Guidance for Implementing the 

January 2001 Methylmercury 

Water Quality Criterion

Final

United States Environmental Protection Agency

Office of Science and Technology (4305T)

1200 Pennsylvania Ave., NW

Washington, DC 20460

EPA-823-R-09-002

www.epa.gov/waterscience

November 2009
DISCLAIMER

This guidance provides advice on how to implement the water quality criterion recommendation for 

methylmercury that the U.S. Environmental Protection Agency (EPA) published in January 2001. This guidance 

does not impose legally binding requirements on EPA, states, tribes, other regulatory authorities, or the regulated 

community, and may not apply to a particular situation based upon the circumstances. EPA, state, tribal, and other 

decision makers retain the discretion to adopt approaches on a case-by-case basis that differ from those in the 

guidance where appropriate. EPA may update this guidance in the future as better information becomes available.

The Office of Science and Technology, Office of Water, U.S. Environmental Protection Agency has approved this 

guidance for publication. Mention of trade names, products, or services does not convey and should not be 

interpreted as conveying official EPA approval, endorsement, or recommendation for use.

The suggested citation for this document is:

USEPA. 2009. Guidance for Implementing the January 2001 Methylmercury Water Quality 

Criterion. EPA 823-R-09-002. U.S. Environmental Protection Agency, Office of Water, Washington, 

DC.
FOREWORD

On January 8, 2001, the Environmental Protection Agency (EPA) announced the availability of its recommended 

Clean Water Act (CWA) section 304(a) water quality criterion for methylmercury. This water quality criterion, 

0.3 milligram (mg) methylmercury per kilogram (kg) fish tissue wet weight, describes the concentration of 

methylmercury in freshwater and estuarine fish and shellfish tissue that should not be exceeded to protect 

consumers of fish and shellfish among the general population. EPA recommends that the criterion be used as 

guidance by states, territories, and authorized tribes in establishing or updating water quality standards for waters 

of the United States and in issuing fish and shellfish consumption advisories. States and authorized tribes remain 

free not to use EPA’s current recommendations, provided that their new or revised water quality criteria for 

methylmercury protect the designated uses and are based on scientifically defensible methodology.

The publication of the 2001 methylmercury criterion was the first time EPA issued a water quality criterion 

expressed as a fish and shellfish tissue value rather than as a water column value. EPA recognizes that this 

approach differs from traditional water column criteria and might pose implementation challenges. In the 

January 8, 2001, notice, EPA stated that it planned to develop more detailed guidance to help states, territories, 

and authorized tribes with implementation of the methylmercury criterion in water quality standards and related 

programs. This document provides that detailed guidance.

EPA wrote the Guidance for Implementing the January 2001 Methylmercury Water Quality Criterion to provide 

technical guidance to states, territories, and authorized tribes exercising responsibility under CWA section 303(c) 

on how to use the new fish tissue-based criterion recommendation in developing their own water quality standards 

for methylmercury and in implementing those standards in Total Maximum Daily Loads (TMDLs) and National 

Pollutant Discharge Elimination System (NPDES) permits. EPA also wrote the guidance to discuss approaches 

for managing the development of TMDLs for waterbodies impaired by mercury and to recommend an approach 

for directly incorporating the methylmercury tissue criterion into NPDES permits.

For more information on the methylmercury criterion, see the criteria page on EPA’s Web site at 

http://www.epa.gov/waterscience/criteria/methylmercury/index.html. For more information on EPA’s water 

quality standards program, see the standards page on EPA’s Web site at http://www.epa.gov/waterscience/

standards. For more information about this guidance document, contact U.S. Environmental Protection Agency, 

Office of Science and Technology (4305T), 1200 Pennsylvania Avenue, NW, Washington, DC 20460.

Peter S. Silva

Assistant Administrator for Water

U.S. Environmental Protection Agency

Contents

i

CONTENTS

1 Executive Summary.....................................................................................1

2 Introduction..................................................................................................9

2.1 What is the interest in mercury?.............................................................................9

2.1.1 What are the health effects of methylmercury?.............................................9

2.1.2 How frequent are the environmental problems? .........................................10

2.2 What are the sources of mercury in fish?.............................................................12

2.3 How does methylmercury get into fish and shellfish? ..........................................14

2.4 Why is EPA publishing this document?................................................................15

2.5 What is the effect of this document? ....................................................................16

3 Water Quality Criteria and Standards Adoption ......................................17

3.1 What must states and authorized tribes include as they adopt the 

methylmercury criterion?......................................................................................17

3.1.1 What do the CWA and EPA’s regulations require?.....................................17

3.1.2 What is the recommended form of the methylmercury criterion? ...............18

3.1.3 What approaches should states or authorized tribes consider when 

developing a water column concentration criterion? ..................................22

3.2 What options are available to address site-specific conditions and concerns? ...36

3.2.1 How can the methylmercury water quality criterion be modified for sitespecific conditions?.....................................................................................36

3.2.2 How do water quality standards variances apply?......................................42

3.2.3 How are use attainability analyses conducted? ..........................................46

4 Monitoring and Assessment .....................................................................49

4.1 What are the analytical methods for detecting and measuring mercury and 

methylmercury concentrations in fish and water?................................................49

4.1.1 Analytical Methods for Methylmercury ........................................................50

4.1.2 Analytical Methods for Mercury...................................................................50

4.1.3 Summary of Recommended Analytical Methods ........................................51

4.2 What is the recommended guidance on field sampling plans for collecting fish 

for determining attainment of the water quality standard?...................................52

4.2.1 What fish species should be monitored? ....................................................53

4.2.2 What sample types best represent exposure?............................................54

4.2.3 What is the recommended study design for site selection?........................54

4.2.4 How often should fish samples be collected? .............................................55

4.2.5 How many samples should be collected?...................................................56

4.2.6 What form of mercury should be analyzed?................................................56

4.2.7 Other sampling considerations....................................................................56

4.3 How should waterbody impairment be assessed for listing decisions? ...............57

4.3.1 How should nondetections be addressed? .................................................58

4.3.2 How should data be averaged across trophic levels?.................................59
Contents

ii

4.3.3 How should older data be assessed? .........................................................61

4.3.4 How should fish consumption advisories be used to determine 

impairment?................................................................................................61

5 Other Water Quality Standards Issues..................................................... 63

5.1 How does this criterion relate to the criteria published as part of the Great 

Lakes Initiative? ...................................................................................................63

5.2 What is the applicable flow for a water column-based criterion? .........................64

5.3 How are mixing zones used for mercury?............................................................65

5.3.1 What is a mixing zone? ...............................................................................65

5.3.2 How does a mixing zone apply for the fish tissue-based 

methylmercury criterion? ............................................................................65

5.3.3 Does the guidance for the fish tissue-based criterion change the Great 

Lakes Initiative approach to mixing zones for bioaccumulative 

pollutants? ..................................................................................................66

5.4 How are fish consumption advisories and water quality standards 

harmonized? ........................................................................................................66

5.4.1 What is the role of state and tribal Fish Advisory Programs? .....................66

5.4.2 How are consumption limits for consumption advisories determined?.......67

5.4.3 How does the criterion differ from the advisory level? ................................67

5.4.4 What if there is a difference between assessing criterion attainment 

and issuance of a fish consumption advisory?...........................................68

5.4.5 Should existing advisories be revised to reflect the new criterion? ............69

5.4.6 What federal agencies issue advisories?....................................................69

5.4.7 How is the criterion related to FDA action levels? ......................................70

5.5 What public participation is recommended for implementing the 

methylmercury criterion?......................................................................................70

6 TMDLs ........................................................................................................ 71

6.1 What is a TMDL?..................................................................................................71

6.2 How have states and tribes approached mercury TMDLs? .................................71

6.2.1 What geographic scales have been used for mercury TMDLs? .................73

6.2.2 What are the considerations in developing mercury TMDLs? ....................74

7 National Pollutant Discharge Elimination System (NPDES) 

Implementation Procedures ..................................................................... 91

7.1 What are the general considerations in NPDES permitting? ...............................91

7.2 What is the EPA-recommended NPDES permitting approach for 

methylmercury? ...................................................................................................92

7.2.1 Developing NPDES permit limits based on the fish tissue criterion............92

7.2.2 Determining reasonable potential ...............................................................93

7.2.3 Implementing antidegradation.....................................................................94

7.2.4 Establishing appropriate WQBELs..............................................................94

7.3 How does EPA recommend implementing the fish tissue criterion for NPDES 

permits? ...............................................................................................................97

7.4 What are the procedures for developing permit limits when the criterion is 

adopted as a water column value or when the criterion is adopted as a fish 
Contents

iii

tissue value and the permitting authority uses a water column translation of 

the fish tissue value?............................................................................................99

7.5 What are the procedures for developing permit limits when the criterion is 

adopted as a fish tissue value and the permitting authority does not use a 

water column translation of the fish tissue value? .............................................101

7.5.1 How to determine the need for permit limits to control mercury (how to 

determine reasonable potential)...............................................................102

7.5.2 Where reasonable potential exists, how can WQBELs be derived from 

a fish tissue value? ...................................................................................113

8 Related Programs ....................................................................................123

8.1 What are EPA and others doing as a whole to address mercury?.....................123

8.2 How does pollution prevention play a role in the methylmercury criterion? .......123

8.3 What regulations has EPA issued pursuant to the CAA to address air 

emissions of mercury? .......................................................................................125

8.3.1 Municipal waste combustors .....................................................................126

8.3.2 Hospital, medical, and infectious waste incinerators ................................126

8.3.3 Chlor-alkali plants......................................................................................127

8.3.4 Hazardous waste combustors...................................................................127

8.3.5 Coal-fired power plants .............................................................................128

8.3.6 Other..........................................................................................................128

9 References ...............................................................................................129

Appendix A. Methylmercury/Mercury Ratio Exhibited in Muscle 

Tissue of Various Freshwater Fish Species...........................................145

Appendix B. Tables from Methylmercury Criteria Document...............147

Appendix C. Analytical Methods ............................................................151

Appendix D. Synopsized Mercury TMDLs Developed or 

Approved by EPA.....................................................................................155

I. Ochlockonee Watershed, Georgia .....................................................................156

Description of the Applicable Water Quality Standards.........................................156

Source Assessment...............................................................................................157

Loading Capacity—Linking Water Quality and Pollutant Sources.........................158

Allocations..............................................................................................................159

II. Arivaca Lake, Arizona.........................................................................................161

Description of the Applicable Water Quality Standards.........................................161

Source Assessment...............................................................................................161

Loading Capacity—Linking Water Quality and Pollutant Sources.........................163

Allocations..............................................................................................................165

III. McPhee and Narraguinnep Reservoirs, Colorado..............................................168

Description of the Applicable Water Quality Standards.........................................168

Source Assessment...............................................................................................168

Loading Capacity—Linking Water Quality and Pollutant Sources.........................169

Allocations..............................................................................................................169
Contents

iv

IV. Clear Lake, California.........................................................................................170

Description of the Applicable Water Quality Standards.........................................170

Source Assessment...............................................................................................170

Loading Capacity––Linking Water Quality and Pollutant Sources ........................172

Allocations .............................................................................................................173

V. Cache Creek, California .....................................................................................176

Description of the Applicable Water Quality Standards.........................................176

Source Assessment...............................................................................................177

Loading Capacity––Linking Water Quality Pollutant Sources ...............................178

Allocations .............................................................................................................179

VI. Minnesota Statewide Mercury Total Maximum Daily Load ................................181

Description of the Applicable Water Quality Standards and TMDL Target ...........181

Source Assessment...............................................................................................182

Loading Capacity...................................................................................................182

Allocations .............................................................................................................184

Appendix E. Model Descriptions ........................................................... 185

BASS (Bioaccumulation and Aquatic System Simulator)......................................185

Community Multi-Scale Air Quality (CMAQ) Model...............................................185

D-MCM (Dynamic Mercury Cycling Model) ...........................................................187

EXAMS2 (Exposure Analysis Modeling System) ..................................................187

GBMM (Grid Based Watershed Mercury Model)...................................................188

GEOS-CHEM Model..............................................................................................188

GWLF (Generalized Watershed Loading Function) ..............................................189

Mercury Maps screening analysis .........................................................................189

MOBILE .................................................................................................................190

NDMMF (National Descriptive Model of Mercury in Fish Tissue) .........................191

NONROAD ............................................................................................................191

QEAFDCHN (Quantitative Environmental Analysis Food Chain) Model...............191

Regional Modeling System for Aerosols and Deposition (REMSAD) ...................192

SERAFM (Spreadsheet-based Ecological Risk Assessment for the Fate of 

Mercury)....................................................................................................193

TOXI5 ..................................................................................................................194

WASP (Water Quality Analysis Simulation Program)............................................194

WCS (Watershed Characterization System) Mercury Loading Model ..................195

Example of Linking Models....................................................................................195

Appendix F. Examples of National Deposition Monitoring 

Networks ............................................................................................ 197

Index .............................................................................................................. 199
Contents

v

TABLES

Table 1a. Recommendations for water quality standards adoption ....................................2

Table 1b. Recommendations for monitoring and assessment ............................................3

Table 1c. Recommendations for total maximum daily loads (TMDLs)................................4

Table 1d. Recommendations for permitting procedures .....................................................5

Table 2. Draft national BAFs for dissolved methylmercury ...............................................30

Table 3. Estimates of freshwater and estuarine combined finfish and shellfish 

consumption from the combined 1994–1996 and 1998 CSFII surveys 

(U.S. population)...................................................................................................41

Table 4. Recommended analytical methods for detecting and measuring low levels of 

methylmercury and mercury in fish tissue and water ...........................................52

Table 5. Example data for calculating a weighted average fish tissue value....................59

Table 6. Suggested content for MMPs based on the type of facility ...............................120

Table B1. Exposure parameters used in derivation of the water quality criterion.

(References cited in this table can be found in the 2001 methylmercury

criterion document.)............................................................................................148

Table B2. Average mercury concentrations in marine fish and shellfisha(References 

cited in this table can be found in the 2001 methylmercury criteria document.) 149

Table B3. Exposure estimates for methylmercury and percent of total exposure 

based on adults in the general population..........................................................150

Table C1. Analytical methods for determining mercury and methylmercury in tissue ....151

Table C2. Analytical methods for determining mercury and methylmercury in water, 

sediment, and other nontissue matrices.............................................................152

Table D1. Annual average mercury load from each subbasin ........................................158

Table D2. Predicted mercury for annual average load and flow .....................................159

Table D3. Annual total mercury load to Arivaca Lake .....................................................164

Table D4. Predicted and observed mercury for annual average load and flow ..............165

Table D5. Summary of TMDL allocations and needed load reductions (in g-Hg/yr).......166

Table D6. Summary of mercury load estimates for McPhee Reservoir ..........................168

Table D7. Summary of TMDL allocations and needed load reductions for McPhee 

Reservoir ............................................................................................................169

Table D8. Summary of TMDL allocations and needed load reductions for 

Narraguinnep Reservoir .....................................................................................170

Table D9. Summary of mercury load allocations.............................................................174

Table D10. Sediment goals for mercury in Clear Lake....................................................174

Table D11. Cache Creek methylmercury allocations ......................................................180

Table D12. Bear Creek methylmercury allocations .........................................................180

Table D13. Approved northeast and southwest mercury TMDLs ...................................184
Contents

vi

FIGURES

Figure 1. Average fish tissue concentrations by HUC watershed (USEPA 2005a). .........11

Figure 2. Total number of state mercury fish consumption advisories 2006.....................12

Figure 3. Percentage of total mercury deposition attributable to global sources 

(USEPA 2005a). ...................................................................................................76

Figure 4. Trends in mercury air emissions between 1990 and 1999 (USEPA 1999a)......76

Figure 5. NPDES permitting approach for methylmercury. ...............................................98

Figure 6. Implementing the fish tissue criterion in NPDES permits...................................99

Figure 7. Determining reasonable potential. ...................................................................101

Figure 8. Implementing tier 2 antidegradation.................................................................109

Figure 9. Determining WQBEL requirements..................................................................113

Figure F-1. MDN data for 2005. ......................................................................................198
Executive Summary

1

1 Executive Summary

In January 2001 EPA published ambient water quality criteria (AWQC) 

recommendations for methylmercury for the protection of people who eat fish and 

shellfish. This criterion, 0.3 milligram (mg) methylmercury per kilogram (kg) fish tissue 

wet weight, marks EPA’s first issuance of a water quality criterion expressed as a fish 

and shellfish tissue value rather than as an ambient water column value.

Research shows that exposure to mercury and its compounds can cause certain toxic 

effects in humans and wildlife (USEPA 1997a). As of 2006, 48 states, 1 territory, and 

2 tribes had issued fish consumption advisories for mercury covering 14.2 million lake 

acres and 883,000 river miles (USEPA 2007a). Mercury is widely distributed in the 

environment and originates from natural and human-induced (anthropogenic) sources, 

including combustion and volcanoes. Methylmercury is highly bioaccumulative, 

especially in aquatic food webs. Nearly 100 percent of the mercury that bioaccumulates

in upper-trophic-level fish (predator) tissue is methylmercury (Akagi et al. 1995; Becker 

and Bigham 1995; Bloom 1992; Kim 1995).

Under section 303(c) of the Clean Water Act (CWA), states and authorized tribes must 

adopt water quality criteria that protect designated uses. This document provides 

technical guidance to states and authorized tribes that exercise responsibility under CWA 

section 303(c) on how to use the new fish tissue-based criterion recommendation as they 

develop their own water quality standards for methylmercury.

EPA expects that, as states adopt methylmercury water quality criteria and as monitoring 

of effluents, receiving waters, and fish tissue with the more sensitive methods 

recommended by EPA increases, the number of waterbodies that states report on CWA

section 303(d) lists as impaired due to methylmercury contamination might increase. This 

guidance is designed to assist states and authorized tribes to address those impairments. 

Furthermore, this guidance addresses coordination across various media and program 

areas in implementing the criterion, which will be important because atmospheric 

deposition and multimedia cycling of mercury are significant in many waterbodies.

EPA recognizes the complexity and comprehensive nature of this guidance. As is always 

the case when EPA issues technical guidance, EPA will provide outreach and technical 

assistance to states and authorized tribes in implementing this guidance.

The following tables (tables 1a through 1d) provide a brief summary of the most 

important recommendations applicable to states and authorized tribes that are contained 

in the guidance.

NOTE: These tables are provided as a convenience to the reader, but are not 

comprehensive and are not a substitute for the full content of the guidance 

contained in the other chapters of this document.
Executive Summary

2

Table 1a. Recommendations for water quality standards adoption

Most 

applicable 

to criteria 

expressed 

as... 

For a full 

discussion 

see 

section...

Recommended form of a methylmercury criterion

EPA recommends that states and authorized tribes adopt a 

methylmercury criterion expressed as a fish tissue value.

When adopting a fish tissue criterion, states and authorized 

tribes will need to decide whether to:

3.1.2 and 

3.1.3

 Implement the fish tissue criterion without water column 

translation, or

FT (fish 

tissue value)

 Translate the fish tissue criterion to a water column 

value using bioaccumulation factors (BAFs). Three 

approaches include:

1. Site-specific BAFs

2. Modeled BAFs

3. BAFs derived using the results of field studies that 

are not site-specific (in limited circumstances); or

WC (water 

column 

value)

 Combination (fish tissue criterion for some or all waters, 

combined with water column criteria for some or all 

waters).

 States and authorized tribes may consider retaining their 

existing water column criteria, on a temporary basis, 

particularly for waters where there is a relatively high 

direct water input of mercury.

Both FT and

WC

FT alone

Adoption considerations

 When adopting a fish tissue criterion, EPA encourages 

states and authorized tribes to develop implementation 

procedures.

FT or WC

3.1.2.1

 This guidance does not supersede requirements in 

EPA’s Great Lakes Initiative (GLI) regulation for waters 

in the Great Lakes system.

5.1

Criterion adjustments

 Adjusting for local fish consumption rates.

 Adjusting for other sources of mercury (marine fish).

FT or WC 3.2.1

Mixing zones

 Not relevant when applying a fish tissue criterion that 

has not been translated to a water column value.

FT alone

5.3

 If the fish tissue criterion is converted to water column 

values, EPA advises caution in the use of any mixing 

zones for mercury. Restricting or eliminating mixing 

zones may be appropriate.

WC

Variances

 Guidance on when variances are appropriate.

 Considerations before granting a variance.

WC 3.2.2
Executive Summary

3

Table 1b. Recommendations for monitoring and assessment

Most 

applicable 

to criteria 

expressed 

as... 

For a full 

discussion 

see 

section...

Recommended analytical methods 4.1

 Methods 1631, revision E and 245.7 for mercury in 

water.

WC

 Appendix A of Method 1631 for mercury in fish tissue. FT

 Method 1630 for methylmercury in water. WC

 Method 1630 (with modifications) for methylmercury in 

fish tissue.

FT

Other available methods are listed in appendix C of this 

guidance.

FT or WC App. C

Field sampling recommendations

 Select fish for monitoring that are commonly eaten in the 

study area.

 Choose large fish because these are typically highest in 

methylmercury.

 If local consumption data are not available, match 

assumed consumption pattern to sampled species, or 

sample trophic level 4 species.

 Use composite samples of fish fillets.

 EPA recommends biennial sampling if resources allow, 

otherwise waterbodies should be screened a minimum 

of every 5 years.

FT alone 4.2

Assessing non-attainment of fish tissue criterion

 Use statistical tests if enough data, or consider sampleby-sample comparisons if very limited data.

FT alone 4.3
Executive Summary

4

Table 1c. Recommendations for total maximum daily loads (TMDLs)

Most 

applicable 

to criteria 

expressed 

as... 

For a full 

discussion 

see 

section...

States’ timing of TMDL development

 States with comprehensive mercury reduction programs 

in place may defer TMDLs for waters impaired by 

mercury mainly from atmospheric sources. 

(Summarizing EPA’s voluntary “5m” category for listing 

impaired waters.)

 The greater the relative contribution to a waterbody from 

mercury sources other than air deposition, such as water 

point sources, the more appropriate it may be to use the 

TMDL process to characterize and address those 

sources sooner, rather than deferring TMDL 

development.

FT or WC 6.2 and 

7.5.2.2

Approaches in approved mercury TMDLs

Examples in guidance text and appendix D discuss:

 Types of mercury sources; tools for assessing point 

sources, atmospheric deposition, past metals mining 

activity, sediments, and natural sources.

 Example allocation scenarios involving waters where 

predominant sources are air deposition or mining.

 Post-TMDL monitoring.

FT or WC 6.2

Geographic scale

Describes scales that have been used for developing 

mercury TMDLs:

 Waterbody-specific.

 Watershed-level.

 Statewide or regional.

FT or WC 6.2.1

Available models and example TMDL applications

 Example models for different situations (steady state, 

dynamic, detail geometry, regression).

 Factors leading to model selection (methylation, BAFs, 

sediments).

FT or WC 6.2.2.2

 Other analytical approaches, e.g., proportionality 

approach: Where air deposition is the only significant 

mercury source and steady-state conditions apply, 

TMDLs have been developed to meet fish tissue targets 

by relying on a proportional relationship between 

mercury deposition and fish tissue methylmercury 

concentration.

FT 6.2.2.2.5

 Use of linked models without having explicit water 

column criteria or translations. 

FT alone 6.2.2.2
Executive Summary

5

Table 1d. Recommendations for permitting procedures

Most 

applicable 

to criteria 

expressed 

as... 

For a full 

discussion 

see 

section...

Two implementation approaches

 If a TMDL or a water column translation derived from a 

fish tissue criterion or site-specific data to translate is 

available at time of permit issuance, implement using the 

approaches described in the Technical Support 

Document (TSD) for Water Quality-based Controls

(USEPA 1991).

WC 7.4

 If a TMDL or water column translation or site-specific

data to translate are not available, implement 

approaches described below.

FT alone 7.5

Finding “reasonable potential” (RP)

a

Depending on the particular facts, a permitting authority may 

reasonably conclude that a facility has RP if:

 There is a quantifiable level of mercury in the discharge,

using a sufficiently sensitive EPA-approved analytical 

method and

 Fish tissue from the receiving water is close to or 

exceeds the criterion.

FT alone 7.5.1

Where mercury effluent levels are unknown

EPA recommends that permitting authorities:

 Require effluent monitoring using a sufficiently sensitive 

EPA-approved analytical method.

 Include a reopener clause in the permit to allow permit to

be modified if effluent data indicate a water quality-based 

effluent limit (WQBEL) is necessary.

FT alone 7.5.1.1.1

Where quantifiable amounts of mercury are not found

 If the permitting authority believes the monitoring data are 

representative of the discharge, no further permit 

conditions may be necessary.

FT alone 7.5.1.1.2

Where fish tissue concentrations are unknown

EPA recommends that permitting authorities:

 Include a special permit condition to conduct a mercury 

fish tissue survey for the receiving waterbody.

 Include a reopener clause in the permit to allow permit to 

be modified if fish tissue data become available 

indicating a WQBEL is necessary.

 Encourage the permittee to develop and implement a 

mercury minimization plan (MMP) tailored to the facility’s 

potential to discharge mercury.

FT alone 7.5.1.2.1
Executive Summary

6

Table 1d. Recommendations for permitting procedures (continued)

Most 

applicable 

to criteria 

expressed 

as...

For a full 

discussion 

see 

section...

Permits with quantifiable mercury but without RP

Where a discharge contains a quantifiable amount of 

mercury but fish tissue in the receiving water does not

exceed the criterion:

 If the discharger will undertake an activity that could 

result in an increase in receiving water or fish tissue 

mercury concentration

o Conduct tier 2 antidegradation analysis and 

develop appropriate permit conditions.

o Require permittee to implement an MMP 

tailored to the facility’s potential to discharge 

mercury.

o Require effluent monitoring.

 If the discharger will not undertake an activity that could 

result in an increase in receiving water or fish tissue 

mercury concentration:

o Encourage the facility to develop and implement 

an MMP tailored to the facility’s potential to 

discharge mercury.

FT alone 7.5.1.2.2

Other factors in determining RP

 EPA recommends that the permitting authority account 

for other factors that may constitute the basis for a 

finding of RP. These include rising fish tissue 

concentrations and the impact on downstream waters, 

FT alone 7.5.1.2.2

Mercury in intake water

 Where the only source of mercury in a discharge may be 

the intake water taken directly from the same body of 

water, and where there are no known sources or 

additional contributions of mercury at the facility, the 

permitting authority may reasonably conclude, based on 

the particular facts, that there is no RP to exceed water 

quality standards.

FT or WC 7.5.1.3
Executive Summary

7

Table 1d. Recommendations for permitting procedures (continued)

Most 

applicable 

to criteria 

expressed 

as...

For a full 

discussion 

see 

section...

WQBELs where there is a finding of RPb

EPA recommends that permitting authorities:

 Require the permittee to implement an MMP tailored to 

its potential to discharge mercury.

 Depending on the particular facts, the permitting 

authority should consider including in an MMP an 

effluent trigger level, a mercury reduction goal, or an 

enforceable numeric level representing existing effluent 

quality or some increment of the mercury reduction 

determined achievable as a result of the measures and 

practices specified in the MMP.

 Require effluent monitoring using a sufficiently sensitive 

EPA-approved method to enable evaluation of the 

effectiveness and implementation of the MMP.

 Include a reopener clause to modify the permit 

conditions if the MMP is found to be not effective or if a 

water column translation of the criterion is developed.

Other considerations and requirements may be necessary:

 Where a discharger undertakes an activity that could 

result in an increase in receiving water or fish tissue 

mercury concentrations, it must be consistent with 

applicable antidegradation requirements. Additional 

requirements may also be necessary under the CWA

and EPA’s NPDES regulations.

 Include appropriate technology-based limits pursuant to 

CWA section 301(b) and 40 CFR sections 125.3 and 

122.44(a)(1).

 For modified or reissued permits with existing effluent 

limits for mercury, any less stringent effluent limit must 

be consistent with anti-backsliding requirements.

FT alone 7.5.2.1

7.2.4
Executive Summary

8

Table 1d. Recommendations for permitting procedures (continued)

Most 

applicable 

to criteria 

expressed 

as...

For a full 

discussion 

see 

section...

Permits with RP where direct water inputs are relatively high

In addition to the above:

 EPA recommends that states and authorized tribes 

specifically consider developing TMDLs in the short 

term.

 Where a state or tribe chooses not to develop a TMDL in 

the short term, the state or tribe should develop an 

analysis of sources and loading capacity similar to what 

a TMDL would provide, or a water column translation of 

the fish tissue criterion.

 EPA recommends that permitting authorities work 

together with mercury dischargers in the watershed to 

collect data necessary to develop:

o A TMDL, or

o An analysis of sources and loading capacity similar 

to what a TMDL would provide, or

o A water column translation of the fish tissue criterion 

for future permitting.

One approach is for the permitting authority to invoke its 

authority under CWA section 308 (or comparable state 

authority).

FT alone 7.5.2.2

Additional requirements that may apply

 Additional requirements for: POTWs with pretreatment 

programs; technology-based limits; anti-backsliding; 

permit documentation

FT or WC 7.5.2.3

Mercury minimization plans (MMPs)

This section provides guidance on appropriate MMPs.

FT 7.5.2.4

Notes:

a

“Reasonable potential” refers to the reasonable potential to cause or contribute to an excursion above a 

numeric or narrative criterion for water quality. 40 CFR 122.44(d)(1)(i). NPDES permits for discharges with 

“reasonable potential” must include water quality-based effluent limits (WQBELs).

b

As noted at the beginning of table 1d, this section refers to situations where neither a TMDL nor a water 

column translation is available at time of permit issuance. Where a TMDL has been developed, the WQBEL for 

that discharge must be consistent with the TMDL’s wasteload allocation. Where a TMDL is not available at the 

time of permit discharge, but where a water column translation of the fish tissue criterion has been developed, 

or where site-specific data to do so are readily available, include a numeric WQBEL.
Introduction

9

2 Introduction

2.1 What is the interest in mercury?

Mercury occurs naturally in the earth’s crust and cycles in the environment as part of 

natural and human-induced activities. The amount of mercury mobilized and released 

into the biosphere has increased since the beginning of the industrial age. Most of the 

mercury in the atmosphere is elemental mercury vapor, which circulates in the 

atmosphere for up to a year and therefore can be widely dispersed and transported 

thousands of miles from sources of emission (USEPA 1997b). Most of the mercury in

water, soil, sediments, plants, and animals is in the form of inorganic mercury salts and 

organic forms of mercury (e.g., methylmercury). Inorganic mercury salts, when bound to 

airborne particles, are readily removed from the atmosphere by precipitation and are also 

dry deposited. Even after mercury deposits, it commonly returns to the atmosphere, as a 

gas or associated with particles, and then redeposits elsewhere. As it cycles between the 

atmosphere, land, and water, mercury undergoes a series of complex chemical and 

physical transformations, many of which are not completely understood (USEPA 1997b).

This guidance focuses on an organic mercury compound known as methylmercury.

Methylmercury most often results from microbial activity in wetlands, the water column, 

and sediments, and it is the form of mercury that presents the greatest environmental risks 

to human health (66 FR 1344; January 8, 2001). The methylation process and 

methylmercury bioaccumulative patterns are discussed in more detail in section 2.3.

2.1.1 What are the health effects of methylmercury?

Exposure to methylmercury can result in a variety of health effects in humans. Children 

that are exposed to low concentrations of methylmercury prenatally might be at risk of 

poor performance on neurobehavioral tests, such as those measuring attention, fine motor 

function, language skills, visual-spatial abilities, and verbal memory (NRC 2000; USEPA 

2002a). In 2000 the National Academy of Sciences (NAS)/National Research Council 

(NRC) reviewed the health studies on mercury (NRC 2000). EPA’s assessment of the 

methylmercury reference dose (RfD) relied on the quantitative analyses performed by the 

NRC (USEPA 2002a). The RfD is an estimate (with uncertainty spanning perhaps an 

order of magnitude) of a daily exposure of the human population, including sensitive 

subgroups, that is likely to be without an appreciable risk of deleterious effects during a 

lifetime (USEPA 2002a). In its review of the literature, NRC found neurodevelopmental 

effects to be the most sensitive endpoints and appropriate for establishing a 

methylmercury RfD (NRC 2000). On the basis of the NRC report, EPA established an 

RfD of 0.0001 mg/kg-day (0.0001 milligram of methylmercury per day for each kilogram 

of a person’s body mass) (USEPA 2002a). EPA believes that exposures at or below the 

RfD are unlikely to be associated with an appreciable risk of deleterious effects. It is 

important to note, however, that the RfD does not define an exposure level corresponding 

to zero risk; mercury exposure near or below the RfD could pose a very low level of risk 

that EPA deems nonappreciable. It is also important to note that the RfD does not define 

a bright line above which individuals are at risk of adverse effects (USEPA 2005a).
Introduction

10

The primary route by which the U.S. population is exposed to methylmercury is through 

the consumption of fish containing methylmercury. The exposure levels at which 

neurological effects have been observed in children can occur through maternal 

consumption of fish (rather than high-dose poisoning episodes) (USEPA 2005a). In 2005 

the National Health and Nutrition Examination Survey (NHANES) published the results 

of a study of blood mercury levels in a representative sample of U.S. women of 

childbearing age (CDC 2005). The report data for the period 1999–2002 show that all 

women of childbearing age had blood mercury levels below 58 μg/L, a concentration 

associated with neurological effects in the fetus. These data show that 5.7 percent of 

women of childbearing age had blood mercury levels between 5.8 and 58 μg/L; that is, 

levels within an order of magnitude of those associated with neurological effects. Typical 

exposures for women of childbearing age were generally within two orders of magnitude 

of exposures associated with these effects, according to data from NHANES (CDC 2005;

USEPA 2005a).

With regard to other health effects of methylmercury, some recent epidemiological 

studies in men suggest that methylmercury is associated with a higher risk of acute 

myocardial infarction, coronary heart disease, and cardiovascular disease in some 

populations (Salonen et al. 1995, as cited in USEPA 2001a). Other recent studies have 

not observed this association. The studies that have observed an association suggest that 

the exposure to methylmercury might offset the beneficial effects of fish consumption 

(USEPA 2005a). There also is some recent evidence that exposures to methylmercury 

might result in genotoxic or immunotoxic effects ([Amorim et al. 2000; ATSDR 1999; 

Silva at al. 2004], as cited in USEPA 2005a). Other research with less corroboration 

suggests that reproductive, renal, and hematological impacts could be of concern. There 

are insufficient human data to evaluate whether these effects are consistent with 

methylmercury exposure levels in the U.S. population (USEPA 2005a).

Deposition of mercury to waterbodies can also have an adverse impact on ecosystems 

and wildlife. Plant and aquatic life, as well as birds and mammalian wildlife, can be 

affected by mercury exposure; however, overarching conclusions about ecosystem health 

and population effects are difficult to make. Mercury contamination is present in all 

environmental media; aquatic systems experience the greatest exposures because of

bioaccumulation. Bioaccumulation refers to the net uptake of a contaminant from all 

possible pathways. It includes the accumulation that might occur by direct exposure to 

contaminated media, as well as uptake from food. Elimination of methylmercury from 

fish is so slow that long-term reductions of mercury concentrations in fish are often due 

to growth of the fish (“growth dilution”), whereas other mercury compounds are 

eliminated relatively quickly. Piscivorous avian and mammalian wildlife are exposed to 

mercury mainly through consuming contaminated fish, and as a result they accumulate 

mercury to levels greater than those in their prey (USEPA 1997a). EPA’s mercury Web

site, at http://www.epa.gov/mercury, provides a broad range of information about 

mercury, including a full discussion of potential human health and ecosystem effects.

2.1.2 How frequent are the environmental problems?

As of the 2006 303(d) listing cycle, 42 states and Puerto Rico reported at least one 

waterbody as impaired due to mercury, and more than 9,000 specific waterbodies were 

listed as impaired due to mercury, either solely or in combination with other pollutants. In 
Introduction

11

2001 EPA mapped concentrations of mercury in fish tissue from fish collected from 

waterbodies all over the country (i.e., not limited to the waters identified by the states as 

impaired) and compared them to the 2001 national recommended water quality criterion,

0.3 mg methylmercury/kg fish tissue wet weight (figure 1). These data were not 

randomly or systematically collected, but rather reflect fish tissue information that states 

had collected as part of their fish consumption advisory programs. Approximately 40

percent of the watershed-averaged fish tissue concentrations exceeded 0.3 mg 

methylmercury/kg fish tissue wet weight (USEPA 2001b).

Figure 1 shows fish tissue mercury concentrations averaged by watershed (by 8-digit 

hydrologic unit code, or HUC).

Figure 1. Average fish tissue concentrations by HUC watershed (USEPA 2005a).

In EPA’s Environmental Monitoring and Assessment Project (EMAP) Western Streams 

and Rivers Statistical Study (USEPA 2005b), 626 streams and rivers were sampled in 12 

states of the western United States. Mercury was detected at 100 percent of sites and 

samples in the study. The 0.3 mg/kg criterion (equivalent to 0.3 parts per million, ppm)

was exceeded in 56.8 percent of waters surveyed, which represent 20–30 percent of 

western rivers (Peterson et al. 2007).
Introduction

12

As of December 2006, 48 states, 1 territory, and 2 tribes had issued fish consumption 

advisories1for mercury covering 14.2 million lake acres and 883,000 river miles 

(figure 2). Twenty-three states had issued advisories for mercury in all freshwater lakes 

and rivers in the state, and 12 states had statewide advisories for mercury in their coastal 

waters (USEPA 2007a). Although states, territories, tribes, and local governments 

continue to issue new fish advisories and most new fish advisories involve mercury, EPA 

believes that the increase in advisories is a result of increased monitoring and assessment

of previously untested waters rather than increased domestic releases of mercury or 

increased levels or frequency of contamination. In fact, U.S. mercury emissions have 

declined by almost 50 percent since 1990 (USEPA 2007a).

Figure 2. Total number of state mercury fish consumption advisories 2006.

2.2 What are the sources of mercury in fish?

Mercury is emitted from both natural and anthropogenic sources. Its residence time in the 

atmosphere is much longer than that of most other metals because mercury can circulate 

for up to a year (USEPA 1997b). Such mobility enables elemental mercury to disperse 

and be transported over thousands of miles from likely sources of emission, across 

regions, and around the globe. As a result, the mercury detected in fish in U.S. surface 

waters is from both U.S. and international sources (USEPA 2005c). EPA estimates that 

1

States and tribes issue their advisories and guidelines voluntarily and have flexibility in which criteria they use and how they collect data. 

As a result, there are significant variations in the numbers of waters tested, the pollutants tested for, and the threshold for issuing advisories. 

Based on self-reporting, the national trend is for states to monitor different waters each year, generally without retesting waters monitored 

in previous years. Note that EPA does not issue fish advisories; states and tribes issue advisories. EPA and the Food and Drug 

Administration (FDA) issue guidance on the level of contaminants in fish, which states and tribes may use in issuing their advisories.
Introduction

13

approximately 83 percent of the atmospheric mercury deposited on land and water in the 

country is from a combination of sources outside the United States and Canada, as well as 

from natural and re-emitted sources. EPA’s current air quality modeling indicates a 

substantial variation across the country: domestic sources influence mercury deposition 

much more in the East, and global sources are a more significant contributor to mercury 

deposition in the West, where relatively few domestic sources exist. This estimate was 

based on an advanced, state-of-the-science modeling assessment of the atmospheric fate, 

transport, and deposition of mercury conducted by EPA for the Clean Air Mercury Rule2

(USEPA 2005d).

Natural sources of mercury include geothermal emissions from volcanoes and crustal 

degassing in the deep ocean, as well as dissolution of mercury from other geologic 

sources (Rasmussen 1994). Anthropogenic sources of mercury in the United States

include combustion (e.g., utility boilers; municipal waste combustors;

commercial/industrial boilers; hospital, medical, and infectious waste incinerators),

manufacturing sources (e.g., chlor-alkali and cement manufacturers), and mining

(USEPA 1997b).

U.S. anthropogenic emissions of mercury to the air have declined more than 45 percent

since passage of the 1990 Clean Air Act (CAA) Amendments. These amendments 

provided EPA new authority to reduce emissions of mercury and other toxic pollutants to 

the air. In 1990 more than two-thirds of U.S. human-caused mercury emissions came 

from just three source categories: coal-fired power plants; municipal waste combustion;

and hospital, medical, and infectious waste incineration (figure 4, section 6.2.2.1).

Regulations were issued in the 1990s to control mercury emissions from waste 

combustion. In addition, actions to limit the use of mercury––most notably voluntary and 

Congressional action to limit the use of mercury in batteries and EPA regulatory limits on 

the use of mercury in paint––contributed to the reduction of mercury emissions from 

waste combustion during the 1990s by reducing the mercury content of waste. More 

recent regulations, including regulation of mercury emissions from chlorine production 

facilities that use mercury cells and regulation of industrial boilers, will further reduce 

emissions of mercury.3

At present, the largest single source of anthropogenic mercury emissions in the country is 

coal-fired power plants. Mercury emissions from U.S. power plants are estimated to 

account for about one percent of total global mercury emissions (70 FR 15994; March 29, 

2005). In May 2005, EPA adopted the Clean Air Mercury Rule (CAMR) to regulate 

mercury emissions from utilities. On February 8, 2008, the D.C. Circuit Court of Appeals 

vacated the CAMR and remanded portions of it to EPA. See EPA’s mercury Web site, at 

2 On February 8, 2008, the D.C. Circuit Court of Appeals vacated the Clean Air Mercury Rule and remanded portions of it to EPA for 

reasons unrelated to the technical analyses in this document.

3 Rules controlling mercury emissions, which implement the 1990 CAA amendments, include standards for municipal waste combustors 

(40 CFR part 60, subpart Da, and parts 72 and 75); standards for hospital, medical, and infectious waste incinerators (40 CFR part 60, 

subpart Ce); standards for chlor-alkali plants (40 CFR part 63, subpart IIIII); standards for existing and new hazardous waste-burning 

incinerators (40 CFR 63.1203 [a][2] and [b][2]); standards for existing and new hazardous waste-burning cement kilns (40 CFR 63.1204 

[a][2] and [b][2]); and standards for existing and new hazardous waste-burning lightweight aggregate kilns (40 CFR 63.1205 [a][2] and 

[b][2]). See also section 8.3 of this document.
Introduction

14

http://www.epa.gov/mercury, for information on current activities related to control of 

power plant emissions.

Point sources of mercury discharging into waters are also regulated by NPDES permits.

Chlor-alkali facilities are subject to effluent guidelines that impose treatment levels 

reflective of the Best Available Technology Economically Achievable (40 CFR part 415).

All NPDES permits must ensure that permitted discharges achieve water quality 

standards (40 CFR 122.44(d)). Nonpoint source discharges are not regulated under 

federal regulations, but to the extent that these sources cause a water to exceed its water 

quality standards, states will develop TMDLs that identify the necessary reductions in 

these sources for achieving the water quality standards.

Anthropogenic emissions, however, are only one part of the mercury cycle. Releases 

from human activities today add to the mercury reservoirs that already exist in land, 

water, and air, both naturally and as a result of previous human activity.

2.3 How does methylmercury get into fish and shellfish?

Mercury is widely distributed in the environment. Understanding the distribution and 

cycling of mercury among the abiotic (nonliving) and biotic (living) compartments of 

aquatic ecosystems is essential to understanding the factors that govern methylmercury 

uptake in fish and shellfish tissue. The following is a synopsis of the current 

understanding of mercury cycling in the environment.

Mercury occurs naturally in the environment as several different chemical species. Most

mercury in the atmosphere (95–97 percent) is present in a neutral, elemental state, Hg0

(Lin and Pehkonen 1999). In water, sediments, and soils, most mercury is found in the 

oxidized, divalent state, HgII (Morel et al. 1998). A small fraction of this pool of divalent 

mercury is transformed by microbes into methylmercury (CH3HgII) (Jackson 1998). 

Methylmercury is retained in fish tissue and is the only form of mercury that 

biomagnifies in aquatic food webs (Kidd et al. 1995). Transformations among mercury 

species within and between environmental media result in a complicated chemical cycle.

The relative contributions of local, regional, and long-range sources of mercury to fish 

mercury levels in a given waterbody are strongly affected by the speciation of natural and 

anthropogenic emission sources. Elemental mercury is oxidized in the atmosphere to 

form the more soluble mercuric ion, HgII (Schroeder et al. 1989). Particulate and reactive 

gaseous phases of HgII are the principal forms of mercury deposited onto terrestrial and 

aquatic systems because they are more efficiently scavenged from the atmosphere 

through wet and dry deposition than is Hg0(Lindberg and Stratton 1998). Because HgII

species or reactive gaseous mercury (RGM) and particulate mercury (Hgp) in the 

atmosphere tend to be deposited more locally than Hg0, differences in the species of 

mercury emitted affect whether the mercury is deposited locally or travels longer 

distances in the atmosphere (Landis et al. 2004).

A portion of the mercury deposited in terrestrial systems is re-emitted to the atmosphere.

On soil surfaces, sunlight might reduce deposited HgII to Hg0, which might then escape

back to the atmosphere (Carpi and Lindberg 1997, Frescholtz and Gustin 2004, Scholtz et 

al. 2003). Significant amounts of mercury can be co-deposited to soil surfaces in 

throughfall and litterfall of forested ecosystems (St. Louis et al. 2001), and exchange of 
Introduction

15

gaseous Hg0 by vegetation has been observed (e.g., Gustin et al. 2004). HgII has a strong 

affinity for organic compounds such that inorganic mercury in soils and wetlands is 

predominantly bound to dissolved organic matter (Mierle and Ingram 1991).

Concentrations of methylmercury in soils are generally very low. In contrast, wetlands 

are areas of enhanced methylmercury production and account for a significant fraction of 

the external methylmercury inputs to surface waters that have watersheds with a large 

portion of wetland coverage (e.g., St. Louis et al. 2001).

In the water column and sediments, HgII partitions strongly to silts and biotic solids, sorbs 

weakly to sands, and complexes strongly with dissolved and particulate organic material.

HgII and methylmercury sorbed to solids settle out of the water column and accumulate 

on the surface of the benthic sediment layer. Surficial sediments interact with the water

column through resuspension and bioturbation. The amount of bioavailable 

methylmercury in water and sediments of aquatic systems is a function of the relative 

rates of mercury methylation and demethylation. In the water, methylmercury is degraded 

by two microbial processes and sunlight (Barkay et al. 2003; Sellers et al. 1996). Mass 

balances for a variety of lakes and coastal ecosystems show that in situ production of 

methylmercury is often one of the main sources of methylmercury in the water and 

sediments (Benoit et al. 1998; Bigham and Vandal 1994; Gbundgo-Tugbawa and Driscoll 

1998; Gilmour et al. 1998; Mason et al. 1995). Changes in the bioavailability of inorganic 

mercury and the activity of methylating microbes as a function of sulfur, carbon, and 

ecosystem-specific characteristics mean that ecosystem changes and anthropogenic 

“stresses” that do not result in a direct increase in mercury loading to the ecosystem, but 

alter the rate of methylmercury formation, might also affect mercury levels in organisms 

(e.g., Grieb et al. 1990).

Dissolved HgII and methylmercury accumulate in aquatic vegetation, phytoplankton, and 

benthic invertebrates. Unlike HgII, methylmercury biomagnifies through each successive 

trophic level in the benthic and pelagic food chains such that mercury in predatory, 

freshwater fish is found almost exclusively as methylmercury (Bloom 1992; Watras et al. 

1998). In fish, methylmercury bioaccumulation is a function of several uptake pathways 

(diet, gills) and elimination pathways (excretion, growth dilution) (Gilmour et al. 1998;

Greenfield et al. 2001). Factors such as pH, length of the aquatic food chain, temperature, 

and dissolved organic carbon (DOC) can affect bioaccumulation (Ullrich et al. 2001). As 

a result, the highest mercury concentrations for a given fish species correspond to 

smaller, long-lived fish that accumulate methylmercury over their life span with minimal 

growth dilution (e.g., Doyon et al. 1998). In general, higher mercury concentrations are 

expected in top predators, which are often large fish relative to other species in a 

waterbody.

2.4 Why is EPA publishing this document?

In a January 8, 2001, Federal Register notice (66 FR 1344), EPA announced the 

availability of its recommended water quality criterion for methylmercury. In that notice, 

EPA also stated that development of the associated implementation procedures and 

guidance documents would begin by the end of 2001. Therefore, EPA makes this 

guidance available to fulfill that commitment to enable states and authorized tribes to 

adopt into their water quality standards the recommendations set forth in Water Quality 

Criterion for the Protection of Human Health: Methylmercury (USEPA 2001a), or other 
Introduction

16

water quality criteria for methylmercury, on the basis of scientifically defensible 

methods.

This nontraditional approach––developing a water quality criterion as a fish and shellfish 

tissue value––raises several implementation questions on both technical and 

programmatic fronts. Development of water quality standards, NPDES permits, and 

TMDLs presents many challenges because these activities have usually been based on a 

water concentration (e.g., as a measure of mercury levels in effluent). This guidance 

addresses issues associated with states’ and authorized tribes’ adoption of the new water 

quality criterion into their water quality standards programs and implementation of the 

revised water quality criterion in TMDLs and NPDES permits. Furthermore, because 

atmospheric deposition is a large source of mercury for many waterbodies,

implementation of this criterion involves coordination across various media and program 

areas, which is also addressed in this guidance.

EPA expects that, as a result of this revised methylmercury water quality criterion, 

together with a more sensitive method for detecting mercury in effluent and the water 

column and increased monitoring of previously unmonitored waterbodies, the number of 

waterbodies that states report on CWA section 303(d) lists as impaired due to mercury 

contamination might increase. This guidance includes recommended approaches for 

relating a concentration of methylmercury in fish tissue to a concentration of mercury in 

ambient water (see chapter 3); a recommended approach for directly using the 

methylmercury tissue criterion as a basis for issuing NPDES permits (see chapter 7); and 

approaches that have been used in approved TMDLs for waterbodies impaired by 

mercury. This guidance includes examples of TMDL approaches for waterbodies where 

much of the mercury comes from atmospheric sources, as well as examples of TMDLs 

for waterbodies where the mercury is predominantly from past mining activity. Finally, 

the guidance describes ongoing EPA efforts to address sources of mercury, such as 

programs under the CAA and pollution prevention activities.

EPA recognizes the complexity and comprehensive nature of this guidance. As is always 

the case when EPA issues technical guidance, EPA will provide outreach and technical 

assistance to states and authorized tribes in implementing this guidance.

2.5 What is the effect of this document?

This guidance document presents suggested approaches––but not the only technically 

defensible approaches––to criteria adoption and implementation. The guidance is not a

substitute for applicable sections of the CWA or EPA’s regulations; nor is it a regulation 

itself. Thus, it cannot impose legally binding requirements on EPA, states, authorized 

tribes, or the regulated community and may not apply to a particular situation. EPA, state, 

territorial, and tribal decision makers retain the discretion to adopt other scientifically 

defensible approaches that differ from this guidance. EPA may change this guidance in 

the future.
Water Quality Criteria and Standards Adoption

17

3 Water Quality Criteria and Standards 

Adoption

3.1 What must states and authorized tribes include as 

they adopt the methylmercury criterion?

3.1.1 What do the CWA and EPA’s regulations require?

The CWA and EPA’s regulations specify the requirements for adoption of water quality 

criteria into state or tribal water quality standards. States and authorized tribes must adopt 

water quality criteria4that protect designated uses. See CWA section 303(c)(2)(A). Water 

quality criteria must be based on a sound scientific rationale and must contain sufficient 

parameters or components to protect the designated uses (see 40 CFR 131.11). States and 

authorized tribes must adopt numeric criteria for all toxic pollutants for which EPA has 

established national recommended ambient water quality criteria (AWQC) and where the 

discharge or presence of these pollutants could reasonably interfere with the designated 

uses (see CWA section 303(c)(2)(B)). EPA issued guidance on how states and authorized 

tribes may comply with CWA section 303(c)(2)(B), which is now contained in the Water 

Quality Standards Handbook: Second Edition (USEPA 1994). This document provides 

three options for compliance:

● Option 1: States and authorized tribes may adopt statewide or reservation-wide 

numeric chemical-specific criteria for all toxic pollutants5for which EPA has 

issued CWA section 304(a) criteria guidance.

● Option 2: States and authorized tribes may adopt numeric chemical-specific criteria 

for those stream segments where the state or tribe determines that the priority toxic 

pollutants for which EPA has issued CWA section 304(a) criteria guidance are 

present and can reasonably be expected to interfere with designated uses.

● Option 3: States or authorized tribes may adopt a chemical-specific translator 

procedure6that can be used to develop numeric criteria as needed.

To protect human health from contaminants in fish, EPA considers the 2001 

methylmercury criterion a sound, scientifically based approach for meeting human health 

designated uses. Thus, EPA strongly encourages states and authorized tribes to adopt the 

2001 methylmercury criterion or any sound, scientifically based approach for 

4 The term water quality criteria has two different definitions under the CWA. Under CWA section 304(a), EPA publishes recommended 

water quality criteria guidance that consists of scientific information regarding concentrations of specific chemicals or levels of parameters 

in water that protect aquatic life and human health. The 2001 methylmercury criterion is an example of a recommended section 304(a) 

criterion. States may use these recommended criteria as the basis for developing water quality standards. Water quality criteria are also 

elements of state water quality standards adopted under CWA section 303(c).

5 CWA section 307(a) identifies a list of toxic pollutants that EPA has published at 40 CFR 401.15.

6 A translator procedure is simply the detailed process adopted by a state or authorized tribe, that explains how the state or authorized tribe 

will interpret its narrative criteria for toxics so that a quantifiable term can be used in assessment, permitting, and TMDL development. For 

example, a state or tribe could use EPA’s water quality criteria as the means for interpreting its narrative criteria.
Water Quality Criteria and Standards Adoption

18

methylmercury or mercury into their water quality standards to fulfill the requirements of 

40 CFR part 131.

Water quality criteria generally consist of three components: magnitude, duration, and 

frequency (USEPA 1994). Water quality criteria for human health are typically expressed 

as an allowable magnitude. A criterion is calculated to protect against long-term chronic,

human health effects. Thus, the duration of exposure assumed in deriving the criterion is 

a lifetime exposure even though the criterion is expressed as a magnitude of contaminant 

per day (USEPA 1991).

3.1.2 What is the recommended form of the methylmercury 

criterion?

EPA’s current recommended CWA section 304(a) water quality criterion for 

methylmercury is expressed as a fish7tissue concentration value (0.3 milligram 

methylmercury per kilogram of wet-weight fish tissue, or 0.3 mg/kg). With the 

publication of the 304(a) criterion, EPA withdrew the previous ambient human health 

water quality criterion for mercury as the recommended section 304(a) water quality 

criterion for states and authorized tribes to use as guidance in adopting water quality 

standards (USEPA 2001c).

States and authorized tribes that adopt a new or revised methylmercury criterion into their 

water quality standards have several options. They may:

 Adopt the criterion as a fish tissue residue concentration, and implement it 

without water column translation; or

 Adopt a water column concentration, using the translation methodologies 

outlined in section 3.1.3.1, and implement it using traditional approaches; or

 Use a combination of the above approaches. For example, states and tribes could 

adopt a fish tissue criterion and implement it without water column translation 

for some or all waters, and translate the criterion to water column values for 

some or all waters.

Collecting site-specific data for reasonable potential determinations or translating the fish 

tissue criterion to water column concentration, where needed, may take time to collect.

Accordingly, states and authorized tribes may consider retaining their existing water 

column criteria, on a temporary basis, particularly for waters where there is a relatively 

high direct water input of mercury. In such a case, where the state has retained the 

existing water column criteria, permits must include both a limit based on the existing 

numeric water column criterion and other requirements based on the fish tissue criterion

(see chapter 7).

7 The criterion applies to both finfish and shellfish. For purposes of simplifying language in this document, the term fish means both finfish 

and shellfish.
Water Quality Criteria and Standards Adoption

19

Where a water column translation of the fish tissue criterion has been developed or where 

site-specific data to do so are readily available using one of the options in Section 3.1.3.1, 

states and authorized tribes should translate the fish tissue criterion, and implement using 

traditional approaches. If data are not available to translate, the state or authorized tribe 

may design data collection activities to obtain the necessary data. States and authorized

tribes should focus data collection activities on water bodies where methylmercury 

impairments are high priorities for action because of high direct water inputs. EPA 

recommends that states and tribes not only focus on data collection but also on the 

development of translators for waters with high direct water inputs of mercury.

Additionally, EPA recommends that states and tribes include such translators in their 

criterion implementation plans.

States and authorized tribes remain free not to use EPA’s current recommendations, 

provided that their new or revised water quality criteria for methylmercury protect the 

designated uses and are based on a scientifically defensible methodology. In doing this, 

states and authorized tribes should consider bioaccumulation, local or statewide fish 

consumption, and exposure to mercury from other sources (relative source contribution, 

or RSC). EPA will evaluate criteria submitted by states and authorized tribes case by

case.

If states and authorized tribes decide to adopt the tissue criterion expressed as a fish 

tissue concentration without translating it to a traditional water column concentration, this 

decision will lead to choices on how to implement the tissue criterion. A state or 

authorized tribe could decide to develop TMDLs and to calculate WQBELs in NPDES 

permits directly without first measuring or calculating a BAF. This guidance provides 

options for such approaches in chapters 6 and 7.

EPA does not require states and tribes to translate the fish tissue criterion into water 

column criteria. For waters with relatively high direct water inputs of mercury (mercury 

from point sources and nonpoint sources other than air deposition), EPA does 

recommend developing TMDLs, an analysis of sources and loading capacity similar to 

what would be provided in a TMDL, or a water column translation of the fish tissue 

criterion, to provide important information for developing appropriate permit limits. See 

section 7.5.2.2 for a further discussion of this situation.

3.1.2.1 Developing a criterion implementation plan

Regardless of the approach a state decides to use to implement its criterion, EPA 

encourages states and authorized tribes to develop a criterion implementation plan to 

ensure environmentally protective and effective administration of all water quality related 

programs with respect to methylmercury. Developing an implementation plan can 

facilitate adoption of the tissue-based criterion and provide transparency on state or tribal 

approaches to the numerous implementation issues associated with this type of criterion. 

This benefits not only the state or tribe but the regulated community and the public.

Developing an implementation plan could also facilitate subsequent regulatory decisions.

Working with stakeholders and the public to develop an appropriate implementation plan 

concurrent with adoption of a tissue-based criterion could facilitate subsequent 

implementation decisions (e.g., application of the criterion in the context of 303(d) listing 

decisions or NPDES permitting actions) and decrease the likelihood of legal challenges.
Water Quality Criteria and Standards Adoption

20

It may be most useful to states and tribes to develop such an implementation plan prior to 

the adoption of the fish tissue criterion. States and tribes could propose draft plans during 

triennial reviews or when they are developing updates or revisions to their water quality 

standards. Additionally, EPA encourages states and tribes to take public comment on 

their draft plan during the time when the state or tribe is proposing to adopt the fish tissue 

criterion.

If a state or tribe develops an implementation plan during adoption of its criterion, the 

state or tribe should submit the plan to EPA with the state’s new criterion. Although the 

plan itself is not subject to EPA review and approval, the plan could facilitate EPA’s 

review of the new criterion.

Examples of potential implementation issues the plan could cover include criterion 

adoption into the water quality standards (e.g., tissue or water column value with 

translators, BAF development methods), reasonable potential and permitting decisions, 

ambient monitoring strategies, and impairment determinations.

3.1.2.2 Why is the fish tissue concentration criterion recommended?

EPA recommends that when states and authorized tribes adopt new or revised 

methylmercury water quality criteria, they adopt the criteria in the form of a fish tissue

methylmercury concentration. This is the preferred form for the following reasons:

● A criterion expressed as a fish tissue concentration is closely tied to the “fishable” 

designated use goal applied to nearly all waterbodies in the United States.

● A fish tissue concentration value is expressed in the same form (fish tissue) through 

which humans are exposed to methylmercury.

● A fish tissue concentration value is more consistent with how fish advisories are 

issued.

● At environmentally relevant concentrations, methylmercury is currently easier to 

detect in fish tissue than in water samples.

3.1.2.3 How is the fish tissue concentration criterion calculated?

The derivation of a methylmercury water quality criterion uses a human health 

toxicological risk assessment (e.g., a reference dose [RfD]), exposure data (e.g., the 

amount of pollutant ingested, inhaled, or absorbed per day), and data about the target 

population to be protected. The methylmercury fish tissue residue criterion (TRC) for the 

protection of human health is calculated as:
Water Quality Criteria and Standards Adoption

21

TRC =

BW RfD - RSC

FIi

i





( )

2

4

(Equation 1)

Where:

TRC = fish tissue residue criterion (in mg/kg) for freshwater and estuarine fish 

and shellfish

RfD = reference dose (based on noncancer human health effects); for 

methylmercury, it is 0.1 μg/kg body weight/day

RSC = relative source contribution (subtracted from the RfD to account for 

methylmercury in marine fish consumed8), estimated to be 0.027 μg/kg 

body weight/day

BW = human body weight (default value of 70 kg for adults)

FI = fish intake at trophic level (TL)i (i = 2, 3, 4); total default intake of 

uncooked freshwater and estuarine fish is 17.5 g fish/day for the general 

U.S. adult population9

This equation and all values used in the equation are described in Water Quality Criterion 

for the Protection of Human Health, Methylmercury (USEPA 2001a). This equation is 

essentially the same equation used in the 2000 Human Health Methodology (USEPA 

2000b) to calculate a water quality criterion for a pollutant that may cause noncancerous

health effects. Here, it is rearranged to solve for a protective concentration in fish tissue 

rather than in water. Thus, it does not include a BAF or drinking water intake value 

(methylmercury exposure from drinking water is negligible (USEPA 2001c)). When all 

the numeric values are put into the generalized equation, the TRC of 0.3 mg 

methylmercury/kg fish is the concentration in fish tissue that should not be exceeded on 

the basis of a consumption rate of 17.5 g fish/day of freshwater or estuarine fish. EPA 

encourages states and authorized tribes to develop a water quality criterion for 

methylmercury using local or regional data to modify the fish consumption rate or the 

RSC rather than using the default values if the state or authorized tribe believes that such 

a water quality criterion would be more appropriate for its target population.

The TRC value is not based on any default breakout of fish consumption by trophic level.

The trophic levels assigned to the fish consumption value should reflect those that each 

target population consumes. For assessing impairment or attainment of the TRC, a state

8 The RSC accounts for exposures from all anticipated sources so that the entire RfD is not apportioned to freshwater/estuarine fish and 

shellfish consumption alone. In the assessment of human exposure in the methylmercury water quality criterion document, EPA found that 

human exposures to methylmercury were negligible except from freshwater/estuarine and marine fish. Therefore, in developing the 

criterion on the basis of consumption of freshwater/estuarine fish, EPA subtracted the exposure due to consumption of marine fish. See 66 

FR 1354–1355; January 8, 2001.

9 The consumption rate value of 17.5 grams uncooked fish per day is the 90th percentile of freshwater and estuarine fish consumed by the 

public according to the 1994–96 Continuing Survey of Food Intakes by Individuals (USEPA 2000a). EPA uses this value as the default 

consumption rate in development of water quality criteria. The default trophic level values for the general population are 3.8 g fish/day for 

TL2, 8.0 g fish/day for TL3, and 5.7 g fish/day for TL4. The rationale behind the selection of this value is described in the Human Health 

Methodology (USEPA 2000b).
Water Quality Criteria and Standards Adoption

22

or authorized tribe may choose to assign the TRC value to only trophic level 4 or to the 

highest trophic level consumed. This approach is conservative in that it assumes that all 

fish consumed are at the highest trophic level, and it will likely protect most, if not all, 

populations at an uncooked freshwater or estuarine fish consumption rate of 17.5 

grams/day. If a state or authorized tribe wishes to calculate the TRC value on the basis of

consumption at each trophic level for monitoring and compliance purposes, it would first 

determine consumption patterns at each trophic level for the target population(s). (For

information on determining consumption patterns, see chapter 4.) This approach might be 

more precise and is less likely to be overprotective; however, developing it could be 

resource-intensive.

3.1.3 What approaches should states or authorized tribes consider 

when developing a water column concentration criterion?

As described in section 3.1.2 above, there may be situations where it is appropriate to 

adopt a criterion expressed as a water column concentration. EPA recognizes that a fish 

tissue residue water quality criterion is new to states and authorized tribes and might pose

implementation challenges for traditional water quality programs. Water quality 

standards, water quality-based effluent limits10 (WQBELs), total maximum daily loads 

(TMDLs), and other activities generally employ a water column value. This section 

provides information for states and authorized tribes that decide to adopt a criterion that 

is expressed as a water column concentration.

Alternatively, a state or authorized tribe may decide to adopt a fish tissue criterion with a

site-specific procedure for translating the tissue criterion to a water column concentration.

Because methylmercury bioaccumulation can vary substantially from one location to 

another, this option allows for the tissue criterion to be translated to a water concentration 

using site-specific information on methylmercury bioaccumulation (i.e., site-specific 

BAFs). Administratively, this option might be more efficient compared to adopting a 

water concentration criterion for an entire state or tribal jurisdiction or adopting or 

approving site-specific criteria on an individual waterbody basis. Approaches for 

translating a tissue concentration-based criterion to a water concentration are provided in 

the following section (section 3.1.3.1).

Developing a water column translation of the fish tissue criterion requires assessment of 

methylmercury bioaccumulation at an appropriate geographic scale. The uncertainty 

associated with differential bioaccumulation of methylmercury across sites within a state

or tribal jurisdiction will be embedded in the state or tribal water-based criterion.

Reducing such uncertainty is one of the primary reasons EPA chose to express its 

national recommended criterion for methylmercury as a tissue concentration rather than 

as a water concentration.

10 A WQBEL is a requirement in an NPDES permit that is derived from, and complies with, all applicable water quality standards and is 

consistent with the assumptions and requirements of any approved wasteload allocation (see 40 CFR 122.44(d)(1)(vii)).
Water Quality Criteria and Standards Adoption

23

To express the methylmercury tissue concentration-based criterion as a water 

concentration, a state or authorized tribe would translate the methylmercury criterion 

concentration in fish tissue to methylmercury concentrations in the water column. To 

accomplish this, the state or authorized tribe would develop BAFs. In the 2001 Federal 

Register notice of the methylmercury criterion, EPA identified three different possible

approaches for developing a BAF. These approaches are discussed in more detail in 

section 3.1.3.1. The basic equations used in developing a water column criterion are 

presented below, and additional discussion of calculating BAFs is presented in the 

following section.

The following equation may be used to translate the tissue concentration-based human 

health AWQC to a water concentration-based methylmercury criterion using a BAF as

AWQC = TRC / BAF (Equation 2)

Where:

AWQC = water concentration-based ambient water quality criterion for 

methylmercury in milligrams per liter (mg/L)

TRC = tissue residue concentration; the water quality criterion for 

methylmercury in fish tissue in mg/kg

BAF = bioaccumulation factor for trophic levels 2, 3, and 4, weighted on the 

basis of fish consumption rates for each trophic level in liters per 

kilogram (L/kg)

The BAF is the ratio of the concentration of the chemical in the appropriate tissue of the 

aquatic organism and the concentration of the chemical in ambient water at the site of 

sampling. BAFs are trophic-level-specific. EPA recommends that they be derived from 

site-specific, field-measured data as

BAF

C

C

t

w



(Equation 3)

Where:

BAF = bioaccumulation factor, derived from site-specific field-collected 

samples of tissue and water in L/kg

Ct = concentration of methylmercury in fish tissue in mg/kg, wet tissue 

weight

Cw = concentration of methylmercury in water in mg/L

When such data are unavailable, other approaches for deriving BAFs may be used, as

outlined in section 3.1.3.1.

In the calculation to derive an AWQC as a water column concentration, the BAFs for the 

different trophic levels are combined to provide a weighted BAF value. For example, if a 

state wants to protect a population that eats on average 17.5 grams per day of uncooked 

fish from a waterbody, and 75 percent of the fish eaten are in trophic level 4 and 25

percent of the fish eaten are in trophic level 3, the weighted BAF would be the sum of 
Water Quality Criteria and Standards Adoption

24

0.25 times the trophic level 3 BAF and 0.75 times the trophic level 4 BAF. Section 

3.2.1.2 provides guidance on estimating fish intake rates.

3.1.3.1 How is the methylmercury fish tissue concentration translated to 

a water concentration?

Should a state or authorized tribe decide to translate the methylmercury fish tissue 

criterion into a water column concentration, it would assess the extent to which 

methylmercury is expected to bioaccumulate in fish tissue for the site(s) of interest.

Assessing and predicting methylmercury bioaccumulation in fish is complicated by a 

number of factors that influence bioaccumulation. These factors include the age or size of 

the organism; food web structure; water quality parameters such as pH, DOC, sulfate, 

alkalinity, and dissolved oxygen; mercury loadings history; proximity to wetlands;

watershed land use characteristics; and waterbody productivity, morphology, and 

hydrology. In combination, these factors influence the rates of mercury bioaccumulation 

in various—and sometimes competing—ways. For example, these factors might act to 

increase or decrease the delivery of mercury to a waterbody, alter the net production of 

methylmercury in a waterbody (through changes in methylation and/or demethylation

rates), or influence the bioavailability of methylmercury to aquatic organisms. Although 

bioaccumulation models have been developed to address these and other factors for 

mercury, their broad application can be limited by the site- or species-specific nature of 

many of the factors that influence bioaccumulation and by limitations in the data 

parameters necessary to run the models.

The bioaccumulation of nonionic organic chemicals11 such as methylmercury can also be 

affected by a number of these same physicochemical factors (e.g., loading history, food 

web structure, dissolved oxygen, DOC). However, a substantial portion of the variability 

in bioaccumulation for nonionic organic chemicals can be reduced by accounting for lipid 

content in tissues and organic carbon content in water and “normalizing” BAFs using

these factors (Burkhard et al. 2003; USEPA 2003). Normalizing to the age or size 

(length, weight) of fish has been shown to reduce variability in measures of 

bioaccumulation (Brumbaugh et al. 2001; Glass et al. 2001; Sonesten 2003; Sorensen et 

al. 1990; Wente 2004). The United States Geological Survey (USGS) developed a 

procedure called the National Descriptive Model for Mercury in Fish Tissue (NDMMF) 

(Wente 2004). This model provides a translation factor to convert a mercury 

concentration taken from one species/size/sample method to an estimated concentration 

for any other user-predefined species/size/sample method.

11 Nonionic organic compounds are those organic compounds that do not ionize substantially when dissolved in water and therefore are 

more likely to associate with sediment compounds, lipids, or other compounds in water (USEPA 2000b).
Water Quality Criteria and Standards Adoption

25

Taking into account the previous discussion, EPA has outlined in this document three 

different approaches that could be considered for relating a concentration of 

methylmercury in fish tissue to a concentration of methylmercury in ambient water, 

should a state decide to develop or implement its standard in this manner:

1. Use site-specific methylmercury BAFs derived from field studies.

2. Use a scientifically defensible bioaccumulation model.

3. Where appropriate, use BAFs derived using the results of field studies that are not 

site-specific. Appropriate situations for using such BAFs include waters where direct 

water inputs are relatively high and where ambient fish tissue data are unavailable, 

where deriving site-specific, field-measured BAFs is not feasible, or where using a 

model is not feasible. Such BAFs may include the draft national BAFs presented in 

appendix A of Water Quality Criterion for the Protection of Human Health: 

Methylmercury (USEPA 2001a) and discussed in more detail below. Alternatively, 

BAFs may be derived using other approaches, such as a combination of national and 

site-specific data in conjunction with other, non-site-specific data, to create better 

estimates.

Of these approaches, 1 and 2 are preferred over 3. Because of the significant uncertainties 

inherent in non-site-specific estimates of BAFs (including the draft national BAFs), they 

should be used as defaults only in limited circumstances such as:

● When a state determines that use of the draft national BAFs are appropriate (for 

example, where direct water inputs are relatively high, where no other data are 

available to derive site-specific field-measured BAFs, and use of an appropriate 

BAF model is not feasible)

● When a state can show that such BAFs are appropriate for its situation (e.g., a state 

has data or analyses that demonstrate that the draft national BAFs would be 

appropriate)

Mercury Terminology

For the purposes of this document, the following definitions apply:

Mercury (or total mercury): The sum of all forms of mercury, including methylmercury, other 

organic forms, inorganic, and elemental mercury. All of these are toxic, and inorganic and 

elemental mercury can be methylated in the environment.

Methylmercury: The organic form of mercury, that bioaccumulates in the food chain. (Other 

organic forms of mercury exist, but exposure to them through environmental pathways is not 

significant.)

Dissolved mercury (or filtered mercury): The portion of mercury that passes through a 

filter.

Dissolved methylmercury (or filtered methylmercury): The portion of methylmercury 

which passes through a filter.

Total recoverable mercury (or unfiltered mercury): The dissolved portion plus the 

particulate portion of mercury in a water sample.

Total recoverable methylmercury (or unfiltered methylmercury): The dissolved portion 

plus the particulate portion of methylmercury in a water sample.
Water Quality Criteria and Standards Adoption

26

● As an interim approach until more appropriate BAFs can be developed using other 

data and/or an alternate approach

The reasons for preferring approaches 1 and 2 are discussed in more detail below. 

However, the hierarchy assigned to the approaches is not intended to be inflexible. For 

example, in some cases, the site-specific information available may be so limited in 

quality or quantity that BAFs derived using other data may be preferable. In other cases, 

there might be enough site-specific information to indicate that the local conditions 

approximate the draft national values.

In situations where the state or tribe has some data available on fish tissue and water 

column levels in its jurisdiction, but data are insufficient to support broad development of 

site-specific translations, the state or tribe may be able to use these data in combination 

with an evaluation of the draft national BAFs to help develop water column translations. 

For example, California’s Office of Environmental Health Hazard Assessment compiled 

mercury concentration data for water and biota, and calculated state-specific BAFs for 

different types of waters and different trophic levels. The office found enough similarities 

between the state-specific BAFs and EPA’s draft national BAFs that it recommended 

using EPA’s draft national values as an interim approach until more complete statespecific data becomes available (Sanborn and Brodberg 2006). The state is in the process 

of deciding whether to adopt this approach.

If the state or tribe chooses to derive BAFs using the third approach above, the state or 

tribe should provide an accompanying rationale that acknowledges an understanding of 

the potential limitations of the approach.

Developing site-specific data to support approaches 1 and 2 can be facilitated by efforts 

involving stakeholders, states, and authorized tribes. Developing site-specific data is one

possible approach EPA recommends permitting authorities consider to help develop 

NPDES permits in watersheds where mercury loadings from point sources are relatively 

high. See section 7.5.2.2.

3.1.3.1.1 Site-specific bioaccumulation factors derived from field studies

The use of site-specific BAFs based on data obtained from field-collected samples of tissue 

from aquatic organisms that people eat and water from the waterbody of concern—referred 

to as a “field-measured site-specific BAF”—is the most direct and most relevant measure 

of bioaccumulation. This approach is consistent with EPA’s bioaccumulation guidance 

contained in the 2000 Human Health Methodology (USEPA 2000b) and the Technical 

Support Document for developing national BAFs (USEPA 2003). Although a BAF is 

actually a simplified form of a bioaccumulation model, the field-measured site-specific 

BAF approach is discussed separately here because of its widespread use and application.

A field-measured site-specific BAF is derived from measurements of methylmercury 

concentrations in tissues of aquatic organisms and the ambient water they inhabit.

Because the data are collected from a natural aquatic ecosystem, a field-measured BAF 

reflects an organism’s exposure to a chemical through all relevant exposure routes (e.g., 

water, sediment, diet). Although a BAF can be measured for the aggregate of fish in a 

location, site-specific BAFs are often specific to trophic level and species of fish. The 

BAF can also be measured based on a predatory indicator species with a high propensity 
Water Quality Criteria and Standards Adoption

27

for bioaccumulation, such as largemouth bass. A field-measured site-specific BAF also 

reflects biotic and abiotic factors that influence the bioavailability and metabolism of a 

chemical that might occur in the aquatic organism or its food web at a given location. By 

incorporating these factors, field-measured site-specific BAFs account for the actual

uptake and accumulation of the chemical.

States and authorized tribes should exercise caution, however, in developing a sitespecific BAF for a migratory fish because its exposure to methylmercury occurred in part 

in areas other than where the fish was caught and therefore might not accurately predict 

the water column mercury concentrations associated with the fish tissue concentration of 

mercury. States and tribes should consider the life history of the migratory fish and the 

consumption patterns of the local population when considering BAFs for migratory 

species. States and tribes should also review how the applicable RSC considers migratory 

fish when considering including those species in BAF calculations (see section 3.2.1.1).

For the purposes of developing a criterion expressed as a water concentration, states and 

authorized tribes should calculate the BAF as the ratio of the concentration of 

methylmercury in the tissue of aquatic organisms that people eat to the concentration of

methylmercury in water12 (Equation 3). To predict the corresponding methylmercury 

concentration in water for a site, the tissue-based methylmercury criterion would then be 

divided by the site-specific BAF (Equation 2). Using the site-specific BAF approach 

assumes that at steady state, the accumulation of methylmercury by the aquatic organism 

varies in proportion to the methylmercury concentration in the water column.

As an example, California is currently employing a site-specific BAF approach in its 

Central Valley Region. In this approach, the state evaluated graphs of average 

concentrations of methylmercury in water and the corresponding concentrations in fish at 

multiple sites in a watershed. Researchers found statistically significant, positive 

relationships between concentrations of unfiltered methylmercury in water and in various 

trophic levels of the aquatic food chain (Slotton et al. 2004). California linearly regressed 

fish tissue methylmercury concentrations for specific trophic level (TL) 3 and 4 fish 

against aqueous methylmercury concentrations (P < 0.001, R

2

= 0.98, and P < 0.01, R2=

0.9, respectively) and determined methylmercury concentrations in unfiltered water that 

correspond to the fish tissue criteria used in the TMDL analyses (0.15 ng/L for TL3 fish 

and 0.14 ng/L for TL4 fish) (Central Valley Water Board 2005). California assumed that 

sites that fit in a statistically significant regression have similar processes controlling 

methylmercury accumulation. In other words, site-specific BAFs for such sites are nearly 

identical.

Strengths associated with using a site-specific BAF approach include simplicity, 

widespread applicability (i.e., site-specific BAFs can be derived for any waterbody, fish 

12 Although BAFs are sometimes calculated to represent the relationship between methylmercury in fish tissue and dissolved 

methylmercury in the water column, data can be collected to determine the relationship between methylmercury in fish tissue and total 

recoverable methylmercury or dissolved or total recoverable mercury in the water column. The Great Lakes Water Quality Initiative (GLI) 

used site-specific BAFs to convert directly from methylmercury in fish to total recoverable mercury in the water column. See 40 CFR part 

132, and appendix B to part 132, Methodology for Deriving Bioaccumulation Factors.
Water Quality Criteria and Standards Adoption

28

species, and the like), and that the net effects of biotic and abiotic factors that affect 

bioaccumulation are incorporated within the measurements used to derive the BAF.

Specifically, it is not required that the exact relationship between methylmercury 

accumulation and the factors that can influence it be understood or quantified to derive a 

site-specific BAF. By measuring the methylmercury concentrations empirically, these 

factors have been incorporated such that site-specific BAFs provide an accounting of the 

uptake and accumulation of methylmercury for an organism in a specific location and at a 

specific point in time.

Limitations to the site-specific BAF approach relate primarily to its cost and empirical 

nature. For example, the level of effort and associated costs of developing site-specific 

BAFs increase as the spatial scale of the site of interest increases. Furthermore, the 

amount of data necessary to obtain a representative characterization of methylmercury in 

the water and fish might take considerable time to gather. (For a discussion on sampling 

considerations for developing a site-specific BAF, see section 3.1.3.2.) The strictly 

empirical nature of this approach is also a barrier to extrapolating BAFs among species, 

across space, and over time because the site-specific factors that might influence 

bioaccumulation are integrated within the tissue concentration measurement and thus 

cannot be individually adjusted to extrapolate to other conditions.

3.1.3.1.2 Bioaccumulation models

Bioaccumulation models for mercury vary in the technical foundation on which they are 

based (empirically or mechanistically based), spatial scale of application (specific to 

waterbodies, watersheds or regions, and species of fish), and level of detail in which they 

represent critical bioaccumulation processes (simple, mid-level, or highly detailed 

representations). Thus, it is critical that states and tribes use a model that is appropriately 

developed, validated, and calibrated for the species and sites of concern.

Empirical bioaccumulation models that explicitly incorporate organism-, waterchemistry-, and waterbody/watershed-specific factors that might affect methylmercury 

bioaccumulation (e.g., fish species, age, length, pH, DOC, sulfate, alkalinity, sediment 

acid-volatile sulfide concentration, proximity to wetlands, land use, morphology, 

hydrology, productivity) usually take the form of multivariate regression models. Many 

examples of such models are available in the literature (e.g., Brumbaugh et al. 2001; 

Kamman et al. 2004; Sorensen et al. 1990). The model developed by Brumbaugh et al. 

(2001) is based on a national pilot study of mercury in 20 watersheds throughout the 

United States. Specifically, Brumbaugh et al. (2001) developed a multiple regression 

relationship between five factors: length-normalized mercury concentration in fish, 

methylmercury concentration in water, percentage of wetland area in the watershed, pH, 

and acid-volatile sulfide concentration in sediments (r2= 0.45; all fish species). When 

data were restricted to a single species (e.g., largemouth bass) and a single explanatory 

variable (e.g., methylmercury in water), a highly significant relationship was found 

(p < 0.001) with a similar degree of correlation (r2= 0.50). This demonstrates the 

importance of species specificity in the strength of such regression relationships and, in 

this case, methylmercury in water as an explanatory variable.

States and tribes should consider several important issues when using regression-based 

bioaccumulation models for translating from a tissue concentration to a water column 
Water Quality Criteria and Standards Adoption

29

concentration. First, a number of such regression models have been developed without 

explicitly incorporating methylmercury (or mercury) concentrations in the water column.

Instead, the models relate fish tissue methylmercury concentrations to variables that serve 

as proxies for methylmercury exposure (e.g., atmospheric deposition rates, ratio of the 

watershed drainage to the wetland area, pH, lake trophic status), often because of the 

costs associated with obtaining accurate measurements of mercury in the water column.

Obviously, such models cannot be directly solved for the parameter of interest 

(methylmercury in water). Second, correlation among independent or explanatory 

variables in these multiple regressions is common and expected (e.g., pH and 

methylmercury concentration in water). Such correlations among explanatory variables 

can cause bias and erroneous estimates of an explanatory variable (in this case, 

methylmercury concentration in water) when back-calculated from the regression 

equation (Neter et al. 1996). In such cases, using the underlying data set to develop a 

separate regression model with methylmercury concentration in water as the dependent 

variable is more appropriate. Last, because these regression models are based on 

empirical data, uncertainty is introduced when the results are extrapolated to aquatic 

ecosystems with different conditions. Only in a few cases have such models been tested 

using independent data sets (e.g., Kamman et al. 2004).

Mechanistic bioaccumulation models are mathematical representations of the natural 

processes that influence methylmercury bioaccumulation. The process of methylation

itself is incompletely understood, and general models for reliably predicting rates of 

methylation do not exist, although EPA’s WASP model might be useful in some 

environments. Three examples of mechanistic bioaccumulation models are the Dynamic 

Mercury Cycling Model, or D-MCM (EPRI 2002); the Bioaccumulation and Aquatic 

System Simulator, or BASS (Barber 2002), and the Quantitative Environmental Analysis 

Food Chain model, or QEAFDCHN (QEA 2000). A conceptual advantage of 

mechanistically based bioaccumulation models is that methylmercury bioaccumulation 

can be predicted under different conditions (e.g., different growth rates of fish, different 

water chemistry conditions, and different mercury loading scenarios) because the models 

include mathematical representations of various processes that affect bioaccumulation.

This advantage comes at the cost of additional input data necessary to run the model.

Notably, only a few models have been used to predict methylmercury bioaccumulation.

Such models have not been widely used and have been applied only to mercury in a few 

aquatic ecosystems under specific environmental conditions. Of the examples listed 

above, only the D-MCM was developed specifically for mercury. The D-MCM has not 

been applied to lotic systems (i.e. streams, rivers, estuaries) and therefore probably 

should be used only for static environments (lakes) at this time. The other models have 

been developed more generally, for nonionic organic chemicals that bioaccumulate, and 

require substantial modification and validation for application to mercury.

Most mechanistic bioaccumulation models use a chemical mass balance approach to 

calculate bioaccumulation in fish or other aquatic organisms. This approach requires 

considerable understanding of mercury loadings to and cycling within the environment.

None of the example models presented can predict bioaccumulation without considerable 

site-specific information, at least some degree of calibration to the waterbody of interest,

and, in some cases, considerable modification of the model. The amount and quality of 

data necessary for proper model application may equal or exceed that necessary to 
Water Quality Criteria and Standards Adoption

30

develop site-specific methylmercury BAFs, although these models might also help in 

determining BAFs if the kinetic condition in the waterbody is not steady state. Because of 

the need for site-specific data and calibration, these models are likely to cost as much to 

implement as a site-specific BAF. Their value comes from the ability to represent a wider 

range of explanatory and policy-relevant variables.

Regardless of the type of model used, states’ and authorized tribes’ methodologies should 

be consistent with the Methodology for Deriving Ambient Water Quality Criteria for the 

Protection of Human Health (section 5.6: National Bioaccumulation Factors for 

Inorganic and Organometallic Chemicals; USEPA 2000b) and Technical Support 

Document Volume 2: Derivation of National Bioaccumulation Factors (USEPA 2003).

These documents provide detailed discussion of topics such as BAF derivation 

procedures, bioavailability, and the steps involved in procedures 5 and 6 of the Human 

Health Methodology. States and tribes should document how they derive the site-specific 

parameters used in the bioaccumulation models and should describe the uncertainty

associated with the BAFs derived using any of the models.

3.1.3.1.3 Draft national bioaccumulation factors

EPA acknowledges that using site-specific BAFs or model-derived BAFs might not be 

feasible in all situations. Without site-specific methylmercury bioaccumulation data or an 

appropriate bioaccumulation model, another approach is to use EPA’s empirically

derived draft national methylmercury BAFs as defaults. EPA used the BAF guidance in 

the 2000 Human Health Methodology (USEPA 2000b, 2003) and the BAF methods in 

volume III, appendix D, of the Mercury Study Report to Congress (USEPA 1997c) to 

derive draft methylmercury BAFs as part of its initial efforts to derive a water columnbased recommended section 304(a) ambient water quality criterion for methylmercury.

These draft national BAFs were developed from field data collected from across the 

United States and reported in the published literature. The draft national BAFs and the 

uncertainties associated with them are discussed in appendix A, section I, of Water 

Quality Criterion for the Protection of Human Health: Methylmercury (USEPA 2001a).

The draft national BAFs (50th percentile values) are listed by trophic level in table 2.

Table 2. Draft national BAFs for dissolved methylmercury

BAF trophic 

level 2 

(L/kg)

BAF trophic 

level 3 

(L/kg)

BAF trophic

level 4

(L/kg)

120,000 680,000 2,700,000

Source: USEPA 2001a.

Note: Expressed as milligrams methylmercury/kilogram fish tissue per milligram methylmercury/liter

water, or liters per kilogram (L/kg).

To develop the draft national BAFs for each trophic level, EPA calculated the geometric 

mean of the field-measured BAFs obtained from the published literature. EPA believes 

the geometric mean BAFs are the best available central tendency estimates of the 

magnitude of BAFs nationally, understanding that the environmental and biological 

conditions of the waters of the United States are highly variable. Specifically, the data 

presented in Water Quality Criterion of the Protection of Human Health: Methylmercury 
Water Quality Criteria and Standards Adoption

31

(USEPA 2001a) indicate that BAFs for trophic levels 3 and 4 vary by a factor of 100 

(two orders of magnitude) between the 5th and 95th percentiles. EPA does not 

recommend basing an AWQC on BAF values associated with the extremes of the 

distribution (e.g., 10th or 90th percentile), unless supported by site-specific data. Such 

values might introduce an unacceptable level of uncertainty into the calculation of a 

water column-based AWQC. States and authorized tribes should consider the magnitude 

of the potential error when proposing to use the draft national BAFs.

When states and authorized tribes calculate a water column-based criterion using draft 

national BAFs that differ greatly from the BAFs for the waterbody of concern, the 

resulting water column-based criterion will be either over- or under-protective. As a 

result, evaluation of the results of the analysis of water samples might result in the false 

conclusion that a fish tissue concentration has been exceeded (when it actually has not) or 

a false conclusion that a fish tissue concentration has not been exceeded (when it actually 

has). For more information on the draft national BAFs, see chapter 6 and appendix A, 

section I, of EPA’s 304(a) water quality criterion for methylmercury (USEPA 2001a). 

The following examples illustrate the potential impact of calculating a water quality 

criterion using a BAF that is substantially different from the actual BAF.

● Underprotective scenario

A state uses the draft national BAF of 2,700,000 L/kg for trophic level 4 fish, but 

the BAF based on site-specific data for the trophic level 4 fish in the waterbody is 

three times that, or 8,100,000 L/kg. In using the draft national BAF, a state would 

consider water column concentrations up to 0.11 nanogram per liter (ng/L)

(0.3 mg/kg / 2,700,000 L/kg) to indicate attainment of the water quality column 

criterion. Using the BAF based on site-specific data, however, a water column 

criterion of 0.11 ng/L would correspond to a fish tissue concentration of 0.9 mg/kg, 

which is three times the 0.3 mg/kg criterion recommended to protect human health.

Thus, load reductions or permits using the draft national BAF of 2,700,000 L/kg 

would be underprotective.

● Overprotective scenario

A state uses the draft national BAF of 2,700,000 L/kg for trophic level 4 fish, but 

the BAF based on site-specific data for the trophic level 4 fish in the waterbody is 

one-third that, or 900,000 L/kg. As a result, a state would consider water column 

concentrations up to 0.11 ng/L (0.3 mg/kg / 2,700,000 L/kg) to indicate attainment 

of the water quality criterion. Using the BAF based on site-specific data, however, 

attainment of the water quality criterion could be achieved at a higher water 

column concentration, 0.33 ng/L. Thus, load reductions or permits using the draft 

national BAF of 2,700,000 L/kg would be overprotective.

EPA cautions water quality managers that methylmercury bioaccumulation is generally 

viewed as a site-specific process and that BAFs can vary greatly across ecosystems. The 

uncertainty in the estimates of a draft national BAF comes from uncertainty arising from 

natural variability, such as size of individual fish, and from uncertainty due to 

measurement error, such as error in measurements of mercury in water or lack of 

knowledge of the true variance of a process (e.g., methylation). Users of the draft national
Water Quality Criteria and Standards Adoption

32

BAFs are encouraged to review appendix A of Water Quality Criterion for the Protection 

of Human Health: Methylmercury (USEPA 2001a), which describes the uncertainties 

inherent in these values. The following is a synopsis of the discussion of uncertainty in 

that appendix.

● Uncertainty due to sampling and chemical analysis: In many cases, water 

methylmercury concentrations reported in the available studies incorporated limited 

or no cross-seasonal variability, incorporated little or no spatial variability, and 

were often based on a single sampling event. Because fish integrate exposure of 

mercury over a lifetime, comparing fish concentrations to a single sample or mean 

annual concentrations introduces bias to the estimates. The geographic range 

represented by the waterbodies was also limited.

● Uncertainty due to estimation method: The approaches used to estimate the draft 

national BAFs have their own inherent uncertainties. The approaches assume that 

the underlying process and mechanisms of mercury bioaccumulation are the same 

for all species in a given trophic level and for all waterbodies. They are also based 

on a limited set of data.

● Uncertainty due to biological factors: With the exception of deriving BAFs on the 

basis of river or lake waterbody type, there were no distinctions in the BAFs as to 

the size or age of fish, waterbody trophic status, or underlying mercury uptake 

processes. In reality, methylmercury bioaccumulation for a given species can vary 

as a function of the age (body size) of the organisms examined.

● Uncertainty due to universal application of BAFs: There is uncertainty introduced 

by failure of a single trophic-level-specific BAF to represent significant real-world 

processes that vary from waterbody to waterbody. The simple linear BAF model 

relating methylmercury in fish to mercury in water simplifies a number of 

nonlinear processes that lead to the formation of bioavailable methylmercury in the 

water column and subsequent accumulation. Much of the variability in field data 

applicable to the estimation of mercury BAFs can be attributed to differences in 

biotic factors (e.g., food chain, organism age or size, primary production, 

methylation or demethylation rates) and abiotic factors (e.g., pH, organic matter, 

mercury loadings, nutrients, watershed type or size) between aquatic systems.

Unfortunately, although the concentration of methylmercury in fish tissue is 

presumably a function of these varying concentrations, published BAFs are 

typically estimated from a small number of measured water values whose 

representativeness of long-term exposure is not completely understood.

Furthermore, although it is known that biotic and abiotic factors control mercury 

exposure and bioaccumulation, the processes are not well understood, and the 

science is not yet available to accurately model bioaccumulation on a broad scale.

Peer reviewers expressed concerns about the use of the draft national BAFs as defaults to 

predict bioaccumulation across all ecosystems and about using them to derive a national 

recommended section 304(a) water quality criterion for methylmercury that would 

suitably apply to waterbodies across the nation. EPA recognized the peer reviewers’

concerns and acknowledges that these draft national BAF values might significantly 

over- or underestimate site-specific bioaccumulation. As a result, EPA decided not to use 
Water Quality Criteria and Standards Adoption

33

the draft national BAFs to develop a national water-column-based AWQC for 

methylmercury. Furthermore, the draft national BAFs are EPA’s least preferred means 

for assessing the BAF. States and tribes should also consider whether more recent data 

and/or data that are more reflective of local conditions are available to supplant or 

supplement the limited database used to derive the draft national BAFs.

Risk managers should also understand that in using the draft national BAFs as defaults, 

one assumes that the biotic and abiotic processes affecting mercury fate and 

bioaccumulation are similar across different waterbodies, and therefore using the draft 

national BAFs does not address site-specific factors that might increase or decrease 

methylation and bioaccumulation. A state’s or tribe’s decision to use the draft national 

BAFs would be a risk management decision. The decision would reflect the state’s or 

tribe’s judgment that, for specific reasons, translating the fish tissue criterion to a water 

column value using such a BAF is preferable to implementing the fish tissue criterion 

directly (e.g., using the approaches discussed in this guidance), conducting studies to 

develop a site-specific BAF (e.g., site-specific field studies or bioaccumulation 

modeling), or not adopting the methylmercury criterion at all.

3.1.3.2 What are the sampling considerations for deriving site-specific 

field-measured BAFs?

For both fish tissue and water, states and authorized tribes should analyze for 

methylmercury when deriving site-specific BAFs. EPA has not yet published analytical 

methods to measure methylmercury in water or fish in 40 CFR part 136. For fish tissue, 

however, states and authorized tribes can estimate methylmercury concentrations by 

using the same analytical method used to measure for mercury, at least for upper-trophiclevel fish (levels 3 and 4). This is because 80 to 100 percent of the mercury found in the 

edible portions of freshwater fish greater than three years of age from these two trophic 

levels is in the form of methylmercury (USEPA 2000c). In fish greater than 

approximately three years of age, mercury has had sufficient time to bioaccumulate to 

roughly steady levels in the fish. Appendix A summarizes eight studies of the relative 

proportion of the mercury concentration in North American freshwater fish that is in the 

form of methylmercury. In six of the eight studies, methylmercury on average accounted 

for more than 90 percent of the mercury concentration in fish tissue. In the remaining two 

studies, methylmercury on average accounted for 80 to 90 percent of the mercury 

concentration in trophic level 3 and 4 fish.

States and tribes should consider a number of issues when sampling aquatic organism 

tissue and water to derive a site-specific BAF. The goal of deriving site-specific 

methylmercury BAFs is to reflect or approximate the long-term bioaccumulation of 

methylmercury in commonly consumed aquatic organisms of a specified trophic level.

Hence, an important sample design consideration is how to obtain samples of tissue and 

water that represent long-term, average accumulation of methylmercury. Methylmercury 

is often slowly eliminated from fish tissue. Therefore, concentrations of methylmercury 

in fish tissue tend to fluctuate much less than the concentration of methylmercury in 

water. Thus, for calculating representative site-specific BAFs, states and tribes should 

consider how to integrate spatial and temporal variability in methylmercury 

concentrations in both water and tissue. States and tribes should address the variability in 

methylmercury concentrations in fish tissue with age or size of the organism either by 
Water Quality Criteria and Standards Adoption

34

restricting sample collection to organisms of similar age or size classes or by using

appropriate normalization techniques. EPA’s fish sampling guidance recommends that 

fish should be of similar size so that the smallest individual in a composite is no less than 

75 percent of the total length (size) of the largest individual (USEPA 2000c). One way of 

normalizing data is by using the National Descriptive Model for Mercury in Fish Tissue, 

or NDMMF (Wente 2004). The NDMMF is a statistical model that normalizes Hg fish 

tissue concentration data to control for species, size, and sample type variability. An 

example use of the NDMMF is in the combination of mercury fish tissue data from two 

databases (USEPA 2005a).

States and tribes should assess the fish consumption patterns of the exposed human 

population when designing a site-specific sampling plan. Because the age and size of 

aquatic organisms are correlated with the magnitude of methylmercury accumulation, the 

types and sizes of aquatic organisms being consumed should be considered when 

determining which fish to sample for deriving BAFs. This information should also guide 

the decision on whether the site-specific BAF should be based on a single trophic level 

(e.g., trophic level 4) or on multiple trophic levels.

States and authorized tribes should review site-specific data used to calculate fieldmeasured BAFs and thoroughly assess the quality of the data and the overall uncertainty

in the BAF values. States and authorized tribes should also consider the following general 

factors when determining the acceptability of field-measured BAFs reported in the 

published scientific literature. The same general issues and questions should also be 

addressed when designing a field study to generate site-specific field-measured BAFs.

● Calculate a field-measured BAF using aquatic organisms that are representative of 

the aquatic organisms commonly consumed at the site of interest (e.g., river, lake, 

ecoregion, state). Review information on the ecology, physiology, and biology of 

the target organisms when assessing whether an organism is a reasonable surrogate 

of a commonly consumed organism.

● Determine the trophic level of the study organism by taking into account its life 

stage, its diet, and the food web structure at the study location. Information from 

the study site (or similar sites) is preferred when evaluating trophic status. If such 

information is lacking, states and authorized tribes can find general information for 

assessing the trophic status of aquatic organisms in Guidance for Assessing 

Chemical Contaminant Data for Use in Fish Advisories, volume 1, Fish Sampling 

and Analysis (USEPA 2000c).

● Collect length, weight, and age data for any fish used in deriving a field-measured 

BAF because current information suggests that variability in methylmercury 

accumulation is dependent on fish age and size (USEPA 2001a). This information 

helps normalize the BAF to a standardized fish size within the range of fish sizes 

and species known to be consumed by the human population of interest.

● Verify that the study used to derive the field-measured BAF contains sufficient 

supporting information from which to determine that tissue and water samples were 

collected and analyzed using appropriate, sensitive, accurate, and precise analytical 

methods.
Water Quality Criteria and Standards Adoption

35

● Verify that the water concentrations used to derive a BAF reflect the average 

exposure of the aquatic organism of concern that resulted in the concentration 

measured in its tissue. Concentrations of methylmercury in a waterbody vary 

seasonally and diurnally (Cleckner et al. 1995) because of a variety of biological 

and physical factors.

● Attempt to design a field sampling program that addresses potential temporal and 

spatial variability and that allows estimation of average exposure conditions. The 

study should be designed to sample an area large enough to capture the more 

mobile organisms and also to sample across seasons or multiple years when 

methylmercury concentrations in waters are expected to have large fluctuations.

Longer sampling durations are necessary for waters experiencing reductions in 

mercury loadings, changes in water chemistry that affect methylation, and changes 

in the composition of the food web.

Volume I of the Guidance for Assessing Chemical Contaminant Data for Use in Fish 

Advisories (USEPA 2000c) provides additional guidance on selecting target species to 

sample, specific sampling design procedures, analytical measurement procedures, and 

quality assurance guidance. Chapter 10 of EPA’s Exposure Factors Handbook provides 

additional guidance on collecting information about local species (USEPA 1997d).

Additional guidance on evaluating existing site-specific bioaccumulation studies for use 

in deriving trophic-level-specific BAFs and designing sampling plans for obtaining data 

for deriving site-specific BAFs is provided in Technical Support Document—Volume 2: 

Developing National Bioaccumulation Factors (USEPA 2003). A publication by 

Burkhard (2003) is also a good source of information on designing BAF field studies and 

on deriving field-measured site-specific BAFs.

3.1.3.3 How is methylmercury in water translated into its mercury

equivalent in water?

Given that permit limits are often derived using a mercury water column concentration 

criterion, a state or tribe may wish to take another step after using a BAF to determine a 

methylmercury water concentration criterion to derive a mercury water column 

concentration criterion. Although not necessary to develop a water quality criterion, a 

state can translate a methylmercury water concentration into a mercury water 

concentration criterion by converting the concentration of methylmercury in water to the 

equivalent concentration of mercury in water. This step might be necessary because 

although the BAF is typically based on the concentration of methylmercury in water, the 

assessment of water quality is typically based on an evaluation of mercury concentrations 

since other forms of mercury are converted to methylmercury in the environment. As a 

result, a relationship between (dissolved or total recoverable) methylmercury and 

(dissolved or total recoverable) mercury in the water needs to be developed. NPDES 

permits and other water quality-based pollution control activities traditionally rely on the 

total recoverable concentration of mercury, not the dissolved methylmercury form.

Many of the issues surrounding the uncertainty in predicting and transferring 

methylmercury BAFs across different waterbodies also apply to translating 

methylmercury concentrations to mercury concentrations. As with BAFs, one approach 

for translating between methylmercury and mercury concentrations is for states and 
Water Quality Criteria and Standards Adoption

36

authorized tribes to measure site-specific concentrations of methylmercury and mercury 

to determine the relative amounts of each form. This field-measured, site-specific 

approach is the most direct and the most appropriate approach to the translation.

Where a site-specific approach is not feasible, states and authorized tribes may consider 

applying EPA’s draft national methylmercury-to-mercury translator factors. In the 2001 

methylmercury criterion document (USEPA 2001c), EPA derived these translator factors 

for rivers/streams and lakes as geometric means from data collected from the literature 

reporting concentrations of mercury in aquatic environments. Thus, like the draft national 

BAFs, the methylmercury-to-mercury translators were empirically derived based on 

various water data from across the United States. As with the draft national BAFs, the 

draft national methylmercury-to-mercury translator factors vary greatly across 

ecosystems and are subject to many of the same uncertainties. Therefore, EPA suggests 

that states and tribes that may be considering using the draft national translator values as 

defaults carefully review the discussion in the 2001 criterion document, particularly the 

discussions concerning uncertainty and limitations, before deciding to apply them in a 

regulatory context (see appendix A, section II, USEPA 2001a). States and tribes should 

consider whether more recent data and/or data that are more reflective of local conditions 

are available to supplant or supplement the limited database used to derive the draft 

national translators.

Alternatively, states and tribes that choose to develop water column criteria can consider 

collecting data to develop BAFs that relate methylmercury in fish tissue directly to total 

mercury in the water column. See the footnote to section 3.1.3.1.1 for more information.

3.2 What options are available to address site-specific 

conditions and concerns?

3.2.1 How can the methylmercury water quality criterion be 

modified for site-specific conditions?

The 2000 Human Health Methodology (USEPA 2000b) describes how states and 

authorized tribes can adopt site-specific modifications of a section 304(a) criterion to 

reflect local environmental conditions and human exposure patterns. “Local” may refer to 

any appropriate geographic area where common aquatic environmental or exposure 

patterns exist. Thus, it may signify a statewide or regional area, a river reach, or an entire 

river. Such site-specific criteria may be developed as long as the site-specific data, either 

toxicological or exposure-related, are justifiable. For example, when using a site-specific 

fish consumption rate, a state or authorized tribe should use a value that represents at 

least the central tendency for the consumption rate of the population surveyed (sport or 

subsistence, or both) that eat fish from the local area.

States and authorized tribes may modify EPA’s recommended 304(a) criterion for 

methylmercury by using different assumptions for certain components of EPA’s criterion 

to derive a criterion that maintains and protects the designated uses. For example, states 

and authorized tribes may:
Water Quality Criteria and Standards Adoption

37

● Use an alternative RSC factor or

● Use a daily uncooked freshwater and estuarine fish consumption rate that is more 

reflective of local or regional consumption patterns than the 17.5 grams/day default 

value. EPA encourages states and authorized tribes to consider using local or 

regional consumption rates instead of the default values if the former would better

reflect the target population.

If a state or authorized tribe intends to modify both the RSC and the fish consumption 

rate, it might find collecting the data at the same time advantageous.

3.2.1.1 How does one modify the RSC?

Section 5 of the methylmercury criterion document (USEPA 2001a) provides detailed 

discussions on how EPA assessed exposure to methylmercury and how EPA derived the 

RSC factor used in calculating the criterion. The methylmercury RSC is an exposure, 

subtracted from the RfD to account for exposure to methylmercury from sources other 

than freshwater or estuarine fish. By accounting for other known exposures, the RSC 

seeks to ensure that methylmercury exposures do not exceed the RfD. To change the RSC 

used by EPA, states and authorized tribes should review section 5 of the methylmercury 

criterion document and modify the media-specific exposure estimates in table 5-30 using 

local data that reflect the exposure patterns of their populations. Of the six exposure 

media presented in table 5-30 (which does not include exposure from fresh and estuarine 

fish), the exposure from ingestion of marine fish (including migratory species such as 

salmonids – see table 5-14, USEPA 2001a) constituted greater than 99.9 percent of the 

total exposure to methylmercury, and thus ingestion of fish would be the focus of any 

modification to the RSC. To modify this factor, states and authorized tribes should 

review the amount of marine fish and shellfish estimated to be consumed (table 5-1,

USEPA 2001a) and the concentration of methylmercury in the commonly consumed 

marine species (table 5-14, USEPA 2001a). States and authorized tribes should document 

the modifications with data supporting the modifications and ideally should share the 

proposed modifications to the RSC with EPA prior to recalculating the criterion. See 

appendix B for the tables from the methylmercury criterion document.

3.2.1.2 How does one modify the daily fish intake rate?

EPA derived the recommended methylmercury water quality criterion on the basis of a 

default fish intake rate for the general population (consumers and nonconsumers) of 

17.5 grams/day13, uncooked (USEPA 2001a). States and authorized tribes may use a 

different intake rate based on local or regional consumption patterns. The fish 

consumption value in the TRC equation may be changed if the target population eats a 

higher or lower amount of fish. For example, if the 90th percentile of a target population 

eats approximately 15 grams/day of freshwater and estuarine fish of various trophic 

13 This value represents the 90th percentile of freshwater and estuarine finfish and shellfish consumption reported by the 1994–1996 

Continuing Survey of Food Intakes by Individuals. For more information, see Methodology for Deriving Ambient Water Quality Criteria 

for the Protection of Human Health (USEPA 2000b).
Water Quality Criteria and Standards Adoption

38

levels, the fish intake value in equation 1 would simply be 15 grams/day, rather than the 

national default value of 17.5 grams/day used in calculating the 0.3 mg/kg TRC.

EPA encourages states and authorized tribes to develop a water quality criterion for 

methylmercury using local or regional fish consumption data rather than the default 

values if they believe that such a water quality criterion would be more appropriate for 

their target population. However, states and authorized tribes should consider whether the 

consumption rates reflect existing public concern about contamination of fish when 

collecting survey data, rather than local preference for fish consumption (i.e., the 

presence of fish advisories limits the consumption of fish). In this instance, the state or 

authorized tribe should take this into account and try to conduct surveys in a manner that 

accounts for the effects of fish advisories on the consumption of fish.

EPA suggests that states and authorized tribes follow a hierarchy when deriving fish 

intake estimates (USEPA 2000b). From highest preferred to lowest preferred, this 

hierarchy is as follows (1) use local data when available, (2) use data reflecting similar 

geography or population groups, (3) use data from national surveys, and (4) use EPA’s 

default fish intake rates. Additional discussion of these four preferences is provided 

below.

When a state or authorized tribe develops a site-specific criterion on the basis of local 

fish consumption, site-specific BAFs, or a site-specific RSC, states and authorized tribes 

might want to include EPA in the development of the study plan and submit the data 

supporting the site-specific criterion for EPA’s consideration when EPA approves or 

disapproves state or tribal water quality standards under CWA section 303(c). Including 

EPA at the study plan development stage may help to avoid problems and facilitate 

development of a defensible site-specific criterion.

3.2.1.2.1 Use local data

If a state or authorized tribe believes a fish consumption rate other than the default would 

be appropriate for their target population, EPA’s first preference is that they use fish 

intake rates derived from studies of consumption of local fish. Such studies could include 

results of surveys designed to obtain information on the consumption of freshwater or 

estuarine species caught from local watersheds within the state or tribal jurisdiction.

When estimating the fish intake rate, all freshwater fish, whether caught recreationally or 

bought commercially, should be included. States and authorized tribes may choose to 

develop either fish intake rates for the local population as a whole, or individual fish 

intake rates for various subpopulations (e.g., sport anglers, subsistence fishers).

States and authorized tribes might wish to conduct their own surveys of fish intake.

Guidance for Conducting Fish and Wildlife Consumption Surveys (USEPA 1998a) 

provides EPA guidance on methods for conducting such studies. States and authorized 

tribes should take care to ensure that the local data are of sufficient quality and scope to 

support development of a criterion and are representative of the population of people that

eat local fish. EPA’s consumption survey guidance offers recommendations on how to 

develop appropriate quality assurance and quality control procedures to help ensure the 

quality of the survey. Results of studies of the broader geographic region in which the 

state or authorized tribe is located can also be used, but they might not be as applicable as 

study results for local watersheds. Because such studies would ultimately form the basis 
Water Quality Criteria and Standards Adoption

39

of a state’s or authorized tribe’s methylmercury criterion, EPA would consider any 

surveys of fish intake as part of its review of the methylmercury criterion’s scientific 

defensibility as part of the Agency’s review of water quality standards under CWA 

section 303(c).

States and authorized tribes may use either high-end (such as 90th or 95th percentile) or 

central tendency (such as median or mean) consumption values for the population of 

interest (e.g., subsistence fishers, sport fishers, or the general population). EPA generally 

recommends that a central tendency value be the lowest value states or authorized tribes 

should use when deriving a criterion. When considering median values from fish 

consumption studies, states and tribes should ensure that the distribution is based on 

survey respondents that reported consuming fish because surveys of both consumers and 

nonconsumers can often result in median values of zero. EPA believes the approach 

described above is a reasonable procedure and is also consistent with other Agency 

positions such as that of the Great Lakes Water Quality Initiative, known as the GLI 

(USEPA 1995a).

3.2.1.2.2 Use similar geography or population groups

If surveys conducted in the geographic area of the state or authorized tribe are not 

available, EPA’s second preference is that states and authorized tribes consider results 

from existing surveys of fish intake in similar geographic areas and population groups 

(e.g., from a neighboring state or authorized tribe or a similar watershed type) and follow 

the method described above regarding target values to derive a fish intake rate. For 

instance, states or tribes with subsistence fisher populations might wish to use 

consumption rates from studies that focus specifically on these groups, or use rates that 

represent high-end values from studies that measured consumption rates for a range of 

types of fishers (e.g., recreational or sport fishers, subsistence fishers, minority 

populations). A state or authorized tribe in a region of the country might consider using 

rates from studies that surveyed the same region; for example, a state or authorized tribe

that has a climate that allows year-round fishing might underestimate consumption if it 

uses rates from studies taken in regions where people fish for only one or two seasons per 

year. A state or authorized tribe that has a high percentage of an age group (such as older 

persons, who have been shown to have higher rates in certain surveys) might wish to use 

age-specific consumption rates, which are available from some surveys. For additional 

information on the use of fish consumption rates, see EPA’s 2000 Human Health 

Methodology (USEPA 2000b). Again, EPA recommends that states and tribes use only

uncooked weight intake values and freshwater or estuarine species data.

3.2.1.2.3 Use national surveys

If applicable consumption rates are not available from local, state, or regional surveys, 

EPA’s third preference is that states and authorized tribes select intake rate assumptions 

for different population groups from national food consumption surveys. EPA has 

analyzed two such national surveys, the 1994–1996 and 1998 Continuing Survey of Food 

Intakes by Individuals (CSFII). These surveys, conducted by the U.S. Department of 

Agriculture (USDA), include food consumption information from a probability sample of 

the population of all 50 states. Respondents to the survey provided 2 days of dietary 

recall data. A separate EPA report provides a detailed description of the combined 1994–
Water Quality Criteria and Standards Adoption

40

1996 and 1998 CSFII surveys, the statistical methodology, and the results and 

uncertainties of the EPA analyses (USEPA 2002b). The estimated fish consumption rates 

in the CSFII report are presented by fish habitat (i.e., freshwater or estuarine, marine, and 

all habitats) for the following population groups: (1) all individuals, (2) individuals age 

18 and over, (3) women ages 15–44, and (4) children age 14 and under. Three kinds of 

estimated fish consumption rates are provided: (1) per capita rates (rates based on 

consumers and nonconsumers of fish from the survey period), (2) by consumers-only 

rates (rates based on respondents that reported consuming finfish or shellfish during the 

2-day reporting period), and (3) per capita consumption by body weight (per capita rates 

reported as mg/kg-day). For purposes of revising the fish consumption rate in the 

methylmercury criterion, EPA recommends using the rates for freshwater and estuarine 

fish and shellfish.

The CSFII surveys (USDA/ARS 1998, 2000) have advantages and limitations for 

estimating per capita fish consumption. The primary advantage of the CSFII surveys is 

that USDA designed and conducted them to support unbiased estimation of food 

consumption across the population in the United States and the District of Columbia. One 

limitation of the CSFII surveys is that individual food consumption data were collected 

for only 2 days—a brief period that does not necessarily depict “usual intake.” Usual 

dietary intake is defined as “the long-run average of daily intakes by an individual.” 

Upper percentile estimates might differ for short-term and long-term data because shortterm food consumption data tend to be inherently more variable. It is important to note, 

however, that variability due to duration of the survey does not result in bias of estimates 

of overall mean consumption levels. Also, the multistage survey design does not support 

interval estimates for many of the subpopulations because of sparse representation in the 

sample. Subpopulations with sparse representation include American Indians on 

reservations and certain ethnic groups. Although these persons were participants in the 

survey, they were not present in sufficient numbers to support fish consumption 

estimates. The survey does support interval estimates for the U.S. population and some 

large subpopulations (USEPA 2002b).

3.2.1.2.4 Use EPA default fish intake rates

EPA’s fourth preference is that states and authorized tribes use as fish intake 

assumptions, default rates on the basis of the 1994–1996 CSFII data for the U.S. 

population, which EPA believes are representative of freshwater and estuarine fish and 

shellfish intake for different population groups. The 1994–1996 CSFII data for U.S. fish 

consumption among both consumers and nonconsumers of fish is delineated below in 

table 3.
Water Quality Criteria and Standards Adoption

41

Table 3. Estimates of freshwater and estuarine combined finfish and shellfish 

consumption from the combined 1994–1996 and 1998 CSFII surveys

(U.S. population)

Mean Median

90th

percentile

95th

percentile

99th

percentile

All ages 6.30 N/a 11.65 41.08 123.94

Age 18 and over 7.50 0.00* 17.53 49.59 142.41

Women ages 15-44 5.78 N/a 6.31 32.37 109.79

Children age 14 and under 2.64 0.00 0.00 13.10 73.70

Note: All values expressed as grams per day for uncooked fish.

* The median value of 0 grams/day might reflect the portion of persons in the population that never eat 

fish, as well as the limited reporting period (2 days) during which intake was measured.

Because the combined 1994–1996 CSFII survey is national in scope, EPA uses the results 

from it to estimate fish intake for deriving national criteria. EPA applies a default rate of 

17.5 grams/day for the general adult population. EPA selected an intake rate that is 

protective of a majority of the population (the 90th percentile of consumers and 

nonconsumers, according to the 1994–1996 CSFII survey data) (USEPA 2000b). EPA 

also recommends a default rate of an average of 17.5 grams/day for sport fishers.

Similarly, EPA believes the 99th percentile of 142.4 grams/day is within the range of 

consumption estimates for subsistence fishers, according to the studies reviewed, and that 

it represents an average rate for subsistence fishers. EPA knows that some local and 

regional studies indicate greater consumption among American Indian, Pacific Asian 

American, and other subsistence consumers and recommends the use of those studies in 

appropriate cases, as indicated by the first and second preferences. Again, states and 

authorized tribes have the flexibility to choose intake rates higher than the average values 

for these population groups. If a state or authorized tribe has not identified a separate 

well-defined population of exposed consumers and believes that the national data from 

the 1994–1996 CSFII are representative, the state or tribe may choose these 

recommended rates.

EPA has made these risk management decisions after evaluating numerous fish intake 

surveys. These values represent the uncooked weight intake of freshwater and estuarine 

finfish and shellfish. As with the other preferences, EPA requests that states and 

authorized tribes routinely consider whether a substantial population of sport fishers or 

subsistence fishers exists in the area when establishing water quality criteria rather than 

automatically using data for the general population.

The CSFII surveys also provide data on marine species, but EPA considered only 

freshwater and estuarine fish intake values for determining default fish consumption rates 

because EPA considered exposure from marine species of fish in calculating an RSC for 

dietary intake.14 States and authorized tribes should ensure that when evaluating overall 

14 See the discussion of the RSC in sections 3.1.2.3 and 3.2.1.1.
Water Quality Criteria and Standards Adoption

42

exposure to a contaminant, marine fish intake is not double-counted with the other dietary 

intake estimate used. Coastal states and authorized tribes that believe accounting for total 

fish consumption (fresh or estuarine and marine species) is more appropriate for 

protecting the population of concern may do so, provided that the marine intake 

component is not double-counted with the RSC estimate (USEPA 2000b).

3.2.2 How do water quality standards variances apply?

A state or authorized tribe may provide NPDES dischargers temporary relief from a 

water quality standard by adopting a temporary water quality standard through a variance

process. The variance would then, in effect, serve as a substitute standard for a point 

source, and the WQBEL contained in an NPDES permit would then be based on the 

variance. As a revision to the otherwise applicable water quality standard (designated use

and criteria), water quality standards variances must be supported by one of the six 

justifications15 under 40 CFR 131.10(g) where a state or authorized tribe believes the 

water quality standard cannot be attained in the immediate future. Variances are generally 

determined based on the discharger’s ability to meet a WQBEL and, therefore, are 

considered after an evaluation of controls necessary to implement water quality 

standards. Typically, variances apply to specific pollutants and facilities, which would 

mean that a water quality standards variance for mercury would apply to only the new 

human health methylmercury criterion in a stated waterbody and specifically to the 

discharger requesting the variance. The state or authorized tribe, however, may provide 

justification for more than one discharger or for an entire waterbody or segment to 

receive a variance (as discussed in section 3.2.2.3 of this document).

3.2.2.1 When is a variance appropriate?

Some regulated point sources discharging mercury might apply for variances for their 

discharges into impaired waters where the largest source of mercury is atmospheric

deposition. In other cases, limits to technology or naturally elevated levels of 

methylmercury in a waterbody could preclude attainment of standards. To address these 

types of issues, the following scenarios are examples of demonstrations that could satisfy 

the requirements under 40 CFR 131.10(g). The demonstrations are more thoroughly 

explained below and in the Water Quality Standards Handbook (USEPA 1994).

● Economic or social impacts (131.10(g)(6)). Demonstrate that, in the short term, the 

costs of constructing controls necessary to meet the methylmercury criterion 

15 These six justifications are allowed for use attainability analyses under 40 CFR 131.10(g): (1) naturally occurring pollutant 

concentrations prevent the attainment of the use; (2) natural, ephemeral, intermittent, or low-flow conditions or water levels prevent the 

attainment of the use, unless these conditions may be compensated for by the discharge of sufficient volume of effluent discharges without 

violating state water conservation requirements to enable uses to be met; (3) human-caused conditions or sources of pollution prevent the 

attainment of the use and cannot be remedied or would cause more environmental damage to correct than to leave in place; (4) dams, 

diversions, or other types of hydrologic modifications preclude the attainment of the use, and it is not feasible to restore the waterbody to 

its original condition or to operate such modification in a way that would result in attainment of the use; (5) physical conditions related to 

the natural features of the waterbody, such as the lack of a proper substrate, cover, flow, depth, pools, riffles, and the like, unrelated to 

water quality, preclude attainment of aquatic life protection uses; and (6) controls more stringent than those required by sections 301(b) and 

306 of the CWA would result in substantial and widespread economic and social impact.
Water Quality Criteria and Standards Adoption

43

(beyond those required by sections 301(b)(1)(A) and (B) and 306 of the CWA) 

would result in substantial and widespread economic and social impact.

● Human-caused conditions that cannot be remedied (131.10(g)(3)). Demonstrate 

that, in the short term, none of the present technologies for improving the quality of 

an effluent are capable of bringing methylmercury levels in the discharge down to a 

level as stringent as necessary to meet the criterion (i.e., there is no technological 

remedy or it is technologically infeasible). For example, atmospheric deposition 

originating overseas could be the source of elevated mercury levels in a local 

stream, yet the lack of an international agreement or treaty to cut mercury 

emissions worldwide prevents attainment of the methylmercury criterion, despite 

local reduction efforts. In this instance, if air deposition modeling shows that the 

atmospheric deposition from outside the United States was a substantial cause of 

the impairment, and there was no immediate expectation that those sources would 

experience reductions, the variance may be warranted.

● Natural conditions that preclude attainment (131.10(g)(1)). Demonstrate that local 

conditions of an aquatic system result in high methylmercury levels. For example, 

elevated methylmercury concentrations might occur naturally in a system because 

of a short-term condition.

During the period the variance applies, any permit issued must be consistent with 

applicable water quality standards (40 CFR 122.44(d)(1)(vii)), which in this case would 

be the temporary standard approved in the variance. The permit would need to be 

modified to derive from and comply with the underlying standard if the variance is not 

re-issued.

3.2.2.2 What should a state or tribe consider before granting a 

variance?16

In general, the temporary revised standard established by a variance should be set at a 

level representing the highest attainable water quality (like all water quality standards). 

Variances may not be set at a level that would not protect the existing uses, and variances 

should ensure progress toward ultimate attainment of the designated use for the

waterbody. Regarding procedural considerations, the same requirements apply for a 

variance as for a new or revised standard (e.g., public review and comment, EPA

approval or disapproval) because a variance is a change to the water quality standards. In 

addition, the following describes more specific issues that states and authorized tribes 

should take into account when considering granting a variance.

● Variance protocols. If a state or authorized tribe anticipates receiving a number of 

variance requests for mercury discharges, it could consider establishing a mercury 

variance protocol, with EPA’s participation and agreement. The protocol would 

govern the development and processing of variance requests. It would specify the 

16 Federal or state regulations also govern the granting of a variance. For example, regulations promulgated under 40 CFR part 132, 

appendix F, procedure 2, specify the conditions for granting variances in the Great Lakes and prohibit the granting of variances to new 

dischargers or recommencing Great Lakes dischargers.
Water Quality Criteria and Standards Adoption

44

information needed and the criteria the state would use in considering whether to 

adopt the variance. Although the state or tribe would need to submit each variance 

to EPA for approval (40 CFR 131.20), EPA’s advance agreement to the protocol 

could streamline EPA’s review of any variances developed in accordance with the 

protocol. Public notice requirements for variances could be satisfied through the 

process of issuing the NPDES permit that incorporates limits based on such 

temporary standards, as long as the variance is identified and all the necessary 

information pertaining to the variance is included.

● Time frames. A variance is typically a time-limited change in the water quality 

standards. Although EPA part 131 regulations do not specify a time limit for 

variances, EPA regulations at 40 CFR 131.20 provide an opportunity to consider 

new information every three years for the purpose of reviewing water quality 

standards and, as appropriate, modifying and adopting standards. For this reason, 

states typically limit the time frame of a variance to three to five years, with 

renewals possible following a sufficient demonstration that the variance is still 

necessary. Variances that extend longer than three years are traditionally revisited 

in the context of a triennial review to justify their continuation. While the permittee

typically makes this demonstration, the permittee also should demonstrate that it

made reasonable progress to control mercury in the discharge during the period of 

the previously approved variance. In terms of methylmercury, EPA anticipates a 

time lag between implementing controls and seeing results (i.e., there may be 

unaddressed sources, continual leaching of mercury from sediments, and so on).

For example, EPA modeled the response in fish tissue to a 50 percent reduction in 

mercury loadings to four lakes and estimated that it would take 1 to 56 years for the 

lakes to reach 90 percent of the estimated steady state fish tissue methylmercury 

concentration (USEPA 2005a). To address this issue, states and authorized tribes 

could develop an expedited variance adoption process, especially if legislative 

deliberations or administrative procedures are necessary to adopt variances into 

water quality standards. For example, a variance provision could allow for 

mercury-specific demonstration for renewals by making use of information already 

available. In the Great Lakes, renewal of a variance may be denied if a permittee 

did not comply with the conditions of the original variance. (40 CFR part 132, 

appendix F, procedure 2, section H).

Another perspective regarding the life span of a variance is that although three 

years is the time frame for water quality standards reviews under the CWA, there is 

no specific federal regulatory requirement for a variance to expire in three years.

Regardless, as with any other revision to the water quality standards, the permit and 

permit conditions implementing the variance do not automatically change back to 

the previous permit conditions if the variance expires, unless that is a condition of a 

variance and permit. Although water quality standards can change with every 

triennial review, states and authorized tribes are not obliged to reopen and modify 

permits immediately to reflect those changes, but may do so where the permit 

contains a reopener condition to address such revised water quality standards. In 

the Great Lakes, however, permits with limits based on variances must include a 

provision enabling the permitting authority to reopen and modify the permit based 

on triennial revisions to water quality standards. (40 CFR part 132, appendix F, 
Water Quality Criteria and Standards Adoption

45

procedure 2, section F.4). Any new or reissued permit must implement the water 

quality standards applicable at time of permit issuance. 40 CFR 122.44(d)(1).

● Antidegradation. Permits with effluent limits based on a variance for 

methylmercury must conform, as do all permits, to the state or authorized tribe’s 

antidegradation policy.

● Pollutant Minimization Program. Pollutant Minimization Programs (PMPs) may 

serve as a pollution prevention measure that states and authorized tribes could 

require of dischargers receiving a variance. By reducing mercury sources up front, 

as opposed to traditional reliance of treatment at the end of a pipe, diligent 

implementation of PMPs might mitigate any adverse effects of a variance by 

improving the water quality.

3.2.2.3 What is involved in granting a variance on a larger scale?

Traditionally, variances are specific to a pollutant and a facility. However, for situations 

where a number of NPDES dischargers are located in the same area or watershed and the 

circumstances for granting a variance are the same, EPA encourages states and 

authorized tribes to consider administering a multiple-discharger variance for a group of 

dischargers collectively. Such a group variance can be based on various scales and may 

depend largely on the rationale for adopting a variance for methylmercury. Possible 

applications of a group variance may include any or some combination of the following:

● Similar discharge processes. A type of industry or effluent treatment process may 

be targeted on the basis of similar treatment requirements and/or available 

technology (e.g., publicly owned treatment works or POTWs, mining operations). 

A state or authorized tribe may choose to adopt a variance with tiered requirements, 

depending on the type of industry requesting coverage. For example, due to the 

differing effluent matrices, one industry would be required to meet a variance of 10 

parts per billion (ppb) above the criterion, whereas another industry would be 

required to meet a variance of 20 ppb above the criterion, based on the availability 

of technology to remove the pollutant from the different types of effluents.

● Watershed basis. A variance on a watershed scale might be a sensible approach, 

particularly for states that issue NPDES permits on a watershed basis. As with 

other pollutants, methylmercury concentrations can be monitored to gain sitespecific information (perhaps for calculating site-specific BAFs) in key watersheds 

for a given year. A state or authorized tribe using a watershed approach to 

permitting should have data available from a watershed in one year for the purpose 

of issuing NPDES permits in the watershed(s) where permits would be issued in 

the subsequent year. The state or authorized tribe could use these data for the 

purpose of evaluating the continuing need for and potential for revision to a 

previously approved water quality variance. Meanwhile, variances for other 

watersheds remain the same or are renewed with unchanged variance requirements 

until monitoring occurs, with variance time frames coinciding with the permitting 

cycle. This way, the variance-based limits will reflect a more “real-time” variance.

● Broader geographic basis. Analogous to a general NPDES permit, a multiplewatershed, areawide, or statewide variance could be made available by the state or 
Water Quality Criteria and Standards Adoption

46

authorized tribe using approaches similar to those above. Individual dischargers

could apply for coverage under such a variance upon fulfilling certain conditions.

It is important to note that, despite the coverage of a multiple-source variance, an 

individual discharger must still demonstrate that the underlying criterion is not attainable 

with the technology-based controls identified by CWA sections 301(b) and 306 and with 

cost-effective and reasonable best management practices (BMPs) for nonpoint sources 

(40 CFR 131.10(h)(2)).

3.2.3 How are use attainability analyses conducted?

3.2.3.1 What is a use attainability analysis?

A use attainability analysis (UAA) is defined in 40 CFR 131.3(g) as a structured 

scientific assessment of the factors affecting the attainment of a use, which may include 

physical, chemical, biological, and economic factors, that must be conducted whenever a 

state wishes to remove a designated use specified in section 101(a)(2) of the CWA, or to 

adopt subcategories of uses specified in section 101(a)(2) of the CWA, which require less 

stringent criteria (see 40 CFR 131.3 and 40 CFR 131.10(g)).

3.2.3.2 What is EPA’s interpretation of CWA section 101(a)?

CWA section 101(a) (2) establishes as a national goal “water quality [that] provides for 

the protection and propagation of fish, shellfish, and wildlife and provides for recreation 

in and on the water,” wherever attainable. These goals are commonly referred to as the 

“fishable/swimmable” goals of the CWA. EPA interprets these goals as providing for the 

protection of aquatic communities and human health related to the consumption of fish 

and shellfish. In other words, EPA views “fishable” to mean that fish and shellfish can 

thrive in a waterbody and, when caught, can also be safely eaten by humans. This 

interpretation also satisfies the CWA section 303(c)(2)(A) requirement that water quality 

standards protect public health. Including human consumption of fish and shellfish as the 

appropriate interpretation of the definition of section 101(a)(2) uses is not new. For 

example, in the National Toxics Rule, all waters designated for even minimal aquatic life 

protection (and therefore a potential fish and shellfish consumption exposure route) are 

protected for human health (57 FR 60859, December 22, 1992).

3.2.3.3 When is a UAA needed for a “fishable” use?

Under 40 CFR 131.10(j) of the Water Quality Standards Regulation, states and 

authorized tribes are required to conduct a UAA whenever the state or authorized tribe

designates or has designated uses that do not include the “fishable/swimmable” use 

specified in CWA section 101 (a)(2); or the state or authorized tribe wishes to remove a 

designated use that is specified in CWA section 101(a)(2) or adopt subcategories of the 

uses specified in that section that require less stringent criteria. An important caveat to 

the process of removing a designated use is that states and authorized tribes may not 

remove an “existing use” as defined by the Water Quality Standards Regulation. An 

existing use is defined in 40 CFR 131.3(c) as any use that has been actually attained on or 

after November 28, 1975, when the CWA regulations regarding use designation were 

originally established. In practical terms, waters widely used for recreational fishing 

would not be good candidates for removing a “fishable” use, especially if the associated 
Water Quality Criteria and Standards Adoption

47

water quality supports, or has until recently supported, the fishable use, on the basis, in 

part, of the “existing use” provisions of EPA’s regulations. In addition, EPA considers 

designated uses attainable, at a minimum, if the use can be achieved (1) through effluent 

limitations under CWA sections 301(b)(1)(A) and (B) and 306 and (2) through 

implementation of cost-effective and reasonable BMPs for nonpoint sources. The federal 

regulations at 40 CFR 131.10(g) further establish the basis for finding that attaining the 

designated use is not feasible, as long as the designated use is not an existing use. EPA

emphasizes that when adopting uses and appropriate criteria, states and authorized tribes 

must ensure that such standards provide for the attainment and maintenance of the 

downstream uses (40 CFR part 131.10(b)). States and tribes are not required to conduct 

UAAs when designating uses that include those specified in CWA section 101(a) (2), 

although they may conduct these or similar analyses when determining the appropriate 

subcategories of uses.

3.2.3.4 What conditions justify changing a designated use?

EPA’s regulations at 40 CFR 131.10(g) list the following six reasons for states or 

authorized tribes to use to support removal of a designated use or adoption of a 

subcategory of use that carries less stringent criteria:

● Naturally occurring pollutant concentrations prevent the attainment of the use.

● Natural, ephemeral, intermittent, or low-flow conditions or water levels prevent the 

attainment of the use, unless these conditions may be compensated for by the 

discharge of sufficient volume of effluent discharges without violating state water 

conservation requirements to enable uses to be met.

● Human-caused conditions or sources of pollution prevent the attainment of the use 

and cannot be remedied or would cause more environmental damage to correct than 

to leave in place.

● Dams, diversions, or other types of hydrologic modifications prevent the 

attainment of the use, and it is not feasible to restore the waterbody to its original 

condition or to operate such modification in a way that would result in attainment 

of the use.

● Physical conditions related to the natural features of the waterbody, such as the lack 

of a proper substrate, cover, flow, depth, pools, riffles, and the like, unrelated to 

water quality, prevent attainment of aquatic protection uses.

● Controls more stringent than those required by CWA sections 301(b) and 306 

would result in substantial and widespread economic and social impact.

In addition to citing one or more of these factors to support removal of a use, states and 

authorized tribes use the same six factors to guide analysis and decision-making with 

respect to establishing an attainable use. Of the six factors above, it is most likely that 

human-caused conditions that cannot be remedied, naturally occurring pollutant 

concentrations, or substantial and widespread social and economic impact resulting from 

additional controls would be the reason cited in a UAA addressing methylmercuryimpacted waters. In all cases, states and authorized tribes must obtain scientifically sound 

data and information to make a proper assessment. It is also recommended that they 
Water Quality Criteria and Standards Adoption

48

conduct pollutant source surveys to define the specific dominant source of mercury in the 

waterbody. Sources may include point source loadings, air deposition, mining waste or 

runoff, legacy levels (e.g., mercury resulting from historical releases), and geologic 

“background levels.” This is similar to source assessments under the TDML program.

Existing documents provide guidance on obtaining data and conducting analyses for the 

other components of a UAA. These documents are at http://www.epa.gov/waterscience/

standards/uaa/info.htm. The Technical Support Manual: Waterbody Surveys and 

Assessments for Conducting Use Attainability Analyses (USEPA 1983) covers the 

physical and chemical components of UAAs. Technical support for assessing economic 

and social impacts is offered through the Interim Economic Guidance for Water Quality 

Standards Workbook (USEPA 1995b).

EPA recognizes that there may be naturally occurring concentrations of methylmercury 

which may exceed the national recommended 304(a) criterion. However, EPA policy, 

whereby criterion may be set at ambient conditions if contaminant levels are due only to 

non-anthropogenic sources, applies only to aquatic life uses. The policy does not apply to 

human health uses. The policy states that for human health uses, where the natural 

background concentration is documented, this new information should result in, at a 

minimum, a re-evaluation of the human health use designation (USEPA 1997e).
Monitoring and Assessment

49

4 Monitoring and Assessment

Water quality monitoring and assessment are essential elements in implementing the 

CWA at the local, state, and national levels. In implementing the water quality-based 

approach, the most obvious uses of monitoring information are in determining attainment 

of water quality standards and in developing TMDLs and permits. In the case of mercury, 

analyzing for mercury and methylmercury in water and fish is particularly important for 

states and tribes that choose to develop BAFs and methylmercury-to-mercury translators. 

This chapter provides guidance on analytical methods, field sampling, and assessment 

considerations for mercury. Additional information on developing site-specific BAFs and 

translators is provided in section 3.1.3 of this guidance.

4.1 What are the analytical methods for detecting and 

measuring mercury and methylmercury 

concentrations in fish and water?

Over the past two decades, EPA and other organizations have developed several 

analytical methods for determining mercury and methylmercury concentrations in fish 

and water. In 2001 EPA conducted a literature review to assess the availability of 

different analytical methods and to determine which of the analytical methods would be 

most useful for implementing the new methylmercury criterion. After the review, EPA

concluded that nearly all current research on low-level concentrations of mercury and 

methylmercury is being performed using techniques that are based on procedures 

developed by Bloom and Crecelius (1983) and refined by Bloom and Fitzgerald (1988), 

Bloom (1989), Mason and Fitzgerald (1990), and Horvat et al. (1993).

To assist states and authorized tribes in selecting an analytical method to use, this chapter 

describes selected analytical methods available (sections 4.1.1 and 4.1.2), and identifies 

five specific methods that EPA recommends for use in implementing this guidance 

(section 4.1.3). In addition, appendix C of this document presents a list of available 

methods in more detail. Table C1 of the appendix summarizes 4 methods to analyze 

mercury and methylmercury in fish tissue, and table C2 summarizes 18 methods for the 

analysis of mercury and methylmercury in water and other nontissue matrices. Each table 

identifies the forms and species of mercury targeted by each method, estimated or known 

sensitivity, the techniques employed in the method, and any known studies or literature 

references that use the techniques employed in the method.

The CWA establishes an EPA approval process for certain methods used in the NPDES 

program and for section 401 certifications. As described in section 4.1.2 below, EPA has 

approved two of the above methods for analysis of mercury in water under 40 CFR part 

136: method 1631, revision E and method 245.7. EPA’s regulations generally require that 

these methods be used whenever such analyses are required for the NPDES program and 

for CWA section 401 certifications issued by states and authorized tribes (40 CFR 136.1). 

Sections 7.4 and 7.5.1.1 of this guidance provide additional information on appropriate 

analytical methods for measuring mercury in water for NPDES permitting purposes.

There are no regulatory requirements for the use of particular methods in setting water 

quality standards, evaluating the attainment of standards, or developing TMDLs, 
Monitoring and Assessment

50

although any methods used need to be scientifically defensible. Although this chapter 

provides recommendations for methods that can be used for these purposes, states and 

tribes are not precluded from using other methods, including those in appendix C.

4.1.1 Analytical Methods for Methylmercury

For measuring methylmercury in water, EPA method 1630 (USEPA 2001d), developed 

by EPA’s Office of Water, reflects the techniques developed by Bloom and Crecelius 

(1983) and refined by Bloom and Fitzgerald (1988), Bloom (1989), Mason and Fitzgerald 

(1990), and Horvat (1993). This method has a quantitation level of 0.06 ng/L.

Three additional methods for measuring methylmercury in water are listed in table C2 

(see appendix C). These methods are UW-Madison’s standard operating procedure, or 

SOP (Hurley et al. 1996), used by the Great Lakes National Program Office for its Lake 

Michigan Mass Balance Study; USGS Wisconsin-Mercury Lab SOPs 004 (DeWild et al.

2002), used by USGS and EPA in the Aquatic Cycling of Mercury in the Everglades 

study; and a recently released USGS method (DeWild et al. 2002). All these procedures 

are based on the same techniques and have detection limits of 0.01 ng/L, 0.05 ng/L and 

0.04 ng/L, respectively.

Because the four methods are nearly identical test procedures, they are expected to 

produce very similar results with sensitivity as low as 0.001 mg/kg in tissue and 0.01 to 

0.06 ng/L in water. These levels are well below the expected range of water column 

concentrations associated with the methylmercury fish tissue criterion.

Modifications to method 1630, described in table C1 (see appendix C) and in Horvat et 

al. (1993), allow for measurement of methylmercury in tissue as low as 0.001 to 0.002 

mg/kg, well below the water quality criterion for methylmercury in tissue (0.3 mg/kg).

EPA recommends using these techniques when direct measurements of methylmercury in 

tissue are desired.

4.1.2 Analytical Methods for Mercury

For measuring low level mercury in water, EPA method 1631, revision E (USEPA 

2002c), developed by EPA’s Office of Water, reflects the techniques developed by 

researchers mentioned previously. It has a quantitation level of 0.5 ng/L. EPA made this 

revision to clarify method requirements, increase method flexibility, and address 

frequently asked questions. The revision includes recommendations for using the clean 

techniques contained in EPA’s Method 1669: Sampling Ambient Water for Trace Metals 

at EPA Water Quality Criteria Levels (USEPA 1996a). The benefits of using method 

1631 are that it has been fully validated, numerous laboratories are routinely using the

method, and it is sensitive enough to measure at the water concentrations expected to be 

associated with the criterion. This method was approved in 2002 under 40 CFR part 136

for NPDES permitting and other purposes under the CWA (67 FR 65876).

In addition, EPA method 245.7 (USEPA 2005e), which has a quantitation level of 

5.0 ng/L, was approved under part 136 in 2007 (72 FR 11200). Developed by EPA’s 

Office of Water, method 245.7 is similar to EPA method 1631E because both methods 

require use of a cold-vapor atomic fluorescence spectrometry (CVAFS) detector to 

measure low levels of mercury. Method 245.7 has been validated in two EPA 
Monitoring and Assessment

51

laboratories, one university laboratory, and an interlaboratory validation study. Results 

from these studies indicate that the method is capable of producing reliable measurements 

of mercury at some toxic criteria levels (40 CFR 136).

Appendix A to method 1631 (64 FR 10596) details the researcher’s techniques for 

determining total and dissolved mercury in tissue, sludge, and sediments. The appendix 

was developed for processing fish tissue samples to be analyzed for mercury using the 

previously validated and approved method 1631 analytical procedures. The procedures 

are expected to be capable of measuring mercury in the range of 0.002 to 5.0 mg/kg.

EPA recognizes that some users might find Method 1631 (appendix A) costly or difficult 

to implement. Appendix C summarizes three other methods available for analyzing 

mercury in fish tissue that are less costly and less difficult to implement, but they have 

not undergone the same extensive interlaboratory validation studies as Method 1631 

(appendix A). Two are listed in table C1 (Methods 245.6 and 7474). The third—Method 

7473 for analyzing mercury in water, listed in table C2—has been adapted by some users 

for analyzing mercury in fish tissue; this approach has been used to measure mercury in 

fish tissue to support state fish consumption advisories.

Because researchers have found that nearly all mercury in fish tissue is in the form of 

methylmercury (USEPA 2000c), EPA also suggests that analysis of tissue for mercury, as 

a surrogate for methylmercury, might be a useful means for implementing the 

methylmercury criterion. If mercury concentrations in tissue exceed the criterion, further 

investigation of the methylmercury component might be desired.

4.1.3 Summary of Recommended Analytical Methods

In summary, on the basis of the available information, EPA believes that the most 

appropriate methods for measuring low levels of mercury concentrations in the water 

column are method 1631, revision E (mercury in water by CVAFS) and method 245.7

(mercury in water by CVAFS). Likewise, EPA believes that the most appropriate method 

for measuring methylmercury concentrations in the water column is method 1630

(methylmercury in water by CVAFS), and the most appropriate methods for measuring 

mercury concentrations in fish tissue are appendix A to method 1631 (mercury in tissue 

by CVAFS) and modifications to method 1630 for handling tissues. EPA recommends 

these procedures for the following reasons:

● EPA developed methods 1631 and 1630 to support implementation of water quality 

criteria for mercury and methylmercury, respectively. Both are already in the 

appropriate EPA format and include all standardized quality control elements 

needed to demonstrate that results are reliable enough to support CWA 

implementation.

● EPA developed method 245.7 specifically to address state needs for measuring 

mercury at ambient water quality criteria levels, when such measurements are 

necessary to protect designated uses. In addition, it has been validated in two EPA 

laboratories, one university laboratory, and an interlaboratory validation study.

● EPA developed appendix A to method 1631 to support its National Study of 

Chemical Residues in Lake Fish Tissue. Appendix A provides information on 
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52

preparing a fish tissue sample for analysis using method 1631. The method was 

validated by Brooks Rand (USEPA 1998b) and was used by Battelle Marine 

Sciences to analyze more than a thousand tissue samples collected during EPA’s 

national study (USEPA 2000d). Successful use of these techniques also has been 

widely reported in the literature. This history, combined with the fact that appendix 

A supplements the already well-characterized and approved method 1631, makes 

this method a good candidate for use with the new fish tissue criterion.

● Method 1630 already has been used in several studies, including EPA’s Cook Inlet 

Contaminant Study (USEPA 2001e) and the Savannah River TMDL study

(USEPA 2001f). The techniques described in the method and in the recommended 

method modifications also have been successfully applied in numerous studies 

described in the published literature. Furthermore, the procedures in method 1630 

are nearly identical to those given in the USGS method and in the University of 

Wisconsin SOP (Hurley et al., 1996), listed in table C2. The University of 

Wisconsin SOP was used in EPA’s Lake Michigan Mass Balance Study (USEPA 

2001g).

Table 4 summarizes the recommendations discussed above.

Table 4. Recommended analytical methods for detecting and measuring low levels 

of methylmercury and mercury in fish tissue and water

Recommended for analysis of:

Methylmercury...

(see section 4.1.1)

Mercury...

(see section 4.1.2)

...in fish tissue

(for additional available methods, 

see appendix C, table C1)

Draft Method 1630 with 

modifications for tissue

Method 1631, draft Appendix 

A

...in water

(for additional available methods, 

see appendix C, table C2)

Method 1630

Method 1631, revision E*

Method 245.7*

*Approved under 40 CFR part 136. See sections 7.4 and 7.5.1.1 for further information on 

appropriate methods for NPDES permitting purposes.

4.2 What is the recommended guidance on field sampling 

plans for collecting fish for determining attainment of 

the water quality standard?

EPA has published guidance providing information on sampling strategies for a fish

contaminant monitoring program in volume 1, Fish Sampling and Analysis, of a 

document series, Guidance for Assessing Chemical Contaminant Data for Use in Fish 

Advisories (USEPA 2000c). This guidance provides scientifically sound 

recommendations for obtaining a representative sample for issuing fish consumption 

advisories, and can be applied for obtaining a representative sample for determining 

attainment. The guidance also includes recommendations for quality control and quality 

assurance considerations. In all cases, states and authorized tribes should develop data 

quality objectives for determining the type, quantity, and quality of data to be collected 

(USEPA 2000e).
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53

4.2.1 What fish species should be monitored?

EPA’s fish sampling guidance (USEPA 2000c) provides recommendations for selecting 

finfish and shellfish species for monitoring to assess human consumption concerns.

According to the guidance, the most important criterion for selecting fish is that the 

species are commonly eaten in the study area and have commercial, recreational, or 

subsistence fishing value. Fish creel data (from data gathered by surveying recreational 

fishers) from state fisheries departments are a justifiable basis for estimating types and 

amounts of fish consumed from a given waterbody. States and authorized tribes should 

ensure that the creel data are of sufficient quality and are representative of the local 

population of people that eat fish.

The fish sampling guidance also identifies recommended target species for inland fresh 

waters and for Great Lakes waters. Walleye and largemouth bass have been identified as 

freshwater fish that accumulate high levels of methylmercury. Reptiles, such as turtle 

species and alligators, are recommended as target species for mercury if they are part of 

the local diet. Larger reptiles can also bioaccumulate environmental contaminants in their 

tissues from exposure to contaminated sediments or consumption of contaminated prey.

The fish sampling guidance further recommends that the size range of the sampled target 

fish ideally should include the larger fish individuals harvested at each sampling site 

because larger (older) fish within a population are usually the most contaminated with 

methylmercury (Phillips 1980, Voiland et al. 1991). In addition, the methylmercury 

concentrations in migratory species are likely to reflect exposures both inside and outside 

the study area, and the state or authorized tribe should take this into account when

determining whether to sample these species. For migratory species, EPA’s fish sampling 

guidance recommends that neither spawning populations nor undersized juvenile stages 

be sampled in fish contaminant monitoring programs (USEPA 2000c). States and 

authorized tribes should consider the life history of migratory species and the 

consumption patterns of the local population when including migratory species in their 

fish sampling protocols. Sampling of target finfish species during their spawning period 

should be avoided because contaminant tissue concentrations might decrease at that time.

If states and authorized tribes do not have local information about the types of fish that 

people eat, the following two options provide an alternative for identifying which fish to 

sample:

● Match assumed or known consumption pattern to sampled species. If the state has 

some knowledge of the fish species consumed by the general population, a 

monitoring sample could be composited to reflect this knowledge. For example, a 

state might decide that 75 percent of the fish consumed by the general population 

are trophic level 4 species, 20 percent are trophic level 3 species, and 5 percent are 

trophic level 2 species. A composite sample (see section 4.2.2) would reflect the 

determined trophic level breakout.

● Use trophic level 4 fish only. Predator species (e.g., trout, walleye, largemouth 

bass, and smallmouth bass) are good indicators for mercury and other persistent 

pollutants that are biomagnified through several trophic levels of the food web.

Increasing mercury concentrations correlate with an increase in fish age, with some 

variability, so that consumption of larger (older) individuals correlates with greater 
Monitoring and Assessment

54

risks to human health. Increasing mercury concentrations also correlate with higher 

trophic levels, and thus consumption of higher-trophic-level species would provide 

greater risks to human health. Therefore, targeting trophic level 4 species should 

serve as a conservative approach (depending on the species most frequently 

consumed by recreational fishers) for addressing waterbodies with highly varying 

concentrations of methylmercury.

4.2.2 What sample types best represent exposure?

EPA recommends using composite samples of fish fillets from the types of fish that 

people in the local area eat because methylmercury is found primarily in fish muscle

tissue (USEPA 2002c). Using skinless fillets is a more appropriate approach for 

addressing mercury exposures for members of the general population and most 

recreational fishers because fish consumers typically eat the fillets without skin. Because 

mercury is differentially concentrated in muscle tissue, leaving the skin on the fish fillet 

actually results in a lower mercury concentration per gram of skin-on fillet than per gram 

of skinless fillet (USEPA 2000c). Analysis of skinless fillets might also be more 

appropriate for some target species, such as catfish and other scaleless finfish species.

Some fish consumers, however, do eat fish with the skin on. In areas where the local 

population eats fish with the skin, the state or authorized tribe should consider including 

the skin in the sample.

Composite samples are homogeneous mixtures of samples from two or more individual 

organisms of the same species collected at a site and analyzed as a single sample. 

Because the costs of performing individual chemical analyses are usually higher than the 

costs of sample collection and preparation, composite samples are most cost-effective for 

estimating average tissue concentrations in target species populations. In compositing 

samples, EPA recommends that composites be of the same species and of similar size so 

that the smallest individual in a composite is no less than 75 percent of the total length 

(size) of the largest individual (USEPA 2000c). Composite samples can also overcome 

the need to determine how nondetections will be factored into any arithmetical averaging 

because the composite represents a physical averaging of the samples. However, 

depending on the objectives of a study, compositing might be a disadvantage because 

individual concentration values for individual organisms are lost. Guidance for Assessing 

Chemical Contaminant Data for Use in Fish Advisories, volume 1, at sections 6.1.1.6 and 

6.1.2.6, provides additional guidance for sampling recommendations.

4.2.3 What is the recommended study design for site selection?

Ideally, states and authorized tribes should collect samples over a geographic area that 

represents the average exposure to those who eat fish from the waterbody. However, if 

there are smaller areas where people are known to concentrate fishing, those areas should 

be used as the sampling area. Fish sampled in locations with mercury point sources 

should be included in the average concentration if fishing occurs in those areas but not 

included if the areas are not used for fishing.

Once the state or tribe identifies the geographic area, EPA recommends that they use a 

probabilistic sampling design to select individual sites or sampling locations. Use of a 

probabilistic design can address the spatial variability of methylmercury levels in fish. 
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55

This approach allows statistically valid inferences to be drawn about tissue levels in the 

area as a whole. EPA’s Guidance on Choosing a Sampling Design for Environmental 

Data Collection, for Use in Developing a Quality Assurance Project Plan (USEPA 

2002d) contains information about probabilistic site selection.

4.2.4 How often should fish samples be collected?

EPA’s Guidance for Assessing Chemical Contaminant Data for Use in Fish Advisories, 

volume 1 (USEPA 2000c), at section 6.1.1.5, provides recommendations for how 

frequently to sample fish tissue. If program resources are sufficient, this guidance 

recommends biennial sampling of fish in waterbodies where recreational or subsistence 

harvesting is commonly practiced. If biennial screening is not possible, waterbodies 

should be screened at least once every five years. Also, the state or authorized tribe

should sample during the period when the target species is most frequently harvested or 

caught.

In fresh waters, the guidance recommends that the most desirable sampling period is from 

late summer to early fall (August to October). Water levels are typically lower during that

time, simplifying collection procedures. Also, the fish lipid content is generally higher,

allowing the data to also provide information for other contaminant levels. The guidance 

does not recommend the late summer to early fall sampling period if it does not coincide 

with the legal harvest season of the target species or if the target species spawns during 

that period. In estuarine and coastal waters, the guidance recommends that the most 

appropriate sampling time is during the period when most fish are caught and consumed 

(usually summer for recreational and subsistence fishers).

EPA recommends that states and authorized tribes sample consistently in a season to 

eliminate seasonal variability as a confounding factor when analyzing fish monitoring 

data. Moreover, focused seasonality studies could be used both to assess the impact of 

seasonal variability on fish concentrations and to normalize concentrations to a standard 

season(s). Several studies have measured seasonality in the mercury concentrations in 

fish fillet muscle in estuaries and reservoirs (Kehrig et al. 1998; Park and Curtis 1997;

Szefer et al. 2003). In these studies, concentrations were generally higher in cold seasons

than in warm seasons by as much as two to three times. Slotten et al. (1995) showed that 

the uptake of methylmercury in zooplankton and fish increased dramatically during the 

fall mixing of Davis Creek Reservoir, a California reservoir contaminated by mercury 

mining activities.

No studies of seasonality of mercury concentrations in fish were found for rivers or 

natural lakes. On the basis of literature-reported fish mercury depuration rates, EPA does 

not expect seasonal fluctuations in fish mercury levels. Though reported mercury 

elimination half-lives cover a wide range of rates, from a few days to several years, the 

central tendency is 100–200 days (Burrows and Krenkel 1973; Giblin and Massaro 1973;

Huckabee et al. 1979 [literature review]; McKim et al. 1976; Rodgers and Beamish 

1982). Such slow depuration rates are expected to dampen strongly any fluctuations in 

methylmercury concentrations in fish. Instead, seasonal variations in fish tissue are likely 

linked to seasonal nutrition variability that affects fish body conditions but not mercury 

body burden.
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4.2.5 How many samples should be collected?

EPA’s Guidance for Assessing Chemical Contaminant Data for Use in Fish Advisories, 

volume 1 (USEPA 2000c), at section 6.1.2.7.2, provides information to help determine 

the number of composite samples needed for comparing fish tissue information to a target 

value. The guidance does not recommend a single set of sample size requirements (e.g., 

number of replicate composite samples per site and number of individuals per composite 

sample) for all fish contaminant monitoring studies, but rather presents a more general 

approach that is both scientifically defensible and cost-effective. The guidance provides 

the means for determining an optimal sampling design that identifies the minimum 

number of composite samples and of individuals per composite necessary to detect a 

minimum difference between a target (in this case, the water quality criterion) and the 

mean concentration of composite samples at a site. Under optimal field and laboratory 

conditions, at least two composite samples are needed at each site to estimate the 

variance. To minimize the risk of a destroyed or contaminated composite sample’s

preventing the site-specific statistical analysis, at least three replicate composite samples 

should be collected at each site.

4.2.6 What form of mercury should be analyzed?

Because of the higher cost of methylmercury analysis (two to three times greater than that 

for mercury analysis), one approach for the states and authorized tribes could be to first 

measure mercury in fish tissue. States and tribes may find that more labs have the 

capability for mercury analysis and that the analysis time may be quicker.

When measuring only mercury, the state or authorized tribe might make the conservative 

assumption that all mercury in fish tissue is methylmercury. Appendix A summarizes 

eight studies of the relative proportion of the mercury concentration in North American 

freshwater fish that is in the form of methylmercury. In six of the eight studies, 

methylmercury, on average, accounted for more than 90 percent of the mercury 

concentration in fish tissue. In the remaining two studies, methylmercury, on average, 

accounted for 80 to 90 percent of the mercury concentration in trophic level 3 and 4 fish. 

If the measured mercury level exceeds the methylmercury criterion, states and tribes may 

wish to repeat the sampling (if sufficient tissue is not left) and analyze for 

methylmercury.

4.2.7 Other sampling considerations

EPA recommends that states and tribes routinely collect both weight and length data 

when assessing the potential influence of fish nutritional state on mercury concentration, 

and potentially for normalizing fish concentrations to a standard body condition. 

Greenfield et al. (2001), Cizdziel et al. (2002, 2003), and Hinners (2004) reported a 

negative correlation between fish body condition (a ratio of weight to cubed length) and 

fish tissue mercury concentration. Regardless of the exact mechanism, body condition 

offers a useful method to explain variability in fish mercury.
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4.3 How should waterbody impairment be assessed for 

listing decisions?

Section 303(d)(1) of the CWA and EPA’s implementing regulations require states and 

authorized tribes to identify and establish priority ranking for waters that do not, or are 

not expected to, achieve or maintain water quality standards. In accordance with this 

ranking, a TMDL for such waters must then be established. For purposes of determining 

impairment of a waterbody and whether to include it on section 303(d) lists, or in 

category 5 of the Integrated Report under sections 303(d) and 305(b)17, states and 

authorized tribes must consider all existing and readily available data and information

(see 40 CFR 130.7).

States and authorized tribes determine attainment of water quality standards by 

comparing ambient concentrations to the numeric and narrative AWQC (40 CFR 130.7 

(b)(3)). Where a fish tissue criterion has been adopted, states and tribes should consider 

observed concentrations in fish tissue in comparison to the criterion. Where a water 

column translation of the fish tissue criterion has been developed and is adopted as part 

of the state’s or tribe’s water quality standards, states and tribes should consider ambient 

water concentrations in comparison to the translation.

For assessment of concentrations in fish tissue, resources may typically be unavailable to 

collect an adequate number of replicate composite samples to support rigorous statistical 

testing, especially where it is desirable to evaluate each individual target species 

separately. In these situations, states should make direct comparisons between composite 

sample concentrations and the criterion, as each composite effectively represents the 

average concentration observed in several fish.

Statistical tests for comparing the average concentration from multiple replicate 

composite samples to the criterion may be conducted where a sufficient number of 

replicates have been collected. EPA’s Guidance for Assessing Chemical Contaminant 

Data for Use in Fish Advisories, volume 1 (USEPA 2000c), at section 6.1.2.7.2,

recommends using the t-test to determine whether the mean concentration of mercury in 

composite fish tissue samples exceeds the screening value. This test involves a statistical 

comparison of the mean of all fish tissue data to the criterion. States and authorized tribes 

can evaluate whether the t-test statistic of the mean exceeds the water quality standards.

This procedure could also be used to determine impairment, provided it is consistent with 

a state’s water quality standards. States and authorized tribes might also want to consider 

the guidance in appendixes C and D of the Consolidated Assessment and Listing 

Methodology: Toward a Compendium of Best Practices (USEPA 2002e). Ultimately, the 

method that states and authorized tribes choose depends on how they express their water 

quality standards and apply their water quality assessment methodology.

17 See EPA’s guidance for Integrated Reports described at http://www.epa.gov/owow/tmdl/2006IRG/.
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4.3.1 How should nondetections be addressed?

When computing the mean of mercury in fish tissue, a state or authorized tribe might

encounter a data set that includes analyzed values below the detection level. EPA does 

not expect this to occur frequently for two reasons. First, if the samples are physically 

composited (see section 4.2.2.), the composite itself provides the average, and there is no 

need to mathematically compute an average. Second, the newer analytical methods 1630

and 1631 can quantify mercury at 0.002 mg/kg, which should be lower than the observed 

mercury in most fish tissue samples being analyzed.

If, however, a state or authorized tribe is mathematically computing an average of a data 

set that includes several values below the detection level, the water quality standards 

and/or assessment methodology should discuss how it will evaluate these values. The 

convention recommended in EPA’s Guidance for Assessing Chemical Contaminant Data 

for Use in Fish Advisories, volume 1, at section 9.1.2, is to use one-half of the method 

detection limit for nondetects in calculating mean values (USEPA 2000c). The guidance 

also recommends that measurements that fall between the method detection limit and the 

method quantitation limit be assigned a value of the detection limit plus one-half the 

difference between the detection limit and the quantitation limit. EPA notes, however, 

that these conventions provide a biased estimate of the average concentration (Gilbert 

1987) and, where the computed average is close to the criterion, might suggest an 

impairment when one does not exist or, conversely, suggest no impairment when one 

does exist.

States or authorized tribes can calculate the average of a data set that includes values 

below the detection level using other statistical methods (e.g., sample median and 

trimmed means) (Gilbert 1987). EPA has published a review of several methods and 

analyzed the potential bias each can introduce into the calculation of the mean (USEPA

2001h).

One approach that a state or authorized tribe could take is to conduct a sensitivity 

analysis to ascertain the consequence of what value is used to quantify samples below the 

detection level. In a sensitivity analysis, the state or authorized tribe would compute the 

mean concentration by first using the value of the detection level to quantify samples 

below the detection level and then using a zero value for samples below the detection 

level. If both calculated means are above or below the criterion, it is clear that the choice 

of how to quantify samples below the detection level does not affect the decision.

However, if one calculated mean is below the criterion and the other is above, it is clear 

that the choice of how to quantify samples below the detection level does affect the 

decision, and a more sophisticated approach such as the ones in Robust Estimation of 

Mean and Variance Using Environmental Data Sets with Below Detection Limit 

Observations (USEPA 2001h) should be used.

All methods have advantages and disadvantages. A state or authorized tribe should 

understand the consequences of which method it uses, especially if the choice makes a 

difference as to whether a waterbody is considered impaired or not. Furthermore, a state

or authorized tribe should be clear about which approach it used. Again, the selected 

methodology must be consistent with the state’s water quality standards and their 

published assessment method.
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4.3.2 How should data be averaged across trophic levels?

If target populations consume fish from different trophic levels, the state or authorized 

tribe should consider factoring the consumption by trophic level when computing the 

average methylmercury concentration in fish tissue. To take this approach, the state or 

authorized tribe would need some knowledge of the fish species consumed by the general 

population so that the state or authorized tribe could perform the calculation using only 

data for fish species that people commonly eat. (For guidance on gathering this 

information, see section 3.2.1.2.) States and authorized tribes can choose to apportion all 

the fish consumption, either a value reflecting the local area or the 17.5 grams fish/day 

national value for freshwater and estuarine fish if a local value is not available, to the 

highest trophic level consumed for their population or modify it using local or regional 

consumption patterns. Fish creel data from state fisheries departments are one reasonable

basis for estimating types and amounts of fish consumed from a given waterbody. The 

state or authorized tribe must decide which approach to use.

As an example of how to use consumption information to calculate a weighted average 

fish tissue concentration, see table 5 and equation 4.

Table 5. Example data for calculating a weighted average fish tissue value

Species Trophic level Number of samples

Geometric mean

methylmercury

concentration (mg/kg)

Cutthroat trout 3 30 0.07

Kokanee 3 30 0.12

Yellow perch 3 30 0.19

Smallmouth bass 4 95 0.45

Pumpkinseed 3 30 0.13

Brown bullhead 3 13 0.39

Signal crayfish 2 45 0.07

These concentrations are used to compute a weighted average of tissue methylmercury 

concentrations for comparison to the 0.3 mg/kg criterion. All fish measured are classified 

as trophic level 3 except signal crayfish, which are trophic level 2, and smallmouth bass, 

which are trophic level 4. The mean methylmercury concentration in trophic level 3 fish 

in this example is 0.15 mg/kg. This is calculated by weighting the geometric mean 

methylmercury concentration in each trophic level 3 species by the number of samples of 

each of the trophic level 3 species, and then averaging the weighted geometric means.

Had the concentrations been averaged without weighting for the number of samples, the 

average concentration would have been 0.18 mg/kg and would have given more weight to 

the methylmercury concentrations in brown bullhead than to the concentrations in the 

other species. (Note that this averaging approach does not consider that the trophic level 

3 fish in this sample are of different sizes, or that some fish might be consumed more or 

less frequently than is represented by the number of samples.) Equation 4 shows how the 

total (all trophic levels) weighted concentration is calculated using the 0.15 mg/kg value 

as representative of trophic level 3 fish and the default consumption for each trophic 

level:
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Cavg = 3.8 * C2 + 8.0 * C3 + 5.7 * C4 = 0.23 mg/kg (Equation 4)

(3.8 + 8.0 + 5.7)

Where:

C2 = average mercury concentration for trophic level 2

C3 = average mercury concentration for trophic level 3

C4 = average mercury concentration for trophic level 4

This calculation is based on apportioning the 17.5 grams/day national default 

consumption rate for freshwater and estuarine fish and shellfish by trophic level 

(5.7 grams/day of trophic level 4 fish, 8.0 grams/day of trophic level 3 fish, and 

3.8 grams/day of trophic level 2 fish18). As noted throughout this document, however, the 

consumption pattern of the target population should be used if available.

If fish tissue concentration data from a trophic level are missing, one would drop the 

consumption factor for that trophic level from both the numerator and denominator. For 

example, if there were no tissue concentration data for trophic level 2 fish in the previous 

example, equation 5 shows the revised calculation:

Cavg = 8.0 * C3 + 5.7 * C4 = 0.27 mg/kg (Equation 5)

(8.0 + 5.7)

This revised calculation preserves the relative contribution of each trophic level to 

consumption patterns. This approach (i.e., dropping a trophic level from Equation 4), 

however, should not be used if there are no fish tissue data for trophic level 4 fish. Since 

level 4 fish are the type of fish that people most often consume, dropping trophic level 4 

from Equation 4 may result in underprotection if trophic level 4 fish are actually 

consumed at the site. Instead, the state or authorized tribe should collect information to 

determine the consumption rate for fish in trophic level 4. If the state or authorized tribe

finds that no trophic level 4 fish are eaten, the state or tribe may drop trophic level 4 from 

Equation 4.

If the state or authorized tribe has developed a site-specific fish consumption rate for the 

criterion, the state or authorized tribe should incorporate this site-specific rate into

equation 4. In this case, the state or authorized tribe would replace the values of 5.7 

grams/day of trophic level 4 fish, 8.0 grams/day of trophic level 3 fish, and 3.8 grams/day

of trophic level 2 fish with the values that the state or authorized tribe developed.

As an alternative approach, states or authorized tribes might wish to translate fish tissue 

sample data to a standard size, length, or species of fish that is more commonly 

consumed or is representative of the risk considerations of the state. Regression models 

18 The values for each trophic level are the same as those discussed in section 3.2.1.2; they can be found in Methodology for Deriving 

Ambient Water Quality Criteria for the Protection of Human Health (USEPA 2000b).
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have been developed for this purpose (Rae 1997; Wente 2003). An inherent assumption is 

that concentrations will differ between samples of two different species/lengths/sample

cuts in a fixed equilibrium distribution relationship among all fish. If this relationship is 

known and at least one tissue sample concentration is measured from a 

species/length/sample cut that is accurately described by this relationship, fish 

consumption risk analyses could be performed for any species/lengths/sample cuts 

described by the relationship at this site.

Such regression models may include independent variables that account for species, 

aquatic environment (e.g., lotic vs. lentic, or other waterbody characteristics), sample cut 

(e.g., whole fish, skin-on fillet, skinless fillet), specific characteristics (e.g., age and 

retention time) of reservoirs, temporal trends, and fish length. The response variable is 

fish mercury concentration, which is typically assumed to be lognormally distributed. In a 

graphic sense, the model shows the covariance of each combination of nominal scale 

variables (e.g., whole fish, lentic waterbody) with fish length, with the slope representing 

the concentration/length ratio. Regression slopes can vary from lake to lake, resulting in 

models that inappropriately retain some fish size covariation (Soneston 2003).

EPA used the USGS National Descriptive Model for Mercury in Fish Tissue in various 

analyses (USEPA 2005a). This model is a statistical model related to covariance, and it 

allows the prediction of methylmercury concentrations in different species, cuts, and 

lengths of fish for sampling events, even when those species, lengths, or cuts of fish were 

not sampled during those sampling events. The model can also prove useful to states and 

authorized tribes in averaging fish tissue across trophic levels.

4.3.3 How should older data be assessed?

For purposes of determining waterbody impairment and inclusion on section 303(d) lists

or category 5 of the Integrated Report, states and authorized tribes must consider all 

existing and readily available water quality-related data and information (40 CFR 130.7).

Ideally, a state or authorized tribe would have collected fish tissue information within the 

past five years, as recommended in section 4.2.4. Such recent information might not 

always be available, however, and the available data often includes mercury samples 

collected and analyzed several years in the past. When the state or authorized tribe

evaluates this information, it should take into account the reliability of this information 

and its compliance with applicable data collection or quality assurance/quality control

program requirements.

4.3.4 How should fish consumption advisories be used to 

determine impairment?

On October 24, 2000, EPA issued guidance on the use of fish advisories in CWA section 

303(d) listing and 305(b) reporting decisions (USEPA 2005f). This guidance notes EPA’s 

general interpretation that fish consumption advisories on the basis of waterbody-specific 

information can demonstrate impairment of CWA section 101(a) “fishable” uses.

Although the CWA does not explicitly direct the use of fish consumption advisories to 

determine attainment of water quality standards, states and authorized tribes must 

consider all existing and readily available data and information to identify impaired 

waterbodies on their section 303(d) lists. For purposes of determining waterbody 
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impairment and inclusion on a section 303(d) list or in an Integrated Report, EPA

considers a fish consumption advisory and the supporting data existing and readily 

available data and information.

A state or authorized tribe should include on its section 303(d) list or in its Integrated 

Report, at a minimum, those waters for which waterbody-specific data that were the basis 

of a fish or shellfish consumption advisory demonstrate nonattainment of water quality 

standards. EPA believes that a fish or shellfish advisory demonstrates nonattainment 

when the advisory is based on tissue data, the data are from the specific waterbody in

question, and the risk assessment parameters of the advisory or classification are 

cumulatively equal to or less protective than those in the water quality standards.19 For 

example, consider a state or authorized tribe that bases its water quality criterion on 

eating two fish meals a month. If the state or authorized tribe finds fish tissue information 

showing that the level of mercury is at a level where it decides to advise people not to eat 

more than one fish meal a month and all other risk assessment factors are the same, the 

advisory also may serve to demonstrate a water quality standard exceedance and that the 

waterbody should be placed on the 303(d) list or in the Integrated Report. In contrast, if 

this same state or authorized tribe finds the level of mercury in fish in another waterbody 

is at a level at which it would advise people to eat no more than eight meals a month, and 

all other risk assessment factors are the same, the advisory is not necessarily the same as 

an impairment and the waterbody might not need to be listed.

When reporting water quality conditions under CWA sections 303(d) or in the Integrated 

Reporting format on the basis of a fish advisory for a migratory fish species, the state or 

authorized tribe should include the waters the migratory fish are known to inhabit 

because those are the waters where the fish potentially would be exposed to mercury. In 

addition, a state or authorized tribe has the discretion to include any other water having a 

fish consumption advisory as impaired on its section 303(d) list if the state or authorized 

tribe believes inclusion is appropriate.

19 The October 2000 EPA guidance assumes that the fish tissue monitoring that supports the advisory is sufficiently robust to provide a 

representative sample of mercury in fish tissue. EPA’s fish tissue guidance (USEPA 2000c) provides recommendations on how public 

health officials can collect sufficient information about contaminants in fish.
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5 Other Water Quality Standards Issues

5.1 How does this criterion relate to the criteria published 

as part of the Great Lakes Initiative?

The 2001 recommended methylmercury fish tissue criterion and EPA’s recommendations 

for its implementation do not supersede the requirements applicable to the Great Lakes at 

40 CFR part 132. The Great Lakes regulatory requirements, known as the Great Lakes 

Initiative, or GLI, apply to all the streams, rivers, lakes and other bodies of water within 

the U.S. portion of the Great Lakes drainage basin. For those waters, a state or authorized 

tribe must adopt requirements (including water quality criteria) that are consistent with 

(as protective as) regulations EPA promulgated on March 23, 1995. See 60 FR 15366 and 

40 CFR 132.1(b) and 132.4.

Under these regulations, if a state or authorized tribe adopts a fish tissue residue

methylmercury criterion for the protection of human health, EPA, in its review of the new 

state or tribal criterion, must determine whether it is as protective as the mercury water

column criterion for human health protection promulgated at 40 CFR 132.6, table 3, and 

whether all implementation procedures are as protective as the implementation 

procedure. See 40 CFR 132.5(g).

As described below, it is unlikely that adoption of EPA’s 2001 recommended

methylmercury fish tissue-based criterion of 0.3 mg/kg to protect human health would 

result in TMDLs or NPDES permit limits addressing mercury impairments in the Great 

Lakes basin less stringent than those that would be required under the existing GLI 

regulations. The reasons for this include the following:

● The GLI requires all states and authorized tribes to adopt the GLI wildlife water 

column criterion. The GLI wildlife criterion has a significantly more stringent 

methylmercury fish tissue basis than either the 2001 criterion or the GLI human 

health criteria and would therefore likely be the controlling basis for any TMDLs 

or NPDES permit limits addressing mercury pollution.

● Even if that were not the case, the 2001 criterion is more stringent than the 

methylmercury fish tissue basis for the GLI human health water column criteria for 

mercury.

Furthermore, using the 2001 fish tissue criterion would not necessarily result in lower 

transaction costs than the GLI. The GLI implementation procedures (e.g., the mixing 

zone prohibition, 40 CFR part 132, appendix F, procedure 3) require the use of water 

column criteria, so the 2001 methylmercury fish tissue criterion would need to be 

converted to a water column criterion following the GLI site-specific modification 

procedures before it could be approved by EPA and implemented using other GLI

implementation procedures.
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The human health criterion for mercury established by the GLI is 3.1 ng/L20. This water 

column criterion for mercury is equivalent to a methylmercury fish tissue residue value of 

0.35 mg/kg using the Great Lakes-specific BAFs for mercury—27,900 L/kg for trophic 

level 3 and 140,000 L/kg for trophic level 4—as well as other Great Lakes-specific 

information (USEPA 1995c). Because EPA’s 2001 methylmercury criterion (0.30 mg/kg) 

is more stringent than the GLI fish tissue residue value, the 2001 criterion would result in 

more stringent water column concentrations than the GLI human health criteria unless 

other, site-specific factors were significantly less stringent. This could occur, for 

example, if a state or authorized tribe applied the GLI site-specific modification 

procedures and found that the current, local BAF is significantly lower than the one used 

to develop the GLI criterion. In that case, the state or tribe could use the lower, local BAF 

and EPA’s recommended fish tissue-based criterion to recalculate the water column 

criterion using the GLI site-specific modification procedures and submit it to EPA for 

review and approval. If the site-specific water column criterion was approved by EPA, 

the state or authorized tribe could use it and the GLI implementation procedures to 

develop TMDLs and NPDES permits.

Finally, as indicated above, if a state or authorized tribe were to adopt the 2001 human 

health criterion in the Great Lakes basin, this action most likely would not result in a 

change to TMDLs or NPDES permits. The GLI also includes a 1.3 ng/L criterion for the 

protection of wildlife, and in most instances, this more stringent criterion will drive the 

calculation of TMDLs or NPDES permit limits.

5.2 What is the applicable flow for a water column-based 

criterion?

If a state or authorized tribe adopts new or revised methylmercury criteria based on a 

water column value rather than a fish tissue value, it should consider the dilution flow

specified in the state’s or tribe’s water quality standards when applying the new mercury 

criterion. Where a state’s or authorized tribe’s water quality standards do not specify the 

appropriate flow for use with the mercury criterion, EPA recommends using a harmonic 

mean flow. EPA used this flow for application of the human health criteria for mercury in 

the Great Lakes (40 CFR part 132). EPA also used this flow for application to the human 

health criteria in the National Toxics Rule (40 CFR 131.36) and the California Toxics 

Rule, or CTR (40 CFR 131.38). The Agency considers this flow to better reflect the 

exposure of fish to mercury. The technical means for calculating a harmonic mean is 

described in section 4.6.2.2.a of the Technical Support Document for Water Qualitybased Toxics Control (USEPA 1991).

20 EPA promulgated the GLI human health criteria of 1.8 ng/L in 40 CFR part 132, table 3, in March 1995, based on an RfD of 0.06

μg/kg/d. In May 1995 EPA revised the RfD to the current 0.1 μg/kg/d, which would result in GLI criteria of 3.1 ng/L. In October 1996 

EPA issued guidance indicating that the 3.1 ng/L criteria were considered as protective as the promulgated 1.8 ng/L.
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5.3 How are mixing zones used for mercury?

5.3.1 What is a mixing zone?

A mixing zone is the area beyond a point source outfall (e.g., a pipe) in which 

concentrations of a pollutant from a wastewater discharge mix with receiving waters.

Under 40 CFR 131.13, states and authorized tribes may, at their discretion, include 

mixing zones in their water quality standards. Within a mixing zone, the water may be

allowed to exceed the concentration-based water quality criterion for a given pollutant.

The theory of allowing mixing zones is based on the belief that by mixing with the 

receiving waters within the zone, the concentration of the pollutant being discharged will 

become sufficiently diluted to meet applicable water quality criteria beyond the borders 

of that zone and fully protect the designated use of the waterbody as a whole. More 

information on mixing zones is available in the Technical Support Document for Water 

Quality-based Toxics Control (USEPA 1991) and the Water Quality Standards 

Handbook (USEPA 1994). States and authorized tribes often authorize mixing zone 

provisions and methodologies for calculating mixing zones for later application to 

NPDES point source discharge points.

5.3.2 How does a mixing zone apply for the fish tissue-based 

methylmercury criterion?

The question of mixing zones is not relevant when applying the fish tissue-based 

criterion, which refers to the level of mercury found in fish flesh. The criterion is fish 

tissue-based, not water column-based. The criterion reflects the exposure of the fish to 

mercury in the water column and food over the life of the fish, and thus it reflects an 

integration of the exposure over time and over spatially varying water column 

concentrations. The total load of mercury in the waterbody, taking into account the 

methylation rate and bioaccumulation of mercury in fish, affects the level of 

methylmercury in the fish tissue.

Some states and authorized tribes, however, might choose to adopt a water column 

criterion based on the fish tissue criterion and thus have a criterion for which a mixing 

zone might apply. In this situation, a state or authorized tribe should follow its existing 

procedures for determining appropriate mixing zones. EPA advises caution in the use of 

mixing zones for mercury. While fish tissue contamination tends to be a far field problem 

affecting entire waterbodies, rather than a narrow scale problem confined to mixing 

zones, EPA’s guidance recommends restricting or eliminating mixing zones for 

bioaccumulative pollutants such as mercury so that they do not encroach on areas often 

used for fish harvesting (particularly for stationary species such as shellfish). Restriction 

or elimination might also be used to compensate for uncertainties regarding the ability of 

aquatic life or the aquatic system to tolerate excursions above the criteria, uncertainties 

inherent in estimating bioaccumulation, or uncertainties in the assimilative capacity of the 

waterbody. See the Water Quality Standards Handbook, section 5.1.3 (USEPA 1994).
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5.3.3 Does the guidance for the fish tissue-based criterion change 

the Great Lakes Initiative approach to mixing zones for 

bioaccumulative pollutants?

To reduce the adverse effects from bioaccumulative chemicals of concern (BCCs) in the 

Great Lakes, on November 13, 2000, EPA promulgated an amendment to the Final Water 

Quality Guidance for the Great Lakes System (40 CFR part 132, appendix F, procedure 

3). The regulation requires prohibition of mixing zones for bioaccumulative pollutants 

from existing discharges in the Great Lakes to the greatest extent technically and 

economically feasible. Specifically, existing discharges of BCCs are not eligible for a 

mixing zone after November 10, 2010 (although under certain circumstances mixing 

zones may be authorized). For new BCC discharges, the rule essentially prohibits mixing 

zones of bioaccumulatives immediately upon commencing discharge. This means that 

NPDES permit limitations for mercury discharged to the Great Lakes system must not 

exceed the water quality criterion. This also limits the flexibility that states and 

authorized tribes would otherwise have to adjust point source controls on the basis of

nonpoint source contributions.

EPA reiterates that the new methylmercury criterion, and EPA’s recommendations on its 

implementation, does not supersede the requirements applicable to the Great Lakes at 40 

CFR part 132. The criteria for the Great Lakes are water column-based, and therefore 

they can be applied as an effluent requirement at the end of a pipe. EPA continues to 

view the prohibition of a mixing zone for mercury and other bioaccumulative pollutants 

for the Great Lakes as appropriately protective for water column-based water quality 

criteria applied to these waters.

If a state or authorized tribe adopts the new fish tissue-based criterion for a Great Lake or 

tributary to the Great Lake, the state or tribe would do this using the site-specific

modification procedures of part 132 (see section 5.1 of this document). The state or tribe

would have determined a site-specific BAF in this process and therefore would have the 

means for calculating a water column-based criterion. Under the part 132 regulations, 

EPA in its review of the new state or tribal implementation procedures would determine 

whether they are as protective as the Great Lakes procedures for human health protection 

(40 CFR 132.5(g)(3)). Specifically, EPA would determine whether the implementation 

procedures are as protective as applying the table 3 (in 40 CFR part 132) criterion for 

protection of human health without a mixing zone, consistent with the prohibition on 

mixing zones for BCCs (40 CFR 132, appendix F.3.c.). In addition, if the state’s or 

tribe’s implementation procedures involve converting the fish tissue-based criterion into 

an equivalent water column-based number, the mixing zone prohibition requirements of 

40 CFR part 132 still apply.

5.4 How are fish consumption advisories and water 

quality standards harmonized?

5.4.1 What is the role of state and tribal Fish Advisory Programs?

States and authorized tribes have the primary responsibility of estimating the human 

health risks from the consumption of chemically contaminated, noncommercially caught 

finfish and shellfish (e.g., where water quality standards are not attained). They do this by 
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67

issuing consumption advisories for the general population, including recreational and 

subsistence fishers, and for sensitive subpopulations (such as pregnant women, nursing 

mothers and their infants, and children). These advisories are nonregulatory and inform 

the public that high concentrations of chemical contaminants, such as mercury, have been 

found in local fish. The advisories recommend either limiting or avoiding consumption of 

certain fish from specific waterbodies or, in some cases, from specific waterbody types 

(e.g., all lakes). In the case of mercury, many states and authorized tribes have calculated 

a consumption limit to determine the maximum number of fish meals per unit of time that 

the target population can safely eat from a defined area.

5.4.2 How are consumption limits for consumption advisories 

determined?

EPA has published guidance for states and authorized tribes to use in deriving their 

recommended fish consumption limits, titled Guidance for Assessing Chemical 

Contaminant Data for Use in Fish Advisories, volumes 1 and 2 (USEPA 2000c, 2000f).

This guidance describes the two main equations necessary to derive meal consumption 

limits on the basis of the methylmercury RfD. Basically, the first equation is used to 

calculate the daily consumption limits of grams of edible fish (in g/day); a second 

equation is used to convert daily consumption limits to meal consumption limits over a 

specified period of time. Variables used to calculate the advisory consumption limits 

include fish meal size and frequency, consumer body weight, contaminant concentration 

in the fish tissue, the time-averaging period selected, and the reference dose for 

methylmercury health endpoints.

In the absence of site-specific fish consumption data, EPA recommends using a fish 

consumption rate of 17.5 grams/day of fish (uncooked) eaten from the local water as a 

screening level. This consumption rate equates to approximately two 8-ounce meals per 

month. Using this consumption rate, and assuming a 70-kg body weight (the same 

assumption used to derive the methylmercury criterion), the concentration of 

methylmercury in locally caught fish that would result in exposures that do not exceed 

the RfD (0.0001 mg/kg-day) is about 0.4 mg/kg and lower ([0.001 mg/kg-day x 70 kg 

bw]/0.0175 kg fish/day).

Advisory limits can differ from one state or tribe to another. This inconsistency is due to 

a host of reasons, some of which speak to the flexibility states and authorized tribes have 

to use different assumptions (chemical concentrations, exposure scenarios and 

assumptions) to determine the necessity for issuing an advisory. The nonregulatory nature 

of fish advisories allows such agencies to choose the risk level deemed appropriate to 

more accurately reflect local fishing habits or to safely protect certain subpopulations 

(e.g., subsistence fishers).

5.4.3 How does the criterion differ from the advisory level?

Although EPA derived its recommended screening value for a fish advisory limit for 

mercury and human health methylmercury criterion from virtually identical 

methodologies, it is important to clarify the distinctions between the two values. They are 

consistently derived, but because each value differs in purpose and scope, they diverge at 

the risk management level. Fish advisories are intended to inform the public about how 
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much consumers should limit their intake of individual fish species from certain 

waterbodies. Alternatively, the Agency uses its methylmercury criterion, like other CWA 

section 304(a) criteria, as a basis for both nonregulatory and regulatory decisions. The 

criterion can serve as guidance to states and authorized tribes for use in establishing water 

quality standards, which, in turn, serve as a benchmark for attainment, compliance, and 

enforcement purposes.

The main risk management difference between EPA’s recommended methylmercury 

water quality criterion and the fish advisory default screening value for mercury is that 

the criterion includes an RSC21 and the screening value does not. In deriving the criterion, 

EPA assumed an RSC value of 2.7x10-5 mg/kg-day to account for exposure from marine 

fish and shellfish. The guidance for setting fish consumption limits also discusses using 

an RSC to account for exposures other than those from noncommercially caught fish, but 

the guidance may be applied without using an RSC. The RSC guidance in the 2000 

Human Health Methodology (USEPA 2000b) provides more detail and specific 

quantitative procedures to account for other exposure pathways. EPA’s advisory 

guidance recommends that states and authorized tribes consider using an RSC to account 

for exposure from other sources of pollutants (such as mercury) when deriving a fish 

consumption limit and setting a fish advisory for mercury.

5.4.4 What if there is a difference between assessing criterion 

attainment and issuance of a fish consumption advisory?

In many states and authorized tribes, numeric water quality criteria and fish and shellfish 

consumption limits differ because of inherent differences in the technical and risk 

assumptions used to develop them. As discussed in section 4.2, EPA considers a fish 

consumption advisory to demonstrate nonattainment of water quality standards when the 

advisory is based on tissue data, the data are from the specific waterbody in question, and 

the risk assessment parameters of the advisory or classification are cumulatively equal to 

or less protective than those in the water quality standards. Two situations in which the 

presence of an advisory might not imply an exceedance of the water quality standard 

(USEPA 2005f) are as follows:

● Statewide or regional advisory. States have issued statewide or regional warnings 

regarding fish tissue contaminated with mercury, on the basis of data from a subset 

of waterbodies, as a precautionary measure. In these cases, fish consumption 

advisories might not demonstrate that a CWA section 101(a) “fishable” use is not 

being attained in an individual waterbody and might not be appropriate for 

determining attainment based on exceedance of water quality criteria.

● Local advisory. States have issued local advisories using a higher fish consumption 

value than that which they use in establishing water quality criteria for protection 

of human health. Again, in this case the fish consumption advisories might not 

demonstrate that a section 101(a) “fishable” use is not being attained in an 

21 See discussion on the RSC in section 3.1.2.3 and 3.2.1.1.
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individual waterbody and might not be as appropriate as comparison with water 

quality criteria as a basis for determining attainment.

For example, consider a state or authorized tribe that adopts EPA’s methylmercury 

criterion of 0.3 mg/kg, which is based on eating approximately two 8-ounce fish meals a 

month. If the state or authorized tribe finds that a waterbody has fish with a mercury level 

of 0.2 mg/kg, this water would not be exceeding the water quality criterion. Yet, this 

mercury concentration is sufficient for the state or authorized tribe to issue a fish 

consumption advisory recommending that people eat no more than four 8-ounce meals a 

month. In this case, because the fish consumption advisory uses a higher fish 

consumption value than that used to develop the water quality criterion (and the fish 

tissue concentration does not exceed the criterion), consistent with EPA’s 2000 guidance, 

the waterbody is not necessarily impaired (USEPA 2005f).

In the case where a local advisory is based on a higher fish consumption value which is 

considered representative of local consumption, the state or authorized tribe should 

consider whether it should adopt a site-specific criterion for the waterbody. A local 

advisory generally reflects actual contaminant monitoring data and may reflect local fish 

consumption patterns, and it might identify more representative fish species. The 

information gathered in developing the advisory might provide valid grounds for revising 

the level of a numeric water quality criterion to match that of the advisory.

5.4.5 Should existing advisories be revised to reflect the new 

criterion?

Although EPA’s screening value for fish advisory studies and the recommended 304(a) 

criterion for mercury are based on similar methodologies and are intended to protect

people who consume mercury-contaminated fish, they do not necessarily have to be the 

same value. As explained above, each limit is predicated on different risk-management 

decisions and thus incorporates different assumptions. A state or tribe may choose to 

revise existing advisories to mirror the methylmercury criterion. Likewise, there is merit 

in adopting a site-specific methylmercury criterion on the basis of a local fish advisory, if 

that advisory is supported by sufficient data that are representative and of acceptable 

quality.

5.4.6 What federal agencies issue advisories?

The Food and Drug Administration’s (FDA’s) mission is to protect the public health with 

respect to levels of chemical contaminants in all foods, including fish and shellfish, sold 

in interstate commerce. To address the levels of contamination in foods, FDA has 

developed both action levels and tolerances. An action level is an administrative 

guideline that defines the extent of contamination at which FDA may regard food as 

adulterated and represents the limit at or above which FDA may take legal action to 

remove products from the marketplace. It is important to emphasize that FDA’s 

jurisdiction in setting action levels is limited to contaminants in food shipped and 

marketed in interstate commerce; it does not include food that is caught locally by 

recreational or subsistence fishers. FDA also issues fish consumption advice on fish and 

shellfish sold in commerce in cases where contaminants have been detected at levels that 

may pose public health concerns for some consumers.
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As described in section 5.4.2, EPA provides guidance to states, tribes, local governments 

and others on scientifically sound, cost-effective methods for developing and managing 

noncommercial fish consumption advisories on local waters. See EPA’s Guidance for 

Assessing Chemical Contaminant Data for Use in Fish Advisories (USEPA 2000c, 

2000f). In addition, EPA has issued advice under CWA section 104(b)(6) to supplement 

state and/or tribal advice on local waters.

In March 2004, EPA and FDA issued a joint national fish advisory about mercury in fish 

and shellfish. The purpose of the advisory is to inform women who may become 

pregnant, pregnant women, nursing mothers, and parents of young children how to get 

the positive health benefits from eating fish and be confident that they have reduced their 

exposure to the harmful effects of mercury. The 2004 advisory lists fish sold in interstate 

commerce that are known to be high in mercury as well as fish that that are low in 

mercury to help consumers choose the most appropriate fish. The advisory also contains 

recommendations about eating fish harvested from local waters where no advice has been 

provided by state or tribal authorities. Information regarding the national advisory is at 

http://www.epa.gov/waterscience/fish/.

5.4.7 How is the criterion related to FDA action levels?

The current FDA action level for mercury in fish is 1 mg/kg. Generally, an action level is 

different from a fish advisory limit—and even more different from a CWA section 304(a) 

criterion. FDA action levels are intended for members of the general population who 

consume fish and shellfish typically purchased in supermarkets or fish markets that sell 

products harvested from a wide geographic area. The underlying assumptions used in the 

FDA methodology were never intended, as local fish advisories are, to be protective of 

recreational, tribal, ethnic, and subsistence fishers who typically consume fish and 

shellfish from the same local waterbodies repeatedly over many years. EPA and FDA 

have agreed that the use of FDA action levels for the purposes of making local advisory 

determinations is inappropriate. Furthermore, it is EPA’s belief that FDA action levels 

and tolerances should not be used as a basis for establishing a state’s or tribe’s 

methylmercury criterion.

5.5 What public participation is recommended for 

implementing the methylmercury criterion?

By applicable regulations, water quality standards, TMDL, and NPDES permit decisions 

require public notice and the opportunity for the public to comment on tentative 

decisions. Some public interest groups might have an interest in decisions related to 

mercury, especially in areas where local citizens rely heavily on locally caught fish as a 

food source. EPA recommends that organizations with an interest in environmental 

justice issues be included in the public notice.
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6 TMDLs

6.1 What is a TMDL?

CWA section 303(d)(1) and EPA’s implementing regulations require states and 

authorized tribes to identify and establish priority rankings for waters that do not, or are 

not expected to, achieve or maintain water quality standards with existing or anticipated 

required controls. This list is known as the state’s or tribe’s list of “impaired” 

waterbodies or 303(d) list. States and authorized tribes then must establish TMDLs for 

the impaired waterbodies.

A TMDL is a calculation of the maximum amount of a pollutant that a waterbody can 

receive and still meet water quality standards. A TMDL also allocates the pollutant loads 

among the contributing sources, both point and nonpoint. The TMDL calculation must 

include a margin of safety to take into account any uncertainty in the TMDL calculation 

and must account for seasonal variation in water quality. The current statutory and 

regulatory framework governing TMDLs includes CWA section 303(d) and the TMDL 

regulations published in 1985 at 40 CFR 130.2 and 130.7 and amended in 1992 (see 50 

FR 1774 (Jan. 11, 1985); 57 FR 33040 (July 24, 1992)).

As of the 2006 303(d) listing cycle, 42 states and Puerto Rico reported at least one 

waterbody as impaired due to mercury, and more than 9,000 specific waterbodies were 

listed as impaired due to mercury, either solely or in combination with other pollutants.

As mentioned previously in section 2.4, with the implementation of the new 

methylmercury fish tissue criterion, monitoring of previously unmonitored waterbodies, 

and use of more sensitive analytical methods, EPA expects that the number of 

waterbodies listed as impaired due to mercury might increase.

6.2 How have states and tribes approached mercury

TMDLs?

Developing TMDLs for waters impaired by mercury raises a number of technical and 

policy issues. For example, air deposition is the predominant source of mercury to many 

waterbodies, especially in the eastern United States. The mercury deposited from air 

comes from local, regional, and international sources, and identifying how each of these 

sources contributes to the mercury load in the waterbody is challenging. In other 

waterbodies, significant loadings might come from other sources, such as past metalmining activity or geologic sources. Frequently, states and authorized tribes do not have 

the authority to address all the sources that contribute mercury to their waterbodies and 

rely on efforts conducted under a variety of programs, such as regulations under the 

CAA, pollution prevention programs, and international efforts to reduce releases and 

emissions from mercury sources. States and EPA have found that, in many cases, it is 

important to coordinate closely with programs other than those under the CWA to 

address these mercury sources.

Given these challenges, EPA is working with states, tribes, and stakeholders to determine 

how best to use TMDLs and the 303(d) listing process to provide a basis for reducing

mercury releases to water, including consideration of air deposition, to meet applicable 
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72

water quality standards and CWA goals. In areas where large numbers of waterbodies are 

impaired due to mercury derived from air deposition, some states have begun to explore 

ways to address mercury impairments efficiently, such as through development of 

TMDLs on various geographic scales. As of December 2008, mercury TMDLs have been 

approved for more than 6,600 waterbodies, including a “statewide” mercury TMDL in 

Minnesota and a multi-state mercury TMDL for the Northeast states (see below).

On March 8, 2007, EPA issued a memorandum describing a voluntary approach for 

listing waters impaired by atmospheric mercury under CWA section 303(d) (USEPA 

2007b) (http://www.epa.gov/owow/tmdl/mercury5m/Mercury5m.pdf). EPA is 

recommending the voluntary approach for states that have in place a comprehensive

mercury reduction program with elements recommended by EPA. These states may 

separate their waters impaired by mercury predominantly from atmospheric sources in a 

subcategory of their impaired waters list (“5m”) and defer the development of TMDLs 

for those waters. A state using the 5m subcategory may also continue to defer the 

development of mercury TMDLs where the state demonstrates continuing progress in 

reducing in-state mercury sources. Recommended elements of a mercury reduction 

program include identification of air and multimedia sources within a state and programs 

to address those sources; mercury reduction goals and target dates; multimedia 

monitoring; public reporting on the state’s mercury reduction efforts; and multistate 

coordination. The 5m subcategory is intended to recognize states with comprehensive 

mercury programs and to allow states to focus on early implementation actions.

Because the 5m subcategory is focused primarily on waterbodies impaired by mercury 

from air deposition, EPA recommends that the 5m subcategory include waters where the 

proportion of mercury from air deposition is high compared to other mercury sources. In 

the 5m memorandum, EPA recommends that states describe how such waterbodies were 

identified. Such information will help determine whether the 5m approach is appropriate. 

EPA also believes that, as the relative contribution to a waterbody from sources other 

than air deposition increases, such as water point sources, it may be more appropriate to 

use the TMDL process to characterize and address those sources sooner, rather than 

deferring TMDL development. As stated in the 5m memorandum, states have the option 

to continue developing mercury TMDLs sooner, whether or not they place waterbodies in 

subcategory 5m.

On September 29, 2008, EPA issued a document titled Elements of Mercury TMDLs 

Where Mercury Loadings Are Predominantly from Air Deposition, to assist states, EPA 

regional staff, and other stakeholders in identifying approaches for the development of 

mercury TMDLs (USEPA 2008). Compiled in a checklist format, approaches described 

in the document are drawn largely from approaches and best practices used in approved 

mercury TMDLs. The checklist summarizes considerations in addressing the required 

and recommended TMDL elements described in the Guidelines for Reviewing TMDLs 

under Existing Regulations Issued in 1992 (USEPA 2002f) when developing mercury 

TMDLs on geographic scales ranging from waterbody-specific to multi-state.

While the checklist is based on existing guidance for reviewing TMDLs, this guidance 

document supplements the checklist by providing additional information and case studies 

on approaches that have been used in approved mercury TMDLs to date, and examples of 

technical tools available to assist in mercury TMDL development. Technical tools 

available to assist in the development of mercury TMDLs include screening-level 
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73

analyses of mercury loadings and sources using the Mercury Maps tool and more 

complex water and air models. Many of these tools are discussed in the sections below.

EPA recommends that states continue to develop TMDLs for mercury-impaired waters 

where appropriate, taking into account the considerations and approaches described in 

this guidance. States may also consider using the 5m subcategory for waters impaired by 

mercury predominantly from air deposition if the state has a comprehensive mercury 

reduction program as described in the 5m memorandum.

6.2.1 What geographic scales have been used for mercury TMDLs?

Many mercury TMDLs approved to date were developed on a waterbody-specific basis. 

They include some of the first approved mercury TMDLs, such as those developed for 

waterbodies in middle and south Georgia. Other examples include TMDLs developed for 

waterbodies in Louisiana, such as the Ouachita River, the Narraguinnup and McPhee 

reservoirs in Colorado, and Pena Blanca and Arivaca lakes in Arizona. Various aspects of 

these TMDLs are described further in appendix D.

In areas of the country where many waterbodies are listed as impaired due to mercury

primarily from atmospheric sources, some states have begun to explore the development 

of mercury TMDLs on a watershed scale or on the basis of a large geographic area, such 

as a state or region. One example of a regional or grouped approach is the mercury 

TMDL for the Coastal Bays and Gulf Waters of Louisiana, approved in June 2005. The 

TMDL covers six segments of coastal Louisiana. Because of the large geographic extent 

of mercury in the coastal waters and the similar extent of mercury contributions from air 

deposition, the TMDL was developed on a watershed basis rather than waterbody by

waterbody. The TMDL used air deposition modeling results from the Regional Modeling 

System for Aerosols and Deposition (REMSAD) to estimate wet and dry deposition of 

mercury for the six segments. The air deposition modeling results, in turn, were used to 

model runoff or nonpoint source mercury loadings. As described in the following section, 

mercury loadings can include direct deposition to waterbodies and deposition to the 

watershed that is subsequently transported to the waterbody via runoff and erosion.

Additional information on this TMDL can be found on EPA’s TMDL webpage at 

http://iaspub.epa.gov/tmdl/waters_list.tmdl_report?p_tmdl_id=11642.

A “statewide” mercury TMDL developed by Minnesota was approved by EPA on 

March 27, 2007. The TMDL report covers 998 mercury impairments and is the first 

approved mercury TMDL covering such a large number of waterbodies and large 

geographic area. (Note: Although called statewide, the TMDL does not cover all 

mercury-impaired waterbodies in the state.) Minnesota used a statewide approach 

because the predominant mercury source in those waterbodies—air deposition—is 

relatively uniform across the state. The final TMDL report includes two TMDLs––one 

for the northeast region of the state and the other for the southwest region of the state. 

Waterbodies were grouped into the two regions on the basis of differences in fish tissue 

concentrations, with higher fish mercury concentrations in the northeast region compared 

to the southwest region. The difference in mercury concentrations is thought to be due to 

the effect of land use and other factors on the methylation of mercury. For example, the 

northeast region is dominated by wetlands, where mercury tends to be methylated more 

readily; the southwest is dominated by cultivated lands. A summary of the Minnesota 
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mercury TMDL approach is provided in appendix D, and the allocation approach is 

described further below. The final TMDL and EPA decision document are at 

http://www.pca.state.mn.us/water/tmdl/tmdl-ercuryplan.html#approval.

On December 20, 2007, EPA approved the Northeast Regional Mercury TMDL covering 

waterbodies in Connecticut, Maine, Massachusetts, New Hampshire, New York, Rhode 

Island and Vermont. In using a regional approach, the TMDL document provides 

aggregate wasteload allocations and load allocations for the region. The regional 

approach was based on an analysis of data showing similar levels of mercury in fish 

throughout waterbodies in the region, and the states’ finding that air deposition is the 

predominant mercury source. The TMDL document focuses on waters impaired by 

mercury primarily from atmospheric sources; it excludes coastal and marine waters and a 

few areas of high localized deposition and high fish mercury levels. The number of 

individual waterbodies covered by the regional TMDL document amounts to over 5,300 

(the specific number of waterbodies covered by the TMDL document vary from state to 

state and are cited in EPA’s approval documents). The TMDL target is EPA’s 

recommended fish tissue criterion of 0.3 ppm methylmercury for each of the states except 

for Connecticut and Maine, where the targets are 0.1 ppm and 0.2 ppm, respectively. The 

TMDL allocates approximately 2.0 percent of the loading capacity to point sources and 

98 percent to nonpoint sources (predominantly atmospheric deposition). The TMDL 

assumes that most of the reductions would need to come from atmospheric sources. The 

Northeast Regional Mercury TMDL are at 

http://www.epa.gov/region1/eco/tmdl/assets/pdfs/ne/Northeast-Regional-MercuryTMDL.pdf, and the EPA approval documents for each of the states are at 

http://www.epa.gov/region1/eco/tmdl/approved.html.

6.2.2 What are the considerations in developing mercury TMDLs?

A TMDL must identify the applicable water quality standards for each listed segment and 

identify the loading capacity of a water (40 CFR 130.2). In addition, a TMDL must 

allocate the pollutant loads among the sources, both point and nonpoint (40 CFR 

130.2(i)). EPA guidance further notes that a TMDL should identify the pollutant sources,

both point and nonpoint, including the location of the sources and quantity of the loading.

Some of the considerations in developing a mercury TMDL and approaches used in 

approved mercury TMDLs are described in more detail in the text below.

6.2.2.1 What are potential mercury sources to waterbodies?

An important step in TMDL development is an evaluation of the loadings from various 

sources. The potential sources of mercury to waterbodies include the following: (1) direct 

discharges of mercury from water point sources, including industrial dischargers and 

wastewater treatment plants; (2) atmospheric deposition, including direct deposition to 

the waterbody surface and deposition to the watershed, which subsequently is transported 

to the waterbody via runoff and erosion, including via stormwater; (3) runoff, ground 

water flow, acid mine drainage, and erosion from mining sites or mining wastes, and 

other waste disposal sites such as landfills and land application units; (4) sediments, 

which might have mercury contamination or hot spots resulting from past discharges; and

(5) “naturally occurring” mercury in soils and geologic materials. Sediments containing 
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mercury from past discharges might continue to contribute mercury to the overlying 

waterbody. Further discussion of each of these types of sources follows.

Point sources. Point source discharges of mercury include POTWs, electric utilities, and 

other industrial facilities. Sources of data on point source discharges of mercury include 

the Permit Compliance System, as well as a study of domestic mercury sources by the 

Association of Metropolitan Sewerage Agencies (AMSA 2000), now called the National 

Association of Clean Water Agencies (NACWA). Without accurate discharge data, a 

sample of a representative portion of dischargers has been used in mercury TMDLs to 

estimate the mercury discharges from point sources. In addition, some point source 

dischargers, such as chlor-alkali plants and POTWs, might have permits requiring 

monitoring for mercury, although most dischargers, especially smaller dischargers, are 

not likely to have such monitoring requirements. NPDES-permitted stormwater sources 

might also include mercury discharges, which in turn might include mercury originating 

from atmospheric deposition.

Atmospheric deposition. Deposition of mercury from the air can be a significant source 

of mercury in many waterbodies. Some waterbodies have been identified as receiving as 

much as 99 percent of their total loading from atmospheric deposition, either directly or 

indirectly via runoff and erosion. (See Ochlockonee, Georgia, TMDL in appendix D.)

The mercury in atmospheric deposition originates from anthropogenic sources, including 

U.S. and international sources, as well as natural sources. Examples of specific 

anthropogenic sources that emit mercury to the air include medical and municipal waste 

incinerators, electric utilities, chlor-alkali plants, and active metals mining, among others.

Mercury is emitted to the air in several chemical forms or species. Common 

measurements of mercury in air differentiate between reactive gaseous mercury (RGM), 

elemental mercury (Hg0), and particulate mercury (Hgp). Some chemical forms of 

mercury emissions to air deposit relatively close to their sources, while others are 

transported over longer distances and even globally. The mix of chemical forms or 

species emitted from a given source determines what fraction of the mercury from that 

source is depositing locally and what proportion is transported over longer distances, 

making the task of identifying sources of deposition to a waterbody challenging. At any 

given location, the mercury deposited from air can originate from several sources.

Figure 3 depicts the current understanding of deposition from U.S. and international 

sources. It shows that in many parts of the United States, the source of deposited mercury 

is not a U.S. source.

Of the approved mercury TMDLs involving atmospheric loadings, most have 

characterized the contributions from air deposition in terms of total or aggregate loadings.

Atmospheric mercury loadings include both direct deposition to the waterbody surface 

and indirect deposition to the watershed. Indirect deposition is that which is deposited to 

the watershed and then transported to the waterbody via runoff and erosion. Atmospheric 

mercury loadings include both wet and dry deposition of mercury.

It is important to use the most current information about deposition because U.S. mercury 

emissions into the air have decreased over time. Older data on deposition might not 

reflect current deposition conditions. For example, figure 4 depicts a summary of U.S. 

mercury air emissions between 1990 and 1999 and shows a 45 percent overall decrease.
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Figure 3. Percentage of total mercury deposition attributable to global sources 

(USEPA 2005a).

Figure 4. Trends in mercury air emissions between 1990 and 1999 (USEPA 1999a).

Additional decreases in mercury air emissions have occurred since 1999 as the result of 

EPA’s regulatory efforts under the CAA. At the same time, however, global emissions 

might have increased.
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The 2002 National Emissions Inventory (NEI) is EPA’s latest comprehensive national 

emission inventory. It contains emission measurements and estimates for 7 criteria 

pollutants and 188 hazardous air pollutants (HAPs). The NEI contains emissions for all 

major contributors to air pollution, including point sources (large industrial sources such 

as electric utilities and petroleum refineries), mobile sources (both onroad sources such as 

cars and trucks and nonroad engines such as those in construction equipment and 

agricultural equipment), and nonpoint sources (small stationary sources such as 

residential fuel use and various types of fires). The NEI includes emission estimates for

the entire United States. For point sources, the NEI inventories emissions for each 

individual process at an industrial facility. For mobile and nonpoint sources, the NEI 

contains county-level emission estimates. The NEI is developed using the latest data and 

best estimation methods, including data from Continuous Emissions Monitors; data 

collected from all 50 states, as well as many local and tribal air agencies; and data 

generated using EPA’s latest models such as the MOBILE and NONROAD models.

More information on the 2002 NEI is at http://www.epa.gov/ttn/chief/net/

2002inventory.html.

Some approved mercury TMDLs have identified the types or categories of sources likely to 

contribute to mercury deposition in a waterbody. An example of this type of source analysis 

is included in the Savannah River mercury TMDLs issued February 28, 2001, and a series 

of mercury TMDLs issued February 28, 2002, for a number of watersheds in middle and 

south Georgia (see http://gaepd.org/Documents/TMDL_page.html). These TMDLs 

included an analysis of the categories of air sources contributing deposition to the 

waterbodies and the reductions in loadings expected from controls in place when the 

TMDL was approved. To estimate the total contributions from air deposition, data from 

the Mercury Deposition Network (MDN) were used. Modelers also used the existing 

Regional Langrangian Model of Air Pollution (RELMAP) deposition results developed 

for the 1997 Mercury Report to Congress to estimate the relative contributions from local 

sources within a 100-kilometer airshed.

EPA has evaluated water and air deposition modeling tools as part of two mercury 

TMDL pilot projects in Wisconsin and Florida. In particular, the pilots examined 

approaches for combining the results of air deposition and water quality modeling, which 

in turn might be used in a TMDL context. In the Florida pilot, air modelers used a 

combination of modeling tools to predict the amount of mercury deposition to the study 

area from local sources in southern Florida. Using the Mercury Cycling Model, aquatic 

modelers then used results from the atmospheric modeling and other data to examine how 

mercury levels in fish might respond to reductions in deposition. The Florida pilot report 

is complete (see ftp://ftp.dep.state.fl.us/pub/labs/assessment/mercury/tmdlreport03.pdf) 

(Atkeson et al. 2002).

In the Wisconsin pilot project, EPA evaluated modeling tools such as the Regional 

Modeling System for Aerosols and Deposition (REMSAD) for identifying the sources or 

categories of sources contributing mercury deposition to a waterbody, as well as how to 

use the deposition results as input to aquatic models, similar to the approach used in the 

Florida pilot. REMSAD is a three-dimensional grid model designed to calculate the 

concentrations of both inert and chemically reactive pollutants by simulating the physical 

and chemical processes in the atmosphere that affect pollutant concentrations (ICF 
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International 2006). REMSAD simulates both wet and dry deposition of mercury. (See 

appendix E for further information on REMSAD.) In the Wisconsin pilot, the results of 

the air deposition modeling were used as input to the Mercury Cycling Model to examine 

how mercury levels in fish might respond to potential changes in deposition.

Other TMDLs in which the results of REMSAD modeling were used include the mercury 

TMDL for the Coastal Bays and Gulf Waters of Louisiana approved in 2005. The results 

of earlier air modeling for the Mercury Study Report to Congress were used in the 

mercury TMDLs for middle and south Georgia approved in 2002 (see Ochlockonee 

TMDL in appendix D). EPA plans to provide each state or authorized tribe with modeled 

estimates of mercury deposition from sources within the state or on the tribal land and 

contributions from sources outside the state or tribe. The modeling results will help EPA

and the states and authorized tribes develop TMDLs and determine the appropriate 

strategies for addressing mercury deposition from sources within their jurisdictions.

Additional tools available for determining mercury deposition loadings include the 

Community Multi-Scale Air Quality (CMAQ) model. The CMAQ modeling system is a 

comprehensive, three-dimensional, grid-based Eulerian air quality model designed to 

estimate pollutant concentrations and depositions over large spatial scales (Dennis et al. 

1996; Byun and Ching 1999; Byun and Schere 2006). The CMAQ model is a publicly 

available, peer-reviewed, state-of-the-science model with a number of science attributes 

that are critical for simulating the oxidant precursors and nonlinear chemical relationships 

associated with mercury formation. Version 4.3 of CMAQ (Bullock and Brehme 2002; 

Byun and Schere 2006) reflects updates to earlier versions in a number of areas to 

improve the underlying science and address comments from peer review. Further 

information on the CMAQ model is provided in appendix E.

As with any analysis based on limited data, uncertainty is inherent in the estimates of all 

analytical outputs of modeling. Model uncertainty results from the fact that models and 

their mathematical expressions are simplifications of reality used to approximate 

real-world conditions, processes, and their relationships. Models do not include all 

parameters or equations necessary to express real-world conditions because of the 

inherent complexity of the natural environment and the lack of sufficient data to describe 

the natural environment. Consequently, models are based on numerous assumptions and 

simplifications and reflect an incomplete understanding of natural processes. As a result, 

there will be some uncertainty when using models to quantify the sources of air-deposited 

mercury.

Other tools available to help states characterize mercury deposition include existing 

national monitoring networks and modeling tools, such as the MDN. Examples of these

tools are provided in appendix F. Published results of national modeling studies could 

also be available to help estimate atmospheric deposition loadings. Further information 

on tools and approaches for characterizing atmospheric deposition to waterbodies can be 

found in the Frequently Asked Questions about Atmospheric Deposition section of 

EPA’s Web site at http://www.epa.gov/oar/oaqps/gr8water/handbook/.

An analysis of deposition should take into account both direct deposition to the 

waterbody, as well as mercury deposited within the watershed (indirect deposition). In 

addition, fires, flooding, and other landscape disturbances could re-mobilize mercury 
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previously deposited within the watershed and cause an increase in mercury transported 

to the waterbody. Studies are underway to examine the extent to which mercury 

deposited to a watershed is transported to a waterbody. For example, the Mercury 

Experiment to Assess Atmospheric Loading in Canada and the United States 

(METAALICUS) project is a mercury loading experiment to examine the timing and 

magnitude of the relationship between mercury loading to ecosystems and mercury 

concentrations in fish (Harris et al. 2006). Using stable mercury isotopes, researchers are 

examining the fate of mercury deposited to uplands, wetlands, and directly to lakes. It is 

being carried out at the Experimental Lakes Area (ELA) in northwestern Ontario by U.S. 

and Canadian researchers. A discussion of factors affecting mercury transport and 

bioavailability is included in chapter 2 of this guidance.

As part of a source evaluation, EPA encourages states to conduct a careful analysis to 

verify and quantify the contributions of air deposition as compared to other sources. Such 

information is important for determining the appropriate management approaches. For 

example, an analysis of the contribution from air sources is the basis for determining 

whether it may be appropriate to defer TMDLs under the 5m approach, or whether it is 

more appropriate to develop TMDLs to address significant local sources.

Although not required for a TMDL, states may wish to examine the contributions to the 

watershed from local air sources within the state as compared to out-of-state sources. 

Such information provides a basis for determining the appropriate allocations. In turn, 

such source information can help to develop a meaningful TMDL implementation plan 

and identify the extent to which state and local programs may be appropriate for 

addressing the mercury sources.

Metals mining activity. Loadings from metals mining activities might reflect both 

historical and recent mining activity within the watershed. Mining areas of interest are 

those involving “placer” deposits, in which mercury itself is present in the ore, or those 

deposits for which mercury is used to extract other metals such as gold. For example, 

sulfide replacement deposits are often associated with mercury. Locations at mining sites 

that might serve as sources of mercury include direct seeps, as well as leachate from 

tailings or spoil piles. In the Clear Lake TMDL (see appendix E), ground water from an 

abandoned mining site was reported to contain mercury that is readily methylated. In 

Clear Lake, acid mine drainage was found to contain high sulfate concentrations, which 

might enhance methylation by sulfate-reducing bacteria. Sources of data on potential 

mercury deposits associated with mining activity include USGS, the U.S. Bureau of 

Mines (for a list of major deposits of gold and silver), the State Inactive Mine Inventory, 

and the EPA Superfund program. Examples of TMDLs involving mercury associated 

with mining are provided in appendix E.

Sediments. A TMDL analysis should account for any mercury present in sediments as a 

result of current and past mercury loadings. Mercury in sediments may be the result of 

past metals mining activity as described above, past industrial activity, and historical air 

deposition. Data on levels of mercury in sediments are important in determining which 

sources are most significant, the most appropriate approach for addressing the sources 

and how long it will take to achieve water quality standards. For example, development 

of appropriate allocations, and in turn development of management strategies, may need 

to address both current sources of deposition as well as legacy sources. An examination 
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of past industrial practices in the watershed could include whether sediments might serve 

as a reservoir for mercury. Various national databases, such as the National Sediments 

Database (USEPA 2002g) and data collected by USGS might help to identify isolated 

locations of elevated mercury in sediments. EPA has also developed a detailed guide on 

sediment source analysis in the first edition of Protocol for Developing Sediment TMDLs:

http://www.epa.gov/owow/tmdl/sediment/pdf/sediment.pdf.

In the absence of sediment data for a waterbody, site-specific monitoring might be 

needed to confirm the levels of mercury in sediments to use as input to water quality 

models. In the sediment TMDL for Bellingham Bay, Washington, site-specific sediment 

analyses for mercury and other pollutants were conducted, including sediment sampling

and toxicity analyses. Two kinds of modeling were also conducted:

● Modeling of contaminant transport and mixing to determine whether loadings from 

a location were contributing to water quality standards violations

● Screening modeling to identify other potential sources of sediment 

contamination (see the TMDL at http://www.epa.gov/waters/tmdldocs/

1991_Bellingham%20Bay%20TMDL.pdf)

Other examples of TMDLs involving an analysis of mercury contributions from 

sediments include the TMDLs for Pena Blanca, Arizona, and the Cache Creek watershed

in California (see appendix D). As described in the section on allocations, the Cache 

Creek watershed TMDL also accounts for methylmercury production in sediments.

Natural or background levels of mercury in soils. Soils and sediments can include 

mercury of geologic origin or mercury produced by the weathering of geologic materials, 

together with mercury of anthropogenic origin (mercury emitted over time from human 

sources and then deposited on soils). Mercury in soils can also re-emit or become resuspended and subsequently redeposit to soils. Local studies have been used in some 

TMDLs to estimate the geologic contributions of mercury to waterbodies. For example, a 

TMDL developed for the Ouachita watershed in Arkansas relied on a study of mercury 

concentrations in the rocks of the Ouachita Mountains (FTN 2002). The mercury 

concentration estimated to be of geologic origin was then subtracted from the total 

concentration of mercury measured in soils to estimate the nongeologic concentration of 

mercury in soils.

6.2.2.2 What modeling tools are available to link mercury sources and 

water quality?

When developing a TMDL, states and authorized tribes should characterize the 

association between the concentration of methylmercury in fish tissue and the identified 

sources of mercury in a watershed. The association is defined as the cause-and-effect 

relationship between the selected targets, in this case the fish tissue-based criterion and 

the sources. The association provides the basis for estimating the total assimilative 

capacity of the waterbody and any needed load reductions. TMDLs for mercury typically 

link models of atmospheric deposition, watershed loading, and mercury cycling with 

bioaccumulation. For example, a watershed model (e.g., Grid Based Watershed Mercury 

Model, GBMM) might be linked with a receiving water mercury model (e.g., Water 

Quality Analysis Simulation Program, WASP) and a bioaccumulation model (e.g., 
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Bioaccumulation and Aquatic Simulator, BASS). These models are described further in 

appendix E. Linking models together can enable a translation between the endpoint for 

the TMDL (expressed as a fish tissue concentration of methylmercury) and the mercury 

loads to the water without having explicit water column criteria or translations. The 

analysis determines the loading capacity as a mercury loading rate consistent with 

meeting the endpoint fish tissue concentration. This section describes some of the 

modeling tools available for use in mercury TMDLs.

When selecting a model or models for developing a mercury TMDL, states and 

authorized tribes should first consider whether the models will effectively simulate the 

management action(s) under consideration. If a percent reduction in mercury load to the 

waterbody is the sole action considered, a simple model might suffice; to answer more 

complex questions, a more complex or detailed model might be needed. Some questions 

decision makers should address include:

● How much do specific mercury loads need to be reduced to meet the criterion?

● What are the relative sources of the mercury load to the segment?

● Are mercury loads to the waterbody from sediments and watershed runoff and 

concentrations in fish at equilibrium with respect to current deposition levels? If 

not, how much will an equilibrium assumption affect the accuracy of predicted 

future fish concentrations?

● Could other pollution-control activities reduce mercury loads to the waterbody or 

affect the mercury bioaccumulation rate?

● After regulatory controls are implemented, how long will it take for fish tissue 

levels to meet the criterion?

Depending on the types of questions states and authorized tribes ask and the management 

approaches they consider, appropriate models could range from a very simple steady state 

model to a comprehensive dynamic simulation model, as described below. In addition, 

models are often used in TMDL analyses but are not required. For more information on 

the specific models described here, see http://www.epa.gov/athens and 

http://www.epa.gov/crem.

6.2.2.2.1 Steady state models and the proportionality approach

Steady state modeling describes the dynamic equilibrium between environmental media 

established in response to constant loads over the long term. Consequently, complex 

mercury cycling processes can be compressed into simple equations. One such approach, 

assumes that a ratio of current to future fish tissue concentration equals the ratio of 

current to future mercury loads to the waterbody. This approach, often referred to as the 

proportionality approach and explained in detail in the Mercury Maps report (USEPA

2001b), assumes that where air deposition is the sole significant source, factors affecting 

methylation remain unchanged. As a result, the ratio of current to future fish tissue 

concentrations can be assumed to equal the ratio of current to future air deposition loads

in this situation. Mercury Maps, and the situations in which the proportionality 

assumption may or may not apply, are described further in appendix E.
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A number of mercury TMDLs where air deposition is the predominant mercury source 

have been developed using an assumption of proportionality between mercury deposition 

and fish tissue methylmercury concentration. Specifically, such TMDLs have reasoned 

that a reduction in deposition will result in a proportional reduction in mercury 

concentrations in fish over time. Such an approach applies to situations where air 

deposition is the only significant mercury source and relies on steady-state conditions. 

This approach may also be used to estimate the reductions needed to meet a fish tissue 

target without necessarily calculating a water column target.

Mercury TMDLs which applied a proportional relationship between reductions in 

deposition and reductions in fish tissue methylmercury concentration include TMDLs for 

waterbodies in Louisiana, such as the Ouachita Basin (FTN 2002), the Mermentau and 

Vermillion-Teche River Basins (USEPA 2001i, 2001j) and the Coastal Bays and Gulf 

Waters of Louisiana (Parsons 2005). Using the Everglades Mercury Cycling Model, the 

pilot mercury TMDL study in the Florida Everglades also reported a linear relationship 

between mercury deposition and the concentrations of mercury in largemouth bass 

(Atkeson et al. 2002).

More recently, the Minnesota statewide mercury TMDL applied the proportionality 

approach. As described in section 6.2.1 above, waterbodies within the state were grouped 

into two regions, and a TMDL developed for each region. Minnesota calculated a 

reduction factor for each region, or the percent reduction in total mercury load needed in 

each region to achieve the fish tissue target of 0.2 mg/kg for the 90th percentile of the 

standard-length fish (MPCA 2007). Using the proportionality assumption, Minnesota 

applied the regional reduction factor (51 percent for the southwest region and 65 percent 

for the northeast region) to the total source loadings to determine the load reduction goal. 

The Minnesota TMDL explains in further detail the basis for using the proportionality 

approach.

Mass balance models are somewhat more complex implementations of the steady state 

approach. In place of a simple ratio, such models describe fluxes of mercury in and out of 

the model domain (e.g., impaired segment) and, optionally, balance fluxes (e.g.,

methylation and demethylation) within the model domain. The advantage provided by

this approach is that individual fate processes can also be simulated. For example, if soil 

erosion and sediment runoff are modeled, decreased mercury soil erosion load can be 

related to decreased fish tissue concentrations (AZDEQ 1999). Where all other aspects of 

a watershed and waterbody remain unchanged, steady state models can produce as 

accurate an estimate of the necessary load reductions as a dynamic model, generally with 

less-intensive data collection and analysis. In addition, such simple approaches might be

less prone to calculation error and are much easier for the public to understand.

6.2.2.2.2 Continuous-simulation and dynamic models

Continuous-simulation and dynamic models take into account time-varying effects such 

as variable pollutant inputs, precipitation, hydrologic responses, seasonal ecosystem 

changes, and effects on fish tissue concentrations. For mercury, they might also include a 

variety of physical and chemical fate and transport processes such as oxidation, 

demethylation, volatilization, sedimentation, resuspension, and adsorption and 

desorption. Dynamic models can be important in establishing cause-and-effect 
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relationships. They assemble available scientific knowledge on mercury fate and 

transport into a single picture. Such models have been used to demonstrate how mercury 

moves from air emission to deposition to watershed runoff to subsequent 

bioaccumulation in fish at observed levels in remote waterbodies (USEPA 1997c).

Dynamic models could be used to describe waterbodies in dis-equilibrium (e.g., a recent 

surface water impoundment with elevated methylation rates). The Everglades Mercury 

TMDL pilot project (USEPA 2000g) simulated the amount of time necessary to attain 

equilibrium in response to reduced mercury loads using the Everglades Mercury Cycling 

Model. The model results predicted that sediments would continue to supply as much as 

5 percent of the mercury load 100 years after air deposition reductions occurred. The 

Dynamic Mercury Cycling Model (D-MCM) was used in the mercury TMDLs for 

McPhee and Narraguinnep reservoirs in Colorado and the TMDLs for Arivaca and Pena 

Blanca lakes in Arizona (see appendix D) (Tetra Tech 2001).

The SERAFM model incorporates more recent advances in scientific understanding and 

implements an updated set of the IEM-2M solids and mercury fate algorithms described 

in the 1997 Mercury Study Report to Congress (USEPA 1997c).

Dynamic models can also describe how fish tissue concentrations are expected to respond 

to environmental variability, such as seasonal or year-to-year changes in meteorology.

Thus, they can be used to better interpret how samples collected in a specific season of a 

specific year would be expected to vary relative to other seasons or years with mercury 

loads being constant.

6.2.2.2.3 Spatially detailed models

Spatially detailed models, such as that used in the Savannah River mercury TMDL 

(USEPA 2001j), can demonstrate how mercury fish tissue concentrations are expected to 

vary with distance downstream of the impaired segment(s). For the Savannah River, EPA

used the Water Quality Analysis Simulation Program (WASP) model. WASP is a 

dynamic, mass balance framework for modeling contaminant fate and transport in surface 

water systems. The model helps users interpret and predict water quality responses to

natural phenomena and man-made pollution for various pollution management decisions.

Another model that has been used for mercury TMDLs is the EPA Region 4 Watershed 

Characterization System (WCS). WCS is a geographic information system (GIS)-based 

modeling system for calculating soil particle transport and pollutant fate in watersheds 

(Greenfield et al. 2002).

As with the steady state mass balance model, including additional processes can allow a 

modeler to determine the impact of different environmental regulatory or management 

controls on mercury fish tissue concentrations. For example, where mercury transport to a 

waterbody occurs predominantly through soil erosion, erosion control might be identified 

as a useful nonpoint source control on mercury to waterbodies (Balogh et al. 1998). As 

another example, controls on acid deposition and, thus, changes in lake pH and their

effect on fish tissue mercury concentrations can also be modeled (Gilmour and Henry 

1991, Hrabik and Watras 2002). Finally, spatially detailed landscape models 

hypothetically could be used to reflect the local effects of wetlands and their impacts on 

mercury methylation rates.
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6.2.2.2.4 Regression models

In general, a regression model is a statistical model describing how a parameter, such as 

mercury levels in fish, is related to one or more variables. Regression models provide 

only approximations of real trends.

One example of a regression model for mercury is the regression-based model under 

development for New England. The model, known as MERGANSER (Mercury 

Geospatial Assessments for the New England Region), is being developed by EPA and 

several partners. The partners include USGS, the Biodiversity Research Institute, the 

State of Vermont, the Clean Air Association of the Northeast States, and the New 

England Interstate Water Pollution Control Commission. The model will integrate recent 

atmospheric mercury-deposition models with many databases on mercury sources, 

mercury levels in fish and bird tissue, and ecosystem features that might be associated 

with the risk of mercury contamination in biota and, ultimately, humans.

The intent of the project is to identify, by using regression modeling, explanatory 

variables that contribute to elevated mercury levels in fish and wildlife in New England. 

The model can then be applied in a predictive mode to lakes throughout New England 

that have no mercury fish tissue or loon blood data. Specifically, the model will 

(1) identify watershed and other factors associated with high mercury levels in fish and 

wildlife; (2) identify likely sources of mercury; (3) provide estimates of mercury levels in 

fish and wildlife at any lake or stream in New England; (4) provide estimates of mercury 

reductions needed from air deposition to meet water-quality criteria; and (5) identify 

optimal locations for long-term monitoring. Modeling will be done within a GIS

environment so that the spatial distribution of data is retained and results can be displayed 

watershed by watershed. Maps from MERGANSER will show the areas in New England 

that are susceptible to high mercury levels in biota and that are, therefore, areas where 

human health impacts (through fish consumption) and ecological impacts (bird tissue 

mercury levels) are potentially occurring. In addition, the model can be used to produce 

maps that identify mercury sources and show the relative magnitude of mercury loading 

from those sources.

6.2.2.2.5 Model selection

When selecting a model, a state or authorized tribe should be aware of the assumptions 

inherent in each type of model and consider the potential effects of those assumptions on 

relationships between loadings and fish tissue levels or water quality. The first 

consideration for model assumptions is methylation. Several factors, including pH, redox, 

potential sulfate concentrations, temperature, dissolved organic carbon (DOC)

concentrations, salinity, and microbial populations, influence the speciation of mercury 

(Ullrich et al. 2001). If these factors fluctuate seasonally around an average condition, a 

waterbody could be at a dynamic equilibrium and the steady state assumption would still 

apply over the long term. If these factors change over time such that they might have a 

significant impact on fish tissue concentrations, the equilibrium assumptions inherent in 

steady state modeling might not hold, and a dynamic model like the D-MCM (EPRI

1999) should be used. In using this model, the state or authorized tribe should consider 

the amount of environmental media concentration data needed to initialize the model to 

represent its non-equilibrium state.
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85

The second consideration for model assumptions is the BAF. As discussed in section 

3.1.3.1, the BAF assumes a constant proportionality between fish tissue methylmercury 

concentrations, water column methylmercury concentrations, and water column mercury 

concentrations. Mercury in a waterbody might not be at a steady state because of ongoing 

reductions in mercury emissions, changes in water chemistry that affect methylation, 

changes in aquatic ecosystem makeup, or changes in fish biomass. If these factors change 

with time, the equilibrium assumptions inherent in steady state modeling might not hold, 

and a dynamic model should be used.

The third consideration for model assumptions is the relative importance of the mercury 

in aquatic sediments to the concentrations in fish tissue. Depending on previous loadings 

to the watershed, the deposition pattern of solids, and the chemistry in the aquatic 

sediments, the mercury in sediments can significantly influence the mercury 

concentrations in fish tissue. Sediments are repositories, and the loading that caused 

sediment mercury could be a legacy source. If so, a simplified steady state approach 

cannot simulate changes in mercury concentrations in fish tissue due to external loading 

reductions, and a dynamic model should be used.

6.2.2.2.6 Model limitations

To effectively estimate fish methylmercury concentrations in an ecosystem, it is 

important to understand that the behavior of mercury in aquatic ecosystems is a complex 

function of the chemistry, biology, and physical dynamics of different ecosystems. The 

majority (95 to 97 percent) of the mercury that enters lakes, rivers, and estuaries from 

direct atmospheric deposition is in an inorganic form (Lin and Pehkonen 1999). Microbes 

convert a small fraction of the pool of inorganic mercury in the water and sediments of 

these ecosystems into methylmercury. Methylmercury is the only form of mercury that 

biomagnifies in organisms (Bloom 1992). Ecosystem-specific factors that affect both the 

bioavailability of inorganic mercury to methylating microbes (e.g., sulfate, DOC) and the 

activity of the microbes themselves (e.g., temperature, organic carbon, redox status) 

determine the rate of methylmercury production and subsequent accumulation in fish 

(Benoit et al. 2003). The extent of methylmercury bioaccumulation is also affected by the 

number of trophic levels in the food web (e.g., piscivorous fish populations) because 

methylmercury biomagnifies as large piscivorous fish eat smaller organisms (Watras and 

Bloom 1992; Wren and MacCrimmon 1986). These and other factors can result in 

considerable variability in fish methylmercury levels among ecosystems at the regional 

and local scales.

The lack of complete knowledge about key mercury process variables, such as the 

functional form of equations used to quantify methylation rate constants, is a major 

contributor to overall uncertainty in models that cannot be quantified at this time.

6.2.2.3 What are the allocation approaches in mercury TMDLs?

A requirement for an approvable TMDL is that the state or authorized tribe allocate the 

pollutant load necessary to achieve water quality standards among point and nonpoint 

sources. EPA’s regulations, however, leave the decision regarding how to allocate 

loadings to the state or authorized tribe developing the TMDL. States and authorized 

tribes have discretion in selecting a method or system for allocating pollutant loads 

among sources, provided that the allocations will result in attainment of water quality 
TMDLs

86

standards represented by the loading capacity (40 CFR 130.2). States and authorized 

tribes could reasonably consider the relative contribution of each source as one factor in 

developing allocations. Other factors might include cost-effectiveness, technical and 

programmatic feasibility, previous experience with the approach being considered, 

likelihood of implementation, and past commitments to load reductions. These same 

considerations apply to mercury TMDLs.

A number of pollutant loading and allocation scenarios have occurred in mercury 

TMDLs, each with a different mix of point and nonpoint sources. The scenarios have 

ranged from situations where mercury loadings are predominantly from air deposition,

with small loadings from point sources or other sources, to situations where mercury 

loadings are predominantly from past mining activity. In addition, allocation approaches 

in mercury TMDLs have included allocations to individual sources as well as allocations 

to sectors and regions where appropriate. Examples of scenarios involving different 

source mixes and allocation approaches in approved mercury TMDLs are provided

below.

Mercury loadings predominantly from air deposition, with very small loadings from 

point sources or other sources

Contributions from air deposition, such as direct deposition to the waterbody and 

deposition to the watershed transported to the waterbody by runoff and erosion, are 

typically included as part of the load allocation. As discussed in EPA guidance on 

reviewing TMDLs, allocations for nonpoint sources may range from reasonably accurate 

estimates to gross allotments (USEPA 2002f). TMDLs where air deposition is the 

predominant mercury source have usually allocated only a small portion of the reductions 

to the point sources or wasteload allocation, as described in the examples below. Many 

mercury TMDLs have included an allocation to air deposition as a whole; in some 

mercury TMDLs, the contributions from air deposition are further allocated to withinstate and out-of-state sources, and contributions from anthropogenic and natural 

contributions are distinguished.

The Savannah River mercury TMDL is one of the first examples of an approach to 

allocating loadings where the predominant mercury source is atmospheric deposition.

Many of the TMDLs developed to date are for situations where air deposition is the 

predominant mercury source. The Savannah River mercury TMDL indicated that NPDES 

point sources contribute 1 percent of the mercury loadings, while atmospheric deposition 

contributes 99 percent of the loadings. The TMDL identified only one point source on the 

Georgia side of the river that has a permit to discharge mercury to the Savannah River. It 

identified 28 point sources in Georgia that might have the potential to discharge larger 

amounts of mercury in their effluent according to the nature of the discharge or the

mercury levels that have been found in their effluents above the water quality standard 

level.

The Savannah River mercury TMDL assigned 99 percent of the load reductions to the air 

sources and 1 percent of the reductions to point sources. The TMDL provides specific 

wasteload allocations for these 28 sources on the basis of meeting the water quality 

criterion at the end of a pipe or, alternatively, implementing a pollutant minimization 

program. In addition, the TMDL identifies about 50 other point sources expected, on the 
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87

basis of their size and nature, to discharge mercury at levels below the water quality 

standard or not add mercury in concentrations above the concentrations in their intake 

water. Individual wasteload allocations are given to these point sources on the basis of 

their holding their effluents at current levels. The wasteload allocations for these point 

sources are expressed in the TMDL as a sum or aggregate allocation.

Note: After the Savannah River mercury TMDL was issued, Georgia adopted a new 

interpretation of its narrative water quality criteria that used EPA’s new recommended 

fish tissue criterion for methylmercury. On the basis of the new interpretation, Georgia 

determined, and EPA agreed, that the Savannah River was meeting water quality 

standards for mercury. EPA therefore withdrew the TMDL. EPA believes, however, that 

the decisions, policies, and interpretations set forth in the TMDL are still valid and 

provide an example of a possible approach to mercury TMDLs. The Savannah River 

mercury TMDL is at http://www.gaepd.org/Files_PDF/techguide/wpb/TMDL/

Savannah/EPA_Savannah_River_Watershed_Hg_TMDL.pdf.

The series of mercury TMDLs issued February 28, 2002, for watersheds in middle and 

south Georgia, such as the Ochlockonee watershed, also illustrate the first scenario. In 

these basins, point source loadings contribute very little to the mercury loadings (the 

cumulative loading of mercury from all point sources is less than 1 percent of the total 

estimated current loading), with the vast majority of loading to the basins as air 

deposition.

The Ochlockonee mercury TMDL assigns most of the load reductions to the air sources, 

with a load allocation of 1.16 kg/yr and a wasteload allocation of 0.06 kg/yr. Although 

point sources collectively contribute a very minute share of the mercury load, the 

Ochlockonee and other mercury TMDLs for middle and south Georgia include wasteload 

allocations for the point sources. The TMDLs include wasteload allocations for each

facility identified as a significant discharger of mercury, with the remainder of the 

allocation assigned collectively to the remaining point sources, considering that these 

smaller point sources would reduce their mercury loadings using appropriate, costeffective minimization measures. The TMDL was written so that all NPDES-permitted 

facilities would achieve the wasteload allocation through discharging mercury at 

concentrations below the applicable water quality standard or through implementing a 

pollutant minimization program. A summary of the Ochlockonee mercury TMDL is 

provided in appendix D and is at http://gaepd.org/Files_PDF/techguide/wpb/

TMDL/Ochlockonee/EPA_Ochlockonee_River_Hg_TMDL.pdf.

The Minnesota “statewide” mercury TMDL document takes a regional approach to 

allocations, providing a single wasteload allocation and a single load allocation that 

applies to each region rather than to individual waterbodies. The TMDL document 

indicates that such a regional allocation serves as a regional “cap.” The predominant 

source is atmospheric deposition, with a small contribution (about 1.2 percent of the total 

source load for both regions combined) from point sources. The wasteload allocation is 

set at 1 percent of the TMDL or the 1990 baseline load, whichever is lower, with the 

remainder allocated to nonpoint sources. Point sources, including NPDES-permitted 

stormwater sources, municipal treatment facilities, and industrial dischargers that impact 

the waterbodies covered by the TMDL, are subject to the wasteload allocation. For the

load allocation, the Minnesota TMDL estimates the contributions to air deposition from 
TMDLs

88

within-state and out-of state sources, as well as from global sources and anthropogenic 

sources. A summary of the Minnesota mercury TMDL is included in appendix D. The 

TMDL and related documents can be found at http://www.pca.state.mn.us/water/tmdl/

tmdl-mercuryplan.html.

Mercury loadings predominantly from past mining activity, with small or no 

contributions from atmospheric deposition and/or NPDES point source contributions

One example of a TMDL for this scenario is the Cache Creek Watershed TMDL. Cache 

Creek is a tributary to the Sacramento-San Joaquin Delta in California. Sources of 

mercury entering the Cache Creek watershed include leaching from waste rock and 

tailings from historical mercury and gold mines, erosion of naturally mercury-enriched 

soils, geothermal springs, and atmospheric deposition. There are multiple inactive 

mercury and gold mines in the Cache Creek watershed and no NPDES-permitted 

discharges. Methylmercury is also produced in situ in the streambed of Cache Creek. The 

TMDL analysis provides load allocations for Cache Creek, as well as each of the 

tributaries. For each waterbody, load reductions are provided for both methylmercury and 

total mercury. Allocations are expressed as a percentage of the existing methylmercury 

loads. Estimated atmospheric contributions of mercury, from direct deposition and runoff 

after deposition, are very small compared to loads of mercury from mine sites or erosion 

of the stream bed and banks, and thus no allocations are made to air deposition. Reducing

the methylmercury loads will require a multifaceted approach that includes controlling

inorganic mercury loads and limiting the entry of inorganic mercury into sites with high 

rates of methylmercury production. The Cache Creek watershed mercury TMDL and the 

allocation approach are summarized further in appendix D.

Mercury loadings from a combination of different sources, including atmospheric 

deposition, past mining, and point sources

The Mercury TMDL for the Willamette Basin, Oregon, identifies atmospheric deposition 

(direct plus indirect deposition: 47.7 percent) and erosion of mercury-containing soils (47.8

percent) as the top sources, along with small contributions from legacy mining (0.6 percent) 

and NPDES-permitted point sources (3.9 percent). The point source loadings consist of 2.7

percent from POTWs and 1.2 percent from industrial discharges. The TMDL assigns 

interim allocations to each of the source categories or sectors, rather than individual 

sources, based on the considerable uncertainty in the loading estimates and other factors. 

The TMDL specifies an across-the-board reduction of 27 percent in each source. After the 

27 percent reduction to each source, the allocations for the Willamette mainstem are 

approximately similar to their relative contribution to the total loadings: 44.7 kg/yr for air 

deposition, 44.8 kg/yr for erosion, 0.6 kg/yr for legacy mine discharges, 2.6 kg/yr for 

POTWs, 1.1 kg/yr for industrial discharges, and 0.8 kg/yr for reserved capacity. Allocations 

are also provided for other waterbodies in the basin. The TMDL is at 

http://www.deq.state.or.us/wq/tmdls/docs/willamettebasin/willamette/chpt3mercury.pdf.

Mercury loadings from point sources predominate or are not insignificant compared to 

other sources

A small number of approved TMDLs have been developed for situations where mercury 

is primarily or exclusively from point sources, including TMDLs for waterbodies in 
TMDLs

89

Colorado. Examples of such TMDLs can be found at http://iaspub.epa.gov/

tmdl_waters10/attains_impaired_waters.control?p_state=CO&p_pollutant_id=693.

6.2.2.4 What kinds of monitoring provisions have been associated with 

approved TMDLs?

Monitoring provisions in approved TMDLs have included point source effluent and 

influent monitoring, as well as water column, fish tissue, sediment, and air deposition 

monitoring. Examples of mercury TMDLs with post-TMDL monitoring are the middle 

and south Georgia mercury TMDLs approved in 2002. For facilities with the potential to 

discharge significant amounts of mercury on the basis of their large flow volume or other 

factors, the TMDL provides the permitting authority with two options for the wasteload 

allocation:

● Implement the criteria-end-of-pipe (i.e., apply the TMDL water quality target to a 

discharger’s effluent at the outfall point).

● Monitor for mercury in the facilities’ influent and effluent using more sensitive 

analytical techniques (e.g., EPA method 1631) and implement cost-effective 

mercury minimization if mercury is present in effluent at concentrations greater 

than source water concentrations and if the discharge exceeds the water quality 

target.

Other facilities expected to discharge at levels below the water quality target will be 

expected to verify through monitoring whether or not they are significant dischargers of 

mercury. Other follow-up activities include further characterization of the air sources and 

additional ambient monitoring of mercury concentrations in water, sediment, and fish.

The mercury TMDL for the coastal bays and gulf waters of Louisiana (approved July 

2005) includes similar monitoring provisions for point source dischargers with flows 

above a specified discharge volume. The TMDL also indicates that Louisiana will 

conduct water, fish tissue, and air deposition monitoring and that the state will develop a 

statewide mercury risk reduction program, including an assessment of all mercury 

sources. (See the TMDL and supporting documents at http://iaspub.epa.gov/tmdl/

waters_list.tmdl_report?p_tmdl_id=11642.)

TMDLs involving past mining activity have also included follow-up monitoring; 

examples include three of the TMDLs described in appendix D (Clear Lake, California;

Arivaca Lake, Arizona; and Cache Creek, California). The mercury TMDL for Arivaca 

Lake lists several follow-up actions and monitoring activities, such as additional 

watershed investigations to identify other potential mine-related mercury sources, 

including sediment sampling; evaluation of livestock BMPs to reduce erosion of soils

containing mercury and follow-up monitoring; and fish tissue monitoring to 

evaluate progress toward the TMDL target (see the TMDL at http://www.epa.gov/waters/

tmdldocs/17.pdf). The Clear Lake, California, mercury TMDL also identifies the need for 

follow-up monitoring of fish tissue and sediment (see appendix D, and the TMDL at 

http://www.swrcb.ca.gov/rwqcb5/water_issues/tmdl/central_valley_projects/

clear_lake_hg/cl_final_tmdl.pdf. The Cache Creek TMDL indicates that monitoring will 

be conducted to determine whether mercury loads have been reduced and to measure 

progress toward the TMDL target, as well as to better characterize areas of 
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90

methylmercury production and mercury loadings from tributaries. Monitoring will 

include fish tissue, sediment, and water monitoring.

EPA recommends that states and authorized tribes periodically review TMDLs during 

implementation to ensure that progress is being made toward achieving water quality 

standards. Such “adaptive implementation” provides the flexibility to refine and improve 

a TMDL as data on the success of implementation activities are collected. States may 

refine information on the contributions from sources such as runoff from abandoned 

mining sites, sediment loading of mercury-laden sediments, and air deposition as data and 

modeling tools improve. States should consider the application of adaptive 

implementation in determining load allocations for these sources. Although a monitoring 

plan is not required in a TMDL, EPA guidance documents recommend using a 

monitoring plan to track the effectiveness of a TMDL; see Guidance for Water QualityBased Decisions: the TMDL Process (EPA 440/4-91-001). Post-TMDL monitoring is an 

important tool for evaluating implementation success and, if necessary, refining the 

TMDL. Follow-up monitoring may include monitoring of water quality, fish tissue, air 

deposition, and sediments.
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7 National Pollutant Discharge Elimination 

System (NPDES) Implementation 

Procedures

7.1 What are the general considerations in NPDES 

permitting?

Section 301(a) of the CWA prohibits the discharge of any pollutant, including mercury,

from a point source into waters of the United States except in compliance with certain 

enumerated provisions of the CWA, among them section 402. CWA section 402 

establishes the NPDES program, under which EPA or states and tribes authorized to 

administer the program issue permits that allow the discharge of pollutants into waters of 

the United States, notwithstanding the general prohibition established by section 301(a).

These permits must contain (1) technology-based effluent limitations, which represent the 

degree of control that can be achieved by point sources using various levels of pollution 

control technology (see CWA sections 301, 304, and 306) and (2) more stringent 

limitations, commonly known as water quality-based effluent limitations (WQBELs), 

when necessary to ensure that the receiving waters achieve applicable water quality 

standards (see CWA section 301(b)(1)(C)).

22

Most WQBELs are expressed as numeric limits on the amounts of specified pollutants 

that may be discharged. However, WQBELs may also be expressed in narrative form 

such as best management practices (BMPs) or pollutant minimization measures (e.g., 

practices or procedures that a facility follows to reduce pollutants to waters of the United 

States) when it is infeasible to calculate a numeric limit (see 40 CFR 122.44(k)(3)). In 

addition, BMPs may be imposed in the form of NPDES permit conditions to supplement 

numeric effluent limitations when the permitting authority determines that such 

requirements are necessary to carry out the purposes and intent of the CWA (see CWA 

section 402(a)(1)(B) and 40 CFR 122.44(k)(4)).

As noted above, NPDES permits must contain WQBELs when necessary to achieve 

applicable water quality standards. The procedure for determining the need for WQBELs 

is called a “reasonable potential” analysis. Under EPA’s regulations at 40 CFR 

122.44(d)(1)(i), effluent limitations must control all pollutants that the permitting 

authority determines “are or may be discharged at a level that will cause, have the 

reasonable potential to cause, or contribute to an exceedance of any applicable water 

quality standard.” Thus, if a pollutant discharge has the reasonable potential to cause or 

contribute to an exceedance of applicable water quality standards, the discharger’s 

NPDES permit must contain a WQBEL for that pollutant (see 40 CFR 122.44(d)(1)(iii)–

(vi)). The procedure for determining reasonable potential must consider the variability of 

the pollutant in the effluent, other loading sources, and dilution (when allowed by the 

22 When developing WQBELs, the permitting authority must ensure that the level of water quality achieved by such limits derives from and 

complies with water quality standards (see 40 CFR 122.44(d)(1)(vii)(A)).
NPDES Implementation Procedures

92

water quality standards) (see 40 CFR 122.44(d)(1)(ii)). The procedure specifies only 

whether a discharge must have a WQBEL; it does not specify the actual permit limits.

The NPDES regulations at 40 CFR 122.44(d)(1)(vii) specify that the level of water 

quality to be achieved by the WQBEL must derive from and comply with water quality 

standards, as required by CWA section 301(b)(1)(C) (requiring “any more stringent 

limitation... necessary to meet water quality standards”). This would necessarily be a 

permit-by-permit determination.

7.2 What is the EPA-recommended NPDES permitting 

approach for methylmercury?

The recommendations below assume that an approved TMDL is not available at the time 

of permit issuance. If EPA has approved or established a TMDL containing wasteload 

allocations for the discharge of mercury (and methylmercury where appropriate), the 

WQBEL for that discharge must be consistent with the wasteload allocation (see 40 CFR 

122.44(d)(1)(vii)(B)).

EPA believes, depending on the particular facts, that a permit writer may reasonably 

conclude that limits on point sources consistent with this guidance are likely to be as 

stringent as necessary to achieve water quality standards. As described in more detail 

below, the permit writer should conduct a reasonable potential analysis to determine 

whether a discharger will cause or contribute to the exceedance of applicable water 

quality standards. Once such a determination is made, limits can be imposed consistent 

with this guidance. In circumstances where waters are not yet impaired, the permit writer 

should consider other factors or conditions when determining whether a facility has 

reasonable potential with the goal of preventing future impairments. (See Sections 7.2.2, 

7.5.1.2.2 and 7.5.1.2.3).

7.2.1 Developing NPDES permit limits based on the fish tissue 

criterion

The first component of the recommended NPDES permitting approach for 

methylmercury is to determine how the methylmercury criterion is expressed in the 

applicable water quality standard and to determine whether a water column translation of 

the fish tissue criterion or site-specific data to translate are available at the time of permit 

issuance. This will inform the selection of the appropriate recommended implementation 

option. If the methylmercury criterion is expressed as a water column value, the permit 

writer should develop permit limits based on this criterion according to procedures 

described in section 5.4.4 of the Technical Support Document for Water Quality-based 

Toxics Control, or TSD (USEPA 1991). If the criterion is expressed as a fish tissue value 

and a water column translation of the fish tissue criterion or site-specific data to translate 

are available at the time of permit issuance, the permit limits based on the translated 

water concentration value should again be developed according to procedures described 

in section 5.4.4 of the TSD.

If, however, the criterion is expressed as a fish tissue value and a water column 

translation of the fish tissue criterion or site-specific data to translate are not available at 

the time of permit issuance, the permitting authority may reasonably conclude that a 

numeric WQBEL is infeasible to calculate. In that instance, EPA recommends that the 
NPDES Implementation Procedures

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permitting authority develop NPDES permit limits based on the criterion using the 

procedures described below. Section 7.3 contains additional information about expressing 

and developing permit limits based on the methylmercury criterion.

7.2.2 Determining reasonable potential

The second component of the recommended NPDES permitting approach for 

methylmercury is to conduct a reasonable potential analysis to determine whether the 

discharge will cause or contribute to an exceedance of applicable water quality standards. 

The recommended reasonable potential analysis consists of two steps. Step one is to 

determine whether there is a quantifiable amount of mercury in the discharge using a 

sufficiently sensitive analytical method (see sections 7.4 and 7.5.1.1 for more information 

on sufficiently sensitive methods.) If this information is unknown, EPA recommends 

including a monitoring requirement in the permit to collect this information and a 

reopener clause to allow establishment of appropriate requirements if the permitting 

authority determines that the discharge has reasonable potential. If, using a sufficiently 

sensitive analytical method, there is not a quantifiable amount of mercury in the 

discharge, depending on the particular facts, the permitting authority may reasonably 

conclude that the discharge does not have reasonable potential and that no water qualitybased limits are necessary. If there is a quantifiable amount of mercury, however, the 

permitting authority should move to step two of the reasonable potential analysis. Section 

7.5.1.1 contains additional information on step one of the reasonable potential analysis.

Step two of the reasonable potential analysis is to determine whether the fish tissue 

concentration of methylmercury in the receiving water is close to or exceeds the criterion. 

If the fish tissue concentration of methylmercury in the receiving water is below and not 

close to the criterion, depending on the particular facts, the permitting authority may 

reasonably conclude that the discharge does not have reasonable potential, but tier 2

antidegradation provisions should be considered. This situation is described below in the 

third component of the NPDES permitting approach. If the fish tissue concentration of 

methylmercury in the receiving water is close to or exceeds the criterion, depending on 

the particular facts, the permitting authority may reasonably conclude that the discharger 

has reasonable potential, and a WQBEL must be included in the permit. Recommended 

WQBELs for this situation are described below in the fourth component of the NPDES 

permitting approach. Section 7.5.1.2 contains additional information on step two of the

reasonable potential analysis. If information for step two is unknown, EPA recommends 

including in the permit a special permit condition to conduct a fish tissue survey of the 

receiving waterbody and a reopener clause so that reasonable potential can be determined 

when the fish tissue data become available. EPA further recommends that in this situation 

the permitting authority encourage permittees to develop and implement mercury 

minimization plans (MMPs) to reduce mercury loading to the waterbody.

In order to prevent future impairments, EPA recommends that a state or authorized tribe 

consider other factors or conditions such as rising fish tissue concentrations or the 

relative contribution of mercury or methylmercury from the source when determining 

whether a facility has reasonable potential in waters that are not yet impaired. Section 

7.5.1.2.2 contains additional examples of other factors, such as downstream impacts, that 

should be considered in a reasonable potential analysis.
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7.2.3 Implementing antidegradation

The third component of the recommended NPDES permitting approach for 

methylmercury is to determine whether the discharger will undertake an activity that can

increase mercury loading to the waterbody. If the discharger will not undertake such an 

activity, no additional permit conditions are necessary. EPA recommends, however, that

in this situation the facility voluntarily develop and implement an MMP to reduce the 

facility’s mercury loading to the receiving water. If the discharger will undertake such an 

activity, EPA recommends that a tier 2 antidegradation analysis be conducted in 

accordance with the state or tribe’s antidegradation policy and that permit conditions 

consistent with the analysis be included in the permit.

As part of conducting a tier 2 antidegradation analysis, the state or authorized tribe would 

evaluate the activity’s potential to lower water quality, whether there are alternatives that 

would avoid lowering water quality, and whether lowering of water quality would be 

necessary to accommodate important economic or social development in the area of the 

discharge. EPA considers analyses of potential pollution prevention and enhanced 

treatment alternatives as an appropriate starting point for the antidegradation review for 

both industrial and municipal dischargers. See 67 FR 68971, 68979. The results of such 

an analysis of potential alternatives could provide the basis for developing an MMP.

EPA further recommends that the permit contain a special condition requiring the 

permittee to implement an MMP and conduct effluent monitoring to allow for evaluation 

of the effectiveness and implementation of the MMP. Section 7.5.1.2.2 contains

additional information on antidegradation considerations.

7.2.4 Establishing appropriate WQBELs

The fourth component of the recommended NPDES permitting approach for 

methylmercury is to develop appropriate WQBEL requirements. Where a TMDL 

containing wasteload allocations for the discharge of mercury (and methylmercury where 

appropriate) has been developed, the WQBEL for that discharge must be consistent with 

the wasteload allocation (see 40 CFR 122.44(d)(1)(vii)(B)). Where a TMDL is not 

available at the time of permit issuance, to satisfy 122.44(d)(1)(vii)(A), EPA recommends 

the following WQBEL requirements, which are explained in greater detail in section 

7.5.2.1:

● Where a water column translation of the fish tissue criterion has been developed, or 

where site-specific data to do so are readily available, include a numeric water 

quality-based limit.

● Where a water column translation or site-specific data are not available and the 

permit writer determines that a numeric limit is infeasible to calculate:

o Require the permittee to implement an MMP tailored to the facility’s potential 

to discharge mercury. Depending on the particular facts, the permitting 

authority may include in the MMP a trigger level, reduction goal, or 

enforceable numeric level (e.g., existing effluent quality) to further manage 

mercury discharges.

o Require effluent monitoring using a sufficiently sensitive EPA-approved 

method to enable evaluation of the effectiveness and implementation of the 
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95

MMP. (See sections 7.4 and 7.5.1.1 for more information on sufficiently 

sensitive methods.)

o Include a reopener clause to modify the permit conditions if the MMP is not 

found to be effective or if a water column translation of the fish tissue 

criterion is developed.

Since permitting authorities need to establish and maintain WQBELs as stringent as 

necessary to meet water quality standards, if a state or tribe has yet to complete the 

transition from an existing water column criterion to a fish tissue-based criterion, states 

may consider retaining their existing water column criteria until translators are 

developed. Alternatively, until a translator is available, EPA recommends that one of the 

approaches outlined in this document for relating a concentration of methylmercury in 

fish tissue to a concentration of methylmercury in ambient water be considered. (See 

section 3.1.3.1)

In modifying or reissuing permits with existing WQBELs for mercury, permit writers 

must also ensure compliance with CWA anti-backsliding requirements. As described 

elsewhere in this Guidance, CWA section 402(o)(1) prohibits the revision of WQBELs to 

make them less stringent than existing permit limits unless a specific exception applies 

under 402(o)(2) or 303(d)(4).

Exceptions under Section 402(o)(2), which would allow for the establishment of less 

stringent limits are:

(1) There have been material and substantial alterations or additions to the permitted 

facility which justify the less stringent limit.

(2) New information (other than revised regulations, guidance, or test methods) is 

available that was not available at the time of permit issuance, and that would have 

justified a less stringent limit.

(3) Good cause exists due to events beyond the permittee’s control (e.g., natural 

disasters) and for which there is no reasonably available remedy.

(4) The permit has been modified under 301(c), 301(g), 301(h), 310(i), 301(k), 301(n), 

or 316(a).

CWA section 303(d)(4) provides additional exceptions to the anti-backsliding

prohibition: paragraph (A), which applies to “non-attainment waters,” and paragraph (B), 

which applies to “attainment waters”.

● Non-attainment water: CWA section 303(d)(4)(A) allows the establishment of a 

less stringent effluent limitation when the receiving water does not meet applicable 

water quality standards (i.e., a “non-attainment water”) if the permittee meets two 

conditions. First, the existing effluent limitation must have been based on a total 

maximum daily load (TMDL) or other wasteload allocation established under 

CWA section 303. Second, relaxation of the effluent limitation is allowed only if 

the cumulative effect of all revised limitations would assure the attainment of water 

quality standards, or the designated use not being attained is removed in 

accordance with the water quality standards regulations.
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● Attainment water: CWA section 303(d)(4)(B) applies to waters where the water 

quality equals or exceeds levels necessary to protect the designated use, or to 

otherwise meet applicable water quality standards (i.e., an “attainment water”). 

Under CWA section 303(d)(4)(B), a limitation based on a TMDL, wasteload 

allocation, other water quality standard, or any other permitting standard may only 

be relaxed where the action is consistent with the state's antidegradation policy.

The application of these exceptions is limited under 402(o)(3), which prohibits the 

relaxation of effluent limitations in all cases if a revised effluent limitation would result 

in a violation of applicable effluent limitation guidelines or water quality standards, 

including antidegradation requirements.

In establishing WQBELs for mercury, permit writers will need to ensure that the CWA 

anti-backsliding requirements are met. The first step of the inquiry is to determine 

whether the WQBEL based on the fish tissue criterion is “less stringent” than the 

WQBEL in the previous permit. If the new permit limit is not less stringent (e.g., if the 

prior numeric WQBEL is included in the MMP as an enforceable numeric level (see 

section 7.5.2.4 for additional information)), then the anti-backsliding prohibition should 

not be triggered and it should be appropriate to include the new limit in the permit. If the 

WQBEL based on the new fish tissue criterion is in fact less stringent than the prior 

WQBEL, then the permit writer must retain the existing numeric WQBEL unless there is 

an available exception to the anti-backsliding prohibition.

Because CWA section 402(o)(2)(B)(i) does not allow backsliding solely because 

regulations are revised (e.g., adoption of the fish tissue criterion), any applicable 

exceptions to the anti-backsliding prohibition for impaired waters would be found under 

section 303(d)(4)(A). In this case, permit limits based on TMDLs or other wasteload 

allocations established under section 303 can be made less stringent only if: a) the 

cumulative effect of all loadings meets the WQS or b) the designated use is removed.

Anti-backsliding requirements are further described in EPA’s NPDES Permit Writers’ 

Manual (USEPA 1996a) and in EPA’s Technical Support Document for Water QualityBased Toxics Control (USEPA 1991).

Other considerations and requirements may be necessary in developing permits. They 

include the following:

● Where a discharger undertakes an activity that could increase mercury loading to 

the receiving water, the WQBEL must be consistent with applicable 

antidegradation requirements (see section 7.5.1.2.2). Additional requirements may

also be necessary under the CWA and EPA’s NPDES regulations (see section 

7.5.2.3).

 The permitting authority would need to include appropriate technology-based 

limits pursuant to CWA section 301(b) and 40 CFR sections 125.3 and 

122.44(a)(1) (see section 7.5.2.3).

The entire recommended NPDES permitting approach is summarized in figure 5 and 

explained in greater detail in the following sections.
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7.3 How does EPA recommend implementing the fish 

tissue criterion for NPDES permits?

As discussed in section 3.1, states and authorized tribes that decide to use the 

recommended criterion as the basis for new or revised methylmercury water quality 

standards have the option of adopting the criterion into their water quality standards as a 

fish tissue concentration, a traditional water column concentration, or both. If states or 

authorized tribes choose to use both approaches, they should clearly describe in their 

standards how each will be used for specific applications and describe applicable 

implementation procedures.

EPA recommends two approaches for implementing the fish tissue-based methylmercury 

water quality criterion in NPDES permits, depending on the form in which the state or 

authorized tribe expresses the criterion––as a fish tissue concentration or as a water 

column concentration. In addition, states and authorized tribes that adopt the 

recommended criterion as a fish tissue value may choose to implement it through NPDES 

permitting as a water column translation of the fish tissue value. Each of these 

approaches is summarized in figure 6 and discussed in more detail in sections 7.4 and 7.5.

The recommendations below assume that an approved TMDL is not available. If EPA has 

approved or established a TMDL containing a wasteload allocation for the discharge of 

mercury (and methylmercury where appropriate), the WQBEL for that discharge must be 

consistent with the wasteload allocation (see 40 CFR 122.44(d)(1)(vii)(B)).

This chapter provides EPA’s guidance on how a permitting authority could implement 

the fish tissue criterion in NPDES permits consistent with the CWA and its implementing 

regulations. States and authorized tribes retain the discretion to develop and use 

procedures for determining reasonable potential and establishing effluent limits in

NPDES permits that differ from those in the guidance. Such procedures may use other 

information relevant to determining reasonable potential and establishing effluent limits,

where appropriate. If a state or authorized tribe develops its own such permitting

procedures, EPA recommends that states and authorized tribes make the procedures
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Figure 5. NPDES permitting approach for methylmercury.
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Figure 6. Implementing the fish tissue criterion in NPDES permits.

public so that all stakeholders can be aware of the requirements and expectations of the 

permit program. In addition, the permit’s fact sheet or statement of basis should also 

explain the basis of the permit conditions and effluent limitations and how these are 

consistent with the state’s or authorized tribes’ permitting procedures, the CWA, and 

applicable federal regulations.

7.4 What are the procedures for developing permit limits 

when the criterion is adopted as a water column value

or when the criterion is adopted as a fish tissue value 

and the permitting authority uses a water column 

translation of the fish tissue value?

This approach assumes that a state or authorized tribe decides to adopt a new or revised 

water quality criterion for methylmercury in one of the following forms:

● Water column concentration value. Expressing a criterion as a water column value 

is very common, and permitting authorities have considerable historical experience 

in developing permit limits based on such criteria in NPDES permits.

● Fish tissue concentration value that is translated into a water column value.

Sections 3.1.3.1 through 3.1.3.3 of this guidance discuss the procedures for 

translating the fish tissue criterion into a water column value for water quality 

standards purposes. These procedures may also be used to translate a fish tissue 

criterion into a water column value for determining reasonable potential and for 

deriving numeric WQBELs.

In either case described above, the permitting authority should determine reasonable 

potential and calculate numeric WQBELs using the procedures described in section 5.4.4 

of the TSD (USEPA 1991) to derive a numeric WQBEL.
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This approach relies on the measurement of mercury in effluent, often at concentrations 

below the quantitation levels of some analytical methods. Therefore, the permitting 

authority should specify that the NPDES regulated discharger use a sufficiently sensitive 

EPA-approved method for the measurement of mercury in the discharge. An analytical 

method is sufficiently sensitive when (1) its method quantitation level is at or below the 

level of the applicable water quality criterion or (2) its method quantitation level is above

the applicable water quality criterion, but the amount of mercury in a discharge is high 

enough that the method detects and quantifies the level of mercury in the discharge. To 

illustrate the latter, if the water column criterion or water column translation of a fish 

tissue criterion for mercury in a particular waterbody is 2.0 parts per trillion (ppt), method 

245.7 (with a quantitation level of 5.0 ppt) would be sufficiently sensitive when it reveals 

that the level of mercury in a discharge is 5.0 ppt or greater. In contrast, method 245.7 

would not be sufficiently sensitive when it resulted in a level of nondetection for that 

discharge because it could not be known whether mercury existed in the discharge at a 

level between 2.0 and 5.0 ppt (less than the quantitation level but exceeding the water 

quality criterion).23

The selection of a sufficiently sensitive method relates method quantitation levels to the 

water column criterion value. If a water column criterion or a water column translation of 

a fish tissue criterion is not available to allow for selecting an alternate sufficiently 

sensitive method, EPA recommends the use of the most recent version of method 1631 to 

characterize discharges from all facilities for which the mercury levels are unknown or 

undetected. Method 1631 is relatively new, and the facilities may not have used it to 

analyze their effluent discharges. As a result, previous monitoring may show 

undetectable levels of mercury when use of method 1631 shows detectable or 

quantifiable amounts. Therefore, EPA recommends monitoring using the most recent 

version of method 1631 to help identify all facilities that contribute to mercury water 

quality impairment, unless another EPA-approved method can be justified as being 

sufficiently sensitive.

EPA’s regulations require that measurements included on NPDES permit applications 

and on reports required to be submitted under the permit must generally be made using 

analytical methods approved by EPA under 40 CFR part 136. Because EPA has approved 

methods for analyzing mercury in water, these approved methods must be used in water 

analyses for NPDES permits involving mercury. See 40 CFR sections 122.21(g)(7), 

122.41(j), 136.1, 136.3, and 136.6. Selection of an approved method should take into 

account the above discussion of method sensitivity. For metals, such as mercury, the 

federal regulations at 40 CFR 122.45(c) generally require effluent monitoring for the total 

form of the metal.

23 For more information on choosing a sufficiently sensitive method, see the memorandum Analytical Methods for Mercury in National 

Pollutant Discharge Elimination System (NPDES) Permits from James A. Hanlon, Director of the Office of Wastewater Management, 

dated August 23, 2007, at http://www.epa.gov/npdes/pubs/mercurymemo_analyticalmethods.pdf.
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The discussion above describes analytical methods for measuring mercury in water. Refer 

to section 4.1 and appendix C for information on analytical methods for measuring 

mercury in fish tissue and for measuring methylmercury in water or fish tissue.

7.5 What are the procedures for developing permit limits 

when the criterion is adopted as a fish tissue value 

and the permitting authority does not use a water 

column translation of the fish tissue value?

This approach assumes that a state or authorized tribe decides to adopt a new or revised 

water quality criterion for methylmercury in the form of a fish tissue concentration and 

that a TMDL, water column translation of the fish tissue criterion, or site-specific data to 

translate are not available at the time of permit issuance. As a result, the permitting 

authority will use a different approach than it has previously used for determining 

reasonable potential and expressing WQBELs. EPA recommends the approach described 

below, which is summarized in figure 7.

Figure 7. Determining reasonable potential.
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7.5.1 How to determine the need for permit limits to control 

mercury (how to determine reasonable potential)

As discussed in section 3.1.2.2 of this document, EPA recommends that states and 

authorized tribes adopt new or revised methylmercury water quality criteria in the form 

of a fish tissue concentration. When a criterion is adopted into standards as a fish tissue 

value, states and authorized tribes may not have sufficient data to translate from a fish 

tissue value to a traditional water column value using BAFs or translators. This section 

provides recommendations for how a permitting authority can determine reasonable 

potential in the absence of data to translate the fish tissue value into a water column 

value.

When determining reasonable potential, the permitting authority must determine whether 

the discharge “causes, has reasonable potential to cause, or contributes” to an exceedance 

of the applicable water quality criterion (see 40 CFR 122.44(d)(1)(ii)). The NPDES

permit fact sheet should provide the rationale and assumptions used in determining 

whether WQBELs proposed in the associated draft permit are appropriate. The 

recommendations in this guidance could be applied on a permit-by-permit basis where 

appropriate to support the reasonable potential determination that satisfies 40 CFR 

122.44(d)(1)(ii) with respect to a water quality criterion for methylmercury expressed as 

a fish tissue value in the absence of a TMDL and a water column translation of that value

at the time of permit issuance.

EPA believes that, depending on the particular facts, a permitting authority could 

reasonably conclude that reasonable potential exists if two conditions are present: (1) the 

NPDES permitted discharger has mercury in its effluent at a quantifiable level and (2) the 

methylmercury level in fish tissue from the receiving waterbody is close to or exceeds the 

fish tissue water quality criterion. Under these circumstances, the effluent data indicate 

that the mercury load in the effluent contributes to the mercury load in the waterbody, 

and the fish tissue concentration indicates that the mercury load in the waterbody causes 

or has the potential to cause an exceedance of the water quality criterion. This approach is 

consistent with federal regulations pertaining to the Great Lakes Basin, which contained 

an approach for determining reasonable potential using fish tissue data (see 40 CFR part 

132, appendix F, procedure 5.F.4). The reasonable potential approach for mercury 

described in this guidance has the advantage of significantly reducing environmental 

monitoring costs and does not involve developing a site-specific BAF for each waterbody 

in a state.

EPA recognizes that the mere presence of mercury at a quantifiable level in an effluent is 

not necessarily an indication that the mercury discharge is the sole cause of the fish 

contamination or even a substantial contributor of such contamination. However, mercury 

in an effluent discharge may contribute to the methylmercury present in fish tissue at 

levels close to or above the fish tissue criterion, and therefore the discharge may be found 

to exhibit the reasonable potential to cause or contribute to the exceedance of applicable 

water quality standards. EPA notes that the reasonable potential procedures as a whole 

are intended as conservative screening procedures to determine when a permit should 

contain a WQBEL to reduce the contribution to existing contamination or to prevent 

further possible degradation.
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EPA notes that, unlike typical water quality criteria that are expressed as water column 

values, the fish tissue water quality criterion integrates spatial and temporal complexity 

and the cumulative effects of mercury loading from point and nonpoint sources that affect 

methylmercury bioaccumulation in aquatic systems. As discussed further in section 

7.5.1.2.2, EPA believes that comparing the fish tissue concentration in steady state 

systems directly to the applicable fish tissue criterion appropriately accounts for the 

factors specified in 40 CFR 122.44(d)(1)(ii) for a criterion expressed as a fish tissue 

value.

Finally, EPA further notes that because of the sensitivity of Method 1631E or other 

sufficiently sensitive methods (as described in section 7.4), it is reasonable to conclude 

that a discharge below quantitation does not have reasonable potential to exceed the 

criterion.

7.5.1.1 Step one of the reasonable potential analysis: Determining 

whether the NPDES-permitted discharger has mercury in its 

effluent at quantifiable levels

The first step in the reasonable potential analysis is to determine whether the discharge 

contains a quantifiable amount of mercury. To determine this, EPA recommends that 

permitting authorities require monitoring using a sufficiently sensitive analytical method 

approved for use by EPA under 40 CFR part 136. Section 7.4 contains additional 

information about sufficiently sensitive EPA-approved methods. If an alternate EPAapproved method cannot be justified as being sufficiently sensitive, EPA recommends 

monitoring using the most recent version of method 1631 to help identify all facilities 

that contribute to mercury water quality impairment. EPA recognizes that using method 

1631 will likely result in a large majority of facilities showing quantifiable mercury 

discharges. This approach, however, is intended to allow permitting authorities to 

determine that facilities without quantifiable levels of mercury may not need step two of 

the reasonable potential analysis (determining whether the fish tissue criterion is being 

attained).

One of three outcomes will be reached in answering the first condition of the reasonable 

potential analysis:

● It is unknown whether the discharge includes a quantifiable amount of mercury.

● The discharge does not include a quantifiable amount of mercury.

● The discharge includes a quantifiable amount of mercury.

The recommended reasonable potential determination and recommended permit 

conditions for each of the outcomes is described in detail below.

7.5.1.1.1 What are the recommended permit conditions when it is unknown whether 

the discharge includes quantifiable amounts of mercury because there are 

limited or no effluent data to characterize the discharge of mercury?

In this situation, EPA recommends that the permitting authority include permit conditions 

that include the following elements:
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● Effluent monitoring using a sufficiently sensitive EPA-approved analytical method 

to characterize the discharger’s effluent for mercury (see sections 7.4 and 7.5.1.1 

for information on sufficiently sensitive methods)

● A reopener clause to identify the actions that the permitting authority may take 

should the monitoring information indicate that a WQBEL for mercury is necessary

EPA recommends that permitting authorities require monitoring, using a sufficiently 

sensitive EPA-approved method, by all facilities for which the mercury levels are 

unknown or previously undetected (using less sensitive methods) to characterize the 

discharger’s effluent for mercury. EPA recommends this monitoring to help identify all 

facilities that contribute to mercury loads in the waterbody. The permitting authority 

could obtain these monitoring data as part of the permit application, by requiring periodic 

(e.g., quarterly to annually) monitoring as part of the permit, or by invoking its authority 

under CWA section 308 (or equivalent state authority) to require NPDES facilities to 

collect information necessary for developing NPDES permit limits. The permit should 

include a reopener clause so that as soon as there is complete information and an 

indication that a more stringent limit is required, the permitting authority can establish the 

necessary requirements. The permitting authority may also decide to no longer require 

the monitoring if the information shows that the facility is not discharging mercury at 

quantifiable levels.

EPA recommends that when selecting the monitoring frequency, permitting authorities 

consider the factors in section 5.7.5 of the TSD (USEPA 1991). This section 

acknowledges that EPA has not recommended a specific monitoring frequency. However, 

the TSD recognizes that the choice of a monitoring frequency is a site-specific decision 

and provides the permitting authority with a number of factors to consider when making 

these decisions.

Until the permitting authority has sufficient data to determine whether the discharge has

reasonable potential, and depending on the particular facts, the permit writer may 

reasonably conclude that the permit conditions described in this section are as stringent as 

necessary to achieve water quality standards, as required by CWA section 301(b)(1)(C).

7.5.1.1.2 What are the recommended permit conditions when the discharge does not 

include quantifiable amounts of mercury?

In this situation, EPA recommends that the permitting authority first review the 

monitoring data to determine whether they are representative of the effluent. If the 

permitting authority believes the monitoring data are representative of the discharge, no 

further permit conditions may be necessary. In contrast, if the permitting authority 

believes the data are not representative, the authority should consider requiring additional 

monitoring, as described in section 7.5.1.1.1.

7.5.1.1.3 What are the recommended actions for discharges that include quantifiable 

amounts of mercury?

In this case, the permitting authority should move to step two of the reasonable potential 

analysis and evaluate data on the concentrations of methylmercury in the fish tissue from 

the receiving waterbody to determine appropriate permit conditions (see section 7.5.1.2).
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7.5.1.2 Step two of the reasonable potential analysis: Determining 

whether the fish tissue concentration of methylmercury in the 

receiving waterbody exceeds the fish tissue criterion

In step two of EPA’s recommended fish tissue criterion reasonable potential procedure, 

the permitting authority has concluded that the first condition of the two-part reasonable 

potential analysis has been satisfied (i.e., the NPDES-permitted discharger has mercury in 

its effluent at a quantifiable level). The permitting authority should then address the 

second condition of the reasonable potential analysis––determining whether the fish 

tissue from the receiving waterbody exceeds (or is close to exceeding) the fish tissue 

water quality criterion.

One of three outcomes will be reached in answering this question:

● The fish tissue concentration of methylmercury is unknown.

● The fish tissue concentration of methylmercury does not exceed the criterion or is 

not close to the criterion.

● The fish tissue concentration of methylmercury exceeds the criterion or is close to 

exceeding the criterion.

For discharges with quantifiable levels of mercury, the recommended reasonable 

potential determination and recommended permit conditions for each outcome is 

described in detail below.

EPA recognizes that when evaluating reasonable potential, the permitting authority 

should exercise discretion and careful judgment in determining whether fish tissue data 

are representative of current ambient conditions. EPA guidance for sampling strategies 

for fish tissue monitoring is provided in section 4.2 of this document.

7.5.1.2.1 What are the recommended permit conditions when a facility discharges

quantifiable amounts of mercury but the fish tissue concentrations of 

methylmercury in the receiving waterbody are unknown?

In waterbodies for which there are insufficient fish tissue data available, a permitting 

authority cannot determine whether there is reasonable potential using a fish tissue 

approach. Therefore, in this case, EPA recommends that the permitting authority take the 

following actions:

● Include a special permit condition to conduct a mercury fish tissue survey for the 

receiving waterbody, unless such information will be available from another source 

in a timely manner.

● Include as a permit condition a reopener clause to identify the actions that the 

permitting authority may take should fish tissue monitoring information become 

available and indicate that a WQBEL for mercury is necessary.

● Encourage the permittee to develop and implement an MMP tailored to the 

facility’s potential to discharge mercury.

In this instance, the permitting authority should start a process for collecting fish tissue 

data in the waterbodies where point source discharges of mercury exist. One approach for 
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collecting this information is for the permitting authority to invoke its authority under 

CWA section 308 (state permitting authorities would use comparable state authorities) to 

require NPDES facilities to collect information necessary for the development of NPDES 

permit limits. In this case, the permitting authority could issue a section 308 letter or 

include special conditions in the permit to require the permittee to conduct a 

methylmercury fish tissue monitoring study. EPA recommends that the study design be 

consistent with the recommendations on conducting ambient monitoring in section 4.2 of 

this guidance.

EPA also recommends that the permitting authority require only one study per 

waterbody. The permitting authority could do this by contacting all facilities that 

discharge into the waterbody and encouraging them to work jointly to conduct the study,

because the outcomes of the study may affect the permit limits of those facilities. For 

example, the State of Idaho has developed a statewide fish tissue monitoring program for 

mercury that provides a standardized approach for collecting reliable data while 

recognizing limited resources for monitoring.

In waterbodies where the permitting authority expects to find high mercury 

concentrations in the water column or believes it will need a site-specific BAF to finish

issuing the permits, the permitting authority should consider requiring the facility to 

include measurement of water column concentrations of mercury as part of the study.

EPA further recommends that the permit include a reopener clause so that as soon as 

there is complete information, the permitting authority can establish any additional 

requirements that are necessary. In this situation EPA recommends that the permitting 

authority encourage the permittee to develop and implement an MMP for the reasons 

discussed in section 7.5.1.2.2.1.

7.5.1.2.2 What are the recommended permit conditions when a facility discharges

quantifiable amounts of mercury but the fish tissue concentrations of 

methylmercury in the receiving waterbody do not exceed and are not close 

to the criterion?

Once the permitting authority has determined that a facility discharges quantifiable 

amounts of mercury and that the concentration of methylmercury in fish tissue in the 

receiving waterbody does not exceed and is not close to the criterion, depending on the 

particular facts, the permitting authority may reasonably conclude that the discharge does 

not have reasonable potential to cause or contribute to an exceedance of the applicable 

fish tissue water quality criterion.

To assist in preventing future impairments, in some situations as outlined below, EPA 

recommends that states and authorized tribes also consider other factors or conditions 

such as a trend of rising fish tissue concentrations or the relative contribution of mercury 

or methylmercury from the source when determining whether a facility has reasonable 

potential in waters that are not yet impaired.

EPA notes that, unlike typical water quality criteria that are expressed as water column 

values, the fish tissue water quality criterion integrates spatial and temporal complexity 

as well as the cumulative effects of variable mercury loading from point and nonpoint 

sources that affect methylmercury bioaccumulation in aquatic systems. EPA believes that 
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comparing the fish tissue concentration in steady state systems directly to the applicable

criterion expressed as a fish tissue value appropriately accounts for the factors specified 

in 40 CFR 122.44(d)(1)(ii) for a criterion expressed as a fish tissue value. Existing tissuebased data are indicators of accumulation that has already occurred. Thus, where fish 

tissue concentrations in a watershed are expected to be constant (i.e., steady state 

conditions) or decreasing over time, data that indicate that the fish tissue criterion is 

currently being attained may be effective indicators of current and potential continued 

future attainment.

However, in dynamic systems where the levels in tissue in a watershed may be expected 

to increase, EPA recommends that the permitting authority account for this as part of the 

reasonable potential determination that is designed to prevent potential future 

impairments. 

Another factor that permitting authorities may consider is the impact of permitted 

discharges to downstream waters (e.g., a discharge to a river that flows into a lake where 

mercury is a concern). In such a circumstance, it may be appropriate to conclude that the 

discharge has reasonable potential on the grounds that its discharge causes or contributes 

to the excursion of the fish tissue criterion in the downstream water.

The presence of these other factors or conditions such as the relative contribution of 

mercury or methylmercury from the source, rising fish tissue concentrations, or potential 

excursion of the criterion downstream, could constitute a basis for concluding that an 

effluent limit is necessary depending on the particular facts.

As discussed in section 7.5.1.2.2.2, for discharges to waters that are not impaired, EPA 

recommends that states and tribes regard any activity that could result in an increase in 

receiving water or fish tissue mercury concentration as a significant lowering of water 

quality for the purposes of triggering an antidegradation review.

Implementing tier 2 antidegradation

If the facility undertakes any activity that could increase mercury loading to the receiving 

waterbody, an antidegradation review may be necessary. Such increases must be 

consistent with the applicable antidegradation policy. Federal regulations at 40 CFR 

131.6 specify that tribal or state water quality standards must include an antidegradation 

policy, and federal regulations at 40 CFR 131.12 identify the elements of an acceptable 

antidegradation policy. Section 303(d)(4)(B) requires that applicable antidegradation 

requirements be satisfied prior to modifying NPDES permits (for example, prior to 

removing a WQBEL or including less stringent effluent limitations).

The federal antidegradation policy is composed of three levels of protection commonly 

referred to as tiers. The first tier, identified at 40 CFR 131.12(a)(1), protects the minimum 

level of water quality necessary to support existing uses and applies to all waters. This 

tier prohibits lowering water quality to the point where existing uses are impaired. The 

second tier, found at 40 CFR 131.12(a)(2), protects water quality where water quality is 

better than that needed to support “fishable/swimmable” uses of the water. Where these 

conditions exist, the waterbody is typically considered not impaired, and water quality 

must be maintained and protected unless it is demonstrated that lowering water quality is 

necessary to support important social and economic development and that existing uses 
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will be fully protected. The third tier, at 40 CFR 131.12(a)(3), involves the protection of 

water quality in waterbodies that are of exceptional ecological, aesthetic, or recreational 

significance. Water quality in such waterbodies, identified and specifically designated by 

states or authorized tribes as Outstanding National Resource Waters, must be maintained 

and protected.

States and authorized tribes should determine whether the discharger will undertake an 

activity that can result in an increase in mercury loading to the receiving waterbody.

One of two outcomes will be reached in answering this question:

● The discharger will not undertake an activity that can increase mercury loading to 

the waterbody.

● The discharger will undertake an activity that can increase mercury loading to the 

waterbody.

As part of conducting a tier 2 antidegradation analysis, the permitting authority would 

evaluate the activity’s potential to lower water quality, whether there are alternatives that 

would avoid lowering water quality, and whether lowering of water quality would be 

necessary to accommodate important economic or social development in the area of the 

discharge. EPA considers analyses of potential pollution prevention and enhanced 

treatment alternatives as an appropriate starting point for the antidegradation review for 

both industrial and municipal dischargers. See 67 FR 68971, 68979. The results of such 

an analysis of potential alternatives could provide the basis for developing an MMP.

EPA’s recommendations for implementing antidegradation provisions and addressing 

increases in mercury loads are summarized in figure 8 and explained in sections 

7.5.1.2.2.1 and 7.5.1.2.2.2. EPA recognizes, however, that states and tribes have the 

flexibility to interpret their antidegradation policies differently. For example, some states 

use limits established at existing effluent quality to implement their antidegradation 

provisions.
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Figure 8. Implementing tier 2 antidegradation.

7.5.1.2.2.1 What are the recommended permit conditions when a facility discharges 

quantifiable amounts of mercury into a waterbody in which the fish tissue 

concentration of methylmercury does not exceed the criterion and the 

facility will not undertake an activity that could increase mercury loading to 

the waterbody?

If the facility discharges a quantifiable amount of mercury and the fish tissue 

concentration of methylmercury in the receiving water does not exceed the criterion, 

depending on the particular facts, the permitting authority may reasonably conclude that 

the discharge does not have reasonable potential to cause or contribute to an exceedance 

of the applicable fish tissue water quality criterion. In such situations, however, EPA 

recommends that the permitting authority encourage the facility to develop and 

implement an MMP.

An MMP helps ensure that the discharge will continue to have no reasonable potential to 

cause or contribute to an exceedance of applicable water quality standards. The 

recommendation to develop a voluntary MMP is also based on the extent of potential 

mercury impairment across the country and the scientific complexities of and

uncertainties associated with assessing mercury loadings and evaluating their effects.

If future monitoring data demonstrate that a discharge does have reasonable potential, 

development of a MMP could assist the permit writer in establishing appropriate permit 
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conditions. Furthermore, EPA believes that simply developing an MMP might provide 

dischargers of mercury with sufficient information to economically reduce the discharge 

of mercury into our Nation’s waters by voluntarily implementing the mercury 

minimization measures identified in the plan. Section 7.5.2.1 provides additional 

information on MMPs.

7.5.1.2.2.2 What are the recommended permit conditions when a facility discharges

quantifiable amounts of mercury into a waterbody in which the fish tissue 

concentration of methylmercury does not exceed the criterion but the facility 

will undertake an activity that could result in an increase in receiving water 

or fish tissue mercury concentration?

In this situation, the receiving water does not currently exceed the fish tissue criterion. 

EPA believes that increases in mercury loading to a waterbody should be allowed at 

levels determined appropriate by an antidegradation analysis and that such dischargers 

should be required to implement MMPs under the authority of CWA section 402(a)(1)(B) 

and 40 CFR 122.44(k)(4).

EPA recommends the following WQBEL requirements:

● Include permit conditions consistent with antidegradation requirements.

● Require the permittee to implement an MMP tailored to the facility’s potential to 

discharge mercury. Depending on the particular facts, the permitting authority may 

include in the MMP a trigger level, reduction goal, or enforceable numeric level to 

further manage mercury discharges.

● Require the permittee to monitor its effluent using a sufficiently sensitive EPAapproved method (see sections 7.4 and 7.5.1.1 for information on sufficiently 

sensitive methods).

Other considerations and requirements might be necessary in developing permits:

● The permitting authority would need to include appropriate technology-based 

limits pursuant to CWA section 301(b) and 40 CFR sections 125.3 and 

122.44(a)(1) .

● For modified or reissued permits with existing effluent limits for mercury, any less 

stringent effluent limit must be consistent with anti-backsliding requirements (see 

section 7.2.4).

Activities that would lower water quality in a high-quality water must be consistent with 

the applicable antidegradation provisions of a state’s or authorized tribe’s water quality 

standards. Consistent with EPA’s antidegradation regulations for water quality standards, 

state and tribal antidegradation regulations are to provide that the quality of waters at 

levels better than the levels necessary to support “fishable/swimmable” uses of the water 

may be lowered only if the state or authorized tribe determines that allowing lower water 

quality is necessary to accommodate important economic or social development in the 

area in which the waters are located (see 40 CFR 131.12(a)(2)). EPA recommends that 

states and authorized tribes regard any activity that could result in an increase in 

receiving water or fish tissue mercury concentration as a significant lowering of water 

quality for the purposes of triggering a tier 2 antidegradation review. If the state’s or 
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111

authorized tribe’s antidegradation analysis determines that the proposed lowering of 

water quality should not be allowed, the permitting authority would not authorize or 

allow any such discharge to occur. If the state’s or authorized tribe’s antidegradation 

analysis determines that a lowering of water quality is allowable, the level to which the 

discharger is ultimately allowed to lower water quality (on the basis of the applicable 

antidegradation requirements) would then be subject to a reasonable potential analysis. 

Also, EPA’s antidegradation regulations for water quality standards require state and 

tribal antidegradation regulations to protect the minimum level of water quality necessary 

to support existing uses by prohibiting lowering of water quality to the point where 

existing uses are impaired (see 40 CFR 131.12(a)(1)).24 For new and increased 

discharges, states have the flexibility to interpret their antidegradation policies 

differently. For example, some states use limits established at existing effluent quality.

EPA expects that fluctuations in mercury loadings arising from normal industrial 

production fluctuations, or loading fluctuations that are not results of change in existing 

POTW service areas, would generally not trigger a tier 2 antidegradation analysis. EPA 

expects that increases in mercury loadings from a POTW arising from adding a new 

subdivision or an unsewered neighborhood to a sewer service area would generally 

trigger a tier 2 antidegradation review. If an antidegradation review is triggered, the 

review should consider the source of the increased mercury loading, the potential for 

source reduction through either treatment, pretreatment or pollution prevention, and the 

expected benefits likely to accrue to the affected community as a result of the activities 

that result in increased mercury loadings. EPA recommends that states and tribes tailor 

the level of detail and documentation for antidegradation demonstrations to the specific 

circumstances. For example, in some instances, as with diffuse domestic sources of 

mercury, available treatment and pollution prevention alternatives may be limited or 

lacking, leaving only the importance of social and/or economic development as the 

primary focus of the review.

EPA recognizes that an increase in the discharge of mercury might be due to mercury 

present in stormwater or input process water that does not originate with and is not under

the reasonable control of a facility. While an MMP, to the extent that there are available 

BMPs to minimize mercury discharges, might still be appropriate in such circumstances, 

EPA would not generally expect that such discharges would trigger the need for an 

antidegradation review, or numeric WQBELs.

In addition to permit conditions consistent with antidegradation requirements, EPA 

recommends that the permit require the dischargers to implement an MMP under the 

authority of CWA section 402(a)(1)(B) and 40 CFR 122.44(k)(4). The MMP should be 

tailored to the individual facility’s potential to discharge mercury. For more information 

on MMPs, see section 7.5.2.1.

24 This part of the antidegradation analysis is similar to the reasonable potential determination and WQBEL development process that a 

permitting authority conducts for an existing discharger.
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7.5.1.2.3 What are the recommended permit conditions when a facility discharges

quantifiable amounts of mercury and the fish tissue concentrations of 

methylmercury in the receiving waterbody are close to or exceed the 

criterion?

EPA believes that, depending on the particular facts, a permitting authority may 

reasonably conclude that reasonable potential exists if two conditions are present: (1) the 

NPDES-permitted discharger has mercury in its effluent at quantifiable levels, and (2) the 

fish tissue concentrations of methylmercury from the receiving waterbody are close to or 

exceed the fish tissue water quality criterion.

Where fish tissue concentrations are below but close to the criterion, EPA recommends 

that a finding of reasonable potential be made since the effect of current discharges and 

other relevant factors may not yet be reflected in fish tissue concentrations. For example, 

where the tissue data are below the water quality criterion, the permitting authority may 

consider applying an appropriate confidence interval (e.g., 95 percent upper confidence 

limit on the mean) to such values and compare that value to the fish tissue criterion to the 

extent necessary to account for variability in fish tissue data. As an example of an 

alternative to this statistical approach, the State of Idaho’s implementation guidance25 for 

its methylmercury fish tissue criterion of 0.3 mg/kg recommends that where the levels in 

fish exceed 0.24 mg/kg, the permitting authority should determine that reasonable 

potential exists. Where methylmercury levels in fish tissue are thought to be relatively 

sensitive to a water point source load of mercury or methylmercury, the permitting 

authority may take that into account in the reasonable potential determination.

When reasonable potential exists, it is necessary to establish an appropriately protective 

WQBEL in the permit. For guidance on recommended WQBELs, see section 7.5.2.1.

7.5.1.3 How to consider mercury in intake water with a reasonable 

potential approach

For some facilities, the only source of mercury in a discharge may be the intake water 

taken directly from the same body of water to which the facility discharges. An example 

of this is a discharge of cooling water where the source of the cooling water is upstream 

of the discharge. In these situations where there are no known sources or additional 

contributions of mercury at the facility, the permitting authority could reasonably 

conclude, based on the particular facts, that there is no reasonable potential to cause or 

contribute to an exceedance of water quality standards. Furthermore, any slight increase 

in concentration after discharge (due to evaporation or other water loss) should not have 

an effect on the bioaccumulation of methylmercury in fish tissue unless the fish are 

known to frequently inhabit the water in the area immediately adjacent to the discharge. 

In making this decision, the permitting authority should consider the monitoring data 

from both the intake and discharge to verify that there are no known sources of additional 

contributions of mercury at the facility. EPA also recommends that permitting authorities 

consider evaluating whether the methylmercury concentration in fish tissue significantly 

25 Implementation Guidance for the Idaho Mercury Water Quality Criteria is available at http://www.deq.state.id.us/water/data_reports/

surface_water/monitoring/idaho_mercury_wq_guidance.pdf.
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increases for facilities with anaerobic conditions in the discharge. This procedure 

represents a comprehensive approach for conducting a site-specific analysis of the 

potential for a discharge to cause or contribute to an excursion above a water quality 

standard, which can lead to a decision to not require a WQBEL. This approach is 

consistent with the rationale for the federal regulations pertaining to the Great Lakes 

Basin, which included consideration of intake pollutants in finding reasonable potential

(see 40 CFR part 132, appendix F, procedure 5.D).

7.5.2 Where reasonable potential exists, how can WQBELs be 

derived from a fish tissue value?

As discussed in section 3.1.2.2 of this document, EPA recommends that states and 

authorized tribes adopt a new or revised methylmercury water quality criterion in the 

form of a fish tissue concentration. When the criterion is adopted into standards as a fish 

tissue value, some states and authorized tribes may not have sufficient data to translate 

from a fish tissue value to a traditional water column value using BAFs or translators. 

When developing WQBELs, the permitting authority must ensure that the level of water 

quality to be achieved by such limits derives from and complies with water quality 

standards (see 40 CFR 122.44(d)(1)(vii)). This section provides recommendations on 

how a permitting authority could derive appropriate WQBELs in the absence of a TMDL 

and a water column translation of the fish tissue criterion at the time of permit issuance. 

The information discussed in this section is summarized in figure 9.

Figure 9. Determining WQBEL requirements.
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7.5.2.1 What are the recommended WQBELs?

If the facility has a quantifiable amount of mercury in its discharge and the concentration 

of methylmercury in fish tissue in the receiving water is close to or exceeds the criterion, 

depending on the particular facts, the permitting authority may reasonably conclude that 

the discharge has reasonable potential to cause or contribute to an exceedance of the 

applicable fish tissue water quality criterion. In this situation, in the absence of a TMDL

and a water column translation of the fish tissue criterion, it may be appropriate to 

conclude that it is infeasible to calculate a numeric WQBEL at the time of permit 

issuance and to instead express the WQBEL as narrative BMPs, as provided in 

122.44(k)(3).

Where a TMDL containing wasteload allocations for the discharge of mercury (and 

methylmercury where appropriate) has been developed, the WQBEL for that discharge 

must be consistent with the wasteload allocation (see 40 CFR 122.44(d)(1)(vii)(B)). 

Where a TMDL is not available at the time of permit issuance, to satisfy 

122.44(d)(1)(vii)(A), EPA recommends that the WQBEL consist of the following 

elements:

● Where a water column translation of the fish tissue criterion has been developed, or 

site-specific data to do so are readily available, include a numeric water qualitybased limit.

● Where a water column translation or site-specific data are not available and the 

permit writer determines that a numeric limit is infeasible to calculate:

o Require the permittee to implement an MMP tailored to the facility’s potential 

to discharge mercury. Depending on the particular facts, the permitting 

authority may include in the MMP a trigger level, reduction goal, or 

enforceable numeric level to further manage mercury discharges.

o Require effluent monitoring using a sufficiently sensitive EPA-approved 

method to enable evaluation of the effectiveness and implementation of the 

MMP. (See sections 7.4 and 7.5.1.1 for more information on sufficiently 

sensitive methods.)

o Include a reopener clause to modify the permit conditions if the MMP is not 

found to be effective or if a water column translation of the fish tissue 

criterion is developed.

Other considerations and requirements may be necessary in developing permits:

● Where a discharger undertakes an activity that could increase mercury loading to 

the receiving water, it must be consistent with applicable antidegradation 

requirements. Additional requirements may also be necessary under the CWA and 

EPA’s NPDES regulations.

● The permitting authority would need to include appropriate technology-based 

limits pursuant to CWA section 301(b) and 40 CFR sections 125.3 and 

122.44(a)(1) .
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115

● For modified or reissued permits with existing effluent limits for mercury, any less 

stringent effluent limit must be consistent with anti-backsliding requirements (see 

section 7.2.4).

7.5.2.2 What does EPA recommend where direct water inputs are 

relatively high?

This section describes EPA’s recommendations where direct water inputs of mercury are 

relatively high. In this section, EPA discusses the recently developed “5m” listing 

approach for waters impaired by mercury from primarily atmospheric sources, as well as 

approaches for developing TMDLs, analyses of sources and loading capacity similar to 

what would be provided in a TMDL, or water column translations of the fish tissue 

criterion, to serve as the basis for permit limits.

As described in section 6.2, EPA recently developed an optional voluntary approach for 

deferring TMDL development for waters impaired by mercury predominantly from 

atmospheric sources pursuant to CWA section 303(d). Under this approach, states with 

comprehensive mercury reduction programs may consider waters appropriate for 

inclusion in a subcategory of their impaired waters lists (category 5m under the 

Integrated Report Guidance) and defer the development of TMDLs for those waters.

EPA’s 5m guidance states that in deciding on the scope of waterbodies proposed for 

subcategory 5m, a contribution for states to consider would be approximately 90 to 95

percent of the loadings or higher from air deposition to the waterbody; the specific 

percent may vary, however. A full description of the 5m approach is at 

http://www.epa.gov/owow/tmdl/mercury5m/.

In watersheds where direct water inputs (mercury from point sources and nonpoint 

sources other than air deposition) represent a relatively high contribution of mercury, 

EPA recommends that states and authorized tribes specifically consider developing 

numeric permit limits for mercury dischargers to these waterbodies. States and authorized 

tribes may develop TMDLs for these waterbodies in the short term to provide important 

information for developing appropriate permit limits. Where a state or authorized tribe 

chooses not to develop a TMDL in the short term for such a waterbody, EPA 

recommends that the state or tribe develop an analysis of sources and loading capacity 

similar to what would be provided in a TMDL or a water column translation of the fish 

tissue criterion using the methods outlined in 3.1.3.1. Consistent with the 5m approach

for establishing priorities for mercury TMDL development, in deciding whether there is a 

relatively high contribution from direct water inputs, a contribution for states to consider 

would be approximately 5 to 10 percent or more of mercury loadings from direct water 

inputs, taking into account that the specific percent may vary by state. At the same time, 

states may consider other factors, such as the complexity of the TMDL, in determining 

schedules for developing TMDLs.

Cumulative loads from point sources and localized nonpoint sources such as abandoned 

mines, contaminated sediments, and naturally occurring sources can potentially combine 

to cause localized mercury impairment. These situations are more complicated because 

the specific location and magnitude of each source could significantly affect fish tissue 

concentrations. In these situations, a TMDL provides the best basis for developing the 

appropriate permit limits.
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Once EPA has approved or established a TMDL containing a wasteload allocation for the 

discharge of mercury (and methylmercury where appropriate), the permitting authority 

develops a WQBEL for a point source discharge that is consistent with the requirements 

and assumptions of the wasteload allocation in the TMDL (see 40 CFR 

122.44(d)(1)(vii)(B)). In addition to developing a WQBEL, the permitting authority

specifies monitoring requirements for the WQBEL (see 40 CFR 122.44(i) and 122.48). 

EPA recommends that permitting authorities require the permittee to use a sufficiently 

sensitive EPA-approved method for monitoring purposes.

In such watersheds where direct water inputs represent a relatively high mercury loading, 

EPA recommends that the permitting authority and the mercury dischargers in the 

watershed work together to collect the data necessary to develop a TMDL, an analysis of 

sources and loading capacity similar to what would be provided in a TMDL, or a water 

column translation of the fish tissue criterion. One approach for collecting information 

for a source analysis described above or a water column translation of the fish tissue 

criterion is for the permitting authority to invoke its authority under CWA section 308 

(state permitting authorities would use comparable state authorities) to require NPDES 

facilities to collect information necessary for the development of NPDES permit limits. 

In the absence of a final TMDL, EPA recommends that a permitting authority conduct an 

analysis of sources and loading capacity similar to what would be provided in a TMDL. 

Such an analysis that applied factors similar to those considered in a TMDL could be 

included in the fact sheet of the draft permit as a justification for the effluent limit being 

as stringent as necessary to attain the water quality standard. The permitting authority 

may also use a water column translation of the fish tissue criterion to derive numeric 

permit limits if such a translation or site-specific data to translate are available.

A water column translation of the fish tissue criterion may not always be necessary in 

developing a TMDL or an analysis of sources and loading capacity similar to what a 

TMDL would provide. For example, section 6.2.2.2.1 of this guidance provides

descriptions of TMDLs that have been developed using steady state models and the 

proportionality approach.

Since permitting authorities need to establish and maintain WQBELs as stringent as 

necessary to meet water quality standards, if a state or tribe has yet to complete the 

transition from an existing water column criterion to a fish tissue-based criterion, states 

may consider retaining their existing water column criteria until translators are 

developed. Alternatively, until a translator is available, EPA recommends that one of the 

approaches outlined in this document for relating a concentration of methylmercury in 

fish tissue to a concentration of methylmercury in ambient water be considered (see 

section 3.1.3.1.)

7.5.2.3 What additional requirements may apply?

Activities that could increase mercury loadings to a receiving waterbody

Permits for sources that are seeking authorization to increase their discharge of mercury 

(or commence the discharge of mercury) must be consistent with applicable 

antidegradation requirements. See discussions of antidegradation elsewhere in this 

chapter, including sections 7.2.3 and 7.5.1.2.2.
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The permitting authority may consider whether an offset of such discharges by other 

pollutant source reductions would support the development of a WQBEL that would 

ensure that the level of water quality to be achieved by such effluent limitation is derived 

from and complies with the water quality standards, as required by 40 CFR 

122.44(d)(1)(vii)(A) and any other applicable NPDES regulations.

Pretreatment

A POTW is required to prohibit discharges from industrial users in amounts that result in 

or cause a violation of any requirement of the POTW’s NPDES permit (see 40 CFR 

403.2(a) and (b), 403.3(i) and 403.3(n)). A POTW that accepts mercury in its collection 

systems may need to ensure that its pretreatment program prevents its effluent from 

contributing to exceedance of the fish tissue criterion. The general pretreatment 

regulations (at 40 CFR part 403) require that each POTW, or combination of POTWs 

operated by the same water authority, with a design flow of 5.0 million gallons per day 

(MGD) or more develop an approved pretreatment program that protects against passthrough and interference, which may be caused by industrial discharges to the treatment 

facilities, by developing local limits for mercury and other pollutants or demonstrating

that limits are not necessary for these pollutants. The POTW is also required to prohibit 

discharges from industrial users in amounts that result in or cause a violation of any 

requirement of the POTW’s NPDES permit (see 403.2(a) and (b), 403.3(i) and 403.3(n)).

Federal categorical pretreatment standards, which are applicable to certain classes of 

industries, establish technology-based minimum pretreatment standards. The categorical 

standards, however, do not address POTW-specific problems that may arise from 

discharges by categorically regulated industries. In addition, many types of industries that 

discharge significant quantities of pollutants are not regulated by the categorical 

standards. Hence, there is a need for many POTWs to establish site-specific discharge 

limits to protect the treatment facilities, receiving water quality, and worker health and 

safety and to allow for the beneficial use of sludge.

Technology-based limits

When developing effluent limits for an NPDES permit, a permit writer must impose 

limits based on the technology available to treat mercury (technology-based limits) as a 

minimum level of control, as required by CWA section 301(b) and 40 CFR sections 

125.3 and 122.44(a)(1). There are two general approaches for developing technologybased effluent limits for industrial facilities: national effluent limitation guidelines 

(ELGs) and best professional judgment (BPJ) on a case-by-case basis (in the absence of 

ELGs). Technology-based effluent limits for municipal facilities (POTWs) are derived 

from secondary treatment standards.

Anti-backsliding

Where a facility has a currently effective effluent limit for mercury and seeks a less 

stringent limit, the permitting authority must also comply with anti-backsliding

requirements (see CWA section 402(o) and 40 CFR 122.44(l); see also CWA section 

303(d)(4)). These requirements are described in EPA’s NPDES Permit Writers’ Manual

(USEPA 1996b).
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Permit documentation

Documentation is an important part of the permit development process. The NPDES

permit fact sheet should provide an explanation of how the limit proposed in the 

associated draft permit is as stringent as necessary to achieve water quality standards

(40 CFR 124.8 and 124.56). The recommendations in this guidance could be applied on a 

permit-by-permit basis, where appropriate, to support effluent limitations and other 

conditions that satisfy CWA section 301(b)(1)(C) and 40 CFR 122.44(d)(1) with respect 

to mercury.

7.5.2.4 Mercury minimization plans

EPA recommends that the permit contain a special condition requiring the permittee to 

implement an MMP that includes effluent monitoring using a sufficiently sensitive EPAapproved method (see sections 7.4 and 7.5.1.1 for information on sufficiently sensitive 

methods), with the expectation that effluent monitoring will allow for evaluation of the 

effectiveness and implementation of the plan. The MMP would be included in the permit 

in addition to a numeric WQBEL in cases where a TMDL, a water column translation of 

the fish tissue criterion, or other water concentration criterion is available at the time of 

issuance. If neither a TMDL nor a water column translation (or other water criterion) is 

available at the time of permit issuance, however, the MMP would be included in the 

permit as part of a narrative WQBEL in lieu of a numeric WQBEL. EPA believes that, 

depending on the particular facts, a permit writer may reasonably conclude that such 

MMPs are as stringent as necessary to achieve water quality standards, for the reasons 

discussed below.

EPA believes that mercury reductions achieved through implementing MMPs tailored to 

the facility’s potential to discharge mercury could result in important reductions in 

mercury loadings. EPA’s basis for this conclusion is its study of pollutant minimization 

programs and their success in reducing mercury loadings to the environment. The reports

Mercury Study Report to Congress (USEPA 1997c) and draft Overview of P2 

Approaches at POTWs (USEPA 1999b) show that POTWs and industrial dischargers 

have implemented source controls, product substitution, process modification, and public 

education programs with great success. These minimization practices focus on sources 

and wastes that originate with and are under the reasonable control of a facility, not on 

pollutants in rainwater or source water.

As an example, POTWs can educate the public to prevent pollution by avoiding 

household products that contain high levels of mercury or substituting for those products 

ones that are mercury-free or more environmentally friendly. The most cost-effective 

approach for POTWs to substantially reduce mercury discharges appears to be pollution 

prevention and waste minimization programs that focus on high-concentration, highvolume discharges to the collection system, with considerable effort also directed at highconcentration, low-volume discharges such as those from medical and dental facilities.

Using pollutant minimization or prevention programs can also reduce the transfer from 

wastewater to other media through disposal of mercury-containing sludge from which 

mercury may subsequently reenter the environment. For example, mercury removed at a 

POTW through treatment is likely to reenter the environment through POTW sludges that 

are then incinerated or applied to land (although some is captured by air emission
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controls on incineration). EPA believes that a better approach for reducing mercury 

releases to the environment is to prevent mercury from entering the wastewater collection 

system at the source through product substitution, waste minimization or process 

modification, or removing and recycling mercury at the source (source controls) using 

state-of-the-art technology. These measures aimed at reducing influent loads to POTWs 

also reduce the use of mercury in the community, which could reduce the amount of 

mercury entering the environment through other media or sources. (For example,

products that contain low levels of mercury may be disposed of as a nonhazardous solid 

waste and incinerated, releasing mercury to the air.) Where pollution prevention

approaches have been implemented, substantial reductions in mercury concentrations in 

POTW influents, sludges, and effluents have been achieved. For a discussion of this 

approach, see the draft Overview of P2 Approaches at POTWs (USEPA 1999a). For an 

example of guidance on developing an MMP, see the EPA Region 5 final document 

Mercury Pollutant Minimization Program Guidance, dated November 2004 

(http://www.epa.gov/region5/water/npdestek/mercury_pmp_nov_04_guidance.pdf). 

Many of the recommendations contained in the document are drawn from existing 

guidance and practice of state permitting authorities in EPA’s Regional Office in 

Chicago. See also the City of Superior’s document, Mercury Pollutant Minimization 

Program Guidance Manual for Municipalities, at http://www.ci.superior.wi.us/

index.asp?NID=129, and EPA’s Local Limits Development Guidance (USEPA 2004) at 

http://www.epa.gov/npdes/pubs/final_local_limits_guidance.pdf.

Finally, as explained in section 2.1.1, mercury is a bioaccumulative, persistent pollutant

that can cause adverse health effects. Given this fact, EPA believes that point sources that 

can cost-effectively reduce their mercury discharges should do so. The fact that air 

sources or historical contamination are likely dominant causes of impairment does not 

mean that point sources should not implement cost-effective, feasible pollution 

prevention measures to reduce their contribution of mercury to the environment, however 

small those contributions may be. In short, EPA believes that it is reasonable to expect 

NPDES permittees to implement cost-effective, feasible, and achievable measures to 

reduce the amount of mercury they discharge into the environment and that, depending 

on the particular facts, permit writers may reasonably conclude that permit limits that 

require such measures derive from and comply with water quality standards as required 

by EPA regulations at 40 CFR 122.44(d)(1)(vii)(A).

In cases where a permittee believes it may have reasonable potential, EPA recommends 

that the permittee provide information that the permitting authority can use in developing 

appropriate permit conditions and would encourage the permittee to provide a draft 

MMP. Alternatively, where a draft MMP is not initially submitted by the permittee, the 

permitting authority may request that the permittee provide a draft MMP. The permitting 

authority retains the final responsibility for determining reasonable potential, and for 

incorporating the appropriate permit conditions, including an effective MMP and its 

implementation, in the permit.

Developing an MMP need not be an intensive or burdensome activity. The content of an 

MMP should be determined on a case-by-case basis and tailored to the individual 

facility’s potential to discharge mercury and implement reasonable controls. The MMP

could be as little as one or two pages or as much as a major engineering study. Table 6
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contains suggestions for the content of an MMP based on the type of facility. Of course, 

MMPs should vary in their level of detail and degree of stringency on the basis of sitespecific factors and the degree to which the facility has the ability to reduce 

environmental releases of mercury. For example, if the mercury analysis performed for 

the permit application shows a much higher concentration than would be expected for the 

type of facility, further investigation would be appropriate and could lead to increased 

requirements. On the other hand, EPA recognizes that MMPs may not be effective in 

certain cases such as when an increase in the discharge of mercury may be due to the 

presence in stormwater or input process water that does not originate with and is not 

under the reasonable control of a facility.

Table 6. Suggested content for MMPs based on the type of facility

Type of facility Suggested content

Publicly (or privately) owned treatment 

works serving a purely residential area. 

No dental or medical offices or hospitals. 

No industrial users.

Recommended distribution of outreach materials on 

fish-consumption advisories and properly disposing of 

mercury-containing products.

POTW whose service area contains 

dental offices.

Recommend or require that dental offices follow 

American Dental Association BMPs.

a

Collect any bulk 

mercury in the offices. Develop an approach for using

amalgam separators.

POTW whose service area contains one 

or more hospitals.

Recommend or require that hospitals follow the 

practices recommended by the American Hospital 

Association.b

POTW whose service area contains 

schools or medical offices.

Recommend or require that schools and medical 

offices properly dispose of bulk mercury in their 

possession (including, for example, mercury-containing 

sphygmomanometers).

Industrial direct or indirect dischargers 

that use mercury as an intentional 

component of their process or recover 

mercury as a by-product of their process.

Generally, such a case would involve a thorough 

analysis of opportunities to reduce their releases of 

mercury.

Industrial direct or indirect dischargers 

that do not use mercury as an intentional 

component of their process and do not 

recover mercury as a by-product of their 

process.

Such facilities should investigate opportunities to 

reduce their incidental releases of mercury such as 

recycling fluorescent lamps, switches, thermostats, etc.

and replacing them with low-mercury or non-mercury 

products.

Notes:

a

For more information on the American Dental Association BMPs, see Best Management Practices for 

Amalgam Waste (September 2005) at http://www.ada.org/prof/resources/topics/

topics_amalgamwaste.pdf.

b

For more information on American Hospital Association practices, see Replacing Mercury in Healthcare 

Facilities––A Step-by-Step Approach at http://www.h2e-online.org/hazmat/mercguide.html.

If a permittee has several of the types of sources listed in table 6, each of these sources 

should be considered in developing an appropriate MMP. For example, if the service area 

of a POTW contains dental offices and medical facilities, the MMP should contain 

appropriate measures for both. The mercury minimization measures suggested in table 6

are expected to reduce mercury levels in the wastewater discharge as well as other waste 

streams and media. Most of the mercury discharged to POTWs, for example, ends up in 

biosolids that may be incinerated or disposed on the land, thus contributing to the overall 
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mercury burden in the environment. In addition, any measures that reduce releases to the 

atmosphere should be encouraged.

When developing MMPs, EPA recommends beginning with any existing best 

management plans and spill prevention and containment control plans for that facility. 

Many of the activities covered by those plans can also reduce mercury sources to 

wastewater. After reviewing many pollutant minimization programs, EPA recommends 

that a plan include at least the following elements:

● Identification and evaluation of current and potential mercury sources

● For POTWs, identification of both large industrial sources and other commercial or 

residential sources that could contribute large mercury loads to the POTW

● Monitoring to confirm current or potential sources of mercury

● Identification of potential methods for reducing or eliminating mercury, including 

requiring BMPs or assigning limits to all potential sources of mercury to a 

collection system, material substitution, material recovery, spill control and 

collection, waste recycling, process modifications, housekeeping and laboratory 

use and disposal practices, and public education

● Implementation of appropriate minimization measures identified in the plan

● Effluent monitoring to verify the effectiveness of pollution minimization efforts

EPA believes that these minimum permit conditions may be appropriate because they 

help to ensure that the discharge does not cause or contribute to an exceedance of water 

quality standards to protect against possible localized impacts and to minimize the 

discharge of mercury. EPA also believes that, depending on the particular facts, a permit 

writer may reasonably conclude that such an MMP is as stringent as necessary to achieve 

water quality standards.

To further manage mercury discharges, the permitting authority should consider 

including an effluent trigger level or reduction goal in an MMP. Such a trigger level or 

goal could be set at a level that would provide a basis for evaluating whether the mercury 

minimization measures or BMPs specified in the MMP are working as anticipated. The 

level or goal could be expressed numerically or in narrative form. For example, the MMP 

might provide a trigger level equal to the existing effluent quality that, if exceeded, would 

indicate that mercury minimization measures may not be effective. Alternately, the MMP 

might provide goals for mercury reductions that are expected to occur as a result of the 

implementation of mercury minimization efforts specified in the MMP. As explained in 

this section and in section 7.5.2.1, an MMP includes a set of BMPs that would be part of 

an enforceable special condition of the permit. The MMP might specify that exceeding a 

trigger level or failing to achieve a mercury reduction goal would prompt actions such as 

reevaluation of the MMP, additional monitoring, or the implementation of additional

BMPs. In this case, the failure of the permittee to undertake the additional actions

identified in the MMP would be a violation of the permit special condition.

Even where it is infeasible to calculate a numeric WQBEL (for the reasons discussed in 

section 7.5.2.1), a permitting authority should consider including in the MMP an
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enforceable numeric level on the discharge of mercury. In this case, the enforceable 

numeric level would not constitute a stand-alone water quality-based effluent limit, but 

rather, a baseline for achieving mercury reductions that, combined with the other 

measures and practices in the MMP, would together constitute the water quality-based 

effluent limit. Such an enforceable numeric level could represent either existing effluent 

quality or a level representing some increment of the mercury reduction determined 

achievable as a result of the measures and practices specified in the MMP. Depending on 

the particular facts, the permit writer may reasonably conclude that the enforceable 

numeric level combined with the other measures and practices in the MMP will result in 

a level of mercury discharge that is controlled as stringently as necessary to meet water 

quality standards. Where the MMP contains an enforceable numeric level for mercury 

and/or methylmercury in the effluent, exceeding that value would be a violation of the 

permit special condition.

The permitting authorities should consider use of effluent trigger levels, effluent 

reduction goals, and enforceable numeric levels in any discharge permits that are based 

on MMPs as water quality-based effluent limits. EPA recommends that permitting 

authorities include such levels or goals in permits where direct water inputs are relatively 

high.
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8 Related Programs

8.1 What are EPA and others doing as a whole to address 

mercury?

A wide variety of actions are under way in the United States and internationally to 

address mercury contamination. EPA’s mercury Web site, at http://www.epa.gov/

mercury, provides a broad range of information about mercury: actions by EPA and 

others, including international actions, effects on people and the environment, and how 

people can protect themselves and their families.

With respect to EPA’s actions, on July 5, 2006, EPA issued a report titled EPA’s 

Roadmap for Mercury (“Roadmap”). It is at http://www.epa.gov/mercury/roadmap.htm. 

EPA’s Roadmap describes the Agency’s progress to date in addressing mercury issues 

domestically and internationally, and it outlines EPA’s major ongoing and planned 

actions to address risks associated with mercury. The Roadmap describes the Agency’s 

most important actions to reduce both mercury releases and human exposure to mercury. 

Creating the Roadmap has enabled EPA to maximize coordination of its many diverse 

efforts, with the goal of improving its mercury program. In addition to providing a 

roadmap for EPA, the report provides important information about mercury to other 

federal agencies; to EPA’s partners in state, tribal, and local governments; and to the 

public.

8.2 How does pollution prevention play a role in the 

methylmercury criterion?

Under the national pretreatment program, POTWs routinely control the volume and 

concentration of pollutants contributed by significant industrial users (SIUs)26 to their 

collection system and wastewater treatment plant. However, as water quality criteria, 

sludge standards, and air emissions standards become more restrictive, even low levels of 

pollutants like mercury might cause noncompliance with these standards. Therefore, 

POTWs must expand pollutant control efforts or install treatment technologies to remove 

the problem pollutants.

In many cases, large-scale treatment technology is either not yet available or not 

economically feasible for controlling mercury at POTWs. Instead, POTWs are choosing 

to develop and implement pollution prevention (P2) strategies to reduce the amount of 

mercury received by the wastewater treatment plant. Although SIUs can contribute a 

significant mercury load to the treatment plant, non-SIU sources can also be identified as 

causing or contributing to the problem. For example, the Western Lake Superior Sanitary

26 EPA defines an SIU as (1) any industrial user (IU) subject to a categorical pretreatment standard (national effluent guidelines); (2) any 

user that discharges an average of 25,000 gallons per day or more of process wastewater or that contributes a process waste stream making 

up 5 percent or more of the average dry weather hydraulic or organic capacity of the POTW treatment plant; or (3) any other user 

designated by the Control Authority (POTW) to be an SIU on the basis that it has a reasonable potential for adversely affecting the 

POTW’s operation or for violating a pretreatment standard or requirement (40 CFR 403.4(v)).
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District (WLSSD) determined that one SIU and many small non-SIUs (dental facilities) 

contribute a major portion of the mercury in its wastewater. Sectors historically more 

difficult to control (e.g., residential) or beyond the POTW’s direct control (e.g., pollutants 

in contaminated inflow/rainfall) can also contribute substantial loadings.

Effective mercury source reduction relies on the POTW’s effectively communicating to 

sector entities that minimal individual efforts can collectively reduce the mercury loading 

to the environment. Forming partnerships and working with sector representatives to 

investigate mercury sources, explore alternatives, and assist in implementing selected 

options is integral to a successful reduction strategy. Permitting authorities developing a 

P2 plan should consider a POTW’s role in compliance assistance. The sections below 

provide summary-level guidance for developing a POTW P2 plan.

Through the pretreatment program, POTWs should communicate with their permitting 

authority, as well as maintain close contact with local sewer dischargers and have a good 

understanding of specific industrial process operations. Thus, they can uniquely promote 

P2 to numerous facilities and provide public awareness and education. In general, the 

success of a POTW P2 effort depends on a behavioral change on the part of the POTW 

and the community. As noted by the City of Palo Alto, “Experience shows that people are 

more likely to change their behaviors if they fully understand environmental problems 

and the range of possible solutions, if they have participated in the process leading to a 

policy decision, and if they believe regulators are dealing with them in good faith....”

(City of Palo Alto 1996). A POTW might minimize community resistance and apathy by 

undertaking the following activities prior to developing its plan:

● Conduct a preliminary investigation of the problem and potential sources. Verify 

that the problem is not a wastewater treatment plant operational issue. Identify 

internal sources and any area government facilities in addition to industrial, 

commercial, and uncontrollable sources that could be contributing to or causing the 

problem.

● Meet with upper management (e.g., utility director, mayor, council) and discuss the 

problem, preliminary findings, and potential ramifications. Upper management 

support will be essential for obtaining necessary resources, funding, equipment, 

and authority for implementing a P2 plan. Their support will also be necessary for 

resolving any wastewater treatment plant and government facility issues. Upper 

management may also advise development of a POTW mission statement that 

declares goals and the chosen approach. Exhibit 1 provides an example of the 

WLSSD mission statement (WLSSD 1997).

● Establish a workgroup composed of representatives from government, industry, 

community, and environmental organizations, preferably those that are familiar 

with P2 strategies or with the pollutant of concern. The workgroup likely will 

develop or help develop the plan, guide plan implementation, and measure plan 

success. Therefore, findings from the preliminary investigation will guide the 

POTW to select appropriate committee members and experts. Bear in mind that the 

workgroup size should ensure representation of most interests but not grow so large 

as to be counterproductive. This group could also prove valuable in disseminating 

information.
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With the support and expertise needed, the POTW and 

workgroup can draft a plan by doing the following:

● State the problem to provide background information 

about the POTW, problems caused by mercury, and 

why the POTW is taking action (described in terms that

most people can understand).

● Identify the goals to determine whether the POTW 

intends to help minimize mercury introduced to all 

environmental media (air, water, solid waste), known as 

“front-end” P2 or merely to minimize the amount of 

mercury discharged to the wastewater treatment plant.

The latter option ignores mercury transfers to other 

media (e.g., air, solid waste) and is the less 

environmentally sound option. It may be essential for 

the POTW to implement a front-end P2 approach and 

establish waste collection programs for the proper 

recycling or disposal of mercury-bearing wastes (e.g., 

thermometers, fluorescent light bulbs).

● Define an approach that outlines the sectors selected for 

P2 efforts, the criteria for targeting efforts (e.g., size of 

the source loading, authority available to control the 

source or sector, time necessary to produce desired results), where efforts will be 

voluntary or mandatory, who will execute the various program efforts, and how the 

POTW will proceed where mercury introduction is beyond its control (e.g., 

contaminated stormwater).

● Identify resources necessary to implement the plan such as staffing, equipment, and 

funding.

● Create contingency plans that describe actions to be taken if the planned efforts do 

not succeed, such as obtaining the authority to mandate and enforce P2 or other 

source control requirements or installing wastewater treatment plant technology.

Plans might develop in response to a specific problem (e.g., elevated mercury levels in 

wastewater treatment plant effluent) or proactively to minimize potential problems. Plans 

will vary in complexity and in resources necessary to achieve goals. Plan updates should 

detail successful and failed efforts, such as in the form of lessons learned.

8.3 What regulations has EPA issued pursuant to the 

CAA to address air emissions of mercury?

As rules and standards pursuant to the CAA have been developed, proposed, and 

promulgated since the Amendments of 1990, compliance by emitting sources and actions 

taken voluntarily have already begun to reduce mercury emissions to the air across the

country. EPA expects that a combination of ongoing activities will continue to reduce 

such emissions over the next decade.

Exhibit 1. Example Mission Statement

The WLSSD Commitment to Zero Discharge

The WLSSD as a discharger to Lake Superior is 

committed to the goal of zero discharge of 

persistent toxic substances and will establish 

programs to make continuous progress toward 

that goal. The District recognizes step-wise 

progress is only possible when pollution 

prevention strategies are adopted and 

rigorously pursued. These approaches will focus 

upon our discharge as well as indirect sources.

WLSSD will work with its users to implement 

programs, practices, and policies which will 

support the goal. We will call upon the 

resources and assistance of the State and 

federal governments for support, including 

financial support of the programs to ensure that 

our users are not penalized unfairly.

WLSSD recognizes that airborne and other 

indirect sources beyond District control must be 

addressed in order for significant reductions to 

occur.
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EPA has made substantial progress in addressing mercury air emissions under the CAA. In 

particular, EPA has issued regulations addressing the major contributors of mercury to the 

air (including, for example, municipal waste combustors; hospital, medical, and infectious

waste incinerators; chlor-alkali plants; and hazardous waste combustors). EPA issued 

regulations for these source categories under different sections of the CAA, including 

sections 111, 112, and 129. Indeed, as the result of EPA’s regulatory efforts, the United 

States achieved a 45 percent reduction in domestic mercury air emissions between 1990 

and 1999 (see figure 4 and http://www.epa.gov/ttn/chief/trends/index.html).

The relevant regulations that EPA has issued to date under the CAA are described briefly 

below. For more information about other CAA actions to control mercury, see 

http://www.epa.gov/mercury under “What EPA and Others Are Doing.”

8.3.1 Municipal waste combustors

In 1995 EPA promulgated final regulations that apply to all new and existing waste-toenergy plants and incinerators with the capacity to burn more than 250 tons of municipal 

solid waste, including garbage, per day (see 60 FR 65,415 [December 19, 1995], codified 

at 40 CFR part 60, subparts Ea and Eb). These regulations cover approximately 130 

existing waste-to-energy plants and incinerators, as well as any new plants and 

incinerators built in the future. The regulations have reduced emissions of a number of 

HAPs, including mercury, by approximately 145,000 tons per year. The regulations have 

resulted in about a 90 percent reduction in mercury emissions from domestic municipal 

waste combustors from 1990 emission levels (see figure 4: 56.7 tons per year of mercury 

emitted from domestic municipal waste combustors in 1990 versus 4.9 tons per year in 

1999).

8.3.2 Hospital, medical, and infectious waste incinerators

Hospital/medical/infectious waste incinerators (HMIWIs) are used by hospitals, health 

care facilities, and commercial waste disposal companies to dispose of hospital waste 

and/or medical/infectious waste. EPA adopted regulations controlling mercury emissions 

from HMIWIs on September 15, 1997 (62 FR 48,348, codified at 40 CFR part 60, 

subparts Ce and Ec). EPA estimated that the regulations would reduce mercury emissions 

from HMIWIs at existing facilities by 93–95 percent, and all existing HMIWIs were

required to comply with the regulations by September 15, 2002 (see figure 4: 49.7 tons 

per year of mercury emitted from domestic HMIWIs in 1990 versus 1.6 tons per year in 

1999). In fact, the actual mercury emission reductions achieved as a result of 

implementing the regulations were approximately 99 percent. At the time the regulations 

were issued, EPA expected that 50 percent to 80 percent of the 2,400 then-existing 

HMIWIs would close in response to the rule. EPA’s rule resulted in a significant change 

in medical waste disposal practices in the United States. Because of the increased cost of 

on-site incineration under the final rule, few health care facilities are likely to install new 

HMIWIs and approximately 97 percent of the 2,400 HMIWIs operating at health care 

facilities in 1997 have shut down. Instead, the facilities have switched to other methods 

of waste disposal, such as off-site commercial waste disposal. There are currently 57

existing HMIWIs at 52 facilities. EPA expected the standards to apply to between 10 and

70 new HMIWIs, most of which would employ mercury control technology by the 
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compliance deadline; only 4 new HMIWI at 3 facilities began operating following the 

1997 rulemaking.

8.3.3 Chlor-alkali plants

On December 19, 2003, EPA issued final regulations to reduce mercury emissions from 

chlorine production plants that rely on mercury cells (see 68 FR 70,904, codified at 40 

CFR part 63, subpart IIIII). These air regulations have reduced mercury air emissions 

from existing chlor-alkali plants by approximately 50 percent since the compliance date 

of December 19, 2006. The regulation requires a combination of controls for point 

sources, such as vents, and BMPs to address fugitive air emissions, that are more 

stringent work practices than those required by a preexisting regulation that covered this 

source category. Today, there are 8 such plants in the United States, compared to 20 

when work on the rule began, with a further decrease to 5 plants expected for 2008. In 

addition, EPA has initiated a study of fugitive mercury emissions at existing chlor-alkali 

plants.

8.3.4 Hazardous waste combustors

In 1999 EPA established standards for HAPs, including air emission standards for 

mercury, for incinerators, cement kilns, and lightweight aggregate kilns that burn 

hazardous waste under CAA section 112 (70 FR 52828, 53011; September 30, 1999).

The 1999 standards were challenged by several parties and subsequently vacated by the 

United States Court of Appeals for the District of Columbia Circuit (the Court) in 2001.

The parties to this proceeding subsequently filed a joint motion with the Court to delay 

issuance of the mandate to vacate the challenged standards to allow time for EPA to 

develop interim standards, which would replace the vacated standards temporarily, until 

EPA could promulgate standards consistent with the Court’s mandate. In 2002 EPA 

published interim standards for these three source categories (67 FR 6792 [February 13, 

2002]), which are under 40 CFR part 63, subpart EEE. The mercury standards for 

existing and new sources, respectively, are under 40 CFR 63.1203(a)(2) and (b)(2) for 

incinerators, 40 CFR 63.1204(a)(2) and (b)(2) for cement kilns, and 40 CFR 

63.1205(a)(2) and (b)(2) for lightweight aggregate kilns. Affected sources were required 

to comply with the interim standards by September 30, 2003.

In 2005 EPA published replacement standards (to replace the 2002 interim standards) for 

hazardous waste-burning incinerators, cement kilns, and lightweight aggregate kilns. At 

the same time, EPA also finalized emission standards for other types of hazardous waste 

combustors, including liquid fuel boilers, solid fuel boilers, and hydrochloric acid 

production furnaces (70 FR 59402 [October 12, 2005]). The mercury standards for 

existing and new sources, respectively, are under 40 CFR 63.1216(a)(2) and (b)(2) for 

solid fuel boilers, 40 CFR 63.1217(a)(2) and (b)(2) for liquid fuel boilers, 40 CFR 

63.1218(a)(2) and (b)(2) for hydrochloric acid production furnaces, 40 CFR 

63.1219(a)(2) and (b)(2) for incinerators, 40 CFR 63.1220(a)(2) and (b)(2) for cement 

kilns, and 40 CFR 63.1221(a)(2) and (b)(2) for lightweight aggregate kilns. Sources are 

required to comply with these emission standards by October 14, 2008.
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8.3.5 Coal-fired power plants

At present, the largest single source of anthropogenic mercury emissions in the country is 

coal-fired power plants. Mercury emissions from U.S. power plants are estimated to 

account for about one percent of total global mercury emissions (70 FR 15994; March 29, 

2005). In May 2005, EPA adopted the Clean Air Mercury Rule (CAMR) to regulate 

mercury emissions from utilities. On February 8, 2008, the D.C. Circuit Court of Appeals 

vacated the CAMR and remanded portions of it to EPA. For information on current 

activities related to control of power plant emissions, see EPA’s mercury Web site, at 

http://www.epa.gov/mercury.

8.3.6 Other

In addition to EPA’s regulatory efforts under the CAA, in 1996 the United States

eliminated the use of mercury in most batteries under the Mercury Containing and 

Rechargeable Battery Management Act. This action reduces the mercury content of the 

waste stream, which further reduces mercury emissions from waste combustion. In 

addition, voluntary measures to reduce use of mercury-containing products, such as the 

voluntary measures to which the American Hospital Association has committed, will 

contribute to reduced emissions from waste combustion.
References

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Water. <http://www.epa.gov/waterscience/humanhealth/method/tsdvol2.pdf>

USEPA (U.S. Environmental Protection Agency). 2004a. Proposed National Emission 

Standards for Hazardous Air Pollutants; and, in the Alternative, Proposed 

Standards for Performance for New and Existing Stationary Sources, Electric Utility 

Steam Generating Units: Notice of Data Availability. Washington, DC: U.S. 

Environmental Protection Agency, Office of Air and Radiation. Fed. Regist., 

December 1, 2004, 69:69864. <http://www.epa.gov/ttn/atw/utility/fr01de04.pdf>

USEPA (U.S. Environmental Protection Agency). 2004. Local Limits Development 

Guidance. EPA-833-R-04-002A. U.S. Environmental Protection Agency, Office of 

Wastewater Management. July 2004. <http://www.epa.gov/npdes/pubs/

final_local_limits_guidance.pdf >

USEPA (U.S. Environmental Protection Agency). 2005a. Regulatory Impact Analysis of 

the Clean Air Mercury Rule. Final report. EPA-452/R-05-003. Research Triangle 

Park, NC: U.S. Environmental Protection Agency, Office of Air Quality Planning 

and Standards, Air Quality Strategies and Standards Division.

<http://www.epa.gov/ttn/atw/utility/ria_final.pdf>

27
References

143

USEPA (U.S. Environmental Protection Agency). 2005b. Environmental Monitoring and 

Assessment Project (EMAP) Western Streams and Rivers Statistical Study. EPA620-R-05-006. Washington, DC: U.S. Environmental Protection Agency, Office of 

Research and Development.

USEPA (U.S. Environmental Protection Agency). 2005c. Technical Support Document, 

Revision of December 2000 Regulatory Finding on the Emissions of Hazardous Air 

Pollutants From Electric Utility Steam Generating Units and the Removal of Coaland Oil-Fired Electric Utility Steam Generating Units from the Section 112(c) List: 

Reconsideration, October 21, 2005.

USEPA (U.S. Environmental Protection Agency). 2005d. Technical Support Document 

for the Final Clean Air Mercury Rule: Air Quality Modeling. Research Triangle 

Park, NC: U.S. Environmental Protection Agency, Office of Air Quality Planning 

and Standards. <http://www.epa.gov/ttn/atw/utility/aqm_oar-2002-0056-6130.pdf>

28

USEPA (U.S. Environmental Protection Agency). 2005e. Method 245.7, Revision 2.0.

Mercury in Water by Cold Vapor Atomic Fluorescence Spectrometry. EPA-821-R05-001. Washington, DC: U.S. Environmental Protection Agency.

USEPA (U.S. Environmental Protection Agency). 2005f. Standards of Performance for 

New and Existing Stationary Sources: Electric Utility Steam Generating Units. U.S. 

Environmental Protection Agency. Fed. Regist., May 18, 2005, 70:28606.

USEPA (U.S. Environmental Protection Agency). 2005g. Method 245.7, Revision 2.0.

Mercury in Water by Cold Vapor Atomic Fluorescence Spectrometry. EPA-821-R05-001. Washington, DC: U.S. Environmental Protection Agency.

USEPA (U.S. Environmental Protection Agency). 2005h. Presentation for the Meeting 

with National Mining Association, Robert J. Wayland, Ph.D., Office of Air Quality 

Planning and Standards, September 28, 2005. EPA-HQ-OAR-2002-0056-6447.

USEPA (U.S. Environmental Protection Agency). 2007a. 2005/2006 National Listing of 

Fish Advisories. EPA-823-F-07-003. Washington, DC: U.S. Environmental 

Protection Agency, Office of Water. <http://www.epa.gov/waterscience/fish/

advisories/2006/tech.html#mercury.pdf>

USEPA (U.S. Environmental Protection Agency). 2007b. Listing Waters Impaired by 

Atmospheric Mercury Under Clean Water Act Section 303(d): Voluntary 

Subcategory 5m for States with Comprehensive Mercury Reduction Programs, Craig 

Hooks, March 8, 2007. <http://www.epa.gov/owow/tmdl/mercury5m/>

27 On February 8, 2008, the D.C. Circuit Court of Appeals vacated the Clean Air Mercury Rule and remanded portions of it to EPA, for 

reasons unrelated to the technical analyses in this document.

28 On February 8, 2008, the D.C. Circuit Court of Appeals vacated the Clean Air Mercury Rule and remanded portions of it to EPA, for 

reasons unrelated to the technical analyses in this document.
References

144

USEPA (U.S. Environmental Protection Agency). 2008. Elements of Mercury TMDLs 

Where Mercury Loadings Are Predominantly from Air Deposition, Craig E. Hooks, 

September 29, 2008. <http://epa.gov/owow/tmdl/guidance.html#1> at

<http://www.epa.gov/owow/tmdl/pdf/cover_memo_mercury_tmdl_elements.pdf> 

and <http://www.epa.gov/owow/tmdl/pdf/document_mercury_tmdl_elements.pdf>

Voiland, M.P., K.L. Gall, D.J. Lisk, and D.B. MacNeill. 1991. Effectiveness of 

recommended fat-trimming procedures on the reduction of PCB and Mirex levels in 

Brown trout (Salmo trutta) from Lake Ontario. J. Great Lakes Res. 17(4):454–460.

Wagemann, R., E. Trebacz, R. Hunt, and G. Boila. 1997. Percent methylmercury and 

organic mercury in tissues of marine mammals and fish using different experimental 

and calculation methods. Environ. Toxicol. Chem. 16(9):1859–1866.

Watras, C.J., R.C. Back, S. Halvorsen, R.J.M. Hudson, K.A. Morrison, and S.P. Wente. 

1998. Bioaccumulation of mercury in pelagic freshwater food webs. Sci. Total 

Environ. 219(2–3):183–208.

Watras, C.J. and N.S. Bloom. 1992. Mercury and methylmercury in individual 

zooplankton: Implications for bioaccumulation. Limnol. Oceanogr. 37:1313–1318.

Watras, C.J., K.A. Morrison, J.S. Host, and N.S. Bloom. 1995. Concentration of mercury 

species in relationship to other site-specific factors in the surface waters of northern 

Wisconsin lakes. Limnol. Oceanogr. 40:556–565.

Wente, S.P. 2003. A Spatially and Temporally Variable Model of Mercury 

Concentrations in Aquatic Communities with Applications to Public Health 

Protection and Water Quality Assessment. Ph.D. thesis, Purdue University, West 

Lafayette, Indiana.

Wente, S.P. 2004. A Statistical Model and National Data Set for Partitioning Fish-Tissue 

Mercury Concentration Variation Between Spatiotemporal and Sample 

Characteristic Effects. Scientific Investigation Report 2004-5199. Reston, Virginia: 

U.S. Geological Survey.

WLSSD (Western Lake Superior Sanitary District). 1997. The WLSSD Commitment to 

Zero Discharge. Duluth, MN: Western Lake Superior Sanitary District.

<http://www.wlssd.duluth.mn.us/Zero_Discharge_Com.pdf.pdf>

Wren, C.D. and H.R. MacCrimmon. 1986. Comparative bioaccumulation of mercury in 

two adjacent freshwater ecosystems. Water Res. 6:763-769.

Yantosca, B. 2004. GEOS-CHEMv7-01-02 User’s Guide. Cambridge, MA: Atmospheric 

Chemistry Modeling Group, Harvard University.
Appendix A. Methylmercury/Mercury Ratio Exhibited in Muscle Tissue 

of Various Freshwater Fish Species

145

Appendix A. Methylmercury/Mercury Ratio Exhibited in 

Muscle Tissue of Various Freshwater Fish 

Species

Source Ecosystem type Fish species

MethylHg/

total Hg ratio

Hammerschmidt et al. 

1999

Freshwater lakes 

in Wisconsin, USA

Yellow perch (Perca flavescens) mean: 0.95

range: 0.84 to 0.97

Becker and Bigham 1995 Onondaga Lake, a 

chemically

contaminated lake 

in New York, USA

Gizzard shad (Dorosoma cepedianum)

White perch (Morone americana)

Carp (Cyprinus carpio)

Channel catfish (Ictalurus punctatus)

Bluegill (Lepomis macrochirus)

Smallmouth bass (Micropterus dolomieui)

Walleye (Stizostedion vitreum)

> 0.90

Note: Authors did not provide 

specific percentages for 

individual species.

Grieb et al. 1990 Lakes in the Upper 

Michigan 

Peninsula, USA

Yellow perch (Perca flavescens)

Northern pike (Esox lucius)

Largemouth bass (Micropterus salmoides)

White sucker (Catostomus commersoni)

0.99

Note: Authors did not provide 

data for each species 

separately—only mean value 

observed over all species.

Bloom 1992 Freshwater fish 

species collected 

from remote 

midwestern lakes 

and one mercury 

contaminated site 

USA

Yellow perch (Perca flavescens)

Northern pike (Esox lucius)

White sucker (Catostomus commersoni)

Largemouth bass (Micropterus salmoides)

0.99

1.03

0.96

0.99

Lasorsa and Allen-Gil 

1995

3 lakes in the 

Alaskan Arctic, 

USA

Arctic grayling

Lake trout

Arctic char

Whitefish

1.00 all for species

Note: Authors did not provide 

species-specific information on 

MeHg/total Hg ratio.

Kannan, et al. 1998 Estuaries in South 

Florida

Hardhead catfish (Arius felis)

White grunt (Haemulon plumieri)

Sand perch (Diplectrum formosum),

Lane snapper (Lutjanus synagris)

Gafftopsail catfish (Bagre marinus)

Pinfish (Lagodon rhomboides)

Spot (Leiostomus xanthurus)

Pigfish (Orthopristis chrysoptera)

Sand seatrout (Cynoscion arenarius)

Brown shrimp (Penaeus aztecus)

0.90

0.91

0.91

0.97

0.71

0.78

0.75

0.82

0.85

0.72

Note: Author sampled the 10

fish species at 20 locations.

Jackson 1991 Lakes and 

reservoirs in 

northern Manitoba, 

Canada

Walleye (Stizostedion vitreum)

Northern pike (Esox lucius)

Lake whitefish (Coregonus clupeaformis)

range: 0.806% to 0.877%

range: 0.824% to 0.899%

range: 0.781% to 0.923%

Note: Author sampled the 3 fish 

species at 4 lake locations.
Appendix A. Methylmercury/Mercury Ratio Exhibited in Muscle Tissue

of Various Freshwater Fish Species

146

Source Ecosystem type Fish species

MethylHg/

total Hg ratio

Wagemann et al. 1997 Sampling location 

not provided; 

presumed to be 

from Canadian 

waters

Walleye (Stizostedion vitreum) mean 1.00

Note: Authors did not provide 

more specific information.

For trophic level assignments for specific fish species, refer to tables 6-4 and 6-6 of the 

2000 Human Health BAF guidance (USEPA 2003). Additional information on trophic 

level assignments is in the appendix of that guidance (http://www.epa.gov/waterscience/

criteria/humanhealth/method/tsdvol2.pdf).
Appendix B. Tables from Methylmercury Criteria Document

147

Appendix B. Tables from Methylmercury 

Criteria Document

This appendix contains several tables taken directly from the 2001 methylmercury 

criteria document. They are repeated here to help the reader understand the development 

of the 2001 criterion.
Appendix B. Tables from Methylmercury Criteria Document

148

Table B1. Exposure parameters used in derivation of the water quality criterion.

(References cited in this table can be found in the 2001 methylmercury criterion document.)

Parameter

Population

Source

Children

(0-14 years)

Women of 

Childbearing Age

(15-44 years)

Adults in the 

General 

Population

Body Weight, kg 30 67 70 USEPA (2000f)

Drinking Water Intake, L/day 1.0 2.0 2.0 USEPA (2000f)

Freshwater/Estuarine Fish Intake, 

g/day 156.3a

165.5a17.5b,c USEPA (2000f)

Inhalation, m3/day 10.4 11 20 USEPA (1994, 1997d)

d

Soil Ingestion, g/day 0.0001, 0.01e

0.00005 0.00005 USEPA (1997d)

Mean Marine Fish Intake, g/day 74.9a

91.04a12.46b USEPA (2000a)

Median Marine Fish intake, g/day 59.71a

75.48a0

b USEPA (2000a)

90th Percentile Marine Fish Intake, 

g/day 152.29a

188.35a49.16b USEPA (2000a)

Notes:

a

For children and women of childbearing age, intake rates are estimates of “consumers only” data (as described in USEPA 2000a).

b

For adults in the general population, intake rates are estimates of all survey respondents to derive an estimate of long-term 

consumption (USEPA).

c

This is the 90th percentile freshwater and estuarine fish consumption value.

d

Inhalation rates for children and women of childbearing age from USEPA, 1997d. Inhalation rates for adults in the general population 

from USEPA (1994).

e

Pica child soil ingestion.
Appendix B. Tables from Methylmercury Criteria Document

149

Table B2. Average mercury concentrations in marine fish and shellfisha

(References cited in this table can be found in the 2001 methylmercury criteria 

document.)

Species

Concentrationb

(μg Hg/g Wet Wt.) Species

Concentration

(μg Hg/g Wet Wt.)

Finfish

Anchovy 0.047 Pompano* 0.104

Barracuda, Pacific 0.177 Porgy* 0.522d

Cod* 0.121 Ray 0.176

Croaker, Atlantic 0.125 Salmon* 0.035

Eel, American 0.213 Sardines* 0.1

Flounder*,c

0.092 Sea Bass* 0.135

Haddock* 0.089 Shark* 1.327

Hake 0.145 Skate 0.176

Halibut* 0.25 Smelt, Rainbow* 0.1

Herring 0.013 Snapper* 0.25

Kingfish 0.10 Sturgeon 0.235

Mackerel* 0.081 Swordfish* 0.95e

Mullet 0.009 Tuna* 0.206

Ocean Perch* 0.116 Whiting (silver hake)* 0.041

Pollock* 0.15 Whitefish* 0.054f

Shellfish

Abalone 0.016 Oysters 0.023

Clam* 0.023 Scallop* 0.042

Crab* 0.117 Shrimp 0.047

Lobster 0.232 Other shellfish* 0.012d

Molluscan Cephalopods

Octopus* 0.029 Squid* 0.026

Notes:

*Denotes species used in calculation of methylmercury intake from marine fish for one or more populations of concern, based 

on existence of data for consumption in the CSFII (USEPA 2000a).

a

More current information on commercial fish and shellfish is provided by the Food and Drug Administration at 

http://www.cfsan.fda.gov/%7Efrf/sea-mehg.html.

b

Mercury concentrations are from NOAA (1978) as referenced in the NMFS database, as reported in USEPA (1997c) unless 

otherwise noted, measured as micrograms (μg) of mercury per gram (g) wet weight of fish tissue.

c

Mercury data for flounder were used to estimate mercury concentration in marine flatfish for intake calculations.

d

Mercury concentration data are from Stern et al. (1996) as cited in USEPA (1997f).

e

Mercury concentration data are from U.S. FDA Compliance Testing as cited in USEPA (1997f).

f

Mercury concentration data are from U.S. FDA (1978) compliance testing as described in the NMFS database, as cited in 

USEPA (1997f).
Appendix B. Tables from Methylmercury Criteria Document

150

Table B3. Exposure estimates for methylmercury and percent of total exposure based on adults in

the general population

Exposure Source

Exposure Estimate 

(mg/kg-day) Percent of Total Exposure Percent of RfD

Ambient water intake 4.3 x 10-90.0047 0.004

Drinking water intakea5.6 x 10-8

0.0605 0.006

Nonfish dietary intake 0 0 0

Marine fish intake 2.7 x 10-529.33 27

Air intake 4.6 x 10-90.005 0.005

Soil intake 1.3 x 10-90.0014 0.001

Note:

a

This represents the high-end of the range of estimates. Because the contribution of ambient water or drinking water intake 

to total exposure is so negligible in comparison to the sum of intake from other sources, there is not difference in the total 

exposure estimated using either of these two alternatives.
Appendix C. Analytical Methods

151

Appendix C. Analytical Methods

Table C1. Analytical methods for determining mercury and methylmercury in tissue

Method

Form/species 

and applicable 

matrices

Quantitation 

Level or MLa

Technique

Known studies or literature 

references using the techniques in 

this method

Draft method 

1630, with 

modifications for 

tissue

(Recommended 

method – see 

section 4.1.3)

Methylmercury 

in tissue

0.001 mg/kg

0.002 mg/kg

Tissue modification: digest tissue with 

acid solution, neutralize with acetate 

buffer, and analyze as per Method 1630, 

i.e., distillation with heat and N2 flow to 

separate methylHg from sample, 

ethylation with sodium tetraethyl borate, 

N2 purging of methylethylHg onto 

graphite carbon (Carbotrap) column, 

thermal desorption of methylethylHg and 

reduction to Hg, followed by CVAFS 

detection.

• EPA Cook Inlet Contaminant Study

• Lake Michigan fish and 

invertebrates, Mason and Sullivan 

1997

• Northeastern Minnesota lake 

plankton, Monson and Brezonik 

1998b

• Method performance testing in 

freshwater and marine fish, Bloom 

1989

Method 1631, 

draft appendix A

(Recommended 

method – see 

section 4.1.3)

Total mercury in 

tissue, sludge, 

and sediment

0.002 mg/kg Digest tissue with HNO3/H2SO4. Dilute 

digestate with BrCl solution to destroy 

remaining organic material. Analyze 

digestate per method 1631: Add BrCl to 

oxidize all Hg compounds to Hg(II). 

Sequentially pre-reduced with 

hydroxylamine hydrochloride to destroy 

the free halogens and reduced with 

SnCl2 to convert Hg(II) to Hg(0). Hg(0) is 

purged from solution onto gold-coated 

sand trap and thermally desorbed from 

trap for detection by CVAFS.

• EPA National Fish Tissue Study 

(>1,000 samples over 4-year period)

• EPA Cook Inlet Contaminant Study

• Lake Michigan fish and 

invertebrates, Mason and Sullivan 

1997

• Northeastern Minnesota lake 

plankton, Monson and Brezonik 

1998b

• Method performance testing in 

freshwater and marine fish, Bloom 

1989

Method 245.6 Total mercury in 

tissue

0.020 mg/kg Sulfuric and nitric acid digestion, 

oxidation with potassium permanganate 

and potassium persulfate, SnCl2

reduction, CVAAS detection

Unknown

Draft method 

7474 

(SW-846)

Total mercury in 

sediment and 

tissue

40 mg/kg Microwave digestion of sample in nitric 

and hydrochloric acids, followed by cold 

digestion with bromate/bromide in HCl. 

Hg purged from sample and determined 

by CVAFS.

Reference materials cited in method.

Niessen et al. 1999.

Notes:

a Quantitation level or minimum level (ML) is considered the lowest concentration at which a particular 

contaminant can be quantitatively measured using a specified laboratory procedure for monitoring of the 

contaminant.

b

Used similar techniques but used a methylene chloride extraction instead of the distillation.
Appendix C. Analytical Methods

152

Table C2. Analytical methods for determining mercury and methylmercury in water, sediment, and other 

nontissue matrices

Method

Forms/species 

and applicable 

matrices

Quantitation 

Level or ML Sample preparation

Known studies or literature 

references using the techniques in 

this method

EPA 1630a

(Recommended 

method – see 

section 4.1.3)

Methylmercury 

in water

0.06 ng/L Distillation with heat and N2 flow, addition 

of acetate buffer and ethylation with 

sodium tetraethyl borate. Purge with N2 

onto Carbotrap. Thermal desorption and 

GC separation of ethylated mercury 

species, reduction to Hg0followed by 

CVAFS detection.

• USEPA Cook Inlet Study

• USEPA Savannah River TMDL study

• Northern Wisconsin Lakes, Watras et 

al. 1995

• Lake Michigan waters, Mason and 

Sullivan 1997

• Anacostia River Study, Mason and 

Sullivan 1998

• Northeastern Minnesota lakes, 

Monson and Brezonik 1998b

• Poplar Creek, TN CERCLA Remedial 

Investigation of surface water, 

sediment, and pore water, Cambell et 

al. 1998c

• Scheldt estuary study of water, 

polychaetes, and sediments, 

Baeyens et al. 1998

UW-Madison 

SOP for MeHg 

Analysisa

Methylmercury 

in water

0.01 ng/L Distillation with heat and N2 flow, with 

potassium chloride, sulfuric acid, and 

copper sulfate. Ethylation with sodium 

tetraethyl borate. Purge with N2 onto 

Carbotrap. Thermal desorption and GC 

separation of ethylated mercury species, 

reduction to Hg0followed by CVAFS 

detection.

• Lake Michigan tributaries to support 

GLNPO’s LMMB Study

• Fox River, WI, waters and sediments, 

Hurley et al. 1998

USGS 

Wisconsin -

Mercury Lab 

SOPs 004a

Methylmercury 

in water

0.05 ng/L Distillation (heat), APDC solution, N2

flow, potassium chloride, sulfuric acid, 

and copper sulfate. Ethylation with 

sodium tetraethyl borate. Purge with N2 

onto Carbotrap. Thermal desorption and 

GC separation of ethylated species, 

reduction to Hg0, and CVAFS detection.

Aquatic Cycling of Mercury in the 

Everglades (ACME). cofunded by 

USGS, EPA, and others

USGS OpenFile Report 01-

445a

Methylmercury 

in water

0.04 ng/L Distillation (heat) and N2 flow, HCl and 

copper sulfate. Addition of acetate buffer 

and ethylation with sodium tetraethyl 

borate. Purge with N2 onto Carbotrap. 

Thermal desorption and GC separation of 

ethylated mercury species, reduction to 

Hg(0) followed by CVAFS detection.

Formalized USGS method version of 

USGS Wisconsin Lab SOP 004. Report 

title is Determination of Methyl Mercury 

by Aqueous Phase Ethylation, Followed 

by GC Separation with CVAFS 

Detection.
Appendix C. Analytical Methods

153

Table C2. Analytical methods for determining mercury and methylmercury in water, sediment, and other 

nontissue matrices (continued)

Method

Forms/species 

and applicable 

matrices

Quantitation 

Level or ML Sample preparation

Known studies or literature references 

using the techniques in this method

EPA 1631, 

revision Ed

(CVAFS)

(Recommended 

method – see 

section 4.1.3)

Total or 

dissolved 

mercury in 

water

ML = 0.5 ng/L

(MDL = 0.2 

ng/L)

Oxidize all Hg compounds to Hg(II) 

with BrCl. Sequentially pre-reduce with 

hydroxylamine hydrochloride to 

destroy the free halogens and reduce 

with SnCl2 to convert Hg(II) to Hg(0). 

Hg(0) is purged from solution with N2

onto gold coated sand trap and 

thermally desorbed from trap for 

detection by CVAFS.

• USEPA Cook Inlet Study

• State of Maine studies

• USEPA Savannah River TMDL study

• USEPA/U.S. Navy study for 

development of Uniform National 

Discharge Standards

• Watras et al. 1995

• Anacostia River Study, Mason and 

Sullivan 1998

• Northeastern Minnesota lakes, Monson 

and Brezonik 1998

• Poplar Creek, TN, CERCLA Remedial 

Investigation Study, Cambell et al. 1998

• Scheldt Estuary Study, Baeyens et al. 

1998

EPA 245.1d

(CVAAS)

Total or 

dissolved 

mercury in 

wastewater

200 ng/L H2SO4 and HNO3 digestion, KMnO4 , 

K2S2O8 oxidation + heat, cool +NaCl-

(NH2OH)2·H2SO4, SnSO4, aeration. 

Detection by CVAAS.

Effluent guideline development studies for 

the Meat Products Industry, Metal 

Products and Machinery Industry, and 

Waste Incinerators

EPA 245.2d

(CVAAS)

Total or 

dissolved 

mercury in 

wastewater and 

sewage

200 ng/L H2SO4 and HNO3 added, SnSO4, 

NaCl-(NH2OH)2·H2SO4, KMnO4 , 

K2S2O8 , heat. Detection by CVAAS.

MPM Industry effluent guideline 

development study

EPA 245.5 

(CVAAS)

Total or 

dissolved 

mercury in 

soils, sludge 

and sediment

200 ng/L Dry sample, aqua regia, heat, KMnO4 

added, cool +NaCl-(NH2OH)2·H2SO4, 

SnSO4, aeration. Detection by CVAAS.

Pharmaceutical industry effluent guideline 

development study

EPA 245.7d

(CVAFS)

(Recommended 

method – see 

section 4.1.3)

Total or 

dissolved 

mercury in 

water

ML = 5 ng/L;

(MDL = 1.8 

ng/L)

e

HCl, KBrO3 /KBr, NH2OH·HCl, SnCl2 , 

liquid-vapor separation. CVAFS

detection

Interlaboratory validation completed

EPA 7470A 

(CVAAS)

Total or 

dissolved 

mercury in 

liquid wastes 

and

ground water

200 ng/L 

(IDL)

H2SO4 and HNO3 added, KMnO4 

added, K2S2O8 added + heat, cool 

+NaCl-(NH2OH)2·H2SO4, SnSO4, 

aeration of sample. CVAAS detection.

Method is similar to and cites performance 

data given in EPA 245.5.
Appendix C. Analytical Methods

154

Table C2. Analytical methods for determining mercury and methylmercury in water, sediment, and other 

nontissue matrices (continued)

Method

Forms/species 

and applicable 

matrices

Quantitation 

Level of ML Sample preparation

Known studies or literature references 

using the techniques in this method

EPA 7471B 

(CVAAS)

Total or 

dissolved 

mercury in solid 

wastes and

semisolid 

wastes

200 ng/L 

(IDL)

H2SO4 and HNO3 added, KMnO4 

added, K2S2O8 added + heat, cool 

+NaCl-(NH2OH)2·H2SO4, SnSO4, 

aeration of sample. CVAAS detection.

Method is similar to and cites performance 

data given in EPA 245.5.

EPA 7472 

(Anodic 

stripping 

voltametry)

Total or 

dissolved 

mercury in water

100-300 ng/L Acidify and chlorinate sample, GCE 

electrode

Unknown

EPA 7473 

(Thermal 

decomposition, 

amalgamation, 

and CVAA )

Mercury in 

water, soil, and 

sediment

estimated to 

be as low as 

20 ng/ L or 

20 ng/kg

Sample aliquot decomposed at 750°C 

in oxygen atmosphere. Decomposition 

products carried into catalytical furnace 

for completed oxidations, then to 

algamated trap. Mercury is thermally 

desorbed and determined by CVAA.

Unknown

Draft Method 

7474 

(SW-846)f

Total mercury in 

sediment and 

tissue

20 ng/g Microwave digestion of sample in nitric 

and hydrochloric acids, followed by 

cold digestion with bromate/bromide in 

HCl. Hg purged from sample and 

determined by CVAFS.

Reference materials cited in method.

Niessen et al. 1999.

EPA 1620 

(CVAAS)

Mercury in 

water, sludge, 

and soil

200 ng/L H2SO4 and HNO3 added, KMnO4 , 

K2S2O8 + heat, cool +NaCl-

(NH2OH)2·H2SO4, SnSO4, aeration. 

CVAAS detection.

Industry effluent guideline development 

studies

SM 3112B

(CVAAS)

Total or 

dissolved 

mercury in water

500 ng/L H2SO4 and HNO3 added, KMnO4 

added, K2S2O8 added + heat, cool 

+NaCl (NH2OH)2·H2SO4, SnCl2 or 

SnSO4, aeration. CVAAS 

determination.

Unknown

ASTM D3223-

97, 02 (CVAAS)

Total or 

dissolved 

mercury in water

500 ng/L H2SO4 and HNO3 added, KMnO4 

added,K2S2O8 added + heat, cool 

+NaCl (NH2OH)2·H2SO4, SnSO4, 

aeration. CVAAS determination.

Unknown

AOAC 977.22

(Atomic 

absorption 

spectrometry)

Total or 

dissolved 

mercury in water

200 ng/L H2SO4 and HNO3 added, KMnO4 

added, K2S2O8 added + heat, cool 

+NaCl (NH2OH)2·H2SO4, SnSO4, 

aeration. Determine mercury by CVAA.

Unknown

Notes: (1) CVAAS = cold vapor atomic absorption spectrometry.

(2) CVAFS = cold-vapor atomic fluorescence spectrometry.

(3) ASTM and AOAC analytical methods are available from the respective organization.

a

All four methylmercury methods above are based on the work of Bloom 1989, as modified by Horvat et al. 1993, and are virtually 

identical as a result.

b

Used similar techniques but used a methylene chloride extraction instead of the distillation.

c

Used similar techniques but omitted the distillation procedure.

d

Promulgated and approved under 40 CFR part 136, Table 1B.

e

The method detection level (MDL) is the minimum concentration of an analyte (substance) that can be measured and reported 

with a 99 percent confidence that the analyte concentration is greater than zero as determined by the procedure set forth in 

appendix B of 40 CFR part 136.

f Provided for reference purposes only. EPA recommends using method 1631 for analyzing mercury for water and fish tissue.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

155

Appendix D. Synopsized Mercury TMDLs 

Developed or Approved by EPA

I. Ochlockonee Watershed, Georgia

II. Arivaca Lake, Arizona

III. McPhee and Narraguinnep Reservoirs, Colorado

IV. Clear Lake, California

V. Cache Creek, California

VI. Minnesota Statewide Mercury Total Maximum Daily Load
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

156

I. Ochlockonee Watershed, Georgia

Description of the Applicable Water Quality Standards

TMDLs are established to attain and maintain the applicable narrative and numerical 

water quality standards. The State of Georgia’s Rules and Regulations for Water Quality 

Control do not include a numeric criterion for the protection of human health from 

methylmercury, but they do provide a narrative “free from toxics” water quality standard. 

Because mercury can cause toxicity in humans, Georgia has used a numeric 

“interpretation” of its narrative water quality standard for toxic substances to ensure that 

a TMDL will protect human health. The numeric interpretation of its narrative water 

quality standard is a concentration of no more than 0.3 mg/kg methylmercury in fish 

tissue. This numeric interpretation protects the “general population,” which is the 

population that consumes 17.5 g/day or less of freshwater fish.

This approach is consistent with EPA’s recommended water quality criterion for the 

protection of human health from methylmercury, described in the document Water 

Quality Criterion for the Protection of Human Health: Methylmercury (USEPA 2001a). 

The methodology uses a “weighted consumption” approach. When only trophic level 3 

and 4 fish have been collected, the methodology assumes that 8 g/day (58.4 percent) of 

the total fish consumption is trophic level 3 fish (e.g., catfish and sunfish) and 5.7 g/day 

(41.6 percent) is trophic level 4 fish (e.g., largemouth bass). EPA collected site-specific 

data from the Ochlockonee River on ambient mercury in fish tissue and in the water 

column in the summer of 2000 and in March and April 2001 at two locations. Using a 

weighted consumption approach, site-specific fish tissue concentration data collected in 

the Ochlockonee River yields a weighted fish tissue concentration of 0.6 mg/kg, which is 

greater than the state’s current applicable water quality criterion of 0.3 mg/kg. This was 

calculated as

Weighted fish tissue concentration = (avg. trophic 4 conc. x .416) +

(avg. trophic 3 conc. x .584)

where:

average trophic level 3 concentration = 0.2 mg/kg

average trophic level 4 concentration = 1.0 mg/kg

weighted fish tissue concentration = 0.6 mg/kg

To establish the TMDL, EPA determined the maximum allowable concentration of 

mercury in the ambient water that will prevent accumulation of methylmercury in fish 

tissue above the applicable water quality standard, 0.3 mg/kg. To determine this 

concentration, EPA used the Methodology for Deriving Ambient Water Quality Criteria 

for the Protection of Human Health (USEPA 2000b). EPA also used the recommended 

national values from the Methodology, including the reference dose of 0.0001 mg/kg-day 

methylmercury, a standard average adult body weight of 70 kg, and the consumption rate 

for the general population of 17.5 g/day. For the other factors in the calculation, 

bioaccumulation and fraction of methylmercury, EPA used site-specific data from the 

Ochlockonee River collected in summer 2000 and March and April 2001. From this site-
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

157

specific data, EPA determined a representative weighted BAF. The BAF was calculated 

by taking the average calculated BAF from each of the two trophic levels. The BAF 

calculation also used 0.17 as the measured fraction of the total mercury as 

methylmercury. Using this approach, an allowable concentration of mercury in the 

ambient water of Ochlockonee River for the protection of human health is 1.6 ng/L. This 

concentration was calculated as

WQS = ((reference dose – RSC) x body weight x units conversion)

(consumption rate x weighted BAF x fraction MeHg)

Where:

WQS = water quality standard = 1.6 ng/L

reference dose = 0.0001 mg/kg-day MeHg

RSC = relative source contribution from other fish species = 

0.000027 mg/kg-day MeHg

body weight = 70 kg

units conversion = 1,000,000 mg/kg

consumption rate = 0.0175 kg/day fish

weighted bioaccumulation factor = 1,063,270 l/kg

fraction of the mercury as methylmercury = 0.17 as measured

Source Assessment

A TMDL evaluation must examine all known potential sources of the pollutant in the 

watershed, including point sources, nonpoint sources, and background levels. The source 

assessment was used as the basis of development of a model and the analysis of TMDL 

allocation options. This TMDL analysis includes contributions from point sources, 

nonpoint sources, and background levels. Sixteen water point sources in the Ochlockonee 

River watershed could have mercury in their discharges.

According to a review of the Mercury Study Report to Congress (USEPA 1997c), 

significant potential air emission sources include coal-fired power plants, waste 

incinerators, cement and lime kilns, smelters, and chlor-alkali factories. In the report, a 

national airshed model (RELMAP) was applied to the continental United States. This 

model provides a distribution of wet and dry deposition of mercury as a function of air 

emissions and global sources, and it was used to calculate wet and dry deposition rates 

for south Georgia.

The MDN includes a national database of weekly concentrations of mercury in 

precipitation and the seasonal and annual flux of mercury in wet deposition. EPA 

reviewed the MDN data for a sampling station near south Georgia. The MDN data were 

compared with the RELMAP deposition predictions and the MDN data were found to be

substantially higher. Using the MDN data, the average annual wet deposition rate was 

determined to be 12.75 μg/square meter. The dry deposition rate was determined to be 

6.375 μg/square meter on the basis of the RELMAP results.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

158

Loading Capacity—Linking Water Quality and Pollutant Sources

The link between the fish tissue endpoint and the identified sources of mercury was the 

basis for the development of the TMDL. The linkage analysis helped estimate the total 

assimilative capacity of the river and any needed load reductions. In this TMDL, models 

of watershed loading of mercury were combined with a model of mercury cycling and 

bioaccumulation in the water. This approach enabled a translation between the endpoint 

for the TMDL (expressed as a fish tissue concentration of mercury) and the mercury 

loads to the water. The loading capacity was then determined by the linkage analysis as a 

mercury loading rate that was consistent with meeting the endpoint fish tissue 

concentration.

Watershed-scale loading of water and sediment was simulated using the WCS. The 

complexity of this loading function model falls between that of a detailed simulation 

model (which attempts a mechanistic, time-dependent representation of pollutant load 

generation and transport) and simple export coefficient models (which do not represent 

temporal variability). The WCS provides a mechanistic, simplified simulation of 

precipitation-driven runoff and sediment delivery, yet it is intended to be applicable 

without calibration. Solids load, runoff, and ground water can then be used to estimate 

pollutant delivery to the receiving waterbody from the watershed. This estimate is based 

on pollutant concentrations in wet and dry deposition, processed by soils in the watershed 

and ultimately delivered to the receiving waterbody by runoff, erosion, and direct 

deposition. The WCS-calculated loads for each subbasin are shown in table D1.

Table D1. Annual average mercury load from each subbasin

Watershed

Total Hg 

load

(mg)

Areal

load 

(mg/ha)

Impervious 

area

(mg/yr)

Sediment 

(mg/yr)

Runoff 

(mg/yr)

Deposition 

on water

(mg/yr)

Barnett Creek 786098.4 25.6 116614.69 422879.88 177553.9 68850

Middle/Lower 

Ochlocklonee 307965.8 21.24 125771.73 89440.3 54786.29 37867.5

Tired Creek 827172.8 22.03 252386.89 317969.16 194751.7 61965

Lower Ochlockonee 359317.5 15.62 100125.11 130407.68 97802.16 30982.5

Little Ochlockonee 873773.4 19.89 140023.69 433136.75 219614.2 80898.75

Bridge Creek 454417.5 23.11 53496.45 261042.44 98468.66 41310

Upper/Middle 

Ochlockonee 627746.1 20.67 152881.42 254746.48 182250.7 37867.5

Upper Ochlockonee 766396.8 20.1 164465.44 320337 186825.6 94668.75

WASP5 (Ambrose et al. 1988) was chosen to simulate mercury fate in the Ochlockonee 

River. WASP5 is a general, dynamic mass balance framework for modeling contaminant 

fate and transport in surface waters. Environmental properties and chemical 

concentrations are modeled as spatially constant within segments. Each variable is 

advected and dispersed among water segments and exchanged with surficial benthic 

segments by diffusive mixing. Sorbed or particulate fractions can settle through water 

column segments and deposit to or erode from surficial benthic segments. Within the bed, 

dissolved variables can migrate downward or upward through percolation and pore water 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

159

diffusion. Sorbed variables can migrate downward or upward through net sedimentation 

or erosion.

The toxics WASP model, TOXI5, combines a kinetic structure adapted from EXAMS2 

with the WASP5 transport structure and simple sediment balance algorithms to predict 

dissolved and sorbed chemical concentrations in the bed and overlying waters. TOXI5

simulates the transport and transformation of chemicals as a neutral compound and up to 

four ionic species, as well as particulate material. Local equilibrium is assumed so that 

the distribution of the chemical among the species and phases is defined by distribution 

or partition coefficients. The predicted mercury concentrations are shown in table D2.

Table D2. Predicted mercury for annual average load and flow

Calculated concentrations

River reach

1 2 3 4 5 6

Total Hg: water column (ng/L) 6.33 5.84 5.55 5.76 5.65 5.17

Total Hg: sediment (ng/g) 7.05 9.07 9.81 8.17 7.63 6.97

Methyl Hg: water column (ng/L) 0.90 0.82 0.77 0.79 0.77 0.71

Allocations

To determine the total maximum load that can enter the Ochlockonee River, the current 

loading conditions were evaluated and the instream concentration was determined using 

the modeling approach described above. This allowed the development of a relationship 

between load and instream mercury concentrations. Using this developed relationship, the 

total maximum load could be determined. Because the water column mercury 

concentration response is linear with respect to changes in load, a proportion could be 

developed to calculate the total maximum mercury load from the watershed that would 

achieve the derived water quality target of 1.6 ng/L. The TMDL was calculated as the 

ratio of the water quality target to the highest segment concentration (1.6 ng/L divided by 

6.3 ng/L) applied to the current annual average load of 5.00 kg/yr. This gave a TMDL 

load of 1.22 kg/yr mercury, which represents a 76 percent reduction from the current 

annual average load.

In a TMDL assessment, the total allowable load is divided and allocated to the various 

pollutant sources. The calculated allowable load of mercury that can come into the 

Ochlockonee River without exceeding the applicable water quality target of 1.6 ng/L is 

1.22 kg/yr. Because EPA’s assessment indicates that over 99 percent of the current 

loading of mercury is from atmospheric sources, 99 percent of the allowable load is 

assigned to the load allocation and 1 percent of the allowable load is assigned to the 

wasteload allocation. Therefore, the load allocation and the wasteload allocation for the 

Ochlockonee River are:

Load allocation (atmospheric sources) = 1.16 kilograms/year

Wasteload allocation (NPDES sources) = 0.06 kilograms/year

EPA estimates that atmospheric deposition contributes over 99 percent of current 

mercury loadings to the river; therefore, significant reductions in atmospheric deposition 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

160

will be necessary if the applicable water quality standard is to be attained. On the basis of 

the total allowable load of 1.22 kg/year, a 76 percent reduction of mercury loading is 

needed to achieve the applicable water quality standard. EPA believes that an estimated 

31 percent to 41 percent reduction in mercury deposition to the Ochlockonee River 

watershed can be achieved by 2010 through full implementation of existing CAA 

requirements. In addition, a number of activities to address remaining sources of mercury

are planned or under way, and EPA expects that further reductions in mercury loadings 

will occur over time as a result of those activities. EPA is not able to estimate the 

reductions in mercury deposition to the Ochlockonee River watershed that will be 

achieved from future activities. As contemplated by CWA section 303(d)(1)(C), 

however, this TMDL quantifies the water quality problem facing the Ochlockonee River 

watershed and identifies the needed reductions in loadings from atmospheric 

deposition—by CAA initiatives or under other authorities—for the watershed to achieve 

applicable standards for mercury. In addition, as EPA collects additional data and 

information for the Ochlockonee River watershed and as new legal requirements are 

imposed under the CAA, EPA will continue to evaluate the effectiveness of regulatory 

and nonregulatory air programs in achieving the TMDL’s water quality target.

The analysis of NPDES point sources in the watershed indicates that the cumulative 

loading of mercury from these facilities is less than 1 percent of the total estimated 

current loading. Even if this TMDL allocated none of the calculated allowable load to 

NPDES point sources (a wasteload allocation of zero), the waterbody would not attain the 

applicable water quality standards for mercury because of the very high mercury loadings 

from atmospheric deposition. At the same time, however, EPA recognizes that mercury is 

an environmentally persistent bioaccumulative toxic with detrimental effects on human 

fetuses even at minute quantities and that it should be eliminated from discharges to the 

extent practicable. Taking these two considerations into account, this TMDL provides a 

wasteload allocation applicable to all Georgia NPDES-permitted facilities in the 

watershed in the amount of 0.06 kg/year. The TMDL was written so that all NPDESpermitted facilities will achieve this wasteload allocation by discharging mercury only at 

concentrations below the applicable water quality standard, 1.6 ng/L, or by implementing 

a pollutant minimization program.

In the context of this TMDL, EPA believes it can reasonably offer the choice of the two 

approaches to the permitting authority for the following reasons. First, on the basis of 

EPA’s analysis, the Agency expects either wasteload allocation option, in the aggregate, 

to result in point source mercury loadings lower than the wasteload allocation. Second, 

EPA believes this flexibility is the best way of ensuring that the necessary load reductions 

are achieved without causing significant social and economic disruption. EPA recognizes 

that NPDES point sources contribute a small share of the mercury contributions to the 

Ochlockonee River. EPA also recognizes, however, that mercury is a highly persistent 

toxic pollutant that can bioaccumulate in fish tissue at levels harmful to human health. 

Therefore, EPA has determined, as a matter of policy, that NPDES point sources known 

to discharge mercury at levels above the amount present in their source water should 

reduce their loadings of mercury using appropriate, cost-effective mercury minimization 

measures to ensure that the total point source discharges are at a level equal to or less 

than the wasteload allocation specified in this TMDL. The point sources’ waste load 

allocation will be applied to the increment of mercury in their discharge that is above the 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

161

amount of mercury in their source water. EPA recommends that the permitting authority 

make this choice between the two options in consultation with the affected dischargers 

because EPA is not able to make the case-by-case judgments in this TMDL that EPA 

believes are appropriate.

II. Arivaca Lake, Arizona

Description of the Applicable Water Quality Standards

Authorities develop TMDLs to meet applicable water quality standards. These standards 

may include numeric water quality standards, narrative standards describing designated 

uses, and other associated indicators supporting designated uses (beneficial uses apply 

only to California). A numeric target identifies the specific goals or endpoints for the 

TMDL that equate to attainment of the water quality standard. The numeric target may be 

equivalent to a numeric water quality standard (where one exists), or it may represent a 

quantitative interpretation of a narrative standard.

The applicable numeric targets for the Arivaca TMDL are the Arizona water quality 

standard of 0.2 μg/L mercury in the water column and the Arizona Fish Consumption 

Guideline criterion of 1 mg/kg mercury concentration in fish tissue. Arizona has adopted 

water quality standards for mercury that apply to a number of the designated uses 

specified for Arivaca Lake, including protection of aquatic life and wildlife and 

protection of human and agricultural uses. Of these numeric criteria, the most stringent is 

the chronic aquatic life criterion of 0.01 μg/L dissolved mercury (see table 7 on page 15 

in the TMDL). Arizona has also issued a fish consumption advisory for this lake because 

mercury concentrations in fish tissue exceed 1 mg/kg.

Mercury bioaccumulates in the food chain. Within a lake fish community, top predators 

usually have higher mercury concentrations than forage fish, and tissue concentrations 

generally increase with age class. Top predators (such as largemouth bass) are often 

target species for sport fishermen. Arizona bases its Fish Consumption Guideline on 

average concentrations in a sample of sport fish. Therefore, the criterion should not apply 

to the extreme case of the most-contaminated age class of fish within a target species; 

instead, the criterion is most applicable to an average-age top predator. Within Arivaca 

Lake, the top predator sport fish is the largemouth bass. The selected target for the 

TMDL analysis is an average tissue concentration in 5-year-old largemouth bass of 1.0 

mg/kg.

Source Assessment

A TMDL evaluation must examine all known potential sources of the pollutant in the 

watershed, including point sources, nonpoint sources, and background levels. The source 

assessment is used as the basis for developing a model and analyzing TMDL allocation 

options. There are no permitted point source discharges and no known sources of 

mercury-containing effluent in the Arivaca watershed. External sources of the mercury 

load to the lake include natural background load from the watershed, atmospheric 

deposition, and possible nonpoint load from past mining activities.

Watershed background load. The watershed background load of mercury was derived 

from mercury in the parent rock and from the net effects of atmospheric deposition of 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

162

mercury on the watershed. Some mercury is also present within the parent rock 

formations of the Arivaca watershed, although no concentrated ore deposits are known. 

The net contributions of atmospheric deposition and weathering of native rock were 

assessed by measuring concentrations in sediment of tributaries to Arivaca Lake. EPA 

collected 25 sediment and rock samples from dry tributaries in the Arivaca watershed and 

analyzed them for mercury. These data show that most of the sediment samples from the 

Arivaca watershed were considered at or near background mercury levels.

Nonpoint loadings from mining. No known mining for mercury itself has occurred in the 

watershed. However, mining activities for minerals other than mercury, especially 

historical mining practices for gold, might contribute to mercury loading in the 

watershed. Gold and silver mining commonly occurred in the area surrounding Arivaca 

Lake but apparently not within the watershed itself. The U.S. Bureau of Mines identified 

only one exploratory prospect, for manganese and uranium, within the Arivaca 

watershed.

Ruby Dump. Ruby Dump is in the southern portion of Arivaca watershed at the very 

upstream end of Cedar Canyon Wash. The dump apparently served the town of Ruby and 

the Montana Mine. The waste is characterized by numerous mining artifacts (e.g., 

crucibles) but also includes many common household items like bottles and plates. 

Samples were taken at three different locations of the Ruby Dump: the top of the hill (just 

below the fire pit), the middle of the hill, and the base of the dump. The mercury results 

for these samples, from the top of the hill to the bottom, were 1,467 ppb, 1,244 ppb (blind 

duplicate was 495 ppb), and 486 ppb. The average of these four samples is 918 ppb, 

which is the number used in the watershed modeling to represent the mercury 

concentration in sediment eroding from this site.

Near-field atmospheric deposition. Significant atmospheric point sources of mercury 

often cause locally elevated areas of near-field atmospheric deposition downwind. A 

review of Mercury Study Report to Congress (USEPA 1997c) and a search of EPA’s 

AIRS database of permitted point sources found no significant U.S. sources of airborne 

mercury within or near the Arivaca watershed. Also, the most nearby parts of Mexico 

immediately to the southwest (prevailing wind direction) of the watershed are sparsely 

populated. Because of the lack of major nearby sources, especially sources along the axis 

of the prevailing wind, EPA does not believe that near-field atmospheric deposition of 

mercury attributable to individual emitters is a major component of mercury loading to 

the Arivaca watershed. Because no significant near-field sources of mercury deposition 

were identified, mercury from atmospheric deposition onto the watershed is treated as 

part of a general watershed background load in this analysis.

Far-field atmospheric deposition. In May 1997 the MDN began collecting deposition 

data at a new station in Caballo, in the southwestern quadrant of New Mexico. This 

station is the closest MDN station to the Arivaca Lake and was used to estimate loads to 

Arivaca Lake. Because the climate at Arivaca is wetter than that at Caballo, the 

distribution of wet and dry deposition is likely to be different. Monthly wet deposition 

rates at Arivaca were estimated as the product of the volume-weighted mean 

concentration for wet deposition at Caballo times the rainfall depth at Arivaca. This 

approach was used because volume-weighted mean concentrations are usually much 

more stable between sites than wet deposition rates, which are sensitive to rainfall 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

163

amount. Dry deposition at Arivaca was then calculated as the difference between the total 

deposition rate at Caballo and the estimated Arivaca wet deposition rate. The estimates 

derived for Arivaca were 5.3 μg/m

2

/yr by wet deposition and 7.1 μg/m2/yr by dry 

deposition. In sum, mercury deposition at Arivaca is assumed to be equivalent to that 

estimated for Caballo, New Mexico, but Arivaca is estimated to receive more wet 

deposition and less dry deposition than Caballo because more of the particulate mercury 

and reactive gaseous mercury that contribute to dry deposition are scavenged at a site 

with higher rainfall.

Loading Capacity—Linking Water Quality and Pollutant Sources

The linkage analysis in a TMDL defines the connection between numeric targets and 

identified sources. The linkage is defined as the cause-and-effect relationship between the 

selected indicators, the associated numeric targets, and the identified sources. This 

linkage analysis provides the basis for estimating total assimilative capacity and any 

needed load reductions. Specifically, for the linkage analysis in the Arivaca TMDL, 

models of watershed loading of mercury were used together with a model of mercury 

cycling and bioaccumulation in the lake. This approach enabled a translation between the 

numeric target (expressed as a fish tissue concentration of mercury) and mercury loading 

rates. The loading capacity was then determined through the linkage analysis as the 

mercury loading rate that is consistent with meeting the target fish tissue concentration.

Watershed model, Watershed-scale loading of water and sediment was simulated using 

the Generalized Watershed Loading Function (GWLF) model. The complexity of this 

loading function model falls between that of detailed simulation models (which attempt a 

mechanistic, time-dependent representation of pollutant load generation and transport) 

and simple export coefficient models (which do not represent temporal variability). 

GWLF provides a mechanistic, simplified simulation of precipitation-driven runoff and 

sediment delivery, yet it is intended to be applicable without calibration. Solids load, 

runoff, and ground water seepage can then be used to estimate particulate and dissolvedphase pollutant delivery to a stream, on the basis of pollutant concentrations in soil, 

runoff, and ground water. Applying the GWLF model to the period from October 1985 

through September 1998 yielded an average of 11.0 cm/year runoff and 2,520,000 kg 

sediment yield by sheet and rill erosion. The sediment yield estimate is likely to be less 

than the actual yield rate from the watershed because mass wasting loads were not 

accounted for; however, mass wasting loads are thought to be of minor significance for 

loading of bioavailable mercury to the lake.

Estimates of watershed mercury loading were based on the sediment loading estimates 

generated by GWLF by applying a sediment potency factor. These estimates are shown in 

table D3. A background loading estimate was first calculated and then combined with 

estimates of loads from individual hot spots. Most of the EPA sediment samples showed 

no clear spatial patterns, with the exception of the hot spot area identified at Ruby Dump. 

Therefore, background loading was calculated using the central tendency of sediment 

concentrations from all samples excluding Ruby Dump. The background sediment
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

164

Table D3. Annual total mercury load to Arivaca Lake

Watershed year

Mercury loading to lake (g/year)

From 

watershed

From Ruby

Dump

From direct 

atmospheric 

deposition to lake Total

1986 170.16 0.65 4.208 175.018

1987 184.34 0.7 4.208 189.248

1988 205.61 0.79 4.208 210.608

1989 70.9 0.27 4.208 75.378

1990 198.52 0.76 4.208 203.488

1991 99.26 0.38 4.208 103.848

1992 163.07 0.62 4.208 167.898

1993 233.97 0.89 4.208 239.068

1994 141.8 0.54 4.208 146.548

1995 219.79 0.84 4.208 224.838

1996 170.16 0.65 4.208 175.018

1997 191.43 0.73 4.208 196.368

1998 276.51 1.06 4.208 281.778

Grand total 2,325.52 8.88 54.704 2,389.10

Annual average 178.89 0.68 4.21 183.78

mercury concentrations were assumed to be distributed lognormally, as is typical for 

environmental concentration samples, and an estimate of the arithmetic mean of 70.9 ppb 

was calculated from the observed geometric mean and coefficient of variation. Applying 

this assumption to the GWLF estimates of sediment transport yields an estimated rate of 

mercury loading from watershed background of 178.9 g/yr.

Loading from the Ruby Dump was calculated separately, but it was also based on the 

GWLF estimate of sediment load generated per hectare of rangeland (the land use 

surrounding the hot spots), as reduced by the sediment delivery ratio for the watershed. 

The extent of the hot spot was observed to be 200 feet by 50 feet. The mercury 

concentration assigned to surface sediments at the dump was the arithmetic average of 

the four EPA samples taken in October 1997, or 918 ppb. From these assumptions, less 

than 1 percent of the watershed mercury load to Arivaca Lake appears to originate from 

Ruby Dump, which is the only identified hot spot in the watershed.

The direct deposition of mercury from the atmosphere onto the Arivaca Lake surface was 

calculated by multiplying the estimated atmospheric deposition rates times the lake 

surface area, resulting in a load of 4.2 g/yr.

Lake hydrology model. The water level in Arivaca Lake is not actively managed, and 

releases occur only when storage capacity is exceeded. Therefore, lake hydrology was 

represented by a simple monthly water balance. Applying the water balance model 

requires pan evaporation data as an input, in addition to the watershed meteorological 

data. Because no evaporation data were available at the local Cooperative Summary of 

the Day meteorological station, pan evaporation data for Tucson were used. Pan 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

165

evaporation data for 1980 through 1995 were obtained from the BASINS 2.0 Region 9 

data files. Later pan evaporation data were not available for Tucson, so monthly averages 

were used for the 1996 through 1998 water balance. The water balance model was run for 

the period 1985 through 1998. This water balance approach provides a rough 

approximation of the seasonal cycle of changes in volume and surface area of Arivaca 

Lake and of the amount of water released downstream over the spillway. It cannot 

capture daily or event-scale movement of water in and out of the lake.

Mercury cycling and bioaccumulation model. Cycling and bioaccumulation of mercury 

within the lake were simulated using the D-MCM (EPRI 1999). D-MCM predicts the 

cycling and fate of the major forms of mercury in lakes, including methylmercury, 

Hg(II), and elemental mercury. D-MCM is a time-dependent mechanistic model, 

designed to consider the most important physical, chemical, and biological factors 

affecting fish mercury concentrations in lakes. It can be used to develop and test 

hypotheses, scope field studies, improve understanding of cause/effect relationships, 

predict responses to changes in loading, and help design and evaluate mitigation options.

Because strong anoxia in the hypolimnion is a prominent feature during summer 

stratification for the Arizona lakes simulated in this study, D-MCM was modified to 

explicitly allow significant methylation to occur in the hypolimnion. In previous 

applications of D-MCM, the occurrence of methylation was restricted to primarily within 

surficial sediments. That the locus of methylation likely includes or is even largely within 

the hypolimnion is supported by (1) the detection of very high methylmercury 

concentrations in the hypolimnia of Arivaca Lake and (2) almost complete losses of 

sulfate in Arivaca Lake in the hypolimnion resulting from sulfate reduction. An input was 

added to the model to specify the rate constant for hypolimnetic methylation, distinct 

from sediment methylation.

The results of the model calibration are shown in table D4. The model calculations are 

the predicted annual ranges after the model has reached steady state. The observed 

concentrations are from July 1997.

Table D4. Predicted and observed mercury for annual average load and flow

Predicted Observed

Methyl Hg: Water column (ng/L) 0.00–12.07 14.3

Hg II: Water column (ng/L) 0.00–6.28 1.46–8.3

Methyl Hg: 5-year-old largemouth bass (mg/kg) 1.18 1.18

Allocations

A TMDL represents the sum of all individual allocations of portions of the waterbody’s 

loading capacity. Allocations may be made to point sources (wasteload allocations) or 

nonpoint sources (load allocations). The TMDL (sum of allocations) must be less than or 

equal to the loading capacity; it is equal to the loading capacity only if the entire loading 

capacity is allocated. In many cases, it is appropriate to hold a portion of the loading

capacity in reserve to provide a margin of safety (MOS), as provided for in the TMDL 

regulation. The allocations and MOS are shown in table D5. These allocations, from the 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

166

best currently available information, predict attainment of acceptable fish tissue 

concentrations within a time horizon of approximately 10 years. A delay in achieving 

standards is unavoidable because time will be required for mercury to cycle through the 

lake and food chain after load reductions occur.

Table D5. Summary of TMDL allocations and needed load reductions (in g-Hg/yr)

Source Allocation Existing load

Needed 

reduction

Wasteload allocations 0.0 0.0 0.0

Load allocations

Atmospheric deposition 4.2 4.2 0

Ruby Dump 0.7 0.7 0

Watershed background 111.2 178.9 67.7

Total 116.1 183.8 67.7

Unallocated reserve 38.7

Loading capacity 154.8

The model was used to evaluate the load reductions necessary to meet the numeric target. 

The response of concentrations of mercury in 5-year-old largemouth bass to changes in 

external mercury loads is nearly linear. This is because the sediment burial rates are high 

and sediment recycling is low, with most of the methylmercury that enters the food chain 

being created in the anoxic portion of the water column. The model calculates that the 

numeric target of 1 mg/kg in 5-year-old largemouth bass is predicted to be met with a 16 

percent reduction in total watershed loads to Arivaca Lake, which results in a loading 

capacity of 154.8 g/year of mercury.

There are uncertainties associated with mercury sources and the linkage between mercury 

sources and fish tissue concentrations in Arivaca Lake. As a result, the TMDL reserves 

38.7 g-Hg/yr (25 percent of the loading capacity) for the MOS and allots the remaining 

load of 116.1 g-Hg/yr for sources. Because no permitted point source discharges occur 

within the Arivaca watershed, the wasteload allocation is zero and the load allocation is 

116.1 g-Hg/yr.

The load allocation provides loads for three general sources: direct atmospheric 

deposition onto the lake surface, hot spot loading from Ruby Dump, and generalized 

background watershed loading, including mercury derived from parent rock and soil 

material, small amounts of residual mercury from past mining operations, and the net 

contribution of atmospheric deposition onto the watershed. Direct deposition to the lake 

surface is a small part of the total load and is believed to derive from long-range transport 

of global sources, which are not readily controllable. The load from Ruby Dump is also 

small. As a result, the TMDL does not require reductions from these sources, and their 

load allocations are their existing loads.

Background watershed loading appears to be the major source of mercury to Arivaca 

Lake. The intensive watershed survey conducted for this TMDL did not identify any 

significant terrestrial sources of mercury. Regarding air deposition to the watershed land 

surface, insufficient data were available to calculate reliable estimates of the proportion 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

167

of mercury deposited from the air that actually reaches Arivaca Lake. Therefore, a load 

allocation of 111.2 g-Hg/yr was established for overall background watershed loading. 

This requires a 38 percent reduction from existing estimated loads from this source. This 

reduction is believed feasible for several reasons.

Potential for erosion control. Reduction of mercury loading from the watershed to 

Arivaca Lake depends on reduction in sediment erosion rates. Improved livestock 

management practices could obtain significant reductions in erosion rates. As a side 

benefit, implementation of livestock BMPs could result in significant reductions in 

loadings of DOC and nutrients to the lake. The availability of high levels of DOC and 

nutrients in the lake appears to affect the methylation process. Reduction of DOC and 

nutrient levels should reduce the efficiency of the methylation process at Arivaca Lake, 

effectively increasing the lake’s mercury loading capacity.

Reductions in atmospheric deposition of mercury. Although no reliable estimates are 

available, new mercury air emissions to the environment appear to be declining. U.S. 

mercury emissions have declined significantly since 1990 and are expected to decline 

further upon implementation of new emission limits on incinerators as required by recent 

EPA regulations. Reductions in air deposition in Arivaca Lake watershed would 

eventually result in decreases in mercury loading to the lake itself.

Potential location and remediation of undiscovered mercury sources. Although 

investigation of the watershed did not reveal any significant localized sources of mercury 

in the watershed (with the possible exception of Ruby Dump), additional site 

investigation is warranted to ensure that no significant sources were missed. From past 

experience with mine site remediation in similar circumstances in Arizona, newly 

discovered sites could be effectively eliminated as ongoing mercury sources.

Alternative management strategies. Any alterations in rates of methylation or in rates of 

mercury loss to deep sediments will change the relationship between external mercury 

load and fish tissue concentration and would thus result in a change in the loading 

capacity for external mercury loads. The loading capacity could be increased by 

management intervention methods that decrease rates of bacterial methylmercury 

production within the lake or increase rates of burial and sequestration of mercury in lake 

sediment. Selection of such an approach would require further research and feasibility 

studies. Some alternative strategies that might be suitable for further investigation include 

the following:

● Hypolimnion aeration or mixing

● Sulfur chemistry modification

● Alum treatment

● Reduction of DOC and nutrient levels

● Dredging of lake sediments
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

168

III. McPhee and Narraguinnep Reservoirs, Colorado

Description of the Applicable Water Quality Standards

The TMDL for McPhee and Narraguinnep Reservoirs in southwestern Colorado was 

based on the Fish Consumption Advisory action level of 0.5 mg/kg mercury 

concentration in fish tissue. Colorado Department of Public Health and the Environment 

listings are based on the risk analysis presented in the May 6, 1991, Disease Control and 

Epidemiology Division position paper for Draft Colorado Health Advisory for 

Consumption of Fish Contaminated with Methylmercury. This paper, using a toxicity 

value RfD of 0.3 μg/kg/day, establishes a fish tissue concentration of 0.5 mg/kg as the 

approximate center of the range at which the safe consumption level is four meals per 

month for nonpregnant adults and one meal per month for women who are pregnant, 

nursing, or planning to become pregnant and children nine years of age or younger. The 

criterion is applied to an average-age top predator. In McPhee Reservoir, the top predator 

among sport fish regularly taken is the smallmouth bass (19 percent of the total catch in 

1993); the top predator sport fish in Narraguinnep Reservoir is the walleye. The lake 

water quality model D-MCM (EPRI 1999) is capable of predicting mercury 

concentrations in fish tissue for each age class at each trophic level. Average mercury 

concentrations in fish tissue of target species are assumed to be approximated by the 

average concentration in 15-inch smallmouth bass in McPhee and the 18-inch walleye in 

Narraguinnep. Therefore, the selected target for the TMDL analysis in McPhee Reservoir 

is an average tissue concentration in 15-inch smallmouth bass of 0.5 mg/kg or less. The 

selected target in Narraguinnep Reservoir is the 18-inch walleye of 0.5 mg/kg or less.

Source Assessment

McPhee and Narraguinnep reservoirs have several sources of mercury. The sources 

external to the reservoirs separate into direct atmospheric deposition onto the lakes (from 

both near- and far-field sources) and transport into the lakes from the watershed. The 

watershed loading occurs in both dissolved and sediment-sorbed forms. Ultimate sources 

in the watershed include mercury in parent rock, mercury residue from mine tailings and 

mine seeps, point source discharges, and atmospheric deposition onto the watershed, 

including deposition and storage in snowpack. A summary of the mercury load estimates 

for McPhee Reservoir is presented in table D6.

Table D6. Summary of mercury load estimates for McPhee Reservoir

Reservoir

Watershed

runoff

(g/yr)

Watershed 

sediment

(g/yr)

Interbasin 

transfer

(g/yr)

Atmos.

deposition

(g/yr)

Total

(g/yr)

Load per 

volume

(mg/ac-ft)

Load per 

surface 

area 

(mg/m2)

McPhee 2,576 222 251 3,049 4.66 0.098

Narraguinnep 2.7 22.7 15.9 36.8 78.1 4.59 0.035

Past mining activities likely provide an important source of mercury load to the McPhee 

and Narraguinnep watershed. There are large mining districts in the Dolores River 

watershed, the LaPlata, the Rico, and the area around Dunton on the West Dolores River. 

The quantity of mercury loading from mining operations has been estimated through a 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

169

combination of observed data in the water column and sediment coupled with the 

watershed linkage analysis.

Significant atmospheric point sources of mercury often cause locally elevated areas of nearfield atmospheric deposition downwind. Two large coal-fired power plants are in the Four 

Corners area within about 50 miles of the McPhee and Narraguinnep reservoirs. The plants 

in the Four Corners area (2,040 megawatt (MW) capacity) and the Navajo plant (1,500 

MW capacity) are upwind of McPhee and Narraguinnep reservoirs. It is likely that the 

mercury emitted from these plants contributes to the mercury loading of the two reservoirs. 

Because no direct measurements of atmospheric deposition of mercury are available, EPA 

cannot assess the significance of this loading and must await further investigation, 

including the establishment of a mercury deposition monitoring site in the area.

Loading Capacity—Linking Water Quality and Pollutant Sources

Models of watershed loading of mercury are combined with a model of mercury cycling 

and bioaccumulation in the lake to translate the numeric target, expressed as a fish tissue 

concentration of mercury, to mercury loading rates. The coupled models estimate mercury 

loading to the reservoirs and predict mercury cycling and speciation within the reservoir. 

An estimated load reduction of 52 percent is needed for long-term average mercury 

concentrations in a standardized 15-inch smallmouth bass to drop to 0.5 mg/kg wet muscle.

Allocations

The loading capacity for McPhee Reservoir was estimated to be 2,592 g/year of mercury. 

Narraguinnep Reservoir’s loading capacity was estimated at 39.1 g/year of mercury. This 

is the maximum rate of loading consistent with meeting the numeric target of 0.5 mg/kg 

in fish tissue. Because of the uncertainties regarding the linkage between mercury sources 

and fish tissue concentrations in McPhee and Narraguinnep reservoirs, an allocation of 70 

percent of the loading capacity was used for this TMDL. The TMDL calculated for 

McPhee Reservoir is equivalent to a total annual mercury loading rate of 1,814 g/yr (70 

percent of the loading capacity of 2,592 g/yr), while that for Narraguinnep Reservoir is 

equivalent to a total annual mercury loading rate of 27.3 g-Hg/yr (70 percent of 39.1 gHg/yr). Summaries of the TMDL allocations and needed load reductions for the McPhee 

and Narraguinnep Reservoirs are presented in tables D7 and D8, respectively.

Table D7. Summary of TMDL allocations and needed load reductions for

McPhee Reservoir

Source Allocation Existing load Needed reduction

Atmospheric deposition 63 251 188

Rico/Silver Creek mining area 507 1030 523

Dunton mining area 348 708 360

La Plata mining area 69 141 72

Watershed background 827 919 92

Total 1,814 3,049 1,235

Unallocated reserve 778

Loading capacity 2,592

Note: Measurements in g/year of mercury.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

170

Table D8. Summary of TMDL allocations and needed load reductions for 

Narraguinnep Reservoir

Source Allocation Existing load Needed reduction

Atmospheric deposition 9.2 36.8 27.6

Interbasin transfer from 

McPhee Reservoir

9.5 15.9 6.4

Watershed background 8.6 25.4 16.8

Total 27.3 78.1 50.8

Unallocated reserve 11.8

Loading capacity 39.1

Note: Measurements in g/year of mercury.

IV. Clear Lake, California

Description of the Applicable Water Quality Standards

EPA promulgated the California Toxics Rule (CTR) in May 2000 (65 FR 31682). The 

CTR contains a water quality criterion of 50 ng/L total recoverable mercury for water and 

organism consumption and is intended to protect humans from exposure to mercury in 

drinking water and through fish and shellfish consumption. This criterion is enforceable 

in California for all waters with a municipal or domestic water supply designated use and 

is applicable to Clear Lake. However, the state of California does not consider this 

criterion sufficiently protective of the consumers of fish from Clear Lake.

The water quality management plan or Basin Plan for the Central Valley Regional Water 

Quality Control Board adopted new water quality standards for mercury for Clear Lake at 

the same time it adopted mercury TMDLs for Clear Lake. The state’s water quality 

criteria are for fish tissue and are intended to protect designated uses for fishing and 

wildlife habitat. The applicable criteria are 0.09 mg/kg and 0.19 mg/kg of mercury in fish 

tissue for trophic levels 3 and 4 fish, respectively. These levels were recommended by the 

U.S. Fish and Wildlife Service to protect wildlife, including osprey and bald eagles, at 

Clear Lake; these levels allow adults to safely consume about 3.5 fish meals per month 

(26 grams/day) if eating mainly trophic level 4 fish such as catfish and bass. The 26 

grams/day assumes a diet composed of 70 percent trophic level 4 fish and 30 percent 

trophic level 3 fish. The 90th percentile consumption rate of a small group of residents of 

Clear Lake, primarily members of the Elem Pomo Indian Tribe, is 30 grams/day of Clear 

Lake fish, as reported in 1997.

Source Assessment

Clear Lake is in Lake County in northern California. It is a shallow, eutrophic waterbody 

that consists of three basins––the Upper, Lower, and Oaks Arms. It is the largest natural 

lake entirely within California’s boundaries. Tourism and sport fishing are important 

sectors of the local economy. Five American Indian tribes use the resources of the lake 

and its watershed.

The Clear Lake watershed lies within a region naturally enriched in mercury. The 

Sulphur Bank Mercury Mine (SBMM) site, on the shores of Oak Arm, was a highly 

productive source of mercury between 1872 and 1957. Similar smaller mines were 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

171

present in the Clear Lake watershed, all of which are now inactive. Levels of mercury in 

Clear Lake sediments rose significantly after 1927, when open-pit operations became the 

dominant mining method at SBMM. EPA declared the SBMM a federal Superfund site in 

1991, and since then several remediation projects have been completed, including 

regrading and vegetation of mine waste piles along the shoreline and construction of a 

diversion system for surface water runoff. EPA is conducting a remedial investigation to 

fully characterize the SBMM site to propose final remedies.

Inorganic mercury loads entering Clear Lake come from ground water and surface water 

from the SBMM site; tributaries and other surface water that flows directly into the lake; 

and atmospheric deposition, including atmospheric flux from SBMM. Some mercury 

deposited historically in the lake due to mining operations or erosion at SBMM might 

also contribute to mercury concentrations in fish today.

Ground water and surface water from the SBMM site. SBMM covers approximately 1 

square mile on the east shore of the Oaks Arm of Clear Lake. The site contains 

approximately 120 acres of exposed mine overburden and tailings (referred to as waste 

rock). Two small unprocessed ore piles are also on the site. Mercury in samples of mine 

materials ranged from 50 to 4,000 mg/kg. All piles of mine materials exhibit the potential 

to generate acid rock drainage. The abandoned mine pit, the Herman Impoundment, is 

filled with 90 feet of acidic water (pH 3) and has a surface area of about 20 acres. The 

average concentrations in the Herman Impoundment of water and sediment are around 

800 ng/L and 26 mg/kg, respectively. A geothermal vent at the bottom of the 

impoundment continues to discharge gases, minerals (including mercury), and fluids into 

the pit.

A large pile of waste rock, known as the waste rock dam (WRD), stretches about 2,000 

feet along the shore of the western side of the SBMM site. The WRD lies between 

Herman Impoundment and Clear Lake. The surface water in the impoundment is 10–14 

feet above the surface of Clear Lake, which creates a gradient of ground water flow 

toward the lake. Surface runoff from the northern side of the site is bounded by a wetland 

that drains to Clear Lake. Surface runoff from the northern waste rock piles is directed 

through culverts into the northern wetland. In 1990 rock and geofabric barriers were 

installed at the culverts to reduce the transport of suspended solids. The northern wetland 

is used for cattle grazing and as a source of fish, tules, and other resources used by the 

members of the Elem Pomo Tribe. Waste rock piles extend into the wetlands.

Inputs of mercury from SBMM are estimated to be between 1 and 568 kg/year. EPA 

Superfund program’s estimate of mercury transported in ground water from the WRD is 

used as the lower-bound input. Regional Board staff estimate that 568 kg/year is the 

maximum upper-bound estimate of all inputs from SBMM, including past and continuing 

contributions to the active sediment layer. This is approximately 96.5 percent of total 

sources.

Ground water from SBMM appears to contribute mercury that is readily methylated, 

relative to mercury from other inputs. Ground water flow from the mine site has been 

detected entering Clear Lake by subsurface flow through lake sediments. Mercury in 

ground water from the WRD is solubilized and likely in chemical forms that are easily 

taken up by methylating bacteria. Acidic drainage from the mine site also contains high 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

172

sulfate concentrations that enhance the rates of methylation by sulfate-reducing bacteria. 

This assertion is supported by data showing that methylation rates near the mine site are 

significantly higher than those in other parts of Clear Lake. In contrast to the mercury in 

SBMM ground water, the mercury in lakebed and tributary sediments originates 

primarily as cinnabar, which has low solubility in water.

Tributaries and other surface water flowing directly into the lake. Mercury entering Clear 

Lake from its tributaries originates in runoff from naturally mercury-enriched soils, sites 

of historical mining activities, and mercury deposited in the watershed from the 

atmosphere. Geothermal springs might contribute to tributary loads, especially in the 

Schindler Creek tributary to Oaks Arm. Tributary and watershed runoff loads of mercury 

range from 1 to 60 kg/year, depending on flow rates. Loads in average water years are 18 

kg/year, approximately 3 percent of the total sources.

Geothermal springs and lava tubes that directly discharge to Clear Lake do not appear to 

be significant sources of mercury. Mercury concentrations in surficial sediment samples 

collected near lakebed geothermal springs were not elevated relative to levels in sediment 

away from geothermal springs.

Atmospheric deposition, including flux from the SBMM site. Small amounts of mercury 

deposit directly on the surface of Clear Lake from the global atmospheric pool and 

potentially from local, mercury-enriched sources. Atmospheric loads to the lake surface 

from the global pool were estimated using data from MDN monitoring stations in 

Mendocino County and San Jose. Estimates ranged from 0.6 to 2.0 kg/year, 

approximately 0.3 percent of the total sources.

Loading Capacity––Linking Water Quality and Pollutant Sources

The Regional Board staff assumes that there is a directly proportional relationship 

between methylmercury in fish and mercury in the surficial sediment. This is a 

simplification of a highly complex process. Many factors, such as sulfide and sulfate 

concentrations, temperature, and organic carbon, affect methylation or concentrations of 

methylmercury. Factors that affect accumulation of methylmercury in fish include 

species, growth rate, prey availability, and the like. To reduce levels of methylmercury in 

fish, loads of mercury to the lake must be reduced. Section 5.3.1 of the Staff Report 

provides examples of remediation projects demonstrating that removal of inorganic 

mercury from a range of aquatic environments has been effective in reducing 

concentrations of mercury in fish.

A set of first-order relationships, each controlled by a single variable of concentration of 

mercury or methylmercury, provide the basis for the assumption of a directly 

proportional relationship between mercury in fish and in surficial sediment in Clear Lake. 

Concentrations of methylmercury in water and methylmercury in biota are related by 

BAFs. Relationships between methylmercury in the water column and in sediment can be 

described as a flux rate of methylmercury from sediment. Concentrations of 

methylmercury and mercury in sediment are related through calculation of a methylation 

efficiency index (ratio of methylmercury to mercury in surficial sediment).

In each of these steps in the linkage analysis, one variable is related to another by a 

simple ratio or linear equation. For example, BAFs are calculated by dividing the 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

173

concentration of methylmercury in fish by the concentration of methylmercury in the 

water. Data are available to determine BAF and methylation indices that are specific for 

Clear Lake. With the current understanding of the transport, methylation, and uptake 

processes in Clear Lake, the Regional Board staff was unable to refine these relationships 

to incorporate the effects of other factors. The end result was that methylmercury in biota 

was related linearly to mercury in surficial sediment.

Meeting the recommended water quality standards would require reducing existing fish 

tissue concentrations by 60 percent. Using the linear relationship, the linkage analysis 

indicates that overall mercury loads to Clear Lake sediment must be reduced by 60 

percent to reduce methylmercury concentrations in fish tissue by the proportional 

amount. The Regional Board is establishing the assimilative capacity of inorganic 

mercury in Clear Lake sediments as 70 percent of existing levels to include a margin of 

safety of 10 percent to account for the uncertainties in the linkage analysis.

Allocations

The strategy for meeting the fish tissue criteria is to reduce the inputs of mercury to the 

lake from tributaries and the SBMM site, combined with active and passive remediation 

of contaminated lake sediments. The load allocations for Clear Lake will result in a 

reduction in the overall mercury sediment concentration by 70 percent of existing 

concentrations. The load allocations are assigned to the active sediment layer of the 

lakebed, the SBMM terrestrial site, the tributary creeks and surface water runoff to Clear 

Lake, and atmospheric deposition. Table D9 summarizes the load allocations. The load 

allocation to the active sediment layer is expressed as reducing concentrations of mercury 

in the active sediment layer to 30 percent of current concentrations. The load allocation to 

the SBMM terrestrial site is 5 percent of the ongoing loads from the terrestrial mine site. 

The load allocation for the mine also includes reducing mercury concentrations in 

surficial sediment to achieve the sediment compliance goals for Oaks Arm, shown in 

table D10. The load allocation to tributary and surface water runoff is 80 percent of 

existing loads. These load allocations account for seasonal variation in mercury loads, 

which vary with water flow and rainfall. The analysis includes an implicit margin of 

safety in the reference doses for methylmercury that were used to develop the fish tissue 

objectives. It also includes an explicit margin of safety of 10 percent to account for 

uncertainty in the relationship between fish tissue concentrations and loads of mercury. 

The reductions in loads of mercury from all sources are expected to result in attainment 

of water quality objectives.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

174

Table D9. Summary of mercury load allocations

Source

Existing load

(kg/year) Needed reduction

Clear Lake sediment

695

70% of existing concentration

Sulphur Bank Mercury 

Mine 95% of existing load

Tributaries 18 20% of existing load

Atmosphere 2 no change

Table D10. Sediment goals for mercury in Clear Lake

Site designation Location

Sediment mercury goal 

(mg/kg dry weight)a

Upper Arm

UA-03

Center of Upper Arm on transect from 

Lakeport to Lucerne 0.8

Lower Arm

LA-03

Center of Lower Arm, north and west of 

Monitor Point 1.0

Oaks Arm

OA-01b

OA-02b

OA-03b

OA-04b

Narrows O1

0.3 km from SBMM 0.3 km from SBMM

0.8 km from SBMM

1.8 km from SBMM

3.0 km from SBMM

7.7 km from SBMM

16c

16c

16

10

3

Notes:

a

Sediment goals are 30 percent of existing concentrations. Existing concentrations are taken as the 

average mercury concentrations in samples collected in 1996–2000 (Clear Lake Basin Plan Amendment 

Staff Report).

b

Sediment goal is part of the load allocation for SBMM.

c

Due to the exceptionally high concentrations existing at the eastern end of Oaks Arm, sediment goals at 

OA-01 and OA-02 are not 70 percent of existing concentrations. These goals are equal to the sediment 

goal established for OA-03.

Clear Lake sediment. Reducing mercury concentrations in surficial sediment by 70 

percent is an overall goal for the entire lake. To achieve water quality objectives, 

extremely high levels of mercury in the eastern end of Oaks Arm near SBMM must be 

reduced by more than 70 percent. To evaluate progress in lowering sediment 

concentrations, the following sediment compliance goals are established at sites that have 

been sampled previously.

Sulphur Bank Mercury Mine. Current and past releases from SBMM are a significant 

source of mercury loading to Clear Lake. Ongoing annual loads from the terrestrial mine 

site to the lakebed sediments occur through ground water, surface water, and atmospheric 

routes. Loads from ongoing releases from the terrestrial mine site should be reduced to 5 

percent of existing inputs. Because of its high potential for methylation relative to 

mercury in lakebed sediments, mercury entering the lake through ground water from the 

mine site should be reduced to 0.5 kg/year.

Past releases from the mine site are a current source of exposure through remobilization 

of mercury that exists in the lakebed sediments as a result of past releases to the lake 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

175

from the terrestrial mine site. Past active mining operations, erosion, and other mercury 

transport processes at SBMM have contaminated sediment in Oaks Arm. The load 

allocation assigned to SBMM includes reducing surficial sediment concentrations in 

Oaks Arm by 70 percent (more at sites nearest the mine site) to meet the sediment 

compliance goals in table D10.

EPA anticipates implementing additional actions to address the ongoing surface and 

ground water releases from SBMM over the next several years. These actions are 

expected to lead to significant reductions in the ongoing releases from the mine pit, the 

mine waste piles, and other ongoing sources of mercury releases from the terrestrial mine 

site. EPA also plans to investigate what steps are appropriate under the Comprehensive 

Environmental Response, Compensation, and Liability Act (CERCLA) to address the 

existing contamination in the lakebed sediments from past releases from SBMM. The 

Regional Board will continue to work closely with EPA on these important activities. In 

addition, the Regional Board will coordinate monitoring activities to investigate other 

sources of mercury loads to Clear Lake. These investigations by EPA and the Regional 

Board should reduce the uncertainty that exists regarding the annual load of mercury to 

the lake, the contribution of each source to that load, and the degree to which those 

sources lead to methylmercury exposure of and mercury uptake by fish in the lake. This 

information should lead to more refined decisions about what additional steps are 

appropriate and feasible to achieve the applicable water quality criteria.

Tributaries and surface water runoff. Past and current loads of mercury from the 

tributaries and direct surface water runoff are also a source of mercury loading to the lake 

and to the active sediment layer in the lakebed. This section excludes loads from surface 

water runoff associated with SBMM, which are addressed separately above. The loads of 

mercury from the tributaries and surface water runoff to Clear Lake should be reduced by 

20 percent of existing levels. In an average water year, existing loads are estimated to be 

18 kg/year. Loads range from 1 to 60 kg/year, depending on water flow rates and other 

factors. The load allocation applies to tributary inputs as a whole, instead of to individual 

tributaries. Efforts should be focused on identifying and controlling inputs from hot spots. 

The U.S. Bureau of Land Management, U.S. Forest Service, other land management 

agencies in the Clear Lake Basin, and Lake County will submit plans for monitoring and 

implementation to achieve the necessary load reductions. The Regional Board will 

coordinate with those agencies and other interested parties to develop the monitoring and 

implementation plans. The purpose of the monitoring is to refine load estimates and 

identify potential hot spots of mercury loading from tributaries or direct surface runoff 

into Clear Lake. Hot spots can include erosion of soils with concentrations of mercury 

above the average for the rest of the tributary. If significant sources are identified, the 

Regional Board will coordinate with the agencies to develop and implement load 

reductions. The implementation plans will include a summation of existing erosion 

control efforts and a discussion of feasibility and proposed actions to control loads from 

identified hot spots. The agencies will provide monitoring and implementation plans 

within five years after the effective date of this amendment and implement load reduction 

plans within five years thereafter. The goal is to complete the load reductions within 10 

years of implementation plan approval.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

176

The Regional Board will work with the American Indian tribes in the Clear Lake 

watershed on mercury reduction programs for the tributaries and surface water runoff. It 

will solicit the tribes’ participation in developing monitoring and implementation plans.

Wetlands. The Regional Board is concerned about the potential for wetland areas to be 

significant sources of methylmercury. Loads and fate of methylmercury from wetlands 

that drain to Clear Lake are not fully understood. The potential for production of 

methylmercury should be assessed during the planning of any wetlands or floodplain 

restoration projects within the Clear Lake watershed. The Regional Board established a 

goal of no significant increases of methylmercury to Clear Lake resulting from such 

activities. As factors contributing to mercury methylation are better understood, the 

Regional Board should examine the possible control of existing methylmercury 

production within tributary watersheds.

Atmospheric deposition. Atmospheric loads of mercury originating outside the Clear 

Lake watershed and depositing locally are minimal. Global and regional atmospheric 

inputs of mercury are not under the jurisdiction of the Regional Water Board. Loads of 

mercury from outside the Clear Lake watershed and depositing from air onto the lake 

surface are established at the existing input rate, estimated to be 1 to 2 kg/year.

V. Cache Creek, California

Description of the Applicable Water Quality Standards

EPA promulgated the California Toxics Rule (CTR) in May 2000 (65 FR 31682). The 

CTR contains a water quality criterion of 50 ng/L total recoverable mercury for waters 

designated for water and organism consumption, and it was intended to protect humans 

from exposure to mercury in drinking water and through fish and shellfish consumption. 

This criterion is enforceable in California for all waters with a municipal or domestic 

water supply designated use, and it is applicable to all waters in the Cache Creek 

watershed. The State of California, however, does not consider this criterion sufficiently 

protective of human and wildlife consumers of fish in the watershed.

The water quality management plan or Basin Plan for the Central Valley Regional Water 

Quality Control Board adopted new water quality standards for mercury for Cache Creek, 

Bear Creek, and Harley Gulch at the same time it adopted mercury TMDLs for those 

waterbodies. The state’s water quality criteria are expressed as concentrations in fish 

tissue and are intended to protect designated uses, which include human and wildlife fish 

consumption. The applicable criteria are as follows: for Cache Creek and Bear Creek, the 

average methylmercury concentration shall not exceed 0.23 mg methylmercury/kg wet 

weight of muscle tissue in trophic level 4 fish 250–350 mm (piscivorous species, 

including bass and catfish), and 0.12 mg methylmercury/kg wet weight of muscle tissue 

in trophic level 3 fish 250–350 mm, or if not available, a minimum of 125 mm (bluegill, 

sunfish, and sucker); for Harley Gulch, the average methylmercury concentration shall 

not exceed 0.05 mg methylmercury/kg wet weight in whole, trophic level 2 and 3 fish 

75–100 mm total length (hardhead, California roach, or other small resident species). 

Because Harley Gulch does not support larger, trophic level 3 and 4 fish, no water quality 

criteria for these larger fish were proposed in that waterbody.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

177

These water quality standards permit safe consumption of about 22–40 g/day of Cache or 

Bear Creek fish (3 to 5.4 meals/month). In Cache and Bear creeks, the standards protect 

wildlife species, including bald eagle, peregrine falcon (state endangered), river otter, 

American mink, mergansers, grebes, and kingfishers. In Harley Gulch, the standards 

protect wildlife species, including small mammals, herons, and kingfishers.

Source Assessment

The Cache Creek watershed is impaired due to elevated levels of mercury in the water 

and in fish tissue. Because Cache Creek is a primary source of mercury to the 

Sacramento-San Joaquin Delta Estuary, lowering mercury levels in the Cache Creek 

watershed will assist in protecting human and wildlife health in the delta. The TMDL 

encompasses the 81-mile reach of Cache Creek between Clear Lake Dam and the outflow 

of the Cache Creek Settling Basin, Bear Creek from its headwaters to its confluence with 

Cache Creek, and the 8-mile length of Harley Gulch.

Sources of mercury entering the watershed include waste rock and tailings from historical 

mercury mines, erosion of naturally mercury-enriched soils, geothermal springs, and 

atmospheric deposition. There are multiple inactive mercury mines in the Cache Creek 

watershed. The Sulphur Bank Mercury Mine contributes mercury to Cache Creek at the 

Clear Lake outflow. The Sulphur Creek mining district includes eight mines that drain 

predominately to Bear Creek via Sulphur Creek and four mines in the Bear Creek Basin. 

Harley Gulch receives inputs from the Turkey Run and Abbott mines. The Reed Mine 

drains to Davis Creek, a tributary to Cache Creek.

Historical mining activities in the Cache Creek watershed discharged and continue to

discharge large volumes of inorganic mercury (termed total mercury) to creeks in the 

watershed. Much of the mercury discharged from the mines is now distributed in the 

creek channels and floodplain downstream from the mines. Natural erosion processes can 

be expected to slowly move the mercury downstream out of the watershed over the next 

several hundred years. However, current and proposed activities in and around the creek 

channel can enhance mobilization of this mercury. Activities in upland areas, such as 

road maintenance and grazing and timber activities, can add to the mercury loads 

reaching Cache Creek, particularly when the activities take place in areas that have 

elevated mercury levels. Mercury can be transformed to methylmercury in sediment by 

sulfate-reducing bacteria.

Cache Creek. In Cache Creek the watershed above Rumsey is the major source of 

methylmercury. The highest concentrations and production rates were observed below 

the mercury mines in Harley Gulch, in Sulphur and Bear creeks, and in the canyon above 

Rumsey. Lower methylmercury concentrations in water were measured in the North Fork 

and Cache Creek below Clear Lake Dam, which have lower inorganic mercury 

concentrations in sediment.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

178

The sources of total mercury in Cache Creek largely parallel the sources of 

methylmercury. Most mercury derives from the watershed upstream of Rumsey. On a 

five-year average, mercury loads from the mine-related tributaries (Bear Creek, Harley 

Gulch, and Davis Creek), North Fork Cache Creek and Clear Lake contributed about 15 

percent of the mercury loads measured in Cache Creek at Rumsey. In years with high 

degrees of runoff or extreme erosional events, inputs from the inactive mines would be 

much greater. The majority of the inorganic mercury loads were from unnamed sources, 

which include smaller, unmeasured tributaries and mercury in the Cache Creek bed and 

banks. Clean sediment entering the watershed below Rumsey diluted sediment mercury 

concentrations.

Bear Creek. The Bear Creek watershed upstream of all mine inputs contributes less than 

10 percent to each of the loads of methylmercury and total mercury in Bear Creek. 

Sulphur Creek contributes about half of each of the methylmercury and total mercury 

loads in Bear Creek. The remainder of the Bear Creek methylmercury likely comes from 

production within the channel and seepage of underground springs. The rest of the 

mercury load in Bear Creek likely derives from the remobilization of mine waste 

deposited in the floodplain.

Harley Gulch. Much of the methylmercury in Harley Gulch is likely produced in a 

wetland area in the West Branch Harley Gulch, downstream of the inactive mercury 

mines. Over 90 percent the total mercury load in Harley Gulch is estimated to come from 

the West Branch, where the mines are. Total mercury loads from the mines may be 

underestimated due to a lack of data collected during heavy rainfall events. An alluvial 

fan, likely containing mine waste, at the confluence of Harley Gulch and Cache Creek, 

might contribute to the unknown source of mercury in the Cache Creek canyon.

Loading Capacity––Linking Water Quality Pollutant Sources

Total mercury in the creeks is converted to methylmercury by bacteria in the sediment. 

The concentration of methylmercury in fish tissue is directly related to the concentration 

of methylmercury in the water. The concentration of methylmercury in the water column 

is controlled in part by the concentration of total mercury in the sediment and the rate at 

which the total mercury is converted to methylmercury. The rate at which total mercury 

is converted to methylmercury varies from site to site; some sites (wetlands and marshes) 

having greatly enhanced methylation rates.

The linkage analysis describes the relationship between methylmercury concentrations in 

water and in large fish. Data collected in 2000 and 2001 show statistically significant 

relationships between concentrations of aqueous unfiltered methylmercury in water and 

large trophic level 3 and 4 fish. In Cache Creek, large trophic level 3 fish tissue 

concentrations (Sacramento sucker), normalized to 290 mm (from Slotten et al. 2004), 

were regressed against aqueous unfiltered methylmercury concentrations (Y= 584.8X + 

30.2; P < 0.001, R2 = 0.98). In Cache Creek, large trophic level 4 fish tissue 

concentrations (largemouth bass, small mouth bass, and pikeminnow, depending on site), 

normalized to 305 mm (from Slotton et al. 2004), were regressed against aqueous 

unfiltered methylmercury concentrations (Y = 2970.8X – 180.6; P < 0.01, R2 = 0.9). 

Using these relationships, staff determined concentrations of unfiltered methylmercury in 

water that correspond to the proposed criteria for trophic levels 3 and 4 fish (0.12 mg/kg 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

179

and 0.23 mg/kg, respectively). These concentrations are 0.15 ng/l for trophic level 3 fish 

and 0.14 ng/L for trophic level 4 fish. To ensure meeting both fish tissue criteria, staff 

selected 0.14 ng/L as the aqueous unfiltered methylmercury goal for Cache Creek.

For Bear Creek, the methylmercury goal of 0.06 ng/L represents the best estimate of the 

annual, median aqueous (unfiltered) concentration of methylmercury needed to attain the 

target of 0.23 mg/kg wet weight in trophic level 4 fish. Harley Gulch has no trophic level 

4 fish, so the above relationships could not be used. Based on bioaccumulation factors 

specific to Harley Gulch, the aqueous methylmercury goal for Harley Gulch is 0.09 ng/L.

Allocations

The TMDL presents a plan to reduce mercury and methylmercury loads. Reducing the 

methylmercury loads will require a multi-faceted approach that includes controlling 

inorganic mercury loads and limiting the entry of inorganic mercury into sites with high 

rates of methylmercury production. Inorganic mercury loads may be controlled through 

remediation of mercury mines, erosion control, removal of highly contaminated 

sediment, and other activities. In addition to addressing inorganic mercury loads, the 

TMDL discusses limits to the production of methylmercury in constructed 

impoundments, such as gravel pits and water storage facilities. Identification and 

evaluation of the unknown mercury source(s) in the upper basin are essential to attain the 

Cache Creek methylmercury targets in fish tissue and to help reduce mercury in sediment 

of the Sacramento-San Joaquin Delta Estuary.

Since methylmercury in the water column is directly related to mercury levels in fish, the 

following methylmercury load allocations are assigned to tributaries and the main stem of 

Cache Creek.

Methylmercury Load Allocations. Tables D11 and D12 provide methylmercury load 

allocations for Cache Creek, its tributaries, and instream methylmercury production. 

Allocations are expressed as a percent of existing methylmercury loads. The 

methylmercury allocations will be achieved by reducing the annual average 

methylmercury (unfiltered) concentrations to site- specific, aqueous methylmercury 

goals, which are 0.14 ng/L in Cache Creek, 0.06 ng/L in Bear Creek, and 0.09 ng/L in 

Harley Gulch. The allocations in tables D11 and D12 apply to sources of methylmercury 

entering each tributary or stream segment. In aggregate, the sources to each tributary or 

stream segment must have reductions of methylmercury loads as shown below.

Table D12 provides the load allocation within Bear Creek and its tributaries to attain the 

allocation for Bear Creek described in table D11. The inactive mines listed in the 

implementation summary are assigned a 95 percent total mercury load reduction. These 

mines include mines in the Harley Gulch Sulphur Creek and Bear Creek watersheds. 

Reductions in mercury loads from mines, erosion, and other sources in the Sulphur Creek 

watershed are expected to reduce in-channel production of methylmercury to meet the 

Sulphur Creek methylmercury allocation.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

180

Table D11. Cache Creek methylmercury allocations

Source

Existing annual load 

(g/yr)

Acceptable annual 

load (g/yr)

Allocation (% of 

existing load)

Cache Creek (Clear Lake to 

North Fork confluence) 36.8 11 30%

North Fork Cache Creek 12.4 12.4 100%

Harley Gulch 1.0 0.04 4%

Davis Creek 1.3 0.7 50%

Bear Creek at Highway 20 21.1 3 15%

Within-channel production 

and ungauged tributaries 49.5 32 65%

7

a

10%a

Total of loads 122 66 54%

Cache Creek at Yolob

72.5 39 54%

Cache Creek Settling Basin 

Outflowc 87 12 14%

Notes:

a

The allocation includes a margin of safety, which is set to 10% of the acceptable loads. In terms of 

acceptable annual load estimates, the margin of safety is 7 g/yr.

b

Cache Creek at Yolo is the compliance point for the tributaries and Cache Creek channel for meeting the 

allocations and aqueous goals. Agricultural water diversions upstream of Yolo remove methylmercury (50 

g/yr existing load).

c

The Settling Basin Outflow is the compliance point for methylmercury produced in the Settling Basin.

Table D12. Bear Creek methylmercury allocations

Source

Existing Annual 

Load (g/yr)

Acceptable Annual 

Load (g/yr)

Allocation (% of 

existing load)

Bear Creek at Bear Valley 

Road 1.7 0.9 50%

Sulphur Creek 8 0.8 10%

In-channel production and 

ungauged tributaries 11.4 1 10%

0.3a10%a

Total of loads 21.1 3 15%

Bear Creek at Highway 20b

21.1 3 15%

Notes:

a

The allocation includes a margin of safety, which is set to 10% of the acceptable loads. In terms of 

acceptable annual load estimates, the margin of safety is 0.3 g/yr.

b

Bear Creek at Highway 20 is the compliance point for Bear Creek and its tributaries.

To achieve the water quality objectives and the methylmercury allocations listed in tables 

D11 and D12, the following actions are needed: (1) reduce loads of total mercury from 

inactive mines; (2) where feasible, implement projects to reduce total mercury inputs 

from existing mercury-containing sediment deposits in creek channels and creek banks 

downstream from historical mine discharges; (3) reduce erosion of soils with enriched 

total mercury concentrations; (4) limit activities in the watershed that will increase 

methylmercury discharges to the creeks and, where feasible, reduce discharges of 

methylmercury from existing sources; and (5) evaluate other remediation actions that are 

not directly linked to activities of a discharger. Because methylmercury is a function of 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

181

total mercury, reductions in total mercury loads are needed to achieve the methylmercury 

load allocations. Methylmercury allocations will be achieved in part by natural erosion 

processes that remove mercury that has deposited in creek beds and banks since the start 

of mining.

The proposed Basin Plan Amendment for mercury in San Francisco Bay assigns a 

reduction in total mercury loads from the Sacramento–San Joaquin River Delta of 110 

kg/yr. Cache Creek is a major source of mercury to the Delta. To attain the San Francisco 

Bay reduction, loads of total mercury exiting Cache Creek should be reduced. Reductions 

in total mercury loads to the inactive mines in Harley Gulch and the Bear Creek 

watershed assigned by this TMDL and proposed changes to the Cache Creek Settling 

Basin, which would increase the mass of mercury retained in the basin, would create 

significant reductions in loads from Cache Creek.

VI. Minnesota Statewide29 Mercury Total Maximum Daily

Load

Description of the Applicable Water Quality Standards and TMDL 

Target

Minnesota Rules Chapters 7050.0222 and 7052.0100 set forth chronic numeric water 

quality standards based on total mercury concentrations in the water column. The 

wildlife-based standard applicable to only the waters of the Lake Superior Basin is 1.3 

ng/L, while the human health-based standard applicable to waters outside the Lake 

Superior Basin is 6.9 ng/L. In addition to these numeric standards, Chapter 7050.0150, 

subpart 7, provides a narrative standard for assessing the contaminants in fish tissue. The 

narrative standard states that a waterbody is impaired when the Minnesota Department of 

Health recommends a consumption frequency of less than one meal per week for any 

member of the population.

To establish the two regional TMDLs, Minnesota selected a target of 0.2 mg/kg fish 

tissue mercury concentration. Fish tissue mercury concentration was selected as the water 

quality target for the TMDLs because it was consistent with EPA’s 2001 methylmercury 

fish tissue criterion. In the 2001 guidance, EPA chose to express the water quality 

criterion as a fish tissue concentration rather than as a water column value because fish 

consumption is the primary route of human exposure. Two aspects of EPA’s criterion are 

toxicity and exposure. Minnesota relied on EPA’s assessments of toxicity to humans but 

selected a more state-specific exposure rate. For purposes of calculating its recommended 

human health-based fish tissue criterion, EPA assumes that people consume 17.5 g/day of

fish. Minnesota selected a higher consumption rate, 30 g/day of fish, based on several 

surveys of the fish-eating habits of upper-Midwest recreational fishers.

29 As described in Section 6 of this guidance, Minnesota divided the state into two regions, a northeast region and a southwest region, and 

developed a TMDL for each region. Although Minnesota’s report is called a “statewide TMDL,” the two regional TMDLs do not address 

all the mercury impairments in the state. The TMDLs address 511 of the lake and river reach impairments in Category 5 of Minnesota’s 

2006 Integrated Report.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

182

Since Minnesota’s water quality standards are water column chronic standards for total 

mercury, not fish tissue concentration standards, Minnesota demonstrated a link from the 

fish tissue mercury concentration TMDL target to the numeric water column water 

quality standards. Bioaccumulation factors for 14 lakes representing agricultural areas, 

urban areas, and forested areas were used to calculate the water column concentration 

that would be equivalent to the 0.2 mg/kg fish tissue mercury concentration target.

Source Assessment

Sources that Minnesota considered in developing the two regional TMDLs included 

atmospheric deposition, wastewater treatment plants, non-municipal waste discharges, 

and stormwater. Atmospheric deposition was the only significant nonpoint source of 

mercury identified by Minnesota. The state identified 99 percent of the total mercury load 

to the state as coming from atmospheric deposition. Both natural and anthropogenic 

sources contribute to the atmospheric deposition mercury load. Minnesota identified 

natural sources as contributing 30 percent to the atmospheric deposition mercury load, 

while the remaining 70 percent is from worldwide anthropogenic sources. Point sources 

that Minnesota considered included wastewater treatment plants, pulp and paper mills, 

taconite mines, coal-fired power plants, and one refinery. The state recognized that 

stormwater is considered a point source and therefore subject to wasteload allocations; 

however, for purposes of estimating a baseline mercury load (referred to in the TMDL 

report as the total source load), the mercury loadings from stormwater were included in 

the estimate of loadings from atmospheric deposition. Using data from two studies in 

Minnesota, the state concluded that the primary source of mercury to stormwater is 

atmospheric deposition rather than specific anthropogenic sources.

Loading Capacity

Minnesota established a loading capacity for each of the two regional TMDLs. Each 

loading capacity was calculated by multiplying a regional reduction factor30 needed to 

achieve the fish tissue mercury concentration target by the total source load31 for each 

region, thus calculating a regional load reduction goal.32 The load reduction goal was 

subtracted from the total source load to arrive at the loading capacities.

The total source load was considered the baseline condition from which reductions would 

be needed to achieve water quality standards. Minnesota selected the year 1990 as the 

baseline to which reductions would be applied. Minnesota selected 1990 as the baseline 

for three reasons. First, the total source load is the sum of the point source load and the 

nonpoint source load. The nonpoint source load is represented by total (wet and dry) 

mercury deposition. Minnesota’s estimate of both wet and dry deposition is from lake 

sediment cores collected in a study conducted from 1988 to 1990. The second reason for 

selecting 1990 was to remain consistent with other mercury reduction baselines. The state 

30 The northeast regional reduction factor is 65 percent, and the southwest regional reduction factor is 51 percent.

31 The baseline load for the northeast region is 1153 kg/yr, and the baseline load for the southwest region is 1628 kg/yr.

32 The load reduction goal for the northeast region is 749 kg/yr, and the load reduction goal for the southwest region is 830 kg/yr.
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

183

uses 1990 as its mercury emission inventory baseline, and other state and federal plans, 

such as the Great Lakes Binational Toxics Strategy and the Lake Superior Lakewide 

Management Plan, use 1990 as a baseline for assessing mercury reductions. Minnesota 

selected a baseline year that was consistent with other reduction goals and targets. Last, 

Minnesota selected 1990 because prior to 1990 mercury use was relatively high, and then 

beginning in around 1990, mercury use dropped precipitously as mercury was removed 

from many products. For this reason Minnesota concluded that 1990 represents the end of 

a period when mercury emissions and fish tissue concentrations were in a steady state.

The sum of the point source load and nonpoint source load are the total source load for 

each region. The total source load for each region simply defines the 1990 baseline 

condition for the region to which the applicable reduction factor is applied.

The existing point source contribution to the total source load was calculated based on the 

sum of design flows for point sources within each region and mean effluent mercury 

concentrations. The design flows were current-day design flows, while the mean effluent 

mercury concentrations were “typical” mercury concentrations unless actual facility 

effluent concentrations were available. Actual facility effluent concentrations were used 

for the coal-fired power plants, the one refinery, and the Metro and Western Lake 

Superior Sanitary District wastewater treatment plants. For all other point sources, typical 

mercury concentrations were used. A typical effluent concentration of 5 ng/L was used 

for wastewater treatment plants. It was based on a study by the Association of 

Metropolitan Sewerage Agencies, a state study of 37 NPDES facilities, and the Mercury 

Maps report. Minnesota relied on the Mercury Maps report in support of the mean 

effluent mercury concentration of 13 ng/L for pulp and paper mills, although effluent 

reports from one Wisconsin and one Minnesota facility show effluent concentrations in 

the range of 1.6 ng/L to 2 ng/L. Minnesota used its discharge monitoring database to 

calculate 1.5 ng/L as the mean mercury effluent concentration for taconite mines.

The existing nonpoint source contribution to the total source load was based on total 

mercury deposition to the state. Minnesota used sediment cores from Minnesota lakes to 

estimate total statewide mercury deposition as 12.5 g km-2yr

-1

. Minnesota used the 

regional surface areas for each of the two regions, along with the total mercury 

deposition, to estimate the nonpoint source contribution to the total source load.

The reduction factor for each region is the percent reduction in total mercury load needed 

to achieve the fish tissue target of 0.2 mg/kg for the 90th percentile of the standard-length 

fish. Fish tissue data were reviewed for the standard-size top predator fish in each region. 

The 90th percentile fish tissue mercury concentration and median concentrations were 

calculated for each region for top predator fish (walleye and northern pike). Minnesota 

used the difference between the 90th percentile mercury concentration in top predator 

fish within each region and the 0.2 mg/kg target to calculate the reduction factors. 

Minnesota used fish tissue data from 1988 to 1992 to establish the reduction factors. The 

state looked at fish tissue data from 1970 to 2002; however, to be consistent with the 

baseline year of 1990, fish tissue data from 1988 to 1992 were selected. Multiyear data 

better represent real conditions over time because they account for year-to-year 

variability in weather, fish populations, and sampling locations. Data for the standard-size 

top predator fish were used to calculate the reduction factor. Mercury bioaccumulates in 

fish; therefore, mercury concentrations are typically highest in the top predator fish. To 
Appendix D. Synopsized Mercury TMDLs Developed or Approved by EPA

184

account for temporal and spatial comparisons of mercury concentrations in these top 

predator fish, Minnesota used the standard-size top predator fish.33 Top predator fish that 

are collected for fish tissue analysis vary in size and age. Because mercury concentrations 

vary with the size of fish and age of fish, it is difficult to make comparisons regarding 

mercury concentrations in fish without establishing a standard of comparison. Use of the 

standard-size fish accounted for differences in mercury concentrations due to age and size 

and allowed Minnesota to compare mercury concentrations across waterbodies.

Allocations

Consistent with the regional approach used to establish the loading capacities, Minnesota 

did not assign waterbody-specific allocations; rather, the state established gross 

allocations for each region.

Minnesota assigned 1 percent of the loading capacity to point sources as the wasteload 

allocation for each regional TMDL. Minnesota chose 1 percent of the loading capacity 

based on an approach used in the Mercury Maps report to screen watersheds for 

significant point source impacts to identify waterbodies impaired primarily by 

atmospheric mercury (see appendix E on Mercury Maps). The northeast region wasteload 

allocation was set at 1 percent of the loading capacity, while the southwest region’s 

wasteload allocation was set equal to the point source load portion of the total source 

load. The state set the southwest region’s wasteload allocation equal to the point source 

load portion of the total source load because it was slightly less than 1 percent of the 

southwest region’s loading capacity and the state chose the more restrictive allocation.

Load allocations for each region were established by subtracting the wasteload allocation 

and any explicit margin of safety from the established loading capacity. The remaining 

load within each region was assigned to the load allocation. The approved loading 

capacity and allocations for both regional TMDLs are shown in table D13.

Table D13. Approved northeast and southwest mercury TMDLs

33 Minnesota uses a standard size of 40 cm (approximately 22 inches) for walleye and 55 cm (approximately 16 inches) for northern pike.

Region

Loading

capacity

Load

allocation

Wasteload 

allocation

Margin of 

safety

Northeast 1.10 kg/day 1.09 kg/day 0.01 kg/day Implicit

Southwest 2.18 kg/day 1.55 kg/day 0.02 kg/day 0.61 kg/day
Appendix E. Model Descriptions

185

Appendix E. Model Descriptions

This appendix describes currently available models discussed in this guidance. These 

models aid in developing bioaccumulation factors and modifying fish tissue criteria (see 

chapter 3), making assessments (see chapter 4), developing total maximum daily loads 

(TMDLs) (see chapter 6), and in carrying out related programs such as 319 Nonpoint 

Source Program activities, watershed management, stormwater permits, and National 

Pollutant Discharge Elimination System (NPDES) discharge evaluations. This appendix

provides a description of each model, some examples of how or where it has been used, 

and a Web site for further information about each model.

BASS (Bioaccumulation and Aquatic System Simulator)

The Bioaccumulation and Aquatic System Simulator (BASS) is a model that simulates 

the population and bioaccumulation dynamics of age-structured fish communities. 

Although BASS was specifically developed to investigate the bioaccumulation of 

chemical pollutants within a community or ecosystem context, it can also be used to 

explore population and community dynamics of fish assemblages that are exposed to a 

variety of non-chemical stressors such as altered thermal regimes associated with 

hydrological alterations or industrial activities, commercial or sports fisheries, and 

introductions of non native or exotic fish species.

BASS is being used to investigate methylmercury bioaccumulation in the Florida 

Everglades and to predict population and community dimensions of “fish health” for a 

regional analysis of the ecological sustainability of the Albemarle Pamlico drainage basin 

in North Carolina and Virginia.

Information on BASS can be found at: http://www.epa.gov/athens/research/

modeling/bass.html.

Community Multi-Scale Air Quality (CMAQ) Model

The CMAQ modeling system is a comprehensive, three-dimensional, grid-based Eulerian 

air quality model designed to estimate pollutant concentrations and depositions over large 

spatial scales (Byun and Ching 1999; Byun and Schere 2006; Dennis et al. 1996). The 

CMAQ model is a publicly available, peer-reviewed, state-of-the-science model 

consisting of a number of science attributes that are critical for simulating the oxidant 

precursors and nonlinear chemical relationships associated with the formation of 

mercury. Version 4.3 of CMAQ (Bullock and Brehme 2002; Byun and Schere 2006) 

reflects updates to earlier versions in a number of areas to improve the underlying science 

and address comments from peer review. The updates in mercury chemistry in version 

4.3 from that described in Bullock and Brehme (2002) are as follows:

1. The elemental mercury (Hg0) reaction with H2O2 assumes the formation of 

100 percent reactive gaseous mercury (RGM) rather than 100 percent particulate 

mercury (HgP).

2. The Hg0reaction with ozone assumes the formation of 50 percent RGM and 

50 percent HgP rather than 100 percent HgP.
Appendix E. Model Descriptions

186

3. The Hg0reaction with OH assumes the formation of 50 percent RGM and 

50 percent HgP rather than 100 percent HgP.

4. The rate constant for the Hg0+ OH reaction was lowered from 8.7 to 7.7 x 10-14 

cm

3 molecules-1

s

-1

.

CMAQ simulates every hour of every day of the year and requires a variety of input files 

that contain information pertaining to the modeling domain and simulation period. These 

include hourly emissions estimates and meteorological data in every grid cell and a set of 

pollutant concentrations to initialize the model and to specify concentrations along the 

modeling domain boundaries.

Meteorological data, such as temperature, wind, stability parameters, and atmospheric 

moisture content influence the formation, transport, and removal of air pollution. The 

CMAQ model requires a specific suite of meteorological input files to simulate these 

physical and chemical processes. For recent CMAQ modeling, meteorological input files 

were derived from a simulation of the Pennsylvania State University’s National Center 

for Atmospheric Research Mesoscale Model (Grell et al. 1994) for the entire year of 

2001. This model, commonly referred to as MM5, is a limited-area, nonhydrostatic, 

terrain-following system that solves for the full set of physical and thermodynamic 

equations that govern atmospheric motions. For this analysis, version 3.6.1 of MM5 was 

used. A complete description of the configuration and evaluation of the 2001 

meteorological modeling is provided by McNally (2003).

These initial and boundary concentrations were obtained from the output of a global 

chemistry model, Harvard’s GEOS-CHEM model (Yantosca 2004), to provide the 

boundary concentrations and initial concentrations. The global GEOS-CHEM model 

simulates atmospheric chemical and physical processes driven by assimilated 

meteorological observations from NASA’s Goddard Earth Observing System (GEOS). 

This model was run for 2001 with a grid resolution of 2 degrees x 2.5 degrees (latitudelongitude) and 20 vertical layers.

The CMAQ modeling domain encompasses all the lower 48 states and extends from 126 

degrees west longitude to 66 degrees west longitude and from 24 degrees north latitude to 

52 degrees north latitude. The modeling domain is segmented into rectangular blocks 

referred to as grid squares. The model predicts pollutant concentrations and depositions 

for each grid cell. For this application the horizontal domain consisted of 16,576 grid 

cells that are roughly 36 km by 36 km. The modeling domain contains 14 vertical layers, 

with the top of the modeling domain at about 16,200 meters, or 100 millibar. The height 

of the surface layer is 38 meters.

A CMAQ modeling run was performed to estimate the impact of global sources on U.S. 

deposition estimates. For this analysis, all non-U.S. mercury input species to the model 

were set to zero. By comparing the results of this analysis with the 2001 Clean Air 

Mercury Rule (CAMR) base case run, which included all U.S. and global mercury 

species, the percent of total mercury deposition attributable to global sources can be 
Appendix E. Model Descriptions

187

estimated.34 The model estimated that over 80 percent on average of total mercury 

deposition in the United States is attributable to global sources.

Due to the evolving nature of mercury modeling science, such deposition estimates have 

associated uncertainties. For example, it remains difficult to distinguish between the 

natural emissions of mercury and the re-emission of previously deposited anthropogenic 

mercury and there remains uncertainty in the scientific community concerning the 

atmospheric processes that control the oxidation state of atmospheric mercury. Thus, 

further advances in the current understanding of mercury chemistry could potentially lead 

to changes in the modeling parameters and assumptions governing the mercury chemistry 

in the models and therefore, changes in the estimate of the fraction deposited in the U.S. 

attributable to global sources.

For more information on CMAQ, see http://www.epa.gov/asmdnerl/CMAQ.

D-MCM (Dynamic Mercury Cycling Model)

D-MCM is a food web simulation of mercury accumulation in lakes. It predicts the 

cycling and fate of major forms of mercury in lakes, including methylmercury, Hg (II), 

elemental mercury, and total mercury. It is a time-dependent mechanistic model which 

considers the most important physical, chemical, and biological factors affecting fish 

mercury concentrations in lakes. D-MCM is meant for lotic (lake) systems, and is not 

meant to be used for lentic (streams, rivers, etc.) systems.

D-MCM can be used to develop and test hypotheses, scope field studies, improve 

understanding of cause and effect relationships, predict responses to changes in loading, 

and support design and evaluation of mitigation options. It was used in the development 

of mercury TMDLs for McPhee and Narraguinnep Reservoirs in Colorado and for the 

TMDLs for Arivaca and Pena Blanca Lakes in Arizona. The Everglades Mercury Cycling 

Model (E-MCM) was developed off of D-MCM and added vegetation processes and the 

ability to simulate multiple sediment layers for wetlands.

Information on D-MCM can be found at: http://rd.tetratech.com/DraftHgBrochurev2.pdf.

EXAMS2 (Exposure Analysis Modeling System)

EXAMS2 is a model for creating aquatic ecosystem models which can evaluate the fate, 

transport, and exposure concentrations of chemicals. Chemicals include synthetic organic 

chemicals like pesticides, industrial materials, and leachates from disposal sites. 

EXAMS2 core is a set of modules that link chemical properties to limnological 

characteristics that control the fate and transport of chemicals in aquatic systems. This 

model allows for both long-term analysis of chronic chemical discharges at constant 

release and varying release over time, and short-term analysis of chemical releases.

34 On February 8, 2008, the D.C. Circuit Court of Appeals vacated the Clean Air Mercury Rule and remanded portions of it to EPA, for 

reasons unrelated to the technical analyses cited in this guidance.
Appendix E. Model Descriptions

188

EXAMS2 has commonly been used to predict pesticide fate in water and soil. This model

has been used to evaluate the role of hydroxyl radicals in degrading pesticides by 

researchers at the University of Georgia. EXAMS2 was also used to simulate mercury 

fate in the Withlacoochee River watershed and the Ohoopee River watershed in Georgia.

Information on EXAMS2 can be found at: http://www.epa.gov/ceampubl/swater/exams/.

GBMM (Grid Based Watershed Mercury Model)

EPA’s Grid Based Watershed Mercury Model (GBMM) is a continuous grid-based 

watershed mercury loading model using the latest ArcGIS platform. It simulates the 

spatial and temporal dynamics of mercury from both point and non-point sources on a 

daily basis. The model calculates the water balance, sediment generation and transport, 

and mercury dynamics within a watershed. The mercury transport and transformation 

module simulates the following key processes:

● Mercury input from atmospheric deposition.

● Mercury assimilation and accumulation in forest canopy and release from forest 

litter.

● Mercury input from bedrock weathering.

● Mercury transformation in soils.

● Mercury transformation in lakes and wetlands including reduction and net 

methylation.

● Mercury transport through sediment and runoff.

● Mercury transport in stream channels.

GBMM accepts input data from atmospheric deposition, point sources, and natural 

background in time series or in digital spatial maps. By using the grid-based technology, 

flow and mercury dynamics can be examined at any of several points in the watershed.

The software has been peer reviewed and tested on two watersheds in Georgia, where it 

was used to calculate mercury TMDLs. GBMM has been used to investigate the mercury 

fate and transport in Brier Creek watershed located in the coastal plain of Georgia. 

GBMM was used to investigate detailed watershed mercury processes. The findings of 

this study were presented in Eighth International Conference on Mercury as a Global 

Pollutant (August 2006), Madison, Wisconsin, USA.

For more information on GBMM please visit: http://www.epa.gov/athens/research/

modeling/mercury/gbmm.html.

GEOS-CHEM Model

The Global GEOS-CHEM model simulates physical and chemical atmospheric processes 

driven by observations by NASA’s Goddard Earth Observing System (GEOS). This 

model is managed and supported by the atmospheric chemistry modeling group at 

Harvard University. This model is used as a tool for atmospheric composition problems.
Appendix E. Model Descriptions

189

This model was run for the 2001 CMAQ model with a grid resolution of 2 degree x 2.5 

degree (latitude-longitude) and 20 vertical layers. GEOS–Chem is a major contributor to 

the NASA Global Model Initiative (GMI). GEOS–Chem has been interfaced with the 

NASA/GISS general circulation model to investigate the effects of climate change. This 

work contributes to the multi-institutional Global Change and Air Pollution (GCAP) 

project. GEOS–Chem provides chemical modules for data assimilation of tropospheric

composition at the NASA GMAO.

For more information on GEOS-CHEM please visit: http://www-as.harvard.edu/

chemistry/trop/geos/geos_overview.html.

GWLF (Generalized Watershed Loading Function)

GWLF simulates mixed land use watersheds to evaluate the effect of land use practices 

on downstream loads of sediment and nutrients (N, P). As a loading function model, it 

simulates runoff and sediment transport using the curve number (CN) and Universal Soil 

Loss Equation (USLE), combined with average nutrient concentration, based on land use. 

Recently, a GIS-interface has been integrated which can use national land use and soil 

GIS data. Also GWLF models in-stream routing using the Muskingum-Cunge method 

and simulates three particle classes of sediment transport.

GWLF has been used in studies and TMDL development nationally. It is suitable for 

application to generalized watershed loading, source assessment, and seasonal and 

interannual variability. It has been extensively used in northeast and mid-Atlantic 

regions. It has been adopted by Pennsylvania as state system for TMDL development and 

agricultural land management. GWLF was used to calculate mercury load from the 

watershed to a lake in several TMDLs in Arizona (e.g., TMDL for Pena Blanca Lake, 

Arizona). GWLF is also applied in West Virginia TMDL projects by Tetra Tech, Inc.

Information on GWLF can be found at: http://www.epa.gov/nrmrl/pubs/

600r05149/600r05149gwlf.pdf and http://www.vims.edu/bio/models/basinsim.html.

Mercury Maps screening analysis

A simple screening-level analysis of the mercury sources affecting a waterbody or 

waterbodies can assist in determining what type of approach to TMDLs is most 

appropriate. EPA’s Mercury Maps (USEPA 2001b) is a geographic information system 

(GIS)-based analysis using national data coverage for watersheds, fish tissue 

concentrations, and non-air deposition source locations.

Mercury Maps uses a simplified form of the IEM-2M model applied in EPA’s Mercury 

Study Report to Congress (USEPA 1997a). By simplifying the assumptions inherent in 

the freshwater ecosystem models described in the report to Congress, Mercury Maps 

showed that these models converge at a steady state solution for methylmercury 

concentrations in fish that are proportional to changes in mercury inputs from 

atmospheric deposition (e.g., over the long term, fish concentrations are expected to 

decline proportionally to declines in atmospheric loading to a waterbody). This analytical 

approach applies only to situations where air deposition is the only significant source of 

mercury to a waterbody and the physical, chemical, and biological characteristics of the 

ecosystem remain constant over time. To predict reductions in fish concentrations, 
Appendix E. Model Descriptions

190

Mercury Maps requires estimates of percent air deposition reductions by watershed, as 

generated from a regional air deposition model, and georeferenced measurements of 

mercury concentrations in fish.

A state or authorized tribe can apply Mercury Maps on a state or watershed scale. For 

example, it could apply Mercury Maps on a statewide scale, using state- or tribe-defined 

watershed boundaries. The state might have its own data on point source effluent loads 

and more detailed information on other significant sources of mercury in the state, e.g., 

erosion of mine tailings or natural geology.

Because Mercury Maps is a simplified approach, it has several limitations.

1. The Mercury Maps approach is based on the assumption of a linear, steady state 

relationship between concentrations of methylmercury in fish and present-day air 

deposition mercury input. This condition might not be met in many waterbodies 

because of recent changes in mercury inputs and other environmental variables that 

affect mercury bioaccumulation. For example, the United States has recently 

reduced human-caused emissions, and international emissions have increased.

2. Environmental conditions might not remain constant over the time required to 

reach steady state inherent in the Mercury Maps methodology, particularly in 

systems that respond slowly to changes in mercury inputs.

3. Many waterbodies, particularly in areas of historical gold and mercury mining in 

western states, contain significant non-air sources of mercury. Mercury Maps’ 

methodology should not be applied to such waterbodies.

4. Finally, Mercury Maps does not provide for a calculation of the time lag between 

a reduction in mercury deposition and a reduction in the methylmercury 

concentrations in fish.

Despite the limitations of Mercury Maps, for those watersheds where mercury comes 

almost exclusively from air deposition, Mercury Maps can be used as a simple screening 

tool to show the watersheds across a region where the current fish tissue concentration on 

average exceeds the new methylmercury fish tissue criterion and, thus, to estimate the 

atmospheric load reductions needed to meet the new criterion. Further information on 

Mercury Maps is at http://www.epa.gov/waterscience/maps and from the Office of Air 

Quality Planning and Standards at http://www.epa.gov/ttn/atw/utility/ria_final.pdf.

MOBILE

MOBILE is an EPA model for estimating air pollution from highway vehicles. MOBILE 

predicts emissions (grams/mile) of air pollutants from cars, trucks, and motorcycles under 

various conditions. MOBILE models emissions of several air toxics, hydrocarbons (HC), 

carbon monoxide (CO), oxides of nitrogen (NOx), carbon dioxide (CO2), and particulate 

matter (PM). MOBILE is based on emissions testing of tens of thousands of vehicles. The 

model accounts for the impact on emissions of factors such as legislative changes in 

vehicle emission standards, variation in local conditions such as temperature, humidity, 

and fuel quality, and changes in the types and use of the vehicles being driven.
Appendix E. Model Descriptions

191

MOBILE has been used to calculate national and local inventories of current and future 

levels of highway vehicle emissions. The inventories are used to inform decision-making 

about air pollution policy and programs at the national, state and local level. Inventories 

based on MOBILE are also used to meet requirement of federal statutes like the Clean 

Air Act (CAA) and the National Environmental Protection Act (NEPA). MOBILE 

contributed to the creation of the National Emissions Inventory (NEI).

Information on MOBILE can be found at: http://www.epa.gov/otaq/mobile.htm.

NDMMF (National Descriptive Model of Mercury in Fish Tissue)

NDMMF is a statistical model which simulates mercury accumulation in varying species 

of fish. It simulates factors representing differences in species, size, and sampling 

method. This model has the ability to control for site factors specific to a location that 

influence mercury concentrations in fish tissue. For example, all fish tissue samples can 

be scaled to a standardized 14” bass for a specific location. The model works in 

association with a national dataset of over 30,000 samples of fish tissue for calibration.

NDMMF could be useful for evaluating spatial and temporal trends in fish mercury 

concentrations and developing fish-consumption advisories. The U.S. Geological Survey 

recently applied this model to study spatial variation in fish-tissue mercury 

concentrations in the St. Croix River Basin, Minnesota and Wisconsin.

Information on NDMMF can be found at: http://emmma.usgs.gov/fishHgAbout.aspx.

NONROAD

NONROAD is an EPA model for estimating air pollution from all engines, equipment, 

and vehicles that is considered “nonroad”. This includes recreational vehicles, 

agricultural equipment, industrial equipment, residential equipment, and construction 

equipment. The NONROAD model is used to predict past, present, and future emissions 

of air pollutants like hydrocarbons (HC), oxides of nitrogen (NOx), carbon monoxide 

(CO), carbon dioxide (CO2), sulfur oxides (SOx), and particulate matter (PM). It has been 

shown that “nonroad” sources contribute a significant amount of air pollutants to the 

environment.

Used in complement to MOBILE, NONROAD has been used to calculate national and 

local inventories of current and future levels of “nonroad” emissions. This model has 

become critical over the past several years in providing state and local pollution control 

agencies the ability to create accurate and consistent inventories of “nonroad” emissions 

to satisfy the requirements of the Clean Air Act Amendments of 1990. NONROAD 

contributed to the creation of the National Emissions Inventory (NEI). The Lake 

Michigan Air Directors Consortium (LADCO) used NONROAD to forecast emissions in 

their region and make appropriate policy recommendations.

Information on NONROAD can be found at: http://www.epa.gov/otaq/nonrdmdl.htm.

QEAFDCHN (Quantitative Environmental Analysis Food Chain) Model

The QEAFDCHN model is a tool for predicting chemical residues in aquatic organisms 

given the concentrations of chemicals in water and sediment. To predict chemical 
Appendix E. Model Descriptions

192

residues, the model requires information on the individual species (bioenergetic and 

physiological) and their diets. The model is designed to determine chemical residue in 

aquatic organisms given varying chemical concentrations in both water and sediment 

over time.

The QEAFDCHN model can be used in a steady-state or dynamic application. The model 

allows the specification of complex food webs, e.g., fish preying on multiple species 

including smaller fish, and even age classes of fishes. The model treats individual 

segments of the greater ecosystem as individual ecosystems and the model has an aquatic 

organism migration feature. QEAFDCHN has been applied to the Lavaca Bay, Texas, 

chlor-alkali facility mercury contamination study by Quantitative Analysis, LLC.

Information on QEADFCHN can be found at: http://www.epa.gov/superfund//health/

conmedia/sediment/pdfs/bsafissue.pdf.

Regional Modeling System for Aerosols and Deposition (REMSAD)

REMSAD is a three-dimensional grid model designed to calculate the concentrations of 

both inert and chemically reactive pollutants by simulating the physical and chemical 

processes in the atmosphere that affect pollutant concentrations (ICF International 2006). 

REMSAD has been peer-reviewed and is designed to support an understanding of the 

distributions, sources, and removal processes relevant to fine particles and other airborne 

pollutants, including soluble acidic components and several toxic species (mercury, 

cadmium, dioxin, polycyclic organic matter [POM], atrazine, and lead).

Mercury can be present in the atmosphere in both the gas and particulate phases. The

mercury species included in REMSAD are Hg

0 (elemental mercury vapor), Hg2+ (divalent 

mercury compounds in gas phase), and HgP (divalent mercury compounds in particulate 

phase). These species represent the oxidation state of mercury, and the gas and particulate 

phases. The reactions in REMSAD, which are based on the studies of Lin and Pehkonen 

(1999) and other recently published studies, simulate the transfer of mercury mass from 

one of these states to another. REMSAD Version 8 uses the full Carbon Bond-V 

mechanism to simulate gas-phase photochemical processes in the atmosphere (micro-CB 

is still available as an option), and it also includes a chemical mechanism to calculate the 

transformations of mercury.

REMSAD simulates both wet and dry deposition of mercury. Wet deposition occurs as a 

result of precipitation scavenging. Dry deposition is calculated for each species based on 

land-use characteristics and meteorological parameters. REMSAD also includes 

algorithms for the reemission of previously deposited mercury (originating from 

anthropogenic and natural sources) into the atmosphere from land and water surfaces due 

to naturally occurring (e.g., microbial) processes.

REMSAD provides estimates of the concentrations and deposition of mercury and all 

other simulated pollutants at each grid location in the modeling domain. Post-processing 

can provide concentration averages and deposition totals for any subset of the time span 

of the simulation for any location within the domain.

The mercury treatment in REMSAD can be expanded to include additional, tagged 

mercury species. The Particle and Precursor Tagging Methodology (PPTM) feature 
Appendix E. Model Descriptions

193

allows the user to tag or track emissions from selected sources or groups of sources and

to quantify their contribution to mercury deposition throughout the modeling domain and 

simulation period.

The REMSAD model is capable of “nesting” one or more finer-scale subgrids within a 

coarser overall grid. This feature uses a fully interactive two-way nesting capability that 

permits high resolution over selected source and/or receptor regions of interest. The 

modeling system can be applied at scales ranging from a single metropolitan area to a 

continent containing multiple urban areas.

REMSAD has been used in identifying the sources contributing mercury deposition to a 

waterbody. In an EPA Wisconsin pilot project, REMSAD was used to input the air 

pollutant deposition results to aquatic models like the Mercury Cycling Model, to 

examine how mercury levels in fish might respond to potential changes in deposition. 

REMSAD has been used to develop TMDLs and determine strategies for addressing 

mercury and other air pollutant deposition. REMSAD was used in developing the 

mercury TMDL for the Coastal Bays and Gulf Waters of Louisiana (approved in 2005) 

and the mercury TMDLs for middle and south Georgia (approved in 2002).

Information on REMSAD can be found at: http://remsad.saintl.com/.

SERAFM (Spreadsheet-based Ecological Risk Assessment for the 

Fate of Mercury)

The SERAFM model is a spreadsheet-based risk assessment tool specifically designed 

for mercury contaminated ecosystems. SERAFM uses a steady-state simplifying 

assumption and includes a series of sequentially linked modules presented on separate 

spreadsheets. These modules include:

● Atmospheric deposition

● Watershed soil erosion

● Watershed mercury loading

● Waterbody solids balance

● Equilibrium partitioning (DOC complexation, solids partitioning)

● Mercury speciation

● Waterbody mercury calculations (historic sediment contamination, background, 

and remedial goal)

● Fish tissue concentrations

● Wildlife hazard quotients

The SERAFM model incorporates more recent advances in scientific understanding and 

implements an updated set of the IEM-2M solids and mercury fate algorithms that were 

described in the 1997 Mercury Study Report to Congress (USEPA 1997c).
Appendix E. Model Descriptions

194

For more information on SERAFM please visit: http://www.epa.gov/athens/research/

modeling/mercury/serafm.html and http://www.epa.gov/nerl/news/forum2005/

knightes.pdf.

TOXI5

TOXI5 is one of two submodels of WASP (Water Quality Analysis Simulation Model), 

the other being EUTRO5, which deals with eutrophication. TOXI5 is a sediment 

transport model which can also simulate the transport and transformation of chemicals. 

The transport of up to three types of sediment and up to three chemicals can be simulated. 

The chemicals may react independently or they may be linked with reaction yields which 

predict the fate of the interaction. Dissolved and sorbed chemical concentrations in the 

waterbody bed and overlying waters can be predicted using TOXI5.

TOXI5 was used to simulate the fate of mercury in the Ochlockonee Watershed in 

Georgia, to help develop mercury TMDLs for the Southeast U.S., and to evaluate the 

feasibility of dam release of water on the Nakdong River in Korea to mitigate frequent 

accidental spills of toxic chemicals.

For more information on TOXI5 please visit: http://smig.usgs.gov/cgi-bin/SMIC/

model_home_pages/model_home?selection=wasp.

WASP (Water Quality Analysis Simulation Program)

The Water Quality Analysis Simulation Program (WASP) is a dynamic compartmentmodeling program for aquatic systems, including both the water column and the 

underlying benthos. It has detailed mercury transformation processes for the water 

column and benthic sediments. The mercury module simulates the following key 

processes:

● Volatilization of Hg0

(aq) to Hg0(air)

● Oxidation of Hg0  HgII

● Reduction of HgII  Hg0

● Methylation of HgII  MeHg

● Demethylation of MeHg  HgII

● Photoreduction of MeHg  Hg0

WASP has been used to examine eutrophication of Tampa Bay, Florida; phosphorus 

loading to Lake Okeechobee, Florida; eutrophication of the Neuse River Estuary, North 

Carolina; eutrophication of the Coosa River and Reservoirs, Alabama; PCB pollution of 

the Great Lakes; eutrophication of the Potomac Estuary; kepone pollution of the James 

River Estuary; volatile organic pollution of the Delaware Estuary; heavy metal pollution 

of the Deep River, North Carolina; and mercury in the Savannah River, Georgia.

Information on WASP can be found at: http://www.epa.gov/athens/research/

modeling/wasp.html.
Appendix E. Model Descriptions

195

WCS (Watershed Characterization System) Mercury Loading Model

The WCS Mercury Loading model is a GIS-based (ArcView 3.x) extension of the WCS 

model based on a soil-mercury mass balance model (IEM v 2.05). The soil-mercury mass 

balance model calculates surface soil concentrations in dissolved, sorbed, and gas phases.

The model accounts for three routes of contaminant entry into the soil:

● Deposition of particle-bound contaminant through dry fall

● Deposition through wet fall

● Diffusion of gas phase contaminant into the soil surface

The model also accounts for four dissipation processes that remove mercury from the 

surface soils:

● Volatilization (movement of gas phase out of the soil surface)

● Runoff of dissolved phase from the soil surface

● Leaching of dissolved phase through the soil horizon

● Erosion of particulate phase from the soil surface

The model assumes that the diffusion and volatilization processes are roughly balanced 

on an annual basis. The WCS Mercury Loading model was used to develop many 

TMDLs in EPA Region 4 including a mercury TMDL for the Middle and Lower 

Savannah River.

Information on the WCS model can be found at: http://www.epa.gov/athens/

wwqtsc/WCS-toolbox.pdf.

Example of Linking Models

Since there is no single model that can simulate all processes involved in TMDLs, some 

TMDLs for mercury have linked together models of atmospheric deposition, watershed 

loading, and mercury cycling with bioaccumulation. For example, a watershed mercury 

model such as GBMM, or the watershed module within SERAFM could be linked to a 

receiving water mercury model such as WASP, and a bioaccumulation model such as 

BASS.

GBMM is a spatially discrete, dynamic watershed mercury loading model which was 

designed for direct linkage to the EPA receiving waterbody model, WASP. GBMM can 

simulate mercury fate and transport within the watershed landscape and transport 

mercury and soils to the receiving waters through the tributaries. WASP can in turn 

simulate mercury dynamics in the receiving water. To predict bioaccumulation of the 

resulting mercury concentrations into fish tissues, WASP can then be linked to BASS. 

SERAFM is a more simplified approach and captures the processes from watershed to 

waterbody to fish bioaccumulation; however, it makes simplifying assumptions such as 

the waterbodies are steady state and it uses the national BAFs presented by EPA for 

trophic level fish.
Appendix E. Model Descriptions

196

Linkage of such models may be a workable solution in some situations. One of the 

limitations of the GBMM-WASP-BASS approach is that it is not an “off-the-shelf” 

model and a high level of expertise might be required to link the models together.
Appendix F. Examples of National Deposition Monitoring Networks

197

Appendix F. Examples of National 

Deposition Monitoring Networks

A number of national deposition monitoring networks might be useful for developing 

TMDLs. The networks include the National Atmospheric Deposition Program–National 

Trends Network (NADP/NTN) and the Mercury Deposition Network (MDN, a subset of 

the NADP network). The NADP/NTN is a nationwide network of precipitation 

monitoring stations. Operating since 1978, it collects data on the chemistry of 

precipitation for monitoring of geographic patterns and temporal long-term trends. 

NADP/NTN measures weekly average concentrations of sulfate, nitrate, ammonium, base 

cations, and acidity at approximately 230 monitoring stations across the United States. 

The MDN measures concentrations of total mercury in precipitation at approximately 45 

monitoring stations across the United States and Canada. NADP/NTN results for 2003 

are shown in figure F-1. For more information about NADP, see http://nadp.sws.

uiuc.edu.

Used in conjunction with NADP/NTN, the Clean Air Status and Trends Network 

(CASTNET) is the nation’s primary source of atmospheric data on the dry deposition 

component of total acid deposition, ground-level ozone, and other forms of atmospheric 

pollution that enters the environment as particles and gases. CASTNET measures weekly 

average atmospheric concentrations of sulfate, nitrate, ammonium, sulfur dioxide, and 

nitric acid, as well as hourly concentrations of ambient ozone levels in rural areas. Dry 

deposition rates are calculated using the measured atmospheric concentrations, 

meteorological data, and information on land use, surface conditions, and vegetation. 

Seventy-nine monitoring stations operate across the United States. For more information 

about CASTNET, see http://www.epa.gov/castnet and http://nadp.sws.uiuc.edu.

Note that these national monitoring networks generally provide only estimates of wet 

deposition; estimates of dry deposition can be obtained from the literature. For more 

information on deposition monitoring networks, see Deposition of Air Pollutants to 

the Great Waters: Third Report to Congress (USEPA 2000h) (http://www.epa.gov/

oar/oaqps/gr8water/3rdrpt) and the Air-Water Interface Plan (http://www.epa.gov/ttn/

caaa/t3/reports/combined.pdf).
Appendix F. Examples of National Deposition Monitoring Network

198

Figure F-1. MDN data for 2005.
Index

199

Index

Note: Bold numbers indicate where the term is defined (if applicable). If the term has been broken 

into subcategories, this is noted with a defined entry.

ambient water quality criteria............See

AWQC

analytical methods..... 3, 34, 49–52, 100, 

101, 151

antidegradation ......... 45, 93, 94, 107–11

atmospheric deposition.... 16, 74, 75–79, 

87, 159, 162

AWQC.............................. 23, 31, 33, 57

BAF

and Great Lakes.............................. 64

and model selection ........................ 85

calculating....................................... 23

defined ............................................ 23

draft national....................... 25, 30–33

sampling considerations ........... 33–35

site-specific....... 2, 35, 38, 45, 66, 102

using.......................................... 20–36

weighted ....................................... 157

best management plan ...................... 121

best management practices...... See BMP

bioaccumulation ................................. 10

bioaccumulation factor.............See BAF

bioaccumulation model ......... 28–30, 80, 

185, 195

biomagnification............... 14, 15, 53, 85

BMP

and designated use.......................... 47

and MMPs ............................ 111, 121

and variances.................................. 46

and WQBELs.......................... 91, 114

dental ............................................ 120

emission........................................ 127

livestock.................................. 89, 167

CAA.................... 13, 16, 71, 76, 125–28

CAMR

analysis supporting ............. 24, 44, 61

and SERA FM model ................... 193

and WCS......................................... 83

defined ............................................ 13

modeling for................................... 78

Clean Air Act .......................... See CAA

Clean Air Mercury Rule....... See CAMR

CMAQ.......................... 78, 185–87, 189

cold-vapor atomic fluorescence 

spectrometry....................See CVAFS

Community Multi-Scale Air Quality.See

CMAQ

composite samples ................... 3, 54–56

Consolidated Assessment and Listing 

Methodology (CALM)................... 57

Continuing Survey of Food Intakes by 

Individuals.......................... See CSFII

criterion, calculating..................... 20, 21

CSFII.............................. 39, 40, 41, 149

CTR.................................... 64, 170, 176

CVAFS..........50, 51, 151, 152, 153, 154

CWA 101(a)........................... 46, 47, 61

CWA 304(a)........................... 17, 18, 68

demethylation................... 15, 24, 32, 82

designated use

and CWA 101(a) ............................ 46

and UAA ........................................ 46

and variances............................ 42, 43

changing ......................................... 47

fishable ........................................... 20

protecting........................ 1, 17, 18, 36

detection level .................................... 58

detection limit....................... 50, 58, 152

dilution flow....................................... 64

D-MCM...........29, 83, 84, 165, 168, 187

emissions

anthropogenic..................... 12, 13, 14

controls......................................... 118

hourly estimates in models........... 186

mobility of...................................... 12

natural........................... 1, 12, 13, 187

regulations................ 13, 123, 125–28

to air ..................... 75, 77, 83, 125–28
Index

200

trends in .............. 76–77, 75, 125, 126

environmental justice ......................... 70

EPA methods

1630.................. 3, 50, 51, 52, 58, 151

1631

and measuring ..... 3, 49, 50, 51, 52, 

151

and nondetects.................... 58, 100

and WQBELs........................... 100

defined.................................. 49–52

in NPDES permits........... 100, 104, 

103–4

in TMDLs................................... 89

1669................................................ 50

245.7............... 3, 49, 50, 51, 100, 153

existing use................... 43, 46, 107, 111

exposure

and BAFs................ 25, 27, 28, 33, 35

and consumption advisories ........... 67

and fish ........................................... 64

and humans........... 123, 170, 176, 181

and sample types ...................... 53, 54

and the RSC........................ 19, 37, 68

data in criterion calculation ..... 20, 36, 

41, 65, 68, 148

duration in criterion calculation...... 18

fish tissue concentration as proxy 

to................................................. 29

from drinking water........................ 21

human health effects............... 1, 9–10

FDA action level ................................ 70

field sampling plan....................... 52–56

fish advisories

and water quality standards...... 66–70

EPA guidance on................ 52, 61–62

issued.................................. 12, 20, 67

map (U.S.) ...................................... 12

revising........................................... 69

statewide................................... 12, 68

Fish Advisory Program ...................... 66

fish intake rate/estimate

and trophic levels................ 21, 34, 59

default........................... 21, 37, 40, 41

fish sampling guidance..............52–56

fish tissue residue criterion... See TRC

in criterion calculation..............21, 36, 

37–42, 148, 150

limits..........................................66–70

modifying ..................................37–42

of subsistence consumers................38

relative source criterion........ See RSC

RSC.........................19, 21, 27, 37, 68

TRC.....................................20, 21, 37

fish sampling guidance........................34

freshwater

and estuarine fish

age........24, 28, 32, 33, 34, 53, 161, 

168, 184, 185, 192

and water quality criterion ...21, 37, 

40, 41, 59, 60, 148

intake..See fish intake rate/estimate

mercury found in.............15, 33, 56

ecosystem models..........................189

lakes and rivers................................12

target species.....................38, 39, 145

Great Lakes Guidance.........................50

Great Lakes Water Quality Initiative 

(GLI) .....................................2, 39, 63

growth dilution..............................10, 15

health effects of methlymercury........See

exposure: human health effects

human health

toxicological risk assessment ..........20

impairment

addressing..............................1, 62, 72

assessing..............................21, 57–62

identifying sources of............115, 119

listing decisions.............................57–62

Mercury Containing and Rechargeable 

Battery Management Act ..............128

Mercury Deposition Network .....77, 197

mercury emissions............ See emissions

Mercury Maps..............73, 81, 183, 184, 

189–90

mercury minimization plan .....See MMP

Mercury Study Report to Congress....10, 

30, 78, 83, 118, 157, 162, 189, 193
Index

201

mercury, forms of defined .................. 25

methylation........... 15, 24, 29, 65, 73, 79

minimum level.......................... 107, 111

mixing zone ........................ 2, 63, 65–66

MMP

and antidegradation........... 94, 108–15

and reasonable potential ............. 7, 93

and type of facility........................ 119

as a permit condition....... 105, 118–22

guidance on................................... 119

implementing.................... 94, 118–22

model D-MCM...... 29, 83, 84, 165, 168, 187

Dynamic Mercury Cycling Model......

SeeD-MCM

empirical bioaccumulation ............. 28

mechanistic bioaccumulation ......... 29

regression................ 28, 29, 60, 61, 84

selecting.......................................... 84

spatially detailed............................. 83

steady state/mass balance ......... 81–85

uncertainty ................................ 78, 85

monitoring and assessment............ 3, 12, 

49–62

National Descriptive Model for Mercury 

in Fish Tissue.................See NDMMF

National Health and Nutrition 

Examination Survey ....................... 10

National Pollutant Discharge 

Elimination System .........SeeNPDES

National Study of Chemical Residues 

in Fish Tissue.................................. 51

National Toxics Rule.................... 46, 64

NDMMF......................... 24, 34, 61, 191

neurological effects ............................ 10

nondetections........................ 54, 58, 100

normalizing factors....................... 24, 34

NPDES

and antidegradation........................See

antidegradation

and WQBELs......... 91–97, 99, 102–3, 

105, 107, 110–17, 121

documentation .............................. 118

effluents, measuring mercury in.... 94, 

100, 102–4, 110, 114

fish tissue criterion, implementing. 97

general considerations.................... 91

new sources and new discharges, 

mercury in .................. 97, 111, 114

permit special condition ............... 105

pollutant minimization plan

defined........................................ 45

recommended conditions ... 106–21

pollutant minimization program.... 45, 

86, 118

reasonable potential determination

and fish tissue data ........ 5, 93, 102, 

105–10, 112

and intakes................................ 112

defined........................................ 91

how to................................. 102–21

process...................................... 101

recommended permitting 

approach ............................... 92–97

reopener clause........ 93–95, 104, 105, 

114

Overview of P2 Approaches at 

POTWs......................................... 118

partition coefficient .......................... 159

persistency................................ 119, 160

pollution prevention ...... 45, 71, 94, 108, 

118–19, 123–25

POTW .........45, 75, 88, 117–21, 123–25

prenatal exposure .................................9

public participation............................. 70

publicly owned treatment works.......See

POTW

quality control .................................... 61

quantitation level................ 50, 100, 151

recommended form of criterion.......... 18

reference dose............................See RfD

regulations under CAA............... 125–28

relative source contribution...... See RSC

REMSAD............................. 73, 77, 192

RfD......9, 21, 37, 67, 150, 156, 157, 168

RSC.......................... 21, 37, 41, 68, 157
Index

202

sampling

and BAFs............................ 23, 33–35

fish............................................ 52–56

guidance on............................... 52–56

sediment.................................... 80, 89

shellfish

advisories.................................. 62, 66

and CWA 101(a) ............................ 46

in criterion calculation.................... 21

intake rates.......................... 14, 40, 41

to be monitored............................... 53

significant industrial users................ 123

site-specific conditions................. 36–48

site-specific procedure..... 22–35, 64, 66, 

80, 102

sources

atmospheric ....... 9, 12–15, 42, 75–79, 

189–90

human activity .................... 13, 14, 80

in fish........................................ 12–14

mining................................. 13, 48, 79

natural............... 13, 14, 43, 47, 74, 80

overseas.................................. 43, 186

point sources..... 14, 42, 48, 65, 74, 75

sediment........................ 9, 15, 79, 115

species.............. 15, 26, 53–54, 146, 149

spill prevention and containment 

control plan................................... 121

tissue concentration-based standard .. 20, 

22

tissue residue value............... 64, 97, 113

TMDL........................................... 71–90

allocation approaches..................... 85

best uses.......................................... 71

challenges....................................... 16

considerations........................... 71–90

defined............................................ 71

examples................... See Appendix A

geographic scale ................... 4, 72, 73

modeling tools.............. 77–79, 80–85

monitoring provisions..................... 89

pollutant loading scenario............... 86

Total Maximum Daily Load. See TMDL

translation factor .............24–33, 97–121

trophic levels

and BAFs...............26, 27, 30–34, 195

and fish intake .........21–23, 34, 38, 59

and fish species .............................146

and food webs .....................15, 53, 85

and GLI ...........................................64

and TMDLs..156, 157, 168, 170, 176, 

178

averaging data across ................59–62

sampling in............................3, 53, 56

UAA..............................................46–47

uncertainty

and margin of safety.......71, 165, 173, 

180

assessing loadings .........................109

BAFs ...................................30, 32, 34

from extrapolating results ...............29

in TMDL .........................................71

model...................................31, 78, 85

reducing...........................................22

RfD....................................................9

use attainability analysis ......... See UAA

variances

and controls.........................42, 44, 46

antidegradation................................45

considerations..................................43

how they apply ..........................42–46

large-scale .......................................45

multiple discharger (group).............46

protocols..........................................43

scenarios..........................................43

time frames......................................44

when appropriate.........................2, 42

water column concentrations........31, 50, 

65, 106

water quality criteria

and BAFs.............................20–36, 38

and fish advisories...........................68

and methods 1630, 1631 ...........51, 52

components ...............................17, 18

water quality standards

and fish advisories.....................66–70
Index

203

water quality-based effluent limits....See

WQBEL

Watershed Characterization System..See

WCS

WCS ................................... 83, 158, 195

weighted consumption...................... 156

WQBEL

and anti-backsliding.... 7, 96, 110, 117

and mercury in intake water ..... 6, 112

and NPDES permits........... 91–97, 99, 

102–3, 106, 107, 117, 118, 119

and pretreatment........................... 117

and technology-based limits...... 7, 91, 

96, 110, 114, 117

and TMDLs............. 94, 97, 101, 102, 

114–17

and variances............................ 42, 45

defined............................................ 91

deriving .............................. 99, 91–97

determining need for .............. 102–13

elements of ................. 94, 96, 113–22

forms of.......................................... 91