823B04001
Draft Guidance for Implementing the
     January 2001 Methyl mercury
         Water  Quality Criterion

               United States Environmental Protection Agency
                Office of Science and Technology (4305T)
                   1200 Pennsylvania Ave., NW
                    Washington, D.C. 20460
                     EPA-823-B-04-001
                   www.epa.gov/waterscience
                      August 2006

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Draft Guidance for Implementing the
     January 2001 Methyl mercury
         Water Quality Criterion
            United States Environmental Protection Agency
             Office of Science and Technology (4305T)
                 1200 Pennsylvania Ave., NW
                  Washington, DC 20460
                   EPA-823-B-04-001
                 www. epa. gov/waterscience
                    August 2006

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                                         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. 2006. Draft Guidance for Implementing the January 2001 Methylmercury Water
        Quality Criterion. EPA 823-B-04-001. U.S. Environmental Protection Agency, Office of
        Water, Washington, DC.

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                                          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 mg methylmercury/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 the criterion to 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.

This is the first time EPA has 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
may 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
the 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 these 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 in 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/methylmercurv/criteria.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.
Benjamin H. Grumbles
Assistant Administrator for Water
U.S. Environmental Protection Agency

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                                                                                           Contents
                               CONTENTS

1  Executive Summary	1
2  Introduction	3
   2.1  What is the interest in mercury?	3
     2.1.1   What are the health effects of mercury?	3
     2.1.2   How frequent are the environmental problems?	5
   2.2  What are the sources of mercury in fish?	6
   2.3  How does  methylmercury get into fish and shellfish?	8
   2.4  Why is EPA publishing this document?	10
   2.5  What is the effect of this document?	10
3  Water Quality Criteria and Standards Adoption	11
   3.1  What must states and authorized tribes include as they adopt the
        methylmercury criterion?	11
     3.1.1   What do the CWA and EPA's regulations require?	11
     3.1.2   What is the recommended form of the methylmercury criterion?	12
     3.1.3   Can  states or authorized tribes adopt a water column concentration
            criterion?	14
   3.2  What options are available to address for site-specific conditions and
        concerns?	26
     3.2.1   How can the methylmercury water quality criterion be modified for site-
            specific conditions?	26
     3.2.2   How do water quality variances apply?	31
     3.2.3   How are use attainability analyses conducted?	37
4  Monitoring and Assessment	41
   4.1  What are the analytical methods for detecting and measuring
        methylmercury concentrations in fish and water?	41
     4.1.1   What is Method 1631 for determination of mercury in water?	43
     4.1.2   What analytical methods are available for determination of
            methylmercury?	43
   4.2  What is the recommended guidance on field sampling plans for collecting
        fish for determining attainment of the water quality standard?	44
     4.2.1   What fish species should be monitored?	44
     4.2.2   What sample types best represent exposure?	45
     4.2.3   What is the recommended study design for site selection?	46
     4.2.4   How often should fish samples be collected?	46
     4.2.5   How many samples should be collected?	48
     4.2.6   What form of mercury should be analyzed?	48
   4.3  How should waterbody impairment be assessed for listing decisions?	49
     4.3.1   How should nondetections be addressed?	49
     4.3.2   How should data be averaged across trophic levels?	50

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Contents
                           4.3.3   How should older data be assessed?	53
                           4.3.4   How should fish consumption advisories be used to determine
                                  impairment?	53
                     5   Other Water Quality Standards Issues	55
                        5.1  How does this criterion  relate to the criteria published as part of the Great
                             Lakes Initiative?	55
                        5.2  What is the applicable flow for a water column-based criterion?	55
                        5.3  How are mixing zones used for mercury?	56
                           5.3.1   What is a mixing zone?	56
                           5.3.2   How does a mixing zone apply for the fish tissue-based
                                  methylmercury criterion?	56
                           5.3.3   Does the guidance for the fish tissue-based criterion change the Great
                                  Lakes Initiative approach to mixing zones for bioaccumulative
                                  pollutants?	56
                        5.4  How are fish consumption advisories and water quality standards
                             harmonized?	57
                           5.4.1   What is the role of the Fish Advisory Program?	57
                           5.4.2   How are consumption limits for consumption advisories determined?	58
                           5.4.3   How does the criterion differ from the advisory level?	58
                           5.4.4   What if there is a difference between  the attainment of a  criterion and
                                  issuance of a fish consumption advisory?	59
                           5.4.5   Should existing advisories be revised to reflect the new criterion?	60
                           5.4.6   How is the criterion related to FDA action levels?	60
                        5.5  What public participation is recommended for implementing the
                             methylmercury criterion?	61
                     6   TMDLs    	63
                        6.1  What is a TMDL?	63
                        6.2  How have states and tribes approached mercury TMDLs?	63
                           6.2.1   How have large-scale approaches been used for mercury TMDLs?	64
                           6.2.2   What is the Mercury Maps screening  analysis?	65
                           6.2.3   What are considerations in developing mercury TMDLs?	67
                     7   NPDES Implementation  Procedures	81
                        7.1  What are the general considerations in NPDES permitting?	81
                        7.2  How does EPA recommend implementing the fish tissue criterion for
                             NPDES permits?	82
                        7.3  What are the implementation  procedures when the criterion is adopted as
                             a water column value?	83
                        7.4  What are the implementation  procedures when the criterion is adopted as
                             a fish tissue value and the permitting authority uses a water column
                             translation of a fish tissue value?	83
                        7.5  What are the implementation  procedures 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?	84
                           7.5.1   How to determine the need for permit limits to control mercury (i.e.,
                                  how to determine reasonable potential)	85

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                                                                                           Contents
     7.5.2  Where reasonable potential exists, how can WQBELs be derived from
            a tissue value?	91
   7.6  What are the recommended analyses for new sources or new dischargers
       discharging quantifiable amounts of mercury?	99
     7.6.1  What are the recommendations for permitting authorities when
            considering issuing permits for new sources or new dischargers
            where the fish tissue concentrations in the receiving waterbody are
            unknown?	99
     7.6.2  What are the recommended permit conditions for new sources or new
            dischargers where the fish tissue in the receiving water does not
            exceed the criterion?	100
     7.6.3  What are recommended permit conditions for new sources or new
            dischargers where the fish tissue in the receiving water exceeds the
            criterion?	102
   7.7  What are the special conditions for mercury in a facility's intake?	103
     7.7.1  How to consider mercury intakes with a reasonable potential approach. 103
     7.7.2  How to consider mercury in intakes in WQBELs	103
8  Related Programs	105
   8.1  How does pollution prevention play a role in the methylmercury criterion?	105
   8.2  What regulations has EPA issued pursuant to the CAA to address air
       emissions of mercury?	107
     8.2.1  Municipal Waste Combustors	108
     8.2.2  Medical Waste Incinerators	108
     8.2.3  Chlor-Alkali Plants	108
     8.2.4  Industrial Boilers	109
     8.2.5  Hazardous Waste Combustors	109
     8.2.6  Coal-fired Power Plants	109
9  References	113
Appendix A.  Synopsized Mercury TMDLs Developed or Approved by
              EPA	127
   I.    Ochlockonee Watershed, Georgia	128
     Description of the Applicable Water Quality Standards	128
     Source Assessment	129
     Loading Capacity—Linking Water Quality and Pollutant Sources	130
     Allocations	131
   II.   Arivaca Lake, Arizona	133
     Description of the Applicable Water Quality Standards	133
     Source Assessment	133
     Loading Capacity—Linking Water Quality and Pollutant Sources	135
     Allocations	137
   III.  McPhee and Narraguinnep Reservoirs, Colorado	140
     Description of the Applicable Water Quality Standards	140
     Source Assessment	140
     Loading Capacity—Linking Water Quality and Pollutant Sources	141
                                                                                                 in

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Contents
                        Allocations	141
                      IV. Clear Lake, California	142
                        Description of the Applicable Water Quality Standards	142
                        Source Assessment	143
                        Loading Capacity-Linking Water Quality and Pollutant Sources	144
                        Allocations	145
                   Appendix B.  Tables from Methylmercury Criteria Document	149
                   Appendix C.  Analytical Methods	153
                   Appendix D.  Examples of National Deposition Monitoring Networks	157
                   Appendix E.  Methylmercury/Mercury Ratio Exhibited in Muscle Tissue
                                of Various Freshwater Fish Species	159
                   Index         	163
IV

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                                                                                          Contents
                                 TABLES

Table 1. National draft BAFs for dissolved methylmercury	21
Table 2. Estimates of freshwater and estuarine combined finfish and shellfish
       consumption from the combined 1994-96 and 1998 CSFII surveys	29
Table 3. Example data for calculating a weighted average fish tissue value	51
Table A1. Annual average mercury load from each subbasin	130
Table A2. Predicted mercury for annual average load and flow	131
Table A3. Annual total mercury load to Arivaca Lake	136
Table A4. Predicted and observed mercury for annual average load and flow	137
Table A5. Summary of TMDL allocations and needed load reductions (in g-Hg/yr)	138
Table A6. Summary of mercury load estimates for McPhee Reservoir	140
Table A7. Summary of TMDL allocations and needed load reductions for  McPhee
       Reservoir	142
Table A8. Summary of TMDL allocations and needed load reductions for
       Narraguinnep Reservoir	142
Table A9. Summary of mercury load allocations	146
Table A10. Sediment goals for mercury in Clear Lake	146
Table 5-1. Exposure parameters used in derivation of the water quality criterion	150
Table 5-14. Average Mercury Concentrations in Marine Fish and Shellfish	151
Table 5-30. Exposure estimates for methylmercury and percent of total exposure
       based on adults in the general population	152
Table C1. Analytical methods for determining mercury and methylmercury in tissue ... 153
Table C2. Analytical methods for determining mercury and methylmercury in water,
       sediment, and other nontissue matrices	154

                                FIGURES

Figure 1. Fish Tissue Mercury Concentrations Averaged by Watershed
       (USEPA2001d)	5
Figure 2 Total Number of State Mercury Fish Consumption Advisories 2004	6
Figure 3. Percent of total mercury deposition attributable to global sources
       (USEPA2005C)	66
Figure 4. Trends in mercury air emissions between 1990 and 1999	69
Figure 5. Implementing the fish tissue criterion in NPDES permits	82
Figure 6. Determining reasonable potential	85
Figure 7. Process for determining the WQBEL	92
Figures. Procedures for addressing new sources and new discharges	100
Figure 9. Mercury deposition in the United States following CAMR and CAIR	110
Figure D-1 Mercury Deposition Network data for 2003	158

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                                                                                    Executive Summary
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 mg methylmercury/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 1997c). As of 2004, 44 states, 1 territory, and
2 tribes have issued fish consumption advisories for mercury covering 13.2 million lake
acres and 765,000 river miles (USEPA 2005a). Mercury is widely distributed in the
environment and originates from both natural and anthropogenic processes, including
combustion and volcanoes. Methylmercury is highly bioaccumulative and is the form of
mercury that bioaccumulates most efficiently in the food web.

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 exercising responsibility under section
CWA 303(c) on how to use the new fish tissue-based criterion recommendation as they
develop their own water quality standards for methylmercury. One approach that States
and authorized tribes may decide to use is to translate the tissue residue value to a water
column value through use of methylmercury bioaccumulation factors (BAFs). If a state or
authorized tribe decides to use  this approach, EPA recommends three potential
approaches for relating a concentration of methylmercury in fish tissue to a concentration
of mercury in ambient water. The approaches are:
   •  Deriving site-specific methylmercury BAFs

   •  Using bioaccumulation models

   •  Using EPA's draft default methylmercury BAFs

All three approaches have limitations, especially in the amount of data necessary to
develop a BAF. This guidance  discusses the advantages and limitations of each approach.
States and authorized tribes may also consider calculating their own fish tissue criteria or
adopting site-specific criteria for methylmercury to reflect local or regional fish
consumption rates or relative source contributions. EPA encourages states and authorized
tribes to develop a water quality criterion for methylmercury using local or regional data
rather than the default values if they believe that such a water quality criterion would be
more appropriate for their target population. This guidance also discusses variances and
use attainability analyses (UAAs) relating to methylmercury.

This document describes methods for measuring mercury and methylmercury in both
tissue and water. These methods can analyze mercury and methylmercury in tissue and
water at very low levels—well below the previous criterion for mercury in water and the
current criterion of methylmercury in fish tissue. This document also provides guidance
for field sampling plans, laboratory analysis protocols, and data interpretation on the
basis of previously published EPA guidance on sampling strategies for contaminant

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Executive Summary
                      monitoring. This document also describes how states can assess the attainment of water
                      quality criteria and protection of designated uses by comparing sampling data to water
                      quality criteria.

                      EPA expects that, as states and authorized tribes adopt the methylmercury criterion, the
                      number of waterbodies states report as impaired due to mercury contamination might
                      increase. EPA expects this to occur because the number of river miles and lake acres
                      under fish consumption advisories due to methylmercury in fish tissue greatly exceeds
                      the number of waters listed by states as impaired. 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 may increase. Thus,
                      this guidance also discusses approaches for managing the development of Total
                      Maximum Daily Loads (TMDLs) for waterbodies impaired by mercury. This includes
                      approaches for addressing waterbodies where much of the mercury is from atmospheric
                      sources and how TMDLs can take into account ongoing efforts to address sources of
                      mercury,  such as programs under the Clean Air Act (CAA) and pollution prevention
                      activities. This guidance also includes a recommended approach for directly
                      incorporating the methylmercury tissue criterion in National Pollutant Discharge
                      Elimination System (NPDES) permits.

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                                                                                         Introduction
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
both 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 hence can be widely dispersed and transported
thousands of miles from sources of emission. 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). Divalent mercury, when bound to airborne
particles, is readily removed from the atmosphere by precipitation and is also dry
deposited. Even after it deposits, mercury commonly returns to the atmosphere either as a
gas or associated with particles, and 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.

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 is the form of mercury that presents the greatest risks to human health.
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 mercury?
Exposure to methylmercury can result in a variety of health effects in humans. Children
who 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 2002e, USEPA 2005b). In 2000, the National Academy of Sciences
(NAS)/National Research Council (NRC) reviewed the health studies on mercury (NRC
2000). EPA's current assessment of the methylmercury reference  dose (RfD) relied on
the quantitative analyses performed by the NRC (USEPA 2002e). The RfD is an estimate
(with uncertainty  spanning perhaps an order of magnitude) of a daily exposure to the
human population, including sensitive subgroups, that is likely to  be without an
appreciable risk of deleterious effects during a lifetime (USEPA 2002e). 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 per day (0.1 microgram of
methylmercury per day for each kilogram of a person's body mass) in 2001 (USEPA
2002e). EPA believes that exposures at or below the RfD are unlikely to be associated
with 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 to be non-appreciable.
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 2005b).

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Introduction
                      The primary route by which the U.S. population is exposed to methylmercury is through
                      the consumption offish containing methylmercury. The exposure levels at which
                      neurological effects have been observed in children can occur via maternal consumption
                      offish (rather than high-dose poisoning episodes) (USEPA 2005b). In 2005, the National
                      Health and Nutrition Examination Survey (NHANES) published 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 (ig/L, a concentration associated
                      with neurologic 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 (ig/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 2005b).

                      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.  Other recent studies have not observed this association. The studies that
                      have observed an association suggest that the exposure to methylmercury might attenuate
                      the beneficial effects offish consumption (USEPA 2005b). There also is some recent
                      evidence  that exposures of methylmercury might result in genotoxic or immunotoxic
                      effects. 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 2005b).

                      Deposition of mercury to waterbodies can also have an adverse impact on ecosystems
                      and wildlife. Plant and aquatic life, as well as fish, 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 with aquatic systems experiencing the greatest exposures due to
                      bioaccumulation. Bioaccumulation refers to the net uptake of a contaminant from all
                      possible pathways and 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 the consumption of contaminated fish and, as a result,
                      accumulate mercury to levels greater than those in their prey (USEPA 1997c). The
                      Regulatory Impact Analysis of the Clean Air Mercury Rule (USEPA 2005b) provides a
                      full discussion of potential ecosystem effects updated since publication of the 1997
                      Mercury Study Report to Congress (USEPA 1997c). Thus, the approach outlined in the
                      Clean Air Mercury Rule provides states an alternative methodology for designing their
                      site-specific  TMDL analyses.

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                                                                                        Introduction
2.1.2   How frequent are the environmental problems?
As of 2004, 42 states reported at least one waterbody as being impaired due to mercury,
and over 8,500 specific waterbodies were listed as being impaired due to mercury, either
solely or in combination with other pollutants. In 2001, EPA mapped concentrations of
mercury in fish tissue from fish collected from waterbodies all over the country (i.e., not
limited to the 595 waters identified by the states) and compared these to the 2001 national
recommended water quality criterion of 0.3 mg methylmercury/kg fish tissue wet weight
(see 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 200Id).

A statistical comparison of the data presented in Figure 1 (from the National Listing of Fish
Advisories (NLFA) fish tissue database), versus data from the National Lake Fish Tissue
Study (NLFTS), a national random sample offish tissue  in 500 lakes and reservoirs
throughout the United States, showed the NLFA data to be biased high (USEPA 2005b).
The bias was found to be the result of sampling bias in the NLFA toward fish of species
and sizes that tended to bioaccumulate more mercury. When data from the NLFA and
NLFTS were normalized to a set of standard species and lengths, the bias was removed.
(See USEPA 2005b, Figure 4-11, page 5-16 which shows fish tissue data averaged by
watershed (i.e., hydrologic unit codes, or HUCs.) As a result, the NLFA data suggest that
fewer watersheds contain fish with methylmercury that exceed the criterion.


                 Fish Tissue Mercury Concentrations
                          Averaged  by Watershed
  Average Fish Cone (pom)
      0.001 -0.149
      0.150 - 0.299
B      0.300 - 0.440
      0.450 - 0.5&S
      0.6-33
      No Data
      Suies

      Note: New Cntenon for mercury in fish is 0.3 pom Po^rt of departure in fish aoVisoHes often r 0.15 ppm to 03 pom rarge
      Average value Qased on Me: sarnies only. See report text for ct:a s
      3:xr;~: Lateral Ls:>r§ o*ris" ,5-ic '.\ 3lfe Ad-.nsi'es (NLrV.'A: Me-s^ry =is* ~issi.* Datata?* iJune CCC1;
Figure 1. Fish Tissue Mercury Concentrations Averaged by Watershed (USEPA 2001 d)

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Introduction
                       As of December 2004, 44 states, 1 territory, and 2 tribes have issued fish consumption
                       advisories1 for mercury covering 13.2 million lake acres and 765,000 river miles (see
                       Figure  2). Twenty-one states have issued advisories for mercury in all freshwater lakes
                       and rivers in their state, and 12 states have statewide advisories for mercury in their
                       coastal waters (USEPA 2005a). EPA believes that the increase in advisories is primarily
                       due to increased sampling of previously untested waters and not necessarily due to
                       increased levels or frequency of contamination. Although states, territories, tribes, and
                       local governments also continue to issue new fish advisories, most new fish advisories
                       involve mercury and are a result of increased monitoring and assessment rather than
                       increased domestic releases of mercury. In fact, U.S. mercury emissions have declined by
                       more than 45 percent since  1990 (USEPA 2005a).
                    Fish Consumption Advisories for Mercury
                                                                                - NH (R,L)
                                                                                WA(R.L)
                                                                                -RKR.LI
                                                                                            NOTE:  ThEira
                                                                                            depicts the prese
                                                                                            arid type offish
                                                                                            the states far mercury
                                                                                            us of December 2004.
                                                                                            Because only selected
                                                                                            wateibodies are
                   Mercury Advisories by Type:
                   • AMuriw tar sp*cinc wiUirtJodlo*
does not reflect the
fall eiieri of chemic al
rnmtflir.TnjHnn cdf fitli
tissues in each state or
province.
                     **n5Grt*f specific
                                                    - AM ntm uiMfer «Mwry
   iSEPA NatbiaiFkl aidnitmRCaitamliatei Pronran
                                                                           soiree: 2DDt Nattaial Lhflig err Fkl Adttkorlei
Figure 2. Total Number of State Mercury Fish Consumption Advisories 2004

                       2.2   What are the sources of mercury in fish?
                       Mercury is emitted from both natural and anthropogenic sources. Mercury's residence
                       time in the atmosphere is much longer than that of most metals, because mercury can
1 States issue their advisories and guidelines voluntarily and have flexibility in what criteria they use and how the data are collected. 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.

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                                                                                              Introduction
circulate for up to a year (USEPA 1997a). 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 derived from both U.S. and international sources. EPA estimates that
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
natural and re-emitted sources. EPA's current air quality modeling does indicate a
substantial variation across the country, with domestic sources influencing mercury
deposition much more in the east and global sources being a more significant contributor
to mercury deposition in the west, where relatively few domestic sources exist. This
estimate was based on the advanced, state-of-the-science modeling  assessment of the
atmospheric fate, transport, and deposition of mercury conducted by EPA for the Clean
Air Mercury Rule (CAMR) (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, MWIs), manufacturing sources (e.g., chlor-alkali, cement,
pulp and paper manufacturing), and mining (USEPA 1997a).

U.S. anthropogenic emissions of mercury to the air have declined more than 45 percent
since passage of the 1990 CAA Amendments. These amendments provided new authority
to EPA  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  medical
waste incineration (see Figure 4). Regulations were  issued in the  1990s to control
mercury emissions from waste combustion. In addition, actions to limit the use of
mercury, most notably 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.2

The largest single source of anthropogenic mercury  emissions in the country  currently 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. In March 2005,  EPA
signed the CAMR to permanently cap and reduce mercury emissions from coal-fired
power plants (USEPA 2005e). This rule makes the United States the first country in the
world to regulate mercury emissions from utilities. CAMR builds on EPA's Clean Air
2 EPA has issued several regulations pursuant to the CAA to address these air emissions, including recent regulations covering coal-fired
power plants. For example, see Title 40 of the Code of Federal Regulations (CFR) Part Cb (standards for municipal waste combustors); 40
CFR Part 60, subpart Ce (standards for MWIs); 40 CFR Part 63 subpart IIIII (standards for chlor-alkali plants); 40 CFR 63.1203 (a)(2) and
(b)(2) (standards for existing and new hazardous waste-burning incinerators), 40 CFR 63.1204 (a)(2) and (b)(2) (standards for existing and
new hazardous waste-burning cement kilns), and § 63.1205 (a)(2) and (b)(2) (standards for existing and new hazardous waste-burning
lightweight aggregate kilns); 40 CFR Part 63, Subpart DDDDD (standards for industrial boilers); and 70 Federal Register 28,606 (May 18,
2005) (codified at 40 CFR Parts 60, 72 and 75) (standards for power plants). See also section 8.2 of this document.

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Introduction
                      Interstate Rule (CAIR) to significantly reduce emissions from coal-fired power plants.
                      When fully implemented, these rules will reduce utility emissions of mercury nearly 70
                      percent.

                      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 CFRPart
                      415). All NPDES permits must assure that permitted discharges achieve water quality
                      standards (40 CFR 122.42(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 are only one part of the mercury cycle, however. 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.
                      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 governing methylmercury
                      uptake in fish and shellfish tissue. The following is a synopsis of the current
                      understanding of mercury cycling in the environment as described in the Regulatory
                      Impact Analysis of the Clean Air Mercury Rule (USEPA 2005b).

                      Mercury occurs naturally in the environment as several different chemical species. The
                      majority of mercury in the atmosphere (95-97 percent) is present in a neutral, elemental
                      state (Hgo) (Lin and Pehkonen 1999), while in water, sediments, and soils, the majority of
                      mercury is found in the oxidized, divalent state (Hg(II)) (Morel et al. 1998). A small
                      fraction of this pool of divalent mercury is transformed by microbes into methylmercury
                      (CH3Hg(II) (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 emissions sources. Elemental mercury is oxidized in the atmosphere to
                      form the more soluble mercuric ion (Hg(II)) (Schroeder et al. 1989). Particulate and
                      reactive gaseous phases of Hg(II) are the principle forms of mercury deposited onto
                      terrestrial and  aquatic systems because they are more efficiently scavenged from the
                      atmosphere through wet and dry deposition than Hgo (Lindberg and Stratton 1998).
                      Because Hg(II) species or reactive gaseous mercury (ROM) and particulate mercury
                      (Hg(p)) in the  atmosphere tend to be deposited more locally than Hgo, differences in the
                      species of mercury emitted affect whether it is deposited locally or travels longer
                      distances in the atmosphere (Landis et al. 2004).

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                                                                                            Introduction
A portion of the mercury deposited in terrestrial systems is re-emitted to the atmosphere.
On soil surfaces, sunlight might reduce deposited Hg(II) to Hgo, which might then evade
back to the atmosphere (Carpi and Lindberg 1997, Frescholtz and Gustin 2004, Scholtz et
al. 2003). Significant amounts of mercury can be codeposited to soil surfaces in
throughfall and litterfall of forested ecosystems (St. Louis et al. 2001), and exchange of
gaseous Hgoby vegetation has been observed (e.g., Gustin et al. 2004). Hg(II) 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, Hg(II) partitions strongly to silts and biotic solids,
sorbs weakly to sands, and complexes strongly with dissolved and particulate organic
material. Hg(II) 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 via 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. 1999).  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.,Griebetal. 1990).

Dissolved Hg(II) and methylmercury accumulate in aquatic vegetation, phytoplankton,
and benthic invertebrates. Unlike Hg(II), methylmercury biomagnifies through each
successive trophic level in both 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 (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.

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Introduction
                      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. As such, EPA makes this  guidance
                      available to fulfill that commitment to enable states and authorized tribes to adopt the
                      recommendations set forth in Water Quality Criterion for the Protection of Human
                      Health: Methylmercury (USEPA 200Ic), or other water quality criteria for
                      methylmercury on the basis of scientifically defensible methods, into their water quality
                      standards.

                      This nontraditional approach in 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 present
                      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 adopting the new water quality
                      criterion into their water quality standards programs and implementation of the revised
                      water quality criterion in TMDLs and NPDES permits. Further, because atmospheric
                      deposition serves as a large source of mercury for many waterbodies, implementation of
                      this criterion involves coordination across various media and program areas.

                      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 discusses approaches for managing the
                      development of TMDLs for waterbodies impaired by mercury. This includes approaches
                      for addressing waterbodies where much of the mercury comes from atmospheric sources
                      and how TMDLs can take into account ongoing efforts to address sources of mercury,
                      such as programs under the CAA and pollution prevention activities. This guidance also
                      includes a recommended approach for directly incorporating the methylmercury tissue
                      criterion in NPDES permits.
                      This guidance document presents suggested approaches, but not the only technically
                      defensible approaches, to criteria adoption and implementation. The guidance does not
                      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 approaches on a case-
                      by-case basis that differ from this guidance where appropriate. EPA may change this
                      guidance in the future.
10

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                                         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. 2006. Draft Guidance for Implementing the January 2001 Methylmercury Water
        Quality Criterion. EPA 823-B-04-001. U.S. Environmental Protection Agency, Office of
        Water, Washington, DC.

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                                                             Water Quality Criteria and Standards Adoption
3    Water Quality Criteria  and Standards
      Adoption
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. States and authorized tribes must adopt water quality criteria3 that 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
criteria for all toxic pollutants for which EPA has established AWQC where the
discharge or presence of these pollutants could reasonably interfere with the designated
uses (see CWA 303(c)(2)(B)). EPA issued guidance on how states and authorized tribes
may comply with 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 pollutants4 for 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
        procedure that 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 into their water
quality standards to fulfill the requirements of 40 CFR Part 131.
 The term "water quality criteria" has two different definitions under the CWA. Under section 304(a), EPA publishes water quality criteria
that consist 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 section 304(a) criterion. States may use these 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).
4 CWA section 307(a) identifies a list of toxic pollutants that EPA has published at 40 CFR 401.16.
5 A translator procedure is simply the detailed process, published 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.

                                                                                                       11

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Water Quality Criteria and Standards Adoption
                      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 304(a) water  quality criterion for methylmercury is
                      expressed as a fish6 tissue concentration value (0.3 milligram methylmercury per
                      kilogram of wet-weight fish tissue, or 0.3 mg/kg). With the publication of this 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 200 Ib).
                      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 as a fish tissue residue concentration into their water quality standards, or
                      adopting it as a traditional water column concentration. However,  if states and authorized
                      tribes choose to use both approaches, they  should clearly describe  how each will be used
                      for specific applications in their standards and describe applicable  implementation
                      procedures. 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 (RSC)). EPA will evaluate criteria submitted by states and authorized
                      tribes on a case-by-case basis.

                      3.1.2.1   Why is the fish tissue concentration criterion recommended?
                      EPA recommends that states and authorized tribes adopt new or revised methylmercury
                      water quality criteria in the form of a fish tissue methylmercury concentration. The
                      following  reasons make this the preferred form:
                         • A fish tissue concentration value water quality criterion 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) that
                           humans are exposed to methylmercury.

                         • A fish tissue concentration value is more consistent with how fish advisories are
                           issued.
6 The criterion applies to both finfish and shellfish. For purposes of simplifying language in this document, the term "fish" means both
finfish and shellfish.

12

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                                                                 Water Quality Criteria and Standards Adoption
    • A fish tissue sample is currently easier to analyze for methylmercury and analysts
      are more experienced in analyzing methylmercury in fish tissue than in water
      samples.

    • A fish tissue concentration avoids the need for BAFs7 that are necessary to translate
      between a tissue concentration and water concentration when deriving a water
      concentration-based criterion. This is significant because bioaccumulation of
      methylmercury by aquatic organisms is temporally and spatially variable such that
      site-specific BAFs, which can be costly to develop, are the preferred approach for
      translating tissue concentrations into water concentrations.

 3.1.2.2   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 criterion for the protection of
 human health is  calculated as:
                                    BWx(Rfl) -RSC)
                                    	^4	
                                         2-«=iFI                       (Equation 1)

 Where:

      TRC =   Fish tissue residue criterion (mg methylmercury/kg fish tissue) for
                 freshwater and estuarine fish and shellfish.
      RfD  =   Reference Dose (based on noncancer human  health effects). For
                 methylmercury it is 0.1 ug/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  ug/kg
                 body weight/day.
      BW  =   Human body weight  (default value of 70 kg for adults).
      FI   =   Fish intake at trophic level (TL) / (/' = 2, 3, 4); total default intake of
                 uncooked freshwater and estuarine fish is  17.5  g fish/day for the general
                 U.S. adult population.9
 This equation and all values used in the equation are described in Water Quality Criterion
for the Protection of Human Health, Methylmercury (USEPA 200 Ic). This equation is
7 A BAF is a ratio (in milligrams/kilogram per milligrams/liter, or liters per kilogram) that relates the expected concentration of a chemical
in commonly consumed aquatic organisms in a specified trophic level to the concentration of the chemical in water (USEPA 2001c).
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
Federal Register 1354-1355.
9 The 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 2000i). 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, nd 5.7 g fish/day for TL4. The rationale behind the selection of this value is described in the Human Health Methodology
(USEPA 2000e).

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Water Quality Criteria and Standards Adoption
                      essentially the same equation used in the 2000 Human Health Methodology 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 200lb)). 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 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
                      rather than the default values if they believe that such a water quality criterion would be
                      more appropriate for their target population.

                      The TRC value is not based on any default breakout offish 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
                      or authorized tribe may choose to assign the TRC value to only trophic level 4 or to the
                      highest trophic level consumed. This will result in a conservative assumption, thereby
                      protecting 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 guidance on determining consumption patterns see
                      section 4.)

                      EPA acknowledges that implementation of a TRC entails more technical steps than
                      implementation of a water column criterion. Although water quality standards programs
                      traditionally use water column values,  states and authorized tribes may not find it
                      necessary to translate this fish tissue based-criterion into a water column value for all
                      implementation methodologies. Later chapters on TMDLs and NPDES permits in this
                      guidance offer some  methodologies that use the fish tissue value without translating from
                      fish tissue to water column values.

                      3.1.3   Can states or authorized tribes adopt a water column
                              concentration criterion?
                      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
                      standards programs. Water quality standards, water quality-based effluent limits
                      (WQBELs), TMDLs, and other activities generally employ a water column value. If
                      states and authorized tribes decide to adopt the tissue criterion expressed as fish tissue
                      concentration, per EPA recommendation, without translating to a traditional water
                      column concentration, they will make a choice on how to implement the tissue criterion.
                      A state or authorized tribe could decide to directly develop TMDLs and to calculate
                      WQBELs  in NPDES permits without first measuring or calculating a BAF. This
                      guidance provides some options for such approaches in sections 6 and 7.
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)).

14

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                                                             Water Quality Criteria and Standards Adoption
Alternatively, a state or authorized tribe may decide to adopt a tissue concentration-based
standard with a site-specific procedure for translating the tissue concentration-based
standard to a water column concentration. Because methylmercury bioaccumulation can
vary substantially from one location to another, this option allows for the tissue
concentration-based standard to be translated to water concentration-based standards
using site-specific information on methylmercury bioaccumulation (i.e., site-specific
BAFs) while ensuring that a water-expressed standard is ultimately developed for the
waterbodies of interest. Administratively, this option might be more efficient when
compared to adopting a water concentration-based standard for an entire state or tribal
jurisdiction adopting or approving site-specific standards on an individual waterbody
basis. Approaches for translating a tissue concentration-based criterion to a water
concentration-based criterion are provided in the following section.

States or authorized tribes may also choose to adopt a standard that is expressed as a
water column concentration. Conversion of the tissue concentration-based criterion to a
water concentration-based criterion may be desirable for various reasons, such as
achieving consistency with traditional water column-based AWQCs and/or regulatory
simplicity. However, note that this approach requires assessment of methylmercury
bioaccumulation on a state or tribal geographic scale. Thus, the uncertainty associated
with differential bioaccumulation of methylmercury across sites within a state or
authorized tribe 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 AWQC
for methylmercury as a tissue concentration rather than as a water concentration.

To express the methylmercury concentration-based criterion as a water concentration, a
state or authorized tribe would translate the methylmercury concentrations in fish tissue
to methylmercury concentrations in the water column. To accomplish this, the state or
authorized tribe will develop BAFs. In the Federal Register notice of the methylmercury
criterion, EPA identified three possible different 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.

States and authorized tribes would translate the tissue concentration-based human health
AWQC to a water concentration-based methylmercury criterion using a BAF as

                A WQC  = TRC • >BAF                               (Equation 2)

Where:

      A WQC =  Water concentration-based ambient water quality criterion for
                methylmercury in mg/L
      TRC =   Tissue concentration  (residue)-based ambient water quality  criterion for
                methylmercury in mg/kg
      BAF =   Bioaccumulation factor for trophic levels 2, 3, and 4, weighted on the
                basis offish consumption rates for each trophic level in 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
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Water Quality Criteria and Standards Adoption
                      sampling. BAFs are trophic level-specific. EPA recommends that they be derived from
                      site-specific, field-measured data as
                                                           w                            (Equation 3)

                      Where:
                           BAF  =   Bioaccumulation factor, derived from site-specific field-collected
                                     samples of tissue and water in L/kg fish
                           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 the following section.

                      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
                      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 methylmercury 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. Some of 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 (i.e., via 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 and by limitations in the data parameters necessary
                      to run the models.

                      The bioaccumulation of nonionic organic chemicals can also be affected by a number of
                      these same physico-chemical factors (e.g., loading history, food web structure, dissolved
                      oxygen, DOC). However, a substantial portion of the variability in bioaccumulation for
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                                                            Water Quality Criteria and Standards Adoption
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 2003b). Normalizing to the age or size (length, weight) offish has
been shown to reduce variability in measures of bioaccumulation (Sorensen et al. 1990,
Glass et al. 2001, Brumbaugh et al. 2001, Sonesten 2003, Wente 2004). The United
States Geological Survey (USGS) developed a procedure called the National Descriptive
Model of Mercury and Fish Tissue (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;
EPA used this model to normalize national data sets offish tissue for analysis supporting
the CAMR (USEPA 2005a).

Taking into account the previous discussion, EPA recommends three different
approaches for relating a concentration of methylmercury in fish tissue to a concentration
of methylmercury in ambient water:
   1.  Use site-specific methylmercury BAFs derived from field studies.

   2.  Use a scientifically defensible bioaccumulation model.

   3.  When derivation of site-specific field-measured BAFs or use of a model are not
      feasible, use national methylmercury BAFs derived from empirical data.

Of these approaches, 1 and 2 are preferred over 3 for reasons discussed below. However,
the hierarchy assigned to the approaches is not intended to be inflexible. Some situations
might indicate that greater uncertainty is likely to occur when applying a BAF derived
from a "more highly preferred" approach (e.g., a field-measure BAF) than with a "less
preferred" approach, for example, when data from the more preferred method have less
representativeness, quantity, or quality relative to the less preferred approach. In these
situations, data from the less preferred, but less uncertain, approach would be used to
derive BAFs.

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 2000e) and its
Technical Support Document for developing national BAFs (USEPA 2003b). 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 that
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). The BAF can be measured for the aggregate
offish in a location or specific to each trophic level. A field-measured site-specific BAF
also reflects biotic and abiotic factors at a location that influence the bioavailability and
metabolism of a chemical that might occur in the aquatic organism or its food web.
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                      However, states should exercise caution in developing a site-specific BAF for a
                      migratory fish because its exposure to methylmercury reflects areas other than where the
                      fish was caught. By incorporating these factors, field-measured site-specific BAFs
                      account for the uptake and accumulation of the chemical.

                      For the purposes of developing a human health water quality criterion, states and 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 water
                      (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. 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 (specifically methylmercury) and that
                      the site-specific BAF is independent of water column concentration.

                      As an example, the State of California is currently employing a site-specific BAF
                      approach in its Central Valley Region. In this approach, California 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, 2004). California linearly regressed fish
                      tissue methylmercury concentrations for specific trophic level 3 and 4 fish against
                      aqueous methylmercury concentrations (PO.001, R2=0.98, and PO.01, R2=0.9,
                      respectively), and determined methylmercury concentrations in unfiltered water that
                      correspond to the fish tissue criteria (0.15 ng/1 for TL3 fish and 0.14 ng/1 for TL4 fish)
                      that were used in the TMDL analyses. (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 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
                      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, such
                      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
                      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 increases 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,
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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 offish), 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-, water
chemistry-, 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., Sorensen et al. 1990,
Kamman et al. 2004, Brumbaugh et al. 2001). 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, percent wetland area in the watershed, pH, and
acid volatile sulfide concentration in sediments (r = 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 (r = 0.50). This demonstrates the
importance of species specificity on 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
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 due to 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, use of 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
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                      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 bioaccumulation. Three examples of mechanistic type
                      bioaccumulation models are: the Dynamic Mercury Cycling Model (D-MCM) (EPRI
                      2002), BA (BASS) (Barber 2002), and the Quantitative Environmental Analysis Food
                      Chain model (QEAFDCHN) (QEA 2000). The conceptual advantage of mechanistically
                      based bioaccumulation models is that predictions of methylmercury bioaccumulation can
                      be made under different conditions (e.g., different growth rates offish, different water
                      chemistry conditions, different mercury loading  scenarios), because the models include
                      mathematical representations of the 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
                      yet been applied to lotic systems. The  other models have been developed more generally,
                      for nonionic organic chemicals that bioaccumulate and that require substantial
                      modification and validation for application to mercury.

                      Most mechanistic bioaccumulation models use a chemical mass balance approach to
                      calculate bioaccumulation into 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
                      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.

                      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 2000e) and Technical Support
                      Document  Volume 2: Derivation of National Bioaccumulation Factors (USEPA 2003b).
                      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 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. EPA used the BAF guidance in the 2000 Human
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Health Methodology (USEPA 2000e, 2003b) and the BAF methods in Volume III,
Appendix D of the Mercury Study Report to Congress (USEPA 1997b) to derive draft
methylmercury BAFs as part of its initial efforts to derive a water column-based
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. These 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 2001c). The draft national BAFs
(50th percentile values) are listed by trophic level in Table  1. The 5th and 95th percentile
values are also provided to show the distribution of national BAF values.

Table 1. National draft BAFs for dissolved methylmercury

5th Percentile
50th Percentile (Geometric mean)
95th Percentile
BAF trophic
level 2
(L/kg)
18,000
117,000
770,000
BAF trophic
level 3
(L/kg)
74,300
680,000
6,230,000
BAF trophic
level 4
(L/kg)
250,000
2,670,000
28,400,000
 (USEPA 2001 c)
 (mg methylmercury/kg fish tissue per mg methylmercury/L water)

To develop the 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. EPA generally does not recommend
basing an AWQC on BAF values near the extremes of the distribution (e.g., 10th or 90th
percentile) because such values might introduce an unacceptable level of uncertainty into
the calculation of a water column-based AWQC.

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). 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 2,670,000 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 • «2,670,000 L/kg) to indicate attainment of the water quality column
       criterion. However, using the BAF based on site-specific data, 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
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Water Quality Criteria and Standards Adoption
                             human health. Thus, load reduction or permits using the national BAF of
                             2,670,000 L/kg would be under-protective.

                             Overprotective scenario
                             A state uses the draft national BAF of 2,670,000 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 of 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 • *2,670,000 L/kg) to indicate
                             attainment of the water quality criterion. However, using the BAF based on site-
                             specific data, attainment of the water quality criterion could be achieved at a
                             higher water column concentration of 0.33 ng/L. Thus, load reductions or permits
                             using the national BAF of 2,670,000 L/kg  would be over-protective.

                      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
                      BAFs are encouraged to review Appendix A of Water Quality Criterion for the
                      Protection of Human Health: Methylmercury (USEPA 200 Ic) that 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 offish, waterbody trophic status, or underlying mercury uptake
                            processes. In reality, methylmercury bioaccumulation for a given species can vary
                            as a function of the ages (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 total mercury in water simplifies a number of
                            nonlinear processes that lead to the formation of bioavailable methylmercury in the
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      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, while the concentration of methylmercury in fish tissue is
      presumably a function of these varying concentrations, published BAFs are
      generally 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.

The peer reviewers of the draft national BAFs expressed concerns about the use of the
draft national BAFs 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 national  BAF
values might significantly over- or underestimate site-specific bioaccumulation. As a
result, EPA decided not to use 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. However, EPA may revise its
guidance should significant new information become available to support developing a
final national BAF.

EPA believes that the draft national methylmercury BAFs  in Table 1 sufficiently
represent bioaccumulation such that they may be used to implement a fish tissue-based
methylmercury water quality criterion in a state's or authorized tribe's water quality
standards in the absence of any other site-specific bioaccumulation data. Thus, EPA is
likely to approve water quality standards for mercury on the basis of these draft national
BAFs in the absence of information indicating that the water quality criteria do not
protect human health in the waters to which the standards apply. Risk managers should
also understand that in using the draft national BAFs, 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. The decision to
allow the use of the draft national BAFs is a risk management decision. It reflects
judgment that human health is better protected if the water quality criteria reflect the new
science associated with methylmercury,  even if that means using a draft national BAF
value, rather than not adopting a criterion because the state or authorized tribe lacks
resources to conduct site-specific studies or to run an appropriate bioaccumulation model.

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 either water or fish  in 40 CFR Part 136. However,
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                      for fish tissue, states and authorized tribes can measure methylmercury concentrations
                      using the same analytical method used to measure for total mercury at least for upper
                      trophic level fish (i.e., levels 3 and 4). This is because 80 to 100 percent of the mercury
                      found in the edible portions of freshwater fish greater than 3 years of age from these two
                      trophic levels is in the form of methylmercury (USEPA 2000c). In fish greater than
                      approximately 3 years of age, mercury has had sufficient time to bioaccumulate to
                      roughly steady levels in the fish. Appendix E summarizes seven 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 seven studies, methylmercury on average accounted
                      for more than 90 percent of the mercury concentration in fish tissue.

                      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
                      restricting sample collection to organisms of similar age or size classes or through
                      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 use of the National Descriptive Model for Mercury in Fish Tissue
                      (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 2005b).

                      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 is correlated with the magnitude of methylmercury accumulation, the
                      types and sizes of aquatic organisms being consumed should be considered when
                      determining what 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 tribes should review site-specific data used to calculate a field-measured
                      BAFs, and thoroughly assess the quality of the data and the overall uncertainty in the
                      BAF values. Consider the following general factors when determining the acceptability
                      of field-measured BAFs reported in the published scientific literature. Address the same
                      general issues and questions also when designing a field study to generate site-specific
                      field-measured BAFs.
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   •  Calculate a field-measured BAF using aquatic organisms that are representative of
      those aquatic organisms that are 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, 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 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 200Ic). 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.

   •  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) due to 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  1997e).
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 2003b). In addition, EPA
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                     expects to publish specific guidance for deriving site-specific BAFs from field studies in
                     the future. Until then, the EPA guidance cited above and a recent publication by
                     Burkhard (2003) are good sources of information on the design of BAF field studies and
                     on deriving field-measured site-specific BAFs.

                     3.2  What options are available to address for 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 2000e) describes how states and
                     authorized tribes can adopt site-specific modifications of a 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, local 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, is 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 of the population surveyed (either sport or subsistence, or both)
                     to eat fish from the local area. 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, EPA will likely review the data supporting the site-specific criterion when EPA
                     approves or disapproves state or tribal water quality standards under section 303(c).

                     States and authorized tribes may modify EPA's recommended 304(a) criteria for
                     methylmercury by using other scientifically defensible methods, or by using different
                     assumptions for certain components of EPA's criterion to derive a criterion that maintains
                     and protects the designated uses. For example:
                        • Use an alternative  RSC factor

                        • 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 these would better reflect
                          the target population.

                     If a state or authorized tribe intends to modify both the RSC and fish consumption rate, it
                     may find it advantageous to collect the data at the  same time.

                     3.2.1.1    How does one modify the RSC?
                     Section 5 of the methylmercury criterion document (USEPA 200 Ic) 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 reference dose 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
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                                                            Water Quality Criteria and Standards Adoption
methylmercury criterion document and modify the media specific exposure estimates
found 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, the exposure from
ingestion of marine fish comprised greater than 99.9 percent of the total exposure to
methylmercury, and thus ingestion offish 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 200Ic) and the
concentration of methylmercury in the commonly consumed marine species (Table 5-14;
USEPA 200Ic). 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
included 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/day11 (uncooked) (USEPA 2001c). States and authorized Tribes can choose to
apportion an intake rate to the highest trophic level consumed for their population or use
a different intake rate based on local or regional consumption patterns. The fish
consumption value in the TRC equation can be changed if the target population eats a
higher or lower amount offish. For example, if the 90* percentile of a target population
eats approximately 15 grams/day of freshwater and estuarine fish of various trophic
levels, the fish intake value in the above equation 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 offish when
collecting survey data, rather than  local preference for fish consumption. In this instance,
the state or authorized tribe should not use the survey data because it does not represent
what the local population would eat if the fish was not already contaminated.

EPA suggests that states and authorized tribes follow a hierarchy when deriving fish
intake estimates (USEPA 2000e). 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.
11 This value represents the 90th percentile of freshwater and estuarine finfish and shellfish consumption reported by the 1994-96
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 2000e).

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Water Quality Criteria and Standards Adoption
                      3.2.1.2.1   Use local data
                      EPA's first preference is that states and authorized tribes modify the water quality
                      criterion using fish intake rates derived from studies of consumption of local fish, such as
                      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. EPA
                      recognizes that states and authorized tribes may choose to develop a fish intake rate for
                      highly exposed subpopulations (e.g., sport anglers, subsistence fishers), and if this is the
                      case, the states and authorized tribes should collect the intake rates from these
                      subpopulations.

                      States and authorized tribes might wish to conduct their own surveys offish 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 who
                      eat local fish. EPA's consumption survey guidance offers recommendations on how to
                      develop appropriate quality assurance and control procedures to help assure the quality of
                      the survey. Results of studies of broader geographic regions in which the state or
                      authorized tribe is located can also be  used, but might not be as applicable as study
                      results for local watersheds. Because such studies would ultimately form the basis of a
                      state or authorized tribe's methylmercury criterion, EPA would review any surveys of
                      fish intake for consistency with the principles of EPA's guidance 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 authorized tribes should ensure that the distribution is
                      based on survey respondents who 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 the recent
                      Great Lakes Water Quality Initiative (known as the "GO") (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 offish 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, at a minimum,
                      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, minority
                      populations). A state or tribe in a region of the country might consider using rates from
                      studies that surveyed the same region; for example, a state or tribe that has a climate that
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                                                            Water Quality Criteria and Standards Adoption
allows year-round fishing may underestimate consumption if rates are used from studies
taken in regions where individuals fish for only one or two seasons per year. A state or
tribe that has a high percentage of an age group (such as elderly individuals, who have
been shown to have higher rates in certain surveys) may wish to use age-specific
consumption rates, which are available from some surveys. EPA has published guidance
for selecting a study from a similar geographic area or population group (USEPA 1998c)
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 n ation al 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 1994-96 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-
96 and 1998 CSFII surveys, the statistical methodology, and the results and uncertainties
of the EPA analyses (USEPA 2002f). The estimated fish consumption rates in the CSFII
report are 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 (i.e.,  rates based on consumers and
nonconsumers offish from the survey period), (2) by consumers-only rates (i.e., rates
based on respondents who reported consuming finfish or shellfish during the 2-day
reporting period), and (3) per capita consumption by body weight (i.e., per capita rates
reported as milligrams offish per kilogram of body weight per 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.

Table 2. Estimates of freshwater and estuarine combined finfish and shellfish
consumption from the combined 1994-96 and 1998 CSFII surveys

All Ages
Age 18 and Over
Women Ages 15-44
Children Ages 14 and Under
Mean
6.30
7.50
5.78
2.64
Median
N/a
O.OO12
N/a
0.00
90tn
11.65
17.37
6.31
0.00
95tn
41.08
49.59
32.37
13.10
99tn
123.94
143.35
109.79
73.70
 Note: (all values as g/day for uncooked fish)

The CSFII surveys 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
12 The median value of 0 grams/day may reflect the portion of individuals in the population who never eat fish as well as the limited
reporting period (2 days) during which intake was measured.
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Water Quality Criteria and Standards Adoption
                      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 short-term 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 Native Americans on reservations and certain ethnic
                      groups. While these individuals 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 2002f).

                      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
                      the following default rates, on the basis of the 1994-96 CSFII data, which EPA believes
                      are representative of freshwater and estuarine fish and shellfish intake for different
                      population groups:  17.5 grams/day for the 90th percentile of the general adult population,
                      an average of 17.5 grams/day for sport fishers, and an average of 142.4 grams/day for
                      subsistence fishers. 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 offish in calculating an
                      RSC for dietary intake.13 States and tribes should ensure that when evaluating overall
                      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 (i.e., fresh or estuarine a«J 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 2000e).

                      Because the  combined 1994-96 CSFII survey is national in scope, EPA uses the results
                      from it to estimate fish intake for deriving national criteria. The estimated mean of
                      freshwater and estuarine uncooked fish intake for adults from the CSFII study is 7.5
                      grams/day, and the median is 0 grams/day. The  estimated 90th percentile is 17.53
                      grams/day; the estimated 95th percentile is 49.59 grams/day; and the estimated 99th
                      percentile is 142.41 grams/day. The median value of 0 grams/day reflects the portion of
                      individuals in the population who never eat fish as well as the limited reporting period (2
13 See the discussion of the RSC in sections 3.1.2.2. and 3.2.1.1.

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                                                               Water Quality Criteria and Standards Adoption
days) during which intake was measured. By applying as a default 17.5 grams/day for the
general adult population, EPA selected an intake rate that is protective of a majority of
the population (again, the 90th percentile of consumers and nonconsumers according to
the 1994-96 CSFII survey data). In apportioning the default consumption rate to fish  in
different trophic levels, EPA uses the following breakout: TL2 = 3.8 grams/day; TL3  =
8.0 grams/day; and TL4 = 5.7 grams/day (USEPA 2000e)

Similarly, EPA believes that the 99*  percentile of 142.4 grams/day is within the range of
consumption estimates for subsistence fishers according to the studies reviewed, and
represents an average rate for subsistence fishers. EPA knows that some local and
regional studies indicate greater consumption among Native American, 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 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
1994-96 CSFII are representative, they may choose these recommended rates.

3.2.2   How do water quality variances apply?
A state or authorized tribe may provide NPDES dischargers temporary relief from a
water quality standard by granting a temporary variance to that standard. 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 change to the
otherwise applicable water quality standard (designated use and criteria), water quality
variances must be supported by one of the six justifications under 40 CFR 131.10(g)
where a state or authorized tribe believes the standard cannot be attained in the immediate
future. Variances are tied to 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 means
that a water quality standard variance for mercury would apply only to the new human
health methylmercury criterion in a stated waterbody and specifically to the discharger
requesting the variance, but the  State 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?
Typically, variances provide a bridge when a state or authorized tribe needs additional
data or analyses before making a determination  of whether the designated use is
14 These six justifications are the ones allowed for use attainability analyses (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 the 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; or (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.

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Water Quality Criteria and Standards Adoption
                      attainable and when the state or authorized tribe adopts an alternative use on the basis of
                      a determination under 40 CFR 131.10(g). In the case of methylmercury, such a variance
                      might also be appropriate where implementation tools are not available or feasible,
                      particularly where a state or authorized tribe has not yet developed a TMDL. With EPA's
                      belief that a number of waterbodies will be added to CWA section 303(d) listings for
                      mercury following adoption of the new methylmercury criterion, variances could provide
                      a short-term solution until development of the TMDL. Further, given limited resources, a
                      state or authorized tribe might decide to focus on controlling significant mercury sources
                      one at a time, beginning with a source other than effluent discharges (e.g., sediment,
                      atmospheric deposition) and employing variances in the interim.

                      EPA believes that a large number of regulated point sources discharging mercury may
                      apply  for variances because they discharge into impaired waters where the largest source
                      of mercury comes from atmospheric deposition, and expects there to be  commonality in
                      the grounds for these variances. The most likely scenarios to prompt a variance request
                      are listed below. Many point source dischargers contribute a relatively small percentage
                      of the  mercury in an aquatic system. These scenarios are examples of demonstrations that
                      could  satisfy the requirements  under 40 CFR 131.10(g). These demonstrations are more
                      thoroughly explained below and in the Water Quality Standards Handbook (USEPA
                      1994).

                             Economic or social impacts—Demonstrate that, in the short term, the costs of
                             constructing controls necessary to meet the methylmercury criterion (beyond
                             those required by sections 301(b)(l)(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—Demonstrate that, in the
                             short term, none of the present technologies for improving the quality of an
                             effluent are capable of bringing methylmercury levels down to the criterion (i.e.,
                             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 mercury
                             criterion, despite local efforts of reduction. In this instance, if air deposition
                             modeling  shows that the atmospheric deposition from outside the United States
                             was a substantial cause of the impairment, the variance may be warranted.

                             Natural conditions preclude attainment—Demonstrate that local conditions of an
                             aquatic system result in high methylmercury levels. This could result from two
                             conditions. The first is that elevated mercury concentrations occur naturally. The
                             second is that conditions of the area or the waterbody itself—whether it be the
                             soil or sediment composition, microbial community, or the aquatic biota
                             interactions—might favor a high level of methylation such that low levels of
                             atmospherically-derived or ambient water column levels of mercury can amplify
                             into high concentrations in fish tissues. In other words, bioaccumulation might
                             occur at a higher rate under certain natural conditions and prevent the criterion
                             from being attained.
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3.2.2.2   What considerations should a state or tribe consider before
          granting a variance?15
In general, the temporary standard established by a variance is set as close as possible to
the numerical criterion for the designated use and is always retained at the level needed to
preserve the existing use. This is done to protect the existing uses, and to ensure progress
toward ultimate attainment of the designated use. 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 in 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.

        Performance-based approach—Unlike the typical numeric chemical criterion,
        EPA based the recommended methylmercury criterion on a fish tissue
        concentration, thus requiring a nontraditional expression of the criterion. States
        and  authorized tribes have flexibility in how a variance is expressed in their
        water quality standard regulations. One approach is to incorporate the temporary
        fish tissue-based criterion established by the variance  directly in the standards,
        and  another is to use a performance-based approach. In the performance-based
        approach, the state or authorized tribe adopts into its water quality standards the
        procedure for calculating a new criterion on the basis  of the variance. Such a
        procedure should fully lay out the calculations and default values necessary to
        derive an alternative fish tissue criterion using more site-specific numbers. To
        implement a performance-based approach, a state or tribe would maintain a
        publicly available, comprehensive list of all site-by-site decisions made using the
        procedures; however, such decisions would not, as a federal matter, have to be
        codified in state or tribal regulations. In addition, the public notice requirements
        for adopting variances could be satisfied through the process of issuing the
        NPDES permit that incorporates such temporary limits.

        States and authorized tribes may find a performance-based approach
        advantageous in the case of variances to the methylmercury criterion because
        once the state or authorized tribe has submitted—and  EPA has approved—these
        procedures, performance-based variances could be issued without subsequent
        individual approvals. The key advantage of this approach is that adoption of
        sufficiently detailed implementation procedures, with suitable safeguards, does
        not require EPA approval of every application of the variance.

        Time frames—A variance is typically a time-limited change in the water quality
        standards. Although EPA 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 3  to 5 years, with renewals


15 Federal or state regulations also govern the granting of a variance. For example, regulations promulgated under 40 CFR Part 132,
Appenidx F, Procedure 2 specifies the conditions for granting variances in the Great Lakes, and prohibits the granting of variances to new
dischargers or recommencing Great Lakes dischargers.

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Water Quality Criteria and Standards Adoption
                             possible following a sufficient demonstration that the variance is still necessary.
                             Variances that extend longer than 3 years are traditionally revisited in the context
                             of a triennial review to justify their continuation. While the discharger makes this
                             demonstration, the discharger also shows that it made reasonable progress to
                             control mercury in the discharge during the period of the previous variance. In
                             terms of methylmercury, there will likely be 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). EPA modeled the response in
                             fish tissue to a 50 percent reduction in mercury loadings to four lakes  as part of
                             the analysis supporting the CAMR and estimated that it would take between 1 to
                             56 years for the lakes to reach 90 percent of the estimated steady state fish tissue
                             methylmercury concentration (USEPA 2005b). 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. Namely, a specific provision
                             within a variance  for methylmercury could describe a less comprehensive
                             demonstration for renewals by making use of information already available.

                             Another perspective regarding the life span of a variance is that a 3-year timeframe
                             is mainly associated with a triennial review; there is no specific federal  regulatory
                             requirement for a variance to expire in 3 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 before issuance of a new permit.

                             Antidegradation—Permits with effluent limits based on a variance for
                             methylmercury must conform to the  state or authorized tribe's antidegradation
                             policy.

                             Pollutant Minimization Plans—Pollution Minimization Plans (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-pipe, PMPs might
                             partially counter the 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:
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                                                             Water Quality Criteria and Standards Adoption
             Case study:

             Ohio  statewide variance for mercury

            Ohio adopted a statewide mercury variance applicable to any point source dischargers in
            the state that meet several criteria. Specifically, Ohio adopted, and EPA approved, a rule
            that finds complying with a mercury WQBEL on the basis of the Great Lakes Guidance
            criteria applied at the end-of-pipe (i.e., without a mixing zone) would result in widespread
            adverse social and economic impacts, relieving individual permittees of the burden of
            making this demonstration on an individual basis. However, to obtain individual coverage
            under the  Ohio group variance, a permittee must do the following:

               1. Demonstrate that it can (or will within 5 years) achieve an average annual
                 effluent concentration no greater than 12 ng/L mercury

               2. Document that it is currently unable to comply with what would be the WQBEL
                 for mercury in the absence of a variance (based on the guidance wildlife
                 criterion of 1.3 ng/L)

               3. Provide a plan of study to document known and suspected sources of mercury

               4. Describe control measures taken to date as well as planned future measures to
                 reduce or eliminate mercury from the discharger's effluent

               5. Explain why there  are not readily available means of complying with the
                 WQBEL for mercury without construction of end-of-pipe controls

            As a condition for receiving the variance, the discharger must accept permit conditions needed
            to implement the plan of study regarding the identification and  evaluation of mercury sources
            and potential control measures. Further, the rule requires public notice of the preliminary
            decision and the supporting materials  (including the plan of study). Ohio also requires
            monitoring as necessary to assess the impacts of the variance on public health, safety, and
            welfare. If the discharger still cannot meet the standard following completion of actions
            addressed in the plan of study and in the PMP, Ohio may take action  (through permit
            modification or permit reissuance) to delete the variance or impose additional pollutant
            minimization steps  (after consideration of public comment). Ohio also retains the right to
            request that a discharger submit an individual variance application.
          Similar costs, discharge processes —A type of industry or effluent treatment
          process may be targeted on the basis of the associated costs or available
          technology (i.e., publicly owned treatment works (POTWs), mining operations,
          and so on). A state or authorized tribe can choose to adopt a variance with tiered
          requirements, depending on the type of industry  requesting coverage. For
          example, due to the differing cost implications, 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.
 Case  study:
 Michigan's  mercury multiple discharger variance

Until recently, analytical methods for detecting mercury in effluents at levels below the

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Water Quality Criteria and Standards Adoption
                   water quality criterion (1.3 ng/L for the protection of wildlife) were lacking. Due to the
                   inability to quantify effluent mercury concentrations at low levels, most monitoring resulted
                   in no detects. Because of these monitoring results, facilities did not receive effluent limits
                   for mercury or were considered in compliance with effluent limits. EPA's new method
                   (1631) makes possible quantification of effluent mercury concentrations to levels less than
                   the criterion (quantification level = 0.5 ng/L).

                   Application of EPA's new method is expected to result in additional permit limits for
                   mercury and better detection of noncompliance with permit limits. Michigan expects that
                   many facilities with mercury limits will be unable to comply with the limits. No known,
                   demonstrated treatment technologies for removing mercury from effluents at low
                   nanogram per liter levels exist. Consequently, efforts intended to achieve compliance with
                   water quality-based effluent limits for mercury focus on the identification and reduction of
                   sources of mercury to a wastewater treatment system. Often, it is difficult to identify such
                   sources and to quantify the expected effects of source controls on  effluent mercury
                   concentrations.  Given the uncertainty in the ability to comply and the timing of compliance,
                   Michigan invoked a provision of this water quality standards (R 323.1103(9)) that
                   authorizes multiple-discharger variances where the Michigan  Department of
                   Environmental Quality determines, "that a  multiple discharger variance is necessary to
                   address widespread UQS compliance issues, including the presence of ubiquitous
                   pollutants or naturally high  background levels of pollutants in  a watershed" for mercury.

                   Where the available data indicate that a limit on mercury is needed, Michigan imposes a
                   limit that reflects the level currently achievable  (10 ng/L expressed as a rolling 12 month
                   average).  The permit requires reasonable  progress towards achieving the limit on the
                   basis of the water quality criterion over the course of the permit. The permit requires the
                   permittee to develop and implement a pollutant minimization plan to identify and eliminate
                   sources of mercury. Effluent data will be generated using Method 1631. The variance is
                   not available to  new dischargers.

                   Rather than having each of these individual facilities apply for and  receive an individual
                   variance, the multiple discharger variance  allows Michigan to respond to this issue
                   consistently and efficiently  and to get in place permits that require  pollutant minimization
                   plans that produce reductions in mercury effluent concentrations.
                              Watershed basis—A variance on a watershed scale might be a sensible approach,
                              particularly for those states that issue NPDES permits on a watershed basis. As
                              with other pollutants, methylmercury concentrations can be monitored to gain
                              site-specific 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 will be collecting data from a watershed in 1 year for the
                              purpose of issuing NPDES permits in a subsequent year. The state or authorized
                              tribe could use these data for the purpose of revising a previously issued 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
                              WQBELs will reflect a more "real-time" variance limit.

                              Statewide—Analogous to a general NPDES permit, a statewide  variance is made
                              available by the state or authorized tribe. Individual dischargers may apply for
                              coverage under the variance upon fulfillment of certain conditions. One example
36

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                                                            Water Quality Criteria and Standards Adoption
       of this approach is Ohio's statewide variance for mercury, which is described
       below.

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
(40CFR131.10(h)(2)).

3.2.3   How are use  attainability analyses conducted?

3.2.3.1   What is a use attainability analysis?
A 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
and40CFR131.10(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 offish, 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 fishable/swimmable as
providing for the protection of aquatic communities and human health related to
consumption offish and shellfish. In  other words, EPA views fishable/swimmable 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 offish and shellfish as the appropriate interpretation of the definition of
section 101(a)(2) fishable/swimmable 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   What is the rebuttable presumption  of CWA section 101 (a)?
EPA regulations effectively establish a rebuttable presumption that fishable/swimmable
uses are attainable and therefore should apply to a waterbody unless it is affirmatively
demonstrated that such uses are not attainable. The rebuttable presumption approach
preserves states' and authorized tribes' paramount role in establishing water quality
standards in weighing any available evidence regarding the  attainable uses of a
waterbody.  If the water quality goals  articulated by Congress cannot be met in a
waterbody,  the regulations simply require that such a determination be based upon a
credible structured scientific assessment (e.g., a UAA). EPA believes that the rebuttable
presumption policy reflected in the federal regulations is an essential foundation for
effective implementation of the CWA as a whole. The use of a waterbody is the most
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Water Quality Criteria and Standards Adoption
                      fundamental articulation of its role in the aquatic and human environments, and all the
                      water quality protections established by the CWA follow from the water's designated
                      use. If a use lower than a fishable/swimmable use is designated on the basis of inadequate
                      information or superficial analysis, water quality-based protections that might have
                      enabled the water to achieve the goals articulated by Congress in section 101 (a) may not
                      be put in place.

                      3.2.3.4   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. Existing
                      uses are 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 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, designated uses
                      are considered by EPA to be attainable, at a minimum, if the use can be achieved (1)
                      through effluent limitations under CWA  sections 301(b)(l)(A) and (B) and 306 and (2)
                      through implementation of cost effective and reasonable BMPs on nonpoint sources. The
                      federal regulation 40 CFR 131.10(g) further establishes 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. States 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.

                      3.2.3.5   What conditions justify changing a designated use?
                      EPA's  regulations at 40 CFR 131.10(g) lists 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
38

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                                                             Water Quality Criteria and Standards Adoption
   •  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 serve the purpose of guiding 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
methylmercury impacted 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 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. 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).
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                                                                         Monitoring and Assessment
4    Monitoring and  Assessment

4.1    What are the analytical methods for detecting and
       measuring methylmercury concentrations in  fish and
       water?
Over the last 2 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 protocols
and to determine which of these protocols would be most useful for implementing the
new methylmercury criterion. After its 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).

EPA Methods 1630 and 1631, developed by EPA's Office of Water, reflect the
techniques developed by these researchers for analyzing methylmercury and mercury in
water, respectively. Appendix A to Method 1631 (64 FR 10596) details the  researcher's
techniques for determining total and dissolved mercury in tissue, sludge, and sediments.
These methods, which are written in EPA Environmental Monitoring Management
Council (EMMC) format,  include all quality control elements that EPA's Office of Water
considers necessary to adequately define data quality.

In Appendix C, Table Cl summarizes these and other methods that EPA knows have
been used to analyze mercury and methylmercury in fish tissue, and Table C2
summarizes methods available for the analysis of mercury and methylmercury in water
and other nontissue matrices. Each table identifies the forms and  species 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.

Modifications to Method 1630 described in Table Cl (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 use of these techniques when direct measurements of methylmercury
in tissue are desired.

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, is 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. Appendix A to Method 1631 allows for
measurement of mercury in tissue at approximately 0.002 mg/kg, well below the tissue
criterion.

Several options are also available for measuring mercury concentrations in water (Table
D2).  Because Method 1631 has already been promulgated for use in CWA applications,
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Monitoring and Assessment
                      EPA strongly recommends use of this method when measuring all species of mercury in
                      water, especially when low-level measurements are expected. When measuring
                      methylmercury in water, three options are Method 1631, developed by the Office of
                      Water (USEPA 2002d); UW-Madison's SOP (Hurley et al. 1996), used by the Great
                      Lakes National Program Office for its Lake Michigan Mass Balance Study; and a
                      recently released USGS method (DeWild et al. 2002). All these procedures are based on
                      the same techniques, and each can meet the most stringent (i.e., Great Lakes Guidance)
                      mercury water quality criterion of 1.3 ng/L for wildlife protection in water. While any of
                      these methods are acceptable, EPA recommends the use of Method 1631, which is
                      documented in EMMC format and includes all quality control criteria considered
                      necessary to define data quality.

                      In summary, on the basis of the available information, EPA believes that the most
                      appropriate methods for measuring compliance with new or revised methylmercury
                      criteria are Method 1631 (mercury in water by cold vapor atomic fluorescence
                      spectrometry (CVAFS)), Method 1630 (methylmercury in water by CVAFS), Appendix
                      A to Method 1631 (mercury in tissue by CVAFS), and modifications to Method 1630 for
                      handling tissues (described in Table Cl—see Appendix C). EPA recommends these
                      procedures for the following reasons:
                        •  Methods 1630 and 1631 were developed by EPA to support implementation of
                           water quality criteria for mercury and methylmercury. Both are already in the
                           appropriate EPA format and include all standardized quality control (QC) elements
                           needed to demonstrate that results are reliable enough to support permitting and
                           enforcement programs.

                        •  Appendix A to Method 1631 was developed by EPA to support its National Study
                           of Chemical Residues in Fish Tissue. Appendix A provides information on
                           preparing a fish tissue sample for analysis using Method 1631. The method was
                           validated by Brooks Rand (USEPA 1998b) and is currently being used by Battelle
                           Marine Sciences to analyze more than a thousand tissue samples collected during
                           EPA's National Fish Tissue Survey (USEPA 2000J). 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 200 Ig) and the Savannah River TMDL study
                           (USEPA 200 le). 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. The procedures in Method 1630 also are
                           nearly identical to those given in the USGS method and in the University of
                           Wisconsin SOP, listed in  Table D2 (Hurley et al.  1996). The University of
                           Wisconsin SOP was used in EPA's Lake Michigan Mass Balance Study (USEPA
                           2001f).
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                                                                           Monitoring and Assessment
4.1.1   What is Method 1631 for determination of mercury in water?
In May 1998, EPA proposed Method 1631 at 40 CFR Part 136 for use in determining
mercury concentrations at AWQC levels in EPA's CWA programs, and subsequently
published a Notice of Data Availability (64 FR 10596) that included additional data
supporting application of the method to effluent matrices. On June 8, 1999, EPA
responded to numerous public comments on the proposed method and promulgated EPA
Method 1631, Revision B: Mercury in Water by Oxidation, Purge and Trap, and Cold
Vapor Atomic Fluorescence Spectrometry at 40 CFR Part 136 for use in EPA's CWA
monitoring programs. EPA promulgated the method on the basis of extensive validation
of the procedures, including four single-laboratory studies and an interlaboratory
validation involving  12 participating laboratories and 1  referee laboratory. The highest
method detection limit (MDL) determined by all laboratories in reagent water was 0.18
ng/L, indicating that  this method is capable of producing reliable measurements of
mercury in aqueous matrices at AWQC levels.

EPA has revised Method 1631 after its promulgation to clarify method requirements,
increase method flexibility, and address frequently asked questions. The current method
(Method 1631, Revision E) includes recommendations for use of clean techniques
contained in EPA's Method 1669: Sampling Ambient Water for Trace Metals at EPA
Water Quality Criteria Levels (USEPA 1996b). The benefits of using Method 1631 are
that it is an approved method under EPA's CWA monitoring programs, has been fully
validated, and numerous laboratories are routinely using this method. However, Method
1631 measures only mercury (total and dissolved) in aqueous samples and is not capable
of measuring the methylmercury species.

Method 1631, Appendix A 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 2 to 5,000 ng/g (0.002 to  5.0 mg/kg). The expected method detection limit
for mercury in fish tissue is 0.002 mg/kg, well below the new water quality criterion for
methylmercury. The  procedures in the appendix are not published in the Code of Federal
Regulations, but were implemented in EPA's National Study of Chemical Residues in
Fish Tissue (USEPA 2000J). Although Appendix A of Method 1631 has not been fully
validated (i.e., via an interlaboratory validation study), it was validated by EPA in a
single laboratory study, and the techniques have been widely reported in the literature.
Also, as discussed above, the analytical component of the method (Method 1631) has
been fully validated and approved for measurement of total or dissolved mercury in
aqueous matrices.

4.1.2   What analytical methods are available for determination of
        methylmercury?
EPA has not published an analytical method specifically for measuring methylmercury.
As technical guidance to assist States and authorized tribes in their selection of an
analytical method to  use, Tables C1 and C2 in Appendix C include four methods that
EPA has seen investigators successfully  use for the determination of methylmercury.
Other methods may be acceptable for use under the appropriate circumstances. As
written, all four of the methods are specific to aqueous matrices and are based on almost
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Monitoring and Assessment
                     identical analytical procedures (i.e., distillation, ethylation, GC separation, and CVAFS
                     detection). These methods have been or are being used in several national or regional
                     studies, but none are yet published in 40 CFR Part 136. Modifications to adapt these
                     procedures for fish tissue have been reported in the literature (e.g., Bloom 1989, and
                     modified by Horvat et al. 1993) and used in EPA's Cook Inlet contaminant study
                     (USEPA 200Ig), the 4-year Lake Michigan Mass Balance study (USEPA 200If), and an
                     extensive study of the Everglades (USEPA 2000b).

                     Because the four methods are nearly identical, they are expected to produce very similar
                     results with sensitivity as low as 0.002 mg/kg in tissue and 0.01 to 0.05 ng/L in water.
                     These levels are well below the methylmercury criterion for fish and the most stringent
                     (i.e., Great Lakes Guidance) mercury water quality criterion of 1.3 ng/L for wildlife
                     protection in water.

                     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 (2000c) 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, thus, offers EPA's current guidance for obtaining a representative sample
                     for determining attainment. This guidance also includes recommendations for quality
                     control and quality assurance considerations. In all cases, states should develop data
                     quality objectives for determining the type, quantity, and quality of data to be collected
                     (USEPA 2000h).

                     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 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 through surveying anglers) from state fisheries
                     departments is one 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 who eat fish.

                     The fish sampling guidance also identifies recommended target species for inland fresh
                     waters and for Great Lakes waters. Seabass, walleye, king mackerel, tilefish, and
                     largemouth bass have been identified as accumulating 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
                     via consumption of contaminated prey.
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The fish sampling guidance recommends that the size range of the sampled fish ideally
should include, from the species of fish that people in the area eat, the larger fish
individuals harvested at each sampling site, because larger (older) fish within a
population are generally the most contaminated with methylmercury (Phillips 1980,
Voiland et al. 1991). This means that small fish such as minnows should be avoided as
target species. 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, for migratory
species, that neither spawning populations nor undersized juvenile stages be sampled in
fish contaminant monitoring programs (USEPA 2000c). Sampling of target finfish
species during their spawning period should be avoided as contaminant tissue
concentrations may decrease during this time and because the spawning period is
generally outside the legal harvest period.

If states and authorized tribes do not have local information about the types of fish
present 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 would reflect the determined
       trophic level breakout. Fish creel data (from data gathered through surveying
       anglers) from state fisheries departments is one justifiable basis for estimating
       types and amounts  offish 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 who eat fish. The state or
       authorized tribe should decide which approach to use.

       Trophic level 4 fish only—Predator species (e.g., trout, walleye, largemouth bass,
       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 higher trophic level species correlates with
       greater risks to human health. (This correlation is less evident in estuarine and
       marine species.) Therefore, targeting trophic level 4 species should serve as a
       conservative approach (depending upon the species most frequently consumed by
       anglers) for addressing waterbodies with highly varying concentrations of
       methylmercury.

4.2.2  What sample types best represent exposure?
EPA recommends using composite samples offish fillets from the types offish people in
the local area eat because methylmercury binds to proteins and is found primarily in fish
muscle. Using skinless fillets is a more appropriate approach for addressing mercury
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Monitoring and Assessment
                      exposures for members of the general population and most recreational fishers because
                      fish consumers generally eat the fillets. 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. However, some fish consumers 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. Besides being cost
                      effective, composite samples also ensure  adequate sample mass to allow analyses for all
                      recommended contaminants. 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 upon 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?
                      To address spatial variability of methylmercury levels in fish, EPA  recommends that
                      states and tribes design a probabilistic sampling by  randomly selecting sites or sampling
                      locations. This approach allows statistically valid inferences to be drawn  on an area as a
                      whole.

                      Ideally, samples should be collected  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, these 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 these areas but not  included if the area is
                      not used for fishing.

                      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 sufficient program resources exist, this guidance
                      recommends biennial sampling offish in waterbodies where recreational  or subsistence
                      harvesting is commonly practiced. If biennial screening is not possible, waterbodies
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should be screened at least once every 5 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 (i.e., August to October). Water levels are typically lower during
this time, thus simplifying collection procedures. Also, the fish lipid content is generally
higher, thus allowing these 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 this period. However, if the target species can be legally harvested during
its spawning period, sampling to determine contaminant concentrations should be
conducted during that time. 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 tribes sample consistently in a season to eliminate
seasonal variability as a confounding factor when analyzing fish monitoring data.
Additionally, 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 fish-fillet muscle mercury
concentrations 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
by as much as a factor of two to three times that in warm seasons. 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 in fish mercury 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. 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 (Giblin and Massaro 1973, Rodgers and Beamish 1982, Huckabee et al. 1979
[literature review], Burrows and Krenkel  1973, McKim et al.  1976). Such slow
depuration rates are expected to dampen strongly any fluctuations in methylmercury
concentrations in fish. Instead, season variations in fish tissue are likely linked to
seasonal nutrition variability that impact fish body conditions but not mercury body
burden.

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. These studies support the concept of starvation
concentration—whereby loss of muscle mass during periods of starvation occurs quicker
than loss of mercury. Burrows and Krenkel (1973) found mercury elimination rate to be
the same for fish that were starved relative to nonstarved fish. The converse phenomenon
of growth dilution, where lower fish-mercury concentrations correlate with higher growth
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Monitoring and Assessment
                      rates, has been described by a number of researchers (Simoneau et al. 2005, Doyon et al.
                      1998, Park and Curtis 1997). The authors of the first two papers hypothesize that slower-
                      growing fish allocate more energy towards maintenance and less to flesh production
                      while faster growing fish add flesh at a lower energy cost and, thus, with proportionally
                      less mercury intake. Park and Curtis (1997) proposed an alternative hypothesis that
                      growth dilution occurs when high growth coincides with periods of low methylmercury
                      concentration. Regardless of the exact mechanism, body condition offers a useful method
                      to explain variability in fish mercury.

                      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 6.1.2.7.1 provides information  to help determine
                      the number of composite samples for comparing fish tissue information to a target value.
                      This guidance does not recommend a single set of sample size requirements (e.g., number
                      of replicate composite samples per site and the 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
                      preventing the site-specific statistical analysis, a minimum of 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 for
                      mercury analysis), states and authorized tribes should first measure mercury in fish tissue.
                      This approach assumes that all mercury in fish tissue is methylmercury and is, thus, a
                      conservative assessment. This approach does not pose a risk of a false positive decision
                      (considering the tissue to exceed the criterion when it does not) where the measured
                      mercury in fish tissue is  less than the 0.3 mg/kg criterion (or a site-specific criterion
                      adopted by a state) nor should it pose a realistic risk of a false positive when the
                      measured mercury exceeds the criterion by 10 percent. Appendix E summarizes seven
                      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 seven studies,
                      methylmercury, on average, accounted for more than 90 percent of the mercury
                      concentration in fish tissue. If the measured mercury level is within 10 percent of the
                      methylmercury criterion, states might wish to repeat the sampling (if sufficient tissue is
                      not left) and analyze for methylmercury.
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 Section 303(d)(l) of the CWA requires 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 with existing or anticipated required controls. In
 accordance to 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, 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 AWQC. EPA's Guidance for Assessing
 Chemical Contaminant Data for Use in Fish Advisories,  Volume 1, at section 6.1.2.7.1
 recommends  using the t-test to determine whether the mean concentration of mercury in
 composite fish tissue samples exceeds the screening value. This involves a statistical
 comparison of the mean of all fish tissue data to the criterion. If the t-test statistic of the
 mean exceeds the water quality standards, there is an exceedence. EPA recommends that
 this procedure also be used for determining impairment. States and authorized tribes
 might also want to consider the guidance in Appendices C and D of the Consolidated
 Assessment and Listing Methodology, Toward a Compendium of Best Practices (USEPA
 2002b). Ultimately, the method that states choose depends on how they express their
 water quality standards.

 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 will
 be no need to mathematically compute  an average. Second, the newer analytical Methods
 1630 and 1631 are able to quantify mercury at 0.002 mg/kg, which should be lower than
 the observed  mercury in fish tissue samples being analyzed.

 However, if a state or authorized tribe is mathematically computing an average of a data
 set that does include 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). This 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 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 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)
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Monitoring and Assessment
                      (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 200 li).

                      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 using first the value of the detection level to quantify samples below
                      the detection level and then again using a zero value for samples below the detection
                      level. If both calculated means are either 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 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 200li) 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.

                      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 performs 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 offish 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 3.
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                                                                              Monitoring and Assessment
Table 3. Example data for calculating a weighted average fish tissue value
Species
Cutthroat Trout
Kokanee
Yellow Perch
Smallmouth Bass
Pumpkinseed
Brown bullhead
Signal crayfish
Trophic Level
3
3
3
4
3
3
2
Number of Samples
30
30
30
95
30
13
45
Geometric Mean
Methyl mercury
Concentration (mg/kg)
0.07
0.12
0.19
0.45
0.13
0.39
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 for 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 be 0.18 mg/kg, and would have given more weight to the
methylmercury concentrations in brown bullhead than 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:


      Qvg  =    3.8 * C2 + 8.0* Q + 5.7* Q   =    0.23 mg/kg     (Equation 4)
Where:
      C2

      C4
                       (3.8 + 8.0 + 5.7)
                average mercury concentration for trophic level 2
                average mercury concentration for trophic level 3
                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 fish  ). However, as noted throughout this document, the consumption
pattern of the target population should be used if available
16 The values for each trophic level are the same as discussed in section 3.2.1.2., and are found in Methodology for Deriving Ambient Water
Quality Criteria for the Protection of Human Health (USEPA 2000e).
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                      If fish tissue 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 data for trophic level 2 fish in the previous example, Equation 5 shows the
                      revised calculation:
                            Cavg   =   8.0* C2 + 5.7* Q    =   0.27mg/kg                (Equations)
                                          (8.0 + 5.7)

                      This revised calculation preserves the relative contribution of each trophic level to
                      consumption patterns. However, this approach should not be used if there are no data for
                      trophic level 4 fish, which is the type offish that is most often eaten. 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 approach can be applied to trophic level 4.

                      If the state or authorized tribe has developed a site-specific fish consumption rate for the
                      criterion, then the state or authorized tribe should incorporate this site-specific rate in
                      Equation 4 above. 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 offish that is more commonly
                      consumed or are representative  of the risk considerations of the state. Regression models
                      have been developed for this purpose (Wente 2003, Rae 1997). 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 log-normally 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 of Mercury and Fish to analyze two
                      data sets for use in analysis supporting the  CAMR (USEPA 2005a). This model is a
                      statistical model related to covariance  and allows the prediction of methylmercury
                      concentrations  in different species, cuts, and lengths offish for sampling events, even
                      when those  species, lengths, or cuts offish were not sampled during those sampling
52

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                                                                              Monitoring and Assessment
events. This 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,
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 last 5 years, as recommended in section
4.2.4. However, such information might not be available, and states and authorized tribes
will often consider mercury from samples collected and analyzed several years in the
past. Although the state and authorized tribe should consider this information, they
should also determine the reliability of this information and its accordance with
applicable data collection or quality assurance/quality control (QA/QC) program
requirements before using these data for listing assessments.

4.3.4  How should fish consumption advisories be used to
        determine impairment?
On October 24, 2000, EPA issued guidance on the use  offish advisories in CWA section
303(d) listing and 305(b) reporting decisions (USEPA 2000g). 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 offish 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
impairment and inclusion on a section 303(d) list, EPA considers a fish consumption
advisory and the supporting data as existing and readily available data and information.

A state or authorized tribe should include on its section 303(d) list, at a minimum, those
waters where waterbody-specific data that was the basis of a fish or shellfish
consumption advisory demonstrates nonattainment of water quality standards. EPA
believes that a fish or shellfish advisory would demonstrate 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.17 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 to not 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 exceedence and that the waterbody should
be placed on the 303(d) list. In contrast, if this same state or authorized tribe finds the
level of mercury in fish in another waterbody is at a level where it would advise  people to
17 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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Monitoring and Assessment
                      eat no more than 8 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 may not need
                      to be listed.

                      When reporting water quality conditions under CWA sections 303(d) or 305(b) on the
                      basis of a fish advisory for a migratory fish species, the state or authorized tribe should
                      include the waters where the migratory fish are known to inhabit because these are the
                      waters where the fish would become potentially 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 it is appropriate.
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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?
As stated in the January 8, 2001, Federal Register notice, EPA encourages states and
authorized tribes to adopt the fish tissue residue water quality criterion for methylmercury
into their water quality standards to protect CWA section 101(a) designated uses related
to human consumption offish. With respect to waterbodies within the Great Lakes basin,
a state or authorized tribe must also follow the requirements promulgated on March 23,
1995, at 40 CFR Part 132. Under these regulations, if a state or authorized tribe adopts
the new methylmercury criterion, EPA, in its review of the new state or tribal criterion,
must determine if it is as protective as the mercury criterion for human health protection
published in Table 3 at 40 CFR 132.5(g)(l) or on the basis of improved science (40 CFR
132.4(h)).

The human health criterion for mercury established by the methodology contained at Part
132 and adopted by the Great Lakes states is 3.1 ng/L. This water column criterion for
mercury is equivalent to a fish tissue residue value of 0.35 ug methylmercury/g fish tissue
using the Great Lakes-specific BAFs for mercury of 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). Therefore, a state or authorized tribe would apply the site-specific
modification procedures of Part 132 to show that the current, local BAF is lower than the
one used to develop the criterion in Part  132 before it could adopt the new fish tissue-
based criterion and methodology.

Also, EPA believes that if a state or authorized tribe adopts the new tissue-based criterion
for protection of human health in the Great Lakes, this action may not always result in a
change to TMDLs or NPDES permits. Part 132 also includes a 1.3 ng/L criterion for the
protection of wildlife, and in some instances, this criterion may 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  a 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 California Toxics Rule (CTR) (40 CFR 131.38) and the National
Toxics Rule (40 CFR 131.36). The Agency considers this flow to better reflect the
exposure offish 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 Quality-
based Toxics Control (USEPA 1991).
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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 is 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. 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 in their water quality
                     standards plans for later application to NPDES point sources 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 both the water column and food over the life of the fish and, thus, 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.

                     However, some states and authorized tribes may choose to adopt a water column criterion
                     based on the fish tissue criterion and, thus, have a criterion where a mixing zone may
                     apply. In this situation, a state  or authorized tribe should follow its existing procedures
                     for mixing zones.

                     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). This 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
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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 through the phased approach to TMDL development.

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, thus, 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
authorized tribe would have determined a site-specific BAF in this process and, thus,
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 if they are as protective as the Great Lakes procedures for human health
protection (40 CFR 132.5(g)(3)). Specifically, EPA would determine if 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 authorized 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 the Fish Advisory Program?
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
issuing consumption advisories for the general population, including recreational and
subsistence fishers,  and 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 offish meals per unit of time that
the target population can safely eat from a defined area.
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                      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, Volume 1 and 2 (USEPA 2000c, 2000d).
                      This guidance describes the two main equations necessary to derive meal consumption
                      limits on the basis of the methylmercury RfD. Basically, a first equation is used to
                      calculate the daily consumption limits of grams of edible fish (in grams per day (gd)); 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.

                      As a default screening-level approach, EPA recommends basing fish consumption
                      advisories on a consumption rate of 17.5 grams/day offish (uncooked) eaten from the
                      local water. 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 (i.e., chemical concentrations, exposure scenarios and
                      assumptions) to determine the necessity for issuing an  advisory. The nonregulatory nature
                      offish 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
                      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 fish consumption limit for mercury is that the criterion
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                                                                   Other Water Quality Standards Issues
includes an RSC18 and EPA's recommended tissue value for a fish advisory does not. In
deriving the criterion, EPA assumed an RSC value of 2.7x10" 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 noncommercially caught
fish, but the guidance can be applied without using an RSC. The RSC guidance in the
2000 Human Health Methodology (USEPA 2000e) 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 the attainment of a
         criterion and issuance of a fish consumption advisory?
In many states and authorized tribes, numeric water quality criteria and fish and shellfish
consumption limits differ due to inherent differences in the technical and risk
assumptions used to their development. 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 where  the
presence of an advisory may not imply an exceedence of the water quality standard
(USEPA 2000g) 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 may not demonstrate that a CWA section 101(a)
       "fishable" use is  not being attained in an individual waterbody and may not be
       appropriate for determining attainment based on exceedence of water quality
       criteria.

       Local advisory—States have issued local advisories using a higher fish
       consumption value than they use in establishing water quality criteria for
       protection of human health. Again, in this  case the fish consumption advisories
       may not demonstrate that a section 101(a) "fishable" use is not being attained in
       an individual waterbody and may not as appropriate as 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 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 eight meals a month. In this case,
  See discussion on the RSC in section 3.1.2.2.

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Other Water Quality Standards Issues
                      because the fish consumption advisory uses a higher fish consumption value than was
                      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 2000g).

                      In the case where a local advisory is based on a higher fish consumption value, 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, local
                      fish consumption patterns, and may identify more representative fish species. The
                      information gathered in developing the advisory may 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 a fish consumption limit and 304(a) criterion for
                      mercury are based on  similar methodologies and are intended to protect human health
                      from consumption of 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   How is the criterion related to FDA action levels?
                      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, not  food that is caught locally by recreational or
                      subsistence fishers

                      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 the general population who consume fish
                      and shellfish typically purchased in supermarkets or fish markets that sell products that
                      are 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
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                                                               Other Water Quality Standards Issues
and tolerances should not be used as a basis for establishing a state'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 are more reliant upon 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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                                                                                              TMDLs
6    TMDLs

6.1    What is a TMDL?
Section 303(d)(l) of the CWA requires 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 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 those 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 2004, 42 states reported at least one waterbody as  being impaired due to mercury,
and over 8,500 specific waterbodies were listed as being impaired due to mercury, either
solely or in combination with other pollutants. With the  implementation of the new
methylmercury fish tissue criterion, EPA expects that the number of waterbodies listed as
impaired due to mercury is likely to increase, although the waterbodies might also be
impaired due to other contaminants.

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 mining 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 to provide a basis for reducing mercury releases to water,
including through  air deposition, to meet applicable water quality standards and Clean
Water Act goals. In areas where large numbers of waterbodies are impaired due to
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TMDLs
                     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. EPA plans to develop further information on approaches to listing
                     mercury impaired waters and developing mercury TMDLs at a later date.

                     In the meantime, states continue to develop mercury TMDLs, with mercury TMDLs
                     approved for over 280 waterbodies. This guidance provides examples of approaches that
                     have been used in approved mercury TMDLs and examples of technical tools available to
                     assist in mercury TMDL development. Note that there are examples beyond those cited in
                     this document. Approaches in approved TMDLs range from waterbody-specific TMDLs
                     to regional-scale approaches. Technical tools available to assist in the development of
                     mercury TMDLs include screening level 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.

                     6.2.1  How have large-scale approaches been used for mercury
                              TMDLs?
                     In areas of the country where many waterbodies are listed as impaired for mercury, some
                     states have begun to explore the development of mercury TMDLs either as a group or on
                     a larger geographic scale, such as statewide or regionally. 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.
                     Due to the large extent of mercury from air deposition, the TMDL was developed on a
                     regional rather than a waterbody-by-waterbody basis. The TMDL used air deposition
                     modeling results to estimate wet and dry deposition of mercury for the six segments. 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, which is subsequently
                     transported to the waterbody via runoff and erosion. Additional information on this
                     TMDL can be found on EPA's detailed TMDL report at http ://oaspub .epa.gov/pls/tmdl/
                     waters  list.tmdl report?p tmdl id= 11642.

                     In New England, EPA is conducting a pilot project to test the feasibility of taking a
                     regionwide approach to mercury contamination. Mercury contamination throughout New
                     England has resulted in statewide fish  consumption advisories and the inclusion of almost
                     all fresh surface water on state lists of impaired waters. The pilot project will involve
                     development of a system to show regionwide information on mercury levels in fish,
                     loadings and sources of mercury, and mercury reductions needed to meet water quality
                     standards. The New England pilot project will consist of two levels of analyses or
                     models—fish tissue concentration predictions and mercury load reduction predictions.
                     EPA will use the regional model to identify factors that contribute to high levels of
                     mercury in fish and to predict the risk of mercury contamination for waterbodies with no
                     fish tissue data. EPA will use the Mercury Maps system, described above, to estimate
                     needed fish tissue concentration reductions.
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                                                                                                TMDLs
6.2.2    What is the Mercury Maps screening analysis?
A simple screening level analysis of the mercury sources impacting a waterbody or
waterbodies can assist in determining what type of approach to TMDLs is most
appropriate. One tool available to help states with such an analysis is EPA's Mercury
Maps (USEPA 200Id). Mercury Maps is a peer-reviewed geographic information system
(GIS) based analysis with 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 that were 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, 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.

Because Mercury Maps is a simplified approach, it has several limitations. First, Mercury
Maps is based on the assumption of a linear, steady-state relationship between
concentrations of methylmercury in fish and present day air deposition mercury inputs.
This condition will likely 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 (see Figure 3).

A second limitation is that the Mercury Maps methodology inherently requires that
environmental conditions remain constant over the time required to reach steady states.
This methodology might not be met, especially in systems that respond slowly to changes
in mercury inputs. For example, fish tissue data might not represent average,  steady-state
concentrations for two major reasons. Fish tissue and deposition rate data for the base
period are not at steady state. Where deposition rates have recently changed, the
watershed or waterbody might not have had sufficient time to fully respond. Also, fish
tissue data do not represent average conditions  (or conditions of interest for forecast fish
levels). Methylation and bioaccumulation are variable and dynamic processes. If fish are
sampled during a period  of high or low methylation or bioaccumulation, they would not
be representative of the average, steady-state or dynamic equilibrium conditions of the
waterbody. Other examples include areas in which seasonal fluctuations in fish mercury
levels are significant, for example due to seasonal runoff of contaminated soils from
abandoned gold and mercury mines or areas geologically rich in mercury. In such a case,
Mercury Maps predictions would be valid for similar conditions (e.g., wet year, dry year,
or season) in the future, rather than typical or average conditions. Alternatively, sufficient
fish tissue should be collected to get an average concentration that represents a baseline
dynamic equilibrium.
                                                                                                     65

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TMDLs
                                                          January 1,00:00:00
                                                Mill- 14.076 at (21,84), Max- 99.985dt(I48,1)
                      Figure 3. Percent of total mercury deposition attributable to global sources
                      (USEPA2005C)

                      Other ecosystem conditions might cause projections from the Mercury Maps approach to
                      be inaccurate for a particular ecosystem. Watershed and waterbody conditions can
                      undergo significant changes in capacity to transport, methylate, and bioaccumulate
                      mercury. Examples of this include regions where sulfate or acid deposition rates are
                      changing (in turn, affecting methylmercury  production independently of mercury
                      loading), and where the trophic status of a waterbody  is changing. A number of other
                      water quality parameters have been correlated with increased fish tissue concentrations
                      (e.g., low pH, high DOC, lower algal concentrations), but these relationships are highly
                      variable among different waterbodies. Mercury Maps will be biased when waterbody
                      characteristics change between  when fish were initially sampled and the new conditions
                      of the waterbody.

                      Third, states should be aware that many waterbodies, particularly in areas of historic gold
                      and mercury mining or areas with known natural mercury deposits, contain significant
                      non-air sources of mercury. The Mercury Maps methodology cannot be applied to these
                      waterbodies.

                      Fourth, 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. If a state or authorized tribe wants know the time over which the methylmercury
                      concentrations would change, they should use a dynamic model to estimate the recovery
                      during the period in which waterbody response lags reductions in mercury loads. A
                      dynamic model is also essential for understanding seasonal fluctuations and year-to-year
                      fluctuations due to meteorological variability.
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                                                                                               TMDLs
Finally, another source of uncertainty in the Mercury Maps forecasts are the atmospheric
deposition rates used to forecast changes in fish mercury concentrations. In the analysis
for the CAMR, EPA compared deposition rates in the Community Multiscale Air Quality
(CMAQ) and Regulatory Modeling System for Aerosols and Deposition (REMSAD) grid
cells to empirically derived loading rates (USEPA 2005b). At the locations chosen for the
analysis, site-specific data suggest somewhat higher deposition rates than the  CMAQ and
REMSAD models. In evaluating the importance of differences in absolute deposition
rates from air quality models and site-specific data, it is important to consider how the
results will be applied. If the results from air quality models are used as inputs to
ecosystem models such as the Dynamic Mercury Cycling Model and the BASS model
then the absolute deposition rates are used and so differences in absolute deposition is
important. However, if the results are used as inputs into models like Mercury Maps then
relative changes in deposition are used. In the latter case, differences in absolute
deposition rates are not directly relevant although such differences are important in model
validation.

EPA recognizes that methylmercury concentrations in fish across all ecosystems might
not reach steady state and that ecosystem conditions affecting mercury dynamics are
unlikely to remain constant over time. EPA further recognizes that many waterbodies,
especially in areas of historic gold and mercury mining in western states, contain
significant non-air sources of mercury. Finally, EPA recognizes that 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, EPA is unaware of any other tool for performing a regional-scale
assessment of the change in fish methylmercury concentrations resulting from reductions
in atmospheric deposition of mercury. Mercury Maps can show the watersheds across a
region where the current fish tissue concentration on average exceeds the new
methylmercury fish tissue criterion and, thus, where mercury load reductions will be
necessary to achieve the criterion. Mercury Maps also can group watersheds by their
major mercury sources, such as those watersheds where air deposition of mercury
predominates and those watersheds where other mercury sources besides air deposition
(e.g.,  POTWs, mining, pulp and paper mills, chlor-alkali chemical plants) have
significant impacts. For those watersheds where mercury comes almost exclusively from
air deposition, Mercury Maps can estimate the atmospheric load reductions needed to
meet the new criterion.

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 tribal-defined
watershed boundaries. The state may have  its own data on point source effluent loads and
more  detailed information on other significant sources of mercury in their state, e.g.,
erosion of mine tailings or natural geology. Further information on Mercury Maps is
available at http://www.epa.gov/waterscience/maps.

6.2.3  What are 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 sources (40 CFR
                                                                                                    67

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TMDLs
                      130.2(i)). EPA guidance further notes that a TMDL should identify the pollutant sources,
                      both point and nonpoint sources, including the location of the sources and quantity of the
                      loading. Some of the considerations in developing a mercury TMDL are described in
                      more detail in the text below.

                      6.2.3.1   What are potential mercury sources to waterbodies?
                      Some of the potential sources of mercury to waterbodies include direct discharges of
                      mercury from water point sources, including industrial dischargers and wastewater
                      treatment plants; 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; 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; sediments, which might have mercury contamination
                      or hot spots resulting from past discharges; and "naturally occurring" mercury in soils
                      and geologic materials. Sediments containing mercury from past discharges might
                      continue to contribute mercury to the overlying waterbody. Below is further discussion of
                      examples of TMDLs involving each of these types of sources.

                      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 (PCS) as well as a study of domestic mercury sources by
                      the National Association of Clean Water Agencies (NACWA), formerly known as the
                      Association of Metropolitan Sewerage Agencies (AMSA 2000). 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.

                      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 the total loadings from atmospheric deposition, either directly or
                      indirectly via runoff and erosion. (See various mercury TMDLs developed by EPA
                      Region 4 at http://www.epa.gov/region4/water/tmdl/georgia/index.htm.) The mercury in
                      atmospheric deposition originates from natural sources and from facilities such as
                      medical and waste  incinerators, electric utilities, and chlor-alkali plants, among others.
                      Mercury is emitted to the air in several chemical forms or species. 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 will determine 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,
                      showing that in many parts of the United States the source of deposited mercury is not
                      from a U.S. source.
68

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                                                                                               TMDLs
In 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.
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, global emissions might have
increased.
                                                      • Other .GodmK
                                                       lri"jtoia KlIsfE, cfiorn=
                                                         urrtKr razsrccus 'XSKS
                                                       Ircnsnton, etc;
                                                     DMedical Waste
                                                       Incinerators
                                                     D Municipal Waste
                                                       Combustors
                                                     D Utility Coal
                                                       Boilers
             1990       1996       1999
          Emissions  Emissions   Emissions
                                 •5a.rc; ^*,
Figure 4. Trends in mercury air emissions between 1990 and 1999

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
electric utilities and petroleum refineries), mobile sources (both onroad sources such as
cars and trucks, and nonroad engines such as construction equipment, agricultural
equipment, and so on), 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 using EPA's
                                                                                                    69

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TMDLs
                      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://www.epa.gov/region4/water/tmdl/
                      georgia/index.htm). 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.

                      EPA has evaluated water and air deposition modeling approaches as part of two mercury
                      TMDL pilot projects in Wisconsin and Florida. The Florida pilot report is complete (see
                      ftp://ftp.dep.state.fl.us/pub/labs/assessment/mercurv/tmdlreport03.pdf) (Atkeson et al.
                      2002). In the Wisconsin pilot project, EPA evaluated modeling tools such as the
                      REMSAD model for identifying the sources or categories of sources contributing
                      mercury deposition to a waterbody.19 The modeling and peer review for the Wisconsin
                      pilot are completed,  and a final report is expected in 2006. The Agency also 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
                      determine the appropriate strategies for addressing mercury deposition from sources
                      within their jurisdictions.

                      Air quality modeling for the CAMR was conducted using the CMAQ. 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 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 (Byun and Schere 2006, Bullock and Brehme 2002) was used for CAMR. This
                      version 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 used
                      for CAMR from that described in (Bullock and Brehme 2002) are  as follows:
                         1. The elemental mercury (HgO) reaction with H2O2 assumes the formation of
                            100 percent ROM rather than 100 percent particulate mercury (HgP).

                        2. The HgO reaction with ozone assumes the formation of 50 percent RGM and
                           50 percent HgP rather than 100 percent HgP.

                        3. The HgO reaction with OH assumes the formation of 50 percent RGM and
                           50 percent HgP rather than 100 percent HgP.
19 The air deposition modeling using REMSAD used an older emissions inventory than was used in CMAQ modeling conducted as part of
the CAMR analysis.

70

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                                                                                               TMDLs
   4.  The rate constant for the HgO + OH reaction was lowered from 8.7 to
      7.7 x ICT^cn^molecules'V1.

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 contents 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 the CAMR CMAQ modeling, meteorological input
files were derived from a simulation of 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 in McNally (2003).

These initial and boundary concentrations were obtained from 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 the NASA's Goddard Earth Observing System
(GEOS). This model was run for 2001 with a grid resolution of 2 degree x 2.5 degree
(latitude-longitude) 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 of these grid cells. 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.

As with any analysis based on limited data, there is inherent uncertainty 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 that are 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.
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                      Other tools available to help states characterize mercury deposition include existing
                      national monitoring networks and modeling tools, such as the Mercury Deposition
                      Network (MDN). Examples of these are provided in Appendix D. 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 at http://www.epa.gov/owow/oceans/airdep/air7.html.

                      Mining activity—Loadings from mining activities might include 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 (e.g., 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 A), 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 may 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 A.

                      Sediments—A  TMDL analysis should account for any mercury present in sediments as  a
                      result of current or past mercury loadings. Data on levels of mercury in sediments are
                      important in determining the extent to which controls on other sources will be effective
                      and how long it will take to achieve water quality standards. An examination of past
                      industrial practices in the watershed could include whether  sediments may serve as a
                      reservoir for mercury. Various national databases, such as the National Sediments
                      Database (USEPA 2002c) and data collected by USGS might also identify isolated
                      locations of elevated mercury in sediments. 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 if loadings from a
                            location  were contribution to water quality standards  violations

                         •  Screening modeling to determine other potential sources of sediment
                            contamination (see the TMDL at http://www.epa.gov/waters/tmdldocs/
                            1991 Bellingham%20Bav%20TMDL.pdf)

                      Natural or  "background" levels of mercury in soils—Soils  and sediments can include
                      mercury of geologic origin or mercury produced by the weathering of geological
                      materials, together with mercury of anthropogenic origin (i.e., mercury emitted overtime
                      from human sources and then deposited on soils). Mercury  in soils can also re-emit and
                      subsequently redeposit to soils. Local studies have been used in some TMDLs to estimate
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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.

6.2.3.2   What modeling tools are available to link mercury sources and
          water quality?
When developing a TMDL states or 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 will typically link
together models of atmospheric deposition, watershed loading, and mercury cycling with
bioaccumulation. This enables a translation between the endpoint for the TMDL
(expressed as a fish tissue concentration of methylmercury) and the mercury loads to the
water. The analysis determines the loading capacity as a mercury loading rate consistent
with meeting the endpoint fish tissue concentration.

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 may 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 accuracy of predicted future
      fish concentrations?

   •  Could other pollution control activities reduce mercury loads to the waterbody or
      affect the mercury bioaccumulation rate?

   •  After implementing regulatory controls, how long will it take for fish tissue levels
      to meet the criterion?

Depending on the types of questions states and 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. For more
information on the specific models described below, see http://www.epa.gov/athens and
http://www.epa.gov/crem.
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                      6.2.3.2.1    Steady state models
                      Steady state modeling describes the dynamic equilibrium between environmental media
                      established in response to constant loads over the long term. As such, complex mercury
                      cycling processes can be compressed into simple equations. One such approach,
                      discussed in the Mermentau/Vermillion Mercury TMDL (USEPA 200Ih), 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, derived in detail in the  Mercury Maps
                      report (USEPA 2001a), assumes that where air deposition is the sole significant source,
                      the ratio of current to future fish tissue concentrations equals the ratio of current to future
                      air deposition loads.  For the Clean Air Mercury Rule the assumptions of the Mercury
                      Maps steady state model were implemented. CMAQ modeled percentage changes in air
                      deposition under the rule were used to  predict changes in fish tissue concentrations.  For
                      example, if the air deposition model showed that the rule would result in a 10 percent
                      reduction air mercury deposition at a given fish tissue sample location,  that sample
                      concentration was reduced by 10 percent.  An advantage of this method is the ability to
                      use measured fish tissue concentrations which, by default, reflect potential variability in
                      bioaccumulation rates between ecosystems.  Examples of the application of the Mercury
                      Maps assumptions can be found in the  Clean Air Mercury Rule (USEPA  2005b and
                      USEPA 2005c).

                      Mass balance models are somewhat more complex implementations of the steady state
                      modeling approach. In  place of a simple ratio, the model would describe fluxes of
                      mercury in and out of the model domain (e.g., impaired segment), and optionally,
                      balancing fluxes (e.g., methylation and demethylation) within the model domain. The
                      advantage of this approach is that  individual fate processes, which could additionally be
                      controlled in a management setting, 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 the
                      watershed and waterbody remain unchanged, steady state models can produce as accurate
                      an estimate of the necessary load reductions as a dynamic model at a fraction of the cost.
                      Additionally, simple approaches, such  as those discussed above, are less prone to
                      calculation errors and much easier to communicate to the public.

                      6.2.3.2.2    Continuous simulation  and dynamic models
                      Continuous simulation or dynamic models take into account time varying effects such as
                      variable pollutant inputs, precipitation, hydrologic response, seasonal ecosystem changes,
                      and other effects on fish tissue concentrations. They might also include a variety of
                      physical and chemical fate and transport processes such as methylation, demethylation,
                      volatilization, sedimentation, re suspension, adsorption and desorption and so on. Such
                      dynamic models are  important in establishing cause and effect relationships.  They
                      assemble all available scientific knowledge on mercury fate and transport into a single
                      picture. Thus, they have been used to demonstrate how mercury moves from air
                      emissions to deposition to watershed runoff to subsequent bioaccumulation in fish at
                      observed levels in remote waterbodies  (USEPA 1997b).

                      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
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TMDL pilot project (USEPA 2000b) simulated the amount of time necessary to attain
equilibrium in response to reduced mercury loads using the Everglades Mercury Cycling
Model. The model results showed sediments continued to supply as much as 5 percent of
the mercury load 100 years after air deposition reductions occurred. The 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 A) (Tetra Tech
2001).

The SERAFM model incorporates more recent advances in scientific understanding
described above and implements an updated set of the IEM-2M solids and mercury fate
algorithms that were described in the 1991 Mercury Study Report to Congress (USEPA
1997b). This model was also used in the watershed characterizations to support the
CAMR (USEPA 2005b).

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 belter 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.

6.2.3.2.3   Spatially detailed models
Spatially detailed models, such as that used in the Savannah River TMDL (USEPA
200la), 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 WASP (Water Quality Analysis Simulation Program) model. WASP is a dynamic,
mass balance  framework for modeling contaminant fate and transport in surface water
systems. This model helps users interpret and predict water quality responses to natural
phenomena and man-made pollution for various pollution management decisions.
Another model used for both mercury TMDLs and watershed characterization in the
CAMR is the  EPA Region 4 Watershed Characterization System (WCS). This is a 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 the
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 is predominantly through soil erosion, erosion control might be identified as a
valid nonpoint source control on mercury to waterbodies (Balogh et al. 1998).
Additionally,  controls on acid deposition and, thus, changes in lake pH and its effect on
fish tissue mercury concentrations, might also be modeled (Gilmour and Henry 1991,
Hrabik and Watras  2002). Finally, spatially detailed models can be used to reflect the
local effects of wetlands, which produce  significantly more methylmercury per unit area
than other types of land use.

6.2.3.2.4   Model selection
When selecting a model,  the state or authorized tribe  should be aware of the assumptions
inherent in each type of model and consider what effect that assumption has on
determining the relationship between loadings and fish tissue levels or water quality. The
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                      first consideration is methylation. Several factors including pH, redox, sulfide
                      concentrations, temperature, DOC concentrations, salinity, and microbial populations
                      influence the speciation of mercury (Ullrich et al. 2001). If these factors vary seasonally
                      or around an average condition, the waterbody could be at a dynamic equilibrium and the
                      steady state assumption still apply. If these factors change with time such that they may
                      have a significant impact on fish tissue concentrations, the equilibrium assumptions
                      inherent in steady state modeling might not hold, and a dynamic model such as 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 out of equilibrium state.

                      The second consideration is the BAF. As discussed in section 3.1.2.2., 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 due to 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 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.

                      6.2.3.2.5   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 the 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., sulfide,
                      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 scale.
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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. In
addition, the expected effect of land-use changes on fish mercury concentrations for a
watershed dominated system illustrates changes like urbanization within a watershed can
alter the magnitude and timing offish mercury concentrations.

6.2.3.3   What are the allocation approaches  in mercury TMDLs?
A requirement for an approvable TMDL is that the state or tribe allocate the pollutant
load necessary to achieve water quality standards among point and nonpoint sources.
However,  EPA's regulations leave the decision regarding how to allocate loadings  to the
state or authorized tribe developing the TMDL. States and authorized tribes may use any
method or system for allocating pollutant loads among sources, provided that the
allocations will result in attainment of water quality 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
may 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 scenarios have occurred in mercury TMDLs, each with a
different mix of point and nonpoint sources. These scenarios have included the following:
   •  Point source loadings are small compared to loadings from nonpoint sources (e.g.,
      atmospheric deposition), but the expected load reductions in the nonpoint sources,
      together with modest reductions from the point  sources, are  sufficient to achieve
      water quality standards.

   •  Point source loadings are small compared to nonpoint sources, but the expected
      nonpoint source reductions are not adequate to achieve water quality standards
      even if point sources cease to discharge.

   •  Point source loadings are not small compared to nonpoint source loadings.

Point source loadings small; nonpoint sources expected to achieve WQS—The Savannah
River mercury TMDL provides an example of the first scenario. On the basis of an
analysis of air loadings for the Savannah TMDL, CAA regulations in place when the
TMDL was developed are expected to achieve the reductions from air loadings needed to
achieve the water quality target in the TMDL. The TMDL determined that a 44 percent
reduction in mercury loadings would be needed to reach the water quality target, and a
38-48 percent reduction in mercury loadings from air sources is expected by 2010  under
air regulations in existence at that time. The air regulations identified in the TMDL
address mercury emissions from medical, municipal, and hazardous waste incinerators.
The TMDL identifies only one point source on the Georgia side of the river that has a
permit to discharge mercury to the Savannah River. It identifies 28 point sources in
Georgia that may have the potential to discharge larger amounts of mercury in their
effluent according to the nature of the discharge or on mercury levels that have been
found in their effluents above the water quality standard level.
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                      The TMDL provides specific wasteload allocations for these sources on the basis of
                      meeting the water quality criterion at the end of pipe or alternatively implementing a
                      pollutant minimization program. In addition, the TMDL identifies about 50 other point
                      sources expected, according to their size and nature, to either discharge mercury 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 them holding their effluents at current levels. The wasteload allocations are
                      expressed in the TMDL by their sum. This TMDL can be found at http://www.epa.gov/
                      region4/water/tmdl/georgia/index.htm.

                      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. However, EPA believes that
                      the decisions, policies, and interpretations set forth in the TMDL are still valid and serve
                      as one example of an approach to mercury TMDLs.

                      Point source loadings small; nonpoint sources not expected to achieve WQS under
                      current regulations—The series of mercury TMDLs issued February 28, 2002 for
                      watersheds in middle and south Georgia illustrate the second scenario. In these basins,
                      point source loadings contribute very little to the mercury loadings (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. In five out of
                      seven basins where load reductions are needed to meet the water quality target, the
                      analysis indicates that CAA air regulations in place at  the time the TMDL was developed
                      will not achieve sufficient load reductions in the air sources to achieve the target. In the
                      Ochlockonee Basin, for example, a 76 percent reduction in mercury loadings is needed to
                      achieve the water quality target, but an analysis conducted for the TMDL indicated that a
                      31-41 percent reduction in air loadings would likely be achieved under air regulations in
                      place at that time (USEPA 2002a). In comparison, the aggregate of point sources is only
                      1 percent of the total load to the basin. The TMDL anticipates that there would be
                      additional reductions in mercury loadings due to current and planned activities. However,
                      as provided for under section 303(d), the TMDL quantifies the reductions needed to meet
                      the water quality standards.

                      Although point sources collectively contributed a very minute share of the mercury load,
                      the Ochlockonee and other mercury TMDLs for middle and south Georgia included
                      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, cost-
                      effective minimization measures. The middle and south Georgia mercury TMDLs issued
                      February 28, 2002, can be found at http://www.epa.gov/region4/water/tmdl/
                      georgia/index.htm.

                      Point sources loadings are not small—For these TMDLs, the reductions in point source
                      loadings, alone or in combination with nonpoint sources, can sufficiently achieve water
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quality standards. In this situation, the TMDL should consider reductions in both the
point sources and nonpoint sources to achieve the water quality standard. Appendix A
provides an example of a TMDL where point source loadings of mercury from mining
areas are large.

6.2.3.4   What kind 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 criteria-end-of-pipe.

   • Monitor for mercury in their influent and effluent using more sensitive analytical
     techniques (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.

For other facilities expected to be discharging below the water quality target, the TMDL
expects that they will verify through monitoring whether 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 by the end of 2005, including an assessment of
all mercury sources. (See the TMDL and supporting documents at
http://oaspub.epa.gov/pls/tmdl/waters list.tmdl  report?p  tmdl  id= 11642.)

TMDLs involving past mining activity have also included follow-up monitoring;
examples include two of the TMDLs described in Appendix A (Clear Lake, California
and Arivaca Lake, Arizona). The mercury TMDL for Arivaca Lake lists several follow-
up actions and monitoring activities, including 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 offish tissue
and sediment (see Appendix A, and the TMDL at http: //www.waterboards. ca. gov/
centralvallev/programs/tmdl/ClearLake/ClkTMDLfinal .pdf).
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                      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 is collected on the success of implementation activities. States may
                      refine information on the contributions from sources, such as runoff from abandoned
                      mining sites, sediment loading of mercury-laden sediments, or air deposition as data and
                      modeling tools improve. Thus, states should consider the application of adaptive
                      implementation in determining load allocations for these sources. Post-TMDL monitoring
                      is an important tool for evaluating implementation success and, if necessary, making
                      refinements in the TMDL.
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                                                                     NPDES Implementation Procedures
7    NPDES Implementation  Procedures
The CWA prohibits the discharge of any pollutant including mercury from a point source
into waters of the United States except in compliance with an NPDES or other CWA
permit (see CWA sections 301 (a) and 402). EPA or states and tribes authorized to
administer the NPDES program issue NPDES permits. 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 WQBELs, when necessary to ensure that the receiving waters achieve applicable water
quality standards (see CWA section 301(b)(l)(C)).20

Most WQBELs are expressed as numerical limits on the amounts of specified pollutants
that may be discharged. However, WQBELs may also be expressed in narrative form,
such as BMPs or pollutant minimization measures (e.g., practices or procedures that a
facility follows that result in a reduction of 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 Act (see CWA section 402(a)(l)(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" determination. Under EPA's regulations at 40 CFR
122.44(d)(l)(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 excursion above any [applicable] water
quality standard." Thus, if a pollutant discharge has the reasonable potential to cause or
contribute to an exceedence of applicable water quality standards, the discharger's
NPDES permit must contain a WQBEL for that pollutant (See 40 CFR 122.44(d)(l)(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
water quality standards) (See 40 CFR 122.44(d)(l)(ii)). The  procedure, while specifying
whether a discharge must have WQBELs, does not specify the actual value of the permit
limitation. The Technical Support Document for Water Quality-based Toxics Control
(TSD) (USEPA 1991)  contains EPA's guidance on determining reasonable potential.
20 When developing WQBELs, the permitting authority must ensure that the level of water quality achieved by such limits is "derived from
and complies with water quality standards (see 40 CFR 122.44(d)(l)(vii)(A)).

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                     7.2   How        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 as a fish tissue residue concentration
                     into their water quality standards, adopting it as a traditional water column concentration,
                     or adopting both the criterion as a fish tissue residue concentration and a traditional water
                     column translation. If states or authorized tribes choose to use both approaches, they
                     should clearly describe how each will be used for specific applications in their standards
                     and describe applicable implementation procedures.

                     EPA recommends three different 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 expressed the criterion (i.e., as a fish tissue value or as
                     a water column concentration). Additionally, states and authorized tribes that adopt the
                     recommended criterion as a fish tissue residue value may choose to implement it through
                     NPDES permitting as a water column translation of the fish tissue value. Each of these
                     approaches is discussed in more detail below and is summarized in Figure 5.
                                   Is the criterion
                                    expressed in
                                    terms offish
                                      tissue?
                                   Is the criterion
                                    implemented
                                     in terms of
                                     fish tissue?
Implement
using the
approaches in
the TSD (EPA,
1991a)

•Must develop
a BAP
Implement
using the
approaches in
the TSD (EPA,
199 la)

•Must develop
a BAP
                             Implement using the
                             approaches in this guidance

                             •Does not require developing
                             a BAP
                             Figure 5. Implementing the fish tissue criterion in NPDES permits
82

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                                                                 NPDES Implementation Procedures
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, the WQBEL for that mercury discharge must be consistent with the wasteload
allocation's assumptions (see 40 CFR  122.44(d)(l)(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
implementation 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 that is relevant to determining reasonable potential and establishing
effluent limits, where appropriate. If a state or authorized tribe develops its own permit
implementation procedures, EPA recommends that states and authorized tribes should
make the procedures 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' implementation procedures,
the CWA, and applicable federal regulations.
7.3  What are the implementation procedures when the
      criterion is adopted as a water column 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 water column concentration
value. Expressing a criterion as a water column value is very common, and permitting
authorities have considerable historical experience in implementing such criteria in
NPDES permits. Under this approach, EPA recommends that the permitting authority
make reasonable potential determinations and calculate numeric effluent limitations using
procedures consistent with those described in the TSD (USEPA 1991) or equivalent state
procedures.

This approach relies upon the measurement of mercury in effluents. Because the level of
mercury in many effluents is often very small, the permitting authority should specify
that the NPDES regulated discharger use the most sensitive analytical method approved
under 40 CFR Part 136 and report the quantitation level associated with that test.
Mercury levels in effluents can often be below the quantitation levels of some analytical
methods. By specifying the most sensitive method, the permitting authority minimizes
the chance that it would not require a WQBEL when one is actually necessary.

7.4  What are the implementation procedures when the
      criterion is adopted as a fish tissue value and the
      permitting authority  uses  a  water  column translation
      of a 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 offish tissue, but translates it into a
water column value for use in making reasonable potential determinations and
developing appropriate numeric WQBEL when necessary. Section 3.1.2.2 of this
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NPDES Implementation Procedures
                     guidance discusses 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 reasonable potential
                     determinations and numeric WQBELs. Once the criterion has been translated into a water
                     column value, the TSD (USEPA 1991) or equivalent state procedures can be used to
                     complete a reasonable potential determination and develop numeric WQBELs.

                     Because the level of mercury in many effluents is often very small, the permitting
                     authority should specify that the NPDES regulated discharger use the most sensitive
                     analytical method approved under 40 CFR Part 136  for total mercury and report the
                     quantitation level associated with that test. Federal regulations at 40 CFR 122.45(c)
                     generally require effluent monitoring for metal using the total form of the metal.

                     In addition, the permitting authority may also specify effluent monitoring using draft
                     EPA Method 1630 where the permitting authority is concerned about the level of
                     methylmercury (as opposed to total mercury) being discharged. Federal regulations at
                     122.41(j)(4) generally require that effluent monitoring results must be conducted
                     according to the test procedures approved under Part 136, unless other test procedures
                     have been specified in the permit.

                     7.5  What are the implementation  procedures 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 offish tissue and directly
                     implements the criterion without translating it into a water column concentration. As a
                     result, the permitting authority will use a different approach than it has used before for
                     determining reasonable potential and expressing effluent limits. EPA recommends the
                     approach described below, which is summarized in Figure 6.
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                                                                          NPDES Implementation Procedures
                               START of Process for Determining
                               Reasonable Potential Directly from a
                                    Fish Tissue Criterion
    • Monitor-asing Method
    1631

    • Permit re opener to
    assess Reasonable
    Potential after data is
    collected
    •  Special condition to
    conduct fish tissue
    survey

    • Permit re opener to
    assess RP once fish
    tissue is collected

    •Develop MMP
    (facilities that use or
    accept mercury
Unknown
  Is there
 quantifiable
  mercury
 discharge?
                               Ho
                                      •Ho necessary
                                      conditions
                                                                   Does feciliry
                                                                   use or accept
                                                                    mercury?
                                                                         Yes
Permit needs a
 WQEELfcr
  Mercury

(see Figure 7)
                                                                • Consider a MMP for
                                                                facilities that use cc
                                                                ac cept mercury
Figure 6. Determining reasonable potential

7.5.1   How to determine the need for permit limits to control
         mercury (i.e., how to determine reasonable potential)

As discussed in section 3.1.2.1. 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 residue concentration. When adopted in standards as a tissue value, states
and authorized tribes do not translate from a traditional water column value to a tissue
residue value using BAFs, which can vary highly by location and can be expensive. This
section provides recommendations  for how a permitting authority could determine
reasonable potential in the absence  of an available translation of the fish tissue value to 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 excursion
above the applicable water quality criterion (see 40 CFR 122.44(d)(l)(ii)). The NPDES
                                                                                                         85

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NPDES Implementation Procedures
                      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 CWA section
                      301(b)(l)(C) and 40 CFR 122.44(d)(l)(ii) with respect to a water quality criteria for
                      methylmercury expressed as fish tissue value, in the absence of a water column
                      translation of that value.

                      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)
                      fish tissue from the waterbody into which the discharger discharges exceeds the fish
                      tissue water quality criterion. Under these circumstances, the effluent data indicates that
                      the mercury loadings in the effluent contribute to the mercury load to the waterbody, and
                      the fish tissue indicates that the mercury load causes a water quality criterion excursion.
                      This approach is also consistent with federal regulations pertaining to the Great Lakes
                      Basin that contained an approach for determining reasonable potential using fish tissue
                      data (see 40 CFR Part 132, Appendix F, Procedure 5.F.4). EPA recommends that
                      permitting authorities use this approach because it 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 mercury present in fish tissue at levels
                      above the fish tissue criterion, and therefore the discharge may be found, in some
                      circumstances, to exhibit the reasonable potential to cause or contribute to the excursion
                      above 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.

                      EPA notes that, unlike typical water quality criteria that are expressed as water column
                      values, the fish tissue residue water quality  criterion integrates spatial and temporal
                      complexity and the cumulative effects of loadings  from point and nonpoint sources that
                      occur in aquatic systems that affect methylmercury bioaccumulation, including the
                      effluent variability of point sources. Therefore, EPA believes that comparing the fish
                      tissue residue concentration in receiving water directly to the applicable criterion
                      expressed as a fish tissue value appropriately accounts for the factors specified in 40 CFR
                      122.44(d)(l)(ii) for a criterion expressed as a fish tissue residual value.

                      7.5.1.1   How to determine that the NPDES permitted discharger has
                                mercury in its effluent at quantifiable levels
                      EPA recommends that permitting authorities require some monitoring using the
                      appropriate version of Method 1631 to  characterize the discharger's effluent for mercury
                      from all facilities for which the mercury levels are unknown or undetected. Method 1631
                      is relatively  new, and the facility might not have used it to analyze  its effluent. As a
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                                                                     NPDES Implementation Procedures
result, previous monitoring might show undetectable levels of mercury when use of
Method 1631 would show detectable or quantifiable amounts. As a result, EPA
recommends monitoring using Method 1631 to help identify all facilities that contribute
to mercury water quality impairment. At time of permit issuance, the permitting authority
should have at least one data point using Method 1631 as part as the permit application
submitted by the facility.

One of three outcomes will be reached in answering the first condition of the above
described reasonable potential analysis:
   • It is unknown whether the discharge includes quantifiable amounts of mercury.

   • The discharge does not include quantifiable amounts of mercury.

   • The discharge includes quantifiable amounts.

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 using
           Method 1631?
In this  situation, EPA recommends the permitting authority include permit conditions
comprised of:
   • Effluent monitoring using the appropriate version of Method 1631 to characterize
     the discharger's effluent for mercury

   • A reopener clause to identify the actions that the permitting authority may take
     should the monitoring information indicate that a mercury effluent limit is
     necessary

EPA recommends that permitting authorities require some monitoring, using the
appropriate version of Method 1631, by all facilities for which the mercury levels are
unknown or previously undetected to characterize the discharger's effluent for mercury,
unless prior testing was done using Method 1631. Method  1631 is relatively new, and the
facility might not have used it to analyze its effluent. As a result, the previous monitoring
might show undetectable levels of mercury when using Method 1631 would show
detectable or quantifiable amounts. As a result, EPA recommends this additional
monitoring to help identify all facilities that contribute to mercury water quality
impairment. The permitting authority could obtain this monitoring data either as part of
the  permit application, by requiring periodic (e.g., quarterly to annually) monitoring  as
part of the permit, or the permitting authority could invoke its authority under CWA
section 308 to require NPDES facilities to collect information necessary for the
development of NPDES permit limits. The permit should include a reopener clause such
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.
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NPDES Implementation Procedures
                      EPA recommends that when selecting the monitoring frequency, permitting authorities
                      consider the factors in section 5.7.5 of the Technical Support Document for Water
                      Quality-based Toxics Control (USEPA  1991). This section acknowledges that EPA has
                      not recommended a specific monitoring frequency, but recognizes that the choice of a
                      monitoring frequency is a site-specific decision and provides the permitting authority a
                      number of factors to consider when making these decisions.

                      Until the permitting authority has sufficient data to determine whether there is reasonable
                      potential, and depending on the particular facts, these permit conditions might be
                      considered as being as stringent as necessary to meet water quality standards, as required
                      by CWA section 301(b)(l)(C).

                      7.5.1.1.2   What are the recommended permit conditions when the discharge is
                                 analyzed using Method 1631 and does not include quantifiable amounts of
                                 mercury?
                      In this situation, EPA recommends the permitting authority first review the monitoring
                      data to determine if it is representative of the effluent.  If the permitting authority believes
                      the monitoring data are representative and all data are  below the level of quantification,
                      no further permit conditions may be necessary. If the discharge is below the level of
                      quantification, EPA does not consider the discharge to have reasonable potential to cause
                      or contribute to an excursion of the applicable fish tissue water quality criterion. 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 above.

                      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 evaluate data on the concentrations of
                      mercury in the fish tissue from the waterbody into which the discharger discharges and
                      determine appropriate permit conditions (see section 7.5.1.2 below).

                      7.5.1.2   How to determine appropriate permit conditions for facilities
                                discharging quantifiable amounts  of mercury
                      When applying EPA's recommended fish tissue reasonable potential procedure, once the
                      permitting authority has concluded that the first condition of the two-part reasonable
                      potential analysis has been satisfied (i.e., that the NPDES permitted discharger has
                      mercury in its effluent at a quantifiable  level),  the permitting authority should then
                      address the second condition. That is, does the fish tissue from the waterbody into which
                      the discharger discharges exceed the fish tissue water quality criterion?

                      One of three outcomes will be reached in answering this question:
                         •  The fish tissue concentration of mercury is unknown.

                         •  The fish tissue concentration of mercury does not exceed the criterion.

                         •  The fish tissue concentration of mercury exceeds the criterion.
88

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                                                                      NPDES Implementation Procedures
For discharges with quantifiable levels of mercury, the recommended reasonable
potential determination and recommended permit conditions for each of the outcomes 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 guidance.

7.5.1.2.1    What are the recommended permit conditions for facilities discharging
            quantifiable amounts of mercury but the concentrations of mercury in
            tissue offish in the receiving waterbody are unknown?
In waterbodies for which there are no fish tissue data, a permitting authority cannot
determine whether there is reasonable potential using a fish tissue approach. Therefore,
EPA recommends the permitting authority include permit conditions comprised of:
   •  A permit special condition to conduct a mercury fish tissue survey for the receiving
      water

   •  A reopener clause to identify the actions that the permitting authority may take
      should the  monitoring information indicate that a mercury effluent limit is
      necessary

   •  A permit special condition under the authority of CWA section 402(a)(l)(B) and
      40 CFR 122.44(k)(4) to develop a mercury minimization plan for facilities that use
      mercury in any aspect of their operations or accept waste waters that may contain
      mercury

In this instance, the permitting authority should start a process for collecting fish tissue
data in the vicinity of the facility. One approach for 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  permitting authority require that the study design be consistent
with the recommendations on conducting ambient monitoring in section 4.2 of this
guidance.

EPA recommends that the  permitting authority require only one study per waterbody.
The authority could do this by contacting all facilities that discharge into the waterbody
and encourage them to jointly work to conduct the study. Additionally, in waterbodies
where the permitting  authority expects to find high water column values or believes it
will need a site-specific BAF to  complete issuing the permits, the authority should
consider requiring the facility to measure water column concentrations  of mercury as part
of the study.

EPA further recommends that the permit should include a reopener clause such 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.
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NPDES Implementation Procedures

                      Additionally, in this situation EPA recommends that the permit should also include a
                      pollutant minimization plan for the reasons as described in section 7.5.1.2.2 below.

                      7.5.7.2.2    What are the recommended permit conditions for facilities discharging
                                  quantifiable amounts of mercury but the concentrations of mercury in
                                  tissue offish in the receiving waterbody do not exceed the criterion?
                      If the concentration of mercury in tissue offish in the receiving water does not exceed the
                      criterion, depending on the particular facts, the permitting authority might reasonably
                      conclude that the discharge does not have reasonable potential to cause or contribute to
                      an excursion of the applicable fish tissue water quality criterion.

                      In such situations,  EPA recommends the permitting authority consider including permit
                      conditions comprised of a permit special condition under the authority of CWA section
                      402(a)(l)(B) and 40 CFR 122.44(k)(4) to develop a mercury minimization plan for
                      facilities that use mercury in any aspect of their operations or accept wastewaters that
                      may contain mercury.

                      A mercury minimization plan helps ensure that the discharge continues to  have no
                      reasonable potential to cause or contribute to an exceedence of water quality standards.
                      The recommendation to consider including in the permit a requirement to develop a
                      mercury minimization plan is also based on the extent of potential mercury impairment
                      across the country  and the scientific complexities of and uncertainties when assessing
                      mercury loadings and evaluating these effects. Given these uncertainties, a permit
                      requirement that a  permittee at least develop a plan to minimize the discharge of mercury
                      would ensure that if the monitoring data demonstrates that a discharge does have
                      reasonable potential, the permittee and the permit writer are prepared to establish a limit
                      as stringent as necessary. Furthermore, EPA believes that a requirement simply to
                      develop a mercury minimization plan may provide dischargers of mercury with sufficient
                      information to voluntarily and economically reduce the discharge of mercury into our
                      nation's waters.

                      EPA recommends  that facilities, when developing mercury minimization plans, start with
                      their existing best management plans and spill prevention and containment control plans.
                      Many of the activities covered by these plans can also serve to reduce mercury sources to
                      wastewater. In addition, for facilities that do not use mercury in any aspect of their
                      operations or accept wastewaters that may contain mercury, EPA does not believe these
                      facilities have pollution prevention opportunities and, thus, should not be required to
                      develop a mercury minimization plan.

                      The facility should determine the content of a mercury minimization plan on a case-by-
                      case basis. After reviewing many PMPs,  EPA recommends that a plan include at least the
                      following elements:
                         •  The identification and evaluation of current and potential mercury sources

                         •  For POTWs, the 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

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                                                                      NPDES Implementation Procedures
   •  The 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, materials 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

   •  Monitoring to verify the results of pollution minimization efforts

7.5.1.2.3   What are the recommended permit conditions for facilities discharging
           quantifiable amounts of mercury and the concentrations of mercury in
           tissue offish in the receiving waterbody exceed the criterion ?
EPA believes that, depending on the particular facts, a permitting authority might
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
concentrations of mercury in tissue offish from the waterbody into which the discharger
discharges exceed the fish tissue water quality criterion. When reasonable potential
exists, it is necessary to establish an appropriately protective WQBEL in the permit. For
guidance on how to develop appropriate WQBELs, see  the following section.

7.5.2   Where reasonable potential exists, how can  WQBELs be
        derived from  a tissue value?
As discussed in section 3.1.2.1 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 residue concentration. When adopted in  standards as a tissue value, states
and authorized tribes do not translate from a tissue residue value to a traditional water
column value using BAFs, which can vary highly by location and can entail extensive
costs to develop. When developing WQBELs, the permitting authority must ensure that
the level of water quality to be achieved by such limits is "derived from and complies
with water quality standards" (see 40 CFR 122.44(d)(l)(vii)). This section provides
recommendations for how a permitting  authority could derive appropriate WQBELs in
the absence of an available translation of the fish tissue  value to a water column value.
The process described in this section is  shown in Figure 7.
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NPDES Implementation Procedures
                                                      S TART of Process for Det*nninmg the
                                                       Water Quality-based Effluent Limit


• Use sere ening tool or
TMDL analysis!*!
determine relative
contribution from point
and non-point sources

/" t the \
/' discharge i \
Unknown \ mercury / j^Q
source ? /

•Numeric limit at
existing effluent
kvels
•Compliance
monitoring using
Method 1631
MMP
                            •Numeric limit based on water
                            quality standards (either water
                            column, translation and applying
                            criteria as limit; or offset; or
                            based OIL TMDL or TMDL-like
                            anafysis
                            • Compliance monitoring using
                            Method 1631
Does the facility
use mercury in
 its process or
   accept
  wastewater
                         Figure 7. Process for determining the WQBEL

                       EPA recommends that the permitting authority, when establishing appropriate WQBELs,
                       first determine whether the discharge is a significant source of mercury. EPA
                       recommends different WQBELs depending on whether the discharge is considered to be
                       significant or not significant, as described in 7.5.2.3 and 7.5.2.2, respectively. EPA's
                       guidance on how to determine whether a discharge is significant is described in section
                       7.5.2.1 below. Additionally, EPA recommends that the permitting authority, when
                       establishing appropriate WQBELs for a significant discharger, consider whether the
                       facility uses or accepts mercury in its process.

                       The NPDES permit fact sheet must provide an explanation that how the 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
                       effluent limitations and other conditions that satisfy CWA section 301(b)(l)(C) and 40
                       CFR 122.44(d)(l) with respect to mercury.

                       7.5.2.1    How to determine if the discharge is a "significant" source of
                                  mercury
                       When determining the sufficiency of a WQBEL to attain and maintain water quality
                       standards, the permitting authority may consider the effluent controls in conjunction with
                       the other point and nonpoint  source controls (including expected mercury reductions
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                                                                       NPDES Implementation Procedures
from airborne deposition as a result of existing or expected controls on air emissions) and
their cumulative effect on water quality standards attainment. Because air deposition and
other nonpoint sources are expected to play a significant role in the mercury loading to
many waters, EPA recommends that permitting authorities take into account these
loadings—and their potential change—when determining what WQBELs are appropriate.
One way of doing this is to use a screening level approach, such as that used in Mercury
Maps21 (USEPA 200Id). The Mercury Maps report identified watersheds where EPA
believed mercury air deposition likely contributed greater than 95 percent of mercury
concentrations in fish tissue. For example, mercury mines, large-producer gold mines,
and mercury-cell chlor-alkali facilities were considered significant sources on the basis  of
simple presence in the watershed. Municipal wastewater treatment plants and pulp and
paper mills were considered significant when their estimated cumulative  load contributed
greater than 5 percent  of the estimated waterbody-delivered air deposition load. Another
option for determining the relative significance of point source discharges is to do a
TMDL, or TMDL-like analysis, as part of the permit. Depending on the facts in each
case, permitting authorities should determine what sources are potentially large sources
of mercury other than  air deposition.

For a discharge not to  be considered "significant," under existing loading conditions,
EPA recommends that the loading of the point source (or cumulative loading of all point
sources) to the receiving water are expected to account for a small  or negligible
component of the current total mercury loadings and that, upon implementation of the
permit's mercury minimization program requirements, any further reductions from the
point source(s) would  result in no discernable improvement in water quality. This is not a
situation where a wasteload allocation to a point source is increased because of an
assumption that loads  from nonpoint sources will be reduced. To the contrary, this is a
situation where mercury minimization activities will maintain or reduce current point
source loadings of mercury to levels at which there are no discernible  impacts to  water
quality.

If permitted discharges are regulated consistent with the recommendations described in
this guidance, EPA believes that the discharge is likely to have no discernible effect on
water quality. EPA believes that discharger mercury loadings that remain following
implementation of the minimization program  requirements would have no discernible
impact to water quality because, due to the large contribution of mercury from
nonpermitted sources, even entirely eliminating the point source discharges of mercury
would cause no discernible improvement to water quality. Therefore, EPA believes,
depending on the particular facts, limits on these point sources consistent with this
guidance are likely to be as stringent as necessary to implement water quality standards.

EPA notes that point source discharges of bioaccumulative chemicals like mercury might
have particular local significance apart from their contribution to the cumulative  load.
Point source discharges by their nature could create hot spots where observed elevated
concentrations have potential impact on human health if fish stay in the immediate  area.
Consequently,  comparing contributions from the air and water sources at long distances
downstream from the point source could conceal the real impact of mercury from point
 21 For more information about Mercury Maps see section 4.2.2.

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NPDES Implementation Procedures
                      source discharges. Instead, permitting authorities should evaluate the relative
                      contributions of point sources to the total load at the point of discharge. In some cases,
                      elevated receiving water concentrations may be dictated caused solely by the mercury
                      concentration in the effluent as opposed to the mercury delivered from air deposition.

                      7.5.2.2    What are EPA's recommended permit conditions for discharges
                                that are not significant sources of mercury?
                      Here, a permitting authority is addressing the situation where there are data showing that
                      there is reasonable potential and thus a WQBEL is necessary. However, if one's mercury
                      discharge is determined to be insignificant, EPA believes that an appropriate WQBEL
                      could be comprised of both of the following:
                         •  A numeric effluent limit for the mass loading of mercury established at the existing
                            effluent level (or any existing numeric limit, whichever is more stringent) including
                            compliance monitoring using the appropriate version of Method 1631

                         •  A permit condition to implement appropriate mercury minimization measures
                            identified in a mercury minimization plan

                      EPA believes these minimum permit conditions may be appropriate because they help to
                      ensure the discharge does not cause or contribute to an exceedence 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, when the discharge is
                      not a significant source of mercury, permit numeric effluent limits established at the
                      existing effluent quality (or any existing numeric limit, whichever is more stringent) and
                      implementation of a mercury minimization plan are likely to be as stringent as necessary
                      to meet water quality standards.

                      EPA believes that mercury reductions achieved through implementation of mercury
                      minimization programs could potentially result  in important reductions in mercury
                      loadings. EPA bases this belief on its study of pollutant minimization programs and their
                      success in reducing loadings of mercury to the environment. See the Mercury Report to
                      Congress (USEPA 1997b) and draft Overview of P2 Approaches atPOTWs (USEPA
                      1999). These reports 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, and not 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 those products for
                      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, high
                      volume discharges to the collection system, with considerable effort also directed at high
                      concentration, low volume discharges such as medical and dental facilities.

                      Using pollutant minimization or prevention programs can also reduce the transfer from
                      wastewater to other media via disposal of mercury-containing sludge that may reenter the
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environment. For example, mercury removed at a POTW through treatment is likely to
reenter the environment through POTW sludges that are then either incinerated or applied
to land (although some is captured by air emission 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 remove and recycle
mercury at the source (i.e., 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 also reduce the amount of mercury entering the environment
through other media or sources (e.g., 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, see the draft Overview of P2 Approaches at POTWs (USEPA
1999). For an example of guidance on how to develop a mercury minimization plan, see
the EPA Region 5 final document Mercury 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 the state permitting
authorities in Region 5.

Finally, mercury is a bioaccumulative, persistent pollutant that has been linked to adverse
health effects. For example, children who 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. In this scenario, EPA believes, as a matter of policy, that
point sources that can cost effectively reduce their mercury discharges should do so.
Because air sources or historical contamination are likely dominant causes of impairment
this does not mean that point sources should not implement cost-effective, feasible
pollution prevention measures to reduce their contribution of mercury, however small, to
the environment. In short, EPA believes it is reasonable to expect that NPDES permittees
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 limits that require such implementation are likely to derive from and comply with
water quality standards as required by EPA regulations at 40 CFR 122.44(d)(vii)(A).

7.5.2.3   What are EPA's recommended permit conditions for discharges
          that are significant sources of mercury?
If a facility is a significant source of mercury, the permitting authority should first
consider whether or not the facility uses mercury in its process or accepts wastewater
containing mercury when deciding on appropriate WQBELs.
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NPDES Implementation Procedures
                      7.5.2.3.1   What are appropriate WQBELsfor significant dischargers that do not use
                                 mercury and do not accept wastewater containing mercury in their
                                 processes?
                      For significant dischargers that do not use mercury in their processes and do not accept
                      wastewater containing mercury, EPA believes that the permitting authority may express
                      the WQBEL that is comprised of the following:
                         • A numeric effluent limit for the  mass loading of mercury established at the existing
                           effluent level of mercury (or any existing numeric limit, whichever is more
                           stringent) including compliance monitoring using the appropriate version of
                           Method 1631

                         • A permit condition to implement appropriate mercury minimization measures
                           identified in a mercury minimization plan

                      If such a discharge has the reasonable  potential to cause or contribute to an exceedence of
                      water quality standards and the discharge is significant, EPA believes that during the first
                      term of the permit and depending on the particular facts, permit terms that limit the
                      discharge of mercury to existing effluent quality (or any existing numeric limit,
                      whichever is more stringent), require the facility to develop and implement a mercury
                      minimization plan, and require monitoring are likely to be as stringent as necessary to
                      meet water quality standards. Given the extent of mercury impairment across the United
                      States mostly due to nonpoint sources such as air deposition or previous contamination,
                      and that assuming these dischargers do not use or accept mercury in their processes but
                      rather receive it from diffuse sources, EPA believes that, depending on the particular
                      facts, permit conditions that prohibit an increase of mass loadings of mercury and
                      mandate a reduction of loadings when consistent with a mercury minimization plan are
                      likely to be as stringent as necessary to meet standards  as required by CWA section
                      301(b)(l)(C). EPA generally believes these minimum permit conditions are appropriate
                      and sufficient to ensure the discharge does not cause  or contribute to an exceedence of
                      water quality standards, protect against possible localized impacts, and minimize the
                      discharge of mercury. EPA believes these permit terms are appropriate in cases where  the
                      facility itself does not use mercury in its processes. EPA expects that the implementation
                      of a mercury minimization plan will reduce the discharge of mercury. However, if at the
                      end of the first permit term,  data and information indicate that a more stringent limit is
                      necessary to ensure that the  discharge  does not cause or contribute to an exceedence of
                      water quality standards, including localized effects, the permit should be revised at
                      renewal.

                      7.5.2.3.2   What are appropriate WQBELsfor significant dischargers that use
                                 mercury in their processes or accept wastewater containing mercury?
                      For significant dischargers that use mercury in their processes or accept wastewater
                      containing mercury, EPA believes that the permitting authority may express the WQBEL
                      that is comprised of the following:
                         • A numeric WQBEL for the mass loading of mercury. Such a limit could be based
                           on a TMDL, a TMDL-like analysis, an offset, or established using the criteria as
                           the effluent limit (through development of a site-specific BAF) including
                           compliance monitoring using the appropriate version of EPA Method 1631
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   •  A permit condition to implement appropriate mercury minimization measures
      identified in a mercury minimization plan

Because there are significant direct water inputs of mercury from these facilities, states
and authorized tribes should carefully consider making these watersheds a priority for
TMDL development so that the TMDL can provide the basis for the appropriate permit
limits. 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 impairment due to mercury. These situations are
more complicated because the specific location and magnitude of each source could be
significant as to its effect on fish tissue concentrations. For these situations, a TMDL
provides the best basis for developing the appropriate permit limits, and thus, these
situations  should receive a higher priority for completion.

Once EPA has approved or established a TMDL containing a wasteload allocation for the
discharge  of mercury, 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)(l)(vii)(B)). Besides developing a
WQBEL,  the permitting authority also 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 the version of Method 1631 then in effect to
assure that even trace levels of mercury are quantified.

In addition, EPA recommends that the permit require the dischargers to implement
appropriate mercury minimization measures  identified through the mercury minimization
plan if the monitoring data shows that mercury is present in the final  effluent. In many
instances,  the mercury minimization plan may be a recommended part of the wasteload
allocation. Where it is not, EPA believes that implementing the plan should help the
facility achieve the WQBEL.

In the absence of a final TMDL, a permitting authority could develop an analysis similar
to what would be provided in a TMDL. Such a TMDL-like analysis that applied similar
factors used 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.

It is also possible for the permitting authority to issue a discharger a permit prior to
TMDL development where it is demonstrated that other pollutant source reductions (such
as nonpoint  source reductions implemented by the discharger or other sources) will offset
the discharge in a manner consistent with water quality standards. The ultimate result of
this type of "offset"  may be a net decrease in the loadings  of the pollutant of concern in
the CWA  section 303(d) listed water, and therefore, the point source being permitted
might be considered as  not causing or contributing to a violation of water quality
standards.

Establishing the proper WQBEL in a specific permit is a fact-based determination. There
are a number of ways to develop a permit that ensure that a discharge does not cause or
contribute to an exceedence of water quality  standards. Historically, EPA has not
considered a discharge with effluent limitations at or below either the numeric water
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NPDES Implementation Procedures
                      quality criteria or a quantification of a narrative water quality criterion to "cause or
                      contribute to a violation of water quality standards."

                      For these significant dischargers, a state or authorized tribe may decide to translate the
                      fish tissue criterion into a water column value for use in making reasonable potential
                      determinations and developing appropriate numeric WQBELs. Section 3.1.2.2 of this
                      guidance discusses 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 reasonable potential
                      determinations and numeric WQBELs. Once the criterion has been translated into a water
                      column value that  accounts for the effects  of bioaccumulation, the TSD (USEPA 1991) or
                      equivalent state procedures can be used to complete a reasonable potential determination
                      and develop numeric WQBELs. Once such a water column criteria concentration value is
                      developed, a WQBEL established at the criterion concentration would be appropriate for
                      receiving waters that exceed the fish tissue criterion.

                      7.5.2.4  What are EPA's recommendations for indirect dischargers to
                                POTWs that are significant sources of mercury?
                      POTWs are 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)). POTWs that accept mercury in their collections
                      systems may need to ensure that their pretreatment program protects the POTW's effluent
                      from contributing to excursions of the fish tissue criterion.  The General Pretreatment
                      Regulations (40 CFR 403) require that each POTW required to develop an approved
                      pretreatment program must protect against pass through 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. POTWs are 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(1) and 403.3(n)).

                      Federal categorical pretreatment standards, which are applicable to certain classes of
                      industries, establish technology-based minimum pretreatment standards. However, the
                      categorical standards do not address POTW-specific problems which 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 in order to protect the treatment facilities, receiving water quality, and worker
                      health and safety, and to allow for beneficial use of sludge.

                      As described above, this guidance typically recommends that permit limits for POTWs
                      consist of a numeric effluent limit and a requirement to develop and implement
                      appropriate mercury minimization measures. EPA expects that a  POTW's numeric limit
                      for mercury would be the basis for the development of local limits in the pretreatment
                      program consistent with guidance on the development of local limits. The mercury
                      minimization program requirements could also be the basis for establishing pollutant
                      minimization program requirements for dischargers to the collection system.
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7.6   What are the recommended analyses for new sources
       or new dischargers discharging quantifiable amounts
       of mercury?
Additional permitting requirements apply to new sources or new dischargers that will be
discharging new or increasing concentrations of pollutants. The NPDES regulations at 40
C.F.R. §122.4(i) currently prohibit the issuance of a permit to a new source or new
discharger whose discharge will cause or contribute to a violation of water quality
standards.

In addition, such increased discharges of mercury 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. Federal Regulations at 40
CFR 131.12 identify the elements of an acceptable  antidegradation policy. The Federal
antidegradation policy is composed of three levels of protection commonly referred to as
tiers. The first element identified at 40 CFR 131.12(a)(l) protects the minimum level of
water quality necessary to support existing uses and applies to all waters. This element
prohibits lowering water quality to the point where existing uses are impaired. The
second element is found at 40 CFR 131.12(a)(2), and protects water quality where water
quality is better than that needed to support designated uses in and on the water. Where
these conditions exist, the water body is 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
will be fully protected. The third element at 40 CFR 131.12(a)(3) involves the protection
of water quality in water bodies that are of exceptional ecological, aesthetic or
recreational significance. Water quality in such water bodies,  identified and specifically
designated by States as Outstanding National Resource Waters must be maintained and
protected.

 One potential means of satisfying antidegradation (and 40 CFR 122.4(i) for new sources
or new dischargers to water quality limited segments) may be a demonstration that other
mercury source reductions (such as nonpoint source reductions implemented by the
discharger) will offset the new or  increased discharge. The ultimate result of this type of
"offset" might be a net decrease in the loadings of mercury to the receiving water, and
therefore, depending on the particular facts, the discharge might not be considered an
increased loading. EPA's recommendations for addressing mercury in new sources and
new discharges are summarized in Figure 8.

7.6.1   What are the recommendations for permitting authorities
        when considering issuing permits for new sources or new
        dischargers where  the fish tissue concentrations in the
        receiving waterbody are unknown?

In waterbodies for which there are no fish tissue data, a permitting authority cannot
determine the applicable antidegradation requirements. In these instances, the permitting
authority should start a process for collecting such data in the vicinity of the facility.  One
approach for collecting this information is for the permitting authority to invoke its
authority under CWA section 308 to require point sources to collect information
necessary for the development of NPDES permit limits. In this case, the permitting
authority could issue a section 308 letter to require  the permittee to conduct a
methylmercury fish tissue monitoring study prior to issuance of a permit. EPA
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NPDES Implementation Procedures
                      recommends that the permitting authority require that the study design be consistent with
                      the recommendations on conducting ambient monitoring in section 4.2 of this guidance.
START Process for New Sources
New Dischargers Expected to
Discharge Mercury



After the study ,
/
• C onduct a fish tis sue
study as part of the
permit application, via a
special condition of the
permit, or via a section
308 authority request
letter
/' Doest
Unknown^,/ tissue
\ water
\ the crit
i
\
he fish \
in the \
ving ^
xceed /
erion? /
and
No
v< 	
                                                   •Numeric limit to prevent new or
                                                   increased loads to the waterbody that
                                                   would cause or contribute to the
                                                   impairment
                                                   •Compliance monitoring using
                                                   Method 1631

                                                   •Develop andimplementMMP
                                                   (facilities that use mercury in their
                                                   process or accept wastewater
                                                   containing mercury)
                                                                                   •Numeric limit based on
                                                                                   antide gradation
                                                                                   requirements (both
                                                                                   prevent significant
                                                                                   degradation and protect
                                                                                   designated use)

                                                                                   •Compliance Monitor
                                                                                   using Method 1631
                                                                                   •Develop and implement
                                                                                   MMP (facilities that use
                                                                                   mercury in their process
                                                                                   or accept wastewater
                                                                                   containing mercury)
                      Figure 8. Procedures for addressing new sources and new discharges

                      Once the permitting authority has determined the appropriate antidegradation
                      requirements on the basis of the fish tissue concentrations in the receiving water, the
                      permitting authority can then determine the appropriate permit requirements for new
                      sources or new dischargers, as described below.

                      7.6.2   What are the recommended permit conditions for new
                               sources or new dischargers where the fish tissue in the
                               receiving water does not exceed the criterion?
                      In this situation, EPA believes that the permitting authority may establish permit
                      conditions that are comprised of the following:

                         «  A numeric effluent limitation, the level to which the discharger is ultimately
                            allowed to lower water quality (on the basis of the applicable antidegradation
                            requirements) including compliance monitoring using the appropriate version of
                            Method 1631
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                                                                       NPDES Implementation Procedures
   •  A permit condition to implement appropriate mercury minimization measures
      identified through the mercury minimization plan if the facility uses mercury in its
      process or accepts wastewater containing mercury

In this case, the receiving water does not currently exceed the fish tissue criterion. EPA
believes that new sources or new dischargers that increase the discharge of mercury
should be required to implement mercury minimization plans and should be allowed to
discharge at levels as determined by the antidegradation analysis.

Permits for proposed new sources or new dischargers of mercury 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. Under EPA's
antidegradation regulations for water quality standards, the  quality of waters belter than
levels necessary to protect human health can 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 encourages states and authorized tribes to regard any
increase in mercury used in a discharger's process or in wastewater accepted by a
discharger as a significant lowering of water quality for the purposes of triggering a tier 2
antidegradation review. If the state's or authorized tribe's antidegradation analysis
determines that the proposed lowering of water quality should not be allowable, the
permitting authority would not authorize or allow any such new or increased discharge.
Where 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 protect the minimum level of
water quality necessary to support existing uses by prohibiting lowering water quality to
the  point where existing uses are impaired (see 40 CFR 131.12(a)(l).

EPA recognizes  that 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.  While an mercury minimization plan, to the extent that
there are available BMPs to minimize mercury discharges, may still be  appropriate in
such circumstances, EPA would not generally expect that such dischargers would trigger
the  need for an antidegradation review or numeric WQBELs, unless they were causing
or contributing to a significant lowering of water quality.

In addition, EPA recommends that the permit require the dischargers to implement
appropriate mercury minimization measures identified through the mercury minimization
plan if the facility uses mercury in its process or accepts wastewater containing mercury.
22 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. See sections 7.5.1.2.2 and 7.5.2 for more details.
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                      7.6.3   What are recommended permit conditions for new sources or
                              new dischargers where the fish tissue in the receiving water
                              exceeds the criterion?
                      In this situation, EPA believes that the WQBEL may be comprised of the following:
                         • A numeric WQBEL for the mass loading of mercury established at levels
                           consistent with 40 CFR 122.4(i) and 122.44(d)(l)(vii). Such a limit could be based
                           on a TMDL, a TMDL like analysis, or via an offset, including compliance
                           monitoring using the appropriate version of Method 1631

                         • A permit condition to implement appropriate mercury minimization measures
                           identified through the mercury minimization plan if the facility uses mercury in its
                           process or accepts wastewater containing mercury

                      Existing EPA regulations do not establish an absolute prohibition on new or increasing
                      discharges for point sources on water quality limited segments. Instead, the NPDES
                      regulations at 40 CFR 122.4(i) prohibit the issuance of a permit to a new source or new
                      discharger whose discharge will cause or contribute to a violation of water quality
                      standards, including the applicable antidegradation policy. A permit may be issued if the
                      discharge would not cause or contribute to the  exceedence of the water quality standards.
                      For example, it is possible for a discharger to be issued a permit, under appropriate
                      circumstances, where it is demonstrated that other pollutant source reductions will  offset
                      the discharge in a manner consistent with water quality standards. The ultimate result of
                      this type of offset may be a net decrease in the  loadings of the pollutant of concern in the
                      impaired water and, therefore, be considered not to "cause or contribute to a violation of
                      water quality standards." This regulation applies only to "new sources"  and "new
                      dischargers" as defined in sections 122.2 and 122.29 of the NPDES regulations. Existing
                      dischargers and increases in existing discharges are not subject to this regulation.

                      Existing dischargers, as well as new sources and new dischargers, are subject to the
                      regulation at 40 CFR 122.44(d)(l)(vii) (A). That regulation provides that when
                      developing water quality-based permit effluent limitations, the permitting authority is to
                      set the limitations to ensure that the level of water quality to be achieved "is derived
                      from, and complies with all applicable water quality standards." This would necessarily
                      be a permit-by-permit determination. After a TMDL has been established, the regulation
                      provides that the effluent limitations must be consistent with the assumptions and
                      requirements of any approved wasteload allocation (see 40 CFR  122.44(d)(l)(vii)(B)).

                      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(1); see also CWA section
                      303(d)(4)). These requirements are described in EPA's NPDES Permit Writers Manual
                      (USEPA 1996a).
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7.7   What are the special  conditions for mercury in a
       facility's intake?

7.7.1   How to consider mercury intakes with a reasonable potential
        approach
For some dischargers, the only source of mercury in a facility's discharge might be the
intake water from the same body of water as where 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 decide that there is
no reasonable potential for the discharge to exceed 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 the fish
unless the fish are known to frequently inhabit the water immediately in the area of the
discharge. In making this decision, the  permitting authority should conduct monitoring of
both the intake and discharge to verify  that there are no known sources of additional
contributions of mercury at the facility. Also, EPA recommends that permitting
authorities consider requiring an evaluation of whether the methylmercury concentration
significantly increases for facilities with anaerobic conditions in the discharge. This
approach is also consistent with federal regulations pertaining to the Great Lakes Basin
that contained an approach for determining reasonable potential using fish tissue data (see
40 CFR Part 132, Appendix F, Procedure 5.D).

7.7.2   How to consider mercury in intakes in WQBELs
For facilities that take in water from the same body of water that they discharge into, a no
net increase limit may be appropriate. This type of effluent limit allows a facility to
discharge into a waterbody no more mercury than it takes out of the waterbody when the
concentration of mercury in the waterbody above the facility already exceeds the water
quality criterion. EPA recommends that permits for these type of facilities contain:
   • Effluent limits that constrain the  mass discharges to not exceed the mass intake of
     mercury from the waterbody, or if proper operation and maintenance of a facility's
     treatment system results in removal of a pollutant, effluent limits that reflect these
     reductions from the influent loading

   • Monitoring of the influent and effluent using the current version of EPA Method
      1631  to quantify the amount of mercury entering and exiting the facility

   • A requirement to develop a mercury minimization plan

This approach is also consistent with federal regulations pertaining to the Great Lakes
Basin that contained an approach for determining reasonable potential using fish tissue
data (see 40 CFR Part 132, Appendix F, Procedure 5.E).
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                                                                                       Related Programs
8    Related  Programs

8.1   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)23 to their
collection system and wastewater treatment plant. However, as water quality criteria,
sludge standards, and air emissions become more restrictive, even low levels of pollutants
such as mercury might cause noncompliance with these standards. As such, POTWs must
either 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
District (WLSSD) determined that one SIU and many small non-SIUs (dental facilities)
contribute a major portion of the mercury in their 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 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 implementation of selected
options is integral to a successful reduction strategy. Permitting authorities developing a
P2 plan should consider a POTWs role in compliance assistance. The sections below
provide summary level guidance for developing a POTW P2 plan.

Through the pretreatment program, POTWs should 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, success of a POTW P2 effort depends  upon 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). By undertaking the
23 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 a 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 403.3(t)).

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Related Programs
                      following activities prior to developing its plan, the POTW might minimize community
                      resistance and apathy:
                         •  Conduct a preliminary investigation of the problem and potential sources. Verify
                            that the problem is not a wastewater treatment plant operational issue. Further,
                            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).
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.
                         •  Establish a workgroup composed of representatives from government, industry,
                            community, and environmental organizations, preferably those that are either
                            familiar with P2 strategies or familiar 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.

                      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 the
                            most people can understand).
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                                                                                       Related Programs
      Identify the goals to determine if the POTW intends to help minimize mercury
      introduced to all environmental media (air, water, solid waste), known as "front-
      end" P2, or merely 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/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 assessment techniques that identify how the POTW will monitor influent,
      effluent, sludge, and sources to assess success and that identify possible follow-up
      activities to ensure  P2 measures continue to be implemented.

      Create contingency plans that describe actions to be taken if planned efforts do not
      succeed, such as obtaining 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 wastewater
treatment plant effluent mercury levels) 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.
As rules and standards pursuant to the CAA have been developed, proposed, and
promulgated since 1990, compliance by emitting sources and actions taken voluntarily
have already begun to reduce emissions of mercury to the air across the country. EPA
expects a combination of ongoing activities will continue to reduce mercury emissions to
the air over the next decade.

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, medical waste incinerators,
chlor-alkali plants, industrial boilers, 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). Most
recently, EPA issued a regulation under CAA section 111 that directly regulates mercury
emissions from coal-fired power plants (see 70 FR. 28,606 (May 18, 2005) (codified at
40 CFR Parts 60, 72, and 75) (standards for power plants)).
                                                                                                    107

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                      The relevant regulations that EPA has issued to date under the CAA are described briefly
                      below.

                      8.2.1   Municipal Waste Combustors
                      In 1995, EPA promulgated final regulations that apply to all new and existing waste-to-
                      energy 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 (Dec. 19, 1995), codified at 40
                      CFR Part 60). These regulations cover approximately 130 existing waste-to-energy plants
                      and incinerators, and any new plants and incinerators built in the future. The regulations
                      have reduced emissions of a number of F£APs, 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 emissions
                      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)).

                      8.2.2   Medical Waste Incinerators
                      Medical waste incinerators (MWIs) are used by hospitals, health care facilities, and
                      commercial waste disposal companies to dispose of hospital waste and medical or
                      infectious waste. EPA adopted regulations controlling mercury emissions from MWIs on
                      September 15, 1997 (62 FR 48,348, codified at 40 CFR Part 60, subpart Ce). EPA
                      estimated that the regulations would reduce mercury emissions from these facilities by
                      about 90 percent, with all existing MWIs required to comply with the regulations by
                      September 15, 2002 (see Figure 4 (49.7 tons per year of mercury emitted from domestic
                      municipal waste incinerators in 1990 versus 1.6 tons per year in 1999)).  At the time the
                      regulations were issued, EPA expected that 50 percent to 80 percent of the 2,400 then-
                      existing MWIs would close in response to the rule. In fact, 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 MWIs and many health care facilities have discontinued use of their
                      existing MWIs.  Instead they have switched to other methods of waste disposal such as
                      off-site commercial waste disposal. EPA expected the standards to apply to between 10
                      and 70 new MWIs, most of which would employ mercury control technology by the
                      compliance deadline.

                      8.2.3   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). The regulations impose requirements for more stringent work
                      practice limits, representing the best practices from the industry, than were required by a
                      preexisting regulation that covered this source category. Today, there are 9 such plants in
                      the United States, as compared to 20 when work on the rule began. The regulations, which
                      require a combination of controls for point sources, such as vents and management
                      practices to address fugitive air emissions, will reduce mercury air emissions from existing
                      chlor-alkali plants by about 50 percent by the compliance date of December 19, 2006. In
108

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                                                                                      Related Programs
addition, EPA is initiating a study of fugitive mercury emissions at existing chlor-alkali
plants, which could result in the proposal of further regulatory changes in the future.

8.2.4  Industrial Boilers
In September 2004, EPA issued a final rule to limit emissions of HAP, including
mercury, from new and existing industrial, commercial, and institutional boilers and
process heaters (ICI boiler and process heaters) at major stationary sources (see 69 FR
55,218 (Sept. 13, 2004), codified at 40 CFR Part 63,  Subpart DDDDD). ICI boilers and
process heaters burn coal and other substances such as wood to produce steam to generate
electricity or mechanical energy and to provide heat.  ICI boilers and process heaters are
used at facilities such as refineries, chemical and manufacturing plants, and paper mills.
In addition, boilers can stand alone to provide heat for shopping malls and university
heating systems. EPA promulgated emissions limitations for mercury for all new solid
fuel boilers and process heaters and for large existing solid fuel units. EPA expects that
this rule will reduce total emissions of HAP from regulated sources by 50,000 to 58,000
tons per year (see 69 FR 55,218, 55,244). The largest segment of emissions and projected
emissions  reductions from these sources involve hydrogen chloride. However, EPA
expects that the standards will reduce mercury emissions from new and existing facilities
by about 2 tons per year.

8.2.5  Hazardous Waste Combustors
In 1999, EPA established standards for HAPs, including mercury, for incinerators,
cement kilns, and lightweight aggregate kilns that burn hazardous waste under CAA
section 112 (64 FR 52,828, 53,011 (September 30, 1999)). The 1999 standards were
challenged and subsequently vacated by the United States Court of Appeals for the
District of Columbia Circuit. In 2002,  EPA issued interim emission standards, which are
found at 40 CFR 63.1203 (a)(2) and (b)(2) (mercury  standards for existing and new
hazardous waste burning incinerators), section 63.1204 (a)(2) and (b)(2) (mercury
standards for existing and new hazardous waste-burning cement kilns), and section
63.1205 (a)(2) and (b)(2) (mercury standards for existing and new hazardous waste-
burning lightweight aggregate kilns). Recently, EPA  issued a rule that supersedes the
interim standards issued in 2002 (see 70 FR 59,402 (Oct. 12, 2005) (final standards for
hazardous air pollutants from hazardous waste combustors: Phase 1 Final Replacement
Standards  and Phase II)). The October 2005 rule sets mercury standards under CAA
section 112 for specific types of sources, including some sources that were not covered
by the interim standards (e.g., liquid fuel fired boilers and solid fuel fired boilers).

8.2.6  Coal-fired Power Plants
On March 15, 2005, EPA issued the first-ever federal rule to permanently cap and reduce
mercury emissions from coal-fired power plants (see 70 FR 28606 (May  18, 2005)
(CAMR)). This rule makes the United States the first country in the world to regulate
mercury emissions from coal-fired power plants. The CAMR, which builds on EPA's
CAIR (70  FR 25,162 (May 12, 2005)), will significantly reduce mercury emissions from
coal-fired power plants. When fully implemented, CAIR and CAMR will reduce coal-
fired utility emissions of mercury  from 48 tons a year to 15 tons, a reduction of nearly 70
                                                                                                   109

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                      percent. EPA expects that air deposition from these utilities will also decrease by nearly
                      70 percent (see Figure 9).
                                          Mercury Deposition in the U.S.
                        _«;•
                        '.-
                        e
                        c
                        i—
                                Total Deposition in 2001
                                        144
                                                                    Deposition from U.S. Power Plants
                                                             12
                                                            "
                          • 2301 depssllHin In tie L S. *oir SQLHCSS sutEtoe :f
                            •he L .3. ard Canada
                          - 2301 total deposltlwi Ir We U.S. Tram Carac 51 aid
                                    r: L
  220" deposition
ffarn U.S power ps/ils
  2121 deposition
frcrn U.S p
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                                                                                      Related Programs
global total, and U.S. coal-fired power plants are estimated to account for only about 1
percent.

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 committed to by the American Hospital Association, will contribute
to reduced emissions from waste combustion.

For more information about CAMR and other CAA actions to control mercury, see
http://www.epa.gov/mercury/control emissions/decision.htm.
                                                                                                   Ill

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                                                                                         References
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                                                                                                 123

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USEPA (U.S. Environmental Protection Agency). 2004a. Proposed National Emission
    Standards for Hazardous Air Pollutants; and, in the Alternative, Proposed Standards
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    2004, 69:69864. 

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    Protection Agency, Office of Air Quality Planning and Standards, Air Quality
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USEPA (U.S. Environmental Protection Agency). 2005c. Technical Support Document.
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    Environmental Protection Agency. Fed. Regist., May 18,  2005, 70:28606.
                                                                                                  125

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126

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                         Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA
Appendix A.  Synopsized Mercury TMDLs
                Developed or Approved by EPA

I.    Ochlockonee Watershed, Georgia
II.    Arivaca Lake, Arizona
III.    McPhee and Narraguinep Reservoirs, Colorado
VI.    Clear Lake, California
                                                                    127

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Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA


                      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, a numeric "interpretation" of the
                      narrative water quality standard was used to assure that a TMDL will protect human
                      health. The state of Georgia has made a numeric  interpretation of their narrative water
                      quality standard for toxic substances at a numeric 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 grams per day or less of
                      freshwater fish.

                      This approach is consistent with EPA's recently adopted guidance value for the
                      protection of human health from methylmercury  described in the document titled, Water
                      Quality Criterion for the Protection of Human Health: Methylmercury (USEPA 2001c).
                      The methodology uses a "weighted consumption" approach. When only trophic level 3
                      and 4 fish have been collected, the methodology  assumes that 8 grams per day (58.4
                      percent) of the total fish consumption is trophic level 3 fish (e.g., catfish and sunfish) and
                      5.7 grams per day (41.6 percent) are 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 Cone, x .416) +
                           (Avg Trophic 3 Cone, x .584)

                      where:
                           Avg. Trophic Level 3 Concentration = 0.2  mg/kg
                           Avg. 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 of 0.3 mg/kg level. To determine this
                      EPA used the Methodology for Deriving Ambient Water Quality Criteria for the
                      Protection of Human Health (USEPA 2000e). EPA also used the recommended national
                      values from the Human Health 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 grams per day. For the other factors
                      in the calculation, bioaccumulation and fraction of methylmercury, EPA used site-


128

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                                   Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA

specific data from the Ochlockonee River collected in summer of 2000 and March and
April of 2001. From this site-specific data, EPA determined a representative weighted
BAF. This BAF was calculated by taking the average calculated BAF from each of the
two trophic levels to determine a "weighted" BAF on the basis of the different
consumption rates for trophic levels and a median measured fraction methylmercury of
0.17. 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 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 I/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. There are 16 water point sources in the
Ochlockonee River watershed that could potentially have mercury in their discharge.

According to a review of the Mercury Study Report to Congress (USEPA 1997a),
significant potential air emission sources include coal-fired power plants, waste
incinerators, cement and limekilns, smelters, pulp and paper mills, 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 both wet and dry
deposition of mercury as a function of air emissions and global sources and was used to
calculate dry and wet deposition rates for south Georgia as derived by RELMAP.

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. This data was
compared with the RELMAP deposition predictions and was found to be substantially
higher. Using the MDN data, the average annual wet deposition rate was determined to
be 12.75 ug/square meter. The dry deposition rate was determined to be 6.375  ug/square
meter on the basis of the RELMAP results.
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                      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. This helped estimate 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 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 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 and 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 Al.
       Table A1. Annual average mercury load from each subbasin
Watershed name
Barnett Creek
Middle/Lower Ochloclonee
Tired Creek
Lower Ochlockonee
Little Ochlockonee
Bridge Creek
Upper/Middle Ochlockonee
Upper Ochlockonee
Total Hg
load
(mg)
786098.4
307965.8
827172.8
359317.5
873773.4
454417.5
627746.1
766396.8
Area!
load
(mg/ha)
25.6
21.24
22.03
15.62
19.89
23.11
20.67
20.1
Impervious
area
(mg/yr)
116614.69
125771.73
252386.89
100125.11
140023.69
53496.45
152881.42
164465.44
Sediment
(mg/yr)
422879.88
89440.3
317969.16
130407.68
433136.75
261042.44
254746.48
320337
Runoff
(mg/yr)
177553.9
54786.29
194751.7
97802.16
219614.2
98468.66
182250.7
186825.6
Deposition
on water
(mg/yr)
68850
37867.5
61965
30982.5
80898.75
41310
37867.5
94668.75
                      WASPS (Ambrose, et al. 1988) was chosen to simulate mercury fate in the Ochlockonee
                      River. WASPS 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
                      diffusion. Sorbed variables can migrate downward or upward through net sedimentation
                      or erosion.
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The toxics WASP model, TOXI5, combines a kinetic structure adapted from EXAMS2
with the WASPS 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, also for particulate material. Local equilibrium is assumed so that the
distribution of the chemical between each of the species and phases is defined by
distribution or partition coefficients. The predicted mercury concentrations are shown in
Table A2.
Table A2. Predicted mercury for annual average load and flow
Calculated concentrations
Total Hg: Water column (ng/L)
Total Hg: Sediment (ng/g)
Methyl Hg: Water column (ng/L)
River reach
1
6.33
7.05
0.90
2
5.84
9.07
0.82
3
5.55
9.81
0.77
4
5.76
8.17
0.79
5
5.65
7.63
0.77
6
5.17
6.97
0.71
Allocations
To determine the total maximum load that can come into the Ochlockonee River, the
current loading conditions are evaluated and the instream concentration is determined
using the modeling approach described above. This allows 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 gives
a TMDL load of 1.22 kg/yr mercury. This 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 kilograms/year. Because EPA's assessment indicates that over 99 percent of the
current loading of mercury is from atmospheric sources, all the load reduction is being
assigned to the load allocation and no reduction is required of 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
will be necessary if the applicable water quality standard is to be attained. On the basis of
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                      the total allowable load of 1.22 kilograms per 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, there are a number of activities planned or underway to address
                      remaining sources of mercury, and EPA expects that further reductions in mercury
                      loadings  will occur overtime as  a result of these  activities. EPA is not able to estimate
                      the reductions in mercury deposition to the Ochlockonee River watershed that will be
                      achieved from future activities. However, as contemplated by CWA section 303(d)(l)(C),
                      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 (i.e., 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 to human fetuses even at minute quantities, and 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 facilities in
                      the watershed in the amount of 0.06 kg/year. The TMDL was written so that all NPDES
                      permitted facilities will achieve this wasteload allocation either through the discharge of
                      mercury  at concentrations below the applicable water quality standard of 1.6 ng/L or
                      through the implementation of a pollutant minimization plan.

                      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 less 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. However, EPA also recognizes 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' WLA will  be
                      applied to the increment of mercury in their discharge that is above the  amount of


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mercury in their source water. EPA recommends that the permitting authority make this
choice between the two options in consultation with the affected discharger 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 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 ug/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 ug/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.

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 offish 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 of development of a model and the analysis of 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 mercury
load to the lake include natural background load from the watershed, atmospheric
deposition, and possible nonpoint loading from past mining activities.
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                      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
                      mercury on the watershed. Some mercury also exists within the parent rock formations of
                      the Arivaca watershed, although no concentrated ore deposits are known. The net
                      contributions of both 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. From these data, 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
                      itself.

                      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 such as bottles and plates.
                      Samples were taken at three  different locations of the Ruby Dump: 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 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. After a
                      review of Mercury Study Report to Congress (USEPA 1997a) and a search of EPA's
                      AIRS database of permitted point sources, there are 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 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

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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 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
ug/m /yr by wet deposition and 7.1 ug/m /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 greater wet deposition and less dry deposition
than Caballo because more of the particulate mercury and reactive gaseous mercury that
contribute to dry deposition will be scavenged at a site with higher rainfall.

Loading  Capacity—Linking Water Quality and Pollutant Sources
The linkage analysis defines the connection between numeric targets and identified
sources. The linkage is defined as the cause-and-effect relationship between the selected
indicators, associated numeric targets, and the identified sources. This provided the basis
for estimating total assimilative capacity and any needed load reductions. Specifically,
models of watershed loading of mercury were combined with a model of mercury cycling
and bioaccumulation in the lake. This 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 via 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 is intended to be applicable without calibration. Solids load, runoff, and
ground water seepage can then be used to estimate particulate and dissolved-phase
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 estimate are shown in
Table A3. A background loading estimate was first calculated, then combined with
estimates of loads from individual hot spots. The majority  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 mercury concentrations were assumed to be distributed lognormally, as is
typical for environmental concentration samples, and an estimate of the arithmetic mean

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                      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 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.
                      Table A3. Annual total mercury load to Arivaca Lake
Watershed year
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
Grand Total
Annual Average
Mercury loading to lake (grams per year)
From
watershed
170.16
184.34
205.61
70.9
198.52
99.26
163.07
233.97
141.8
219.79
170.16
191.43
276.51
2,325.52
178.89
From Ruby
Dump
0.65
0.7
0.79
0.27
0.76
0.38
0.62
0.89
0.54
0.84
0.65
0.73
1.06
8.88
0.68
From direct
atmospheric
deposition to lake
4.208
4.208
4.208
4.208
4.208
4.208
4.208
4.208
4.208
4.208
4.208
4.208
4.208
54.704
4.21
Total
175.018
189.248
210.608
75.378
203.488
103.848
167.898
239.068
146.548
224.838
175.018
196.368
281.778
2,389.10
183.78
                      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
                      evaporation for 1980 through 1995 was 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
136

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                                    Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA

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 significant 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.

Results of the model calibration are shown in Table A4. The model calculations are the
predicted annual ranges after the model has reached steady state. The observed
concentrations are from July  1997.

Table A4. Predicted and observed mercury for annual average load and  flow

Methyl Hg: Water column (ng/L)
Hg II: Water column (ng/L)
Methyl Hg: 5-year-old largemouth bass (mg/kg)
Predicted
0.00-12.07
0.00-6.28
1.18
Observed
14.3
1.46-8.3
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 in reserve a portion of the
loading capacity to provide a margin of safety (MOS), as provided for in the TMDL
regulation. The allocations and MOS are shown in Table A5. These allocations, from the
best currently available information, predict attainment of acceptable fish tissue
concentrations within a time horizon of approximately 10 years. A delay in achieving
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                      standards is unavoidable because time will be required for mercury to cycle through the
                      lake and food chain after load reductions occur.
                      Table AS. Summary of TMDL allocations and needed load reductions (in g-Hg/yr)
Source
Wasteload allocations
Allocation
0.0
Existing load
0.0
Needed
reduction
0.0
                       Load allocations
Atmospheric deposition
Ruby dump
Watershed background
Total
Unallocated reserve
Loading capacity
4.2
0.7
111.2
116.1
38.7
154.8
4.2
0.7
178.9
183.8
0
0
67.7
67.7

                      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 the majority 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 grams mercury per year.

                      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
                      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.
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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 reliable estimates are
unavailable, 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 may be suitable for further investigation include
the following:
   •  Hypolimnion aeration or mixing

   •  Sulfur chemistry modification

   •  Alum treatment

   •  Reduce DOC and nutrient levels

   •  Dredge lake sediments
                                                                                                    139

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Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA
                      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 atoxicity
                      value RfD of 0.3  ug/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 4 meals per month
                      for nonpregnant adults and 1 meal per month for women who are pregnant, nursing, or
                      planning to become pregnant and children 9 years of age or younger. The criterion is
                      applied to an average-age top predator. Within 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 on to the watershed,
                      including deposition and storage in snowpack.
      Table A6. Summary of mercury load estimates for McPhee Reservoir
Reservoir
McPhee
Narraguinnep
Watershed
runoff
(g/yr)
2,576
2.7
Watershed
sediment
(g/yr)
222
22.7
Interbasin
transfer
(g/yr)

15.9
Atmos.
deposition
(g/yr)
251
36.8
Total
(g/yr)
3,049
78.1
Load per
volume
(mg/ac-ft)
4.66
4.59
Load per
surface area
(mg/m2)
0.098
0.035
                     Past mining activities likely provide an important source of mercury load to the McPhee
                     and Narraguinnep watershed. Three large mining districts exist 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
140

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                                   Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA

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
near-field 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
McPhee and Narraguinnep Reservoirs. No direct measurements of atmospheric
deposition of mercury are available, therefore 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 O.g mg/kg
wet muscle.

Allocations
The loading  capacity for McPhee Reservoir was estimated to be 2.59 kilograms mercury
per year. Narraguinnep Reservoir loading capacity was estimated at 39.1 grams of
mercury per year.  This is the maximum rate of loading consistent with meeting the
numeric target  of 0.5 mg/kg in fish tissue. Due to 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 kg/yr), while
Narraguinnep Reservoir is equivalent to a total annual mercury loading rate of 27.3 g-
Hg/yr (70 percent of 39.1 g-Hg/yr).
                                                                                                  141

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Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA
                     Table A7. Summary of TMDL allocations and needed load reductions for
                     McPhee Reservoir
Source
Atmospheric deposition
Rico/Silver Creek mining area
Dunton mining area
La Plata mining area
Watershed background
Total
Unallocated reserve
Loading capacity
Allocation
63
507
348
69
827
1,814
778
2,590
Existing load
251
1030
708
141
919
3,049
Needed reduction
188
523
360
72
92
1,235

                     Measurements in g-Hg/yr
                     Table A8. Summary of TMDL allocations and needed load reductions for
                     Narraguinnep Reservoir
Source
Atmospheric Deposition
Interbasin Transfer from
McPhee Reservoir
Watershed Background
Total
Unallocated Reserve
Loading Capacity
Allocation
9.2
9.5
8.6
27.3
11.8
39.1
Existing load
36.8
15.9
25.4
78.1
Needed reduction
27.6
6.4
16.8
50.8

                      Measurements in g-Hg/yr
                      IV.  Clear Lake, California

                      Description of the Applicable Water Quality Standards
                      The EPA promulgated the 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 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 to be
                      sufficiently protective of the  consumers offish 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 comprised of 70 percent trophic level 4 fish and 30 percent
142

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                                    Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA

trophic level 3 fish. The 90th percentile consumption rate of a small group of residents of
Clear Lake, primarily members of the Elem Porno 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 is comprised 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 Native American Indian Tribes use
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 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 located 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 transport of suspended solids. The northern wetland is

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Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA

                      used for cattle grazing and as a source offish, tules, and other resources used by the
                      members of the Elem Porno 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
                      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 in  other parts of the Clear Lake. In contrast to  mercury in
                      SBMM ground water, 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 upon flow rates. Loads in average water years are
                      18 kg/year. This is approximately 3 percent of 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. This is
                      approximately 0.3 percent of 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 affect methylation or
                      concentrations of methylmercury, including sulfide and sulfate concentrations,
                      temperature, organic carbon, and so on. Factors that affect accumulation of
                      methylmercury in fish include species, growth rate, prey availability, and the like. To

144

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                                    Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA

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 that
demonstrate 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 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
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, staff is unable to refine these relationships to incorporate 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 reduction of 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 in order 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  A9 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 A10. 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


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Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA

                      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.
                       Table A9. Summary of mercury load allocations
Source
Clear Lake sediment
Sulphur Bank Mine
Tributaries
Atmosphere
Existing load
(kg/year)
695
18
2
Needed reduction
70% of existing concentration
95% of existing load
20% of existing load
no change
                       Table A10. Sediment goals for mercury in Clear Lake
Site designation
Upper Arm
UA-03
Lower Arm
LA-03
Oaks Arm
OA-01C
OA-02C
OA-03C
OA-04C
Narrows O1
Location
Center of Upper Arm on transect from
Lakeportto Lucerne
Center of Lower Arm, north and west of
Monitor Point
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
Sediment mercury goal
(mg/kg dry weight)3
0.8
1.0
16b
16b
16
10
3
                       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. 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.
                       c. Sediment goal is part of the load allocation for SBMM.
                       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 the 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.
146

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                                    Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA

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
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 A10.

EPA anticipates implementing additional actions to address the ongoing surface and
ground water releases from the 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 Comprehensive
Environmental Response, Compensation, and Liability Act (CERCLA) to address the
existing contamination in  the lakebed sediments from past releases from the 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 currently 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 to 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 the 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 upon 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 5 years after the effective date of this amendment and
                                                                                                    147

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Appendix A. SynopsizedMercury TMDLs Developed or Approved by EPA

                      implement load reduction plans within 5 years thereafter. The goal is to complete the load
                      reductions within 10 years of implementation plan approval.

                      The Regional Board will work with the Native American Tribes in the Clear Lake
                      watershed on mercury reduction programs for the tributaries and surface water runoff.
                      They will solicit the tribes' participation in the development of 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 of 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 of the Clear Lake watershed and depositing from air onto the lake
                      surface are established at the existing input rate, which is estimated to be  1 to 2 kg/year.
148

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                                      Appendix B. Tables from Methylmercury Criteria Document
Appendix B. Tables from  Methylmercury
                  Criteria Document
This appendix contains several tables taken directly from the 2001 methylmercury
criteria document. These are repeated here to help the reader understand the development
of the 2001 criterion.
                                                                              149

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Appendix B. Tables from Methylmercury Criteria Document
Table 5-1. Exposure parameters used in derivation of the water quality criterion. (References cited in this
table can be found in the 2001 methylmercury criteria document.)
Parameter
Body Weight, kg
Drinking Water Intake, L/day
Freshwater/Estuarine Fish Intake,
g/day
Inhalation, m3/day
Soil Ingestion, g/day
Mean Marine Fish Intake, g/day
Median Marine Fish intake, g/day
90th Percentile Marine Fish Intake,
g/day
Population
Children
(0-14 years)
30
1.0
156.3b
10.4
0.0001,0.01s
74. 9b
59.71b
152.29b
Women of
Childbearing
Age
(15-44 years)
67
2.0
165.5b
11
0.00005
91.04b
75.48b
188.35b
Adults in the
General
Population
70
2.0
17.5c24
20
0.00005
12.46C
Oc
49.16C
Source
U.S. EPA
(2000a)
U.S. EPA
(2000a)
U.S. EPA
(2000a)
U.S. EPA
(1994, 1997h)d
U.S. EPA
(1997h)
U.S. EPA
(2000b)
U.S. EPA
(2000b)
U.S. EPA
(2000b)
  aPica child soil ingestion
  bFor children and women of childbearing age, intake rates are estimates of "consumers only" data (as described in U.S. EPA,
  2000b)
  Tor adults in the general population, intake rates are estimates of all survey respondents to derive an estimate of long-term
  consumption (U.S. EPA).
  dlnhalation rates for children and women of childbearing age from U.S. EPA, 1997h. Inhalation rates for adults in the general
  population from U.S. EPA (1994).
24 This is the 90th percentile freshwater and estuarine fish consumption value.

150

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                                                      Appendix B.  Tables from Methylmercury Criteria Document
Table 5-14. Average Mercury Concentrations in Marine Fish and Shellfish25
(References cited in this table can be found in the 2001 methylmercury criteria
document)
Species
Concentration"
(jigHg/gWetWt.)
Species
Concentration
(jigHg/gWetWt.)
Finfish
Anchovy
Barracuda, Pacific
Cod*
Croaker, Atlantic
Eel, American
Flounder*'6
Haddock*
Hake
Halibut*
Herring
Kingfish
Mackerel*
Mullet
Ocean Perch*
Pollock*
0.047
0.177
0.121
0.125
0.213
0.092
0.089
0.145
0.25
0.013
0.10
0.081
0.009
0.116
0.15
Pompano*
Porgy*
Ray
Salmon
Sardines*
Sea Bass*
Shark*
Skate
Smelt, Rainbot
Snapper
Sturgeon
Swordfish*
Tuna*
Whiting (silver hake)*
Whitefish*
0.104
0.522b
0.176
0.035
0.1
0.135
1.327
0.176
0.1
0.25
0.235
0.95C
0.206
0.041
0.054d
Shellfish
Abalone
Clam*
Crab*
Lobster
0.016
0.023
0.117
0232
Oysters
Scallop*
Shrimp
Other shellfish*
0.023
0.042
0.047
0.012b
Molluscan Cephalopods
Octopus
0.029
Squid
0.026
 '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 (U.S. EPA, 2000b).
 a Mercury concentrations are fromNMFS (1978) as reported in U.S. EPA (1997d) unless otherwise noted, measured as jig of
 mercury per gram wet weight of fish tissue.
 b Mercury concentration data are from Stem et al. (1996) as cited in U.S. EPA (1997c).
 0 Mercury concentration data are from U.S. FDA Compliance Testing as cited in U.S. EPA (1997c).
 d Mercury concentration data are from U.S. FDA (1978) as cited in U.S. EPA (1997c).
 e Mercury data for flounder were used to estimate mercury concentration in marine flatfish for intake calculations.
25 More current information on commercial fish and shellfish is provided by the Food and Drug Administration at
http: //www.cfsan. fda. go v/%7Efrf/sea-mehg .html
                                                                                                                151

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Appendix B. Tables from Methylmercury Criteria Document
Table 5-30. Exposure estimates for methylmercury and percent of total exposure based on adults in the
general population
Exposure Source
Ambient water intake
Drinking water intake3
Nonfish dietary intake
Marine fish intake
Air intake
Soil intake
Exposure Estimate
(mg/kg-day)
4.3 x ID'9
5.6 xlO'8
0
2.7 xlO'5
4.6 xlO'9
1.3 xlO'9
Percent of Total
Exposure
0.0047
0.0605
0
29.33
0.005
0.0014
Percent of RfD
0.004
0.006
0
27
0.005
0.001
  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.
152

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                                                                         Appendix C. Analytical Methods
Appendix  C.  Analytical Methods
Table C1. Analytical methods for determining mercury and methylmercury in tissue
Method
Draft Method
1630, with
modifications
for tissue
Method 1631,
Draft
Appendix A
Method
245. 628
Draft Method
7474
(SW-846)29
Form/species
and applicable
matrices
Methylmercury in
tissue
Total mercury in
tissue, sludge,
and sediment
Total mercury in
tissue
Total mercury in
sediment and
tissue
Sensitivity
Methylmercury
in tissue
Total mercury in
tissue, sludge,
and sediment
Total mercury in
tissue
Total mercury in
sediment and
tissue
Technique
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,
etnylation 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.
Digest tissue with HNO3/H2SO4. Dilute
digestate with BrCI solution to destroy
remaining organic material. Analyze
digestate per Method 1631 (i.e., add BrCI
to oxidize all Hg compounds to Hg(ll).
Sequentially pre-reduced with
hydroxylamine hydrochloride to destroy
the free halogens and reduced with
SnCI2 to convert Hg(ll) to Hg(0). Hg(0) is
purged from solution onto gold-coated
sand trap and thermally desorbed from
trap for detection by CVAFS.
Sulfuric and nitric acid digestion,
oxidation with potassium permanganate
and potassium persulfate, SnCb
reduction, CVAAS detection
Microwave digestion of sample in nitric
and hydrochloric acids, followed by cold
digestion with bromate/bromide in HCI.
Hg purged from sample and determined
by CVAFS.
Known studies or literature
references using the techniques
in this method
• EPA Cook Inlet Contaminant
Study
• Lake Michigan fish and
invertebrates, Mason and Sullivan
1997
• NE Minnesota lake plankton,
Monson and Brezonik 199826
• Method performance testing in
freshwater and marine fish, Bloom
1989
• 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
• NE Minnesota lake plankton,
Monson and Brezonik 199827
• Method performance testing in
freshwater and marine fish, Bloom
1989
unknown
Reference materials cited in method.
Niessen etal. 1999.
26 Used similar techniques but used a methylene chloride extraction instead of the distillation.
27 Used similar techniques but used a methylene chloride extraction instead of the distillation.
28 Provided for reference purposes only. EPA recommends use of Method 1631 for mercury for analyzing water and fish tissue.
29 Provided for reference purposes only. EPA recommends using Method 1631 for analyzing mercury for water and fish tissue.
                                                                                                   153

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Appendix C. Analytical Methods
Table C2. Analytical methods for determining mercury and methylmercury in water, sediment, and other
nontissue matrices
Method
EPA 1630
UW-Madison
SOPforMeHg
Analysis
USGS
Wisconsin -
Mercury Lab
SOPs 004
USGS Open-
File Report 01-
445:
Forms/species
and applicable
matrices
Methylmercury
in water
Methylmercury in
water
Methylmercury in
water
Methylmercury in
water
Sensitivity
0.02 ng/L
0.01 ng/L
0.05 ng/L
Detection
limit cited as
0.04 ng/L
Sample preparation
Distillation with heat and N2 flow,
addition of acetate buffer and etnylation
with sodium tetraethyl borate. Purge
with N2 onto Carbotrap. Thermal
desorption and GC separation of
ethylated mercury species, reduction to
Hg followed by CVAFS detection.
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 Hg° followed by
CVAFS detection.
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 Hg°, and CVAFS
detection.
Distillation (heat) and N2 flow, HCI 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.
Known studies or literature
references using the techniques in
this method
• 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
• NE Minnesota lakes, Monson and
Brezonik199830
• Poplar Creek, TN CERCLA Remedial
Investigation of surface water,
sediment, and pore water, Cambell et
al. 199831
• Scheldt estuary study of water,
polychaetes, and sediments, Baeyens
etal. 1998
• Lake Michigan tributaries to support
GLNPO's LMMB Study
• Fox River, Wl waters and sediments,
Hurley etal. 1998
Aquatic Cycling of Mercury in the
Everglades, (ACME) cofunded by USGS,
EPA, and others
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.
 Note:    The four methylmercury methods above are all based on the work of Bloom 1989 as modified by Horvat et al. 1993, and are
 virtually identical as a result.
30 Used similar techniques but used a methylene chloride extraction instead of the distillation.
31 Used similar techniques but omitted the distillation procedure
154

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                                                                    Appendix C. Analytical Methods
Table C2. (continued)
Method
EPA 1631
(CVAFS)
EPA 245.1
(CVAAS)
EPA 245.2
(CVAAS)
EPA 245.5
(CVAAS)
EPA 245.7
(CVAFS)
EPA 7470A
(CVAAS)
EPA 7471 B
(CVAAS)
EPA 7472
(Anodic
Stripping
Voltametry)
EPA 7473
(Thermal
decomposition,
amalgamation,
and CVAA )
Forms/species
and applicable
matrices
Total or
dissolved
mercury in water
Total or
dissolved
mercury in
wastewater
Total or
dissolved
mercury in
wastewater and
sewage
Total or
dissolved
mercury in soils,
sludge and
sediment
Total or
dissolved
mercury in water
Total or
dissolved
mercury in liquid
wastes and
Ground water
Total or
dissolved
mercury in solid
wastes
semisolid wastes
Total or
dissolved
mercury in water
Mercury in water,
soil, and
sediment
Sensitivity
MDL=0.2 ng/L
ML=0.5 ng/L
200 ng/L
200 ng/L
200 ng/L
ML = 5 ng/L;
MDL= 1.8
ng/L
200 ng/L
(IDL)
200 ng/L
(IDL)
100-300 ng/L
estimated to
be as low as
20 ng/L or 20
ng/kg
Sample preparation
Oxidize all Hg compounds to Hg(ll)
with BrCI. Sequentially pre-reduce with
hydroxylamine hydrochloride to
destroy the free halogens and reduce
with SnCI2 to convert Hg(ll) 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.
H2SO4 and HNO3 digestion, KMnO4,
K2S2C>8 oxidation + heat, cool +NaCI-
(NH2OH)2H2SO4, SnSO4, aeration.
Detection by CVAAS.
H2SO4 and HNO3 added, SnSO4,
NaCI-(NH2OH)2H2SO4, KMnO4,
K2S2O8 , heat. Detection by CVAAS.
Dry sample, aqua regia, heat, KMnO4
added, cool +NaCI-(NH2OH)2 H2SO4,
SnSO4, aeration. Detection by CVAAS.
HCI, KBr03/KBr, NH2OH HCI, SnCI2 ,
liquid-vapor separation. CVAFS
detection
H2SO4 and HNO3 added, KMnO4
added, K2S2Os added + heat, cool
+NaCI-(NH2OH)2H2S04, SnSO4,
aeration of sample. CVAAS detection.
H2SO4 and HNO3 added, KMnO4
added, K2S2Os added + heat, cool
+NaCI-(NH2OH)2H2SO4, SnSO4,
aeration of sample. CVAAS detection.
Acidify and chlorinate sample, GCE
electrode
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 AA.
Known studies or literature
references using the techniques in
this method
• 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
• Watrasetal. 1995
• Anacostia River Study, Mason and
Sullivan 1998
• Northeastern Minnesota lakes, Monson
and Brezonik1998
• Poplar Creek, TN CERCLA Remedial
Investigation Study, Cambell etal. 1998
• Scheldt Estuary Study, Baeyens et al.
1998
Effluent guideline development studies for
the Meat Products Industry, Metal
Products and Machinery Industry, and
Waste Incinerators
MPM Industry effluent guideline
development study
Pharmaceutical industry effluent guideline
development study
Interlaboratory validation completed.
Method is similar to and cites performance
data given in EPA 245.5
Method is similar to and cites performance
data given in EPA 245.5
Unknown
Unknown
                                                                                             155

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Appendix C. Analytical Methods
 Table C2. (continued)
Method
EPA 1620
(CVAAS)
SM3112B
(CVAAS)
*ASTM D3223-
91 (CVAAS)
*AOAC 977.22
(Atomic
absorption
spectrometry)
Forms/species
and applicable
matrices
Mercury in water,
sludge, and soil
Total or
dissolved
mercury in water
Total or
dissolved
mercury in water
Total or
dissolved
mercury in water
Sensitivity
200 ng/L
500 ng/L
500 ng/L
200 ng/L
Sample preparation
H2S04 and HNO3 added, KMnO4 ,
K2S2O8 + heat, cool +NaCI-
(NH2OH)2H2SO4, SnSO4, aeration.
CVAAS detection.
H2SO4 and HNO3 added, KMnO4
added, K2S2Os added + heat, cool
+NaCI (NH2OH)2 H2S04, SnCI2 or
SnSO4, aeration. CVAAS
determination.
H2SO4 and HNO3 added, KMnO4
added, K2S2Os added + heat, cool
+NaCI (NH2OH)2 H2S04, SnSO4,
aeration. CVAAS determination.
H2S04 and HNO3 added, KMnO4
added, K2S2Os added + heat, cool
+NaCI (NH2OH)2 H2SO4, SnSO4,
aeration. Determine mercury by CVAA.
Known studies or literature
references using the techniques in
this method
Industry effluent guideline development
studies
Unknown
Unknown
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
156

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                                       Appendix D. Examples of National Deposition Monitoring Networks
Appendix D.  Examples of National

                      Deposition  Monitoring Networks

There are a number of national deposition monitoring networks that may be useful for
developing TMDLs. Networks include the National Atmospheric Deposition Program -
National Trends Network (NADP/NTN) and the 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
geographical 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 NADP/NTN results for 2003 are shown in Figure D-l. 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, and 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 2000f)
(http ://www.epa.gov/oar/oaqps/gr8water/3rdrpt) and the Air-Water Interface Plan
Oittp ://www. epa. gov/owow/oceans/airdep/airwater_plan 16 .pdf).
                                                                                              157

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Appendix D. Examples of National Deposition Monitoring Networks
                       Total Mercury Concentration, 2003
                                                   n 1     •   *».
                                                 'V'v       -
                                     f
      Natoonal Atmosphenc Depositor PrograrrVMercury Deposition Network
184
     Figure D-1 MDN data for 2003
158

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      Appendix E. Methylmercury/Mercury Ratio Exhibited in Muscle Tissue of Various Freshwater Fish Species
Appendix  E.   Methylmercury/Mercury  Ratio Exhibited  in
                       Muscle Tissue of Various Freshwater Fish
                       Species
Source
Ecosystem type
Fish species
                                    MethylHg/
                                    total Hg ratio
C.R. Hammerschmidt,
J.G. Wiener, B.E.
Frazier and R.G.
Rada(1999)
Freshwater lakes in
Wisconsin, USA
Yellow perch (Perca flavescens)
                                    mean 0.95; range from 0.84 to
                                    0.97
D.S. Becker and G.N.
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
T.M. Grieb, C.T.
Driscoll, S.P. Gloss,
C.L Schofield, G.L
Bowie, and D.B.
Porcella(1990)
Lakes in the Upper
Michigan Peninsula,
USA
Yellow perch (Perca flavescens)
Northern pike (Esoxlucius)
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
N.S. 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
B. Lasorsa and S.
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
T. A. 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
R. Wagemann, E.
Trebacz, R. Hunt, and
G. Boila(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
                                                                                                 159

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-------
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.
                                                                                              Index
ambient water quality criteria       See
  AWQC
antidegradation
AWQC
BAF
  and Great Lakes
  and model selection
  and WQBEL
  calculating
  defined
  using
  weighted
best management plan
   34,97-99
15,21,23,48

          55
          75
          95
          15
          15
      13-26
         125
          89
best management practices    See BMP
bioaccumulation factor       See BAF
biomagnification           8, 9, 45, 76
BMP                   37,38,79,81
CAA                 2, 69, 77, 103-7
CAIR                     7, 105, 106
CAMR
  analysis supporting    17, 33, 52, 67
  and coal-fired power plants    105-7
  and SERA FM model            74
  and WCS                       75
  defined                         7
  modeling for                    70
Clean Air Act               See CAA
Clean Air Interstate Rule      See CAIR
Clean Air Mercury Rule    See CAMR
CMAQ                     67,70-71
cold vapor atomic fluorescence
  spectrometry (CVAFS)           42
Community Multiscale Air Quality  See
  CMAQ
composite samples
45, 46, 48
Consolidated Assessment and Listing
  Methodology (CALM)           49
Continuing Survey of Food Intakes by
  Individuals              See CSFII
CSFII                 27, 29, 30, 147
designated use
  andCWAlOl(a)             37,55
  and UAA                       37
  and variances                31,32
  changing                       38
  fishable                        12
  protecting                 1, 11, 26
detection level                   2, 49
dilution flow                      55
emissions
  anthropogenic                 7,77
  controls                     92,93
  hourly estimates in models        70
  mobility of                      6
  natural                          7
                     regulations
                     to air
                     trends in
                   environmental justice
                   EPA Method 1630
                   EPA Method 1631
                     and nondetects
                          101, 103-7
                     3, 68, 74, 103-7
                                 69
                                 61
                           41,42,49

                                 49
                                                                                               161

-------
Index
                        defined                      41-43
                        in Michigan case study           36
                        in NPDES permits      86-87, 93, 99
                        in TMDLs                      79
                        inWQBELs                     100
                      EPA Method 1669                 43
                      existing use                    32, 38
                      exposure
                        and sample types                45
                        and the RSC                    26
                        data in criterion calculation       13
                        duration in criterion calculation    12
                        fish tissue concentration as proxy to
                                                       19
                        from drinking water              14
                        human health effects          1, 3-4
                        to fish                    18,22,55
                        to humans                26, 46, 59
                      facility intake                     100
                      FDA action level                  60
                      FDA tolerances                    60
                      field sampling plan          25, 44-48
                      fish advisories
                        and water quality standards    57-61
                        EPA guidance on          44, 52-53
                        issued                          1,6
                        national listing of                 5
                        statewide                       64
                        updating                       60
                      fish intake rate/estimate
                        and trophic levels          14, 24, 50
                        in criterion calculation  13, 26, 27-31
                        limits                        57-61
                      fish sampling guidance       24, 44-45
                      freshwater
                        and estuarine fish
    age                           23
    and water quality criterion      13
    intake See fish intake rate/estimate
    mercury found in        9, 23, 48
  ecosystem models                65
  lakes and rivers                   6
Great Lakes Guidance       35, 42, 44
human health toxicological risk
  assessment                      13
impairment
  addressing                       64
  assessing                 14,48-53
  identifying sources of   2, 86, 94, 95
  nationwide                  89,95
laboratory analysis protocols     41-44
listing decisions                48-53
market-based cap-and-trade program 106
Mercury Deposition Network        71
mercury emissions
  anthropogenic                 7,77
  controls                     92,93
  hourly estimates in models        70
  mobility of                       6
  natural                 7, 32, 38, 68
  regulations              101, 103-7
  to air               3, 68, 74, 103-7
  trends in                        69
Mercury Maps       64, 65-67, 73, 92
Mercury Study Report to Congress    4,
  20, 34, 65, 74
mixing zone                   56-57
model
  continuous simulation/dynamic 66, 74
  Dynamic Mercury Cycling Model (D-
    MCM)                    19,74
  empirical bioaccumulation
18
162

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                                                                                                Index
  mechanistic bioaccumulation      19
  regression                    19,51
  selection of                     75
  spatially detailed                 75
  steady state/mass balance      73, 76
  uncertainty               22, 71, 76
monitoring and assessment    6,41-53
National Descriptive Model of Mercury
  and Fish                     17,52
National Health and Nutrition
  Examination Survey               4
National Lake Fish Tissue Study      5
National Listing of Fish Advisories    5
National Pollutant Discharge
  Elimination System     See NPDES
                Overview of P2 Approaches at POTWs
                                                  93
neurological effects
nondetections
normalizing factors
NPDES
       4
   46,49
17, 24, 47
  effluents, measurement of mercury in
                               36,83
  fish tissue criterion, implementing  82
  new sources and new discharges,
    mercury in                 97-99
  permit special condition           88
  pollutant minimization plan
    defined                       34
    in case study                  36
    recommended conditions    89-90
  reasonable potential determination
    and fish tissue data           100
    and intakes                  100
    defined                       81
    how to                     84-97
    process                    83, 84
  reopener clause           86, 87, 88
                partition coefficient
                                 127
persistent, bioaccumulative, and toxic
                                 128
persistent, bioaccumulative, and toxic
  (PBT)                          94
pollution prevention      34,94,101-3
POTW           35, 68, 90, 93, 101-3
prenatal                         3,94
public participation                 61
publicly owned treatment works     See
  POTW
quality control              41,42,52
reference dose            3, 13, 26, 58
REMSAD                      67,70
sampling
  and BAFs                16, 23-25
  and increased advisories           6
  bias                              5
  guidance on                  44-48
  sediment                     72,79
shellfish
  advisories                    53,57
  andCWAlOl(a)                 37
  in criterion calculation            13
  intake rates                   29, 30
  to be monitored                  44
significant industrial users          101
site-specific procedure 15-25, 55, 57, 72,
  85
sources
  atmospheric3, 6-9, 32, 65-67, 68-71,
     106
  background levels in soil       39, 72
  human activity           7, 8, 72, 77
  mining         7, 39, 66, 67, 72, 78
                                                                                                  163

-------
Index
                        natural
7, 8, 32, 38, 68, 72
use attainability analysis (UAA)  37-39
                        point sources 8, 31, 39, 56, 67, 68, 77,
                          92
                        variances
                        sediment
   3, 9, 72-76, 95
                      spill prevention and containment control
                        plan                           89
                      tissue concentration-based standard  12,
                        13, 15
                      tissue residue value
                      TMDL
                        allocation approaches
                        best uses
                        challenges
                        defined
                        examples
  1,55,82,85,90
           63-79
              76
              63
              10
              63
  See Appendix A
                        monitoring provisions            78
                        pollutant loading scenario        77
                      Total Maximum Daily Loads See TMDL
                      translation factor      1, 16-23, 82-97
                      uncertainty
                        accuracy                       13
                        assessing loadings               89
                        BAFs                 17, 20, 22, 24
                        from extrapolating results        19
                        in TMDL                       63
                        Mercury Maps                  67
                        model                    22, 71, 76
                        precision                       13
                        reducing                       15
                        RfD                            3
                                  34
                                  31
                                  34
                                  36
                                  33
                                  34
                                  33
                                  31
                                  34
  antidegradation
  how they apply
  large-scale
  multiple discharger
  performance-based approach
  pollutant minimization plans
  time frames
  when appropriate
  wildlife
water column concentrations  21, 56, 89
water quality criteria   See also AWQC
  and BAFs                       23
  and fish advisories                59
  and Methods 1630, 1631      42, 43
  components                     12
  defined                         11
  recommendation           12, 30, 84
water quality limited segments   97, 99
water quality-based effluent limits   See
  WQBEL
                        WQBEL
                          and mercury intakes
                          and variances
                          components
                          defined
                          deriving
                          determining need for
                          forms of
                                 100
                                  31
                                  99
                                  14
                              90-97
                              84-86
                                  81
164

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