As such, the commenters supported the overall methodology outlined in the NODA, which will enable EPA to assess the potential public health benefits associated with reducing Hg emissions from power plants, i. e., the benefits to public health that are related to the purpose of this rule. According to the commenters, such quantification is admittedly difficult, as there is no direct correlation between power plant Hg emissions and human MeHg exposure ( which, as EPA recognizes, is the public health outcome of concern). As such, the commenters concurred with EPA that the Agency must undertake the following five step assessment in order to translate 9 168 power plant Hg emissions reductions into public health outcomes: ( I) quantify Hg emissions projected from power plants under the Base Case as compared to after implementation of the Hg rule, including quantifying Hg emissions that result from sources other than U. S. coal fired plants; ( ii) model changes atmospheric dispersion, atmospheric speciation, and Hg deposition as a result of Hg reductions from power plants; ( iii) model the link between changes in Hg deposition and changes in MeHg concentrations in fish; ( iv) assess types and amounts offish consumed by U. S. customers and extrapolate resulting changes in MeHg exposures resulting from reduced power plant Hg emissions; and ( v) assess how reductions in MeHg exposure affect human health.

Although the commenters supported the overall methodology EPA has articulated, and concurred that the Agency has properly identified the five necessary steps to enable quantification of the public health benefits associated with the proposed rule, the commenters also supported the concerns articulated by the Electric Power Research Institute (" EPRI") and others with respect to specific aspects of how EPA proposes to conduct specific aspects of the benefits assessment. For example, the commenters concurred with EPRI that there are some significant weaknesses in how the MMAPs model quantifies the link between changes in Hg deposition and changes in MeHg concentrations in fish. The commenters urged EPA to seriously consider these critiques of specific aspects of the benefits assessment before it finalizes this analysis.

Response:

The benefits analysis completed for the RIA is not intended to model local scale changes in fish tissue concentrations and exposures in support of site specific risk analysis. Instead, modeling conducted for the RIA is intended to capture generalized regional changes in methlmercury exposure resulting from reductions in power plant mercury emissions in order to support a national scale benefits assessment focusing on the 37 state eastern US study area. For additional details on the benefits analysis modeling framework see Section 10 of the RIA.

EPA recognizes the complexities associated with methylation of mercury deposited in waterbodies and watersheds and subsequent biomagnification within the aquatic foodweb. While there are dynamic fate/ transport models that can be used to conduct detailed site specific modeling of mercury in aquatic and terrestrial environments for purposes of predicting mercury fish tissue concentrations ( e. g., the dynamic mercury cycling model), it is not feasible to utilize these resource intensive models for a regional or nationals scale analysis. Therefore, EPA selected the MMAPs models for application in the RIA. EPA fully recognizes the limitations and simplifying assumptions associated with this model, but believes that it has sufficient precision to support a benefits analysis conducted at the regional or national scale ( i. e., it can capture general trends in mercury fish tissue response to changes in mercury deposition from power plants). However, to provide additional perspective on the relationship between mercury deposition and fish tissue concentration changes ( especially in relation to MMAPS linearity assumption and the lag time required for systems to reach steady state), EPA has conducted several detailed local scale sensitivity analyses. The results of these case studies are presented and discussed in Appendix D.

The RIA includes an assessment, to the extent possible given our scientific understanding 9 169 of mercury and its behavior in the environment and impacts on human health, of the health benefits associated with the proposed regulatory options. Due to limitations in our current understanding of these technical areas related to mercury this benefit analysis is limited to the self caught freshwater fish consumption pathway and to IQ deficits in prenatally exposed infants. In keeping with precedent in evaluating benefits of air regulations ( REFERENCE), cobenefits ( in this case resulting from potential reductions in direct PM2.5) are also included in the RIA.

Comment:

One commenter ( OAR 2002 0056 5535) referenced EPA's Revised Benefits Methodology Must Account Fully for the Risks and Public Health Costs of Hg Pollution EPA's NODA seeks public comment on a suite of technical issues, supposedly to assist the agency in developing its final assessment of the benefits of Hg control pursuant to Executive Order 12866. Specifically, EPA asks for public input on " the U. S. power plant contribution to total Hg deposition within the U. S.," the agency's planned approach to " evaluate how Hg moves through the atmosphere and how it ultimately will be deposited," " the strengths and weaknesses of different approaches for modeling the anticipated response offish tissue [ MeHg] concentrations to declines in deposition," the " consumption data" to be used in EPA's " analysis concerning the relationship between reductions in MeHg concentrations in fish tissue and reductions of human exposure to MeHg," and " all aspects of the methodology for estimating the relationship between reductions in MeHg exposure and improvements in health."

These sweeping requests for information suggest that EPA is interested in more than simply calculating the benefits of a Hg control regime. Instead, the agency appears to be giving the utility industry another chance to advance claims that power plants are neither a significant cause of U. S. Hg deposition nor a public health concern, and to argue that EPA should weaken its proposed Hg rule before finalizing it. Indeed, EPA acknowledges that this information may be used to influence the stringency of the final rules, saying:

" To the extent that we receive any comments or other information in the process of completing the benefits assessment for purposes of EO12866 and to the extent that such information bears on the statutory factors relevant to setting either a beyond the floor standard for Hg under CAA Section 112( d) or a standard of performance for Hg under CAA Section 111, we intend to evaluate and consider that information as we make a final decision as to which regulatory approach to pursue."

This open ended invitation for comment reveals how far EPA has veered from its statutory responsibilities under § 112 of the Act. At this stage of the rulemaking process, the agency should be refining its understanding of the issues central to the development of a proper MACT standard by correcting the problems created by its proposal to subcategorize by coal rank, by abandoning its inflation of the emission standards to account for " variability," by making an unbiased assessment of the state of Hg control technology in light of manufacturers' comments, and by conducting the modeling ( and additional technical work) necessary to evaluate the cost, 9 170 energy impacts, and non air quality impacts of numerous above the floor control options. Outrageously, EPA instead seeks to revisit issues considered four years ago when the agency concluded that regulating power plants' hazardous air emissions was " appropriate and necessary," and gives the public a mere month to make   again   the scientific case for stringent controls on utility units.

Response:

EPA is finalizing a cap and trade approach under section 111. Please see Chapters 10 and 11 of the CAMR Regulatory Impact Analysis for a discussion of the exposure modeling and benefit methodologies used to analyze the benefits associated with this rulemaking.

1. Step 1: Analyzing Hg Emissions from Other Sources. EPA plans to do fate and transport modeling of emissions originating in other countries to allow an estimated of U. S. power plant contribution to total Hg deposition in the U. S. EPP received comments relevant to this issue from the Center for Energy and Economic Development ( OAR 2002 0056 2256), Electric Power Research Institute ( OAR 2002 0056 2578), Hubbard Brook Research Foundation ( OAR 2002 0056 2038), National Mining Association ( OAR 2002 0056 2434), TXU Energy ( OAR 2002 0056 1831), and Utility Air Regulatory Group ( OAR 2002 0056 2922). Some of these comments used different approaches for stimulating boundary conditions for apportioning Hg exposure from domestic and international sources, and EPA asks for input on these alternative approaches and analyses.

Comment:

One commenter ( OAR 2002 0056 5458) noted that EPA requested that stakeholders identify sources of Hg emissions other than coal fired utilities for inclusion into the IPM. The lack of Hg source data in the 1999 National Emission Inventory ( NEI) is a direct result of the Agency's decision not to include hazardous air pollutants in its Consolidated Emission Reporting Rule. Other source categories not regulated for Hg emissions include sewage sludge incineration and Portland cement plants that burn coal. Of combustion sources, coal fired utilities make up half the Hg emitted in New York State. The Department has promulgated and implemented more stringent state regulations that contain stricter emission limits for Hg emissions for the municipal waste combustion source category than promulgated by the EPA. The current estimated Hg emissions inventory for stationary sources in New York is attached as Table 1 ( see OAR 2002 0056 5458). The Department is evaluating other source categories for reductions and hopes that EPA will propose a NESHAP for the utility sector that will provide meaningful reductions of national Hg emissions.

Resources:

EPA appreciates the additional information submitted by the commenter. EPA is finalizing a cap and trade approach under section 111. Please see Chapter 8 of the CAMR 9 171 Regulatory Impact Analysis for a discussion of the air quality modeling for the CAMR regulatory options and the resulting change in mercury deposition.

Comment:

One commenter ( OAR 2002 0056 5488) stated that studies from the Florida Everglades and Wisconsin, as well as the heterogeneity among the states in Hg deposition highlight the localized nature of Hg deposition. Any benefits estimation must model local deposition, methylation, and the consumption of fish contaminated by such local deposition, whether that consumption occurs locally or more nationally. The NODA refers to the EPA's intent " to estimate the U. S. power plant contribution to total Hg deposition within the U. S." From a public health perspective, this estimate is meaningless, as Hg deposition, methylation, and human exposure from U. S. coal fired power plants depend on highly localized conditions and are unrelated to the U. S. fraction of total Hg deposition averaged over the entire U. S. land mass.

Response:

Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition. Please also see Section 8 for a discussion of the change in mercury deposition based on air quality modeling and Chapters 11 for a benefits analysis of the CAMR. Comment:

One commenter ( OAR 2002 0056 5535) presented several specific issues below that would benefit from such careful peer review. They did not intend this list to be comprehensive, but merely included it to illustrate that the agency must address critical scientific issues prior to making changes to its deposition and watershed models or conducting other components of the proposed analyses. This overview serves to demonstrate that it would be foolish to hold up final Hg regulations while EPA further investigates certain technical issues concerning the emissions, fate, and toxicity of Hg from power plants.

° Global model of Hg transport. The global model of Hg transport developed by the Electric Power Research Institute needs thorough vetting by the broader scientific community before it, or results generated by it, are adopted by the EPA. In particular, the spatial resolution of EPRI's model is coarse and the effect of that resolution profoundly affects the modeling results.

Response:

EPA appreciates the commenters input. EPA is finalizing a cap and trade approach under section 111. Please see Chapter 8 of the Regulatory Impact Analysis for a discussion of the air quality modeling used to analyze the CAMR rule.

Comment: 9 172 One commenter ( OAR 2002 0056 5423) notes that natural sources of Hg emissions dominate the small amount of Hg emissions from U. S. power plants. This commenter believes that this fact brings into stark reality that any meaningful control of Hg emissions toward a realistic " reduction" in Hg deposited on U. S. soils will be almost impossible. There are well over 5,000 surface and submarine volcanoes in the world, with about 50 to 60 eruptions each month, according to the Smithsonian Institution. Volcanic degassing may be the single largest source of ocean and atmospheric Hg. For example, at Roaming Mountain, Wyoming, researchers measured Hg emanating from the clay hillside at up to 2,400 nanograms per square meter per hour. By comparison, background levels away from geothermal areas range from zero to ten. So Hg emissions from active geothermal areas could be tens and hundreds times more than from other background areas.

For obvious reasons, volcanic degassing and other geothermal activities as dominant sources of Hg have not received much attention or have been downplayed. For example, EPA staff provided the Administrator with an outdated volcanic accounting study. Figure A1 ( see OAR 2002 0056 5423) clearly shows that EPA's current adopted value for the annual contribution of atmospheric Hg by volcanic eruptions and degassing is significantly under accounted for by about a factor of 6 to 7. When adjusted to reflect a more accurate accounting of volcanic Hg emission ( Figure A2, see OAR 2002 0056 5423), U. S. power plant contributions to the annual estimated Hg budget world wide fall to an insignificant 0.8 percent or less.

Figure A3 ( see OAR 2002 0056 5423) maps the range of potential volcanic activity in the Western U. S. These are also potential sources of enormous Hg degassing and deposition, especially Yellowstone National Park.

Figure A4 ( see OAR 2002 0056 5423) emphasizes that the pools of Hg stored in U. S. forests and peat lands ( covering less than 2 percent of U. S. area) swamp the 100 150 tons total annual anthropogenic Hg emission from U. S. sources.

Taken together, these figures support EPA's admission of poor accounting for ( a) natural sources of Hg emission and ( b) the large pool of background Hg at all times available for emission from the natural ecosystems and geological settings within the U. S.

Yellowstone National Park is just one such geological reservoir of Hg. A report issued last fall by the Idaho National Engineering and Environmental Lab showed that several places in Yellowstone Park have higher levels of airborne Hg than power plants. It went on to say that Yellowstone could emit or exceed as much Hg as all of Wyoming's eight coal fired power plants combined.

At Yellowstone Lake, researchers have discovered submerged faults, explosion craters, domal features, hydrothermal vents, lava flows extending far out into the lake and much more. Mercury may propagate from these natural features up through the food chain transforming into MeHg in native cutthroat trout.

And since Yellowstone is the headwaters of important tributaries to the Missouri 9 173 ( Yellowstone River) and Columbia ( Snake River), no one knows how far the natural contamination carries through the Earth's air and water systems.

However, the Hg presence and emissions were noted by experts to pose no danger to park rangers or visitors. Even native grizzly bears who consume up to 400 lb of spawning cutthroat trout exhibit no ill effects, according to researchers with the Interagency Grizzly Bear Study Team.

Thus, the most important question for EPA's Hg emission and deposition modeling team to answer confidently is whether the proposed CAMR to control Hg emissions from U. S. power plants can assure any consequential " reduction" of Hg deposition in U. S. soils, leading to any reputed public health " benefits."

Response:

Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition. Please also see Section 8 for a discussion of the change in mercury deposition based on air quality modeling and Chapters 11 for a benefits analysis of the CAMR.

Comment:

One commenter ( OAR 2002 0056 5423) provides an important challenge for EPA's Hg emission and deposition modeling team ( Figure B1, see OAR 2002 0056 5423). It shows the results from recent measurements of Hg content in Illinois soils clearly suggesting that atmospheric deposition from any past or current U. S. power plants is insignificant compared to the large quantity of background Hg already resided in soils across the state. This is why it is extremely difficult for EPA to convincingly show that the Hg from U. S. power plant emissions will be selectively filtered by the ecosystem components in Illinois ( or anywhere else) to bring about increased levels of MeHg in freshwater fish while ignoring the large pool of natural Hg in the native ecosystem. It is therefore extremely difficult, if not impossible, for EPA to plausibly demonstrate the assertion that its proposed CAMR can/ will bring about any direct, measurable improvement in public health.

In Figure B2 ( see OAR 2002 0056 5423), the commenter provides a recent Hg emission and deposition budget analysis for the northeastern Chinese city of Changchun. Scientists found that of that 7.1 tons of Hg emitted by the city of Changchun, only less than 12 percent of coal fired power plant Hg was deposited back into the local area, while most escaped as contributions to regional and global cycling of Hg.

This scenario of the local Hg emission and deposition budget at Changchun, China may serve as a useful model verification target for the EPA's Hg emission and deposition modeling team under a wide range of meteorological and climatic conditions and settings. 9 174 Man made atmospheric deposition of Hg is a very small contributor to the huge amount of natural Hg in Illinois and U. S. soils ( 1) It has been estimated that " anthropogenic activities could have increased world soil Hg content by [ only] 0.02 percent."

( 2) From the measured high Hg content in Illinois soils, it would take 9000 years at the currently measured atmospheric deposition rate to dump all the Hg to the top 380 cm of Illinois soils.

( 3) If assuming the average Hg in the top 140 cm of U. S. soils to be about 10 ppb, it would take 14,000 years at the current atmospheric deposition rate to do it.

Response:

EPA appreciates the commenters input. Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition. Please also see Section 8 for a discussion of the change in mercury deposition based on air quality modeling. Comment:

One commenter ( OAR 2002 0056 5423) presents an important observational target for EPA's modelers ( Figure C1, see OAR 2002 0056 5423). First, it is important to point out that the Hg in rainwater or moist air exists mostly in the dissolved ionic form of Hg ( Hg2+) rather than MeHg   the biologically active form of Hg that may affect human health at extraordinary dose levels. Figure C1 ( see OAR 2002 0056 5423) shows that most ( 72.5 percent) of the MeHg in the Chesapeake Bay ecosystems comes from in situ production. Remote transport of MeHg from rivers contributes about another 20 percent, and atmospheric deposition sources may contribute toward production of as little as 7.5 percent of MeHg in the Chesapeake Bay's ecosystems. Such a scenario of the MeHg budget clearly emphasizes the need for better scientific understanding of the complex physical, chemical and biological factors controlling the production and destruction of MeHg, and why the levels of MeHg in an ecosystem do not depend directly on available amounts of inorganic Hg ( i. e., from background or power plant emissions).

Sources of MeHg in the Chesapeake Bay: Atmospheric deposition is a not an important contributor Percentage contribution of 70 percent MeHg sources 60 percent in the Chesapeake Bay In situ Production Rivers Atmospheric Deposition Reference: Mason et al. ( 1999)

Response:

EPA appreciates the commenters input. Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition. Please also see Section 8 for a discussion of the change in mercury deposition based on air quality modeling. 9 175 Comment:

The commenter ( OAR 2002 0056 5460) stated that UARG, following EPRI, argues that Hg pollution is a global problem and, therefore, EPA should not regulate Hg emissions pursuant to Section 112. See UARG Comments at 25 26. The commenter noted that, in particular, UARG contends that only 25 percent of domestic Hg deposition stems from domestic anthropogenic sources. Id. ( citing EPRI Comments at 13). But little, the commenter stated, if any, basis exists to have confidence in UARG's estimates given EPA's conclusion that Hg's fate in the environment can not be tracked with precision and that Hg emitted from any source may be re emitted into the environment. See 69 Fed. Reg. 4652, 4658 ( Jan. 30, 2004). The commenter stated that indeed, UARG itself contends that " the state of the science on Hg cycling is too imprecise to predict" the consequences of changes in Hg deposition. UARG Comments at 26 n. 57.

The commenter stated that, more importantly, even if UARG is correct that Hg is a global problem, it does not follow that EPA should not regulate domestic power plant emissions pursuant to section 112. The commenter further stated that to the contrary, the Court of Appeals for the D. C. Circuit has already made clear that whether a pollutant has local, regional or global consequences is not, standing alone, a sufficient legal basis for concluding that the pollutant should not be regulated. See American Lung Ass'n, 134 F. 3d at 392 ( rejecting EPA's argument that it could avoid revising a national ambient air quality standard simply by claiming that the pollutant at issue had local effects only). The commenter added that simply because EPA cannot prevent Hg from entering the U. S. from other countries is not a reason for EPA to avoid regulating domestic Hg emissions. The commenter stated that UARG's argument that local emissions should be exempt from regulation because they cause only part of the problem makes no sense: Just because pollution enters our borders from somewhere else is not a reason to allow domestic sources to create even more pollution. The commenter further stated that, indeed, it is well established that a polluter may be enjoined from contributing to a public nuisance even where the polluter's contributions alone are insufficient to create the nuisance. See, e. g., The Law of Torts § 52 (" Pollution of a stream to even a slight extent becomes unreasonable when similar pollution by others makes the condition of the stream approach the danger point."); Cox v. City of Dallas, 256 F. 3d 281,292 n. 19 ( 5th Cir. 2001) ( explaining that nuisance liability attaches where one simply contributes to the nuisance); City of New York v. Beretta U. S. A. Corp., 315 F. Supp. 2d 256, 282 ( E. D. N. Y. 2004) (" Where it is difficult or impossible to separate the injury caused by one contributing actor from that caused by another and where each contributing actor's responsibility individually does not constitute a substantial interference with a public right, defendants may still be found liable for conduct creating in the aggregate a public nuisance if the suit is one for injunctive relief").

The commenter stated that at bottom, EPRI and UARG appear to be arguing that EPA needs greater proof that power plant Hg emissions cause harm before it can regulate those emissions. But, the commenter stated, no reason exists for EPA to allow harm from Hg emissions to continue until it has yet more concrete proof of that harm. The commenter further stated that to the contrary, existing science already confirms the harm is sufficient for EPA to act. The 9 176 commenter stated that, finally, given the overarching flaws in EPRI's and UARG's comments, EPA should look with skepticism upon their more detailed technical claims.

Response:

EPA appreciates the commenters input. Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition. Please also see Section 8 for a discussion of the change in mercury deposition based on air quality modeling.

Comment:

One commenter ( OAR 2002 0056 5497) stated that any emissions inventory must do a good job of defining the global sources of Hg emissions. Recent modeling by EPRI and others has revealed that a substantial majority of Hg that is deposited in the U. S. ( about 75 percent) originates from natural sources and anthropogenic sources outside the U. S. Asian sources may provide 20 percent of the Hg deposited in the U. S. EPRI's NODA comments ( see OAR 2002 0056 5469) offer a number of sources of information on global Hg emissions.

Modeling of Hg emissions from global and domestic sources remains a challenging exercise. To date, Hg model results have not matched the levels of Hg in water bodies and fish in various parts of the U. S. The atmospheric chemistry of Hg may hold the key to this disparity. As EPRI notes in its NODA comments, atmospheric Hg reactions can move in different directions. For major urban sources, including mobile sources, elemental Hg can be rapidly oxidized to ionic Hg which can then be removed form the atmosphere by wet and dry processes. By contrast, ionic Hg emitted from coal fired power plants appears to be reduced to elemental Hg shortly after leaving the stack. Recent results from Edgerton suggest that models that do not account for this atmospheric reduction of ionic Hg will over predict the local and regional Hg deposition from coal fired power plants.

Future EPA modeling should use the results of global modeling to define the boundary conditions for domestic modeling. Global modeling is a better scientific approach for setting boundary conditions than arbitrarily setting those conditions. If EPA uses the results of global modeling to set boundary conditions, it should still review those results to ensure they are reasonable.

Response:

EPA appreciates the commenters input. Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition. Please also see Section 8 for a discussion of the change in mercury deposition based on air quality modeling. 9 177 2. Step 2: Analyzing Air Dispersion Modeling Capabilities. EPA plans on modeling the atmospheric dispersion, atmospheric speciation, and deposition of Hg using the REMSAD and CMAQ models, with the GEOS CHEM global model for boundary conditions input. The simulated results will be compared with ambient monitoring data from the Mercury Deposition Network ( MDN). EPA requested comments on the use of these models and approach. EPA received comments on the use of models for assessing the impacts of the proposed programs on Hg deposition patterns from the Center for Energy and Economic Development ( OAR 2002 0056 2256), Clean Air Task Force et al ( OAR 2002 0056 3460), Electric Power Research Institute ( OAR 2002 0056 2578), and Utility Air Regulatory Group ( OAR 2002 0056 2922). EPA also sought comment on the alternative approaches suggested by some of these commenters.

Comment:

One commenter ( OAR 2002 0056 5464) highlighted another concern related to EPA's proposed rule, which is the issue of hot spots. The commenter believed the definition of a " hot spot" in the proposal is insufficient. The proposal states, " a power plant may lead to a hot spot if the contribution of the plant's emissions of Hg to local deposition is sufficient to cause blood Hg levels of highly exposed individuals near the plant to exceed the RID" ( 69 Federal Register 4702). The commenter was very concerned that the proposal considers the effects of only one plant at a time in determining if there is a hot spot. This method of determining hot spots ignores the cumulative, localized impacts of Hg deposition caused by multiple, nearby or co located, coal fired utility boilers by individually quantifying and analyzing the air quality impact of each boiler in the absence of the others. This is contrary to previously adopted, long standing, peer reviewed EPA procedures for performing air quality modeling on stationary sources for criteria pollutants and is not acceptable for performing air quality modeling for more hazardous pollutants like Hg.

The commenter had an additional concern related to the improvement of existing hot spots and stated that 45 states have issued fish consumption advisories due to Hg. Clearly a contaminated water body is a hot spot that already exists. In fact, the current situation indicates that we have " hot states" and " hot regions" and not just localized hot spots. The commenter feared that EPA's proposal, which calls for a nationwide market based cap and trade program, will not ameliorate the problem of existing hot spots or areas.

The commenter categorically disagreed with UARG's position and strongly urged EPA, if the agency decides to proceed with a rule under Section 111, to maintain the ability of state and local agencies to opt out of the trading program.

The Clean Air Act explicitly allows state and local air pollution control agencies to adopt programs more stringent than those of the federal government. Specifically, section 116 states that air quality agencies are not precluded from adopting or enforcing any standards, limitations or requirements as long as they are at least as stringent as those required under the federal program. 9 178 The only exceptions are found in section 119 of the Clean Air Act, which preempts certain state and local regulation of mobile sources. Therefore, UARG's suggested approach, in which EPA would preempt state and local agencies' ability to adopt a more stringent program that does not permit trading, is in direct conflict with section 116 of the Clean Air Act.

The commenter stated that for a variety of reasons, maintaining the ability of state and local agencies to adopt more stringent programs is essential. Not the least of these reasons is that state and local agencies will need some way of addressing and preventing hot spots in their areas. In fact, EPA appropriately acknowledged this in its January 30, 2004, proposal by stating, "[ s] tates retain the power under the proposed section 111 rule to adopt stricter regulations to address local hot spots or other problems" ( 69 Federal Register 4702).

UARG expresses concern in its comments that allowing states to opt out of the program would result in a " patchwork approach". The commenter contended that a federal standard, such as what EPA has proposed, that is less stringent than the law requires would be to blame for any patchwork effect. In fact, state and local agencies have already begun to adopt their own programs to ensure adequate public health protection in their states ( see attached table). If EPA and industry wish there to be more national consistency, EPA can accomplish this by adopting a protective standard consistent with the requirements of Section 112( d). Fewer state and local agencies may then feel compelled to adopt different approaches ( see OAR 2002 0056 5464).

Response:

EPA has examined the commenter's concerns in context of the final rulemaking. EPA is finalizing a cap and trade approach under section 111. EPA has addressed the hot spots issue in the Revision of December 2000 Regulatory Finding on the Emissions of Hazardous Air Pollutants from Electric Utility Steam Generating Units and the Removal of Coal and Oil fired Electric Utility Steam Generating Units from the Section 112( c) List for a discussion of the Agency's rationale for not proceeding under Section 112 Notice and in the Technical Support Document: Methodology Used to Generate Deposition, Fish Tissue Methylmercury Concentrations, and Exposure for Determining Effectiveness of Utility Emission Controls in the docket.

Comment:

One commenter ( OAR 2002 0056 5559), beginning in 2001, began a two year effort to develop a Hg atmospheric modeling system for Wisconsin and the Great Lakes region. Partial funding for this work was provided by a grant, # X97599601, from EPA.

Based on this experience, the commenter had the following comments on the modeling of Hg deposition that EPA is proposing to conduct as part of the revised benefits assessment.

Step 1   Analyzing Mercury Emissions from Other Sources. The commenter used 1999 National Emission Inventory ( NEI) to build emissions estimates for Hg and conducted considerable quality assurance of the Hg emissions data. Based on experience with the 1999 NEI 9 179 Hg emissions data, if EPA chooses to use the 2002 NEI data, this commenter recommends that EPA conduct considerable quality assurance of these data, before they are used in model simulations.

In simulations, the commenter added Canadian and Mexican emission sources. Although those data are somewhat incomplete, the commenter would be happy to share these data with EPA.

Step 2   Analyzing Air Dispersion Modeling Capability. There is a significant amount of uncertainty associated with the simulation of Hg deposition. EPA should conduct an uncertainty analysis with the modeling system to evaluate the effect on the modeling simulations for changes to the underlining assumptions in the model. In particular, EPA should evaluate changes to the wet deposition algorithms, dry deposition algorithms and emissions speciation profiles and processes that form MeHg in water bodies.

There is some anecdotal evidence such as around the White Pine smelter in Michigan and around certain Florida point sources, that reductions in Hg emissions at large sources results in a reduction in Hg concentration in nearby water bodies. To date, this phenomenon has not been satisfactorily simulated with Eulerian grid models. EPA should further investigate this phenomenon, since it seems to highlight a significant problem with the current state of the science Hg modeling systems and puts into doubt the interpretation of the results of model simulations. If a significant improvement in nearby water quality results from a reduction in Hg emissions, this commenter will realize a much greater benefit from power plant Hg reductions than has been demonstrated by previous model simulations.

EPA should reconsider its selection of the Regional Modeling System for Aerosols and Deposition ( REMSAD) for one of the models to use in its analyses. Under a grant, AER and Environ worked with the commenter to create a version of the Comprehensive Air Quality Model with extensions ( CAMx) that includes the most up to date Hg chemistry and deposition algorithms. The model development and simulations were peer reviewed by Alpine Geophysics. When measured wet deposition at Mercury Deposition Network ( MDN) sites was compared with model results, the commenter found that CAMx provided much better model performance than REMSAD.

Deposition is very sensitive to rainfall simulation in MM5. Since, in the eastern U. S., a significant portion of annual Hg wet deposition occurs in convective rain events, it is important to properly simulate convection in the meteorological model. In model performance testing for MM5, the commenter found that using a 12 Km grid spacing rather than 36 Km, 46 vertical layers rather than 34, and the Reisner Graupel Ice scheme rather than Simple Ice provided superior performance for a convective rainfall event in the Great Lakes area. These changes will adversely affect model run time, but may be worth employing for comparison purposes.

Response:

Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion 9 180 of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition. Please also see Section 8 for a discussion of the change in mercury deposition based on air quality modeling.

Comment:

The commenter ( OAR 2002 0056 5563) wished to specifically address the issue of utilizing modeling under the notice of data availability for the Clean Air Mercury Rule. The commenter felt that modeling should only compliment a comprehensive monitoring program of Hg pollution. Monitoring should not be limited to the Great Lakes and Eastern portion of the country it should be extended to all parts of the U. S., especially to tribal lands and communities. The utilization of the Community Multiscale Air Quality ( CMAQ) and Regional Modeling System for Aerosols and Deposition ( REMSAD) models that the EPA had used and intended to use in the future for modeling the atmospheric dispersion, speciation, and deposition of Hg did not address tribal concerns effectively. The EPA's plan was to use a 36 kilometer modeling grid for both CMAQ and REMSAD in assessing the aforementioned items. However, these models were equipped to handle 12 and 4 kilometer modeling grids. In order to assess the real effects of Hg on tribal lands, the commenter urged the EPA to utilize the latter modeling grids. This rational was related to the size of tribal lands and communities. Most tribes occupied land closer to these grid sizes. Furthermore, the proportion of the proposed models for both CMAQ and REMSAD models failed to distinguish between different jurisdictions such as tribal, state, and county lands. This issue failed in carrying out the trust responsibility of the federal government. Tribal lands, as sovereign Nations, had the right to be indicated as such in modeling practices. In addition, the NTAA also feels that proposed modeling techniques will error in properly addressing jurisdictional point sources of Hg. Therefore, modeling output could be adversely affected.

Response:

Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition. Please also see Section 8 for a discussion of the change in mercury deposition based on air quality modeling and Chapters 11 for a benefits analysis of the CAMR.

Comment:

One commenter ( OAR 2002 0056 5458) agreed with the subcategorization of coal types for regulatory purposes in response to the NODA section II. C. 3. b., but disagrees with the standard ( or average) speciation profile in modeling analysis. Work conducted by the Department on the 1999 Information Collection Request ( ICR) shows that the majority of Hg emitted in the eastern half of the US is predominantly in the oxidized form. The speciation profile cited in the NODA, 54 percent elemental, and 43 percent oxidized and 3 percent particulate Hg, is a national average and does not represent the Northeast States potential deposited burden. The Department's analysis of the ICR data indicates a profile closer to 30 percent elemental, 65 percent oxidized and 5 percent particulate Hg for sources which are impacting New York. 9 181 The NODA focuses on industry sponsored research about the speciation of atmospheric Hg after it exits the stack and how emissions rapidly converted to elemental Hg in the plume will not be deposited in the U. S. due to their long atmospheric residence time ( Air Docket OAR 2002 0056 2928 and OAR 2002 0056 2848). This entire data set was not included in the docket and the Department could not locate any information which indicates the data had undergone any public and peer review. The EPA must cautiously evaluate the results and conclusions drawn from this research in light of the unknown and non linear relationship between Hg emissions and deposition. There are many factors which need to be assessed when evaluating atmospheric Hg transformation and potential deposition scenarios. The NODA must consider the emerging science about the atmospheric chemistry and deposition of Hg. These recent findings indicate that the abundance of oxidized Hg increases with altitude in the troposphere, and airborne halogens and oxidants, such as ozone facilitate the oxidation of elemental Hg, which in turn increases both wet and dry deposition of Hg. The reactions are occurring in areas with high seasonal ozone levels and also appear to take place in the temperate coastal zone. Researchers have concluded that a short half life for elemental Hg is probable.

Response:

EPA appreciates the commenters concerns. Please see EPA ' s Office of Research and Development white paper ( see Control of Emissions from Coal Fired Electric Utility Boilers: An Update, EPA/ Office of Research and Development, March 2005) for a discussion of the importance of speciation for mercury capture. Also, please see Chapter 7 of the Regulatory Impact Analysis for a discussion of how we took speciation into account in our power sector modeling and see Chapter 8 for a discussion of how we took speciation into account in our air quality and deposition modeling. Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition.

Comment:

One commenter ( OAR 2002 0056 5425) said their comments were specific to the Community Multiscale Air Quality ( CMAQ) and Regional Modeling System for Aerosols and Deposition ( REMSAD) models that the EPA has used and intends to use in the future for modeling the atmospheric dispersion, speciation, and deposition of Hg. The EPA is planning to use 36 kilometer modeling grids for both CMAQ and REMSAD in assessing the aforementioned items although these same models are equipped to handle 12 and 4 kilometer modeling grids. In the interest of assessing the true effects of Hg dispersion, speciation and deposition on tribal lands, the EPA should use the latter modeling grids because most tribes occupy lands closer to these grid sizes as opposed to the former grid size being proposed by the EPA for modeling. Furthermore, the spatial resolution of the gridded output for both the CMAQ and REMSAD models fails to distinguish between different jurisdictions such as tribal, state and county lands. Such a failure could likely cause the models to incorrectly attribute ( e. g., mislocate) one jurisdiction's Hg point source to another jurisdiction ( such as a tribe's land). This incorrect attribution of a point source could then adversely affect the results of the modeling output. A means for mitigating this problem is to assign a numeric code to the jurisdiction within the grid 9 182 cell having the dominant contribution to emissions in that cell. This would help the models to produce more accurate results otherwise impossible under current model designs.

Response:

EPA appreciates the commenters input. Please see Chapter 8 of the Regulatory Impact Analysis for a discussion of EPA's air quality modeling analysis. Please also see EPA's Technical Support Document for the Final Clean Air Mercury Rule: Air Quality Modeling.

Comment:

One commenter ( OAR 2002 0056 5502) referenced research results reported to U. S. EPA in June 2004 analyzed Hg deposition for a 2004 Base Case emissions inventory and two 2020 scenarios, the proposed Maximum Achievable Control Technology ( MACT) rule and the proposed Cap and Trade ( C& T) rule. Additional analyses of Hg deposition patterns for 2004 and equivalent patterns under the MACT and C& T rules have now been carried out.

These additional analyses have examined the deposition that would result from utility sources that are projected under MACT and/ or C& T to not implement controls specifically to reduce Hg emissions. Mercury emissions from these sources ( in addition to all the other utility sources that will be reducing their Hg emissions) are found to have insignificant impacts on nearby and distant deposition patterns for the C& T case, relative to the MACT case. Receiving waters downwind from these locations are calculated to experience no significant additional deposition due solely to these sources individually or as a group not initiating Hg specific controls. Thus, under EPA and other definitions of utility Hg deposition " hot spots," these particular sources do not in themselves bring about " hot spot" conditions under a C& T rule relative to either a MACT rule or 2004 emissions conditions.

Measurements of speciated Hg in the ambient atmosphere are needed to improve our understanding of the atmospheric fate and transport of Hg. The commenter recommended that the available information on speciated Hg concentrations be used to ( 1) provide insights on the processes that appear to influence Hg speciation in the atmosphere, ( 2) evaluate the performance of global and continental models of atmospheric Hg vs. surface and aloft data, and ( 3) quantify the limitations and possible biases of those models. Those limitations and biases should then be taken into account when evaluating the potential benefits of Hg emission reductions.

The MDN data are extremely useful to evaluate the ability of models to reproduce the regional patterns of Hg wet deposition. The MDN data have also highlighted the fact that Hg deposition is significantly different from sulfate deposition. Sulfate wet deposition shows a clear west to east increasing gradient whereas Hg wet deposition shows primarily a north to south increasing gradient. These results suggest that sulfate wet deposition is strongly influenced by regional emission sources whereas Hg is not. Instead, Hg wet deposition appears to be influenced primarily by oxidant concentrations that are conducive to the oxidation of Hg0 to Hg+ 2.

Comment: 9 183 One commenter ( OAR 2002 0056 5497) suggested that the Mercury Deposition Network ( MDN) data are useful in evaluating the ability of models to reproduce regional patterns of wet deposition of Hg. However, the MDN was never designed to monitor the local deposition of Hg. Consequently, MDN data should not be used to evaluate model predictions of near field deposition.

For the reasons detailed in EPRI's NODA comments ( see OAR 2002 0056 5469), the commenter agrees that the Industrial Source Complex ( ISC) and Regional Lagrangian Model of Air Pollution ( RELMAP) models used for the Hg Study are not the best means of assessing local and global Hg deposition. These models are outdated and do not reflect the current state of the science. Whatever model( s) EPA ultimately chooses, EPA must recognize that a grid based model may over predict local Hg deposition by a factor of two or more when compared to the results of a plume model.

Response:

EPA appreciates the commenters input. Please see Chapter 8 of the Regulatory Impact Analysis for a discussion of EPA's air quality modeling analysis. Please also see EPA's Technical Support Document for the Final Clean Air Mercury Rule: Air Quality Modeling.

3. Step 3: Modeling Ecosystem Dynamics. EPA plans to quantify Hg deposition associated with Hg reductions to estimate changes in human exposure to methylmercury that may result from reductions in power plant emissions. This requires quantifying the linkage between different levels of Hg deposition and fish tissue methylmercury concentration. EPA is currently considering using the Water Office's Mercury Maps ( Mmaps) for this purpose, supplemented by case studies, and requested comment on this approach. To complement the case studies, EPA asked for both empirical information and modeled scenarios that show the effects of ecosystem properties other than total Hg loading on accumulation in organisms in different ecosystems and, specifically, on new knowledge related to factors affecting methylation and demethylation in a range of aquatic ecosystem types. EPA also sought comment on data and/ or analytic tools that can be used to forecast methylation rates and bioaccumulation rates in aquatic ecosystems. EPA received analyses of the changes in fish concentrations expected as a result of changes in deposition from Chippewa Indians ( OAR 2002 0056 2118), Environmental Defense ( OAR 2002 0056 2878), Electric Power Research Institute ( OAR 2002 0056 2578,  2589,  2593), Hubbard Brook Research Foundation ( 2038), NESCAUM ( OAR 2002 0056 2887,  2890), and TXU Energy ( OAR 2002 0056 1831) and asked for comment on their analyses.

Comment:

One commenter ( OAR 2002 0056 5544) has been monitoring Hg in sediments and fish in reservoirs on one particular river and its tributaries over the last 30 years. Based on this extensive 9 184 data set, several observations can be made:

° Mercury levels in this reservoir sediment have declined substantially since 1973. ° Mercury content in fish in the reservoirs was varied but has generally shown constant or reducing trends. ° Generation of power from coal fired facilities in the region has increased over this period.

The commenter has been working in cooperation with state agencies in the Tennessee Valley to determine the overall condition of streams and lakes and the level of contaminants in fish for three decades. Mutual efforts specifically related to Hg began in 1970 in response to discovery of Hg contaminated fish in the Great Lakes and adoption of guidelines for Hg in fish by the U. S. Food and Drug Administration. The commenter collected a variety of fish species and sediment samples from throughout the are in the early 1970' s and reported current fish tissue monitoring program began in 1987 and continued as part of their Vital Signs Monitoring Program. The commenter has monitored hundreds of locations throughout the Tennessee Valley over the years.

Mercury levels in the commenter's area reservoir sediments have declined substantially since 1973. The commenter conducted a special study in 1973 of sediments from the forebay area ( i. e., the area just upstream of a dam) of 26 reservoirs. A comparison of the results from 1973 with recent Vital Signs Monitoring data from the same locations indicates a substantial decrease in Hg concentrations throughout the commenter's area from the early 1973s to the late 1990s. The large reduction in sediment Hg occurred in the 1970s as many large industrial users of Hg changed production methods to less Hg intensive operations. In locations without historical industrial point sources of Hg, no increases have been observed in sediment Hg concentration.

Mercury levels in fish tend to be low in largemouth bass and catfish in the reservoirs of the commenter's main river. In samples collected since 1990' s in the reservoirs, largemouth bass show a mean concentration of total Hg around 0.2 mg/ kg. Largemouth bass are routinely included in many studies because they are at the top of the food chain. Since 1970, the Hg concentrations in largemouth bass and channel catfish have remained relatively steady with no apparent increase.

In furtherance of its congressional mandate to promote the safe and efficient use of electric energy, the commenter has long supported and conducted research into various subjects associated with the generation of electricity. This includes the publication of various technical reports and studies. Among these publications are a series of " On the Air" reports that are technical notes on important air topics. The November 2004 " On the Air" focused on TVA's sampling of Hg levels in the Tennessee Valley region and was entitled, " Three decades of Mercury Levels in the Tennessee River System". The commenter is enclosing a copy of this document with our comments. It presents the above information in more detail and shows that a decline in Hg concentrations in the Tennessee River has occurred over a period of time when coal combustion has increased substantially ( see OAR 2002 0056 5544).

Response: 9 185 EPA has examined the commenter's concerns in context of the final rulemaking. As described in the Technical Support Document: Methodology Used to Generate Deposition, Fish Tissue Methylmercury Concentrations, and Exposure for Determining Effectiveness of Utility Emission Controls in the docket, we have limited the fish tissue samples to more recent years for some analysis in part for reasons similar to those raised by the commenter.

Comment:

One commenter's ( OAR 2002 0056 5559) studies at Little Rock Lake in northern Wisconsin indicated that environmental improvement may be occurring from actions already taken to reduce Hg air emissions. For a five year period beginning in 1995, researchers measured Hg air deposition, and levels of Hg in lake water and in fish tissue. The studies documented declines in Hg levels for all these parameters with reductions of Hg levels in yellow perch tissue averaging 5 percent annually. The declines measured show that Little Rock Lake is responding more rapidly to changes in deposition of Hg than the decline in acidity from sulfur dioxide emission reductions. This is a promising indication that reducing Hg deposition will lead to lower lake Hg levels as well as a reduction in fish tissue.

Response:

EPA appreciates the commenter's input to the record. We pursued several case studies as described in Chapter 3 of the Regulatory Impact Analysis that address time lags at various ecosystems.

Comment:

One commenter ( OAR 2002 0056 5517) notes that as demonstrated by peer reviewed research performed by scientists at Princeton University, the level of methymercury in the world's oceans is not controlled by atmospheric Hg. In this respect, a particularly significant omission of the NODA is its failure to note the observation of these Princeton University scientists whose findings provide strong evidence that the concentration of MeHg in the world's oceans does not respond to anthropogenic emissions of Hg. Rather, oceanic biogeochemical processes, possibly involving deep ocean sediments, control the level of methymercury in the world's oceans. Thus, this commenter believes that regardless of what Hg emissions controls are put in place at coal fired power plants, the levels of MeHg in ocean fish will remain virtually unchanged.

The commenter points out that EPA, in its NODA, has produced no peer reviewed data that would contradict the finding of the Princeton study. Moreover, the Princeton study aside, given the fact that the world's oceans contain millions of tons of Hg, reducing some or even all of the less than 50 tons of Hg emissions released from U. S. coal fired power plants will leave the levels of Hg in the world's oceans virtually unchanged. Consequently, the levels of MeHg in the ocean fish that Americans eat every day will be virtually unchanged. Unless EPA can produce peer reviewed data that convincingly contradicts the Princeton study findings, it must be presumed as prima facie evidence arguing against any assertion by EPA that curtailing Hg emissions from power plants would have any effect on the level of MeHg found in ocean fish. 9 186 If the Princeton study is correct concerning the constancy of the amount of Hg in the world's oceans, then the modeling EPA proposes to use to estimate reductions of MeHg in marine fish is entirely inappropriate and must not be used. This is because the models, by their very design, would incorrectly predict that lowering emissions of Hg from coal fired power plants will in fact lower the levels of MeHg in the ocean.

However, actual oceanic field data taken by the Princeton University scientists clearly indicate that this is not the case and that regardless of whether atmospheric levels of Hg change, the level of MeHg in the ocean remains unchanged. No model that EPA proposes to use reflects this fact. If these scientists are correct in their surmisal, then limiting emissions of Hg from coal fired power plants will have absolutely no significant effect on the levels of MeHg in marine fish, consumption of which constitutes the major route of human exposure to this chemical. The commenter states that this important finding has a very significant bearing on estimates of the benefits of reducing power plant emissions of Hg and must be taken into account.

Response:

EPA appreciates the commenter's input to the record. For several reasons described in the Regulatory Impact Analysis, EPA focused its analysis on recreationally caught freshwater fish.

Comment:

One commenter ( OAR 2002 0056 5458) did not agree with the EPA formal definition of a Hg hot spot. The definition based on modeling Hg deposition rates by utilities and determining how the removal of those emissions will hypothetically reduce the amount of MeHg in fish is flawed logic. A Hg hot spot is an existing area or region which already has had its natural resources ( fish and wildlife) adversely impacted by Hg emissions resulting in the issuance of fish advisories to protect public health. Existing Hg concentrations in fish and wildlife should be used to define Hg hot spots.

The proposed scale of modeling responses of fish tissues to changes in Hg emissions to the atmosphere is unclear in the NODA. National scale modeling would appear to have a tendency to dilute or obscure the benefits of reduced Hg emissions, particularly for the northeastern and east coast states. This dilution or obscuring of benefits is not scientifically sound or acceptable. For fresh waters of the U. S., modeling should be scaled to at least a regional basis although benefits of Hg emission reductions will be even greater in localities and waters with closer proximity to the sources of Hg emissions.

Response:

EPA has addressed the hot spots issue in the revision Federal Register notice and in the Technical Support Document: Methodology Used to Generate Deposition, Fish Tissue Methylmercury Concentrations, and Exposure for Determining Effectiveness of Utility Emission Controls in the docket. 9 187 Comment:

One commenter ( OAR 2002 0056 5502) stated that power plant emission reductions will reduce Hg deposition in the Eastern U. S., and MeHg concentrations in wild freshwater fish are likely to drop in response. Much of the freshwater fish consumed in the U. S. is farm raised, and does not contribute significantly to exposure. But because so much more marine fish than wild freshwater fish is consumed, the affect on total exposure is modest. The range of exposure reduction from Hg controls under any of the scenarios under consideration was from 0.4 6.5 percent, depending on the state. The average of the calculated state reduction was about 1.6 percent. These reductions apply to those who eat a typical mix of marine and wild freshwater fish, based on state recreational fishing data. Some members of the population such as subsistence fishers may experience greater exposure reductions.

EPA's MMaps tool assumes that 1) there is a linear relationship between atmospheric deposition and MeHg in fish, 2) atmospheric deposition is the only significant source of Hg to lakes, and 3) fish tissue Hg is at steady state equilibrium with the water. Data ( from the METAALICUS study) and modeling results negating these assumptions are provided. Furthermore, it is still not possible to accurately forecast either the lag time or the level of response of fish tissue following reductions in Hg deposition. Modeling runs with the commenter's Dynamic Mercury Cycling Model have indicated lag times of decades to more than 100 years. Regardless of the complexity of models describing the behavior of Hg in the environment, characteristics of both terrestrial and aquatic ecosystems such as microbial activity, pH, dissolved organic matter, algal productivity, redox potential, temperature, hydrodynamics, etc. make it very difficult to accurately conduct broad brush regional modeling of the responses of fish tissues to reductions in atmospheric Hg deposition.

Reductions in Hg emissions from U. S. coal fired power plants will have only a small effect on U. S. exposures to MeHg. This is because most exposure to MeHg occurs through consumption of marine fish. Marine fish will be largely unaffected by U. S. power plant emission reductions because the reductions are so small, less than 1 percent, of global emissions. The predominance in U. S. commerce of North Pacific marine fish implies an even lower sensitivity to U. S. Hg reductions, due to prevailing winds and the uncertainty about the availability of MeHg in marine environments.

Response:

The benefits analysis completed for the RIA is not intended to model local scale changes in fish tissue concentrations and exposures in support of site specific risk analysis. Instead, modeling conducted for the RIA is intended to capture generalized regional changes in methlmercury exposure resulting from reductions in power plant mercury emissions in order to support a national scale benefits assessment focusing on the 37 State eastern U. S. study area. For additional details on the benefits analysis modeling framework see Section 10 of the RIA.

EPA recognizes the complexities associated with methylation of mercury deposited in waterbodies and watersheds and subsequent biomagnification within the aquatic foodweb. While 9 188 there are dynamic fate/ transport models that can be used to conduct detailed site specific modeling of mercury in aquatic and terrestrial environments for purposes of predicting mercury fish tissue concentrations ( e. g., the dynamic mercury cycling model), it is not feasible to utilize these resource intensive models for a regional or nationals scale analysis. Therefore, EPA selected the MMAPs models for application in the RIA. EPA fully recognizes the limitations and simplifying assumptions associated with this model, but believes that it has sufficient precision to support a benefits analysis conducted at the regional or national scale ( i. e., it can capture general trends in mercury fish tissue response to changes in mercury deposition from power plants). However, to provide additional perspective on the relationship between mercury deposition and fish tissue concentration changes ( especially in relation to MMAPS linearity assumption and the lag time required for systems to reach steady state), EPA has conducted several detailed local scale sensitivity analyses. The results of these case studies are presented and discussed in Appendix D.

EPA recognizes both ( a) the technical challenges in predicting the change in saltwater fish mercury concentrations resulting from reductions in US power plant emissions and ( b) the relatively small contribution that US power plants make to total deposition over saltwater habitats for commercial fish ( Note, however that because of the high proportion of US fish consumption associated with saltwater fish, even a relatively low impact of US power plant emissions on those fish could produce significant benefits). Because primarily of technical challenges in translating mercury emissions reductions from US power plants into changes in saltwater fish mercury concentrations, EPA has not included this exposure pathway ( saltwater fish consumption) in the primary benefits analysis.

EPA agrees with the commentor that there is significant uncertainty associated with predicting the lag ( time) required for fish to reach new steady state mercury concentrations following reductions in mercury deposition to watersheds/ waterbodies. EPA also recognizes, that, depending on the specific aquatic system under consideration, lag times could extend over decades. Consequently, EPA has provided benefits results in the RIA reflecting a range of lag periods ( from 0 years to 100 years).

Comment:

One commenter ( OAR 2002 0056 5465) stated that even working within the narrowly framed revised benefits assessment proposed by EPA, numerous issues arose. The commenter hereby incorporated by reference the arguments elaborated in the attached articles insofar as they spoke to the questions raised by the NODA. In particular, the commenter drew EPA's attention to the discussion and sources cited by Professor O'Neill regarding the biological, chemical, and physical processes relevant to determining Hg exposure for those in the upper Great Lakes.

Response: EPA appreciates the commenters concerns. EPA addresses ecosystem response to mercury loading in Chapter 3 of the Regulatory Impact Analysis.

Comment: 9 189 One commenter ( OAR 2002 0056 5535) presented several specific issues that would benefit from such careful peer review. The commenter did not intend this list to be comprehensive, but merely include it to illustrate that the agency must address critical scientific issues prior to making changes to its deposition and watershed models or conducting other components of the proposed analyses. This overview serves to demonstrate that it would be foolish to hold up final Hg regulations while EPA further investigates certain technical issues concerning the emissions, fate, and toxicity of Hg from power plants.

Hg Speciation in Deposition Modeling. The NODA points out that HBRF said that an average speciation profile should not be used when modeling or estimating deposition. This statement was made in the context of Hg deposition modeling because using an average speciation profile for emissions would likely underestimate the amount of local, regional and continental deposition. This is because speciation drives assumptions about the residence time of Hg in the atmosphere, which controls transport and the likelihood that Hg emitted in the U. S. will deposit in outside the U. S. For example, the more Hg emitted as elemental, the higher the proportion of Hg that is transported longer distances. In particular, the EPA should incorporate new speciation data for Northeast plants. A revised speciation profile for Northeast plants is: 70 percent oxidized Hg and 30 percent elemental Hg.

Hg Maps ( MMAPS). MMAPS is a useful tool for roughly evaluating the impacts of Hg deposition on fish Hg levels. However, as the agency notes, it is still undergoing external peer review and the commenter expects the agency will need time to address peer review comments. Hindering the applicability of MMAPs is the lack of fish concentration data for many watersheds and the " averaging" of results across large watersheds. The commenter thought MMAPS was best suited to the application for which it was developed   evaluating specific watersheds where additional information on population exposure can be considered when evaluating the results of the assessment.

Response:

EPA appreciates the commenters concerns. Please see EPA ' s Office of Research and Development white paper ( see Control of Emissions from Coal Fired Electric Utility Boilers: An Update, EPA/ Office of Research and Development, March 2005) for a discussion of the importance of speciation for mercury capture. Also, please see Chapter 7 of the Regulatory Impact Analysis for a discussion of how we took speciation into account in our power sector modeling and see Chapter 8 for a discussion of how we took speciation into account in our air quality and deposition modeling. See the Regulatory Impact Analysis and the Technical Support Document: Methodology Used to Generate Deposition, Fish Tissue Methylmercury Concentrations, and Exposure for Determining Effectiveness of Utility Emission Controls in the docket for a discussion of the application of Mercury Maps assumption.

Comment:

One commenter ( OAR 2002 0056 5423) presented a figure illustrating no evidence of 9 190 increasing trend or any change in Hg of deep sea fish ( blue hake): 1970s versus 1880s fish ( Figure E1, see OAR 2002 0056 5423).

" To test for a change in mercury content in the last century, two samples of the deep sea fish named blue hake ( Antimora rostrata) were analyzed. Antimora rostrata is resident throughout the world's oceans at depths of 1000 3000 m but does not venture into depths shallower than 800 m [ actually about 200 m in the cold waters of the polar region]; therefore, this deep sea species is not exposed to local estuarine, coastal, or atmospheric inputs of mercury. A sample of 21 specimens collected in the 1880s was compared with a sample of 66 specimens collected in the 1970s in the western North Atlantic Ocean. In both recent and old fish mercury increased as a function of length, but comparison of the two concentration vs. length relationships shows that there has not been an increase in mercury concentration in deep sea fish in the last century. This result supports the idea that the relatively high concentration of mercury found in marine fish that inhabit the surface and deep waters of the open ocean result from natural processes, not 20th century industrial pollution." [ Barber et al. ( 1984) Environmental Science & Technology, vol. 18, 552 555; Barber et al. ( 1972) Science, vol. 178, 636 639]

This commenter presented a figure ( Figure E1, see OAR 2002 0056 5423) showing that although one can find a clear increase of Hg concentration in the tissue of the deep sea fish ( blue hake) caught from western Atlantic waters as the size of the fish increases, one can hardly see any significant changes in the fish tissue Hg size relation for fish samples caught in 1880s when compared to the modern samples caught in the 1970s. This research clearly suggests that Hg concentration in world ocean fish is not likely to be changed or modified by any amount of alteration of inorganic Hg sources ( either anthropogenic or natural). This is why the claim that the current EPA CAMR will lead to a measurable reduction in MeHg accumulated in world ocean fish or even fish from local U. S. lakes is factually misleading. Additional evidence and comments by the commenter follow.

Figure E2 , ( see OAR 2002 0056 5423) shows recent results by Kraepiel et. al., ( 2003, Environmental Science & Technology, vol. 37, 5551 5558) which found no clear increase in the Hg levels of Yellowfin tuna caught in 1998 relative to a similar cohort caught in 1971.

The theoretical expectation ( similar to EPA's) was that the MeHg concentration " should have increased by 9 to 26 percent" over the interval " if methylation occurred in the mixed layer or in the thermocline [ of the Pacific oceans]." The theory was not proven. The commenter further noted that Zhang et al. ( 2002, Ambio, vol. 31,482 484) has recently estimated that China's Hg emissions from coal combustion are increasing at the rate of 5 percent per year ( from available data from 1978 through 1995), which is consistent with the theoretical expectation of increase in the amount of MeHg in the waters of the Pacific Ocean if the Hg to MeHg conversion process is sensitive to industrial emissions. To the contrary, Kraepiel et al. ( 2003) clearly concluded that "[ s] uch an increase is statistically inconsistent with the constant Hg concentrations measured in 9 191 tuna. The commenter concluded tentatively that Hg methylation in the oceans occurs in deep waters or in sediments." ( p. 5551). This is why the relatively small man made sources of Hg emissions can neither overwhelm nor directly alter the natural cycling of Hg in the environment and biosphere.

Independent results shown in Figure E3 ( see OAR 2002 0056 5423) support and update the finding of Zhang et al. ( 2002) that industrial Hg emissions from China ( and India) are increasing significantly from 1990 to 2000 and that amount, both in the absolute amount and the rate of increase, dwarfed the rather small amount of industrial Hg emissions from the U. S. EPA's CAMR should seriously consider and weighing this important fact if there is to be any effective Hg emission management rulings.

Figure E4 ( see OAR 2002 0056 5423) shows additional new evidence against any increasing trend in Hg levels in fish by examining concentrations in tissue of striped bass from the San Francisco Bay area over the period 1970 2000. The study's findings also clearly show that in any given year there is at least one striped bass sample containing Hg values above EPA's consumption advisory threshold value of 0.5 ppm. Perhaps even more significant, those striped bass with Hg concentration values above 0.5 ppm had no apparent connection to power plant Hg emissions.

Figure E4 ( see OAR 2002 0056 5423) illustrates no evidence of increasing trend in Hg concentration in striped bass caught off San Francisco Bay area from 1970 2000. The commenter notes that at any given year there is at least one striped bass with Hg level above the EPA's consumption advisory threshold of 0.5 ppm since 1970 with no apparent tie to any U. S. power plant Hg emission sources. Also although no increasing trend is clear for Hg, declines were noted in the fish tissue's DDT and chlordane in the late 90s. Those declines may be related to the use curtailment of these two chemicals in the 70s and 80s.

Figure E4 ( see OAR 2002 0056 5423) reveals another important finding from this new study. Even though no accumulation trend was noted for Hg in striped bass in the 1970 2000 period, significant declines in the late 1990s were noted for other contaminants like DDT and chlordane in San Francisco Bay's fish tissues. The authors suggest that the declines may be linked to known curtailed usage of the two chemicals in the 1970s and 1980s. Thus, the combined findings suggest a more complicated and complex chain of biomethylation and bioaccumulation for Hg in fish. That is, compared to other contaminants it appears that the pathway and behavior of Hg transformation and accumulation in fish differs significantly.

A similar tendency was recently reported ( Yamaguchi et al., 2003, Chemosphere, vol. 50, 265 273) for levels of contaminants in fish from upper River Thames in Britain by a group of zoologists from Oxford University and Cornell University. These authors concluded that although the recent decrease in environmental contamination level of PCBs may be partly associated with industrial and human activities, it was difficult to find such associations for Hg.

Response: 9 192 EPA appreciates the commenter's input to the record. For several reasons described in the Regulatory Impact Analysis, EPA focused its analysis on recreationally caught freshwater fish.

Comment:

One commenter ( OAR 2002 0056 5423) presented a figure ( Figure F1, see OAR 2002 0056 5423) that confirms admission by EPA that trace levels of MeHg in fish depend on the complex physical, chemical, and biological factors within each unique ecosystem. More importantly, it evidences that despite the relatively constant level of total inorganic Hg available in all four ( 3 open water and 1 salt marshland) of the sampling sites ( the four blue bars in Figure F1) in this study, the production and concentration levels of MeHg were significantly enhanced at the biologically active and organically rich marsh wetland site ( the tallest red bar marked " marsh" in Figure F1). The authors concluded that " sediment geochemistry ( redox, sulfide, pH, organic content, etc.) is a much more important control on MeHg production than is the absolute total mercury concentration" ( p. 266 of Marvin DiPasquale et. al., 2003, Environmental Geology, vol. 43, 260 267).

The San Francisco Bay findings add to the body of evidence showing that either adding or reducing Hg atmospheric deposition from any coal fired power plant would not measurably affect MeHg levels in the San Francisco Bay ecosystems. To the contrary, MeHg levels are naturally self limited by specific ecosystem dynamics, water quality variables like dissolved sulfate, parameters like the population of algae and/ or zooplankton, availability of nutrients and/ or sunlight and so on.

Response:

EPA appreciates the commenter's input to the record. For several reasons described in the Regulatory Impact Analysis, EPA focused its analysis on recreationally caught freshwater fish.

Comment:

Generally agreeing with EPA's assessment, one commenter ( OAR 2002 0056 5423) offers an additional new and important model validation target for EPA's modeling team to factor. Figure G1 ( see OAR 2002 0056 5423) shows a very important observation concerning the accumulation of MeHg in various watershed systems. It shows a particular " MeHg accumulation paradox" in that the relative percentage of MeHg converted from raw Hg available actually decreases as the amount of raw Hg available increases. The authors of these important findings suggest that as the amount of raw Hg increases in a watershed system, there are actually more bacterial operons ( i. e., bacterial enzymes encoded by the Hg resistance ( mer) operon) available to significantly break down the MeHg produced, and thus explaining the observed " MeHg accumulation paradox." Again, the proposed EPA's CAMR needs to fully account for the underlying science before making costly and ineffectual compliance rulings. 9 193 Response:

EPA appreciates the commenter's input to the record. EPA's ecosystem modeling is described in detail in Chapter 3 of the Regulatory Impact Anaysis.

Comment:

One commenter ( OAR 2002 0056 5423) believed it is very important to emphasize that the best available science suggests repeatedly that the stated assumptions in EPA's MMaps model are likely to be wrong. Therefore, they cannot be meaningfully applied for a realistic assessment of how a change in Hg emissions from power plants can possibly affect concentrations of MeHg in fish.

First, Figure D1 ( see OAR 2002 0056 5423) provides real data that " the physical, chemical, and biological characteristics of the ecosystem( s)" never remained constant over time. That alone invalidates the critical model assumption.

Figure H1 ( see OAR 2002 0056 5423) offers another important target for validating the assumptions in EPA's MMaps model. It shows that local atmospheric deposition of Hg has negligible contribution to the annual budget of Hg to the Lake Whatcom ecosystems, thus directly challenging EPA's MMaps assumption that " air deposition is the only significant source of Hg to a water body."

No local man made Hg " pollution" at Lake Whatcom, Bellingham, WA: Annual input of Hg from local industrial sources is negligible Paulson ( 2004) Sources of mercury in sediments, water, and fish of the Lakes of Whatcom County, Washington, U. S. Geological Survey Scientific Investigations Report 2004 5084 ( August 2004)

The author of the new Lake Whatcom study further noted:

" Concerns about mercury ( Hg) contamination in Lake Whatcom, Washington, were raised in the late 1990s after a watershed protection survey reported elevated concentrations of Hg in smallmouth bass. The USGS ... cooperated to develop a study to review existing data and to collect new data that would lead to a better understanding of Hg deposition to Lake Whatcom and other lakes in Whatcom County. Of all the lakes examined, basin 1 of Lake Whatcom would have been most affected by the Hg emissions from the chlor alkali plant and the municipal sewage sludge incinerator in the City of Bellingham. The length adjusted concentrations of Hg in largemouth and smallmouth bass were not related to estimated deposition rates of Hg to the lakes from local atmospheric sources. Hg concentrations in dated sediment core samples indicate that increase in Hg sedimentation were largest during the first half of the 20th century. Increases in 9 194 Hg sedimentation were smaller after the chlor alkali plant and the incinerators began operating between 1964 and 1984. Analysis of sediments recently deposited in basin 1 of Lake Whatcom, Lake Terrell, and Lake Samish indicates a decrease in Hg sedimentation." ( p. 1 of Paulson, Sources of mercury in sediments, water, and fish of the Lakes of Whatcom County, Washington, U. S. Geological Survey Scientific Investigations Report 2004 5084, August 2004)

Response:

EPA appreciates the commenter's input to the record. EPA's ecosystem modeling is descbribed in detail in Chapter 3 of the Regulatory Impact Analysis. Also see the Technical Support Document: Methodology Used to Generate Deposition, Fish Tissue Methylmercury Concentrations, and Exposure for Determining Effectiveness of Utility Emission Controls in the docket.

Comment:

One commenter ( OAR 2002 0056 5423) believed that it should be fairly clear from the peer reviewed literature exhibited in comments ( A) through ( H) that EPA's MMaps isn't simply suffering from " limitations," but is instead terminally overwhelmed by numerous demonstrably flawed assumptions in its irrational determination to claim a reduction of Hg emissions from U. S. power plants can or will lead to a reduction in accumulation of MeHg in ocean or U. S. freshwater fish.

Figure I1 ( see OAR 2002 0056 5423) presents for EPA's modelers yet additional, recently published fish Hg data sets of various sport fish species caught from 17 " areas of concern for Hg contamination" in the Canadian Great Lakes from 1971 to 1997. The results again evidence that historical changes in Hg concentrations are not simply to be expected from local industrial Hg emissions. In fact, the author concluded that, " Differences observed [ among different areas of concern] did not consistently parallel expectations associated with historical presence of chlor alkali plants in the vicinities of some locations." More importantly, the author also noted that " An attempt to correlate the fish tissue Hg with the frequency of occurrence of infantile cerebral palsy at AOC [ areas of concern] was unsuccessful." Further Hg related health issues are addressed in comments ( K) through ( Q) in e docket text.

" The tissue mercury concentration in six species of fish collected at the 17 Areas of Concern [ AOC] were analyzed. A linear increase in Hg concentration with fish length was found, but slopes differed among locations. The temporal pattern over the period 1971 1997 differed across species in fish collected in Lake St. Clair; in at least two species there was evidence of increased Hg concentration during the 1990s that had been suggested in an earlier analysis. AOC differed significantly in observed tissue concentrations. Differences observed did not consistently parallel expectations associated with historical presence of chlor alkali plants in the vicinities of some locations. An attempt to correlate the fish tissue Hg with the frequency of occurrence of infantile cerebral palsy at AOC was unsuccessful." 9 195 Response:

EPA appreciates the commenter's input to the record. The limitations and aplication of Mercury Maps is described in detail in the RIA and Technical Support Document: Methodology Used to Generate Deposition, Fish Tissue Methylmercury Concentrations, and Exposure for Determining Effectiveness of Utility Emission Controls in the docket.

Comment:

One commenter ( OAR 2002 0056 5423) notes that this partial admission on the highly limited use of MMaps by EPA goes a long way toward illustrating the commenter's concerns regarding the scientifically questionable direction of EPA's modeling effort. Figure J1 ( see OAR 2002 0056 5423) confirms that the top 15 fish and sea foods consumed in the U. S., representing about 90 percent of the U. S. commercial market, is accounted for by marine and farm raised species. ( According to UN statistics, domestic fresh water fish may account for as little as 0.05 percent of total U. S. consumption.) This alone renders EPA's MMaps modeling results on MeHg levels in freshwater fish almost irrelevant or largely insignificant.

Considering the insignificant Hg emissions from U. S. coal fired power plants ( i. e., less than 1 percent of annual global emissions budget) and the millions of tons of natural Hg available in world oceans from deep venting, it is clear that there will be no detectable change in trace MeHg in oceanic fish even if EPA were to impose zero emission standards for all U. S. Hg sources. Evidence provided in comment ( E) ( i. e., Figures El, E2 and E4, see OAR 2002 0056 5423) alone should be adequate for our hypothesis. ( If trace levels of MeHg did not increase in a wide variety of fish along with rapidly growing worldwide anthropogenic emissions ( See figure E3), what rationale is there that fish MeHg levels would drop in response to falling U. S. emissions?) Meanwhile, the strictest burden of proof for EPA rule making demands a clear demonstration ( not invalid modeling assumptions) that its CAMR rulings can deliver a clear and meaningful reduction in MeHg in world ocean fish. Seafood Consumed in the U. S. Accounts for 90 percent of the Commercial Market. Source: Carrington and Bolger ( 2002) Risk Analysis, vol. 22,689 699 + updates in Carrington and Bolger ( 2003) ` s Intervention Analysis Draft Report.

Response:

EPA appreciates the commenter's input to the record. The limitations and aplication of Mercury Maps is described in detail in the RIA and Technical Support Document: Methodology Used to Generate Deposition, Fish Tissue Methylmercury Concentrations, and Exposure for Determining Effectiveness of Utility Emission Controls in the docket.

Comment:

Using a global model such as the GEOS CHEM model to provide the boundary conditions for a continental model ( REM SAD or CMAQ) is a more scientifically sound approach than prescribing those regional boundary conditions. It is necessary to conduct a performance 9 196 evaluation of the global model to ensure that the boundary conditions provided to the continental model are realistic. It is also desirable to have a consistent formulation of the physical and chemical processes governing the Hg species concentrations in both the global and continental models.

Response:

EPA appreciates the commenter's input to the record. For discussion of the air quality modeling, please see chapter 8 of the RIA and EPA's Technical Support Document for the Final Clean Air Mercury Rule: Air Quality Modeling.

Comment:

One commenter ( OAR 2002 0056 5535) presented several specific issues that would benefit from such careful peer review. They did not intend this list to be comprehensive, but merely included it to illustrate that the agency must address critical scientific issues prior to making changes to its deposition and watershed models or conducting other components of the proposed analyses. This overview serves to demonstrate that it would be foolish to hold up final Hg regulations while EPA further investigates certain technical issues concerning the emissions, fate, and toxicity of Hg from power plants.

Atmospheric Hg reactions. Another problem with EPRI's global Hg model is the incorporation of atmospheric Hg reactions that are very uncertain. In particular, the incorporation and effect of Hg reduction reactions needs investigation. It is also unclear whether the EPRI or EPA models fully account for the many conversion processes that can occur in the atmosphere or the conversion/ uptake of Hg by the forest canopy. Both of these processes would increase local deposition. Specifically, it is now well documented that elemental Hg is converted to oxidized Hg in the presence of ozone and chloride. This is particularly important along the eastern seaboard where both ozone and sea salt are high.

Community Multiscale Air Quality ( CMAQ) model. The commenter understood that EPA has been developing CMAQ over a number of years as a potential replacement for REMSAD. However, the commenter stated that the CMAQ is not yet in the public domain and has not been externally peer reviewed to the extent it must be prior to its adoption by EPA.

Response:

EPA appreciates the commenter's input to the record. For discussion of the air quality modeling, please see chapter 8 of the RIA and EPA's Technical Support Document for the Final Clean Air Mercury Rule: Air Quality Modeling.

Comment:

One commenter ( OAR 2002 0056 5423) agrees with EPA's call for increased 9 197 understanding about how dry Hg deposition adds to the total ecosystem deposition. Figure D1 shows a new set of Hg deposition data obtained from sedimentary cores representative of the past 11,000 years in Elk Lake in Minnesota. These data might serve as an important benchmark in wet and dry depositions under a very wide range of meteorological and climatic conditions as well as a variety of Hg sources ( for example, Hg enriched dusts and sands from nearby Nebraska sand hills or Hg from local and regional forest fires) for EPA's REMSAD and CMAQ models to demonstrate both the correctness and robustness of their atmospheric transport, chemistry and deposition modules. Measurements of Hg over the past 11,000 years in Elk Lake, MN show that today's Hg level is neither exceptional nor alarming.

Response:

EPA appreciates the commenter's input to the record. For discussion of the air quality modeling, please see chapter 8 of the RIA and EPA's Technical Support Document for the Final Clean Air Mercury Rule: Air Quality Modeling. Please see Chapter 3 of the CAMR Regulatory Impact Analysis for a detailed discussion of mercury in the environment, including an assessment of the response time for systems after a change in mercury deposition.

Comment:

One commenter ( OAR 2002 0056 5497) noted that EPA was the major co funder of a technical workshop on Mercury Monitoring and Assessment held in Pensacola, Florida, on September 14 17,2003. The Workshop was also co funded by EPRI and sponsored by the Society of Environmental Toxicology and Chemistry ( SETAC). Participants included 32 Hg scientists from academia, industry, government, and nonprofit organizations. The purpose of the workshop was to identify a variety of environmental and ecological indicators to assess trends due to changes in Hg emissions and fluxes to the environment. Participants at the workshop were also charged with the preliminary development of a network of monitoring stations with emphasis on North America in order to evaluate the effects of Hg regulatory actions being implemented by Canada and the U. S.

Two products from this workshop will be published in 2005. A paper, Monitoring the Environmental Response to Changing Atmospheric Mercury Deposition ( Mason et. al., in press), is scheduled for publication in Environmental Science and Technology in its January issue. In addition, a book, Mercury Monitoring and Assessment ( Newman et. al., in preparation), will be published by SET AC Publications later this year. This major, EPA initiated effort is not mentioned in EPA's proposed revised benefits assessment. Since the direct monitoring of Hg impacts on human, wildlife, and ecosystem health would provide the most irrefutable evidence of the success ( or lack of success) of EPA's regulatory strategy, this initiative should be given a primary role in EPA's benefits assessment.

The Relationship of Reductions in Air Deposition of Mercury to Methylmercury in Fish Tissue 9 198 EPA's benefits analysis requires that it quantify how a reduction in Hg emissions to the air translates into reductions of MeHg in fish tissue. The most important thing for EPA to recognize is that a reduction in air emissions of Hg cannot be expected to result in a reduction in fish tissue Hg in a manner that is predictable by a generally applicable rule   whether linear, sub linear, or time dependent. The entire array of physical, chemical, and biological processes that influence the conversion of elemental Hg in the air to MeHg in fish tissue is complex. Moreover, the relative importance of these factors differs on a site specific basis.

EPA proposes to use a steady state source receptor model ( Mercury Maps or MMaps) as a tool to predict the biological responses ( changes in Hg content) to reductions in emissions of Hg from coal fired utility boilers. As explained in more detail below, Mercury Maps should only be used as a preliminary screening tool whose results must be groundtruthed by subsequent, more definitive investigations.

Additionally, any screening tool must recognize that only Hg deposited directly to a water body ( or flowing overland without reacting with soil) is readily and quickly incorporated into biota. Mercury deposition to uplands does not readily migrate to waterbodies but instead becomes incorporated into the humic matter of soil. While some very small amount of this Hg in soil may eventually leach to a water body, such a process occurs over decades, so that any newly deposited Hg has an insignificant effect on the total amount of Hg that has already accumulated in soil over thousands of years. Any modeling process that EPA uses must treat deposition directly to the lake surface differently from deposition to the surrounding uplands.

The Florida Mercury Report Does Not Demonstrate that Changes in Air Emissions Result in Rapid Changes in Fish Tissue Mercury EPA should not give undue weight to a study in Florida that shows large reductions in fish tissue Hg in the aftermath of reductions in air emissions. For one thing, the Everglades, where this study was conducted, are a unique ecosystem, and results from the Everglades cannot be extrapolated to other locales.

In November 2003, the Florida Department of Environmental Protection combined two earlier Hg reports into a single document that the commenter will call the " Florida Study". This report has been widely publicized and widely misinterpreted. It does not show that reductions in air deposition will rapidly be followed by reductions in fish tissue Hg, and certainly not outside the unique environment of the Everglades.

Analyses of the Florida Study by EPRI reveal the following important facts:

° The Hg emissions released by municipal and medical waste incinerators are different from those released by power plants.

° There are two major forms of Hg in emissions   oxidized ( ionic), which is water 9 199 soluble, and elemental, which is not water soluble.

° Most incinerator Hg is in the water soluble form, whereas the form of Hg released from power plants depends upon many factors such as the type of coal being burned. Recent research has shown that most of the Hg released by utilities ( at least 60 percent) is the non water soluble elemental form and that a significant amount of the remainder converts to this non soluble form shortly after leaving the stack.

° The form of Hg emitted is critical, because oxidized Hg can be washed into local rivers, lakes, and streams by rainfall, whereas elemental Hg is carried away by wind and enters the global Hg cycle.

° The Florida Everglades Study represents a unique ecological system not typical of, and in fact strikingly different from, other US waterways. Thus, the results from this study are not necessarily applicable to other areas.

° The Everglades are in a tropical zone ( no seasons), the water is shallow, and the bottom sediments are much different from those in other water bodies throughout, the U. S. Other waterways also have different levels of acidity, biological activity" dissolved oxygen, and turbidity. These differences can dramatically affect Hg cycling and uptake by biological organisms. Thus, it is unlikely that the changes in Hg in fish, both the amount and the rate of decline, would be observed in other U. S. waters.

° The claim that changes in Hg emissions result in rapid changes in fish Hg content is not supported by the data or findings.

° The Florida Study assumes that Hg deposition in the Everglades originates only from local sources ( primarily incinerators and power plants). However, data measurements and long range transport modeling indicate otherwise.

° In fact, despite decreases in Hg emissions from incinerators, no deposition data exist spanning the reduction period, and other regional and global changes were occurring during the same period. Therefore, it is not possible to determine how much, if any, the amount of Hg being deposited in the Everglades has changed.

° Indeed, both EPA and EPRI have modeled Hg transport and concluded that over 60 percent of Hg currently deposited in Florida originates outside the U. S. Guentzel et al. ( 200 I) hypothesized that long range ( global) transport of reactive gaseous Hg ( RGM), coupled with strong thunderstorm activity during summer months, represents > 50 percent of the Hg deposition in southern Florida. 58 The authors highlight the importance of non atmospheric loading factors and global sources of Hg: 9 200 ° The contrast between uniform Hg deposition and geographical " hot spots" in fish Hg concentrations from the Everglades region further suggests that aquatic/ terrestrial Hg cycling processes, rather than atmospheric source strength, are responsible for the hot spots... . From our analysis of the data, we conclude that significant reduction in rainfall Hg deposition over the Florida Everglades will likely require reductions in local and global Hg emissions.

° In addition, not all the change in fish Hg content can be attributed to the unknown change in deposition because other factors that mediate the ecosystem response to Hg load have also changed. Several theories have been suggested involving changing nutrient levels and water flows in the Everglades. Further research is needed to understand this situation.

° The atmospheric transport model used by the State of Florida to estimate Hg deposition has limitations.

° The model does not incorporate chemical reactions in the atmosphere. It also does not include global sources of Hg, only local emissions. Thus, it cannot effectively simulate the actual Hg deposition.

° EPRI's recent research findings indicate that power plant Hg controls would not significantly change the amount of Hg contained in fish, or the human exposure to it.

° In early 2003, EPRI completed a comprehensive study of U. S. power plant Hg emissions, potential Hg controls, and responses of fish to changes in Hg in their habitat waters. The results showed that reducing Hg emissions from power plants by approximately 50 percent would result in a reduction of Hg in fish of about 1 ½ percent. This study, combining atmospheric data and models, fish consumption information from U. S. government studies, and an economic model of the U. S. utility industry, relies on more recent information than the two year old Florida study report released on November 6, 2003.

The Steady State Linear Relationship Between Air Deposition and Methylmercury Concentrations in Fish as Reflected in the Mercury Maps Model Is Not Generally Appropriate EPA requests comment on the use of the steady state linear relationship between air deposition and MeHg concentrations in fish, as reflected in the MMaps model. The MMaps model was developed by EPA's Office of Water to assess what levels of reduction in Hg deposition would be needed to achieve the fish Hg criterion of 0.3 ppm. It was intended to apply only to watersheds where the principle source of Hg is atmospheric deposition and where there is no known land based source of Hg discharge into the waterways.

The MMaps model is simplified compared to other models and for that reason can be 9 201 applied over a broader geographic scale. But it has limitations, among which are the following:

° The model uses the arithmetic average of all fish Hg data in each Hydrologic Unit Code ( HUC) watershed. This averaging may blur the substantial differences in water chemistry and fish Hg, both spatially and temporally, that are known to exist within a single watershed. An alternative approach may be to associate the fish Hg level with both the deposition loads as well as other relevant water chemistry parameters ( pH; nutrient levels; extent of anoxia in waterbody; water temperature; etc.). This may provide a better prediction of the responses in specific waterbodies as opposed to making a prediction of the fish Hg response for an entire HUC watershed. Further, the analysis should be confined to a particular species of fish, preferably with a similar size or weight range.

° The output of MMaps is highly dependent on the extent of state specific fish tissue data records. For states that have a relatively long record offish Hg levels from a variety of water body types, the accuracy of the true existing Hg levels is relatively high. For states that have monitored Hg levels in fish only recently, or sporadically, the comparison of " average" HUC fish tissue Hg to EPA's MeHg fish tissue criterion incorporates high uncertainty. EPA's summary of state fish tissue monitoring data for 1990 95 indicates considerable state to state variation in the number of water bodies sampled, the number of total collections, the species sampled for, the number of fish analyzed, and the frequency of monitoring. The report also indicates the distribution of species sampled for Hg levels. Two sport fish species ( largemouth bass and channel catfish) were sampled more extensively than any other species. Only two states ( Kansas and Washington State) do not report Hg levels for largemouth bass. EPA should consider standardizing MMaps by using Hg analysis data for two species only: largemouth bass and channel catfish. A standardization of fish length or age would further increase the accuracy of what watersheds actually may have fish that exceed the human health fish tissue criterion.

° The EPA report uses the modeling results from the Everglades TMDL to help corroborate the assumption of a linear response. However, there are two concerns, as noted above. First, the Everglades TMDL assumed all the fish changes were caused by local emission changes and did not consider other regional and global changes or any local ecosystem changes that would mediate that assumption. Thus, the actual reduction in fish Hg due to atmospheric deposition reduction in the U. S. would be less than predicted by Mmaps if the simulated changes in Hg deposition from changes in U. S. emissions do not account for global sources. Second, the intercept was considered by the TMDL researchers to be due to the fact that sediment Hg is recycled to the water column by macrophyte roots. The redistribution of Hg from sediments may be an important source to the water column in some waterbodies. Matty and Long found that, in the Great Lakes, the most important processes influencing levels of Hg in sediments ( decay of organic matter, iron and manganese redox cycling, exchange of Hg from porewater to sediments) would result in the long term retention of Hg near the sediment water interface. This Hg would be available for assimilation by benthic organisms. This source ( legacy sediment Hg) would not be influenced by cutbacks in atmospheric deposition. The TMDL report also stated 9 202 that the intercept is partly a function of the length of simulation time   the longer the time, the smaller the intercept.

° The assumptions that are critical in causing the model to exhibit a linear response to air deposition were discussed in the Everglades TMDL report but were not discussed in the EPA report. These assumptions include:

° Methylation was allowed to occur only in sediment, based on porewater Hg( II) concentration. Large stratified reservoirs and lakes with anoxic zones can have methylation in the water column. Thus, the Everglades system is not representative of all types of waterbodies.

° Geochemical factors that could influence the Hg( II) concentration in the porewater such as cinnabar formation were not included. If such factors are controlling the Hg( II) concentration, then a reduction in deposition load may not cause a linear change in Hg( II) concentration.

° Methylation/ demethylation rates were limited only by Hg substrates. Other factors such as carbon supply are important and can influence the rates. Watras et. al., studied 15 lakes in northern Wisconsin, and found relatively low concentrations of sediment MeHg. The authors suggested that a high rate of demethylation accounted for this observation. Based on recent work by Marvin DiPasquale et. al. ( 2000), demethylation rates become nonlinear at high concentrations of Hg. This latter possibility was not considered in the Everglades TMDL, although it may be less important in that system than in waterbodies with natural elevated Hg.

° The model is based on a uniform linkage between atmospheric deposition of Hg and the amount that reaches a waterbody for eventual uptake by fish. This type of analysis does not consider the effect of varying retention based on different types of land uses and the effect of canopy type on dry deposition. The actual response to the same decrease in atmospheric load would vary across watersheds because the proportion of the load reaching the waterbody would not be the same.

° The apparent linear response of fish to decreases in atmospheric loading does not seem compatible with a hypothesis of differing methylation efficiency in waterbodies. Other factors such as temperature are important in controlling fish metabolism, which then influences the response of fish to a given MeHg concentration. A more detailed comparison of fish data with wide geographic coverage to the Hg deposition data may reveal that the response is not as linear as thought.

° Variability in the fish data could be used to derive an estimate of the uncertainty in fish response to any l ad . cutbacks. The assessment of variability in an explicit form. allows policy makers to determine whether small changes in fish tissue Hg as a result of changing deposition loads will be masked by the inherent variability of the data and so may be 9 203 unmeasurable or even indiscernible. Due to multiple factors such as fish species, age, size, and changes in diet during their lifetime, fish data for a given waterbody are extremely variable.

° The fish data set could be extended to include other available data, particularly to close the data gap in several of the western states, where no geo referenced fish data were presented in Mercury Maps.

° EPA acknowledges the need for higher resolution modeling in order for predictions of fish Hg changes to have some accuracy:

° Estimates of percent air deposition reductions, by watershed, as generated from a regional air deposition model, would be needed to predict fish concentration changes.

In summary, the commenter believed that the scientific knowledge of Hg source and receptor processes should compel EPA to use MMaps as a preliminary screening tool only, subject to verification by watershed specific information. All in all, the key assumptions of MMaps ( all Hg from atmospheric sources and equilibrium conditions for Hg levels in fish and other compartments) make the predictive analysis highly sensitive to variable watershed processes, physical and chemical factors affecting methylation, and biological interactions. All of these factors are known to control, or influence, Hg cycling and bioavailability.

An Evaluation of the EPA's Assumed Linear Relationship Between Changes in Atmospheric Emissions and Resulting Fish Tissue Mercury Concentrations The Great Lakes The commenter stated that there is a considerable amount of information on Hg sources to the Great Lakes, as well as Hg deposition, rates of sediment accumulation, and measured levels in Great Lakes fish. Using this information, one can evaluate the key assumptions of the linear source receptor response by comparing the long term ( and new) information on the Hg in the Great Lakes, against the predictions inherent in the linear model.

1. Sources of Hg and deposition to the Great Lakes Cohen et. al., modeled the total annual deposition of Hg to the Great Lakes, and the relative contributions by specific sources ( industry sector, U. S. vs. Canadian sources). They estimated that the annual atmospheric deposition of Hg to Lake Michigan was highest ( 750 kg/ yr), followed by Lakes Superior, Erie, and Huron ( 425 490 kg/ yr), and Lake Ontario ( 220 kg/ yr); however, current and historic additions of Hg are much greater than atmospheric deposition in the case of Lake Superior. In an earlier publication, Shannon and Voldner reported estimates of annual deposition to the Great Lakes. Lake Erie and Lake Michigan had the highest estimated deposition ( 728 1,012 kg/ yr), followed by Lake Superior. Lake Ontario had the lowest estimated annual deposition ( 297 kg/ yr). Interestingly, the authors also found that the estimated annual rate of Hg volatilization from all lakes ( 2.3 13.7 t/ yr) was within the range of total Hg loading to all lakes 9 204 ( 4.7 t/ yr).

2. Sediment Hg levels in the Great Lakes In a recent publication, Marvin et. al., reported on long term sediment Hg levels in the Great Lakes. Based on the most recent surveys, Lake Huron had the lowest sediment Hg levels. The western basin of Lake Erie had the second highest sediment Hg levels, while Lake Ontario had the highest levels. These Hg level patterns would not be expected based on total atmospheric load ( deposition) estimates. If annual deposition of Hg alone dictated sediment Hg, the highest Hg levels would be expected in Lake Michigan, while the lowest sediment Hg levels would be expected in Lake Ontario. The authors, however, clearly pointed out the differences in Hg sources to each of the Great Lakes. While estimates indicate Lake Superior receives most of the external Hg loading from atmospheric sources, the relatively high sediment Hg levels in Lake Ontario are due to localized ( major urban) sources, and Hg loads coming from the Niagra River. Clearly, for the Great Lakes, Hg deposition information alone cannot be used to predict resulting comparative sediment levels. The authors indicate that sediment Hg levels have declined drastically across the Great Lakes since the 1960s: lakewide decreases in mean sediment levels ranged from 25 percent ( Lake Ontario) to 80 percent ( Lake Huron).

3. Mercury levels in Great Lakes fish If EPA's linear source receptor response assumption were valid, then one would expect that Hg levels in Great Lakes fish would mirror comparative deposition rates, when compared across identical species and age classes. The Canadian Department of Fisheries and Oceans summarized information on long term Hg level trends for two species sampled in all of the Great Lakes: rainbow smelt and lake trout. Mean concentrations for the years 1977 1995 were presented. The lowest Hg levels are found in lake trout from Lake Erie   ( average concentration during 1990 1995 ( 0.6 ppm); lake trout from Lake Ontario had the second lowest mean concentration ( 0.8 0.9 ppm), and lake trout from Lakes Superior and Huron had the highest mean Hg levels ( 1.2 1.3 ppm). For smelt, once again Lake Erie samples contained the least amount of Hg ( 0.2 ppm), Lake Ontario samples had the second highest level ( 0.21 0.25 ppm), and Lakes Huron and Superior fish samples had the highest Hg levels ( 0.4 and 0.55 ppm, respectively). Long term Hg concentration data for two sentinel fish species in the Great Lakes indicate, like sediment data, that atmospheric deposition information alone provides no useful information for resulting levels of Hg in lake biotic and abiotic compartments.

Factors Affecting Methylation The factors affecting methylation of Hg were summarized in a 2003 report entitled " Implementation of EPA's Methylmercury Criterion for Fish Tissue," by AMEC Earth and Environmental and ENVIRON with support from the EPRI.

Formation of MeHg in aquatic systems is influenced by a number of environmental factors. While the microbial activity and the concentration of bioavailable Hg primarily determine 9 205 methylation rates, parameters such as temperature, pH, redox potential, and the presence of inorganic and organic complexing agents playa complex, yet poorly understood, role in the methylation process. The following is a list of some technical publications that indicate the most important factors controlling Hg mobility and bioavailability:

Microbial Activity: It generally is believed that anaerobic sulfate reducing bacteria are the principal methylators of inorganic Hg in both freshwater and estuarine environments ( e. g., Gilmour et. al., 1992). Recent studies also indicate that these same bacteria also are capable of mediating MeHg degradation. Not all sulfate reducing bacteria are capable of Hg methylation, and methylation rates are not always correlated with sulfate concentration or with sulfatereduction rates. The efficiency of microbial MeHg production appears to depend chiefly on the activity and structure of the bacterial community, bioavailable Hg concentration, and the availability of nutrients and electron acceptors such as sulfate ( Choi and Bartha 1994). According to Compeau and Bartha ( 1985), the methylation potential of sulfate reducing bacteria is greatest in sulfate limiting environments, but at high sulfate concentrations, sulfide produced in respiration may inhibit methylation through the formation of HgS precipitates or charged Hg S complexes that are not readily bioavailable.

Sulfide: Several studies have reported that high sulfide concentrations inhibit MeHg formation, and an inverse relationship between sulfide concentration and MeHg production in sediments and pore waters also has been observed. On the other hand, increased MeHg production also has been observed under certain sulfide concentrations ( e. g., Craig and Moreton 1983). This suggests that, while high concentrations of sulfide can greatly reduce MeHg production, methylation is not usually completely inhibited ( Ullrich et. at., 2001). It is generally believed that the inhibitory effect of sulfide on methylation is due to the formation of insoluble HgS precipitates that are not bioavailable. However, high concentrations of dissolved Hg observed in sulfidic porewaters suggests that sulfide may actually help mobilize Hg through the formation of soluble Hg sulfide complexes. Sulfide may also affect methylation through the formation of neutral HgS0 species that can diffuse readily through cell membranes ( Benoit et. al., 1999). On the other hand, formation of charged polysulfide complexes actually can decrease bioavailability, but its effect on methylation is not clear. As the primary pathway for methylation is by sulfate reducing bacteria, more research is needed to identify the role of various sulfur species and other parameters on MeHg formation.

Temperature: Several studies have indicated that maximum methylation activity occurs during mid or late summer ( e. g., Watras et. al., 1995). Other studies have found higher MeHg concentrations in spring than in summer. While increased temperature can contribute to increased microbial activity, it also affects seasonal changes in productivity/ nutrient supply, redox conditions, and demethylation rates.

pH: There has been concern that low pH values may lead to increases in the production and/ or bioaccumulation of MeHg because elevated Hg levels have been observed from fish in acidified lakes. Enhanced methylation has been observed in low pH waters and sediments; however, this process is dependent on the redox state of the system ( in anaerobic systems, acidic 9 206 pH lowers MeHg production) and other factors. pH may indirectly affect methylation by altering the mobility and partitioning of Hg and MeHg in soils, stimulating MeHg production through the addition of sulfate ( in acid rain) and by changing microbial activity ( particularly the 49 sulfatereducing species) or cellular uptake of Hg+ 2. Changes in pH also can alter Hg speciation ( e. g., enhanced production of elemental Hg, altering the binding of Hg to organic matter and other ligands), which in turn can affect the amount of ionic Hg available for microbial methylation. Demethylation rates are also pH sensitive, albeit to a lesser extent than methylation rates.

Organic Matter: The role of organic matter in methylation also is very complex and poorly understood. Observed increases in MeHg concentrations with higher dissolved organic carbon ( DOC) concentrations have been attributed to a stimulating effect of organic nutrients on microbial methylation activity ( i. e., microbes utilizing organic matter as energy source when sulfate is limiting). Direct abiotic methylation of Hg by humic and fulvic acids ( the refractory portions of dissolved organic matter) also could be very important, particularly in wetlands where high generation of MeHg has been observed. This mechanism largely has been ignored and, to date, it is not clear to what extent abiotic methylation contributes to MeHg production in organicrich sediments and lake waters. It may be hypothesized that where organic matter is labile and readily biodegradable, it may promote methylation by stimulating microbial growth, and where the organic matter is recalcitrant and consists of high molecular weight humic and fulvic acids, it may contribute to abiotic methylation.

Decreased methylation has also been observed at high concentrations of organic matter in both natural systems and experimental studies, and it has been suggested that DOC may strongly bind with inorganic Hg at sulfur containing functional groups, rendering them unavailable for bacterial methylation. Even if MeHg forms, it may be complexed by DOC and, therefore, not available for bioaccumulation. DOC also can compete with sulfide for Hg binding and favor the mobilization of Hg through the formation of Hg DOC complexes. In Hg binding with DOC, pH may play an important role where protons compete with metal binding sites in organic matter. Humic substances, which are recalcitrant, high molecular weight fractions of organic matter, also can reduce Hg+ 2 to the volatile Hg0 species, both directly as well as by enhancing the reduction rates in photochemical reactions, thus reducing the Hg burden available for methylation.

Redox Conditions: Even though Hg methylation occurs in both aerobic and anaerobic conditions in the natural environment, methylation rates are highest in anoxic sediments and waters, and the stability of MeHg is greatest in anaerobic environments. This may be due to the reduced activity of sulfate reducing bacteria under aerobic conditions and the enhanced degradation of MeHg in aerobic conditions. It appears that anaerobic methylation is predominantly microbial in nature and, therefore, enhanced by the presence of organic matter; whereas abiotic methylation is favored under aerobic conditions and is suppressed by the presence of organic matter ( possibly due to complexation with organic matter rendering Hg unavailable for methylation). Methylmercury concentrations usually are highest in the moderately anaerobic surface sediments ( mostly at the oxic anoxic interface) and rapidly decline with depth. Likewise, in stratified lakes and estuaries, MeHg concentrations are usually highest at the oxic/ anoxic boundary layer. Changes in redox conditions in water column and sediment layers also result in 9 207 seasonal variations in MeHg concentrations. Organic matter, nutrients, pH, and sulfides significantly influence the redox effects on MeHg production.

Salinity: The methylating activity in marine and estuarine sediments is usually lower than in freshwater sediments, partly due to salinity effects. The negative effect appears to be a result of formation of charged sulfide complexes ( from sulfate in sea salt) in seawater and charged Hgchloride complexes such as HgCl 4  2 that limit the methylation process. Thus, estuarine fish tend to have lower MeHg in their tissue than comparable species in freshwater fish ( Gilmour and Riedel 2000).

In summary, Hg methylation is primarily a microbially mediated process, and the precise mechanism of MeHg formation still is unclear. Mercury methylation and demethylation rates in aquatic systems are influenced by both the speciation and biochemical availability of Hg and by a large number of interrelated environmental variables, such as biological activity, nutrient availability, pH, temperature, redox potential, and inorganic and organic complexing agents. The importance of each of these parameters and their complex interactions varies across different ecosystems and even within the same type of water bodies. Different mechanisms of methylation may occur in sediments and in water. Seasonal variations in MeHg production appear to be related to temperature, redox effects, seasonal changes in nutrient availability and Hg availability. Sulfur speciation and dissolved organic matter complexation are other important factors that are not well understood.

Knowledge Gaps Despite the vast body of literature on the subject ( 348 publications cited in Ullrich et. al., 2001), we still are unable to predict Hg methylation rates or the likely effects of environmental perturbations on methylation processes in natural systems due to the complexity of the systems described above. Since laboratory studies look at simple systems with few variables at a time, it also is difficult to directly compare the results of the laboratory studies published to date with the processes and rates in the natural environment. Knowledge gaps exist in the following areas:

Biotic vs. Abiotic Methylation: While it widely is believed that Hg methylation is biologically mediated, review of literature by Ullrich et. al. ( 2001) suggests that there may be more than one mechanism of MeHg formation. Abiotic methylation, particularly that mediated by humic substances, could be very important in wetlands and other ecosystems, but the significance of such processes in natural environments is unknown. Methylation vs. Demethylation: A portion of MeHg generated is demethylated by microorganisms, photochemical reactions, and other processes. Sulfate reducing bacteria, which were considered to be important methylating agents, also now are considered to be active demethylators. It is not clear what environmental conditions cause these microbes to carry out methylation instead of demethylation.

Biomethylation: Review of the literature suggests that methylation can be caused by sulfate reducing bacteria as well as a number of other types of bacteria that have not yet been identified. In the case of methylation by sulfate reducing bacteria, the optimum sulfate 9 208 concentrations required for methylation vary widely between different ecosystems and are difficult to predict. For example, bacteria in estuarine systems can methylate Hg at much higher sulfate concentrations than in freshwater systems. In addition, since bacteria that methylate Hg also are capable of demethylating, we are unable to predict biomethylation rates in natural systems.

Role of Organic Matter: Natural organic matter in soils, sediments, and water affect methylation in several ways. While natural organic matter can provide a stimulating effect on bacterial methylation in some systems, it may promote abiotic methylation in other systems or inhibit Hg methylation ( due to strong complexation) under other environmental conditions. The exact role of organic matter in a given system often is ignored in predicting methylation rates. Because of the complex structure and composition of organic matter and due to the paucity of thermodynamic data for organic matter   Hg complexes, the role of organic matter on the speciation and bioavailability of Hg has not been well described or modeled.

Sulfur Chemistry: Sulfur speciation is an important variable in the methylation process. In addition to the role of sulfate on the methylation process, reduced sulfur can complex with Hg and form charged or uncharged Hg sulfide complexes determining whether or not Hg becomes available to the microbes for methylation. Various stoichiometries of Hg sulfide complexes have only been speculated, and competitive reactions between sulfide, organic matter, and Hg are not well defined.

Synergistic and Antagonistic Effects: From previous discussions it is apparent that each of the variables discussed above have multiple influences on the methylation and demethylation process. For example, low concentrations of sulfate can limit microbial methylation, while high concentrations of sulfate can result in the formation of excess sulfide concentrations that complex with Hg and inhibit Hg methylation. Some of the above parameters can alter the effect of other influencing factors on Hg methylation. For example, pH and redox can directly affect methylation and bioaccumulation as well as altering Hg speciation, sulfur chemistry, and microbial activity. Due to the complex role of anyone of the above parameters on methylation, it is difficult to predict their combined effect in natural systems with existing models. There are many factors affecting methylation, but the current science is not adequate to resolve which factors are most important and allow models to move towards a more predictive capability. Research into factors affecting methylation is ongoing, however, with significant progress expected over the next several years.