, which is 0.02 

mg/L and might be a limit to a possible 

MCLG of 0.002 mg/L (i.e., analytical 

feasibility). Passing rates for PE data 

available through late 1999 for oxamyl 

are below 75 percent for most studies 

with true concentrations below the 


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22The Six-Year Review ICR occurrence data are 

based on the Standardized Monitoring Framework 

for synthetic organic compounds, which is designed 

to evaluate long-term exposure to contaminants 

with chronic exposure health endpoints. As a 

result, EPA recognizes that short-term seasonal 

peaks, which correspond to oxamyl application as 

a pesticide, cannot be readily detected in this 

dataset. Nonetheless and as noted, EPA used the 

peak concentrations to evaluate occurrence for 

oxamyl because the health endpoint is associated 

with acute exposure. 

current PQL of 0.02 mg/L. More recent 

PT data from late 1999 through 2004, 

supplied by a PT provider, show no 

results below the current PQL but had 

most passing rates above 75 percent 

with true values at or above the current 

PQL. Given the variable results from the 

PE and PT studies, and the lack of PT 

data below the current PQL, PE and PT 

data are insufficient to support a PQL 

reduction (USEPA, 2009c). 

While the PT data are not sufficient to 

support a lowering of the PQL for 

oxamyl at this time, the present PQL of 

0.02 mg/L is greater than the possible 

MCLG. It would therefore limit a 

possible revision to the MCL. EPA 

evaluated two alternative sources of 

information to determine whether they 

indicate any potential to quantitate at 

levels as low as the possible MCLG: 

laboratory minimum reporting levels in 

the Six-Year Review ICR dataset, and 

the MDLs for approved methods for the 

detection of oxamyl (Methods 531.1 and 

531.2). While EPA prefers to use 

laboratory performance data to calculate 

the PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 52,201 

samples. Of these, 45,290 (87 percent) 

have an MRL value of 0.002 mg/L or 

lower. Because more than 80 percent of 

the MRL values are at or below the 

possible MCLG of 0.002 mg/L, EPA 

selected that value as the minimum 

threshold for the occurrence and 

exposure analysis (USEPA, 2009e). 

Method 531.1 has an MDL of 0.00086 

mg/L, and Method 532.2 has a detection 

limit (DL) of 0.000065 mg/L. Applying 

a multiplier of 10 would give a possible 

PQL range from 0.00065 to 0.0086 mg/ 

L, which contains the possible MCLG 

(USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, 

there is evidence of a potential to lower 

the PQL for oxamyl even though the PE 

and PT data are insufficient to support 

a PQL reduction. To determine whether 

any MCL revision is likely to provide a 

meaningful opportunity to improve 

public health protection, EPA evaluated 

the occurrence of oxamyl at the possible 

MCLG of 0.002 mg/L (USEPA, 2009f). 

Table VI–22 shows the results of the 

occurrence and exposure analysis for 

the current MCL and the possible 

MCLG. The analysis uses single sample 

or peak results instead of system average 

results because the health endpoint is 

associated with acute exposure.22 The 

occurrence and exposure analysis shows 

that individual sample concentrations 

exceed the current MCL of 0.2 mg/L for 

one of 30,876 systems (0.003 percent) 

serving 200 people (or 0.000 percent of 

167 million people). Note that these 

results are based on the subset of 

monitoring data provided in response to 

the Six-Year Review ICR and do not 

necessarily reflect MCL violations, 

which are based on running annual 

average concentrations at entry points; 

SDWIS/FED indicates no MCL 

violations for oxamyl between 1998 and 

2005 (USEPA, 2007g). Individual 

sample concentrations at 18 of 30,876 

systems (0.058 percent), serving fewer 

than 0.3 million people (0.177 percent), 

exceeded the possible MCLG of 0.002 

mg/L at least one time between 1998 

and 2005. 

TABLE VI–22—NUMBER AND PERCENT OF SYSTEMS WITH PEAK CONCENTRATIONS EXCEEDING OXAMYL THRESHOLDS

AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or health-based threshold 

Systems with any sample that is greater than the regulatory or healthbased threshold 

(Percentages based on 30,876 systems with oxamyl data in the sixyear review ICR occurrence dataset) 

MCL (0.2 mg/L) ........................................................................................ 1 (0.003%) 

Possible MCLG (0.002 mg/L) ................................................................... 18 (0.058%) 

Corresponding population served 

(Percentages based on 167,378,400 people served by the systems 

Regulatory or health-based threshold with oxamyl data in the Six-Year Review ICR occurrence dataset) 

MCL (0.2 mg/L) ........................................................................................ 200 (0.0001%) 

Possible MCLG (0.002 mg/L) ................................................................... 297,000 (0.177%) 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

oxamyl, EPA does not believe a revision 

to the NPDWR for oxamyl is appropriate 

at this time. In making this decision, the 

Agency considered whether any 

possible revision to the NPDWR for 

oxamyl is likely to provide a meaningful 

opportunity for health risk reductions. 

Taking into consideration the low 

occurrence of this contaminant, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

52. Pentachlorophenol 

a. Background. EPA published the 

current NPDWR for pentachlorophenol 

on July 1, 1991 (56 FR 30266 (USEPA, 

1991b)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.001 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

pentachlorophenol. The revised health 

effects assessment will consider relevant 


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studies on the toxicity of 

pentachlorophenol, including its 

potential developmental and 

reproductive toxicity. The new health 

effects assessment was not completed by 

March 1, 2009, the review cutoff date for 

this notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

Although a risk assessment is in 

process for pentachlorophenol, the 

existing MCLG is zero and the current 

MCL of 0.001 mg/L is based on the PQL. 

Therefore, EPA reviewed whether there 

is potential to revise the PQL. EPA 

reviewed PE data from the first Six-Year 

Review cycle and then analyzed more 

recent PT data to determine if the PQL 

can be revised (i.e., analytical 

feasibility). Several passing rates in the 

PE data for pentachlorophenol available 

through late 1999 are below 75 percent, 

and none of the true concentrations 

were below the current PQL. There are 

six PE studies with passing rates equal 

to or less than the 75 percent criterion, 

and only one of 16 true values in the PE 

data is below the current PQL. More 

recent PT data from late 1999 through 

2004, supplied by a PT provider, show 

passing rates below the 75 percent 

criterion for eight studies, and all of the 

true concentrations in the PT data were 

higher than the current PQL. Because of 

the variability in passing rates and the 

lack of data points below the current 

PQL, a lowering of the PQL for 

pentachlorophenol is not appropriate at 

this time (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: Laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of pentachlorophenol (Methods 515.1, 

515.2, and 525.2). While EPA prefers to 

use laboratory performance data to 

calculate the PQL, the MRL and MDL 

information can be valuable for this 

review to indicate whether it is possible 

to quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 59,594 

samples. Fewer than 80 percent of these 

values are less than or equal to the 

modal MRL: 26,666 (45 percent) equal 

the modal MRL of 0.00004 mg/L and an 

additional 2,399 (4 percent) are lower 

than 0.00004 mg/L. Therefore, EPA did 

not set the EQL equal to the modal MRL 

(USEPA, 2009e). The MDLs of approved 

methods are 0.000032, 0.00016, and 

0.001 mg/L. EPA selected the median 

value, applied a multiplier of 10, and 

rounded up to 0.002 mg/L. The result is 

higher than the current PQL and, 

therefore, EPA did not estimate an EQL 

(USEPA, 2009e). Based on these varied 

and unrelated approaches/sources of 

information, EPA believes that there is 

no potential to lower the PQL for 

pentachlorophenol. Since the MCL is 

constrained by the PQL, and the PQL is 

unchanged, EPA does not believe it is 

necessary to conduct an occurrence 

analysis at this time. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

pentachlorophenol is appropriate at this 

time because a reassessment of the 

health risks resulting from exposure to 

pentachlorophenol is in progress 

(USEPA, 2009b). Furthermore, a review 

of analytical feasibility did not identify 

a potential to revise the MCL, which is 

limited by feasibility. 

53. Picloram 

a. Background. EPA published the 

current NPDWR for picloram on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.5 mg/L. EPA based the MCLG 

on a reference dose of 0.07 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. In the first SixYear Review cycle, EPA evaluated new 

information from a health effects 

assessment completed in 1995 (USEPA, 

1995b). At that time, the Agency could 

not determine that a revision to the 

NPDWR would provide a meaningful 

opportunity for cost savings to public 

water systems or their customers, and 

decided that any revision would be a 

low priority activity for the Agency 

because of competing workload 

priorities, the administrative costs 

associated with rulemaking, and the 

burden on States and the regulated 

community to implement any regulatory 

change (67 FR 19030 (USEPA, 2002c); 

68 FR 42908 (USEPA, 2003e)). The 1995 

assessment considered relevant studies 

on the toxicity of picloram including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.07 mg/kg-day to 0.2 mg/kg-day 

and classified picloram as Group E, 

evidence of noncarcinogenicity (USEPA, 

1995b). In the current review cycle, EPA 

conducted a literature search through 

June 2007 for relevant data on the 

toxicology of picloram, including its 

potential developmental and 

reproductive toxicity. The literature 

search did not identify any new data 

that would affect the RfD or cancer 

classification (USEPA, 2009b). Based on 

the 1995 OPP assessment and RfD of 0.2 

mg/kg-day, and assuming a 70-kg adult 

body weight and 2 liters water intake 

per day, the DWEL could be 7 mg/L. An 

RSC of 20 percent results in a possible 

MCLG of 1 mg/L (USEPA, 2009b). 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor if EPA 

were to raise the MCLG. EPA evaluated 

the results of the occurrence and 

exposure analyses for picloram to 

determine whether a revised MCLG/ 

MCL would be likely to result in a 

meaningful opportunity to achieve cost 

savings for PWSs and their customers 

while maintaining, or improving, the 

level of public health protection 

(USEPA, 2009f). Although the Agency 

obtained and evaluated the finished 

water occurrence data for picloram, its 

usefulness is limited for determining 

potential cost savings to PWSs and their 

customers because the Agency does not 

know which systems are treating for this 

contaminant. As an alternative, the 

Agency evaluated available data on 

source water quality and conducted a 

qualitative assessment of treatment cost 

savings. 

Table VI–23 provides summary data 

for contaminant occurrence based on 

maximum sample values for the 

locations included in the STORET and 

NAWQA data. Although the degree to 

which these occurrence rates represent 

national drinking water source 

occurrence is uncertain, the information 

shows no to low occurrence at threshold 

levels of interest. This information 

indicates that any resulting NPDWR 

change would not affect systems that 

rely on source water at any of the 

NAWQA or STORET locations. 

TABLE VI–23—AMBIENT WATER QUALITY MONITORING OCCURRENCE SUMMARY FOR PICLORAM

Maximum concentration 

Number of locations (% of locations) 

STORET 1 NAWQA2

Total ................................................................... 870 (100%) ....................................................... 5,772 (100.0%) 

Nondetect .......................................................... 745 (85.6%) ...................................................... 5,733 (99.3%) 

Detected ............................................................ 125 (14.4%) ...................................................... 39 (0.7%) 


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TABLE VI–23—AMBIENT WATER QUALITY MONITORING OCCURRENCE SUMMARY FOR PICLORAM—Continued 

Maximum concentration 

Number of locations (% of locations) 

STORET 1 NAWQA2

Exceeds current MCLG of 0.5 mg/L ................. 0 (0%) ............................................................... 0 (0.0%) 

Exceeds alternative value of 1.0 mg/L .............. 0 (0%) ............................................................... 0 (0.0%) 

1STORET database 2002–2007. 

2 NAWQA database 1992–2005. 

Source: USEPA, 2009d. 

The BATs and small system 

compliance technologies for picloram 

have other beneficial effects, e.g., 

reduction of other co-occurring 

contaminants, precursors for DBPs, or 

other common impurities. Therefore, if 

EPA were to consider a higher level, the 

Agency does not know how many PWSs 

that are currently treating to comply 

with the existing MCL of 0.5 mg/L 

would be likely to discontinue 

treatment that is already in place 

(USEPA, 2009d). Also, the Agency does 

not know to what extent affected 

systems might be able to reduce costs 

given that capital costs are not 

recoverable. However, the Agency 

recognizes that there may be 

opportunities to achieve operational 

cost savings if these systems are able to 

re-optimize current treatment. 

Given these considerations, the 

Agency believes that any resulting 

revision is not likely to provide a 

meaningful opportunity for cost savings. 

In view of this, any revision would be 

a low priority activity and not 

appropriate at this time. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

picloram, EPA does not believe a 

revision to the NPDWR for picloram is 

appropriate at this time. In making this 

decision, the Agency considered 

whether any possible revision to the 

NPDWR for picloram is likely to provide 

a meaningful opportunity for cost 

savings to public water systems and 

their customers. Taking into 

consideration the low occurrence of this 

contaminant in source waters, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

54. Polychlorinated Biphenyls (PCBs) 

a. Background. EPA published the 

current NPDWR for PCBs on January 30, 

1991 (56 FR 3526 (USEPA, 1991c)). The 

NPDWR established an MCLG of zero 

based on a cancer classification of B2, 

probable human carcinogen. The 

NPDWR also established an MCL of 

0.0005 mg/L, based on analytical 

feasibility. 

b. Technical Reviews. EPA has 

initiated a reassessment of the cancer 

health risks resulting from exposure to 

PCBs. The revised health effects 

assessment will consider relevant 

studies on the toxicity of PCBs, 

including its potential developmental 

and reproductive toxicity. The new 

health effects assessment was not 

completed by March 1, 2009, the review 

cutoff date for this notice (USEPA, 

2009b). On December 21, 2007 (72 FR 

72715 (USEPA, 2007c)), the Agency 

noted that the health effects assessment 

for PCBs is in process. 

Although a risk assessment is in 

process for PCBs, the existing MCLG is 

zero and the current MCL of 0.0005 mg/ 

L is based on the PQL. Therefore, EPA 

reviewed whether there is potential to 

revise the PQL. EPA reviewed PE data 

from the first Six-Year Review cycle and 

then analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). The PE data for 

PCBs available through late 1999 

includes only one true concentration 

below the current PQL, and the passing 

rate for that concentration is below 75 

percent. The passing rates for studies 

above the PQL are above 75 percent. 

More recent PT data from late 1999 

through 2004, supplied by a PT 

provider, show passing rates above 75 

percent for all studies, but includes no 

studies below the current PQL. Because 

of the lack of data points below the 

current PQL, a lowering of the PQL for 

PCBs is not appropriate at this time 

(USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDL 

for the approved method for the 

detection of PCBs (Method 508A). While 

EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. The SixYear Review ICR dataset contains MRL 

values for 35,178 samples. Fewer than 

80 percent of these values are less than 

or equal the modal MRL: 23,785 (68 

percent) equal the modal MRL of 0.0001 

mg/L and an additional 2,355 (7 

percent) are lower than 0.0001 mg/L. 

Therefore, EPA did not set the EQL 

equal to the modal MRL (USEPA, 

2009e). The MDL of approved method is 

0.00008 mg/L. Applying a multiplier of 

10 would give a possible PQL of 0.0008 

mg/L. The result is higher than the 

current PQL, and therefore, EPA did not 

estimate an EQL (USEPA, 2009e). Based 

on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is no potential to 

lower the PQL for PCBs. Since the MCL 

is constrained by the PQL, and the PQL 

is unchanged, EPA does not believe it is 

necessary to conduct an occurrence 

analysis at this time. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

PCBs is appropriate at this time because 

a reassessment of the health risks 

resulting from exposure to PCBs is in 

progress (USEPA, 2009b). Furthermore, 

a review of analytical feasibility did not 

identify a potential to revise the MCL, 

which is limited by feasibility. 

55. Combined Radiums (226 and 228) 

a. Background. EPA published an 

interim NPDWR and set an MCL of 5 

pCi/L for combined radium 226 and 228 

on July 9, 1976 (41 FR 28402 (USEPA, 

1976)). As noted in the August 14, 1975 

proposal (40 FR 34324 (USEPA, 1975)) 

and a subsequent September 30, 1986 

FR notice, EPA considered the 

feasibility of treatment techniques, 

analytical methods and monitoring 

when establishing the MCL of 5 pCi/L. 

EPA also considered the risks associated 

with exposure to radium 226 and 228, 

which generally fell within the Agency’s 

acceptable risk range of 10¥4 to 10¥6 at 

the MCL of 5 pCi/L. On December 7, 

2000 (65 FR 76708 (USEPA, 2000c)), 

EPA established an MCLG of zero based 

on a cancer classification of A (known 


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23After the December 7, 2000 final regulation, 

two trade associations and several municipal water 

systems challenged EPA’s standard for combined 

radiums by claiming that the Agency did not use 

the best available science when finalizing the 

standard. In February of 2003, the DC Circuit Court 

of Appeals upheld EPA’s regulation for combined 

radiums (as well as beta and photon emitters and 

uranium). 

24The 0.4 mg/day safe level was based on data 

(Yang et al., 1989a, 1989b) that extrapolated from 

blood selenium levels to estimated dietary intake in 

the studied population. As described in the January 

30, 1991 FR (56 FR 3526 (USEPA, 1991c)), the 

Agency partially considered selenium’s status as a 

nutrient and did not use the typical procedure for 

deriving the MCLG. Hence, there is no specific 

reference to an RfD for selenium in the 1991 FR 

notice. After the publication of the regulation, IRIS 

(USEPA, 1991a) posted an RfD of 0.005 mg/kg-day 

for selenium using the same data that are the basis 

of the regulation. 

human carcinogen) and finalized the 

NPDWR by retaining the MCL of 5 pCi/ 

L. EPA noted in the December 7, 2000 

FR notice that new risk estimates from 

Federal Guidance Report 13 reaffirmed 

that the 5 pCi/L MCL was appropriate 

and protective.23 EPA also tightened the 

monitoring requirements for combined 

radiums by requiring that systems 

monitor for radium 226 and 228 

separately. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

radiums. The revised health effects 

assessment will consider relevant 

studies on the toxicity of radiums, 

including its potential developmental 

and reproductive toxicity. The new 

health effects assessment was not 

completed by March 1, 2009, the review 

cutoff date for this notice (USEPA, 

2009b). 

Although there is an ongoing health 

effects assessment, the MCLG is zero 

and the current MCL is higher than the 

MCLG. Therefore, EPA reviewed 

whether there is potential to revise the 

MCL based on new information 

regarding analytical and treatment 

feasibility for radiums. EPA 

promulgated detection limits of 1 pCi/ 

L for both radium 226 and radium 228 

in 1976 (41 FR 28402 (USEPA, 1976)) 

and retained the use of a detection limit 

as the required measure of sensitivity 

for radiochemical analysis in lieu of an 

MDL or PQL in the final rule (65 FR 

76708, December 7, 2000 (USEPA, 

2000c)). EPA did not identify new 

analytical methods during the current 

review that would feasibly lower the 

detection limits. In addition, since the 

December 7, 2000, regulation, there is 

no new information regarding treatment 

feasibility. Since there is no new 

information regarding analytical or 

treatment feasibility that suggests 

changes to the MCL, EPA does not 

believe it is necessary to conduct an 

occurrence analysis at this time. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

combined radiums is appropriate at this 

time because a reassessment of the 

health risks resulting from exposure to 

radium is in progress (USEPA, 2009b). 

Furthermore, there is no new 

information regarding analytical or 

treatment feasibility that would warrant 

reconsideration of the MCL. 

56. Selenium 

a. Background. EPA published the 

current NPDWR for selenium on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG and 

an MCL of 0.05 mg/L. EPA based the 

MCLG on a maximum safe intake24 of 

0.4 mg/person/day and a cancer 

classification of D, not classifiable as to 

human carcinogenicity. 

b. Technical Reviews. The health 

effects technical review identified new 

data that relate to the biological 

properties of selenium in mammalian 

species, as well as data regarding its 

cancer and anticancer properties, that 

may indicate the need to update the 

Agency’s health effects assessment 

(USEPA, 2009b). Hawkes and Keim 

(2003) reported thyroid hormone and 

related metabolism changes in subjects 

treated with deficient, sufficient, and 

excess dietary selenium. The excess 

selenium dose was associated with a 

slight decrease in triiodothyronine (T3) 

levels, a thyrotropin increase, and an 

increase in body weight compared to the 

selenium-sufficient subjects. The 

opposite responses occurred in the 

selenium-deficient subjects. Several 

studies identified changes in sperm 

parameters and fertility in mice fed 

either selenium-deficient or excessselenium diets compared to diets with 

adequate selenium. In addition, new 

information about the metabolism of 

selenium since the IRIS review (USEPA, 

1991a, 1993a) suggests that it may be 

appropriate to differentiate between 

inorganic selenium and organic 

selenium in the form of selenoproteins 

and selenoaminoacids for an assessment 

that applies to drinking water. Although 

selenium is not a candidate for an 

MCLG of zero because of its status as a 

micronutrient, new data relevant to the 

cancer assessment are now available 

(e.g., Duffield-Lillico et al., 2003; Su et 

al., 2005) and may need further 

evaluation. 

In light of this information, EPA 

considers selenium as a potential 

candidate for a new health effects 

assessment. The Agency solicits general 

feedback on its plans to reassess health 

risks resulting from exposure to 

selenium. The Agency also welcomes 

any scientific information related to 

selenium health risks from the public. 

Because EPA considers selenium as a 

candidate for a new assessment, EPA 

does not believe it is appropriate to 

consider any revisions to the MCLG (as 

well as the MCL) at this time. 

A review of analytical or treatment 

feasibility is not necessary for selenium 

because changes to the MCLG are not 

warranted at this time and the current 

MCL is set at the MCLG. Since EPA did 

not identify a health or technology basis 

for revising the selenium NPDWR, the 

Agency did not conduct a detailed 

occurrence and exposure analysis. 

c. Review Result. The Agency is 

considering whether to initiate a new 

health assessment for selenium and 

therefore does not believe a revision to 

the NPDWR is appropriate at this time. 

57. Simazine 

a. Background. EPA published the 

current NPDWR for simazine on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG and an 

MCL of 0.004 mg/L. EPA based the 

MCLG on a reference dose of 0.005 mg/ 

kg-day and a cancer classification of C, 

possible human carcinogen. 

b. Technical Reviews. In 2006, the 

Agency finalized a health effects 

assessment for the reregistration of 

simazine as a pesticide (USEPA, 2006i). 

Because the database for simazine’s 

potential neuroendocrine effects is less 

robust than the atrazine database, and 

because simazine and atrazine share a 

common neuroendocrine mechanism of 

toxicity, the atrazine data were used as 

bridging data for simazine. Thus, the 

2006 assessment established a new RfD 

of 0.018 mg/kg-day for simazine, based 

on the attenuation of pre-ovulatory LH 

surge from atrazine exposure. Similarly, 

simazine was reclassified in 2006 as 

‘‘not likely to be carcinogenic to 

humans’’ based on weight-of-evidence 

that it is not genotoxic and because the 

tumor response in the Sprague-Dawley 

rats was determined to be a strain 

specific mechanism which is not 

relevant to humans. 

c. Review Result. The Agency believes 

it is not appropriate to consider 

revisions to the NPDWR for simazine at 

this time and has placed simazine in the 

emerging information/data gap category 

because of an impending re-evaluation 

of the Agency’s risk assessment for 

atrazine and the assessment for 

simazine is based on atrazine data. See 

section VI.7 (atrazine) for additional 

information. 


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58. Styrene 

a. Background. EPA published the 

current NPDWR for styrene on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG and 

an MCL of 0.1 mg/L. EPA based the 

MCLG on a reference dose of 0.2 mg/kgday and a cancer classification of C, 

possible human carcinogen. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to styrene. 

The revised health effects assessment 

will consider relevant studies on the 

toxicity of styrene, including its 

potential developmental and 

reproductive toxicity. The new health 

effects assessment was not completed by 

March 1, 2009, the review cutoff date for 

this notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

c. Review Result. Since the MCL for 

styrene is set at its MCLG and a 

reassessment of the health risks 

resulting from exposure to styrene is in 

progress, the Agency does not believe a 

revision to the NPDWR is appropriate at 

this time. 

59. 2,3,7,8-TCDD (Dioxin) 

a. Background. EPA published the 

current NPDWR for dioxin on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG of zero 

based on a cancer classification of B2, 

probable human carcinogen. The 

NPDWR also established an MCL of 

3×10¥8 mg/L, based on analytical 

feasibility. 

b. Technical Reviews. In 2003, the 

Agency prepared a draft human health 

reassessment for dioxin and its related 

compounds (USEPA, 2003c) that 

underwent external review by the 

National Academy of Science. In their 

peer review report (NAS, 2006), NAS 

recommended that EPA reevaluate its 

conclusions regarding the 

carcinogenicity of dioxin based on the 

criteria set out in the 2005 cancer 

guidelines; that EPA should consider 

developing more information on the 

noncancer effects of dioxin; and that 

EPA evaluate new dose-response data 

released by the NTP. The Agency is 

currently considering the NAS 

recommendations. The Agency does not 

expect any new health effects 

assessment to be completed in the time 

frame of the current Six-Year Review 

cycle (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

Although a health effects assessment 

is in process for dioxin, the existing 

MCLG is still zero and the current MCL 

is based on a PQL of 3×10¥8 mg/L. 

Therefore, EPA reviewed whether there 

is potential to revise the PQL. The PT 

data currently available for dioxin are 

not sufficient to evaluate the potential 

for PQL revision (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: Laboratory MRLs in the SixYear Review ICR dataset, and the MDL 

for the approved method for the 

detection of dioxin (Method 1613). 

While EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. The SixYear Review ICR dataset contains dioxin 

data for fewer than 2,500 systems, 

which is an insufficient sample size to 

derive an EQL based on MRL data. The 

MDL of the approved method is 1×10¥8

mg/L. Applying a multiplier of 5 would 

yield an EQL of 5×10¥8 mg/L. The 

result is slightly higher than the current 

PQL and, therefore, EPA did not 

estimate an EQL. Based on these varied 

and unrelated approaches/sources of 

information, EPA believes that a PQL 

reduction for dioxin is not appropriate 

at present. Since the MCL is constrained 

by the PQL, and the PQL is unchanged, 

EPA does not believe it is necessary to 

conduct an occurrence analysis at this 

time. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

dioxin is appropriate at this time 

because a reassessment of the health 

risks resulting from exposure to dioxin 

is in progress (USEPA, 2009b). 

Furthermore, a review of analytical 

feasibility did not identify a potential to 

revise the MCL, which is limited by 

feasibility. 

60. Tetrachloroethylene 

a. Background. EPA published the 

current NPDWR for tetrachloroethylene 

on January 30, 1991 (56 FR 3526 

(USEPA, 1991c)). The NPDWR 

established an MCLG of zero based on 

a cancer classification of B2, probable 

human carcinogen. The NPDWR also 

established an MCL of 0.005 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

tetrachloroethylene. The revised health 

effects assessment will consider relevant 

studies on the toxicity of 

tetrachloroethylene, including its 

potential developmental and 

reproductive toxicity. The new health 

effects assessment was not completed by 

March 1, 2009, the review cutoff date for 

this notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment and indicates that 

tetrachloroethylene is currently 

undergoing review by NAS. 

Although a risk assessment is in 

process for tetrachloroethylene, the 

existing MCLG is zero and the current 

MCL of 0.005 mg/L is based on the PQL. 

Therefore, EPA reviewed whether there 

is potential to revise the PQL. EPA 

reviewed PE data from the first Six-Year 

Review cycle and then analyzed more 

recent PT data to determine if the PQL 

could be revised (i.e., analytical 

feasibility). Passing rates for PE data 

available through late 1999 for 

tetrachloroethylene are above 95 percent 

at the lowest concentrations. However, 

the true concentrations were all higher 

than the current PQL of 0.005 mg/L. 

More recent PT data from late 1999 

through 2004, supplied by a PT 

provider, also show greater than 90 

percent passing rates for studies around 

the current PQL, including 13 with true 

values below the PQL. Because most of 

the laboratory passing rates from PE and 

PT studies exceeded the 75 percent 

criterion typically used to derive a PQL, 

including several with true values 

below the PQL, a lowering of the PQL 

for tetrachloroethylene might be 

possible. These results, however, are 

insufficient to recalculate a revised PQL 

for tetrachloroethylene because not 

enough data points are available below 

the current PQL to derive a value at the 

75 percent passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: Laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of tetrachloroethylene (Methods 502.2, 

524.2, and 551.1). While EPA prefers to 

use laboratory performance data to 

calculate the PQL, the MRL and MDL 

information can be valuable for this 

review to indicate whether it is possible 

to quantitate at levels below the current 

PQL. EPA also noted that the State of 

New Jersey uses a PQL of 0.001 mg/L, 

based on a 1987 study of laboratory 

performance at low concentrations that 

used criteria similar to those in the PT 

data (NJDWQI, 1987). The Six-Year 

Review ICR dataset contains MRL 

values for 138,348 samples. More than 

80 percent of these values are less than 


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or equal the modal MRL: 117,033 (85 

percent) equal the modal MRL of 0.0005 

mg/L and an additional 15,848 (11 

percent) are lower than 0.0005 mg/L. 

Therefore, EPA selected the modal MRL 

as the EQL (USEPA, 2009e). The MDLs 

of approved method are 0.00005, 

0.00014, and 0.000008 mg/L. Applying 

a multiplier of 10 would give a possible 

PQL range from 0.00008 to 0.0014 mg/ 

L, which contains the EQL (USEPA, 

2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is potential to lower 

the PQL for tetrachloroethylene. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of tetrachloroethylene at the 

EQL of 0.0005 mg/L and additional 

thresholds of 0.001 and 0.0025 mg/L. 

Table VI–24 shows the results of the 

occurrence and exposure analysis for 

the current MCL and these thresholds. 

The occurrence and exposure analysis 

shows that average concentrations 

exceed the current MCL for 23 to 25 out 

of 50,436 systems (0.046 to 0.050 

percent) serving approximately 630, 000 

to 1.1 million people (or 0.277 to 0.473 

percent of 227 million people). Note 

that these results are based on the subset 

of monitoring data provided in response 

to the Six-Year Review ICR and do not 

necessarily reflect MCL violations, 

which are based on annual average 

concentrations at entry points; SDWIS/ 

FED indicates 174 MCL violations for 

tetrachloroethylene between 1998 and 

2005, with annual violations ranging 

from 10 to 33 (USEPA, 2007g). Average 

concentrations at 412 to 519 of 50,436 

systems (0.817 to 1.029 percent), serving 

12.4 to 14.6 million people (or 5.466 to 

6.419 percent of 227 million people), 

exceed the lowest EQL of 0.0005 mg/L. 

While these systems are widely 

distributed and located in most of the 

States providing data, a few large 

systems (serving 500,000 or more 

people) account for almost half of the 

exposed population. 

TABLE VI–24—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING TETRACHLOROETHYLENE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold (Percentages based on 50,436 systems with tetrachloroethylene data in the Six-Year Review ICR occurrence 

dataset) 

Nondetect values = MRL1 Nondetect values = 1⁄2 MRL2 Nondetect values = 03

MCL (0.005 mg/L) ........................... 25 (0.050%) .................................. 23 (0.046%) .................................. 23 (0.046%) 

1/2 MCL (0.0025 mg/L) ................... 75 (0.149%) .................................. 71 (0.141%) .................................. 68 (0.135%) 

2xEQL (0.001 mg/L) ........................ 286 (0.568%) ................................ 251 (0.498%) ................................ 220 (0.437%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 519 (1.030%) ................................ 412 (0.818%) 

Corresponding population served (Percentages based on 227,009,000 people served by the systems with 

tetrachloroethylene data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL1 Nondetect values = 1⁄2 MRL2 Nondetect values = 03

MCL (0.005 mg/L) ........................... 1,074,000 (0.473%) ...................... 628,000 (0.277%) ......................... 628,000 (0.277%) 1⁄2 MCL (0.0025 mg/L) .................... 1,706,000 (0.752%) ...................... 1,692,000 (0.745%) ...................... 1,647,000 (0.726%) 

2xEQL (0.001 mg/L) ........................ 10,706,000 (4.716%) .................... 10,177,000 (4.483%) .................... 9,625,000 (4.240%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 14,572,000 (6.419%) .................... 12,408,000 (5.466%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that a revision to the MCL may 

provide a meaningful opportunity to 

improve the level of public health 

protection, EPA considered whether 

treatment feasibility is likely to pose any 

limitations if the MCL were lowered 

(USEPA, 2009g). The current BATs for 

tetrachloroethylene are packed tower 

aeration (PTA) and granular activated 

carbon (GAC). Small system compliance 

technologies (SSCTs) for 

tetrachloroethylene include GAC and 

several aeration technologies. EPA’s 

assessment shows that PTA and GAC 

are effective enough to achieve 

concentrations as low as the EQL. 

EPA is not currently able to assess the 

potential health benefits from a revised 

MCL for tetrachloroethylene, because 

the revised health effects assessment is 

not yet available. However, based on its 

B2 cancer classification (MCLG of zero) 

and the occurrence and exposure 

analysis at possible MCL values, the 

Agency believes that a revision to the 

MCL may provide a meaningful 

opportunity to reduce public health 

risks. 

c. Review Result. The Agency believes 

it is appropriate to revise the NPDWR 

for tetrachloroethylene although a 

health effects assessment is currently in 

progress. The existing MCLG is zero 

(based on the current B2 cancer 

classification) and the current MCL is 

based on a PQL (i.e., analytical 

feasibility) of 0.005 mg/L. The Agency’s 

review indicates that analytical 

feasibility could be as much as 10 times 

lower (∼ 0.0005 mg/L) and occurrence at 

this level appears to be relatively 

widespread. Hence, revisions to the 

tetrachloroethylene NPDWR may 

provide a meaningful opportunity for 

health risk reduction. If the updated 

health effects assessment is completed 

in time to consider for the regulatory 

revision of tetrachloroethylene, the 

Agency will consider this assessment in 

its evaluation of public health benefits 

associated with any revision. As 

discussed in Section VII, the Agency 

solicits public comment and/or relevant 

information that may inform the 

regulatory revision for 

tetrachloroethylene. EPA is also 

requesting that stakeholders provide 

information/data about the lowest level 

of quantitation (including the analytical 

method used) that laboratories can 

reliably and consistently achieve. 


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61. Thallium 

a. Background. EPA published the 

current NPDWR for thallium on July 17, 

1992 (57 FR 31776 (USEPA, 1992)). The 

NPDWR established an MCLG of 0.0005 

mg/L. EPA based the MCLG on a 

reference dose of 0.00007 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

The NPDWR also established an MCL of 

0.002 mg/L, based on analytical 

feasibility. 

b. Technical Reviews. EPA completed 

the risk reassessment for thallium in 

September of 2009 (USEPA, 2009k). 

Because the new health effects 

assessment was not completed by March 

1, 2009, the review cutoff date for this 

notice (USEPA, 2009b), the outcome of 

this assessment has not been included 

in the current review effort. EPA will 

consider the updated assessment in the 

next review cycle. 

The current MCL is based on a PQL 

of 0.002 mg/L. Therefore, EPA reviewed 

whether there is potential to revise the 

PQL. EPA reviewed PE data from the 

first Six-Year Review cycle and then 

analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

thallium are above 80 percent around 

the current PQL of 0.002 mg/L, 

including one study with a true 

concentration less than the current PQL. 

More recent PT data from late 1999 

through 2004, supplied by a PT 

provider, show passing rates at or above 

75 percent, but tending to fall below 80 

percent as the true concentration 

approaches the current PQL. No studies 

had true concentrations below the 

current PQL. Given the lack of data 

points below the current PQL and the 

low PT passing rates close to the PQL, 

a lowering of the PQL for thallium is not 

appropriate at this time (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of thallium (Methods 200.8 and 200.9). 

While EPA prefers to use laboratory 

performance data to calculate the PQL, 

the MRL and MDL information can be 

valuable for this review to indicate 

whether it is possible to quantitate at 

levels below the current PQL. The SixYear Review ICR dataset contains MRL 

values for 73,409 samples. Fewer than 

80 percent of these values are less than 

or equal the modal MRL: 46,273 (63 

percent) equal the modal MRL of 0.001 

mg/L and an additional 11,032 (15 

percent) are lower than 0.001 mg/L. 

Therefore, EPA did not set the EQL 

equal to the modal MRL (USEPA, 

2009e). The MDLs of approved methods 

range from 0.0003 to 0.0007 mg/L. 

Applying a multiplier of 10 would give 

a possible PQL range from 0.003 to 

0.007 mg/L. The result is higher than 

the current PQL and, therefore, EPA did 

not estimate an EQL (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is no potential to 

lower the PQL for thallium. Since the 

MCL is constrained by the PQL, and the 

PQL is unchanged, EPA does not believe 

it is necessary to conduct an occurrence 

analysis at this time. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

thallium is appropriate at this time 

because a reassessment of the health 

risks resulting from exposure to 

thallium was in progress (USEPA, 

2009k) and did not meet the March 1, 

2009 cutoff date for this review. 

Furthermore, a review of analytical 

feasibility did not identify a potential to 

revise the MCL, which is limited by 

feasibility. 

62. Toluene 

a. Background. EPA published the 

current NPDWR for toluene on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG and 

an MCL of 1 mg/L. EPA based the MCLG 

on a reference dose of 0.2 mg/kg-day 

and a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. In 2005, the 

Agency updated its health effects 

assessment of toluene (USEPA, 2005b). 

The change in this assessment could 

lead to a change in the MCLG. This 

assessment considered relevant studies 

on the toxicity of toluene including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 0.2 mg/kg-day to 0.08 mg/kg-day 

and concluded that there is inadequate 

information to assess the carcinogenic 

potential of toluene (USEPA, 2005b). 

Although there were no changes in the 

critical study or effect, there were 

changes in the toxicity database that 

increase concern for immunotoxicity 

and neurotoxicity via the oral exposure 

route and justified the higher 

uncertainty factor for the revised RfD 

(USEPA, 2005b). Based on the new IRIS 

assessment and RfD of 0.08 mg/kg-day, 

and assuming a 70-kg adult body weight 

and 2 liters water intake per day, the 

DWEL could be 2.8 mg/L. An RSC of 20 

percent results in a possible MCLG of 

0.6 mg/L. 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor for the 

possible MCLG decrease under 

consideration. EPA evaluated the results 

of the occurrence and exposure analyses 

for toluene to determine whether a 

revised MCLG/MCL would be likely to 

result in a meaningful opportunity to 

improve the level of public health 

protection (USEPA, 2009f). Table VI–25 

shows the results of the occurrence and 

exposure analysis for the current MCL 

and the possible MCLG set equal to 0.6 

mg/L based on the new health effects 

information. The occurrence and 

exposure analysis shows that average 

concentrations exceed the current MCL 

for one system out of 50,451 (0.002 

percent) serving approximately 500 

people (0.0002 percent of 227 million 

people). Note that these results are 

based on the subset of monitoring data 

provided in response to the Six-Year 

Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; SDWIS/FED indicates MCL 

violations for toluene at only one system 

in one year between 1998 and 2005 

(USEPA, 2007g). Average concentrations 

at two of 50,451 systems (0.004 percent), 

serving 800 people (or 0.0004 percent of 

227 million people), exceed the possible 

MCLG based on new health effects 

information (0.06 mg/L). 


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TABLE VI–25—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING TOLUENE THRESHOLDS

AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or health-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or health-based threshold (percentages based on 50,451 systems with toluene data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (1 mg/L) .................................. 1 (0.002%) .................................... 1 (0.002%) .................................... 1 (0.002%) 

Possible MCLG (0.6 mg/L) ............. 2 (0.004%) .................................... 2 (0.004%) .................................... 2 (0.004%) 

Corresponding population served (percentages based on 226,955,000 people served by the systems with 

toluene data in the Six-Year Review ICR occurrence dataset) 

Regulatory or health-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (1 mg/L) .................................. 500 (0.0002%) .............................. 500 (0.0002%) .............................. 500 (0.0002%) 

Possible MCLG (0.6 mg/L) ............. 800 (0.0004%) .............................. 800 (0.0004%) .............................. 800 (0.0004%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset 

2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

toluene, EPA does not believe a revision 

to the NPDWR for toluene is appropriate 

at this time. In making this decision, the 

Agency considered whether any 

possible revision to the NPDWR for 

toluene is likely to provide a meaningful 

opportunity for health risk reductions. 

Taking into consideration the low 

occurrence of this contaminant, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

63. Toxaphene 

a. Background. EPA published the 

current NPDWR for toxaphene on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.003 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the carcinogenicity of 

toxaphene as well as its potential 

developmental and reproductive 

toxicity. EPA has not identified any new 

information that indicates that it is 

appropriate to consider revisions to the 

cancer classification for toxaphene at 

this time (USEPA, 2009b). Because the 

MCLG remains at zero, the Agency 

believes that a further review of the 

health effects of toxaphene is not 

warranted at this time. 

The current MCL for toxaphene is 

based on a PQL of 0.003 mg/L. For the 

Six-Year Review, the Agency considered 

whether changes in the analytical 

feasibility of toxaphene might lead to a 

lower MCL. EPA reviewed PE data from 

the first Six-Year Review cycle and then 

analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

toxaphene are generally above 90 

percent around the current PQL of 0.003 

mg/L, including three studies with true 

values below the current PQL. All 

passing rates in the PE data exceeded 80 

percent. More recent PT data from late 

1999 through 2004, supplied by a PT 

provider, show greater than 80 percent 

passing rates for a majority of studies, 

but there are no studies with true values 

below the current PQL. There are two 

PT studies with passing rates equal to or 

below 75 percent, at true values well 

above the current PQL. Despite this 

variability, most of the laboratory 

passing rates from PE and PT studies 

exceeded the 75 percent criterion 

typically used to derive a PQL, 

including three with true values below 

the PQL. Therefore, a lowering of the 

PQL for toxaphene might be possible. 

These results, however, are insufficient 

to recalculate a revised PQL for 

toxaphene because not enough data 

points are available below the current 

PQL to derive a value at the 75 percent 

passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of toxaphene (Methods 505, 508.1, and 

525.2). While EPA prefers to use 

laboratory performance data to calculate 

the PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 54,529 

samples. More than 80 percent of these 

values are less than or equal the modal 

MRL: 36,763 (67 percent) equal the 

modal MRL of 0.001 mg/L and an 

additional 8,525 (16 percent) are lower 

than 0.001 mg/L. Therefore, EPA 

selected the modal MRL as the EQL 

(USEPA, 2009e). The MDLs of approved 

methods are 0.0017, 0.001, and 0.00013 

mg/L. Applying a multiplier of 10 

would give a possible PQL range from 

0.0013 to 0.017 mg/L, which is above 

the EQL, but includes values below the 

PQL (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there may be potential to 

lower the PQL for toxaphene. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of toxaphene at the EQL of 

0.001 mg/L and an additional threshold 

of 0.0015 mg/L (USEPA, 2009f). Table 

VI–26 shows the results of the 

occurrence and exposure analysis for 

the current MCL and these thresholds. 

The occurrence and exposure analysis 


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shows that average concentrations 

exceed the current MCL for three to four 

of 30,387 systems (0.010 to 0.013 

percent) serving 23,000 people (or 0.014 

percent of 160 million people). Note 

that these results are based on the subset 

of monitoring data provided in response 

to the Six-Year Review ICR and do not 

necessarily reflect MCL violations, 

which are based on annual average 

concentrations at entry points; SDWIS/ 

FED indicates three MCL violations for 

toxaphene between 1998 and 2005 

(USEPA, 2007g). Average concentrations 

at five of 30,387 systems (0.016 percent), 

serving 23,000 people (or 0.015 percent 

of 160 million people), exceed the EQL 

of 0.001 mg/L. 

TABLE VI–26—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING TOXAPHENE THRESHOLDS

AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold (percentages based on 30,387 systems with toxaphene data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.003 mg/L) ........................... 4 (0.013%) .................................... 3 (0.010%) .................................... 3 (0.010%) 1⁄2 MCL (0.0015 mg/L) .................... 5 (0.016%) .................................... 5 (0.016%) .................................... 5 (0.016%) 

EQL (0.001 mg/L) ........................... not applicable ............................... 5 (0.016%) .................................... 5 (0.016%) 

Corresponding population served (percentages based on 160,012,000 people served by the systems with 

toxaphene data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.003 mg/L) ........................... 23,000 (0.014%) ........................... 23,000 (0.014%) ........................... 23,000 (0.014%) 1⁄2 MCL (0.0015 mg/L) .................... 23,000 (0.014%) ........................... 23,000 (0.014%) ........................... 23,000 (0.014%) 

EQL (0.001 mg/L) ........................... not applicable ............................... 23,000 (0.014%) ........................... 23,000 (0.014%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.001 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate 

of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 

toxaphene is appropriate at this time. 

The occurrence and exposure analysis 

based on possible changes in analytical 

feasibility indicates that any revision to 

the MCL is unlikely to provide a 

meaningful opportunity to improve 

public health protection. Taking into 

consideration the low occurrence of this 

contaminant, EPA has decided that any 

revision to the NPDWR would be a low 

priority activity for the Agency, and, 

thus, is not appropriate to revise at this 

time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

64. 2,4,5-TP (Silvex; 2,4,5- 

Trichlorophenoxypropionic Acid) 

a. Background. EPA published the 

current NPDWR for 2,4,5-TP on January 

30, 1991 (56 FR 3526 (USEPA, 1991c)). 

The NPDWR established an MCLG and 

an MCL of 0.05 mg/L. EPA based the 

MCLG on a reference dose of 0.008 mg/ 

kg-day and a cancer classification of D, 

not classifiable as to human 

carcinogenicity. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the toxicology of 2,4,5- 

TP, including its potential 

developmental and reproductive 

toxicity. The literature search did not 

identify any studies that warrant a 

review of the RfD or the cancer 

classification (USEPA, 2009b). 

A review of analytical or treatment 

feasibility is not necessary for 2,4,5-TP 

because changes to the MCLG are not 

warranted at this time and the current 

MCL is set at the MCLG. Since EPA did 

not identify a health or technology basis 

for revising the 2,4,5-TP NPDWR, the 

Agency did not conduct a detailed 

occurrence and exposure analysis. 

c. Review Result. EPA’s review shows 

that there are no data supporting a 

change to the 2,4,5-TP NPDWR. As a 

result, a revision to the NPDWR would 

not be appropriate at this time. 

65. 1,2,4-Trichlorobenzene 

a. Background. EPA published the 

current NPDWR for 1,2,4- 

trichlorobenzene on July 17, 1992 (57 

FR 31776 (USEPA, 1992)). The NPDWR 

established an MCLG and an MCL of 

0.07 mg/L. EPA based the MCLG on a 

reference dose of 0.01 mg/kg-day and a 

cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. The health 

effects technical review identified 

information regarding the 

carcinogenicity of 1,2,4- 

trichlorobenzene, as well as its 

noncancer effects, that may indicate the 

need to update the Agency’s health 

effects assessment (USEPA, 2009b). Two 

chronic carcinogenicity studies of 1,2,4- 

trichlorobenzene, one in mice (Moore, 

1994a) and one in rats (Moore, 1994b), 

reported liver effects in both mice and 

rats, as well as kidney effects in rats. 

Mice appeared more sensitive than rats 

for noncancer effects, and mice also 

demonstrated a significant treatmentrelated increase in the incidence of 

hepatocellular carcinomas. No increased 

incidence of any tumor type was 

observed in rats. These health effect 

data could have implications for the 

1,2,4-trichlorobenzene MCLG because 

they identify effect levels for noncancer 

effects in the liver and kidney, as well 

as evidence of carcinogenicity in mice. 


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In light of this information, EPA 

considers 1,2,4-trichlorobenzene as a 

potential candidate for a new health 

effects assessment. The Agency solicits 

general feedback on its plans to reassess 

health risks resulting from exposure to 

1,2,4-trichlorobenzene. The Agency also 

welcomes any scientific information 

related to 1,2,4-trichlorobenzene health 

risks from the public. Because EPA 

considers 1,2,4-trichlorobenzene as a 

candidate for a new assessment, EPA 

does not believe it is appropriate to 

consider revisions to the MCLG (as well 

as the MCL) at this time. 

A review of analytical or treatment 

feasibility is not necessary for 1,2,4- 

trichlorobenzene because changes to the 

MCLG are not warranted at this time 

and the current MCL is set at the MCLG. 

Since EPA did not identify a health or 

technology basis for revising the 1,2,4- 

trichlorobenzene NPDWR, the Agency 

did not conduct a detailed occurrence 

and exposure analysis. 

c. Review Result. The Agency is 

considering whether to initiate a new 

health assessment for 1,2,4- 

trichlorobenzene and therefore does not 

believe a revision to the NPDWR is 

appropriate at this time. 

66. 1,1,1-Trichloroethane 

a. Background. EPA published the 

current NPDWR for 1,1,1- 

trichloroethane on July 8, 1987 (52 FR 

25690 (USEPA, 1987)). The NPDWR 

established an MCLG and an MCL of 

0.20 mg/L. EPA based the MCLG on a 

reference dose of 0.035 mg/kg-day and 

a cancer classification of D, not 

classifiable as to human carcinogenicity. 

b. Technical Reviews. In 2007, the 

Agency updated its health effects 

assessment of 1,1,1-trichloroethane 

(USEPA, 2007d). The Agency identified 

a change in this assessment that could 

lead to a change in the MCLG. This 

assessment considered relevant studies 

on the toxicity of 1,1,1-trichloroethane 

including developmental and 

reproductive toxicity. The assessment 

revised the RfD from 0.035 mg/kg-day to 

2 mg/kg-day and concluded that there is 

inadequate information to assess the 

carcinogenic potential of 1,1,1- 

trichloroethane (USEPA, 2007d). Based 

on the new IRIS assessment and RfD of 

2 mg/kg-day, and assuming a 70-kg 

adult body weight and 2 liters water 

intake per day, the DWEL could be 70 

mg/L. An RSC of 20 percent results in 

a possible MCLG of 14 mg/L (USEPA, 

2009b). 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor if EPA 

were to raise the MCLG. EPA evaluated 

the results of the occurrence and 

exposure analyses for 1,1,1- 

trichloroethane to determine whether a 

revised MCLG/MCL would be likely to 

result in a meaningful opportunity to 

achieve cost savings for PWSs and their 

customers while maintaining, or 

improving, the level of public health 

protection (USEPA, 2009f). Although 

the Agency obtained and evaluated the 

finished water occurrence data for 1,1,1- 

trichloroethane, its usefulness is limited 

for determining potential cost savings to 

PWSs and their customers because the 

Agency does not know which systems 

are treating for this contaminant. As an 

alternative, the Agency evaluated 

available data on source water quality 

and conducted a qualitative assessment 

of treatment cost savings. 

Table VI–27 provides summary data 

for contaminant occurrence based on 

maximum sample values for the 

locations included in the STORET and 

NAWQA data. Although the degree to 

which these occurrence rates represent 

national drinking water source 

occurrence is uncertain, the information 

shows no to low occurrence at threshold 

levels of interest. This information 

indicates that any resulting NPDWR 

change would affect systems that rely on 

source water at none of the NAWQA 

locations and at less than 0.1 percent of 

the STORET locations. 

TABLE VI–27—AMBIENT WATER QUALITY MONITORING OCCURRENCE SUMMARY FOR 1,1,1-TRICHLOROETHANE

Maximum concentration 

Number of locations 

(% of locations) 

STORET 1 NAWQA2

Total ................................................................................................................................................................... 3,429 (100.0%) 5,788 (100.0%) 

Nondetect ........................................................................................................................................................... 2,304 (67.2%) ... 5,290 (91.4%) 

Detected ............................................................................................................................................................. 1,125 (32.8%) ... 498 (8.6%) 

Exceeds current MCLG of 0.2 mg/L .................................................................................................................. 5 (0.1%) ............ 0 (0.0%) 

Exceeds alternative value of 14 mg/L ............................................................................................................... 0 (0.0%) ............ 0 (0.0%) 

1STORET database 2002–2008. 

2 NAWQA database 1992–2008. 

Source: USEPA, 2009d. 

The BATs and small system 

compliance technologies for 1,1,1- 

trichloroethane have other beneficial 

effects, e.g., reduction of other cooccurring contaminants, precursors for 

DBPs, or other common impurities. 

Therefore, if EPA were to consider a 

higher level, the Agency does not know 

how many PWSs that are currently 

treating to comply with the existing 

MCL of 0.2 mg/L would be likely to 

discontinue treatment that is already in 

place (USEPA, 2009d). Also, the Agency 

does not know to what extent affected 

systems might be able to reduce costs 

given that capital costs are not 

recoverable. However, the Agency 

recognizes that there may be 

opportunities to achieve operational 

cost savings if these systems are able to 

re-optimize current treatment. 

Given these considerations, the 

Agency believes that any resulting 

revision is not likely to provide a 

meaningful opportunity for cost savings. 

In view of this, any revision would be 

a low priority activity and not 

appropriate at this time. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

1,1,1-trichloroethane, EPA does not 

believe a revision to the NPDWR for 

1,1,1-trichloroethane is appropriate at 

this time. In making this decision, the 

Agency considered whether any 

possible revision to the NPDWR for 

1,1,1-trichloroethane is likely to provide 

a meaningful opportunity for cost 

savings to public water systems and 

their customers. Taking into 

consideration the low occurrence of this 

contaminant in source waters, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 


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• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

67. 1,1,2-Trichloroethane 

a. Background. EPA published the 

current NPDWR for 1,1,2- 

trichloroethane on July 17, 1992 (57 FR 

31776 (USEPA, 1992)). The NPDWR 

established an MCLG of 0.003 mg/L. 

EPA based the MCLG on a reference 

dose of 0.004 mg/kg-day and a cancer 

classification of C, possible human 

carcinogen. The NPDWR also 

established an MCL of 0.005 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the toxicology of 1,1,2- 

trichloroethane, including its potential 

developmental and reproductive 

toxicity. The literature search did not 

identify any studies that warrant a 

review of the RfD or the cancer 

classification (USEPA, 2009b). 

The current MCL for 1,1,2- 

trichloroethane is based on a PQL of 

0.005 mg/L. For the Six-Year Review, 

the Agency considered whether changes 

in the analytical feasibility of 1,1,2- 

trichloroethane might lead to a lower 

MCL. EPA reviewed PE data from the 

first Six-Year Review cycle and then 

analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

1,1,2-trichloroethane are above 95 

percent near the current PQL of 0.005 

mg/L, but there were no PE studies with 

true values below the current PQL. More 

recent PT data from late 1999 through 

2004, supplied by a PT provider, show 

greater than 90 percent passing rates 

around the current PQL, including 

twelve studies with true values below 

the PQL. Because most of the laboratory 

passing rates from PT studies— 

including several with true 

concentrations below the PQL— 

exceeded the 75 percent criterion 

typically used to derive a PQL, a 

lowering of the PQL for 1,1,2- 

trichloroethane might be possible. These 

results, however, are insufficient to 

recalculate a revised PQL for 1,1,2- 

trichloroethane because not enough data 

points are available below the current 

PQL to derive a value at the 75 percent 

passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether 

they indicate any potential to quantitate 

at levels as low as the current MCLG: 

laboratory MRLs in the Six-Year Review 

ICR dataset, and the MDLs for approved 

methods for the detection of 1,1,2- 

trichloroethane (Methods 502.2 and 

524.2). While EPA prefers to use 

laboratory performance data to calculate 

the PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 139,672 

samples. Of these, 117,788 (84 percent) 

equal the modal MRL of 0.0005 mg/L. 

An additional 17,142 (12 percent) are 

lower than 0.0005 mg/L. Because more 

than 80 percent of the of MRLs are equal 

to or less than the current MCLG of 

0.003 mg/L, EPA selected that value as 

the minimum threshold for the 

occurrence and exposure analysis 

(USEPA, 2009e). The MDLs of approved 

methods range from 0.00004 to 0.0001 

mg/L. Applying a multiplier of 10 

would give a possible PQL range from 

0.0004 to 0.001 mg/L, which is below 

the current MCLG (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is potential to lower 

the PQL for 1,1,2-trichloroethane. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of 1,1,2-trichloroethane at 

the current MCLG of 0.003 mg/L 

(USEPA, 2009f). Table VI–28 shows the 

results of the occurrence and exposure 

analysis for the current MCL and the 

current MCLG of 0.003 mg/L. The 

occurrence and exposure analysis shows 

that average concentrations do not 

exceed the current MCL for any system 

in the analysis. Note that these results 

are based on the subset of monitoring 

data provided in response to the SixYear Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; SDWIS/FED indicates six 

MCL violations for 1,1,2-trichloroethane 

between 1998 and 2005 (USEPA, 

2007g). The average concentration at 

one out of 50,195 systems (0.002 

percent), serving approximately 700 

people (or 0.0003 percent of 227 million 

people), exceeds the current MCLG of 

0.003 mg/L. 

TABLE VI–28—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING 1,1,2-TRICHLOROETHANE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or health-based threshold 

Systems with mean concentrations that are greater than the regulatory or health-based threshold 

(percentages based on 50,195 systems with 1,1,2-trichloroethane data in the Six-Year Review ICR 

occurrence dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect 

values = 0 3

MCL (0.005 mg/L) ..................................... 0 (0.000%) ................................................ 0 (0.000%) ................................................ 0 (0.000%) 

Current MCLG (0.003 mg/L) ..................... 1 (0.002%) ................................................ 1 (0.002%) ................................................ 1 (0.002%) 

Corresponding population served (Percentages based on 226,852,000 people served by the systems 

with 1,1,2-trichloroethane data in the Six-Year Review ICR occurrence dataset) 

Regulatory or health-based threshold Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect 

values = 0 3

MCL (0.005 mg/L) ..................................... 0 (0.000%) ................................................ 0 (0.000%) ................................................ 0 (0.000%) 

Current MCLG (0.003 mg/L) ..................... 700 (0.0003%) .......................................... 700 (0.0003%) .......................................... 700 

(0.0003%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. 

2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 


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Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 

1,1,2-trichloroethane is appropriate at 

this time. The occurrence and exposure 

analysis based on possible changes in 

analytical feasibility indicates that any 

revision to the MCL is unlikely to 

provide a meaningful opportunity to 

improve public health protection. 

Taking into consideration the low 

occurrence of this contaminant, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

68. Trichloroethylene 

a. Background. EPA published the 

current NPDWR for trichloroethylene on 

July 8, 1987 (52 FR 25690 (USEPA, 

1987)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of B2, probable human 

carcinogen. The NPDWR also 

established an MCL of 0.005 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

trichloroethylene. The revised health 

effects assessment will consider relevant 

studies on the toxicity of 

trichloroethylene, including its 

potential developmental and 

reproductive toxicity. The new health 

effects assessment was not completed by 

March 1, 2009, the review cutoff date for 

this notice (USEPA, 2009b). The IRIS 

Substance Assessment Tracking System 

Web site (http://cfpub.epa.gov/iristrac/ 

index.cfm) has the most up-to-date 

information on the status of the health 

effects assessment. 

Although a risk assessment is in 

process for trichloroethylene, the 

existing MCLG is zero and the current 

MCL of 0.005 mg/L is based on the PQL. 

Therefore, EPA reviewed whether there 

is potential to revise the PQL. EPA 

reviewed PE data from the first Six-Year 

Review cycle and then analyzed more 

recent PT data to determine if the PQL 

can be revised (i.e., analytical 

feasibility). Passing rates for PE data 

available through late 1999 for 

trichloroethylene are above 95 percent 

at the lowest concentrations. However, 

the true concentrations were all higher 

than the current PQL of 0.005 mg/L. 

More recent PT data from 1999 to 2004, 

supplied by a PT provider, also show 

greater than 95 percent passing rates for 

studies around the current PQL, 

including 6 with true values below the 

PQL. Because most of the laboratory 

passing rates from PE and PT studies 

exceeded the 75 percent criterion 

typically used to derive a PQL, 

including several with true values 

below the PQL, a lowering of the PQL 

for trichloroethylene might be possible. 

These results, however, are insufficient 

to recalculate a revised PQL for 

trichloroethylene because not enough 

data points are available below the 

current PQL to derive a value at the 75 

percent passing rate (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of trichloroethylene (Methods 502.2, 

524.2, and 551.1). While EPA prefers to 

use laboratory performance data to 

calculate the PQL, the MRL and MDL 

information can be valuable for this 

review to indicate whether it is possible 

to quantitate at levels below the current 

PQL. EPA also noted that the State of 

New Jersey uses a PQL of 0.001 mg/L, 

based on a 1987 study of laboratory 

performance at low concentrations that 

used criteria similar to those in the PT 

data (NJDWQI, 1987). The Six-Year 

Review ICR dataset contains MRLs for 

138,439 samples. More than 80 percent 

of these values are less than or equal the 

modal MRL: 118,193 (85 percent) equal 

the modal MRL of 0.0005 mg/L and an 

additional 17,057 (12 percent) are lower 

than 0.0005 mg/L. Therefore, EPA 

selected the modal MRL as the EQL 

(USEPA, 2009e). The MDLs of approved 

methods range are 0.00006, 0.00019, 

and 0.000042 mg/L. Applying a 

multiplier of 10 would give a possible 

PQL range from 0.00042 to 0.0019 mg/ 

L, which contains the EQL (USEPA, 

2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there is potential to lower 

the PQL for trichloroethylene. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of trichloroethylene at the 

EQL of 0.0005 mg/L and additional 

thresholds of 0.0010 and 0.0025 mg/L 

(USEPA, 2009f). Table VI–29 shows the 

results of the occurrence and exposure 

analysis for the current MCL and these 

thresholds. The occurrence and 

exposure analysis shows that average 

concentrations exceed the current MCL 

for 25 out of 50,432 systems (0.050 

percent) serving approximately 410,000 

people (or 0.181 percent of 227 million 

people). Note that these results are 

based on the subset of monitoring data 

provided in response to the Six-Year 

Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; SDWIS/FED indicates 191 

MCL violations for trichloroethylene 

between 1998 and 2005 (USEPA, 

2007g), with annual violations ranging 

from 12 to 31. Average concentrations at 

310 to 388 of 50,432 systems (0.615 to 

0.769 percent), serving approximately 

12.0 to 13.0 million people (or 5.237 to 

5.670 percent of 227 million people), 

exceed the EQL of 0.0005 mg/L. While 

these systems are widely distributed 

and located in most of the States 

providing data, a few large systems 

(serving 500,000 or more people) 

account for almost half of the exposed 

population. 


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25After the December 7, 2000 final regulation, 

two trade associations and several municipal water 

systems challenged EPA’s standard for uranium by 

claiming that the Agency did not use the best 

available science when finalizing the standard. In 

February of 2003, the DC Circuit Court of Appeals 

upheld EPA’s regulation for uranium (as well as 

combined radiums, and beta particle and photon 

emitters). 

TABLE VI–29—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING TRICHLOROETHYLENE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold (percentages based on 50,432 systems with trichloroethylene data in the Six-Year Review ICR occurrence 

dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.005 mg/L) ........................... 25 (0.050%) .................................. 25 (0.050%) .................................. 25 (0.050%) 1⁄2 MCL (0.0025 mg/L) .................... 70 (0.139%) .................................. 68 (0.135%) .................................. 64 (0.127%) 

2xEQL (0.001 mg/L) ........................ 239 (0.474%) ................................ 208 (0.412%) ................................ 182 (0.361%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 388 (0.769%) ................................ 310 (0.615%) 

Corresponding population served (percentages based on 226,908,000 people served by the systems with 

trichloroethylene data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect Values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.005 mg/L) ........................... 410,000 (0.181%) ......................... 410,000 (0.181%) ......................... 410,000 (0.181%) 1⁄2 MCL (0.0025 mg/L) .................... 4,765,000 (2.100%) ...................... 4,691,000 (2.067%) ...................... 4,598,000 (2.026%) 

2xEQL (0.001 mg/L) ........................ 10,367,000 (4.569%) .................... 8,282,000 (3.650%) ...................... 7,399,000 (3.261%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 12,866,000 (5.670%) .................... 11,884,000 (5.237%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that a revision to the MCL may 

provide a meaningful opportunity to 

improve the level of public health 

protection, EPA considered whether 

treatment feasibility is likely to pose any 

limitations if the MCL were lowered 

(USEPA, 2009g). The current BATs for 

trichloroethylene are packed tower 

aeration (PTA) and granular activated 

carbon (GAC). Small system compliance 

technologies for trichloroethylene 

include GAC and several aeration 

technologies. EPA’s assessment shows 

that PTA and GAC are effective enough 

to achieve concentrations as low as the 

EQL. 

EPA is not currently able to assess the 

potential health benefits from a revised 

MCL for trichloroethylene, because the 

revised health effects assessment is not 

yet available. However, based on its B2 

cancer classification (MCLG of zero) and 

the occurrence and exposure analysis at 

possible MCL values, the Agency 

believes that a revision to the MCL may 

provide a meaningful opportunity to 

reduce public health risks. 

c. Review Result. The Agency believes 

it is appropriate to revise the NPDWR 

for trichloroethylene although a health 

effects assessment is currently in 

progress. The existing MCLG is zero 

(based on the current B2 cancer 

classification) and the current MCL is 

based on a PQL (i.e., analytical 

feasibility) of 0.005 mg/L. The Agency’s 

review indicates that analytical 

feasibility could be as much as 10 times 

lower (∼ 0.0005 mg/L) and occurrence at 

this level appears to be relatively 

widespread. Hence, revisions to the 

trichloroethylene NPDWR may provide 

a meaningful opportunity for health risk 

reduction. If the updated health effects 

assessment is completed in time to 

consider for the regulatory revision of 

trichloroethylene, the Agency will 

consider this assessment in its 

evaluation of public health benefits 

associated with any revision. As 

discussed in Section VII, the Agency 

solicits public comment and/or relevant 

information that may inform the 

regulatory revision for 

trichloroethylene. EPA is also 

requesting that stakeholders provide 

information/data about the lowest level 

of quantitation (including the analytical 

method used) that laboratories can 

reliably and consistently achieve. 

69. Uranium 

a. Background. EPA published the 

current NPDWR for uranium on 

December 7, 2000 (65 FR 76708 

(USEPA, 2000c)). The NPDWR 

established an MCLG of zero based on 

a cancer classification of A, known 

human carcinogen. As noted in the 

December 2000 FR, uranium has also 

been identified as a nephrotoxic metal 

(kidney toxicant) and EPA derived a 

drinking water equivalent level of 20 μg/ 

L as a noncancer health endpoint for 

kidney toxicity. The NPDWR also 

established an MCL of 30 μg/L, which 

is higher than the feasible level of 20 μg/ 

L and the level associated with kidney 

toxicity. In December 2000, EPA 

exercised its discretionary authority to 

set an MCL at a level higher than 

feasible (SDWA Section 1412(b)(6)), 

based on the finding that ‘‘benefits do 

not justify the costs at the feasible level 

(20 μg/L) and that the net benefits are 

maximized at a level (30 μg/L) that is 

still protective of health with an 

adequate margin of safety’’ (65 FR 76708 

(USEPA, 2000c)) 25. 

b. Technical Reviews. EPA has 

initiated a reassessment of the health 

risks resulting from exposure to 

uranium. The revised health effects 

assessment will consider relevant 

studies on the toxicity of uranium, 

including its potential developmental 

and reproductive toxicity. The new 

health effects assessment was not 

completed by March 1, 2009, the review 

cutoff date for this notice (USEPA, 

2009b). The IRIS Substance Assessment 

Tracking System Web site (http:// 

cfpub.epa.gov/iristrac/index.cfm) has 

the most up-to-date information on the 

status of the health effects assessment. 

c. Review Result. The Agency does not 

believe a revision to the NPDWR for 

uranium is appropriate at this time 

because a reassessment of the health 

risks resulting from exposure to 


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uranium is ongoing (USEPA, 2009b). As 

noted previously, the uranium MCL is 

based on the SDWA cost benefit 

provision (Section 1412(b)(6)) and the 

health effects assessment is important 

for reviewing the benefits associated 

with the basis of the MCL. 

70. Vinyl Chloride 

a. Background. EPA published the 

current NPDWR for vinyl chloride on 

July 8, 1987 (52 FR 25690 (USEPA, 

1987)). The NPDWR established an 

MCLG of zero based on a cancer 

classification of A, known human 

carcinogen. The NPDWR also 

established an MCL of 0.002 mg/L, 

based on analytical feasibility. 

b. Technical Reviews. As part of the 

Six-Year Review process, EPA 

conducted a literature search for 

relevant data on the carcinogenicity of 

vinyl chloride as well as its potential 

developmental and reproductive 

toxicity. EPA has not identified any new 

information that indicates that it is 

appropriate to consider revisions to the 

cancer classification for vinyl chloride 

at this time (USEPA, 2009b). Because 

the MCLG remains at zero, the Agency 

believes that a further review of the 

health effects of vinyl chloride is not 

warranted at this time. 

The current MCL for vinyl chloride is 

based on a PQL of 0.002 mg/L. For the 

Six-Year Review, the Agency considered 

whether changes in the analytical 

feasibility of vinyl chloride might lead 

to a lower MCL. EPA reviewed PE data 

from the first Six-Year Review cycle and 

then analyzed more recent PT data to 

determine if the PQL can be revised (i.e., 

analytical feasibility). Passing rates for 

PE data available through late 1999 for 

vinyl chloride are generally in the 75 to 

80 percent range near the current PQL 

of 0.002 mg/L, but there were no results 

for PE studies with true values below 

the current PQL. More recent PT data 

from late 1999 through 2004, supplied 

by a PT provider, also show greater than 

80 percent passing rates for studies 

around the current PQL, including two 

studies with true values below the PQL. 

Despite the limited data below the PQL, 

most of the laboratory passing rates from 

PE and PT studies—including two with 

true concentrations below the PQL— 

exceeded the 75 percent criterion 

usually used to derive a PQL. Therefore, 

a lowering of the PQL for vinyl chloride 

might be possible (USEPA, 2009c). 

EPA evaluated two alternative sources 

of information to determine whether an 

EQL below the current PQL could be 

estimated: laboratory MRLs in the SixYear Review ICR dataset, and the MDLs 

for approved methods for the detection 

of vinyl chloride (Methods 502.2 and 

524.2). While EPA prefers to use 

laboratory performance data to calculate 

the PQL, the MRL and MDL information 

can be valuable for this review to 

indicate whether it is possible to 

quantitate at levels below the current 

PQL. The Six-Year Review ICR dataset 

contains MRL values for 139,494 

samples. More than 80 percent of these 

values are less than or equal the modal 

MRL: 105,410 (76 percent) equal the 

modal MRL of 0.0005 mg/L and an 

additional 25,723 (18 percent) are lower 

than 0.0005 mg/L. Therefore, EPA 

selected the modal MRL as the EQL 

(USEPA, 2009e). The MDLs of approved 

methods range from 0.00017 to 0.00018 

mg/L. Applying a multiplier of 10 

would give a possible PQL range from 

0.0017 to 0.0018 mg/L, which is higher 

than the EQL, but below the current 

PQL (USEPA, 2009e). 

Based on these varied and unrelated 

approaches/sources of information, EPA 

believes that there may be potential to 

lower the PQL for vinyl chloride. To 

determine whether any MCL revision is 

likely to provide a meaningful 

opportunity to improve public health 

protection, EPA evaluated the 

occurrence of vinyl chloride at the EQL 

of 0.0005 mg/L and an additional 

threshold of 0.001 mg/L (USEPA, 

2009f). Table VI–30 shows the results of 

the occurrence and exposure analysis 

for the current MCL and these 

thresholds. The occurrence and 

exposure analysis shows that average 

concentrations exceed the current MCL 

for 8 to 11 of 50,411 systems (0.016 to 

0.022 percent) serving fewer than 14,000 

people (or 0.003 to 0.006 percent of 226 

million people). Note that these results 

are based on the subset of monitoring 

data provided in response to the SixYear Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; SDWIS/FED indicates 25 

MCL violations for vinyl chloride 

between 1998 and 2005 (USEPA, 

2007g). Average concentrations at 32 to 

49 of 50,411 systems (0.063 to 0.097 

percent), serving 483,000 to 766,000 

people (or 0.213 to 0.338 percent of 226 

million people), exceed the EQL of 

0.0005 mg/L. 

TABLE VI–30—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING VINYL CHLORIDE

THRESHOLDS AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or feasibility-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or feasibility-based threshold (percentages based on 50,411 systems with vinyl chloride data in the Six-Year Review ICR occurrence 

dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.002 mg/L) ........................... 11 (0.022%) .................................. 10 (0.020%) .................................. 8 (0.016%) 1⁄2 MCL (0.001 mg/L) ...................... 21 (0.042%) .................................. 18 (0.037%) .................................. 15 (0.030%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 49 (0.097%) .................................. 32 (0.063%) 

Corresponding Population Served (Percentages based on 226,464,000 people served by the systems with 

vinyl chloride data in the Six-Year Review ICR occurrence dataset) 

Regulatory or feasibility-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (0.002 mg/L) ........................... 14,000 (0.006%) ........................... 12,000 (0.005%) ........................... 6,000 (0.003%) 1⁄2 MCL (0.001 mg/L) ...................... 56,000 (0.025%) ........................... 23,000 (0.010%) ........................... 18,000 (0.008%) 

EQL (0.0005 mg/L) ......................... not applicable ............................... 766,000 (0.338%) ......................... 483,000 (0.213%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset. Results are not reported at the 

EQL of 0.0005 mg/L because this is the modal MRL and setting a majority of the results equal to this value results in an upwardly biased estimate of the number of systems with mean concentrations that exceed this value. 2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 


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Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

a possibly lower PQL (and therefore a 

possibly lower MCL), EPA does not 

believe a revision to the NPDWR for 

vinyl chloride is appropriate at this 

time. The occurrence and exposure 

analysis based on possible changes in 

analytical feasibility indicates that any 

revision to the MCL is unlikely to 

provide a meaningful opportunity to 

improve public health protection. 

Taking into consideration the low 

occurrence of this contaminant, EPA has 

decided that any revision to the NPDWR 

would be a low priority activity for the 

Agency, and, thus, is not appropriate to 

revise at this time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

71. Xylenes (Total) 

a. Background. EPA published the 

current NPDWR for total xylenes on 

January 30, 1991 (56 FR 3526 (USEPA, 

1991c)). The NPDWR established an 

MCLG and an MCL of 10 mg/L. EPA 

based the MCLG on a reference dose of 

2 mg/kg-day and a cancer classification 

of D, not classifiable as to human 

carcinogenicity. 

b. Technical Reviews. In 2003, the 

Agency updated its health effects 

assessment of xylenes (USEPA, 2003d). 

The change in this assessment could 

lead to a change in the MCLG. This 

assessment considered relevant studies 

on the toxicity of xylenes including 

developmental and reproductive 

toxicity. The assessment revised the RfD 

from 2 mg/kg-day to 0.2 mg/kg-day and 

concluded that there is inadequate 

information to assess the carcinogenic 

potential of xylenes (USEPA, 2003d). 

Based on the new IRIS assessment and 

RfD of 0.2 mg/kg-day, and assuming a 

70-kg adult body weight and 2 liters 

water intake per day, the DWEL could 

be 7 mg/L. An RSC of 20 percent results 

in a possible MCLG of 1 mg/L. 

Analytical feasibility does not pose 

any limitations for the current MCL and 

would not be a limiting factor for the 

possible MCLG decrease under 

consideration. EPA evaluated the results 

of the occurrence and exposure analyses 

for total xylenes to determine whether a 

revised MCLG/MCL would be likely to 

result in a meaningful opportunity to 

improve the level of public health 

protection (USEPA, 2009f). Table VI–31 

shows the results of the occurrence and 

exposure analysis for the current MCL 

and the possible MCLG set equal to 1 

mg/L based on the new health effects 

information. The occurrence and 

exposure analysis shows that average 

concentrations do not exceed the 

current MCL for any system in the 

analysis. Note that these results are 

based on the subset of monitoring data 

provided in response to the Six-Year 

Review ICR and do not necessarily 

reflect MCL violations, which are based 

on annual average concentrations at 

entry points; SDWIS/FED indicates two 

MCL violations for xylene between 1998 

and 2005 (USEPA, 2007g). The 

occurrence and exposure analysis shows 

that average concentrations do not 

exceed the possible MCLG based on 

new health effects information (1 mg/L). 

TABLE VI–31—NUMBER AND PERCENT OF SYSTEMS WITH MEAN CONCENTRATIONS EXCEEDING XYLENE THRESHOLDS

AND CORRESPONDING ESTIMATES OF POPULATION SERVED

Regulatory or health-based 

threshold 

Systems with mean concentrations that are greater than the regulatory or health-based threshold (percentages based on 47,698 systems with xylene data in the Six-Year Review ICR occurrence dataset) 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (10 mg/L) ................................ 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Possible MCLG (1 mg/L) ................ 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Corresponding population Served (percentages based on 218,072,000 people served by the systems with 

xylene data in the Six-Year Review ICR occurrence dataset) 

Regulatory or health-based 

threshold 

Nondetect values = MRL 1 Nondetect values = 1⁄2 MRL 2 Nondetect values = 0 3

MCL (10 mg/L) ................................ 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

Possible MCLG (1 mg/L) ................ 0 (0.000%) .................................... 0 (0.000%) .................................... 0 (0.000%) 

1 Results are based on setting all nondetect results equal to MRL values in the Six-Year Review ICR dataset 

2 Results are based on setting all nondetect results equal to 1⁄2 MRL values in the Six-Year Review ICR dataset. 3 Results are based on setting all nondetect results equal to zero. 

Source: USEPA, 2009f. 

Since the occurrence analysis 

indicates that any revision to the MCL 

is unlikely to provide a meaningful 

opportunity to improve the level of 

public health protection, it was not 

necessary to perform any additional 

reviews on treatment feasibility or 

economic considerations. 

c. Review Result. Although there are 

new data that support consideration of 

whether to revise the MCLG/MCL for 

total xylenes, EPA does not believe a 

revision to the NPDWR for total xylenes 

is appropriate at this time. In making 

this decision, the Agency considered 

whether any possible revision to the 

NPDWR for total xylenes is likely to 

provide a meaningful opportunity for 

health risk reductions. Taking into 

consideration the low occurrence of this 

contaminant, EPA has decided that any 

revision to the NPDWR would be a low 

priority activity for the Agency, and, 

thus, is not appropriate to revise at this 

time because of: 

• Competing workload priorities; 

• The administrative costs associated 

with rulemaking; and 

• The burden on States and the 

regulated community to implement any 

regulatory change that resulted. 

VII. EPA’s Request for Comments 

A. Request for Comment and/or 

Information on the Candidates for 

Revision 

EPA invites commenters to submit 

any new, relevant peer-reviewed data or 

information pertaining to the four 


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15568 Federal Register / Vol. 75, No. 59 / Monday, March 29, 2010 / Notices 

26Note that new health effects studies/ 

information for acrylamide, PCE and TCE are being 

considered as part of the IRIS update to these health 

assessments. 

NPDWRs identified in today’s action as 

candidates for revision (i.e., acrylamide, 

epichlorohydrin, tetrechloroethylene 

and trichloroethylene). This information 

will inform EPA’s evaluation as the 

Agency moves forward with the 

regulatory revisions for these four 

NPDWRs. Peer reviewed data are 

studies/analyses that have been 

reviewed by qualified individuals (or 

organizations) who are independent of 

those who performed the work, but who 

are collectively equivalent in technical 

expertise (i.e., peers) to those who 

performed the original work. A peer 

review is an in-depth assessment of the 

assumptions, calculations, 

extrapolations, alternate interpretations, 

methodology, acceptance criteria, and 

conclusions pertaining to the specific 

major scientific and/or technical work 

products and of the documentation that 

supports them (USEPA, 2000d). 

Relevant data include studies/analyses 

pertaining to analytical feasibility, 

treatment feasibility, and occurrence/ 

exposure related to the four NPDWRs 

candidates for revision listed in today’s 

action.26 Table VII–1 provides a list of 

the specific items for which EPA is 

requesting comment and/or information 

for the four candidates for revision. It 

also provides a cross-reference to the 

section addressing the issue. 

TABLE VII–1—ITEMS FOR WHICH EPA IS REQUESTING COMMENT AND/OR INFORMATION FOR THE FOUR CANDIDATES FOR

REVISION

Issue Notice section 

Any new, relevant peer-reviewed data or information that would inform the revision of the NPDWR for acrylamide, including information pertaining to extent of use of polyacrylamide in drinking water facilities.Section VI.B.1. 

Any new, relevant peer-reviewed data or information that would inform the revision of the NPDWR for epichlorohydrin, including information pertaining to extent of use of epichlorohydrin-based polymers/co-polymers in drinking water facilities.Section VI.B.36. 

Any new, relevant peer-reviewed data or information that would inform the revision of the NPDWR for tetrachloroethylene, 

including information/data about the lowest level of quantitation (and analytical method used) that laboratories can reliably and consistently achieve.

Section VI.B.60. 

Any new, relevant peer-reviewed data or information that would inform the revision of the NPDWR for trichloroethylene, including information/data about the lowest level of quantitation (and the analytical method used) that laboratories can reliably and consistently achieve.Sections VI.B.65. 

B. Request for Information/Data on 

Other Review Topics 

EPA also invites commenters to 

submit new, relevant information on 

several other review topics referenced in 

this notice and listed in Table VII–2. 

TABLE VII–2—ISSUES FOR WHICH EPA IS REQUESTING PUBLIC INPUT AND/OR INFORMATION

Issue Notice section 

Location for nitrate and nitrite monitoring .................................................................................................................. Section V.B.6. 

Monitoring frequency for ground water systems with low nitrate and nitrite concentrations .................................... Section V.B.6. 

Monitoring requirements for non-community water systems ..................................................................................... Section V.B.6. 

Detection limits that serve as triggers to determine compliance monitoring frequency for SOCs ............................ Section V.B.6. 

New, relevant health effects information that will help the Agency decide whether to initiate a new health effects 

assessment for chromium.

Section VI.B.17. 

New, relevant health effects information that will help the Agency decide whether to initiate or nominate nitrate 

and nitrite for a new health effects assessment.

Sections VI.B.49 and VI.B.50. 

New, relevant health effects information that will help the Agency decide whether to initiate or nominate selenium for a new health effects assessment.Sections VI.B.56. 

New, relevant health effects information that will help the Agency decide whether to initiate or nominate 1,2,4- 

trichlorobenzene for a new health effects assessment.

Sections VI.B.65. 

C. Requests for Information on the 

Impacts of Climate Change on Water 

Quality 

The Agency recognizes that changes 

in global climate can 

impacttemperature, rainfall patterns, 

and snow and ice cover. Changes in 

these climate indicators can impact 

water quantity and water quality. In an 

effort to assess the impacts of climate 

change on water quality, EPA is asking 

if public water systems and/or States 

have any information or data that 

illustrates the impact of climate change 

(e.g., changes in rainfall, drought, 

temperature, and snow/ice cover) on the 

occurrence of contaminants in drinking 

water, both in source water and in 

finished water. EPA also requests data 

on changes in the variability of 

occurrence and impacts on drinking 

water treatment to address occurrence 

or variability changes. 

VIII. EPA’s Next Steps 

EPA will consider the public 

comments and/or any new, relevant, 

peer-reviewed data submitted for the 

four NPDWRs listed as candidates for 

revision as the Agency proceeds with 

the regulatory revisions for these 

regulations. The announcement that the 

Agency intends to revise an NPDWR 

(pursuant to SDWA section 1412(b)(9)) 

is not a regulatory decision. Instead, it 

initiates a regulatory process that will 

involve more detailed analyses of health 

effects, analytical and treatment 

feasibility, occurrence, benefits, costs, 

and other regulatory matters relevant to 

deciding whether an NPDWR should be 

revised. The Six-Year Review results do 

not obligate the Agency to revise an 

NPDWR in the event that EPA 

determines during the regulatory 

process that revisions are no longer 


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appropriate and discontinues further 

efforts to revise an NPDWR. Similarly, 

the fact that an NPDWR has not been 

selected for revision means only that 

EPA believes that regulatory changes to 

a particular NPDWR are not appropriate 

at this time for the reasons given in 

today’s action; future reviews may 

identify information that leads to an 

initiation of the revision process. 

IX. References 

ATSDR (Agency for Toxic Substances and 

Disease Registry). 2007. Toxicological 

profile for benzene. Atlanta, GA: Agency 

for Toxic Substances and Disease 

Registry, Public Health Service, U.S. 

Department of Health and Human 

Services. Available on the Internet at: 

http://www.atsdr.cdc.gov/toxprofiles/ 

tp3.html. 

Cocco, P, G. Broccia, et al. 2003. Nitrate in 

community water supplies and 

incidence of non-Hodgkin’s lymphoma 

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Primary Drinking Water Regulations; 

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Sulfone, Pentachlorophenol, and 

Barium; Final Rule. Federal Register. 

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USEPA. 1991c. National Primary Drinking 

Water Regulations—Synthetic Organic 

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Prevention, Pesticides, and Toxic 

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0096.pdf. 

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summary information on the Integrated 

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summary information on the Integrated 

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03–003. March 2003. 

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Existing National Primary Drinking 

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Cumulative Risk Assessment for the 

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Office of Pesticide Programs. March 8, 

2006. Available on the Internet at: 

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USEPA. 2006b. Addendum to the 2002 

Lindane Reregistration Eligibility 

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028. Washington, DC: Office of 

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Internet at: http://www.epa.gov/

oppsrrd1/REDs/lindane_red_

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IRED, and Completion of Tolerance 

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Eligibility Process. Washington, DC: 

Office of Prevention, Pesticides, and 

Toxic Substances. April 2006. Available 

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USEPA. 2006d. Interim Reregistration 

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USEPA. 2006e. Lindane; Cancellation Order. 

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USEPA. 2006f. National Primary Drinking 

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USEPA. 2007c. Integrated Risk Information 

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USEPA. 2007e. Unregulated Contaminant 

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USEPA. 2007f. National Primary Drinking 

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Dated: December 17, 2009. 

Lisa P. Jackson, 

Administrator. 

[FR Doc. 2010–6624 Filed 3–26–10; 8:45 am] 

BILLING CODE 6560–50–P 


sroberts on DSKD5P82C1PROD with NOTICES