### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

1 **ENVIRONMENTAL PROTECTION AGENCY** 1 **40 CFR Part 50** 2 **[EPA -**  **HQ -**  **OAR -**  **2007 -**  **1145]** 3 **RIN: 2060 -**  **AO72** 4 **Secondary National Ambient Air Quality Standards**  **for Oxides of Nitrogen and Sulfur** 5 6 **AGENCY:** Environmental Protection Agency (EPA). 7 **ACTION:** Proposed rule. 8 **SUMMARY:** Based on its review of the air quality criteria and the secondary national ambient 9 air quality

standards (NAAQS) for oxides of nitrogen and oxides of sulfur, EPA proposes to 10 retain the current nitrogen dioxide (NO 2 ) and sulfur dioxide (SO 2 ) secondary standards to 11 provide requisite protection for the direct effects on vegetation resulting from exposure to 12 gaseous oxides of nitrogen and sulfur in the ambient air. With regard to protection from the 13 deposition of oxides of nitrogen and sulfur to sensitive aquatic a nd terrestrial ecosy stems, 14 including acidification and nutrient enrichment effects, EPA is proposing to add secondary 15 standards identical to the NO 2 and SO 2 primary 1

- hour standards and not to set a new multi

16 pollutant secondary standard in this review

. In addition , EPA has decided to undertake a field 17 pilot program to gather and anal yz e additional relevant data so as to enhance the Agency ’s 18 understanding of the degree of protectiveness that a new multi

- pollutant approach , defined in 19 term s of an aquatic acidification index (AAI), would afford and to support development of an 20 appropriate monitoring network for such a standard. The EPA solicits comment on the 21 framework of such a standard and on the design of the field pilot program. 22

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

2 **DATES:** Written comments on this proposed rule must be received b y **[insert date 60**  **days** 1 **after date of publication in the Federal Register]**

. 2 Public Hearings : The EPA intends to hold a public hearing around the end of August to 3 earl y September and will announce in a separate Federal Register notice the date , time, and 4 address of the public hearing on this proposed rule. 5 **ADDRESSES:** Submit your comments, identified by Docket ID No. EPA

- HQ

- OAR

- 2007

- 6 1145, by

one of the following methods: 7

 www.regulations.gov : Follow the on

- line instructions for submitting comments. 8

 Email: a - and

- r - Docket@epa.gov

. 9

 Fax: 202

- 566

- 1741. 10

 Mail: Docket No. EPA

- HQ

- OAR

- 2007

- 1145, Environmental Protection Agen cy , Mail 11 code 6102T, 1200 Pennsylvania Ave., NW., Washington, DC 20460. Please include a 12 total of two copies. 13

 Hand Delivery : Docket No. EPA

- HQ

- OAR

- 2007

- 1145, Environmental Protection 14 Agency , EPA West, Room 3334, 1301 Constitution Ave., NW, Washington, D C. Such 15 deliveries are onl y accepted during the Docket’s normal hours of operation, and special 16 arrangements should be made for deliveries of boxed information. 17 Instructions: Direct your comments to Docket ID No. EPA

- HQ

- OAR

- 2007

- 1145. The 18 EPA’s policy

i s that all comments received will be included in the public docket without change 19 and may

be made available online at www.regulations.gov , including an y personal information 20 provided, unless the comment includes inf ormation claimed to be Confidential Business 21 In formation (CBI) or other information whose disclosure is restricted b y statute. Do not submit 22 information that you consider to be CBI or otherwise protected through www .regulations.gov or 23

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

3 email. The www.regulations.gov website is an “anony mous access” sy stem, which means EPA 1 will not know your identity or contact information unless you provide it in the body of your 2 comment. If you send an email comment directl y to EPA without going through 3 www.regulations.gov , your email address will be automatically captured and included as part of 4 the comment that is placed in the public docket and mad e available on the Internet. If you 5 submit an electronic comment, EPA recommends that you include your name and other contact 6 information in the body of your comment and with any disk or CD

- ROM you submit. I f EPA 7 cannot read your comment due to technical difficulties and cannot contact you for clarification, 8 EPA may

not be able to consider your comment. Electronic files should avoid the use of special 9 characters, an y form of encry ption, and be free of any

defects or viruses. For additional 10 information a

bout EPA’s public docket, visit the EPA Docket Center homepage at 11 http://www.epa.gov/epahome/dockets.htm

. 12 Docket : All documents in the docket are listed in the www.regulations.gov index. 13 Although listed in the index, some information is not publicly available, e.g., CBI or other 14 information whose disclosure is restricted b y statute. Certain other material, such as cop yri ghted 15 material, will be publicly availab le onl y in hard copy . Publicly available docket materials are 16 available either electronically in www.regulations.gov or in hard cop y at the Air and Radiation 17 Docket and Information Center, EPA/DC, EPA West, Room 33 34, 1301 Constitution Ave., NW, 18 Washington, DC. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday 19 through Friday , excluding legal holiday s. The telephone number for the Public Reading Room is 20 (202) 566

- 1744 and the telephone number for the Air and Radiation Docket and Information 21 Center is (202) 566

- 1742. 22

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

4 **FOR FURTHER INFORMATION CONTACT:** Dr. Richard Scheffe, Office of Air Quality 1 Planning and Standards, U.S. Environmental Protection Agency , Mail code C304

- 02, Research 2 Triangle Park, NC 27711; telephone: 919

- 541

- 4650; fax: 919

- 541

- 2357; email: 3 scheffe.rich@epa.gov. 4 **SUPPLEMENTARY INFORMATION:** 5 **General Information** 6 What Should IConsider as I Prepare M y Comments for EPA? 7 1. Submitting CBI. Do not submit this information to EPA through www.regulations.gov or 8 email. Clearl y mark the part or all of the information that you claim to be CBI. For CBI 9 information in a disk or CD ROM that you mail to EPA, mark the outside of the disk or 10 CD ROM as CBI and th en identify electronically

within the disk or CD ROM the specific 11 information that is claimed as CBI. In addition to one complete version of the comment 12 that includes information claimed as CBI, a cop y of the comment that does not contain 13 the information claimed as CBI must be submitted for inclusion in the public docket. 14 In formation so marked will not be disclosed except in accordance with procedures set 15 forth in 40 CFR part 2. 16 2. Tips for Preparing Your Comments . When submitting comments, remember to: 17

 Id e ntify

the rulemaking by

docket number and other identifying information (subject 18 heading, Federal Register date and page number). 19

 Follow directions

– The Agency may ask you to respond to specific questions or 20 organize comments by referencing a Code of Fed eral Regulations (CFR) part or section 21 number. 22

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

5  Explain why

you agree or disagree, suggest alternatives, and substitute language for your 1 requested changes. 2

 Describe an y assumptions and provide an y technical information and/or data that you 3 used. 4

 If you est imate potential costs or burdens, explain how you arrived at your estimate in 5 sufficient detail to allow for it to be reproduced. 6

 Provide specific examples to illustrate your concerns, and suggest alternatives. 7

 Explain your views as clearl y as possible

. Make sure to submit your comments by the 8 comment period deadline identified. 9 Availability

of Related Information 10 A number of documents relevant to this rulemaking are available on EPA web sites. The 11 Integrated Science Assessment for Oxides of Nitrogen and Sulfur

- Ecological Criteria: FINAL 12 REPORT (I SA) is available on EPAs National Center for Environmental Assessment web site. 13 To obtain this document, go to http://www .epa.gov/ncea , and click on Air Quality then click on 14 Oxides of Nitrogen and Sulfur. The Policy Assessment, Risk and Exposure Assessment (REA) , 15 and other related technical documents are available on EPA’s Office of Air Quality Planning and 16 Standards (OAQ PS) Technology Transfer Network (TTN) web site. The Policy Assessment is 17 available at http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_pa.html, and the exposure and 18 risk assessments and other related technical documents are available at 19 http://www.epa.gov/ttn/naaqs/standards/no2so2sec/cr_rea.html

. These and other related 20 documents are also available for inspection and copying in the EPA docket identified above. 21 **Table of Contents** 22 The following topics are discussed in this preamble: 23

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

6 I. Background 1 A. Legislative Requirements 2 B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides 3 1. NAAQS for Oxides of Nitrogen 4 2. NAAQS for Oxides of Sulfur 5 C. History of Related Assessments and Agency Actions 6 D. History of the Current Review 7 E. Scope of the Current Review 8 II. Rationale for Proposed Decision on the Adequacy of the Current Secondary Standards 9 A. Ecological Effects 10 1. Effects Associated with Gas

- Phase Oxides of Nitrogen and Sulfur 11 a. Nature of ecos ys tem responses to gas
- phase nitrogen and sulfur 12 b. Magnitude of ecos ys tem response to gas
- phase nitrogen and sulfur 13 2. Acidification Effects Associated with Deposition of Oxides of N itrogen 14 and Sulfur 15 a. Nature of Acidification

–relate d Ecosy stem Responses 16 i. Aquatic ecos ys tems 17 ii. Terrestrial ecos ys tems 18 iii. Ecosy stem sensitivity 19 b. Magnitude of Acidification

- related Ecos ys tem Responses 20 i. Aquatic acidification 21 ii. Terrestrial acidification 22 c. Key Uncertainties Associated with Acidification 23 i. Aquatic acidificati on 24 ii. Terrestrial acidification 25 3. Nutrient Enrichment Effects Associated with Deposition of Oxides of 26 Nitrogen 27 a. Nature of Nutrient Enrichment

- related Ecos ys tem Responses 28 i. Aquatic ecos ys tems 29 ii. Terrestrial ecos ys tems 30 iii. Ecosy stem sensitiv ity to nutrient enrichment 31 b. Magnitude of Nutrient Enrichment

- related Ecos ys tem Responses 32 i. Aquatic ecos ys tems 33 ii. Terrestrial ecos ys tems 34 c. Key Uncertainties Associated with Nutrient Enrichment 35 i. Aquatic ecos ys tems 36 ii. Terrestrial ecos ys tems 37 4. Other Ecological Effects 38 B. Risk and Exposure Assessment 39 1. Overview of Risk and Exposure Assessment 40 2. Key Findings 41 a. Air quality anal ys es 42 b. Deposition

- related aquatic acidification 43 c. Deposition

- related terrestrial acidification 44 d. Deposition

- related aquatic nutrient enrichment 45 e. Deposition

- related terrestrial nutrient enrichment 46

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

7 f. Additional effects 1 3. Conclusions on Effects 2 C. Adversity of Effects to Public Welfare 3 1. Ecosy stem Services 4 2. Effects on Ecosy stem Services 5 a. Aquatic Acidification 6 b. Terrestrial Acidification 7 c. Nutrient Enrichment 8 3. Summary 9 D. Adequacy of the Current S tandards 10 1. Adequacy of the Current Standards for Direct Effects 11 2. Appropriateness and Adequacy of the Current Standards for Deposition

12 related Effects 13 a. Appropriateness 14 b. Adequacy of Protection 15 i. Aquatic Acidification 16 ii. Terrestrial acidification 17 iii. Terrestrial nutrient enrichment 18 iv. Aquatic nutrient e nrichment 19 v. Other effects 20 3. CASAC Views 21 4. Administrator’s Proposed Conclusions Concerning Adequacy of Current 22 Standard 23 III. Rationale for Proposed Decision on Alternative Multi

- pollutant Approach to Secondary 24 Standard s for Aquatic Acidific ation 25 A. Ambient Air Indicators 26 1. Oxides of Sulfur 27 2. Oxides of Nitrogen 28 B. Form 29 1. Ecological Indicator 30 2. Linking ANC to Deposition 31 3. Linking Deposition to Ambient Air Indicators 32 4. Aquatic Acidification Index 33 5. Spatial Aggregation 34 a. Ecoregion Sensitivity 35 b. Representative Ecoregion

- specific Factors 36 i. Factor F1 37 a) Acid

- sensitive Ecoregions 38 b) Non
- acid sensitive Ecoregions 39 ii. Factor F2 40 iii. Factors F3 and F4 41 c. Factors in Data

- limited Ecoregions 42 d. Application to Hawaii, Alaska, and the U.S. Territories 43 6. Summary of the AAI Form 44 C. Averaging Time 45 D. Level 46

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

8 1. Associat ion Between pH Levels and Target ANC Levels 1 2. ANC Levels Related to Effects on Aquatic Ecos ys tems 2 3. Consideration of Episodic Acidity 3 4. Consideration of Ecos ys tem Response Time 4 5. Prior Examples of Target ANC Levels 5 6. Consideration of Public Welfare Benefits 6 7. Summary of Alternative Levels 7 E. Combined Alternative Levels and Forms 8 F. Characterization of Uncertainties 9 1. Overview of Uncertainty 10 2. Uncertainties Associated with Data Gaps 11 3. Uncertainties in Modeled Processes 12 G. CASAC Advice 13 H. Administrator’s Proposed Conclusions 14 IV
. Field Pi lot Program and Ambient Monitoring 15 A. Field Pilot Program 16 a. Objectives 17 b. Overview of Field Pilot Program 18 c. Complementary Measurements 19 d. Complementary Research Efforts 20 e. Implementation Challenges 21 f. Final Monitoring Plan Development and Stakeholder Participation 22 B. Evaluatio n of Monitoring Methods 23 1. Potential FRMs for SO 2 and p

- SO 4 24 2. Potential FRM for NO y 25 V. Statutory

and Executive Order Reviews 26 A. Executive Order 12866: Regulatory Planning and Review 27 B. Paperwork Reduction Act 28 C. Regulatory Flexibility

Act 29 D. Unfunded Mandates Reform Act 30 E. Exe cutive Order 13132: Federalism 31 F. Executive Order 13175: Consultation and Coordination with Indian Tribal 32 Governments 33 G. Executive Order 13045: Protection of Children From Environmental Health and 34 Safety Risks 35 H. Executive Order 13211: Actions that Significantl y Affect Energy Suppl y, 36 Distribution, or Use 37 I. National Technology Transfer and Advancement Act 38 J. Executive Order 12898: Federal Actions to Address Environmental Justice in 39 Minority Populations and L ow - Income Populations 40 References 41 42

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

9 **I. Background**  1 A. Legislati ve Requirements 2 Two sections of the Clean Air Act (CAA) govern the establishment and revision of the 3 NAAQS. Section 108 (42 U.S.C. section 7408) directs the Administrator to identify and list 4 certain air pollutants and then to issue air quality criteria for those pollutants. The Administrator 5 is to list those air pollutants that in her “judgment, cause or contribute to air pollution which may 6 reasonabl y be anticipated to endanger public health or welfare;” “the presence of which in the 7 ambient air result s from numerous or diverse mobile or stationary sources;” and “for which . . . 8 [the Administrator] plans to issue air quality criteria...” Air quality criteria are intended to 9 “accuratel y reflect the latest scientific knowledge useful in indicating the kind and extent of all 10 identifiable effects on public health or welfare which may be expected from the presence of [a] 11 pollutant in the ambient air . . .” 42 U.S.C. § 7408(b). Section 109 (42 U.S.C. 7409) directs the 12 Administrator to propose and promulgate “primary” and “secondary” NAAQS for pollutants for 13 which air quality criteria are issued. Section 109(b)(1) defines a primary standard as one “the 14 attainment and maintenance of which in the judgment of the Administrator, based on such 15 criteria and allowing an adequate margin of safety , are requisite to protect the public health.” 1 A 16 secondary

standard, as defined in section 109(b)(2), must “specify a level of air quality the 17 attainment and maintenance of which, in the judgment of the Administrator, based o n such 18 criteria, is requisite to protect the public welfare from any known or anticipated adverse effects 19 associated with the presence of [the] pollutant in the ambient air.” Welfare effects as defined in 20 section 302(h) (42 U.S.C. § 7602(h)) include, but are not limited to, “effects on soils, water, 21

1 The legislative history of section 109 indicates that a primary standard is to be set at “the maximum permissible am bient air level . . . which will protect the health of any [sensitive] group of the population ,” and that for this purpose “reference should be made to a representative sample of persons comprising the sensitive group rather than to a single person in such a group” S. Rep. No. 91

- 1196, 91st Cong., 2d Sess. 10 (1970).

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

10 crops, vegetation, man

- made materials, animals, wildlife, weather, visibility and climate, damage 1 to and deterioration of property , and hazards to transportation, as well as effects on economic 2 values and on per sonal comfort and well

- being.” 3 In setting standards that are “requisite” to protect public health and welfare, as provided 4 in section 109(b), EPA’s task is to establish standards that are neither more nor less stringent 5 than necessary

for these purposes. In so doing, EPA may not consider the costs of implementing 6 the standards. See generally , Whitman v. American Trucking Associations, 531 U.S. 457, 465

- 7 472, 475

- 76 (2001). Likewise, “[a]ttainability and technological feasibility are not relevant 8 considerations in the promulgation of national ambient air quality standards.” American 9 Petroleum Institute v. Costle, 665 F. 2d at 1185. Section 109(d)(1) requires that “not later than 10 December 31, 1980, and at 5

- ye ar intervals thereafter, the Administrator shal l complete a 11 thorough review of the criteria published under section 108 and the national ambient air quality 12 standards . . . and shall make such revisions in such criteria and standards and promulgate such 13 new standards as may be appropriate . . . .” Sec tion 109(d)(2) requires that an independent 14 scientific review committee “shall complete a review of the criteria . . . and the national primary 15 and secondary

ambient air quality

standards . . . and shall recommend to the Administrator any 16 new . . . standards and revisions of existing criteria and standards as may be appropriate . . . .” 17 Since the earl y 1980's, this independent review function has been performed by the Clean Air 18 Scientific Advisory Committee (CASAC). 19 B. History of Reviews of NAAQS for Nitrogen Oxides and Sulfur Oxides 20 1. NAAQS for Oxides of Nitrogen 21 After reviewing the relevant science on the public health and welfare effects associated 22 with oxides of nitrogen, EPA promulgated identical primary and secondary NAAQS for NO2 in 23

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

11 April 1971. Th ese standards were set at a level of 0.053 parts per million (ppm) as an annual 1 average (36 FR 8186). In 1982, EPA published Air Quality Criteria for Oxides of Nitrogen (US 2 EPA, 1982), which updated the scientific criteria upon which the initial standards were based. In 3 February

1984 EPA proposed to retain these standards (49 FR 6866). After taking into account 4 public comments, EPA published the final decision to retain these standards in June 1985 (50 FR 5 25532). 6 The EPA began the most recent previous review of the oxides of nitrogen secondary 7 standards in 1987. In November 1991 EPA released an updated draft air quality criteria 8 document (AQCD ) for CASAC and public review and comment (56 FR 59285), which provided 9 a comprehensive assessment of the availa ble scientific and technical information on health and 10 welfare effects associated with NO2 and other oxides of nitrogen. The CASAC reviewed the 11 draft document at a meeting held on July 1, 1993 and concluded in a closure letter to the 12 Administrator that th e document “provides a scientifically balanced and defensible summary of 13 current knowledge of the effects of this pollutant and provides an adequate basis for EPA to 14 make a decision as to the appropriate NAAQS for NO2” (Wolff, 1993). The Air Quality Crite ria 15 for Oxides of Nitrogen was then finalized (US EPA, 199 5 a). EPA’s OAQPS also prepared a 16 Staff Paper that summarized and integrated the key studies and scientific evidence contained in 17 the revised AQCD for oxides of nitrogen and identified the critical elements to be considered in 18 the review of the NO2 NAAQS. CASAC reviewed two drafts of the Staff Paper and concluded 19 in a closure letter to the Administrator that the document provided a “scientifically adequate 20 basis for regulatory decisions on nitrogen dioxide” (Wolff, 1995). 21 In October 1995 the Administrator announced her proposed decision not to revise either 22 the primary

or secondary NAAQS for NO2 (60 FR 52874; October 11, 1995). A year later, the 23

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

12 Administrator made a final determination not to revis e the NAAQS for NO2 after careful 1 evaluation of the comments received on the proposal (61 FR 52852; October 8, 1996). While the 2 primary

NO2 standard was revised in January 2010 by supplementing the existing annual 3 standard with the establishment of a new 1

- hour standard (75 FR 6474), the secondary NAAQS 4 for NO2 remains 0.053 ppm (100 micrograms per cubic meter [μg/m3] of air), annual arithmetic 5 average, calculated as the arithmetic mean of the 1

- hour NO2 concentrations. 6 2. NAAQS for Oxides of Sulfur 7 EPA p romulgated primary and secondary NAAQS for SO2 in April 1971 (36 FR 8186). 8 The secondary

standards included a standard set at 0.02 ppm, annual arithmetic mean, and a 3

- 9 hour average standard set at 0.5 ppm, not to be exceeded more than once per year. Thes e 10 secondary

standards were established solel y on the basis of evidence of adverse effects on 11 vegetation. In 1973, revisions made to Chapter 5 (“Effects of Sulfur Oxide in the Atmosphere on 12 Vegetation”) of Air Quality Criteria for Sulfur Oxides (US EPA, 19 73) indicated that it could not 13 properl y be concluded that the vegetation injury reported resulted from the average SO2 14 exposure over the growing season, rather than from short

- term peak concentrations. Therefore, 15 EPA proposed (38 FR 11355) and then final ized (38 FR 25678) a revocation of the annual mean 16 secondary

standard. At that time, EPA was aware that then

- current concentrations of oxides of 17 sulfur in the ambient air had other public welfare effects, including effects on materials, 18 visibility , soils, and water. However, the available data were considered insufficient to establish 19 a quantitative relationship between specific ambient concentrations of oxides of sulfur and such 20 public welfare effects (38 FR 25679). 21 In 1979, EPA announced that it was rev ising the AQCD for oxides of sulfur concurrentl y 22 with that for particulate matter (PM) and would produce a combined PM and oxides of sulfur 23

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

13 criteria document. Following its review of a draft revised criteria document in August 1980, 1 CASAC concluded that a cid deposition was a topic of extreme scientific complexity because of 2 the difficulty

in establishing firm quantitative relationships among (1) emissions of relevant 3 pollutants (e.g., SO2 and oxides of nitrogen), (2) formation of acidic wet and dry deposit ion 4 products, and (3) effects on terrestrial and aquatic ecos ys tems. CASAC also noted that acid 5 deposition involves, at a minimum, several different criteria pollutants: oxides of sulfur, oxides 6 of nitrogen, and the fine particulate fraction of suspended particles. CASAC felt that any 7 document on this subject should address both wet and dry deposition, since dry

deposition was 8 believed to account for a substantial portion of the total acid deposition problem. 9 For these reasons, CASAC recommended that a separate, comprehensive document on 10 acid deposition be prepared prior to any consideration of using the NAAQS as a regulatory 11 mechanism for the control of acid deposition. CASAC also suggested that a discussion of acid 12 deposition be included in the AQCDs for oxides of nitrogen and PM and oxides of sulfur. 13 Following CASAC closure on the AQCD for oxides of sulfur in December 1981, EPA’s OAQPS 14 published a Staff Paper in November 1982, although the paper did not directly assess the issue of 15 acid deposition. Instead, EPA subsequently prepared the following documents to address acid 16 deposition: The Acidic Deposition Phenomenon and Its Effects: Critical Assessment Review 17 Papers, Volumes I and II (US EPA, 1984a, b) and The Acidic Deposition Phenomenon and Its 18 Ef fects: Critical Assessment Document (US EPA, 1985) (53 FR 14935

- 14936). These 19 documents, though they were not considered criteria documents and did not undergo CASAC 20 review, represented the most comprehensive summary of scientific information relevant to acid 21 deposition completed by EPA at that point. 22

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

14 In April 1988 (53 FR 14926), EPA proposed not to revise the existing primary and 1 secondary

standards for SO 2 . This proposed decision with regard to the secondary SO 2 NAAQS 2 was due to the Administrator’s co nclusions that (1) based upon the then

- current scientific 3 understanding of the acid deposition problem, it would be premature and unwise to prescribe any 4 regulatory

control program at that time and (2) when the fundamental scientific uncertainties had 5 been decreased through ongoing research efforts, EPA would draft and support an appropriate set 6 of control measures. Although EPA revised the primary SO 2 standard in June 2010 by 7 establishing a new 1

- hour standard and revoking the existing 24

- hour and annual standards (75 8 FR 35520), no further decisions on the secondary SO 2 standard have been published. 9 C. History of Related Assessments and Agency Actions 10 In 1980, the Congress created the National Acid Precipitation Assessment Program 11 (NAPAP) in response to growing concern about acidic deposition. The NAPAP was given a 12 broad 10

- ye ar mandate to examine the causes and effects of acidic deposition and to explore 13 alternative control options to alleviate acidic deposition and its effects. During the course of the 14 program, the NAPAP issued a series of publicl y available interim reports prior to the completion 15 of a final report in 1990 (NAPAP, 1990). 16 In spite of the complexities and significant remaining uncertainties associated with the 17 acid deposition problem, it soon became clear that a program to address acid deposition was 18 needed. The Clean Air Act Amendments of 1990 included numerous separate provisions related 19 to the acid deposition problem. The primary and most important of the provisions, the 20 amendments to Title IV of the Act, established the Acid Rain Program to reduce emissions of 21 SO 2 by

10 million tons and emissions of nitrogen oxides by 2 million tons from 1980 emission 22 levels in order to achieve reductions over broad geographic regions. In this pro vision, Congress 23

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

15 included a statement of findings that led them to take action, concluding that (1) the presence of 1 acid compounds and their precursors in the atmosphere and in deposition from the atmosphere 2 represents a threat to natural resources, ecos ys tems, materials, visibility , and public health; (2) 3 the problem of acid deposition is of national and international significance; and (3) current and 4 future generations of Americans will be adversely affected by

delaying measures to remedy the 5 problem. 6 S econd, Congress authorized the continuation of the NAPAP in order to assure that the 7 research and monitoring efforts already undertaken would continue to be coordinated and would 8 provide the basis for an impartial assessment of the effectiveness of the Title IV program. 9 Third, Congress considered that further action might be necessary in the long term to 10 address an y problems remaining after implementation of the Title IV program and, reserving 11 judgment on the form that action could take, included Section 4 04 of the 1990 Amendments 12 (Clean Air Act Amendments of 1990, Pub. L . 101

- 549, § 404) requiring EPA to conduct a study 13 on the feasibility

and effectiveness of an acid deposition standard or standards to protect 14 “sensitive and critically sensitive aquatic an d terrestrial resources.” At the conclusion of the 15 study , EPA was to submit a report to Congress. Five years later, EPA submitted its report, 16 entitled Acid Deposition Standard Feasibility Study : Report to Congress (US EPA, 1995 b ) in 17 fulfillment of this requirement. That Report concluded that establishing acid deposition 18 standards for sulfur and nitrogen deposition may at some point in the future be technically 19 feasible, although appropriate deposition loads for these acidifying chemicals could not be 20 def ined with reasonable certainty at that time. 21 Fourth, the 1990 Amendments also added new language to sections of the CAA 22 pertaining to the scope and application of the secondary NAAQS designed to protect the public 23

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

16 welfare. Specifically , the definition o f “effects on welfare” in Section 302(h) was expanded to 1 state that the welfare effects include effects “...whether caused by transformation, conversion, or 2 combination with other air pollutants.” 3 In 1999, seven Northeastern states cited this amended langu age in Section 302(h) in a 4 petition asking EPA to use its authority under the NAAQS program to promulgate secondary 5 NAAQS for the criteria pollutants associated with the formation of acid rain. The petition stated 6 that this language “clearly references th e transformation of pollutants resulting in the inevitable 7 formation of sulfate and nitrate aerosols and/or their ultimate environmental impacts as wet and 8 dry deposition, clearl y signaling Congressional intent that the welfare damage occasioned b y 9 sulfur and nitrogen oxides be addressed through the secondary standard provisions of Section 10 109 of the Act.” The petition further stated that “recent federal studies, including the NAPAP 11 Biennial Report to Congress: An Integrated Assessment, document the continued and increasing 12 damage being inflicted by acid deposition to the lakes and forests of New York, New England 13 and other parts of our nation, demonstrating that the Title IV program had proven insufficient.” 14 The petition also listed other adverse welfare e ffects associated with the transformation of these 15 criteria pollutants, including impaired visibility , eutrophication of coastal estuaries, global 16 warming, and tropospheric ozone and stratospheric ozone depletion. 17 In a related matter, the Office of the Se cretary of the U.S. Department of Interior (DOI) 18 requested in 2000 that EPA initiate a rulemaking proceeding to enhance the air quality in 19 national parks and wilderness areas in order to protect resources and values that are being 20 adversel y affected b y air pollution. Included among the effects of concern identified in the 21 request were the acidification of streams, surface waters, and/or soils; eutrophication of coastal 22 waters; visibility

impairment; and foliar injury from ozone. 23

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

17 In a Federal Register notic e in 2001, EPA announced receipt of these requests and asked 1 for comment on the issues raised in them. EPA stated that it would consider any relevant 2 comments and information submitted, along with the information provided by the petitioners and 3 DOI, before making any decision concerning a response to these requests for rulemaking (65 FR 4 48699). 5 The 2005 NAPAP report states that “... scientific studies indicate that the emission 6 reductions achieved b y Title IV are not sufficient to allow recovery of acid

- sen sitive ecos ys tems. 7 Estimates from the literature of the scope of additional emission reductions that are necessary in 8 order to protect acid

- sensitive ecosy stems range from approximately 40

- 80% beyond full 9 implementation of Title IV.... ” The results of the modeling presented in this Report to Congress 10 indicate that broader recovery is not predicted without additional emission reductions” (NAPAP , 11 2005 ). 12 Given the state of the science as described in the ISA, REA, and in other recent reports, 13 such as the NAPAP reports noted above, EPA has decided, in the context of evaluating the 14 adequacy

of the current NO2 and SO2 secondary standards in this review, to revisit the question 15 of the appropriateness of setting secondary NAAQS to address remaining known or anti cipated 16 adverse public welfare effects resulting from the acidic and nutrient deposition of these criteria 17 pollutants. 18 D. History of the Current Review 19 The EPA initiated this current review in December 2005 with a call for information (70 20 FR 73236) for th e development of a revised Integrated Science Assessment for Oxides of 21 Nitrogen and Oxides of Sulfur

- Ecological Criteria (henceforth the "ISA"). An Integrated 22 Review Plan (IRP) was developed to provide the framework and schedule as well as the scope of 23

### Interagency Working Comments on Draft Language under EO12866 and 13563 Interagency Review. Subject to Further Policy Review.

18 the review and to identify policy

- relevant questions to be addressed in the components of the 1 review. The IRP was released in 2007 (US EPA , 2007) for CASAC and public review. EPA 2 held a workshop in July 2007 on the I SA to obtain broad input from the relev ant scientific 3 communities. This workshop helped to inform the preparation of the first draft ISA , which was 4 released for CASAC and public review in December 2007; a CASAC meeting was held on April 5 2

- 3, 2008 to review the first draft ISA. A second draft ISA was released for CASAC and public 6 review in August 2008, and was discussed at a CASAC meeting held on October 1

- 2, 2008. T he 7 final I SA (US EPA, 2008) was released in December 2008. 8 Based on the science presented in the I SA, EPA developed a Risk and Exposure 9 Assessment for Review of the Secondary National Ambient Air Quality Standards for Oxides of 10 Nitrogen and Oxides of Sulfur (henceforth the “REA”) to further assess the national impact of 11 the effects documented in the I SA. The Draft Scope and Meth ods Plan for Risk/Exposure 12 Assessment: Secondary NAAQS Review for Oxides of Nitrogen and Oxides of Sulfur outlining 13 the scope and design of the future REA was prepared for CASAC and public in March 2008. A 14 first draft REA was presented to CASAC and the pu blic for review in August 2008 and a second 15 draft was presented for review in June 2009. The final REA (US EPA 2009) was released in 16 September 2009. A first draft Policy Assessment was released in March 2010 and reviewed by 17 CASAC on April 1

- 2, 2010. In a June 22, 2010 letter to the Administrator, CASAC provided 18 advice and recommendations to the Agency concerning the first draft Policy Assessment (Russell 19 and Samet, 2010 a). A second draft Policy Assessment was released to CASAC and the public 20 in Septemb er 2010 and reviewed b y CASAC on October 6

- 7, 2010. CASAC provided advice 21 and recommendations to the Agency regarding the second draft Policy Assessment in a 22 December 9, 2010 letter (Russell and Samet 2010 b ). CASAC and public comments on the 23

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19 second draft Policy

Assessment were considered by EPA staff in developing a final Policy 1 Assessment (US EPA, 2011). CASAC requested an additional meeting to provide additional 2 advice to the Administrator based on the final Policy Assessment on February 15

– 16, 2011. 3 EPA released on January 14, 2011, the final Policy Assessment prior to final document 4 production, to provide sufficient time for CASAC review of the document in advance of this 5 meeting. A final Policy Assessment, incorporating final reference checks and do cument 6 formatting, was released in February 2011. In a May

17, 2011 letter (Russel l and Samet, 2011 a), 7 CASAC offered additional advice to the Administrator with regard to recommendations and 8 revisions to the secondary NAAQS for oxides of nitrogen and oxid es of sulfur. 9 On February 3, 2006, the Center for Biological Diversity and four other plaintiffs filed 10 an amended complaint alleging that EPA had failed to complete the current review within the 11 period provided b y statute. 2 The schedule for completion of this review is governed b y a consent 12 decree resolving that lawsuit and the subsequent extension granted to the Agency by the 13 plaintiffs. The schedule presented in the original consent decree that governs this review, entered 14 by the court on November 19, 2007, was revised on October 22, 2009 to allow for a 17 month 15 extension of the schedule. The current decree provides that EPA sign for publication notices of 16 proposed and final rulemaking concerning its review of the oxides of nitrogen and oxides of 17 sulf ur NAAQS no later than July 12, 2011 and March 20, 2012, respectively

. 18 This action presents the Administrator’s proposed decisions on the review of the current 19 secondary

oxides of nitrogen and oxides of sulfur standards. Throughout this preamble a numb er 20 of conclusions, findings, and determinations proposed by the Administrator are noted. While 21 they

identify the reasoning that supports this proposal, they are not intended to be final or 22 conclusive in nature. The EPA invites general, specific, and/or t echnical comments on all issues 23

2 Center for Biological Diversity, et al. v. Johnson, No. 05

– 1814 (D.D.C.)

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20 involved with this proposal, including all such proposed judgments, conclusions, findings, and 1 determinations. 2 E. Scope of the Current Review 3 In conducting this periodic review of the secondary NAAQS for oxides of nitrogen and 4 oxides of sulfur, as discussed in the IRP and REA, EPA decided to assess the scientific 5 information, associated risks, and standards relevant to protecting the public welfare from 6 adverse effects associated jointly with oxides of nitrogen and sulfur. Although EPA has 7 historically

adopted separate secondary standards for oxides of nitrogen and oxides of sulfur, 8 EPA is conducting a joint review of these standards because oxides of nitrogen and sulfur, and 9 their associated transformation products are linked from an atmospheric chemistry perspective, 10 as well as from an environmental effects perspective. The National Research Council (NRC) has 11 recommended that EPA consider multiple pollutants, as appropriate, in forming the scientific 12 basis for the NAAQS (N RC, 2004). As discussed in the I SA and REA, there is a strong basis for 13 considering these pollutants together, building upon EPA’s past recognition of the interactions of 14 these pollutants and on the growing body of scientific information that is now avail able related to 15 these interactions and associated ecological effects. 16 In defining the scope of this review, it must be considered that EPA has set secondary 17 standards for two other criteria pollutants related to oxides of nitrogen and sulfur: ozone and 18 p articulate matter (PM). Oxides of nitrogen are precursors to the formation of ozone in the 19 atmosphere, and under certain conditions, can combine with atmospheric ammonia to form 20 ammonium nitrate, a component of fine PM. Oxides of sulfur are precursors to the formation of 21 particulate sulfate, which is a significant component of fine PM in many parts of the U.S. There 22 are a number of welfare effects directl y associated with ozone and fine PM, including ozone

- 23

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21 related damage to vegetation and PM

- related visibility impairment. Protection against those 1 effects is provided b y the ozone and fine PM secondary standards. This review focuses on 2 evaluation of the protection provided b y secondary standards for oxides of nitrogen and sulfur 3 for two general types of e ffects: (1) direct effects on vegetation associated with exposure to 4 gaseous oxides of nitrogen and sulfur in the ambient air, which are the effects that the current 5 NO 2 and SO 2 secondary standards protect against and (2) effects associated with the depos ition 6 of oxides of nitrogen and sulfur to sensitive aquatic and terrestrial ecos ys tems, including 7 deposition in the form of particulate nitrate and particulate sulfate. 8 The I SA focuses on the ecological effects associated with deposition of ambient oxides of 9 nitrogen and sulfur to natural sensitive ecos ys tems, as distinguished from commercially managed 10 forests and agricultural lands. This focus reflects the fact that the majority of the scientific 11 evidence regarding acidification and nutrient enrichment i s based on studies in unmanaged 12
ecos ys tems. Non
- managed terrestrial ecos ys tems tend to have a higher fraction of N deposition 13 resulting from atmospheric nitrogen (US EPA , 2008, section 3.3.2.5). In addition, the I SA notes 14 that agricultural and commercial forest lands are routinely fertilized with amounts of nitrogen 15 that exceed air pollutant inputs even in the most polluted areas (US EPA , 2008, section 3.3.9). 16 This review recognizes that the effects of nitrogen deposition in managed areas are viewed 17 diff erentl y from a public welfare perspective than are the effects of nitrogen deposition in 18 natural, unmanaged ecosystems, largely due to the more homogeneous, controlled nature of 19 species composition and development in managed ecos ys tems and the potential fo r benefits of 20 increased productivity in those ecos ys tems. 21 In focusing on natural sensitive ecosy stems, the Policy Assessment primarily

considers 22 the effects of ambient oxides of nitrogen and sulfur via deposition on multiple ecological 23

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22 receptors. The ISA highlights effects including those associated with acidification and nitrogen 1 nutrient enrichment. With a focus on these deposition

- related effects, EPA’s objective is to 2 develop a framework for oxides of nitrogen and sulfur standards that incorporates e cologicall y 3 relevant factors and that recognizes the interactions between the two pollutants as they deposit to 4 sensitive ecos ys tems. The overarching policy objective is to develop a secondary standard(s) 5 that is based on the ecological criteria described in the I SA and the results of the assessments in 6 the REA, and is consistent with the requirement of the CAA to set secondary standards that are 7 requisite to protect the public welfare from any known or anticipated adverse effects associated 8 with the prese nce of these air pollutants in the ambient air. Also consistent with the CAA, this 9 policy

objective necessarily

includes consideration of “variable factors . . . which of themselves 10 or in combination with other factors may alter the effects on public welf are” of the criteria air 11 pollutants included in this review. 12 In addition, we have chosen to focus on the effects of ambient oxides of nitrogen and 13 sulfur on ecological impacts on sensitive aquatic ecosy stems associated with acidifying 14 deposition of nitrog en and sulfur, which is a transformation product of ambient oxides of 15 nitrogen and sulfur. Based on the information in the I SA, the assessments presented in the REA, 16 and advice from CASAC on earlier drafts of this Policy Assessment (Russell and Samet, 201 0a, 17 2010b), and as discussed in detail in the Policy Assessment , we have the greatest confidence in 18 the causal linkages between oxides of nitrogen and sulfur and aquatic acidification effects 19 relative to other deposition

- related effects, including terrestrial acidification and aquatic and 20 terrestrial nutrient enrichment. 21 22

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23 **II. Rationale for Proposed Decision on the Adequacy of the Current Secondary**  1 **Standards** 2 D ecisions on retaining or revising the current secondary standards for oxides of nitrogen 3 and sulfur are largely public welfare policy

judgments based on the Administrator’s informed 4 assessment of what constitutes requisite protection against adverse effects to public welfare. A 5 public welfare policy decision should draw upon scientific information and anal ys es about 6 welfare effects, exposure and risks, as well as judgments about the appropriate response to the 7 range of uncertainties that are inherent in the scientific evidence and analy ses. The ultimate 8 determination as to what level of damage to ecos yst ems and the services provided by those 9
ecos ys tems is adverse to public welfare is not wholly a scientific question, although it is 10 informed b y scientific studies linking e cos ys tem damage to losses in ecosy stem services, and 11 information on the value of thos e losses of ecos ys tem services. In reaching such decisions, the 12 Administrator seeks to establish standards that are neither more nor less stringent than necessary 13 for this purpose. 14 This section presents the rationale for the Administrator’s proposed conclusions with 15 regard to the adequacy of protection and ecological relevance of the current secondary standards 16 for oxides of nitrogen and sulfur. As discussed more full y below, this rationale considered the 17 latest scientific information on ecological effects associated with the presence of oxides of 18 nitrogen and oxides of sulfur in the ambient air. This rationale also takes into account: (1) staff 19 assessments of the most policy

- relevant information in the I SA and staff analy ses of air quality , 20 exposure, and ecological risks, presented more fully in the REA and in the Policy Assessment , 21 upon which staff conclusions on revisions to the secondary oxides of nitrogen and oxides of 22 sulfur standards are based; (2) CASAC advice and recommendations, as reflected in discussions 23

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24 of drafts of the I SA, REA, and Policy Assessment at public meetings, in separate written 1 comments, and in CASAC’s letters to the Administrator; and (3) public comments received 2 during the development of these documents, either in connection with CASAC meetings or 3 separately

. 4 In developing this rationale, EPA has drawn upon an integrative s ynthesis of the entire 5 body of evidence, published through early 2008 , on ecological effects associated with the 6 deposition of oxides of nitrogen and oxides of sulfur in the ambient air (US EPA, 2008)

. As 7 discussed below in section II.A, this body of evidence addresses a broad range of ecological 8 endpoints associated with ambient levels of oxides of nitrogen and oxides of sulfur

. In 9 considering this evidence, EPA focuses on those ecological endpoints, such as aquatic 10 acidification, for which the IS A judges associations with oxides of nitrogen and oxides of sulfur 11 to be causal, likel y causal, or for which the evidence is highl y suggestive that oxides of nitroge n 12 and/or sulfur contribute to the reported effects

. 13 Crucial to this review is the development of a form for an ecologicall y relevant standard 14 that reflects both the geographicall y variable and deposition

- dependent nature of the effects. 15 The atmospheric levels of oxides of nitrogen and sulfur that afford a particular level of 16
ecos ys tem protection are those levels that result in an amount of deposition that is less than the 17 amount of deposition that a given ecos ys tem can accept without defined levels of d egradation of 18 the ecological indicator for a targeted effect. 19 Drawing from the framework developed in the REA, the framework we used to structure 20 an ecologically

meaningful secondary

standard in the Policy

Assessment and to further develop 21 the indicator, f

orm, level, and averaging time of such as standard in section III of this proposal is 22

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25 depicted below and highlights the three key linkages that need to be considered in developing an 1 ecologicall y relevant standard. 2 3

4 **Figure II -**  **1. Sim**  **plified conceptual des ign of the form of an aquatic acidification standard**  5 **for oxides of nitrogen and sulfur.** 6 7 The following discussion relies heavil y on Chapters 2 and 3 of the Policy Assessment

. 8 The Policy

Assessment includes staff’s evaluation of the policy implications of the scientific 9 assessment of the evidence presented and assessed in the I SA and the results of quantitative 10 assessments based on that information presented and assessed in the REA. Taken together, this 11 information informs staff conclusions and the developm ent of policy options in the Policy 12 Assessment for consideration in addressing public and welfare effects associated with the 13 presence of oxides of nitrogen and oxides of sulfur in the ambient air. Of particular note, chapter 14 2 of the Policy

Assessment presents information not repeated here that characterizes emissions, 15 air quality , deposition and water quality. It includes discussions of the sources of nitrogen and 16 sulfur in the atmosphere as well as current ambient air quality monitoring networks and mod els. 17 Additional information in this section includes ecological modeling and water quality data 18
sources. 19 Section II.A presents a discussion of the effects associated with oxides of nitrogen and 20 sulfur in the ambient air. The discussion is organized around the ty pes of effects being 21

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26 considered, including direct effects of gaseous oxides of nitrogen and sulfur , deposition

- related 1 effects related to acidification and nutrient enrichment , and other effects such as materials 2 damage, climate

- related effects an d mercury methylation

. 3 Section II
. B presents a summary and discussion of the risk and exposure assessment 4 performed for each of the four major effects categories. The REA uses case studies representing 5 the broad geographic variability of the impacts from oxides of nitrogen and sulfur to conclude 6 that there are ongoing adverse effects in many ecosy stems from deposition of oxides of nitrogen 7 and sulfur and that under current emissions scenarios these effects are likely to continue. 8 Section II
. C presents a discussion of adversity linking ecological effects to measures that 9 can be used to characterize the extent to which such effects are reasonabl y considered to be 10 adverse to public welfare. This involves consideration of how to characterize adver sity from a 11 public welfare perspective. In so doing, consideration is given to the concept of ecos ys tem 12 services, the evidence of effects on ecos ys tem services, and how ecos ys tem services can be 13 linked to ecological indicators. 14 Section II
. D presents an a ssessment of the adequacy of the current oxides of nitrogen and 15 oxides of sulfur secondary standards. Consideration is given to the adequacy of protection 16 afforded b y the current standards for both direct and deposition

- related effects, as well as to the 17 appropriateness of the fundamental structure and the basic elements of the current standards for 18 providing protection from deposition

- related effects. Considerations as to the extent to which 19 deposition

- related effects that could reasonably be judged to be adverse to public welfare are 20 occurring under current conditions which are allowed b y the current standards is also considered

. 21 Discussion of the structures and basic elements of the current NO 2 and SO 2 secondary standards 22 and whether they are adequate to protect against such effects is presented

. 23

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27 A. Ecological Effects 1 This section discusses the known or anticipated ecological effects associated with oxides 2 of nitrogen and sulfur, including the direct effects of gas

- phase exposure to oxides of nitrogen 3 and sulfur (section II.A.1) and effects associated with deposition

- related exposure (sections 4 II.A.2 and 3)

. Section II .A. 2 addresses effects related to acidification of aquatic and terrestrial 5
ecos ys tems and s ection II A.3 addre sses effects related to nutrient enrichment of aquatic and 6 terrestrial ecos ys tems
. These sections also address questions about the nature and magnitude of 7
ecos ys tem responses to reactive nitrogen and sulfur deposition, including responses related to 8 acidi fication, nutrient depletion, and, in Section II.A 4 the mobilization of toxic metals in 9 sensitive aquatic and terrestrial ecos ys tems . T he uncertainties and limitations associated with the 10 evidence of such effects are also discussed throughout this sectio n . 11 1. Effects Associated with Gas

-Phase Oxides of Nitrogen and Sulfur 12 E cological effects on vegetation as discussed in earlier reviews as well as the I SA can be 13 attributed to gas

- phase oxides of nitrogen and sulfur. Acute and chronic exposures to gaseou s 14 pollutants such as sulfur dioxide (SO 2 ), nitrogen dioxide (NO 2 ), nitric oxide (NO), nitric acid 15 (HNO 3 ) and perox yacetyl nitrite (PAN) are associated with negative impacts to vegetation. The 16 current secondary

NAAQS were set to protect against direct damag e to vegetation by exposure to 17 gas - phase oxides of nitrogen and sulfur , such as foliar injury , decreased photosy nthesis, and 18 decreased growth. The following summary is a concise overview of the known or anticipated 19 effects to vegetation caused by gas phas e nitrogen and sulfur

. Most phototoxic effects associated 20 with gas phase oxides of nitrogen and sulfur occur at levels well above ambient concentrations 21 observed in the U.S. (US EPA, 2008, section 3.4.2.4). 22

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28 a. Nature of ecosystem responses to gas

- phase nitrogen and sulfur 1 The 2008 I SA found that gas phase N and S are associated with direct phy totoxic effects 2 (US EPA, 2008, section 4.4). The evidence is sufficient to infer a causal relationship between 3 exposure to SO 2 and injury to vegetation (US EPA, 2008, section 4.4.1 and 3.4.2.1). Acute foliar 4 injury

to vegetation from SO 2 may occur at levels above the current secondary standard (3

- h 5 average of 0.50 ppm). Effects on growth, reduced photosy nthesis and decreased yi eld of 6 vegetation are also associated with increased SO 2 exposure concentration and time of exposure. 7 The evidence is sufficient to infer a causal relationship between exposure to NO, NO 2 8 and PAN and injury

to vegetation (US EPA, 2008, section 4.4.2 and 3.4.2.2). At sufficient 9 concentrations, NO, NO 2 and PAN can decrease photosy nthesis and induce visible foliar injury 10 to plants. Evidence is also sufficient to infer a causal relationship between exposure to HNO 3 11 and changes to vegetation (US EPA, 2008, section 4.4.3 and 3.4.2.3). Phytotoxic effec ts of this 12 pollutant include damage to the leaf cuticle in vascular plants and disappearance of some 13 sensitive lichen species. 14 b. Magnitude of ecosystem response to gas

- phase nitrogen and sulfur 15 Vegetation in ecos ys tems near sources of gaseous oxides of nitrogen and sulfur or where 16 SO 2 , NO, NO 2 , PAN and HNO 3 are most concentrated are more likel y to be impacted b y these 17 pollutants. Uptake of these pollutants in a plant canopy is a complex process involving 18 adsorption to surfaces (leaves, stems and soil) and a bsorption into leaves (US EPA, 2008, section 19 3.4.2). The functional relationship between ambient concentrations of gas phase oxides of 20 nitrogen and sulfur and specific plant response are impacted b y internal factors such as rate of 21 stomatal conductance an d plant detoxification mechanisms, and external factors including plant 22

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29 water status, light, temperature, humidity , and pollutant exposure regime (US EPA, 2008, section 1 3.4.2). 2 Entry

of gases into a leaf is dependent upon phy sical and chemical processes o f gas phase 3 as well as to stomatal aperture. The aperture of the stomata is controlled largely by the 4 prevailing environmental conditions, such as water availability , humidity , temperature, and light 5 intensity . When the stomata are closed, resistance to gas uptake is high and the plant has a very 6 low degree of susceptibility to injury . Mosses and lichens do not have a protective cuticle barrier 7 to gaseous pollutants or stomata and are generall y more sensitive to gaseous sulfur and nitrogen 8 than vascular p lants (US EPA, 2008, section 3.4.2). 9 The appearance of foliar injury can vary significantly

across species and growth 10 conditions affecting stomatal conductance in vascular plants (US EPA, 200 9 , section 6.4.1). For 11 example, damage to lichens from SO 2 exp osure include s decrease d photosynthesis and 12 respiration, damage to the algal component of the lichen, leakage of electroly tes, inhibition of 13 nitrogen fixation, decreased K+ absorption, and structural changes. 14 The phy

totoxic effects of gas phase oxides of nitrogen and sulfur are dependent on the 15 exposure concentration and duration and species sensitivity to these pollutants. Effects to 16 vegetation associated with oxides of nitrogen and sulfur are therefore variable across the U.S. 17 and tend to be higher near sources of photochemical smog. For example, SO 2 is considered to be 18 the primary

factor contributing to the death of lichens in many urban and industrial areas

. 19 The I SA states there is very limited new research on phy totoxic effects of NO, NO 2 , PAN 20 and HNO 3 at concentrations currentl y observed in the United States with the exception of some 21 lichen species ( US EPA, 2008, section 4.4). Past and current HNO 3 concentrations may be 22 contributing to the decline in lichen s pecies in the Los Angeles basin . Mos t phototoxic effects 23

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30 associated with gas phase oxides of nitrogen and sulfur occur at levels well above ambient 1 concentrations observed in the U.S. (US EPA, 2008, section 3.4.2.4). 2 2. Acidification Effects Associated with Deposition of Oxides of Nitrogen and Sulfur 3 Sulfur oxides and nitrogen oxides in the atmosphere undergo a complex mix of reactions 4 in gaseous, liquid, and solid phases to form various acidic compounds. These acidic compounds 5 are removed from the atmosphere through deposition: either wet (e. g., rain, snow), fog or cloud, 6 or dry

(e.g., gases, particles). Deposition of these acidic compounds to ecosy stems can lead to 7 effects on ecos ys tem structure and function. Following deposition, these compounds can, in 8 some instances unless retained b y soil or biota, leach out of the soils in the form of sulfate (SO 4 2

- 9 ) and nitrate (NO 3 - ), leading to the acidification of surface waters. The effects on ecosy stems 10 depend on the magnitude and rate of deposition, as well as a host of biogeochemical processes 11
occ urring in the soils and waterbodies ( US EPA, 200 9 , section 2.1). The chemical forms of 12 nitrogen that may

contribute to acidifying deposition include both oxidized and reduced chemical 13 species , including NH x . 14 When sulfur or nitrogen leaches from soils to s urface waters in the form of SO 4 2

- or NO 3 - , 15 an equivalent amount of positive cations, or countercharge, is also transported. This maintains 16 electroneutrality . I f the countercharge is provided by base cations, such as calcium (Ca2+ ), 17 magnesium (Mg 2+ ), sodium (Na+ ), or potassium (K+ ), rather than h ydrogen (H + ) and dissolved 18 inorganic aluminum, the acidity of the soil water is neutralized, but the base saturation of the soil 19 decreases. Continued SO 4 2

- or NO 3 - leaching can deplete the available base cation pool in soil. 20 As the base cations are removed, continued deposition and leaching of SO 4 2

- and/or NO 3 - (with 21 H + and Al 3+ ) leads to acidification of soil water, and by connection, surface water. Introduction 22 of strong acid anions such as sulfate and nitrate to an already acidic soil, whether naturally or due 23

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31 to anthropogenic activities, can lead to instantaneous acidification of waterbodies through direct 1 runoff without an y significant change in base cation saturation. The ability of a watershed to 2 neutralize aci dic deposition is determined by a variety of biogeoph ys ical factors including 3 weathering rates, bedrock composition, vegetation and microbial processes, phy sical and 4 chemical characteristics of soils and hydrologic flowpaths. (US EPA, 200 9 , section 2.1) S ome of 5 these factors such as vegetation and soil depth are highl y variable over small spatial scales such 6 as meters, but can be aggregated to evaluate patterns over larger spatial scales. Acidifying 7 deposition of oxides of nitrogen and sulfur and the chem ical and biological responses associated 8 with these inputs vary temporally

. Chronic or long

- term deposition processes in the time scale of 9 ye ars to decades result in increases in inputs of N and S to ecos ys tems and the associated 10 ecological effects. Episo dic or short term (i.e., hours or day s) deposition refers to events in which 11 the level of the acid neutralizing capacity (ANC) of a lake or stream is temporaril y lowered. In 12 aquatic ecosy stems, short

- term (i.e., hours or day s) episodic changes in water ch emistry can have 13 significant biological effects. Episodic acidification refers to conditions during precipitation or 14 snowmelt events when proportionately more drainage water is routed through upper soil horizons 15 that tend to provide less acid neutralizing than is passing through deeper soil horizons (US EPA, 16 200 9 , section 4.2). In addition, the accumulated sulfate and nitrate in snow packs can provide a 17 surge of acidic inputs. Some streams and lakes may have chronic or base flow chemistry that is 18 sui table for aquatic biota, but may be subject to occasional acidic episodes with deleterious 19 consequences to sensitive biota. 20 The following summary is a concise overview of the known or anticipated effects caused 21 by acidification to ecos ys tems within the Un ited States. Acidification affects both terrestrial and 22 freshwater aquatic ecosystems. 23

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32 a. Nature of acidification

- related ecosystem responses 1 The I SA concluded that deposition of oxides of nitrogen and sulfur and NH x leads to the 2 varying degrees of acidif ication of ecos ys tems (US EPA, 2008 ). In the process of acidification, 3 biogeochemical components of terrestrial and freshwater aquatic ecos ys tems are altered in a way 4 that leads to effects on biological organisms. Deposition to terrestrial ecosy stems oft en moves 5 through the soil and eventually leaches into adjacent water bodies. 6 i. Aquatic ecosystems 7 The scientific evidence is sufficient to infer a causal relationship between acidifying 8 deposition and effects on biogeochemistry and biota in aquatic ecos ys te ms (US EPA, 2008, 9 section 4.2.2). The strongest evidence comes from studies of surface water chemistry in which 10 acidic deposition is observed to alter sulfate and nitrate concentrations in surface waters, the sum 11 of base cations, ANC, diss olved inorganic aluminum and pH (US EPA, 2008, section 3.2.3.2). 12 ANC is a key

indicator of acidification with relevance to both terrestrial and aquatic ecosy stems. 13 ANC is useful because it integrates the overall acid

- base status of a lake or stream and reflects 14 how aquati c ecos ys tems respond to acidic deposition over time. There is also a relationship 15 between ANC and the surface water constituents that directl y contribute to or ameliorate acidity

16 related stress, in particular, concentrations of hydrogen ion (as pH), Ca2+ , and aluminum. 17 Moreover, low pH surface waters leach aluminum from soils, which is quite lethal to fish and 18 other aquatic organisms. In aquatic s ys tems, there is a direct relationship between ANC and fish 19 and phy

to
- zooplankton diversity

and abundance

. 20 Lo w ANC coincides with effects on aquatic s ys tems (e.g., individual species fitness loss 21 or death, reduced species richness, altered community structure). At the community level, 22 species richness is positively correlated with pH and ANC because energy cost i n maintaining 23

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33 phy siological homeostasis, growth, and reproduction is high at low ANC levels. For example, 1 there is a logistic relationship between fish species richness and ANC class for Adirondack Case 2 Study

Area lakes that indicates the probability of oc currence of an organism for a given value of 3 ANC

. Biota are generally not harmed when ANC values are >100 microequivalents per liter 4 (μeq/L). The number of fish species also peaks at ANC values >100 μeq/L. Below 100 μeq/L 5 ANC , fish fitness and community diversity begin to decline (US EPA, section 4.2) **.** At ANC 6 levels between 100 and 50 μeq/L , the fitness of sensitive species (e.g., brook trout, zooplankton) 7 also begins to decline . When ANC concentrations are <50 μeq/L , they are generall y associated 8 with d eath or loss of fitness of biota that are sensitive to acidification. 9 Consistent and coherent documentation from multiple studies on various species from all 10 major trophic levels of aquatic sy stems shows that geochemical alteration caused by acidification 11 can result in the loss of acid

- sensitive biological species (US EPA, 2008, section 3.2.3.3). This is 12 most often discussed with relation to pH. For example, in the Adirondacks, of the 53 fish species 13 recorded in Adirondack lakes about half (26 species) we re absent from lakes with pH below 6.0. 14 Biological effects are linked to changes in water chemistry including decreases in ANC and pH 15 and increases in inorganic Al concentration. The direct biological effects are caused b y lowered 16 pH which leads to in creased inorganic Al concentrations (US EPA, 20 11 , Figures 3

- 1 and 3

- 2 ). 17 While ANC level does not cause direct biological harm it is a good overall indicator of the risk 18 of acidification (US EPA, 20 11 , section 3.1.3). 19 There are clear associations between ANC, pH, and aquatic species mortality and health 20 which are summarized in section 3.1.1 of the Policy Assessment

. Significant harm to sensitive 21 aquatic species has been observed at pH levels below 6. Normal stream pH levels with little to 22 no toxicity

ran ge from 6 to 7 (MacAvo y et al, 1995). Baker et al (1990) observed that “lakes 23

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34 with pH less than approximately 6.0 contain significantly

fewer species than lakes with pH levels 1 above 6.0”. As noted in Chapter 3, ty picall y at pH <4.5 and an ANC <0 μeq/L , co mplete to 2
near - complete loss of many taxa of organisms occur, including fish and aquatic insect 3 populations, whereas other taxa are reduced to only acidophilic species. 4 Additional evidence can help refine the understanding of effects occurring at pH leve ls 5 between 4.5 and 6. When pH levels are below 5.6, relativel y lower trout survival rates were 6 observed in the Shenandoah National Park. In field observations, when pH levels dropped to 5, 7 mortality

rates went to 100 percent. (Bulger et al, 2000). At pH levels ranging from 5.4 to 5.8, 8 cumulative mortality continues to increase. Several studies have shown that trout exposed to 9 water with varying pH levels and fish larvae showed increasing mortality as pH levels decrease. 10 In one study

almost 100 percent mortality was observed at a pH of 4.5 compared to almost 100 11 percent survival at a pH of 6.5. Intermediate pH values (6.0, 5.5) in all cases showed reduced 12 survival compared with the control (6.5), but not by statisticall y significant amounts (US EPA, 13 200 8, section 3.2.3.3). 14 One important indicator of acid stress is increased fish mortality . The response of fish to 15 pH is not uniform across species. A number of s yn optic survey s indicated loss of species 16 diversity

and absence of several fish species in the pH range of 5.0 to 5.5. If pH is lower, there is 17 a greater likelihood that more fish species could be lost without replacement, resulting in 18 decreased richness and diversity . In general, populations of salmonids are not found at pH levels 19 less than 5. 0, and smallmouth bass (Micropterus dolomieu) populations are usually not found at 20 pH values less than about 5.2 to 5.5. From Table 3

- 1, only one study

showed significant 21 mortality

effects above a pH of 6, while a number of studies showed significant mort ality when 22 pH levels are at or below 5.5. 23

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35 The highest pH level for any of the studies reported in the I SA, is 6.0, suggesting that pH 1 above 6.0 is protective against mortality effects for most species. Most thresholds are in the 2 range of pH of 5.0 to 6 .0, which suggests that a target pH should be no lower than 5.0. 3 Protection against mortality in some recreationally important species such as lake trout (pH 4 threshold of 5.6) and crappie (pH threshold of 5.5), combined with the evidence of effects on 5 lar val and embryo survival suggests that pH levels greater than 5.5 should be targeted to provide 6 protection against mortality effects throughout the life stages of fish. 7 Non
- lethal effects have been observed at pH levels as high as 6. A study in the 8 Shenan doah National Park found that the condition factor, a measure of fish health expressed as 9 fish weight/length multiplied by a scaling constant, is positively correlated with stream pH 10 levels, and that the condition factor is reduced in streams with a pH of 6.0 ( US EPA, 2008, 11 section 3.2.3.3). 12 Biodiversity is another indicator of aquatic ecos ys tem health. A key study in the 13 Adirondacks found that lakes with a pH of 6.0 had only half the potential species of fish (27 of 14 53 potential species). There is often a positive relationship between pH and number of fish 15 species, at least for pH values between about 5.0 and 6.5, or ANC values between about 0 to 16 100 μeq/L . Such observed relationships are complicated, however, by the tendency

for smaller 17 lakes and stream s, having smaller watersheds, to also support fewer fish species, irrespective of 18 acid

- base chemistry . This pattern may be due to a decrease in the number of available niches as 19 stream or lake size decreases. Nevertheless, fish species richness is relative ly easily determined 20 and is one of the most useful indicators of biological effects of surface water acidification. 21 Changes in stream water pH and ANC also contribute to declines in taxonomic richness 22 of zooplankton, and macroinvertebrates which are ofte n sources of food for fish, birds and other 23

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36 animal species in various ecos ys tems. These fish may also serve as a source of food and 1 recreation for humans. Acidification of ecos ys tems has been shown to disrupt food web 2 dy namics causing alteration to the di et, breeding distribution , and reproduction of certain species 3 of birds (US EPA, 2008, section 4.2.2.2. and Table 3

- 9). For example, breeding distribution of 4 the common goldeneye (Bucephala clangula ) , an insectivorous duck , may be affected b y 5 changes in a cidifying deposition. Similarly , decreases in prey diversity and quantity

have been 6 observed to create feeding problems for nesting pairs of loons on low

- pH lakes in the 7 Adirondacks. 8 ii. Terrestrial ecosystems 9 In terrestrial ecos ys tems, the evidence is suf ficient to infer a causal relationship between 10 acidifying deposition and changes in biogeochemistry ( US EPA, 2008, section 4.2.1.1). The 11 strongest evidence comes from studies of forested ecos ys tems, with supportive information on 12 other plant taxa, includi ng shrubs and lichens (US EPA, 2008, section 3.2.2.1.). Three useful 13 indicators of chemical changes and acidification effects on terrestrial ecos ystems, showing 14 consistency

and coherence among multiple studies are: soil base saturation, Al concentrations in 15 soil water and soil C:N ratio (US EPA, 2008, section 3.2.2.2). 16 As discussed in the I SA and REA, i n soils with base saturation less than about 15 to 20%, 17 exchange chemistry is dominated by

Al. Under these conditions, responses to inputs of sulfuric 18
ac id and HNO 3 largel y involve the release and mobilization of dissolved inorganic Al. The 19 effect can be neutralized by weathering from geologic parent material or base cation exchange. 20 The Ca2+ and Al concentrations in soil water are strongl y influenced b y soil acidification and 21 both have been shown to have quantitative links to tree health, including Al interference with 22 Ca2+ uptake and Al toxicity to roots. Effects of nitrification and associated acidification and 23

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37 cation leaching have been consistently sh own to occur onl y in soils with a C:N ratio below about 1 20 to 25. 2 Soil acidification caused by acidic deposition has been shown to cause decreased growth 3 and increased susceptibility to disease and injury

in sensitive tree species. Red spruce ( Picea 4 rube ns ) dieback or decline has been observed across high elevation areas in the Adirondack, 5 Green and White mountains. The frequency of freezing injury

to red spruce needles has 6 increased over the past 40 years, a period that coincided with increased emission s of S and N 7 oxides and increased acidifying deposition. Acidifying deposition can contribute to dieback in 8 sugar maple (

Acer saccharum ) through depletion of cations from soil with low levels of available 9 Ca. Grasslands are likel y less sensitive to acidif ication than forests due to grassland soils being 10 generall y rich in base cations. 11 iii. Ecosystem sensitivity 12 The intersection between current deposition loading, historic loading, and sensitivity 13 defines the ecological vulnerability to the effects of acidifi cation. Freshwater aquatic and some 14 terrestrial ecos ys tems, notably forests, are the ecosy stem ty pes which are most sensitive to 15 acidification. The IS A reports that the principal factor governing the sensitivity of terrestrial and 16 aquatic ecosy stems to ac idification from sulfur and nitrogen deposition is geology (particularly 17 surficial geology). Geologic formations having low base cation supply generall y underlie the 18 watersheds of acid

- sensitive lakes and streams. Other factors that contribute to the sensi tivity of 19 soils and surface waters to acidifying deposition include topography , soil chemistry , land use, 20 and hydrologic flowpaths. Episodic and chronic acidification tends to occur in areas that have 21 base

- poor bedrock, high relief, and shallow soils (US E PA, 2008, section 3.2.4.1). 22

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38 b. Magnitude of acidification

- related ecosystem responses 1 Terrestrial and aquatic ecosy stems differ in their response to acidifying deposition. 2 Therefore the magnitude of ecos ys tem response is described separatel y for aquatic and terrestrial 3
ecos ys tems in the following sections. The magnitude of response refers to both the severity of 4 effects and the spatial extent of the U.S. which is affected. 5 i. Aquatic acidification 6 Freshwater ecos ys tem survey s and monitoring in the eastern Uni ted States have been 7 conducted b y man y programs since the mid

- 1980s, including EPA’s Environmental Monitoring 8 and Assessment Program (EMAP), National Surface Water Survey (NSWS), Temporall y 9 Integrated Monitoring of Ecosy stems (TIME), and L ong - term Monitori ng (LTM) programs. 10 Based on anal ys es of surface water data from these programs, New England, the Adirondack 11 Mountains, the Appalachian Mountains (northern Appalachian Plateau and Ridge/Blue Ridge 12 region), and the Upper Midwest contain the most sensitive lakes and streams (i.e., ANC less than 13 about 50 μeq/L). Portions of northern Florida also contain many acidic and low

- ANC lakes and 14 streams, although the role of acidifying deposition in this region is less clear. The western U.S. 15 contains many

of the surfac e waters most sensitive to potential acidification effects, but with the 16 exception of the L os Angeles Basin and surrounding areas, the levels of acidifying deposition are 17 low in most areas. Therefore, acidification of surface waters b y acidic deposition i s not as 18 prevalent in the western U.S., and the extent of chronic surface water acidification that has 19 occurred in that region to date has likel y been very limited relative to the Eastern U.S. (US EPA, 20 2008, section 3.2.4.2 and US EPA, 200 9 , section 4.2.2)

. 21 There are a number of species including fish, aquatic insects, other invertebrates and 22 algae that are sensitive to acidification and cannot survive, compete, or reproduce in acidic 23

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39 waters ( US EPA, 2008, section 3.2.3.3). Decreases in ANC and pH have been shown to 1 contribute to declines in species richness and declines in abundance of zooplankton, 2 macroinvertebrates, and fish. Reduced growth rates have been attributed to acid stress in a 3 number of fish species including Atlantic salmon (Salmo salar), Chin ook salmon (Oncorh ynchus 4 tshawy

tscha), lake trout (Salvelinus namaycush), rainbow trout (Oncorhy nchis my kiss), brook 5 trout (Salvelinus Fontinalis), and brown trout (Salmo trutta). In response to small to moderate 6 changes in acidity , acid

- sensitive species are often replaced b y other more acid

- tolerant species, 7 resulting in changes in community composition and richness. The effects of acidification are 8 continuous, with more species being affected at higher degrees of acidification. At a point, 9 ty picall y a pH <4.5 and an ANC <0 μeq/L , complete to near

- complete loss of many taxa of 10 organisms occur, including fish and aquatic insect populations, whereas other taxa are reduced to 11 only

acidophilic species. These changes in taxa composition are associated with the high energy 12 cost in maintaining ph ys iological homeostasis, growth, and reproduction at low ANC levels (US 13 EPA, 2008, section 3.2.3.3) . Decreases in species richness related to acidification have been 14 observed in the Adirondack Mountains and Catskill Mountains of New York, New England and 15 Pennsylvania, and Virginia. From the sensitive areas identified by the ISA, further “case study” 16 anal ys es on aquatic ecosy stems in the Adirondack Mountains and Shenandoah National Park 17 were conducted to better characteri ze ecological risk associated with acidification (US EPA, 18 200 9 , section 4). 19 ANC is the most widely used indicator of acid sensitivity and has been found in various 20 studies to be the best single indicator of the biological response and health of aquatic 21
c ommunities in acid

- sensitive sy stems (Lien et al., 1992; Sullivan et al., 2006; US EPA, 2008). 22 In the REA, surface water trends in SO 4 2

- and NO 3 - concentrations and ANC levels were 23

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40 anal yz ed to affirm the understanding that reductions in deposition could in fluence the risk of 1 acidification. ANC values have been categorized according to their effects on biota, as shown in 2 the table below . Monitoring data from TIME/LTM and EMAP programs were assessed for the 3 ye ars 1990 to 2006, and past, present, and future water quality levels were estimated by both 4 steady

- state and dyn amic biogeochemical models. 5

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41 1 **Table II -**  **1. Ecological effects associated with alternative levels of acid neutralizing capacity** 2 **(ANC). (source: USEPA, Acid Rain Program)** 3 Category

L abel ANC Levels and Expected Ecological Effects
Acute Concern <0 μeq/L Complete loss of fish populations is expected. Planktonic communities have extremel y low diversity and are dominated by

acidophilic taxa. The numbers of individuals in plankton species that are present are greatl y reduced. Severe Concern 0

–20 μeq/L Highl y sensitive to episodic acidification. During episodes of high acidifying deposition, brook trout populations may experience lethal effects. The diversity and distribution of zooplankton communities decline sharpl y. Elevated Concern 20

– 50 μeq/L Fish species richness is greatl y reduced (i.e., more than half of expected species can be missing). On average, brook trout populations experience sublethal effects, including loss of health, ability to repro duce, and fitness. Diversity and distribution of zooplankton communities decline. Moderate Concern 50

– 100

μeq/L Fish species richness begins to decline (i.e., sensitive species are lost from

lakes). Brook trout populations are sensitive and variable, with possible sublethal effects. Diversity and distribution of zooplankton communities also begin to decline as species that are sensitive to acidifying deposition are affected. Low Concern >100 μeq/L Fish species richness may be unaffected. Reproducing brook trout populations are expected where habitat is suitable. Zooplankton communities are unaffected and exhibit expected diversity and distribution. 4 Studies on fish species richness in the Adirondacks Case Study Area demonstrated the 5 effect of acidificati on. Of the 53 fish species recorded in Adirondack Case Study Area lakes, 6 only 27 species were found in lakes with a pH <6.0. The 26 species missing from lakes with a 7 pH <6.0 include important recreational species, such as Atlantic salmon, tiger trout (Salm o trutta 8

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42 X Salvelinus fontinalis ), redbreast sunfish ( Lepomis auritus ), bluegill ( Lepomis macrochirus ), 1 tiger musky (

Esox masquinongy X lucius ), walleye (Sander vitreus ), alewife ( Alosa 2 pseudoharengus ), and kokanee ( Oncorhynchus nerka ), as well as ecologic ally important 3 minnows that are commonly consumed b y sport fish. A survey of 1,469 lakes in the late 1980s 4 found 346 lakes to be devoid of fish. Among lakes with fish, there was a relationship between 5 the number of fish species and lake pH, ranging from ab out one species per lake for lakes having 6 a pH <4.5 to about six species per lake for lakes having a pH >6.5. In the Adirondacks, a 7 positive relationship exists between the pH and ANC in lakes and the number of fish species 8 present in those lakes ( US EPA, 2008, section 3.2.3.4). 9 Since the mid

- 1990s, streams in the Shenandoah Case Study Area have shown slight 10 declines in NO 3 - and SO 4 2

- concentrations in surface waters. The 2006 concentrations are still 11 above pre

- acidification (1860) conditions. MAGIC mode ling predicts surface water 12 concentrations of NO 3 - and SO 4 2

- are10

- and 32

- fold higher, respectively , in 2006 than in 1860. 13 The estimated average ANC across 60 streams in the Shenandoah Case Study Area is 57.9 μeq/L 14 (± 4.5 μeq/L). 55% of all monitored stre ams in the Shenandoah Case Study Area have a current 15 risk of Elevated, Severe, or Acute. Of the 55%, 18% are chronically acidic today

( US EPA, 16 200 9 , section 4.2.4.3). 17 Based on a deposition scenario for this study area that maintains current emission leve ls 18 from 2020 to 2050, the simulation forecast indicates that a large number of streams still have 19 Elevated to Acute problems with acidity in 2050 . In fact, from 2006 to 2050, the percentage of 20 streams with Acute Concern is predicted to increase by 5%, whil e the percentage of streams in 21 Moderate Concern decreases b y 5%. 22

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43 Biological effects of increased acidification documented in the Shenandoah Case Study 1 Area include a decrease in the condition factor in blacknose dace and a decrease in fish 2 biodiversity

as sociated with decreasing stream ANC. On average, the fish species richness is 3 lower b y one fish species for every 21 μeq/L decrease in ANC in Shenandoah National Park 4 streams (US EPA, 2008, section 3.2.3.4). 5 ii. Terrestrial acidification 6 The I SA identified a variety of indicators that can be used to measure the effects of 7 acidification in soils. Most effects of terrestrial acidification are observed in sensitive forest 8
ecos ys tem in the U.S. Tree health has been linked to the availability of base cations (Bc ) in soil 9 (such as Ca2+ , Mg 2+ and potassium), as well as soil Al content. Tree species show a range of 10 sensitivities to Ca/Al and Bc/Al soil molar ratios, therefore these are good chemical indicators 11 because they

directly relate to the biological effects. Critical Bc/Al molar ratios for a large 12 variety of tree species ranged from 0.2 to 0.8. This range is similar to critical ratios of Ca/Al. 13 Plant toxicity

or nutrient antagonism was reported to occur at Ca/Al molar ratios ranging from 14 0.2 to 2.5 (US EPA, 200 9 ). 15 There has been no s ys tematic national survey of terrestrial ecos ys tems to determine the 16 extent and distribution of terrestrial ecos ys tem sensitivity to the effects of acidifying deposition. 17 However, one preliminary national evaluation estimated th at ~15% of forest ecosy stems in the 18 U.S. exceed the estimated critical load based on soil ANC leaching for S and N deposition by 19 >250 eq ha

- 1 y

r - 1 (McNulty

et al., 2007). Forests of the Adirondack Mountains of New York, 20 Green Mountains of Vermont, White M ountains of New Hampshire, the Allegheny Plateau of 21 Pennsylvania, and high

- elevation forest ecos ys tems in the southern Appalachians are the regions 22 most sensitive to terrestrial acidification effects from acidifying deposition (US EPA, 2008, 23

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44 section 3.2.4. 2). While studies show some recovery of surface waters, there are widespread 1 measurements of ongoing depletion of exchangeable base cations in forest soils in the 2 northeastern U.S. despite recent decreases in acidifying deposition, indicating a slow recovery 3 time. 4 In the REA, a critical load anal ys is was performed for sugar maple and red spruce forests 5 in the eastern United States by using Bc/Al ratio in acidified forest soils as an indicator to assess 6 the impact of nitrogen and sulfur deposition on tree h ealth. These are the two most commonly 7 studied tree species in North America for effects of acidification. At a Bc/Al ratio of 1.2, red 8 spruce growth can be decreased b y 20%. Sugar maple growth can be decreased by 20% at a 9 Bc/Al ratio of 0.6 (U S EPA, 2009 , section 4.4). The REA anal ys is determined the health of at 10 least a portion of the sugar maple and red spruce growing in the United States may have been 11 compromised with acidifyingtotal nitrogen and sulfur deposition. Specifically , total nitrogen and 12 sulf ur deposition levels exceeded three selected critical loads for tree growth in 3% to 75% of all 13 sugar maple plots across 24 states

-- that is, it exceeded the highest (least stringent) of the three 14 critical loads in 3% of plots, and the lowest (most stringent) in 75% of plots . For red spruce, total 15 nitrogen and sulfur deposition levels exceeded three selected critical loads in 3% to 36% of all 16 red spruce plots across eight states ( US EPA, 200 9 , section 4.4). 17

c. Key uncertainties associated with acidification 18 There are different levels of uncertainty associated with relationships between deposition, 19 ecological effects and ecological indicators. In Chapter 7 of the REA, the case study analy ses 20 associated with each targeted effect area were s ynthesized by identi fying the strengths, 21 limitations, and uncertainties associated with the available data, modeling approach, and 22 relationship between the selected ecological indicator and atmospheric deposition as described 23

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45 by the ecological effect function (US EPA, 200 9 , F igure 1

- 1). A further discussion of 1 uncertainty in aquatic and terrestrial ecos ys tems is presented below. The key uncertainties were 2 characterized as follows to evaluate the strength of the scientific basis for setting a national 3 standard to protect again st a given effect (US EPA, 200 9 , section 7): 4 (1) Data Availability: high, medium or low quality . This criterion is based on the availability and 5 robustness of data sets, monitoring networks, availability of data that allows for extrapolation 6 to larger assessme nt areas, and input parameters for modeling and developing the ecological 7 effect function. The scientific basis for the ecological indicator selected is also incorporated 8 into this criterion. 9 (2) Modeling Approach: high, fairl y high, intermediate, or low confi dence. This value is based 10 on the strengths and limitations of the models used in the anal ys is and how accepted they are 11 by

the scientific community for their application in this analy sis. 12 (3) Ecological Effect Function: high, fairl y high, intermediate, or low confidence. This ranking is 13 based on how well the ecological effect function describes the relationship between 14 atmospheric deposition and the ecological indicator of an effect. 15 i. Aquatic acidification 16 The REA concludes that the available data are robust and considered high quality . There 17 is high confidence about the use of these data and their value for extrapolating to a larger 18 regional population of lakes. The EPA TIME/LTM network represents a source of long

- term, 19 representative sampling. Data on sulf ate concentrations, nitrate concentrations and ANC from 20 1990 to 2006 used for this analy sis as well as EPA EMAP and REMAP survey s, provide 21 considerable data on surface water trends. 22

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46 There is fairly

high confidence associated with modeling and input param eters. 1 Uncertainties in water quality estimates (i.e., ANC) from MAGIC were derived from multiple 2 site calibrations. The 95% confidence interval for pre

- acidification of lakes was an average of 15 3 μeq/Ldifference in ANC concentrations or 10% and 8 μeq/L or 5% for streams (US EPA, 200 9 , 4 section 7.1.2). The use of the critical load model to estimate aquatic critical loads is limited by 5 the uncertainties associated with runoff and surface water measurements and in estimating the 6 catchment suppl y of bas e cations from the w eathering of bedrock and soils (McNulty et al., 7 2007). To quantify uncertainty in the model parameters, Monte Carlo methods were employed 8 to develop an inverse function of exceedances

. There is high confidence associated wi th the 9 ecological effect function developed for aquatic acidification

. Incalculating the ANC function, 10 the depositional load for N or S is fixed by the deposition of the other, so deposition for either 11 will never be zero ( US EPA, 200 9 , Figure 7.1

- 6)

. 12 i i. Terrestrial acidification 13 The available data used to quantify the targeted effect of terrestrial acidification are 14 robust and considered high quality. The USFS

- Kane Experimental Forest and significant 15 amounts of research work in the Allegheny Plateau have produced extensive, peer

- reviewed data 16 sets. A meta

- analy sis of laboratory studies showed that tree growth was decreased by 20% 17 relative to controls for BC/Al ratios (US EPA, 2008, section 7.2.1 and Figure 7.2

- 1). Sugar 18 maple and red spruce were the focus of the REA since they are demonstrated to be negativel y 19 affected b y soil available Ca2+ depletion and high concentrations of available Al, and occur in 20 areas that receive high acidifying deposition, There is high confidence about the use of the RE A 21 terrestrial acidification data and their value for extrapolating to a larger regional population of 22 forests. 23

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47 There is high confidence associated with the models, input parameters, and assessment of 1 uncertainty used in the case study for terrestrial ac idification. The Simple Mass Balance (SMB) 2 model, a commonly

used and widel y applied approach for estimating critical loads, was used in 3 the REA anal ys is (US EPA, 2008, section 7.2.2). There is fairly high confidence associated with 4 the ecological effect function developed for terrestrial acidification (US EPA, 200 9 , section 5 7.2.3). 6 3. Nutrient Enrichment Effects Associated with Deposition of Oxides of Nitrogen 7 The following summary is a concise overview of the known or anticipated effects caused 8 by nitroge n nutrient enrichment to ecosy stems within the United States. Nutrient

- enrichment 9 affects terrestrial, freshwater and estuarine ecos ys tems. Nitrogen deposition is a major source of 10 anthropogenic nitrogen. For many terrestrial and freshwater ecos ys tems o ther sources of 11 nitrogen including fertilizer and waste treatment are greater than deposition. Nitrogen deposition 12 often contributes to nitrogen

- enrichment effects in estuaries, but does not drive the effects since 13 other sources of N greatly exceed N depo sition. Both oxides of nitrogen and reduced forms of 14 nitrogen (NH x ) contribute to nitrogen deposition. For the most part, nitrogen effects on 15
ecos ys tems do not depend on whether the nitrogen is in oxidized or reduced form. Thus, this 16 summary

focuses on the effects of nitrogen deposition in total. 17

a. Nature of nutrient enrichment

- related ecosystem responses 18 The I SA found that deposition of nitrogen, including oxides of nitrogen and NH x , leads to 19 the nitrogen enrichment of ecos ys tems (US EPA 2008). In the process of nitrogen enrichment, 20 biogeochemical components of terrestrial and freshwater aquatic ecos ys tems are altered in a way 21 that leads to effects on biological organisms. 22 i. Aquatic ecosystems 23

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48 In freshwater ecos ys tems, the evidence is sufficient to in fer a **causal** relationship between 1 N deposition and the alteration of biogeochemical cycling in freshwater aquatic ecos ys tems (US 2 EPA, 2008, section 3.3.2.3 ). N deposition is the main source of N enrichment to headwater 3 streams, lower order streams and high elevation lakes. The most common chemical indicators 4 that were studied included NO 3 − and dissolved inorganic nitrogen (DIN) concentration in surface 5 waters as well as Chl a:total P ratio. Elevated surface water NO 3 − concentrations occur in both 6 the eastern and western U.S. Studies report a significant correlation between N deposition and 7 lake biogeochemistry by identifying a correlation between wet deposition and [DI N] and Chl a: 8 Total P. Recent evidence provides examples of lakes and streams that are limited by N and show 9 signs of eutrophication in response to N addition. 10 The evidence i s sufficient to infer a **causal** relationship between N deposition and the 11 alteration of species richness, species composition and biodiversity in freshwater aquatic 12
ecos ys tems (US EPA, 2008, section 3.3.5.3 ). Increased N deposition can cause a shift in 13
community

composition and reduce algal biodiversity , especially in sensitive oligotrophic lakes. 14 In the IS A, the evidence is sufficient to infer a **causal** relationship between N deposition 15 and the biogeochemical cycling of N and carbon (C) in estuaries (US EP A, 2008, section 4.3.4.1 16 and 3.3.2.3 ). In general, estuaries tend to be nitrogen

- limited, and many currentl y receive high 17 levels of nitrogen input from human activities (US EPA, 200 9 , section 5.1.1 ). It is unknown if 18 atmospheric deposition alone is suffici ent to cause eutrophication; however, the contribution of 19 atmospheric nitrogen deposition to total nitrogen load is calculated for some estuaries and can be 20 >40% (US EPA, 200 9 , section 5.1.1 ). 21 The evidence is sufficient to infer a **causal** relationship betw een N deposition and the 22 alteration of species richness, species composition and biodiversity in estuarine ecos ys tems (US 23

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49 EPA, 2008, section 4.3.4.2 and 3.3.5.4 ). Atmospheric and non

- atmospheric sources of N 1 contribute to increased phy toplankton and algal productivity , leading to eutrophication. Shifts in 2 community

composition, reduced h ypolimnetic DO, decreases in biodiversity , and mortality of 3 submerged aquatic vegetation are associated with increased N deposition in estuarine s ys tems. 4 ii. Terrestrial Ecosy stems 5 The evidence is sufficient to infer a **causal** relationship between N deposition and the 6 alteration of biogeochemical cycling in terrestrial ecos ys tems (US EPA, 2008, section 4.3.1.1 7 and 3.3.2.1 ). This is supported by numerous observational, depositio n gradient and field addition 8 experiments in sensitive ecos ys tems. T he leaching of NO 3 - in soil drainage waters and the export 9 of NO 3 - in stream water w ere identified as two of the primary indictors of N enrichment. Several 10 N
- addition studies indicate that NO 3 - leaching is induced b y chronic additions of N

. Studies 11 identified in the I SA found that surface water NO 3 - concentrations exceeded 1 μeq/Lin 12 watersheds receiving about 9 to 13 kg N/ha/yr of atmospheric N deposition. N deposition 13 disrupts the nutri ent balance of ecos ys tems with numerous biogeochemical effects. The chemical 14 indicators that are typically measured include NO 3 − leaching, soil C:N ratio, rates of N 15 mineralization, nitrification, denitrification, foliar N concentration, and soil water NO 3 − and 16 NH 4 + concentrations. Note that N saturation (N leaching from ecos ys tems) does not need to 17 occur to cause effects. Substantial leaching of NO 3 − from forest soils to stream water can acidify 18 downstream waters, leading to effects described in the previ ous section on aquatic acidification. 19 Due to the complexity of interactions between the N and C cycling, the effects of N on C budgets 20 (quantified input and output of C to the ecosy stem) are variable. Regional trends in net 21
ecos ys tem productivity (NEP) of forests (not managed for silviculture) have been estimated 22 through models based on gradient studies and meta

- anal ys is. Atmospheric N deposition has been 23

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50 shown to cause increased litter accumulation and carbon storage in above

- ground woody 1 biomass. In the West, this has lead to increased susceptibility to more severe fires. Less is 2 known regarding the effects of N deposition on C budgets of non

- forest ecosy stems. 3 The evidence is sufficient to infer a **causal** relationship between N deposition on the 4 alterati on of species richness, species composition and biodiversity in terrestrial ecos ys tems (US 5 EPA, 2008, section 4.3.1.2 ). Some organisms and ecos ys tems are more sensitive to N deposition 6 and effects of N deposition are not observed in all habitats. The most sensitive terrestrial taxa to 7 N deposition are lichens. Empirical evidence indicates that lichens in the U.S. are affected by 8 deposition levels as low as 3 kg N/ha/yr. Alpine ecosystems are also sensitive to N deposition, 9 changes in an individual species ( Carex rupestris ) were estimated to occur at deposition levels 10 near 4 kg N /ha/y

r and modeling indicates that deposition levels near 10 kg N/ha/yralter plant 11 community

assemblages. In several grassland ecosy stems, reduced species diversity and an 12 increase in non

- native, invasive species are associated with N deposition. 13 iii. Ecosystem sensitivity to nutrient enrichment 14 The numerous ecos ys tem ty pes that occur across the U.S. have a broad range of 15 sensitivity

to N deposition (U S EPA, 2008, Table 4

- 4 ). Increase d deposition to N

- limited 16
ecos ys tems can lead to production increases that may be either beneficial or adverse depending 17 on the sy stem and management goals. 18 Organisms in their natural environment are commonly adapted to a specific regime of 19 nutrient av ailability . Change in the availability of one important nutrient, such as N, may result 20 in an imbalance in ecological stoichiometry , with effects on ecos ys tem processes, structure and 21 function. In general, N deposition to terrestrial ecosy stems causes acce lerated growth rates in 22 some species deemed desirable in commercial forests but may lead to altered competitive 23

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51 interactions among species and nutrient imbalances, ultimately affecting biodiversity . The onset 1 of these effects occurs with N deposition level s as low as 3 kg N/ha/yr in sensitive terrestrial 2
ecos ys tems to N deposition. In aquatic ecos ys tems, N that is both leached from the soil and 3 directly

deposited to the water surface can pollute the surface water. This causes alteration of the 4 diatom commun ity

at levels as low as 1.5 kg N/ha/y r in sensitive freshwater ecos ys tems. 5 The degree of ecos ys tem effects lies at the intersection of N loading and N

- sensitivity

. 6 N
- sensitivity

is predominately

driven by the degree to which growth is limited by nitroge n 7 availability . Grasslands in the western United States are typically N

- limited ecos ys tems 8 dominated by

a diverse mix of perennial forbs and grass species. A meta

- analy sis discussed in 9 the I SA (US EPA, 2008, section 3.3.3), indicated that N fertilization i ncreased aboveground 10 growth in all non

- forest ecosy stems except for deserts. In other words, almost all terrestrial 11
ecos ys tems are N
- limited and will be altered b y the addition of anthropogenic nitrogen. 12 Likewise, a freshwater lake or stream must be N

- limi ted to be sensitive to N

- mediated 13 eutrophication. There are many examples of fresh waters that are N

- limited or N and 14 phosphorous (P) co

- limited (US EPA, 2008, section 3.3.3.2 ). A large dataset meta

- analy sis 15 discussed in the I SA (US EPA, 2008, section 3.3. 3.2), found that N

- limitation occurred as 16 frequentl y as P
- limitation in freshwater ecos ys tems. Additional factors that govern the 17 sensitivity

of ecos ys tems to nutrient enrichment from N deposition include rates and form of N 18 deposition, elevation, climat e, species composition, plant growth rate, length of growing season, 19 and soil N retention capacity (US EPA, 2008, section 4.3 ). Less is known about the extent and 20 distribution of the terrestrial ecos ys tems in the U.S. that are most sensitive to the effects of 21 nutrient enrichment from atmospheric N deposition compared to acidification. 22

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52 Because the productivity

of estuarine and near shore marine ecos ys tems is generall y 1 limited by

the availability

of N, they

are susceptible to the eutrophication effect of N d eposition 2 (US EPA, 2008, section 4.3.4.1 ). A recent national assessment of eutrophic conditions in 3 estuaries found the most eutrophic estuaries were generall y those that had large watershed

- to
4 estuarine surface area, high human population density, high rai nfall and runoff, low dilution, and 5 low flushing rates. In the REA, the National Oceanic and Atmospheric Administration’s 6 (NOAA) National Estuarine Eutrophication Assessment (NEEA) assessment tool, Assessment of 7 Estuarine Tropic Status (ASSETS) categorical Eutrophication Index (EI) was used to evaluate 8 eutrophication due to atmospheric loading of nitrogen. ASSETS EI is an estimation of the 9 likelihood that an estuary is experiencing eutrophication or will experience eutrophication based 10 on five ecological indicators: chlorophyll a , macroalgae, dissolved oxygen, nuisance/toxic algal 11 blooms and submerged aquatic vegetation (SAV). 12 In the REA, two regions were selected for case study anal ys is using ASSETS EI , the 13 Chesapeake Bay

and Pamlico Sound. Both region s received an ASSETS EIrating of Bad 14 indicating that the estuary had moderate to high pressure due to overall human influence and a 15 moderate high to high eutrophic condition (US EPA, 200 9 , section s 5.2.4.1 and 5.2.4.2 ). These 16 results were then considered with SPAtially Referenced Regression (SPARROW) modeling to 17 develop a response curve to examine the role of atmospheric nitrogen deposition in achieving a 18 desired decrease in load. To change the Neuse River Estuary ’s EI score from Bad to Poor not 19 only

mus t 100% of the total atmospheric nitrogen deposition be eliminated, but considerabl y 20 more nitrogen from other sources as well must be controlled (US EPA, 200 9 , section 5.2.7.2). In 21 the Potomac River estuary, a 78% decrease of total nitrogen could move the EIscore from Bad 22 to Poor (US EPA, 200 9 , section 5.2.7.1 ). The results of this analy sis indicated decreases in 23

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53 atmospheric deposition alone could not eliminate coastal eutrophication problems due to 1 multiple non

- atmospheric nitrogen inputs (US EPA, 200 9 , section 7.3.3 ). However, the 2 somewhat arbitrary

discreteness of the EI scale can mask the benefits of decreases in n itrogen 3 between categories. 4 In general, estuaries tend to be N

- limited, and many currentl y receive high levels of N 5 input from human activities to cause eutrophication. As reported in the IS A (US EPA, 2008, 6 section 3.2.2.2), atmospheric N loads to estuari es in the U.S. are estimated to range from 2

- 8% 7 for Guadalupe Bay , TX on the lowest end to as high as 72% for St Catherines

- Sapelo estuary , 8 GA. The Chesapeake Bay is an example of a large, well

- studied and severely eutrophic estuary 9 that is calculated to receive as much as 30% of its total N load from the atmosphere. 10 b. Magnitude of ecosystem responses 11 i. Aquatic ecosystems 12 The magnitude of ecos ys tem response may be thought of on two time scales, current 13 conditions and how ecosystems have been altered since the onset of anthropogenic N deposition. 14 As noted previousl y, studies found that N

- limitation occurs as frequently as P
- limitation in 15 freshwater ecos ys tems ( US EPA, 2008, section 3.3.3.2 ). Recentl y, a comprehensive study of 16 available data from the northern h emisphere survey s of lakes along gradients of N deposition 17 show increased inorganic N concentration and productivity to be correlated with atmospheric N 18 deposition. The results are unequivocal evidence of N limitation in lakes with low ambient inputs 19 of N, and increased N concentrations in lakes receiving N solel y from atmospheric N deposition. 20 It has been suggested that most lakes in the northern hemisphere may have originall y been N

21 limited, and that atmospheric N deposition has changed the balance of N and P in lakes. 22

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54 Available data suggest that the increases in total N deposition do not have to be large to 1 elicit an ecological effect. For example, a hindcasting exercise determined that the change in 2 Rocky

Mountain National Park lake algae that occurred between 1850 and 1964 was associated 3 with an increase in wet N deposition that was only about 1.5 kg N/ha. Similar changes inferred 4 from lake sediment cores of the Beartooth Mountains of Wyoming also occurred at about 1.5 kg 5 N/ha deposition. Pre

- industrial inorganic N deposition is estimated to have been onl y 0.1 to 0.7 6 kg N/ha based on measurements from remote parts of the world. In the western U.S., pre

7 industrial, or background, inorganic N deposition was estimated b y to range from 0.4 to 0.7 kg 8 N/ha/y

r. 9 Eutrophication effects from N deposition are most likely to be manifested in undisturbed, 10 low nutrient surface waters such as those found in the higher elevation areas of the western U.S. 11 The most severe eutrophication from N deposition effects is expect ed downwind of major urban 12 and agricultural centers. High concentrations of lake or streamwater NO 3 − , indicative of 13
ecos ys tem saturation, have been found at a variety of locations throughout the U.S., including the 14 San Bernardino and San Gabriel Mountains within the L os Angeles Air Basin, the Front Range 15 of Colorado, the Allegheny mountains of West Virg inia, the Catskill Mountains of New York, 16 the Adirondack Mountains of New York, and the Great Smoky Mountains in Tennessee (US 17 EPA, 2008, section 3.3.8 ). 18 In contrast to terrestrial and freshwater s ys tems, atmospheric N load to estuaries 19 contributes to the total load but does not necessarily drive the effects since other combined 20 sources of N often greatly exceed N deposition. In estuaries, N

- loading from multiple 21 anthropogenic and non

- anthropogenic pathway s leads to water quality deterioration, resulting in 22 numerous effects including h ypoxic zones, species mortality , changes in community composition 23

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55 and harmful algal blooms that are indicative of eutrophication. The following summary is a 1 concise overview of the known or anticipated effects of nitrogen enrichment on estuaries within 2 the United States. 3 There is a scientific consensus (US EPA, 2008, section 4.3.4) that nitrogen

- driven 4 eutrophication in shallow estuaries has increased over the past several decades and that the 5 environmental degradation of co astal ecos ys tems due to nitrogen, phosphorus, and other inputs is 6 now a widespread occurrence. For example, the frequency of phyt oplankton blooms and the 7 extent and severity

of hypoxia have increased in the Chesapeake Bay and Pamlico estuaries in 8 North Carolina and along the continental shelf adjacent to the Mississippi and Atchafalaya 9 rivers’ discharges to the Gulf of Mexico. 10 A recent national assessment of eutrophic conditions in estuaries found that 65% of the 11 assessed s ys tems had moderate to high ove rall eutrophic conditions and generall y received the 12 greatest N loads from all sources, including atmospheric and land

- based sources . Most eutrophic 13 estuaries occurred in the mid

- Atlantic region and the estuaries with the lowest degree of 14 eutrophication were in the North Atlantic. Other regions had mixtures of low, moderate, and high 15 degrees of eutrophication (US EPA, 2008, section 4.3.4.3 ). 16 The mid

- Atlantic region is the most heavily impacted area in terms of moderate or high 17 loss of submerged aquatic v egetation due to eutrophication (US EPA, 2008, section 4.3.4.2 ). 18 Submerged aquatic vegetation is important to the quality of estuarine ecosy stem habitats because 19 it provides habitat for a variety of aquatic organisms, absorbs excess nutrients, and traps 20
s ediments (US EPA, 2008, section 4.3.4.2). It is partly because many estuaries and near

- coastal 21 marine waters are degraded by nutrient enrichment that they are highl y sensitive to potential 22 negative impacts from nitrogen addition from atmospheric depositio n. 23

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56 ii. Terrestrial ecosystems 1 Little is known about the full extent and distribution of the terrestrial ecos ystems in the 2 U.S. that are most sensitive to impacts caused b y nutrient enrichment from atmospheric N 3 deposition. As previously stated, most terrestri al ecos ys tems are N
- limited, therefore they are 4 sensitive to perturbation caused b y N additions (US EPA, 2008, section 4.3.1) . Effects are most 5 likely

to occur where areas of relativel y high atmospheric N deposition intersect with N

- limited 6 plant communiti es. The alpine ecos ys tems of the Colorado Front Range, chaparral watersheds of 7 the Sierra Nevada, lichen and vascular plant communities in the San Bernardino Mountains and 8 the Pacific Northwest, and the southern California coastal sage scrub (CSS) communi ty are 9 among the most sensitive terrestrial ecos ys tems. There is growing evidence (US EPA, 2008, 10 section 4.3.1.2) that existing grassland ecos ys tems in the western United States are being altered 11 by elevated levels of N inputs, including inputs from atmosp heric deposition. 12 In the eastern U.S., the degree of N saturation of the terrestrial ecos ys tem is often 13 assessed in terms of the degree of NO 3 − leaching from watershed soils into ground water or 14 surface water. Studies have estimated the number of surface waters at different stages of 15 saturation across several regions in the eastern U.S. Of the 85 northeastern watersheds examined 16 60% were in Stage 1 or Stage 2 of N saturation on a scale of 0 (background or pretreatment) to 3 17 (visible decline). Of the northe astern sites for which adequate data were available for assessment, 18 those in Stage 1 or 2 were most prevalent in the Adirondack and Catskill Mountains. Effects on 19 individual plant species have not been well studied in the U.S. More is known about the 20 sensi tivity

of particular plant communities. Based largel y on results obtained in more extensive 21 studies conducted in Europe, it is expected that the more sensitive terrestrial ecos ys tems include 22

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57 hardwood forests, alpine meadows, arid and semi

- arid lands, and grassland ecos ys tems (US 1 EPA, 2008, section 3. 3.5)

. 2 The REA used published research results (US EPA, 200 9 , section 5.3.1 and US EPA, 3 2008, Table 4.4) to identify meaningful ecological benchmarks associated with different levels of 4 atmospheric nitrogen depo sition. These are illustrated in Figure 3

- 4 of the Policy Assessment

. 5 The sensitive areas and ecological indicators identified by the IS A were analyzed further in the 6 REA to create a national map that illustrates effects observed from ambient and experimental 7 atmospheric nitrogen deposition loads in relation to Community Multi

- scale Air Quality 8 (CMAQ) 2002 modeling results and NADP monitoring data. This map, reproduced in Figure 3

- 9 5 of the Policy

Assessment , depicts the sites where empirical effects of terrestrial nutrient 10 enrichment have been observed and site proximity to elevated atmospheric N deposition. 11 Based on information in the I SA and initial anal ys is in the REA, further case study 12 anal ys es on terrestrial nutrient enrichment of ecosy stems were developed for the CCS 13 community

and Mixed Conifer Forest (MCF) (US EPA, 2009). Geographic information sy stems 14 (GI S) analy sis supported a qualitative review of past field research to identify ecological 15 benchmarks associated with CSS and mycorrhizal commun ities, as well as MCF nutrient

16 sensitive acidoph yt e lichen communities, fine

- root biomass in Ponderosa pine, and leached 17 nitrate in receiving waters. 18 The ecological benchmarks that were identified for the CSS and the MCF communities 19 are included in the suite of benchmarks identified in the I SA ( US EPA, 2008, section 3.3 ). There 20 are sufficient data to confidently relate the ecological effect to a loading of atmospheric nitrogen. 21 For the CSS community , the following ecological benchmarks were identified: 22

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58 (1) 3.3 kg N/ha/yr
– the amount of nitrogen uptake b y a vigorous stand of CSS; above this level, 1 nitrogen may

no longer be limiting 2 (2) 10 kg N/ha/yr
– mycorrhizal community changes 3 For the MCF community, the following ecological benchmarks were identified: 4 (1) 3.1 kg N/ha/yr
– shift from sensitive to tolerant lichen species 5 (2) 5.2 kg N/ha/yr
– dominance of the tolerant lichen species 6 (3) 10.2 kg N/ha/yr
– loss of sensitive lichen species 7 (4) 17 kg N/ha/yr
– leaching of nitrate into streams. 8 These benchmarks, ranging from 3.1 to 17 kg N/ha/y r, were compared to 2002 9 CMAQ/NADP data to discern an y associations between atmospheric deposition and changing 10 communities. Evidence supports the finding that nitrogen alters CSS and MCF communities

. 11 Key

findings include the following: 2002 C MAQ/NADP nitrogen deposition data show that the 12 3.3 kg N/ha/yrbenchmark has been exceeded in more than 93% of CSS areas (654,048 ha). 13 These deposition levels are a driving force in the degradation of CSS communities. Although 14 CSS decline has been observed in the absence of fire, the contributions of deposition and fire to 15 the CSS decline require further research. CSS is fragmented into many small parcels, and the 16 2002 CMAQ/NADP 12

- km grid data are not fine enough to fully validate the relationship 17 between CSS distribution, nitrogen deposition, and fire. 2002 CMAQ/NADP nitrogen deposition 18 data exceeds the 3.1 kg N/ha/y r benchmark in more than 38% (1,099,133 ha) of MCF areas, and 19 nitrate leaching has been observed in surface waters. Ozone effects confound nit rogen effects on 20 MCF acidoph yt e lichen, and the interrelationship between fire and nitrogen cycling requires 21 additional research. 22

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59 c. Key uncertainties associated with nutrient enrichment 1 There are different levels of uncertainty associated with relationships between deposition, 2 ecological effects and ecological indicators. The criteria used in the REA to evaluate the degree 3 of confidence in the data, modeling and ecological effect function are detailed in Chapter 7 of the 4 REA . Below is a discussion of uncer tainty relating aquatic and terrestrial ecosystems to nutrient 5 enrichment effects. 6 i. Aquatic ecosystems 7 The approach for assessing atmospheric contributions to total nitrogen loading in the 8 REA was to consider the main

- stem river to an estuary (including the estuary) rather than an 9 entire estuary

s ys tem or bay . The biological indicators used in the NOAA ASSETS EIrequired 10 the evaluation of many national databases including the US Geological Survey National Water 11 Quality

Assessment (NAWQA) files, EPA’s STO Rage and RETrieval (STORET) database, 12 NOAA’s Estuarine Drainage Areas data, and EPA’s water quality standards nutrient criteria for 13 rivers and lakes (US EPA, 200 9 , Appendix 6 and Table 1.2.

- 1 ). Both the SPARROW modeling 14 for nitrogen loads and assessment o f estuary conditions under NOAA ASSETS EI , have been 15 applied on a national scale. The REA concludes that the available data are medium quality with 16 intermediate confidence about the use of these data and their values for extrapolating to a larger 17 regional area ( US EPA, 200 9 , section 7.3.1 ). Intermediate confidence is associated with the 18 modeling approach using ASSETS EIand SPARROW. The REA states there is low confidence 19 with the ecological effect function due to the results of the anal ys is which indicat ed that 20 reductions in atmospheric deposition alone could not solve coastal eutrophication problems due 21 to multiple non

- atmospheric nitrogen inputs (US EPA, 200 9 , section 7.3.3 ). 22

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60 ii. Terrestrial ecosystems 1 Ecological thresholds are identified for CSS and MCF areas and these data are considered 2 to be of high quality , however, the ability to extrapolate these data to larger regional areas is 3 limited (U S EPA, 2009 , section 7.4.1 ). No quantitative modeling was conducted or ecological 4 effect function developed for terrestrial nutrient enrichment reflecting the uncertainties 5 associated with these depositional effects. 6 4. Other Ecological Effects 7 It is stated in the ISA ( US EPA, 2008, section 3.4.1 and 4.5 ) that mercury is a highl y 8 neurotoxic contaminant that enter s the food web as a methylated compound, methylmercury

. 9 Mercury

is principally

methylated by

sulfur

- reducing bacteria and can be taken up by 10 microorganisms, zooplankton and macroinvertebrates. The contaminant is concentrated in higher 11 trophic levels, inclu ding fish eaten b y humans. Experimental evidence has established that only 12 inconsequential amounts of methylmercury can be produced in the absence of sulfate. Once 13 methylmercury

is present, other variables influence how much accumulates in fish, but elevat ed 14 mercury

levels in fish can only

occur where substantial amounts of methylmercury are present. 15 Current evidence indicates that in watersheds where mercury is present, increased oxides of 16 sulfur deposition very likely results in additional production of m eth yl mercury which leads to 17 greater accumulation of MeHg concentrations in fish. With respect to sulfur deposition and 18 mercury

methylation, the final I SA determined: The evidence is sufficient to infer a causal 19 relationship between sulfur deposition and in creased mercury methylation in wetlands and 20 aquatic environments. 21 The production of meaningful amounts of methyl mercury (MeHg) requires the presence 22 of SO 4 2

- and mercury , and where mercury is present, increased availability of SO 4 2

- results in 23

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61 increased production of MeHg. There is increasing evidence on the relationship between sulfur 1 deposition and increased methylation of mercury in aquatic environments; this effect occurs onl y 2 where other factors are present at levels within a range to allow methylat ion. The production of 3 methylmercury

requires the presence of sulfate and mercury , but the amount of methylmercury 4 produced varies with ox ygen content, temperature, pH, and supply of labile organic carbon (US 5 EPA, 2008, section 3.4 ). In watersheds where ch anges in sulfate deposition did not produce an 6 effect, one or several of those interacting factors were not in the range required for meaningful 7 methylation to occur (US EPA, 2008, section 3.4 ). Watersheds with conditions known to be 8 conducive to mercury m ethylation can be found in the northeastern United States and 9 southeastern Canada. 10 While the relationship between sulfur and methylmercury production was concluded to 11 be causal in the IS A, the REA concluded that there was insufficient evidence to quantif y the 12 relationship between sulfur and meth yl mercury . Therefore onl y a qualitative assessment was 13 included in Chapter 6 of the REA

. The Policy Assessment was then unable to make a 14 determination as to the adequacy of the existing SO 2 standards in protecting against welfare 15 effects associated with increased mercury methylation. 16 B. Risk and Exposure Assessment 17 The risk and exposure assessment conducted for the current review was developed to 18 describe potential risk from current and future deposition of oxide s of nitrogen and sulfur to 19 sensitive ecos ys tems. The case study anal ys es in the REA show that there is confidence that 20 known or anticipated adverse ecological effects are occurring under current ambient loadings of 21 nitrogen and sulfur in sensitive ecosy st ems across the United States. An overview of the 22 material covered in the REA , a summary of the key

findings from the air quality analy ses, 23

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62 acidification and nutrient enrichment case studies, and general conclusions from evaluating 1 additional welfare effec ts, are presented below. 2 1. Overview of the Risk and Exposure Assessment 3 The REA evaluates the relationships between atmospheric concentrations, deposition, 4 biologicall y relevant exposures, targeted ecos ys tem effects, and ecos ys tem services. To evaluate 5 the nature and magnitude of adverse effects associated with deposition , the REA also examines 6 various way s to quantify the relationships between air quality indicators, deposition of 7 biologicall y available forms of nitrog en and sulfur, ecologicall y relevant indicators relating to 8 deposition, exposure and effects on sensitive receptors, and related effects resulting in changes in 9
ecos ys tem structure and services. The intent is to determine the exposure metrics that incorpor ate 10 the temporal considerations (i.e., biologically relevant timescales), pathways, and ecologicall y 11 relevant indicators necessary to determine the effects on these ecos ys tems. To the extent feasible, 12 the REA evaluates the overall load to the s ys tem for nitrogen and sulfur, as well as the variability 13 in ecosy stem responses to these pollutants . It also evaluates the contributions of atmosphericall y 14 deposited nitrogen and sulfur individually relative to the c ombined atmospheric loadings of both 15 elements together . Since oxidized nitrogen is the listed criteria pollutant (currentl y measured b y 16 the ambient air quality indicator NO 2 ) for the atmospheric contribution to total nitrogen, the REA 17 examines the contribution of nitrogen oxides to total reactive nitrogen in the atmosphere, relative 18 to the contributions of reduced forms of nitrogen (e.g., ammonia, ammonium), to ultimately 19 assess how a meaningful secondary National Ambient Air Quality Standards (NAAQS) might be 20 structured. 21 The REA focuses on ecosy stem welfare effects that result from the deposition of total 22 reactive nitrogen and sulfur. Because ecos ys tems are diverse in biota, climate, geochemistry , and 23

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63 hydrology , response to pollutant exposures can vary greatl y between ecos ystems. In addition, 1 these diverse ecos ys tems are not distributed evenly across the United States. To target nitrogen 2 and sulfur acidification and nitrogen and sulfur enrichment, the REA addresses four main 3 targeted ecos ys tem effects on t errestrial and aquatic sy stems identified by the IS A (US EPA, 4 2008 ): a quatic acidification due to nitrogen and sulfur ; t errestrial acidification due to nitrogen 5 and sulfur ; a quatic nutrient enrichment, including eutrophication ; and t errestrial nutrient 6
enrichment. 7 In addition to these four targeted ecos ys tem effects, the REA also qualitatively addresses 8 the influence of sulfur oxides deposition on methylmercury production; nitrous oxide (N 2 O) 9 effects on climate; nitrogen effects on primary productivity and biogenic greenhouse gas fluxes; 10 and phy

totoxic effects on plants. 11 Because the targeted ecosy stem effects outlined above are not evenl y distributed across 12 the United States, the REA identified case studies for each targeted effects based on ecos ys tems 13 id entified as sensitive to nitrogen and/or sulfur deposition effects. Eight case study areas and 14 two supplemental study areas (Rocky Mountain National Park and Little Rock Lake, WI) are 15 summarized in the REA based on ecos ys tem characteristics, indicators, and ecos ys tem service 16 information. Case studies selected for aquatic acidification effects were the Adirondack 17 Mountains and Shenandoah National Park. Kane Experimental Forest in Pennsylvania and 18 Hubbard Brook Experimental Forest in New Hampshire were se lected as case studies for 19 terrestrial acidification. Aquatic nutrient enrichment case study locations were selected in the 20 Potomac River Basin upstream of Chesapeake Bay and the Neuse River Basin upstream of the 21 Pamlico Sound in North Carolina. The Coas tal Sage Scrub Communities in southern California 22 and the Mixed Confer Forest Communities in the San Bernardino and Sierra Nevada Mountains 23

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64 of California were selected as case studies for terrestrial nutrient enrichment. Two supplemental 1 areas were also c hosen, one in Rocky Mountain National Park for terrestrial nutrient enrichment 2 and one in L ittle Rock Lake, Wisconsin for aquatic nutrient enrichment. 3 2. Key findings 4 In summary , based on case study anal ys es, the REA concludes that known or anticipated 5
a dverse ecological effects are occurring under current conditions and further concludes that these 6 adverse effects continue into the future

. Key findings from the air quality analyses, acidification 7 and nutrient enrichment case studies, as well as general conclusions from evaluating additional 8 welfare effects, are summarized below. 9

a. Air quality analyses 10 The air quality analyses in the REA encompass the current emissions sources of nitrogen 11 and sulfur, as well as atmospheric concentrations, estimates of dep osition of total nitrogen, 12 policy

- relevant background, and non - ambient loadings of nitrogen and sulfur to ecos ys tems, both 13 nationwide and in the case study areas. Spatial fields of deposition were created using wet 14 deposition measurements from the National Atmospheric Deposition Program (NADP) National 15 Trends Network and dry deposition predictions from the 2002 Community Multi

- Scale Air 16 Quality

(CMAQ) model simulation. Some key conclusions from this analysis are: 17 (1) Total reactive nitrogen deposition and sul fur deposition are much greater in the East 18 compared to most areas of the West. 19 (2) These regional differences in deposition correspond to the regional differences in oxides of 20 nitrogen and SO2 concentrations and emissions, which are also higher in the East. Oxides of 21 nitrogen emissions are much greater and generally more widespread than NH 3 emissions 22 nationwide; high NH 3 emissions tend to be more local (e.g., eastern North Carolina) or sub

- 23

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65 regional (e.g., the upper Midwest and Plains states). The relative amo unts of oxidized versus 1 reduced nitrogen deposition are consistent with the relative amounts of oxides of nitrogen 2 and NH 3 emissions. Oxidized nitrogen deposition exceeds reduced nitrogen deposition in 3 most of the case study areas; the major exception bein g the Neuse River/Neuse River Estuary 4 Case Study

Area. 5 (3) R educed nitrogen deposition exceeds oxidized nitrogen deposition in the vicinity of local 6 sources of NH 3 . 7 (4) There can be relativel y large spatial variations in both total reactive nitrogen deposition and 8 sulfur deposition within a case study area; this occurs particularl y in those areas that contain 9 or are near a high emissions source of oxides of nitrogen , NH 3 , and/or SO 2 . 10 (5) The seasonal patterns in deposition differ between the case study areas. For the c ase study 11 areas in the East, the season with the greatest amounts of total reactive nitrogen deposition 12 correspond to the season with the greatest amounts of sulfur deposition. Deposition peaks in 13 spring in the Adirondack, Hubbard Brook Experimental Forest , and Kane Experimental 14 Forest case study areas, and it peaks in summer in the Potomac River/Potomac Estuary , 15 Shenandoah, and Neuse River/Neuse River Estuary case study

areas. For the case study areas 16 in the West, there is less consistency in the seasons w ith greatest total reactive nitrogen and 17 sulfur deposition in a given area. In general, both nitrogen and/or sulfur deposition peaks in 18 spring or summer. The exception to this is the Sierra Nevada Range portion of the Mixed 19 Conifer Forest Case Study Area, in which sulfur deposition is greatest in winter. 20 b. Deposition

- related aquatic acidification 21 The role of aquatic acidification in two eastern United States areas

— northeastern New 22 York’s Adirondack area and the Shenandoah area in Virginia

— was anal yz ed in the REA to 23

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66 assess surface water trends in SO 4 2

- and NO 3 - concentrations and acid neutralizing capacity (ANC) 1 levels and to affirm the understanding that reductions in deposition could influence the risk of 2 acidification. Monitoring data from the EPA

- administered Temporall y Integrated Monitoring of 3 Ecosy stems (TIME)/Long

- Term Monitoring (LTM) programs and the Environmental Monitoring 4 and Assessment Program (EMAP) were assessed for the years 1990 to 2006, and past, present, 5 and future water quality levels were es timated using both steady

- state and dy namic 6 biogeochemical models. 7 Although wet deposition rates for SO 2 and oxides of nitrogen in the Adirondack Case 8 Study

Area have reduced since the mid

- 1990s, current concentrations are still well above pre

9 acidifi cation (1860) conditions. Model of Acidification of Groundwater in Catchments (MAGIC) 10 modeling predicts NO 3 - and SO 4 2

- are 17

- and 5

- fold higher today , respectively . The estimated 11 average ANC for 44 lakes in the Adirondack Case Study Area is 62.1 microequi valents per liter 12 (μeq/L) (± 15.7 μeq/L); 78 % of all monitored lakes in the Adirondack Case Study Area have a 13 current risk of Elevated, Severe , or Acute . Of the 78%, 31% experience episodic acidification, 14 and 18% are chronicall y acidic today

. 15 (1) Based on the steady- state critical load model for the year 2002, 18%, 28%, 44%, and 58% of 16 169 modeled lakes received combined total sulfur and nitrogen deposition that exceeded 17 critical load s corresponding to ANC limits of 0, 20, 50, and 100 μeq/ L respectivel y. 18 (2) Based on a deposition scenario that maintains current emission levels to 2020 and 2050, the 19 simulation forecast indicates no improvement in water quality in the Adirondack Case Study 20 Area. The percentage of lakes within the Elevated to Acute Concern classes remains the same 21 in 2020 and 2050. 22

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67 (3) Since the mid

- 1990s, streams in the Shenandoah Case Study Area have shown slight declines 1 in NO 3 and SO 4 2

- concentrations in surface waters. ANC levels increased from about 50 2 μeq/Lin the earl y 1990 s to >75 μeq/Luntil 2002, when ANC levels declined back to 1991

– 3 1992 levels. Current concentrations are still above pre

- acidification (1860) conditions. 4 MAGI C mod eling predicts surface water concentrations of NO 3 and SO 4 2

- are 10

- and 32

- 5 fold higher today , respectively . The estimated average ANC for 60 streams in the 6 Shenandoah Case Study Area is 57.9 μeq/L (± 4.5 μeq/L). 55% of all monitored streams in 7 the Shenand oah Case Study Area have a current risk of Elevated, Severe , or Acute . Of the 8 55%, 18% experience episodic acidification, and 18% are chronically acidic today

. 9 (4) Based on the steady- state critical load model for the year 2002, 52%, 72%, 85%, and 93% of 10 60 mo deled streams received combined total sulfur and nitrogen deposition that exceeded 11 critical load s corresponding to ANC limits of 0, 20, 50, and 100 μeq/Lrespectivel y. 12 (5) Based on a deposition scenario that maintains current emission levels to 2020 and 2050, the 13 simulation forecast indicates that a large number of streams still have Elevated to Acute 14 problems with acidity. Infact, from 2006 to 2050, the percentage of streams with Acute 15 Concern increases b y 5%, while the percentage of streams in Moderate Concern decreases 16 by

5%. 17
c. Deposition

- related terrestrial acidification 18 The role of terrestrial acidification was examined in the REA using a critical load 19 anal ys is for sugar maple and red spruce forests in the eastern United States by using the base 20 cation to aluminum (Bc/Al) ratio in acidified forest soils as an indicator to assess the impact of 21 nitrogen and sulfur depos ition on tree health. These are the two most commonly studied species 22 in North America for impacts of acidification. At a Bc/Al ratio of 1.2, red spruce growth can be 23

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68 reduced b y 20%. Sugar maple growth can be reduced b y 20% at a Bc/Al ratio of 0.6. Key 1 fi ndings of the case study are summarized below. 2 (1) Case study

results suggest that the health of at least a portion of the sugar maple and red 3 spruce growing in the United States may have been compromised with acidifying total 4 nitrogen and sulfur deposition in 2002

. 2002 CMAQ/NADP total nitrogen and sulfur 5 deposition levels exceeded three selected critical loads in 3% to 75% of all sugar maple plots 6 across 24 states. The three critical loads ranged from 107 to 6,008 eq/ha/y r for the Bc/Al 7 ratios of 0.6, 1.2, and 10.0 (increasing levels of tree protection)

. The 2002 CMAQ/NADP 8 total nitrogen and sulfur deposition levels exceeded three selected critical loads in 3% to 36% 9 of all red spruce plots across 8 states. The three critical loads ranged from 180 to 4,278 10 eq/ha/y

r for the Bc/Al ratios of 0.6, 1 .2, and 10.0 (increasing levels of tree protection). 11 (2) The Simple Mass Balance model assumptions made for base cation weathering (Bcw) and 12 forest soil ANC input parameters are the main sources of uncertainty since these parameters 13 are rarel y measured and require researchers to use default values. Bcw contributed 49% to 14 the total variability in the critical load estimates, and forest soil ANC contributed 46% to the 15 total variability

. 16 (3) The pattern of case study results sug gests that nitrogen and sulfur acidifying deposition in the 17 sugar maple and red spruce forest areas studied were very close to, if not greater than, the 18 critical loads for those areas and both ecos ys tems are likel y to be sensitive to any future 19 changes in the levels of deposition

. 20 d. Deposition

- related aquatic nutrient enrichment 21 The role of nitrogen deposition in two main stem rivers feeding their respective estuaries 22 was anal yz ed in the REA to determine if decreases in deposition could influence the risk of 23

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69 eutrophication as predicted using the Assessment of Estuarine Trophic Status eutrophication 1 index (ASSETS EI) scoring s ys tem in tandem with SPAtially Referenced Regression on 2 Watershed Attributes (SPARROW) modeling. This modeling approach provides a tra nsferrable, 3 intermediate

- level anal ys is of the linkages between atmospheric deposition and receiving waters, 4 while providing results on which conclusions could be drawn . Future application of the methods 5 to case study areas where atmospheric deposition plays a larger role in the nitrogen loading to an 6 estuary

will likely

provide more tangible results. A summary of findings follows: 7 (1) 2002 CMAQ/NADP results showed that an estimated 40,770,000 kg of total nitrogen was 8 deposited in the Potomac River watershed. SPARROW modeling predicted that 7,380,000 kg 9 N/y

r of the deposited nitrogen reached the estuary (20% of the total load to the estuary). The 10 overall ASSETS EIfor the Potomac River and Potomac Estuary was Bad (based on all 11 sources of N)

. 12 (2) To improve the Pot omac River and Potomac Estuary ASSETS EIscore from Bad to Poor , 13 there is a slim chance that a decrease of at least 78% in the 2002 total nitrogen atmospheric 14 deposition load to the watershed would be required. 15 (3) 2002 CMAQ/NADP results showed that an estimated 18,340,000 kg of total nitrogen was 16 deposited in the Neuse River watershed. SPARROW modeling predicted that 1,150,000 kg 17 N/y

r of the deposited nitrogen reached the estuary (26% of the total load to the estuary). The 18 overall ASSETS EIfor the Neuse Rive r/Neuse River Estuary was Bad

. 19 (4) It was found that the Neuse River/Neuse River Estuary ASSETS EIscore could not be 20 improved from Bad to Poor with decreases onl y in the 2002 atmospheric deposition load to 21 the watershed. Additional reductions would be require d from other nitrogen sources within 22 the watershed. 23

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70 The small effect of decreasing atmospheric deposition in the Neuse River watershed is 1 because the other nitrogen sources within the watershed are more influential than atmospheric 2 deposition to the total nitrogen loadings to the Neuse River Estuary as estimated with the 3 SPARROW model . A waterbody’s response to nutrient loading depends on the magnitude (e.g., 4 agricultural sources have a high influence in the Neuse), spatial distribution, and other 5 charact eristics of the sources within the watershed. 6

e. Deposition

- related terrestrial nutrient enrichment 7 California C oastal S age S crub (CSS) and M ixed C onifer F orest (MCF) communities 8 were the focus of the Terrestrial Nutrient Enrichment Case Studies of the REA. Geographic 9 information sy stems (GIS) analy sis supported a qualitative review of past field research to 10 identify

ecological benchmarks associated with CSS and mycorrhizal communities, as well as 11 MCF’s nutrient

- sensitive acidophy te lichen communities, fine

- root biomass in Ponderosa pine, 12 and leached nitrate in receiving waters. These benchmarks, ranging from 3.1 to 17 kg N/ha/yr, 13 were compared to 2002 CMAQ/NADP data to discern any associations between atmospheric 14 deposition and changing communities. Evidence supports the finding that nitrogen alters CSS 15 and MCF. Key

findings include the following: 16 (1) 2002 CMAQ/NADP nitrogen deposition data show that the 3.3 kg N/ha/yrbenchmark has 17 been exceeded in more than 93% of CSS areas (654,048 ha). These deposition levels are a 18 driving force in the degradation of CSS communities. Although CSS decline has been 19 observed in the absence of fire, the contributions of deposition and fire to the CSS decline 20 require further research. CSS is fragmented into many small parcels, and the 2002 21 CMAQ/NADP 12

- km grid data are not fine enough to fully validate the relationship between 22 CSS distribution, nitrogen deposition, and fire. 23

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71 (2) 2002 CMAQ/NADP nitrogen deposition data exceeds the 3.1 kg N/ha/yr benchmark in more 1 than 38% (1,099,133 ha) of MCF areas, and nitrate leaching has been observed in surface 2 waters. Ozone effects confound nitrogen effects on MCF acidoph yt e lichen, and the 3 interrelationship between fire and nitrogen cycling requires additional research. 4 f. Additional effects 5 Ecol ogical effects have also been documented across the United States where elevated 6 nitrogen deposition has been observed, including the eastern slope of the Rocky Mountains 7 where shifts in dominant algal species in alpine lakes have occurred where wet nitrog en 8 deposition was only

about 1.5 kg N/ha/yr. High alpine terrestrial communities have a low 9 capacity

to sequester nitrogen deposition, and monitored deposition exceeding 3 to 4 kg N/ha/ yr 10 could lead to community

- level changes in plant species, lichens, and mycorrhizae. 11 Additional welfare effects are documented, but examined less extensively , in the REA. 12 These effects include qualitative discussions related to visibility and materials damage, such as 13 corrosion, erosion, and soiling of paint and buildings which are being addressed in the particulate 14 matter (PM) NAAQS review currently underway . A discussion of the causal relationship 15 between sulfur deposition (as sulfate, SO 4 2

- ) and increased mercury methylation in wetlands and 16 aquatic environments is also i ncluded in the REA. On this subject the REA concludes that 17 decreases in sulfate deposition will likely result in decreases in methyl mercury concentration; 18 however, spatial and biogeochemical variations nationally hinder establishing large scale dose

19 respo nse relationships. 20 Several additional issues concerning oxides of nitrogen were addressed in the REA. 21 Consideration was also given to nitrous oxide (N 2 O), a potent GHG. The REA concluded that it 22 is most appropriate to analyze the role of N 2 O in the co ntext of all of the GHGs rather than as 23

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72 part of the REA for this review. The REA considered nitrogen deposition and its correlation 1 with the rate of photos ynthesis and net primary productivity

. Nitrogen addition ranging from 2 15.4 to 300 kg N/ha/yr is documented as increasing wetland N 2 O production by an average of 3 207%. Nitrogen addition ranging from 30 to 240 kg N/ha/y increased CH 4 emissions by 115%, 4 averaged across all ecosystems, and methane uptake was reduced b y 38% when nitrogen addition 5 ranged from 10 to 560 kg N/ha/y r, but reductions were onl y significant for coniferous and 6 deciduous forests. The heterogeneity of ecos ys tems across the United States, however, 7 introduces variations into dose

- response relationships. 8 The p hy

totoxic effects of oxides o f nitrogen and sulfur on vegetation were also briefl y 9 discussed in the REA which concluded that since a unique secondary NAAQS exists for SO 2 , 10 and concentrations of nitric oxide (NO ) , NO 2 , and PAN are rarely high enough to have 11 phy totoxic effects on vegetation , further assessment was not warranted at this time. 12 3. Conclusions on E ffects 13 For aquatic and terrestrial acidification effects, a similar conceptual approach was used 14 (critical loads) to evaluate the impacts of multiple pollutants on an ecologic al e ndpoint, whereas 15 the approaches used for aquatic and terrestrial nutrient enrichment were fundamentally distinct. 16 Although the ecological indicators for aquatic and terrestrial acidification (i.e., ANC and Bc/Al) 17 are very

different, both ecological indicators are well

- correlated with effects such as reduced 18 biodiversity

and growth. While aquatic acidification is clearl y the targeted effect area with the 19 highest level of confidence, the relationship between atmospheric deposition and an ecological 20 indicator is also quite strong for terrestrial acidification. The main drawback with the 21 understanding of terrestrial acidification is that the data are based on laboratory responses rather 22

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73 than field measurements. Other stressors that are present in the field b ut that are not present in 1 the laboratory

may confound this relationship. 2 For nutrient enrichment effects, the REA utilized different types of indicators for aquatic 3 and terrestrial effects to assess both the likelihood of adverse effects to ecosy stems an d the 4 relationship between adverse effects and atmospheric sources of oxides of nitrogen. The 5 ecological indicator chosen for aquatic nutrient enrichment, the ASSETS EI, seems to be 6 inadequate to relate atmospheric deposition to the targeted ecological ef fect, likely

due to the 7 many

other confounding factors. Further, there is far less confidence associated with the 8 understanding of aquatic nutrient enrichment because of the large contributions from non

9 atmospheric sources of nitrogen and the influence of both oxidized and reduced forms of 10 nitrogen, particularl y in large watersheds and coastal areas. However, a strong relationship exists 11 between atmospheric deposition of nitrogen and ecological effects in high alpine lakes in the 12 Rocky

Mountains because atm ospheric deposition is the only source of nitrogen to these 13 sy stems. There is also a strong weight

- of

- evidence regarding the relationships between 14 ecological effects attributable to terrestrial nitrogen nutrient enrichment; however, ozone and 15 climate chang e may be confounding factors. In addition, the response for other species or species 16 in other regions of the United States has not been quantified. 17 C. Adversity of Effects to Public Welfare 18 Characterizing a known or anticipated adverse effect to public welfare is an important 19 component of developing any secondary

NAAQS. According to the Clean Air Act, welfare 20 effects include: “ effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, 21 weather, visibility , and climate, damage to and dete rioration of property, and hazards to 22 transportation, as well as effect on economic values and on personal comfort and well

- being, 23

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74 whether caused b y transformation, conversion, or combination with other air pollutants ” (CAA, 1 Section 302(h)). While the text above lists a number of welfare effects, these effects do not 2 define public welfare in and of themselves. 3 Although there is no specific definition of adversity to public welfare, the paradigm of 4 linking adversity to public welfare to disruptions in ecos ys tem structure and function has been 5 used broadl y b y EPA to categorize effects of pollutants from the cellular to the ecos ys tem level. 6 An evaluation of adversity to public welfare might consider the likelihood, ty pe, magnitude, and 7 spatial scale of the e ffect as well as the potential for recovery and any uncertainties relating to 8 these considerations. 9 Similar concepts were used in past reviews of secondary NAAQS for ozone and PM 10 (relating to visibility ), as well as in initial reviews of effects from l ead deposition. Because 11 oxides of nitrogen and sulfur are deposited from ambient sources into ecosy stems where they 12 affect changes to organisms, populations and ecosystems, the concept of adversity to public 13 welfare as a result of alterations in structure and function of ecos ys tems is an appropriate 14 consideration for this review. 15 Based on information provided in the Policy Assessment , the following section discusses 16 how ecological effects from deposition of oxides o f nitrogen and sulfur relate to adversity to 17 public welfare

. In the Policy Assessment , p ublic welfare was discussed in terms of loss of 18
ecos ys tem services (defined below) , which in some cases can be monetized. Each of the four 19 main effect areas (aquatic and terrestrial acidification and aquatic and terrestrial nutrient over

20 enrichment) are discussed including current ecological effects and associated ecos ys tem services, 21 including potential value of the services. The quantified indicators of value discuss ed below 22

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75 relate to ecos ys tem services generall y that are impacted b y deposition related acidification, not to 1 the specific effects of this action. 2 1. Ecosystem Services 3 The Policy

Assessment defines ecosy stem services as the benefits individuals and 4 organizations obtain from ecosy stems. Ecos ys tem services can be classified as provisioning 5 (food and water), regulating (control of climate and disease), cultural (recreational, existence, 6 spiritual, educational), and supporting (nutrient cycling). Conceptually , changes in ecos ys tem 7 services may

be used to aid in characterizing a known or anticipated adverse effect to public 8 welfare. In the REA and Policy Assessment ecosy stem services are discussed as a method of 9 assessing the magnitude and significance to the public of resource s affected by ambient 10 concentrations of oxides of nitrogen and sulfur and deposition in sensitive ecos ys tems
. 11 EPA has in previous NAAQS reviews defined ecological goods and services for the 12 purposes of a Regulatory Impact Anal ys is as the “outputs of ecological functions or processes 13 that directly

or indirectl y contribute to social welfare or have the potential to do so in the future. 14 Some outputs may

be bought and sold, but most are not marketed.” It is e specially important to 15
a cknowledge that it is difficult to measure and/or monetize the goods and services supplied by 16
ecos ys tems. It can be informative in characterizing adversity to public welfare to attempt to 17 place an economic valuation on the set of goods and services that h ave been identified with 18 respect to a change in policy however it must be noted that this valuation will be incomplete and 19 illustrative only

. 20 Knowledge about the relationships linking ambient concentrations and ecosy stem 21 services is considered in the Pol icy Assessment as one method by which to inform a policy 22 judgment on a known or anticipated adverse public welfare effect. For example, a change in an 23

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76 ecos ys tem structure and process, such as foliar injury , would be classified as an ecological effect, 1 with the associated changes in ecos ys tem services, such as primary productivity , food 2 availability , forest products, and aesthetics (e.g., scenic viewing), classified as public welfare 3 effects. Additionally , changes in biodiversity would be classified as an ecological effect, and the 4 associated changes in ecosy stem services

— productivity , existence (nonuse) value, recreational 5 viewing and aesthetics

—would also be classified as public welfare effects. 6 As described in Chapters 4 and 5 of the REA , case study analy ses were performed that 7 link deposition in sensitive ecos ys tems to changes in a given ecological indicator (e.g., for 8 aquatic acidification, to changes in acid neutralizing capacity [ANC]) and then to changes in 9
ecos ys tems. Appendix 8 of the REA links the changes in ecos ys tems to the services they 10 provide (e.g., fish species richness and its influence on recreational fishing). To the extent 11 possible for each targeted effect area, the REA linked ambient concentrations of nitrogen and 12 sulfur (i.e., ambient air quality indicators) to deposition in sensitive ecosy stems (i.e., exposure 13 pathway s), and then to system response as measured by a given ecological indicator (e.g., lake 14 and stream acidification as measured b y ANC). The ecological effect (e.g., chang es in fish 15 species richness) was then, where possible, associated with changes in ecosy stem services and 16 the corresponding public welfare effects (e.g., recreational fishing). 17 2. Effects on Ecosystem Services 18 The process used to link ecological indicator s to ecosy stem services is discussed 19 extensively

in Appendix 8 of the REA. In brief, for each case study area assessed the ecological 20 indicators are linked to an ecological response that is subsequently linked to associated services 21 to the extent possible . For example in the case study for aquatic acidification the chosen 22 ecological indicator is ANC which can be linked to the ecos ys tem service of recreational fishing. 23

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77 Although recreational fishing losses are the onl y s ervice effects that can be independentl y 1 quantified or monetized at this time, there are numerous other ecos ys tem services that may be 2 related to the ecological effects of acidification. 3 While aquatic acidification is the focus of this proposed standard, the other effect areas 4 were also analy

zed in the REA and these ecos ys tems are being harmed by nitrogen and sulfur 5 deposition and will obtain some measure of protection with any decrease in that deposition 6 regardless of the reason for the decrease. The following summarizes the current levels of specific 7
ecos ys tem services for aquatic and terrestrial acidification, and aquatic and terrestrial nutrient 8
over - enrichment and attempts to quantify and when possible monetize the harm to public 9 welfare, as represented by ecos ys tem services, due to nitrogen and sulfur deposition. 10
a. Aquatic Acidification 11 Acidification of aquatic ecosy stems primarily affects the ecos ys tem services that are 12 derived from the fish and other aquatic life found in surface waters. In the northeastern United 13 States, the surface waters affected by acidification are not a major source of commerciall y raised 14 or caught fish; however, they are a source of food for some recreational and subsistence fishers 15 and for other consumers. Although data and models are available for examining th e effects on 16 recreational fishing, relatively little data are available for measuring the effects on subsistence 17 and other consumers. Inland waters also provide aesthetic and educational services along with 18
non - use services, such as existence value (prote ction and preservation with no expectation of 19 direct use). In general, inland surface waters such as lakes, rivers, and streams also provide a 20 number of regulating services, playing a role in hydrological regimes and climate regulation. 21 There is little ev idence that acidification of freshwaters in the northeastern United States has 22 significantl y degraded these specific services; however, freshwater ecos ys tems also provide 23

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78 biological control services by providing environments that sustain delicate aquatic f ood chains. 1 The toxic effects of acidification on fish and other aquatic life impair these services b y disrupting 2 the trophic structure of surface waters. Although it is difficult to quantify these services and how 3 they are affected by

acidification, it is worth noting that some of these services may be captured 4 through measures of provisioning and cultural services. For example, these biological control 5 services may

serve as “intermediate” inputs that support the production of “final” recreational 6 fishin g and other cultural services. 7 As summarized in Chapter 4 of the Policy Assessment , recent studies indicate that 8 acidification of lakes and streams can result in significant loss in economic value. For example, 9 data indicate that more than 9% of adults i n the northeastern part of the country participate 10 annually

in freshwater fishing yielding 140 million freshwater fishing day s. Each fishing day 11 has an estimated average value per day of $35. Therefore, the implied total annual value of 12 freshwater fishing in the northeastern United States was $5 billion in 2006. Embedded in these 13 numbers is a degree of harm to recreational fishing services due to acidification that has occurred 14 over time. These harms have not been quantified on a regional scale; however, a case study was 15 conducted in the Adirondacks area ( US EPA, 20 11 , section 4.4.2). 16 In the Adirondacks case study , estimates of changes in recreational fishing services were 17 determined, as well as changes more broadl y in “cultural” ecos ys tem services (in cluding 18 recreational, aesthetic, and nonuse services). First, the MAGIC model (U S EPA, 2009, 19 Appendix 8 and s ec tion 2.2) was applied to 44 lakes to predict what ANC levels would be under 20 both “business as usual” conditions (i.e., allowing for some declin e in deposition due to existing 21 regulations) and pre

- emission (i.e., background) conditions. Second, to estimate the recreational 22 fishing impacts of aquatic acidification in these lakes, an existing model of recreational fishing 23

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79 demand and site choice was applied. This model predicts how recreational fishing patterns in the 1 Adirondacks would differ and how much higher the average annual value of recreational fishing 2 services would be for New York residents if lake ANC levels corresponded to background 3 (rather than business as usual) conditions. To estimate impacts on a broader category of cultural 4 (and some provisioning) ecos ys tem services, results from the Banzhaf et al (2006) valuation 5 survey

of New York residents were adapted and applied to this conte xt. The survey used a 6 contingent valuation approach to estimate the average annual household WTP for future 7 reductions in the percent of Adirondack lakes impaired b y acidification. The focus of the survey 8 was on impacts on aquatic resources. Pretesting o f the survey indicated that respondents 9 nonetheless tended to assume that benefits would occur in the condition of birds and forests as 10 well as in recreational fishing. 11 By

extrapolating the 44 lake Adiriondack case study to all 3,000 Adirondack lakes and by 12 applying the willingness to pay survey results to all New York residents, the study estimated 13 aggregated benefits between $300 and $800 million annually for the equivalent of improving 14 lakes in the Adirondacks region to an ANC level of 50 μeq/L . The REA estimated 44% of the 15 Adirondack lakes currently fall below an ANC of 50 μeq/L . Several states have set goals for 16 improving the acid status of lakes and streams, generall y targeting ANC in the range of 50 to 60 17 μeq/L , and have engaged in costly activiti es to decrease acidification. 18 These results imply significant value to the public in addition to those derived from 19 recreational fishing services. Note that the results are onl y applicable to improvements in the 20 Adirondacks valued b y residents of New Y ork. If similar benefits exist in other acid

- impacted 21 areas, benefits for the nation as a whole could be substantial. The anal ys is provides results on 22

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80 only a subset of the impacts of acidification on ecosy stem services and suggests that the overall 1 impact on these services is likely to be substantial. 2 b. Terrestrial Acidification 3 Chapter 4.4.3 and 4.4.4 of the Policy Assessment review several economic studies of 4 areas sensitive to terrestrial acidification. Forests in the northeastern United States provide 5 several important and valuable provisioning ecos ystem services, which are reflected in the 6 production and sales of tree products. Sugar maples are a particularly important commercial 7 hardwood tree species in the United States, producing timber and maple s y rup that provide 8 hundreds of millions of dollars in economic value annually . Red spruce is also used in a variety 9 of wood products and provides up to $100 million in economic value annually . Although the 10 data do not exist to directly link acidification damages to economic values of lost recreational 11
ecos ys tem services in forests, these resources are valuable to the public. A recent study, 12 reviewed in the Policy Assessment, suggests that the total annual value of recreational off

- road 13 driving was more th an $9 billion and the value of hunting and wildlife viewing was more than $4 14 billion each in the northeastern United States. EPA is not able to quantify at this time the 15 specific effects on these values of acid deposition, or of any specific reductions i n deposition, 16 relative to the effects of many other factors that may

affect them. 17

c. Nutrient Enrichment 18 Chapter 4.4.5 and 4.4.6 of the Policy Assessment summarize economic studies of east 19 coast estuaries affected by nutrient over

- enrichment or eutrophicatio n. Estuaries in the eastern 20 United States are important for fish and shellfish production. The estuaries are capable of 21 supporting large stocks of resident commercial species, and they serve as the breeding grounds 22 and interim habitat for several migratory species. To provide an indication of the magnitude of 23

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81 provisioning services associated with coastal fisheries, from 2005 to 2007, the average value of 1 total catch was $1.5 billion per year in 15 East Coast states. Estuaries also provide an important 2 and substantial variety of cultural ecos ys tem services, including water

- based recreational and 3 aesthetic services. For example, data indicate that 4.8% of the population in coastal states from 4 North Carolina to Massachusetts participated in saltwater fishing, with a total of 26 million 5 saltwater fishing day s in 2006. Based on estimates in the Policy Assessment , total recreational 6 value from these saltwater fishing days was approximately $1.3 billion. Recreational 7 participation estimates for 1999

–2000 showed almost 6 million individuals participated in 8 motorboating in coastal states from North Carolina to Massachusetts. The aggregate value of 9 these coastal motorboating outings was $2 billion per year. EPA is not able to quantify at this 10 time the specific effe cts on these values of acid deposition, or of any specific reductions in 11 deposition, relative to the effects of many other factors that may affect them. 12 Terrestrial ecos ys tems can also suffer from nutrient over

- enrichment. Each ecos ys tem is 13 different in its composition of species and nutrient requirements. Changes to individual 14
ecos ys tems from changes in nitrogen deposition can be hard to assess economically . Relative 15 recreational values are often determined b y public use information. Chapter 4.4.7 of the Policy 16 Assessment reviewed studies related to park use in California. Data from California State Parks 17 indicate that in 2002, 68.7% of adult residents participated in trail hiking for an average of 24.1 18 day s per year. The anal ys es in the Policy Assess ment indicate that the aggregate annual benefit 19 for California residents from trail hiking in 2007 was $11.59 billion. EPA is not able to quantify 20 at this time the specific effects on these values of acid deposition, or of any specific reductions in 21 depos ition, relative to the effects of many other factors that may affect them. 22

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82 The Policy

Assessment also identified fire regulation as a service that could be affected 1 by nutrient over

- enrichment of the Coastal Sage Scrub and Mixed Conifer Forest ecos ys tems by 2 encouraging growth of more flammable grasses, increasing fuel loads, and altering the fire cycle. 3 Over the 5

- ye ar period from 2004 to 2008, Southern California experienced, on average, over 4 4,000 fires per ye ar, burning, on average, over 400,000 acres p er ye ar. It is not possible at this 5 time to quantify

the contribution of nitrogen deposition, among many other factors, to increased 6 fire risk. 7 3. Summary 8 Adversity

to public welfare can be understood by looking at how deposition of oxides of 9 nitro gen and sulfur affect the ecological functions of an ecos ys tem (see II.A.), and then 10 understanding the ecosy stem services that are degraded. The monetized value of the ecos ys tem 11 services provided b y ecosystems that are sensitive to deposition of oxides of nitrogen and sulfur 12 are in the billions of dollars each ye ar , though it is not possible to quantify or monetize at this 13 time the effects on these values of acid deposition or of any changes in deposition that may result 14 from new secondary stan dards . Man y lakes and streams are known to be degraded b y acidic 15 deposition which affects recreational fishing and tourism. Forest growth is likely suffering from 16 acidic deposition in sensitive areas affecting red spruce and sugar maple timber production , sugar 17 maple sy

rup production, hiking, aesthetic enjoy ment and tourism. Nitrogen deposition 18 contributes significantly to eutrophication in many estuaries affecting fish production, 19 swimming, boating, aesthetic enjo ym ent, and tourism. Ecosy stem services are likel y affected by 20 nutrient enrichment in many natural and scenic terrestrial areas , affecting biodiversity , including 21 habitat for rare and endangered species, fire control, hiking, aesthetic enjoyment, and tourism. 22 D. Adequ acy of the Current Standards 23

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83 An important issue to be addressed in the current review of the secondary standards for 1 oxides and nitrogen and sulfur standard is whether, in view of the scientific evidence reflected in 2 the I SA, additional information on exp osure and risk discussed in the REA, and conclusions 3 drawn from the Policy Assessment , the existing standards provide adequate protection. The 4 Administrator therefore, has considered the extent to which the current standards are adequate 5 for the protecti on of public welfare. Having reached th e general conclusion that aquatic and 6 terrestrial ecos ys tems can be degrade b y deposition of oxides of nitrogen and sulfur , it is then 7 necessary

to evaluate the adequacy of the current secondary standards for oxides of nitrogen and 8 sulfur to provide requisite protection by considering to what degree risks to sensitive ecos ys tems 9 would be expected to occur in areas that meet the current standards. Conclusions regarding the 10 adequacy

of the current standards are based on the available ecological effects, exposure and 11 risk

- based evidence. In evaluating the strength of this information, EPA has taken into account 12 the uncertainties and limitations in the scientific evidence. This section addresses the adequacy 13 of the current standards to protect against direct exposure effects on plants from oxides of 14 nitrogen and sulfur, the appropriateness of the current structure of the standards to address 15 deposition

- related effects of oxides of nitrogen and sulfur on sensitive ecosy stems and finally , 16 the adequacy

of such standards to protect against adverse effects related to the deposition of 17 oxides of nitrogen and sulfur. 18 1. Adequacy of the Current Standards for Direct Effects 19 The current secondary oxides of nitrogen and sulfur standards are intended to protect 20 against adverse effects to public welfare. For oxides of nitrogen, the current secondary standard 21

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84 was set identical to the primary standard 3 , e.g. an annual standard set for NO 2 to protect against 1 adverse effects on vegetation from direct exposure to ambient oxides of nitrogen. For oxides of 2 sulfur, the current secondary standard is a 3

- hour standard intended to provide protection for 3 plants from the direct foliar damage associated with atmospheric concentrations of SO 2 . It is 4 appropriate to consider whether the current standards are adequate to protect against the direct 5 effects on vegetatio n resulting from ambient NO 2 and SO 2 which were the basis for the current 6 secondary

standards. The I SA concluded that there was sufficient evidence to infer a causal 7 relationship between exposure to SO 2 , NO, NO 2 and PAN and injury to vegetation. Addition al 8 research on acute foliar injury has been limited and there is no evidence to suggest foliar injury 9 below the levels of the current secondary standards for oxides of nitrogen and sulfur. There is 10 sufficient evidence to suggest that the levels of the cur rent standards are likely

adequate to 11 protect against direct ph ytotoxic effects. 12 2. Appropriateness and Adequacy of the Current Standards for Deposition

- related Effects 13 This section addresses two concepts necessary to evaluate the current standards in the 14 context of deposition related effects. First, appropriateness of the current standards is considered 15 with regard to indicator, form, level and averaging time. This discussion centers around the 16 ability

of the current standards to evaluate and provide protection against deposition related 17 effects that vary

spatiall y and temporall y. It includes particular emphasis on the indicators and 18 forms of the current standards and the degree to which they are ecologically

relevant with regard 19 to deposition related effects. Second, this section evaluates the current standards in terms of 20 adequacy

of protection. 21
a. Appropriateness 22

3 The current primary NO 2 standard has recently been changed to the 3 year average of the 98 th percentile of the annual distribution of the 1 hour daily maximum of the concentration of NO 2 . The current secondary standard remains as it was set in 1971.

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85 The I SA has established that the major effects of concern for this review of the oxides of 1 nitrogen and sulfur standards are a ssociated with deposition of N and S caused by atmospheric 2 concentrations of oxides of nitrogen and sulfur. The current standards are not directed toward 3 depositional effects, and none of the elements of the current NAAQS

– indicator, form, 4 averaging time , and level

– are suited for addressing the effects of N and S deposition. 5 Four issues arise that call into question the ecological relevance of the structure of the 6 current secondary

standards for oxides of nitrogen and sulfur. 7 (1) The current SO 2 seco ndary standard (0.5 ppm SO 2 over a 3

- hour average) does not utilize an 8 exposure period that is relevant for ecos ys tem impacts. The majority of deposition related 9 impacts are associated with depositional loads that occur over periods of months to years. 10 T his differs significantl y from exposures associated with hourly concentrations of SO 2 as 11 measured b y the current secondary standard. By addressing short

- term concentrations, the 12 current SO 2 secondary standard, while protective against direct foliar effect s from gaseous 13 oxides of sulfur, does not take into account the findings of effects in the I SA, which notes the 14 relationship between annual deposition of S and acidification effects which are likel y to be 15 more severe and widespread than ph yt otoxic effects under current ambient conditions, and 16 include effects from long term deposition as well as short term. Acidification is a process 17 which occurs over time, as the ability of an aquatic sy stem to counteract acidic inputs is 18 reduced as natural buffers are use d more rapidly than they can be replaced through geologic 19 weathering. The relevant period of exposure for ecosy stems is therefore not the exposures 20 captured in the short averaging time of the current SO 2 secondary standard. The current 21 secondary

standard f

or oxides of nitrogen is an annual standard (0.053 ppm averaged over 1 22 ye ar) and as such is more ecologicall y relevant. Current standards do not utilize appropriate 23

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86 atmospheric indicators. NO 2 and SO 2 are used as the component of oxides of nitrogen and 1
s ulfur that are measured, but they do not provide a complete link to the direct effects on 2
ecos ys tems from deposition of oxides of nitrogen and sulfur as they do not capture all 3 relevant chemical species of oxidized nitrogen and oxidized sulfur that contrib ute to 4 deposition. The I SA provides evidence that deposition related effects are linked with total 5 nitrogen and total sulfur deposition, and thus all forms of oxidized nitrogen and oxidized 6 sulfur that are deposited will contribute to effects on ecos ys tem s. Thus, by using 7 atmospheric NO 2 and SO 2 concentrations as indicators, the current standards address onl y a 8 fraction of total atmospheric oxides of nitrogen and sulfur, and do not take into account the 9 effects from deposition of total atmospheric oxides of nitrogen and sulfur. This suggests that 10 more comprehensive atmospheric indicators should be considered in designing ecologicall y 11 relevant standards. 12 (2) Current standards reflect separate assessments of the two individual pollutants, NO 2 and SO 2 , 13 rather than assessing the joint impacts of deposition to ecos ys tems. Recognizing the role that 14 each pollutant play s in jointly affecting ecos ys tem indicators, functions, and services is vital 15 to developing a meaningful standard. The clearest example of this int eraction is in 16 assessment of the impacts of acidifying deposition on aquatic ecosy stems. Acidification in 17 an aquatic ecos ys tem depends on the total acidifying potential of the deposition of both N 18 and S from both atmospheric deposition of oxides of nitrog en and sulfur as well as the inputs 19 from other sources of N and S such as reduced nitrogen and non

- atmospheric sources. It is 20 the joint impact of the two pollutants that determines the ultimate effect on organisms within 21 the ecos ys tem, and critical ecos ys t em functions such as habitat provision and biodiversity

. 22 Standards that are set independentl y are less able to account for the contribution of the other 23

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87 pollutant. This suggests that interactions between oxides of nitrogen and oxides of sulfur 1 should be a critical element of the conceptual framework for ecologically relevant standards. 2 There are also important interactions between oxides of nitrogen and sulfur and reduced 3 forms of nitrogen, which also contribute to acidification and nutrient enrichment. It is 4 important that the structure of the standards address the role of reduced nitrogen in 5 determin ing the ecological effects resulting from deposition of atmospheric oxides of 6 nitrogen and sulfur. Consideration will also have to be given to total loadings as ecos ys tems 7 respond to all sources of N and S. 8 (3) Current standards do not take into account varia bility in ecos ys tem sensitivity . Ecos ys tems 9 are not uniforml y distributed either spatiall y or temporally in their sensitivity to oxides of 10 nitrogen and sulfur . Therefore, failure to account for the major determinants of variability , 11 includin g geological and soil characteristics related to the sensitivity to acidification or 12 nutrient enrichment as well as atmospheric and landscape characteristics that govern rates of 13 deposition, may lead to standards that do not provide requisite levels of pro tection across 14
ecos ys tems. The current structures of the standards do not address the complexities in the 15 responses of ecos ys tems to deposition of oxides of nitrogen and sulfur. Ecosy stems contain 16 complex groupings of organisms that respond in various ways to the alterations of soil and 17 water that result from deposition of nitrogen and sulfur compounds. Different ecos ys tems 18 therefore respond in different way s depending on a multitude of factors that control how 19 deposition is integrated into the sy stem. For example, the same levels of deposition falling 20 on limestone dominated soils have a very different effect from those falling on shallow 21 glaciated soils underlain with granite. One s ys tem may over time display

no obvious 22 detriment while the other m ay experience a catastrophic loss in fish communities. This 23

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88 degree of sensitivity

is a function of many

atmospheric factors which control rates of 1 deposition as well as ecological factors which control how an ecosy stem responds to that 2 deposition. The current standards do not take into account spatial and seasonal variations not 3 only

in depositional loadings but also in sensitivity of ecos ys tems exposed to those loadings. 4 Based on the discuss ion summarized above, the Policy Assessment concludes that the current 5 secondary

standards for oxides of nitrogen and oxides of sulfur are not ecologically relevant 6 in terms of averaging time, form, level or indicator. 7 b. Adequacy of Protection 8 As described in the Policy Assessment , 2005 ambient conditions indicate that the current 9 SO 2 and NO 2 secondary standards are not exceeded (US EPA, 20 11 , Figures 6

- 1 and 6

- 2) in 10 locations where ecological effects have been observed. In many locations, SO 2 and NO 2 11 concentrations are substantially below the levels of the secondary standards. This pattern 12 suggests that levels of deposition and any effects on ecos ys tems due to deposition of oxides of 13 nitrogen and sulfur under recent conditions are occurring even though areas meet or are below 14 current standards. In addition, these l evels based on conclusions in the REA will not decline in 15 the future to levels below which it is reasonable to anticipate effects. 16 Indetermining the adequacy of the current secondary standards for oxides of nitrogen 17 and sulfur the Policy Assessment cons idered the extent to which ambient deposition contributes 18 to loadings in ecos ys tems. Since the last review of the secondary standard for oxides of nitrogen, 19 a great deal of information on the contribution of atmospheric deposition associated with 20 ambient oxides of nitrogen has become available. The REA presents a thorough assessment of 21 the contribution of oxidized nitrogen to nitrogen deposition throughout the U.S., and the relative 22 contributions of ambient oxidized and reduced forms of nitrogen. The REA concludes that based 23

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89 on that anal ys is, ambient oxides of nitrogen are a significant component of atmospheric nitrogen 1 deposition, even in areas with relatively high rates of deposition of reduced nitrogen. In 2 addition, atmospheric deposition of oxidized nitrogen contributes significantly to total nitrogen 3 loadings in nitrogen sensitive ecosy stems. 4 The I SA summarizes the available studies of relative nitrogen contribution and finds that 5 in much of the U.S., oxides of nitrogen contribute from 50 to 75 percent of total atmospheric 6 deposition relative to total reactive nitrogen that includes oxidized and reduced nitrogen species 7 (US EPA, 2008, section 2.8.4 ). Although the proportion of total nitrogen loadings associated 8 with atmospheric deposition of nitrog en varies across locations (N deposition in the eastern U.S. 9 includes locations with greater than 9 kg N/ha

- yr , and in the central U.S. high deposition 10 locations with values on the order of 6 to 7 kg N/ha

- yr) , the IS A indicates that atmospheric N 11 depositi on is the main source of new anthropogenic N to most headwater streams, high elevation 12 lakes, and low

- order streams. Atmospheric N deposition contributes to the total N load in 13 terrestrial, wetland, freshwater, and estuarine ecosy stems that receive N throu gh multiple 14 pathway s. In several large estuarine s ys tems, including the Chesapeake Bay , atmospheric 15 deposition accounts for between 10 and 40 percent of total nitrogen loadings (U S EPA, 2008 ). 16 Atmospheric concentrations of oxides of sulfur account for nearl y all S deposition in the 17 US. For the period 2004

–2006, mean S deposition in the U.S. was greatest east of the 18 Mississippi River with the highest deposition amount, 21.3 kg S/ha

- yr, in the Ohio River Valley 19 where most recording stations reported 3 ye ar averages >10 kg S/ha

- yr . Numerous other stations 20 in the East reported S deposition >5 kg S/ha

- yr. Total S deposition in the U.S. west of the 100th 21 meridian was relativel y low, with all recording stations reporting <2 kg S/ha

- yr and man y 22 reporting <1 kg S/ha

- yr. S was primarily deposited in the form of wet SO 4 2

− followed in 23

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90 decreasing order b y a smaller proportion of dry SO 2 and a much smaller proportion of deposition 1 as dry

SO 4 2

−

. 2 As discussed throughout the REA (US EPA, 200 9 and s ection IIB above) , there are 3 several key

areas of risk that are associated with ambient concentrations of oxides of nitrogen 4 and sulfur. As noted earlier, in previous reviews of the secondary standards for oxides of 5 nitrogen and sulfur, the standards were designed to prot ect against direct exposure of plants to 6 ambient concentrations of the pollutants. A significant shift in understanding of the effects of 7 oxides of nitrogen and sulfur has occurred since the last reviews, reflecting the large amount of 8 research that has b een conducted on the effects of deposition of nitrogen and sulfur to 9
ecos ys tems. The most significant current risks of adverse effects to public welfare are those 10 related to deposition of oxides of nitrogen and sulfur to both terrestrial and aquatic ecosy stems. 11 These risks fall into two categories, acidification and nutrient enrichment, which were 12 emphasized in the REA as most relevant to evaluating the adequacy of the existing standards in 13 protecting public welfare from adverse ecological effects. 14 i. Aqu atic acidification 15 The focus of the REA case studies was on determining whether deposition of sulfur and 16 oxidized nitrogen in locations where ambient oxides of nitrogen and sulfur were at or below the 17 current standards was resulting in acidification and related effects, including episodic 18 acidification and mercury meth yl ation. Based on the case studies conducted for lakes in the 19 Adirondacks and streams in Shenandoah National Park (case studies are discussed more full y in 20
s ection IIB and US EPA, 2009 ), th ere is significant risk to acid sensitive aquatic ecos ys tems at 21 atmospheric concentrations of oxides of nitrogen and sulfur at or below the current standards. 22 The REA also supports strongl y a relationship between atmospheric deposition of oxides of 23

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91 nitrog en and sulfur and loss of ANC in sensitive ecosy stems and indicates that ANC is an 1 excellent indicator of aquatic acidification. The REA also concludes that at levels of deposition 2 associated with oxides of nitrogen and sulfur concentrations at or below the current standards, 3 ANC levels are expected to be below benchmark values that are associated with significant 4 losses in fish species richness. 5 Significant portions of the U.S. are acid sensitive, and current deposition levels exceed 6 those that would all ow recovery of the most acid sensitive lakes in the Adirondacks (US EPA, 7 2008, Executive Summary). In addition, because of past loadings, areas of the Shenandoah are 8 sensitive to current deposition levels (US EPA, 2008, Executive Summary). Parts of the W est 9 are naturall y less sensitive to acidification and subjected to lower deposition (particularl y SO x ) 10 levels relative to the eastern United States, and as such, less focus in the ISA is placed on the 11 adequacy

of the existing standards in these areas, with the exception of the mountainous areas of 12 the West, which experience episodic acidification due to deposition. 13 In describing the effects of acidification in the two case study areas the REA uses the 14 approach of describing benchmarks in terms of ANC valu es. Man y locations in sensitive areas of 15 the U.S. have ANC levels below benchmark levels for ANC classified as severe, elevated, or 16 moderate concern (US EPA, 20 11 , Figure 2

- 1). The average current ANC levels across 44 lakes 17 in the Adirondack case study are a is 62.1 μeq/L (moderate concern). However, 44 percent of 18 lakes had deposition levels exceeding the critical load for an ANC of 50 μeq/L , and 28 percent of 19 lakes had deposition levels exceeding the (higher) critical load for an ANC of 20 μeq/L(US 20 EPA, 20 0 9 , section 4.2.4.2). This information indicates that almost half of the 44 lakes in the 21 Adirondacks case study area are at an elevated concern level , and almost a third are at a severe 22 concern level. These levels are associated with greatly diminished f

ish species diversity , and 23

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92 losses in the health and reproductive capacity of remaining populations. Based on assessments of 1 the relationship between number of fish species and ANC level in both the Adirondacks and 2 Shenandoah areas, the number of fish speci es is decreased b y over half at an ANC level of 20 3 μeq/Lrelative to an ANC level at 100 μeq/L (US EPA, 200 9, Figure 4.2

- 1). When extrapolated 4 to the full population of lakes in the Adirondacks area using weights based on the EMAP 5 probability

survey (US E PA, 200 9 , section 4.2.6.1 **)** , 36 percent of lakes exceeded the critical load 6 for an ANC of 50 μeq/L and 13 percent of lakes exceeded the critical load for an ANC of 20 7 μeq/L

. 8 Many

streams in the Shenandoah case study area also have levels of deposition that are 9 associated with ANC levels classified as severe, elevated, or moderate concern. The average 10 ANC under recent conditions for the 60 streams evaluated in the Shenandoah case study area is 11 57.9 μeq/L , indicating moderate concern. However, 85 percent of these streams had recent 12 deposition exceeding the critical load for an ANC of 50 μeq/L , and 72 percent exceeded the 13 critical load for an ANC of 20 μeq/L

. As with the Adirondacks area, this information suggests 14 that significant numbers of sensitive streams in the Shenandoah area are at risk of adverse 15 impacts on fish populations under recent conditions. Many other streams in the Shenandoah area 16
are also likel y to experience conditions of elevated to severe concern based on the prevalence in 17 the area of bedro ck geology associated with increased sensitivity to acidification suggesting that 18 effects due to stream acidification could be widespread in the Shenandoah area (US EPA, 200 9 , 19 section 4.2.6.2). 20 In addition to these chronic acidification effects, the I SA n otes that “consideration of 21 episodic acidification greatly increases the extent and degree of estimated effects for acidifying 22 deposition on surface waters.” (US EPA, 2008, section 3.2.1.6 ) Some studies show that the 23

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93 number of lakes that could be classifi ed as acid

- impacted based on episodic acidification is 2 to 3 1 times the number of lakes classified as acid

- impacted based on chronic ANC. These episodic 2 acidification events can have long term effects on fish populations (US EPA, 2008, section 3 3.2.1.6 ). Under recent conditions, episodic acidification has been observed in locations in the 4 eastern U.S. and in the mountainous western U.S. (US EPA, 2008, section 3.2.1.6 ). 5 The I SA, REA and Policy Assessment all conclude that the current standards are not 6 ade quate to protect against the adverse impacts of aquatic acidification on sensitive ecos ys tems. 7 In the IS A it is noted that significant portions of the U.S. are acid sensitive, and that current 8 deposition levels exceed those that would allow recovery of th e most acid sensitive lakes in the 9 Adirondacks ( US EPA, 2008, Executive Summary). In addition, because of past loadings, areas 10 of the Shenandoah are sensitive to current deposition levels (US EPA, 2008, Executive 11 Summary). A recent survey , as reported in the ISA, found sensitive streams in many locations in 12 the U.S., including the Appalachian Mountains, the Coastal Plain, and the Mountainous West 13 (US EPA, 2008, section 4.2.2.3). In these sensitive areas, between 1 and 6 percent of stream 14 kilometers are chro nically acidified. The REA further concludes that both the Adirondack and 15 Shenandoah case study areas are currentl y receiving deposition from ambient oxides of nitrogen 16 and sulfur in excess of their ability to neutralize such inputs. In addition based on the current 17 emission scenarios, forecast modeling out to the year 2020 as well as 2050 indicates a large 18 number of streams in these areas will still be adversely impacted ( s ection IIB). Based on these 19 considerations, the Policy Assessment concludes that the current secondary NAAQS for oxides 20 of nitrogen and sulfur do not provide adequate protection of sensitive ecos ystems with regard to 21 aquatic acidification

. 22 ii. Terrestrial acidification 23

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94 Based on the terrestrial acidification case studies (Kane Experimental Forest in 1 Pennsylvania and Hubbard Brook Experimental Forest described in section IIB) on sugar maple 2 and red spruce habitat, the REA concludes that there is significant risk to sensitive terrestrial 3
ecos ys tems from acidification at atmospheric concentrations of NO x and SO X at or below the 4 current standards. The ecological indicator selected for terrestrial acidification is the base 5 cation to aluminum ratio (BC:Al), which has been linked to tree health and growth. The results 6 of the REA strongly support a relationship between atmospheric deposition of oxides of nitrogen 7 and sulfur and BC:Al, and that BC:Al is a good indicator of terrestrial acidification. At levels of 8 deposition associated with oxides of nitrogen and sulfur conce ntrations at or below the current 9 standards, BC:Al levels are expected to be below benchmark values that are associated with 10 significant effects on tree health and growth. Such degradation of terrestrial ecos ys tems could 11 affect ecos ys tem services such as habitat provisioning, endangered species, goods production 12 (timber, sy

rup, etc.) among others. 13 Many

locations in sensitive areas of the U.S. have BC:Al levels below benchmark levels 14 classified as providing low to intermediate levels of protecti on to tree health. At a BC:Al ratio of 15 1.2 (intermediate level of protection), red spruce growth can be reduced b y 20 percent. At a 16 BC:Al ratio of 0.6 (low level of protection), sugar maple growth can be decreased b y 20 percent. 17 The REA did not evaluat e broad sensitive regions. However, in the sugar maple case study area 18 (Kane Experimental Forest), recent deposition levels are associated with a BC:Al ratio below 19 1.2, indicating between intermediate and low level of protection, which would indicate the 20 potential for a greater than 20 percent reduction in growth. In the red spruce case study area 21 (Hubbard Brook Experimental Forest), recent deposition levels are associated with a BC:Al ratio 22

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95 slightly above 1.2, indicating slightl y better than an intermedi ate level of protection (US EPA, 1 200 9 , section 4.3.5.1 ). 2 Over the full range of sugar maple, 12 percent of evaluated forest plots exceeded the 3 critical loads for a BC:Al ratio of 1.2, and 3 percent exceeded the critical load for a BC:Al ratio 4 of 0.6. However, there was large variability across states. In New Jersey , 67 percent of plots 5 exceeded the critical load for a BC:Al ratio of 1.2, while in several states on the outskirts of the 6 range for sugar maple (e.g. Arkansas, Illinois) no plots exceeded t he critical load for a BC:Al 7 ratio of 1.2. For red spruce, overall 5 percent of plots exceeded the critical load for a BC:Al ratio 8 of 1.2, and 3 percent exceeded the critical load for a BC:Al ratio of 0.6 . In the major red spruce 9 producing states (Maine , New Hampshire, and Vermont), critical loads for a BC:Al ratio of 1.2 10 were exceeded in 0.5, 38, and 6 percent of plots. 11 The I SA, REA and Policy Assessment all conclude that the current standards are not 12 adequate to protect against the adverse impacts of terrestrial acidification on sensitive 13
ecos ys tems. The ISA reported one study that estimated 15 percent of U.S. forest ecos ys tems 14 exceeded the critical loads for acidity for N and S deposition by >250 eq/ha/year under current 15 conditions (US EPA, 2008, sect ion 4.2.1.3 ) . The main drawback, as stated in the REA and 16 Policy

Assessment , with the understanding of terrestrial acidification lies in the availability of 17 data b y which we can predict critical loads and that the data are based on laboratory responses 18

rather than field measurements. Other stressors that are present in the field but that are not 19 present in the laboratory may confound this relationship. The REA does however, conclude that 20 the case study results suggest that nitrogen and sulfur acidifying d eposition in the sugar maple 21 and red spruce forest areas studied were very close to, if not greater than, the critical loads for 22

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96 those areas, and both ecosy stems are likel y to be sensitive to any future changes in the levels of 1 deposition. 2 iii. Terrestrial nu trient e nrichment 3 Nutrient enrichment effects are due to nitrogen loadings from both atmospheric and non

4 atmospheric sources. Evaluation of nutrient enrichment effects requires an understanding that 5 nutrient inputs are essential to ecosy stem health and th at specific long term levels of nutrients in 6 a sy stem affect the types of species that occur over long periods of time. Short term additions of 7 nutrients can affect species competition, and even small additions of nitrogen in areas that are 8 traditionally nutrient poor can have significant impacts on productivity as well as species 9 composition. Most ecosy stems in the United States are nitrogen

- limited, so regional decreases 10 in emissions and deposition of airborne nitrogen compounds could lead to some decrease in 11 growth of the vegetation that surrounds the targeted aquatic s ys tem but as discussed below 12 evidence for this is mixed. Whether these changes in plant growth are seen as beneficial or 13 adverse will depend on the nature of the ecosy stem being assesse d. 14 In formation on the effects of changes in nitrogen deposition on forestlands and other 15 terrestrial ecos ys tems is very limited. The multiplicity of factors affecting forests, including other 16 potential stressors such as ozone, and limiting factors such a s moisture and other nutrients, 17 confound assessments of marginal changes in any one stressor or nutrient in forest ecos ys tems. 18 The I SA notes that only a fraction of the deposited nitrogen is taken up by the forests, most of 19 the nitrogen is retained in the soils (US EPA, 2008, section 3.3.2.1 ). In addition, the I SA 20 indicates that forest management practices can significantly affect the nitrogen cycling within a 21 forest ecos ys tem, and as such, the response of managed forests to NO x deposition will be 22 variable depending on the forest management practices employed in a given forest ecos ys tem 23

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97 (US EPA, 2008, Annex C C.6.3 ). Increases in the availability of nitrogen in N

- limited forests 1 via atmospheric deposition could increase forest production over large non

- man aged areas, but 2 the evidence is mixed, with some studies showing increased production and other showing little 3 effect on wood production (US EPA, 2008, section 3.3.9) . Because leaching of nitrate can 4 promote cation losses, which in some cases create nutrie nt imbalances, slower growth and 5 lessened disease and freezing tolerances for forest trees, the net effect of increased N on forests 6 in the U.S. is uncertain ( US EPA, 2008, section 3.3.9). 7 The scientific literature has many examples of the deleterious eff ects caused by

excessive 8 nitrogen loadings to terrestrial sy stems. Several studies have set benchmark values for levels of 9 N deposition at which scientifically adverse effects are known to occur. Large areas of the 10 country

appear to be experiencing depos ition above these benchmarks. The I SA indicates that 11 studies have found that at 3.1 kg N/ha

- yr , the community of lichens begins to change from 12 acidoph yt ic to tolerant species; at 5.2 kg N/ha

- yr, the typical dominance b y acidoph yt ic species 13 no lo nger occurs; and at 10.2 kg N/ha

- yr, acidophytic lichens are totall y lost from the 14 community . Additional studies in the Colorado Front Range of the Rocky Mountain National 15 Park support these findings. These three values (3.1, 5.2, and 10.2 kg/ha

- yr) are on e set of 16 ecologicall y meaningful benchmarks for the mixed conifer forest (MCF) of the pacific coast 17 regions. Nearl y all of the known sensitive communities receive total nitrogen deposition levels 18 above the 3.1 N kg/ha

- yr ecological benchmark according to the12 km, 2002 CMAQ/NADP 19 data, with the exception of the easternmost Sierra Nevadas. MCFs in the southern portion of the 20 Sierra Nevada forests and nearl y all MCF communities in the San Bernardino forests receive 21 total nitrogen deposition levels above the 5. 2 N kg/ha

- yrecological benchmark. 22

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98 Coastal Sage Scrub communities (CSS) are also known to be sensitive to community 1 shifts caused b y excess nitrogen loadings. Studies have investigated the amount of nitrogen 2 utilized by

healthy and degraded CSS sy stems. In healthy stands, the authors estimated that 3.3 3 kg N/ha

- yr was used for CSS plant growth. It is assumed that 3.3 kg N/ha

- yr is near the point 4 where nitrogen is no longer limiting in the CSS community . Therefore, this amount can be 5 considered an ecologic al benchmark for the CSS community . The majority of the known CSS 6 range is currentl y receiving deposition in excess of this benchmark. Thus, the REA concludes 7 that recent conditions where oxides of nitrogen ambient concentrations are at or below the 8
curre nt oxides of nitrogen secondary standards are not adequate to protect against anticipated 9 adverse impacts from N nutrient enrichment in sensitive ecosy stems. 10 iv. Aquatic nutrient enrichment 11 The REA aquatic nutrient enrichment case studies focused on coastal estuaries and 12 revealed that while current ambient loadings of atmospheric oxides of nitrogen are contributing 13 to the overall depositional loading of coastal estuaries, other non

- atmospheric sources are 14 contributing in far greater amounts in total, althou gh atmospheric contributions are as large as 15 some other individual source types. The ability of current data and models to characterize the 16 incremental adverse impacts of nitrogen deposition is limited, both by the available ecological 17 indicators, and b y the inability to attribute specific effects to atmospheric sources of nitrogen. 18 The REA case studies used as the ecological indicator for aquatic nutrient enrichment an index 19 of eutrophication known as the Assessment of Estuarine Trophic Status Eutrophication Index 20 (ASSETS EI). This index is a six level index characterizing overall eutrophication risk in a 21 waterbody. This indictor is not sensitive to changes in nitrogen deposition within a single level 22

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99 of the index . In addition, this typ e of indicator does not reflect the impact of nitrogen deposition 1 in conjunction with other sources of nitrogen. 2 Based on the above considerations, the REA concludes that the ASSETS EI is not an 3 appropriate ecological indicator for estuarine aquatic eutrophication and that additional analy sis 4 is required to develop an appropriate indicator for determining the appropriate levels of 5 protection from N nutrient enrichment effects in estuaries related to deposition of oxides of 6 nitrogen . As a result, EPA i s unable to make a determination as to the adequacy of the existing 7 secondary oxides of nitrogen standard in protecting public welfare from N nutrient enrichment 8 effects in estuarine aquatic ecos ys tems. 9 Additionally , nitrogen deposition can alter species composition and cause eutrophication 10 in freshwater s ys tems. In the Rocky Mountains, for example, deposition loads of 1.5 to 2 kg/ha

11 yrwhich are well within current ambient levels are known to cause changes in species 12 composition in diatom communities indicating impaired water quality (US EPA, 2008, section 13 3.3.5.3 ). This suggests that the existing secondary standard for oxides of nitrogen may not 14 protect such ecos ys tems and their resulting services from impairment. 15

v. Oth er effects 16 An important consideration in looking at the effects of deposition of oxides of sulfur in 17 aquatic ecosy stems is the potential for production of methylmercury , a neurotoxic contaminant

. 18 The production of meaningful amounts of methyl merc ury (MeHg) requires the presence of SO 4 2

- 19 and mercury , and where mercury is present, increased availability of SO 4 2

- results in increased 20 production of MeHg. There is increasing evidence on the relationship between sulfur deposition 21 and increased meth yl ati on of mercury in aquatic environments; this effect occurs onl y where 22 other factors are present at levels within a range to allow methylation. The production of 23

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100 methylmercury

requires the presence of sulfate and mercury , but the amount of methylmercury 1 prod uced varies with ox ygen content, temperature, pH, and supply of labile organic carbon (US 2 EPA, 2008, section 3.4). In watersheds where changes in sulfate deposition did not produce an 3 effect, one or several of those interacting factors were not in the rang e required for meaningful 4 methylation to occur (US EPA, 2008, section 3.4). Watersheds with conditions known to be 5 conducive to mercury methylation can be found in the northeastern United States and 6 southeastern Canada (US EPA, 200 9 , section 6). 7 With res pect to sulfur deposition and mercury methylation, the final ISA determined: The 8 evidence is sufficient to infer a causal relationship between sulfur deposition and increased 9 mercury

methylation in wetlands and aquatic environments. However, EPA did not co nduct a 10 quantitative assessment of the risks associated with increased mercury methylation under current 11 conditions. As such, EPA is unable to make a determination as to the adequacy of the existing 12 SO 2 secondary standards in protecting against welfare eff ects associated with increased mercury 13 methylation. 14 vi. Summary of Adequacy Considerations 15 In summary , the Policy Assessment concludes that currently available scientific evidence 16 and assessments clearl y call into question the adequacy of the current stan dards with regard to 17 deposition

- related effects on sensitive aquatic and terrestrial ecos ys tems, including acidification 18 and nutrient enrichment. Further, the Policy Assessment recognizes that the elements of the 19 current standards

-- indicator, averaging time, level and form

– are not ecologically relevant, and 20 are thus not appropriate for standards designed to provide such protection. Thus, the Policy 21 Assessment concludes that consideration should be given to establishing a new ecologically 22 relevant mult i

- pollutant, multimedia standard to provide appropriate protection from deposition

- 23

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101 related ecological effects of oxides of nitrogen and sulfur on sensitive ecosystems, with a focus 1 on protecting against adverse effects associated with acidifying deposition in sensitive aquatic 2
ecos ys tems. 3 3. CASAC Views 4 In a letter to the Administrator (Russell and Samet 2011 a), the CASAC Oxides of 5 Nitrogen and Oxides of Sulfur Panel, with full endorsement of the chartered CASAC, 6 unanimously

concluded that : 7 EPA staff has dem onstrated through the Integrated Science Assessment (I SA), 8 Risk and Exposure Characterization (REA) and the draft PA that ambient NOx 9 and SOx can have, and are having, adverse environmental impacts. The Panel 10 views that the current NOx and SOx secondary st andards should be retained to 11 protect against direct adverse impacts to vegetation from exposure to gas phase 12 exposures of these two families of air pollutants. Further, the ISA, REA and draft 13 PA demonstrate that adverse impacts to aquatic ecosy stems are also occurring due 14 to deposition of NOx and SOx. Those impacts include acidification and 15 undesirable levels of nutrient enrichment in some aquatic ecosy stems. The levels 16 of the current NOx and SOx secondary NAAQS are not sufficient, nor the forms 17 of those standards appropriate, to protect against adverse depositional effects; thus 18 a revised NAAQS is warranted. 19 20 Inaddition, with regard to the joint consideration of both oxides of nitrogen and oxides 21 of sulfur as well as the consideration of deposition rel ated effects, CASAC concluded that the 22 Policy

Assessment had developed a credible methodology for considering such effects. The 23 Panel stated that “the Policy Assessment develops a framework for a multi

- pollutant, multimedia 24 standard that is ecologically relevant and reflects the combined impacts of these two pollutants as 25 they

deposit to sensitive aquatic ecosy stems.” 26 4. Administrator’s Proposed Conclusions Concerning Adequacy of Current Standard 27 Based on the above considerations and taking into account CASA C advice, the 28 Administrator recognizes that the purpose of the secondary standard is to protect against 29 “adverse” effects resulting from exposure to oxides of nitrogen and sulfur, discussed above in 30

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102 section IIA. The Administrator also recognizes the need f or conclusions as to the adequacy of the 1 current standards for both direct and deposition related effects as well as conclusions as to the 2 appropriateness and ecological relevance of the current standards. 3 In considering what constitutes an ecologi cal effect that is also adverse to the public 4 welfare, the Administrator took into account the ISA conclusions regarding the nature and 5 strength of the effects evidence, the risk and exposure assessment results, the degree to which the 6 associated uncertainties should be considered in interpreting the results, the conclusions 7 presented in the Policy Assessment , and the views of CASAC and members of the public. On 8 these bases, the Administrator concludes that the current secondary standard s are adequate to 9 p rotect against direct ph ytotoxic effects on vegetation. Thus , the Administrator proposes to 10 retain the current secondary standard for oxides of nitrogen at 53 ppb, 4 annual average 11 concentration, measured in the ambient air as NO 2 , and the current secondary standard for oxides 12 of sulfur at 0.5 ppm , 3

- hour average concentration, measured in the ambient air as SO 2 . 13 With regard to deposition

- related effects, the Administrator has first to consider the 14 appropriateness of the structure of the current standards to address ecological effects of concern. 15 Based on the evidence as well as considering the advice given b y CASAC on this matter, the 16 Administrator conclud es that the elements of the current standards are not ecologically relevant 17 and thus are not appro priate to provide protection of ecos ys tems
. On the subject of adequacy of 18 protection with regard to deposition

- related effects, the Administrator considered the full nature 19 of ecological effects related to the deposition of ambient oxides of nitrogen and s ulfur into 20 sensitive ecos ys tems across the country . Her conclusions are based on the evidence presented in 21 the I SA with regard to acidification and nutrient enrichment effects, the findings of the REA with 22

4 The annual secondary standard for oxides of nitrogen is being specified in units of ppb to conform to the current version of the annual primary standard, as specified in the final rul e for the most recent review of the NO 2 primary NAAQS (75 FR 6531; February 9, 2010).

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103 regard to scope and severity of the current and l ikely

future effects of deposition, the sy nthesis of 1 both the scientific evidence and risk and exposure results in the Policy Assessment as to the 2 adequacy

of the current standards, and the advice of both CASAC and the public. After such 3 consideration, the Administrator concludes that current levels of oxides of nitrogen and sulfur are 4 sufficient to cause acidification of both aquatic and terrestrial ecos ys tems, nutrient enrichment of 5 terrestrial ecos ys tems and contribute to nutrient enrichment effects in estuaries that could be 6 considered adverse, and the current secondary standards do not provide adequate protection from 7 such effects. 8 Having reached these conclusions, the Administrator determines that it is appropriate to 9 consider alternative standards that are ecologically relevant. These considerations support the 10 conclusion that the current secondary standard s is neither appropriate nor adequate to protect 11 against deposition related effects

. The Administrator’s consideration of such alternative 12
s tandards is discussed below in Section III. 13