NPS: Nature & Science» Air Resources Division

FEDERAL LAND MANAGERS' AIR QUALITY RELATED VALUES WORKGROUP (FLAG)
PHASE I REPORT
(December 2000)

D. 4. Deposition

a. Introduction

Atmospheric deposition has been studied extensively throughout the world, beginning in the 1800's in England, Sweden, Norway, and Germany. Research has primarily focused on the deposition of acidic pollutants and long-term acidification. Many publications describe current conditions, monitoring and modeling methods, and the results of acidification experiments. In the United States, research on acidification was first begun in 1962 at Hubbard Brook, New Hampshire. Subsequent work in the Adirondack lakes and other areas furthered the understanding of acid deposition effects. It is now recognized that, in addition to causing acidification, deposition of pollutants can affect many ecosystem characteristics, including nutrient cycling and biological diversity.

Although much progress has been made to control sulfur dioxide and nitrogen oxide emissions, deposition of sulfur (S) and nitrogen (N) compounds continues to be a problem in North America and Europe (Hedin and Likens 1996). As a result, certain sensitive freshwater lakes and streams continue to lose acid-neutralizing capacity (ANC) and sensitive soils continue to be acidified. Other ecosystems, including forests, grasslands, estuaries, and N-limited lakes exhibit unwanted fertilization and other effects from excess N deposition.

Federal Land Managers (FLMs) have documented the effects of S and N deposition on many air quality related values (AQRVs). Documented effects include acidification of lakes, streams, and soils; leaching of nutrients from soils; injury to high-elevation spruce forests; changes in terrestrial and aquatic species composition and abundance; changes in nutrient cycling; unnatural fertilization of terrestrial ecosystems; and eutrophication of estuarine and some lake systems. FLMs recognize that other undocumented effects may also be occurring.

The FLAG deposition subgroup was formed to identify common approaches among these agencies for evaluating atmospheric deposition and its effects on AQRVs. In addition, the subgroup was directed to recommend methods for establishing critical deposition loading values ("critical loads") and, where possible, recommend such critical loads for specific areas. These tasks were assigned to Phase I or Phase II, depending on their degree of difficulty.

During the scoping process, the FLAG Deposition Subgroup determined that Phase I tasks would include the summarization of information currently available about deposition and its effects on FLM areas and the development of recommendations on methods to model and evaluate current and future deposition and its effects on AQRVs. In addition, critical load values, where available from previous FLM guidance documents, would be referenced. FLMs agreed that site-specific AQRV and critical load information would be maintained on FLM web sites, rather than included in the Phase I report. In this way, the information can be updated and the most recent versions made quickly available to the public. Some of this information is already available on FLM web sites, and the FLMs are committed to entering remaining available information as soon as possible.

The subgroup recognizes that the development and refinement of site-specific critical load values for all FLM areas are crucial for AQRV protection. However, because of the complexity of this undertaking, and the lack of information for many areas, it was deferred to Phase II.

During Phase II, the subgroup will focus efforts on developing methods for establishing critical deposition loading values for FLM areas, and establishing critical loads for areas with adequate information. For areas lacking sufficient information to determine critical loads, strategies will be developed to obtain needed information. Previously established critical loads will be reviewed and refined as necessary. The subgroup will also explore alternative methods for estimating background deposition rates, including extrapolation techniques or modeling that considers the spatial scale of ecosystems and differences in elevation. Methods for addressing problems with dry deposition and cloud and fog deposition measurements will also be considered. In addition, Phase II will provide research or monitoring recommendations to improve our understanding of deposition and its effects, including effects on cultural resources.

b. Current Trends in Deposition

From 80%-99% of S emissions and from 83%-95% of nitrogen oxides emissions are anthropogenic (NAPAP 1991). As a result, most S and N deposition is anthropogenic in origin. The Clean Air Act mandated reductions in S and N emissions that should result in decreases in S and N deposition. Deposition monitoring data can be used to identify decreases in deposition. The National Atmospheric Deposition Program (NADP) provides one of the best and most comprehensive long-term records of wet deposition chemistry in the U.S. Results from a trends analysis of 1983-1994 data from 153 NADP sites (Lynch et al. 1996) are shown on Figures D-2, D-3, and D-4.

Figure D-2 shows that, between 1983-1994, wet precipitation sulfate concentrations declined significantly at 38% of monitored sites, presumably as a result of the Clean Air Act and mandated emissions reductions. Numeric declines were observed at the majority of the remaining sites. Sulfate concentrations increased significantly at only one site. A more recent analysis of 1995-1997 NADP data from eastern sites showed continuing declines in sulfate concentrations and wet depositions over much of the northeastern U.S. (Lynch et al. 2000). The EPA's "National Air Quality and Emissions Trends Report, 1998," also reported declines in the concentrations of sulfate in wet deposition, noting that the reductions were directly related to the large regional decreases in sulfur dioxide emissions resulting from Phase I of the Acid Rain program (U.S. EPA 2000).

Despite emissions reductions, many FLM areas are located where current sulfate, and therefore total S, deposition remains high and where deposition is in excess of estimated critical loads. A workgroup convened by the Ecological Society of America (ESA) in 1999 concluded that in some regions and ecosystems, current reductions of S emissions may be insufficient for ecological recovery to occur (ESA 2000). Surface water sulfate concentrations are generally not decreasing because of the desorption of previously deposited S from soils (Johnson and Henderson 1979), and surface S mineralization with pond and lake water level changes (Schindler 1998). Elevated surface water sulfate concentrations from past deposition is expected to persist for decades even with continued emission declines. Chronic high surface water sulfate levels reduce ANC, making ecosystems more vulnerable to chemical change during episodic events.

Figure D-3 shows that there is no consistent trend in nitrate concentrations. The number of sites exhibiting increasing trends is nearly equal to the number exhibiting decreasing trends. However, the number of sites that experienced a statistically significant increase in nitrate (14%) was much greater than the number of sites that experienced a significant decrease in nitrate (1.3%). Figure D-4 shows an increase in ammonium concentrations at most sites, with statistically significant increases at 22% of the sites, mostly in the West. Only one site had a significant decreasing trend. Given the increases in nitrate and ammonium, total N concentrations are clearly increasing at some locations.

Estimates of natural background S and N precipitation concentrations and deposition can be made from certain reliable early precipitation chemistry data (Junge 1958), precipitation data from carefully selected remote areas such as Alaska and Argentina, and to some extent from present NADP data from coastal Oregon and Alaska (NADP 1982-1997). Except for coastal Oregon, present precipitation S and N concentrations throughout the contiguous states exceed these estimates of natural background levels, primarily due to anthropogenic emissions of S and N compounds.

In this chapter, it is assumed that S is deposited into the environment primarily as sulfate ion and N is deposited primarily as nitrate and ammonium ions. Other ionic forms of S and N occur in the atmosphere, but information on their deposition into ecosystems is limited. For example, organic N may be important in some areas, but reliable measurement methods for organic N in atmospheric deposition are not widely available.

c. Identification and Assessment of AQRVs

AQRVs sensitive to pollutant deposition have been identified in various documents published by the USDA/FS, NPS, and FWS, which are listed in the "General References" of Appendix H of this report. The FLMs have previously used a combination of approaches to identify AQRVs, including national and regional workshops, regional reviews, and site-specific studies. AQRV identification was based on information from peer-reviewed scientific literature and expert judgment. Because information on AQRVs may change as new data becomes available, the FLMs agree that AQRV information will be made available on FLM web sites to allow for updating and improve accessibility, as discussed in the Introduction to this chapter.

Information on AQRVs for many USDA/FS Class I areas can be found at

http://www.fs.fed.us/r6/aq/natarm

The USDA/FS is currently adding to and updating this information.

NPS and FWS are currently developing a web site with AQRV information that will be linked to the NPS Air Resources Division website at

/air/

and to the FWS National Wildlife Refuge System web site at

http://refuges.fws.gov

FLMs recommend that permit applicants consult with the appropriate FLM (Appendix F) to determine the need for an AQRV analysis and, if applicable, the methods for the analysis.

All FLMs use a similar conceptual approach to identify AQRVs that reflects the FLMs' interest in maintaining the integrity of ecosystem structure and function and protecting the most sensitive ecosystem components. AQRVs can be categorized by the type of ecosystem in which they are found, such as terrestrial, freshwater, and estuarine ecosystems. Each ecosystem and its AQRVs responds somewhat differently to deposition and approaches to evaluating deposition effects must therefore be developed accordingly. In terrestrial ecosystems, detection of changes in production, decomposition, and nutrient cycling processes provide information on deposition stress. In aquatic and estuarine ecosystems, detection of changes in water chemistry and aquatic community composition and structure provide similar information. Table D-1 summarizes AQRV indicators that may be used to assess effects in various ecosystems.

Terrestrial, freshwater, and estuarine AQRVs are discussed below. In addition, methods to evaluate S- and N- induced deposition stress are discussed.

Terrestrial Ecosystems

Terrestrial ecosystem AQRVs include flora, fauna, and soils. FLMs have identified, where possible, AQRVs, or characteristics of AQRVs, most likely to be sensitive to S and N deposition ("sensitive receptors"). For example, high-elevation spruce forests may be sensitive receptors. FLMs assess the condition of these sensitive receptors by evaluating some aspect of the receptor (the "sensitive receptor indicator", or "indicator"). For example, an indicator for high-elevation red spruce forests is the occurrence and extent of winter foliar injury. In general, the FLM has focused on deposition effects to vegetation and chemical receptors in terrestrial ecosystems, with little emphasis on fauna. In addition, there is increasing awareness among FLMs that certain soil fauna (e.g., microorganisms and invertebrates) are very sensitive to deposition and can be used as sensitive receptors.

In terrestrial ecosystems, sulfate production is regulated primarily by chemical processes (Johnson et al. 1983) and it is rarely a limiting nutrient. Soil response to acidic deposition can be evaluated by monitoring the leaching of essential soil cations, soil acidification, and mobilization of ionic aluminum. These processes have been studied both in field and laboratory experiments, and are defined in detail in the literature (Mollitor and Raynal 1983, Richter et al. 1983, Johnson et al. 1983, Reuss and Johnson 1986). Effects of S deposition can be detected by monitoring calcium and magnesium ions and S in the litter layer and surface soils; calcium, magnesium, potassium, and sulfate ions in soil solution; cation exchange capacity (CEC); and base saturation.

In general, biological AQRVs do not provide reliable indicators of S deposition in terrestrial ecosystems except under extreme S deposition. Lichens have been used in some areas as biomonitors to demonstrate spatial trends in S deposition, particularly in areas with pronounced S deposition gradients. For example, isotopic analysis of lichens from Mt. Zirkel Wilderness, Colorado, indicated that power plants in the nearby Yampa Valley were the source of elevated S in the lichens (Jackson et al. 1996).

Unlike S, the production and mobility of N in ecosystems is regulated almost entirely by biological processes. N is a limiting nutrient in most terrestrial and estuarine ecosystems, and is seasonally limiting in many freshwater ecosystems. Most ecosystems can retain and process significant additions of N, with resulting increases in production and changes in species diversity, biomass, and nutrient cycling. However, these changes are usually considered to be undesirable in natural ecosystems. The ability to retain and process N varies significantly depending on watershed successional status, site and fire history, soil conditions, vegetation, and other non-human factors. When N inputs exceed an ecosystem's assimilation capacity, N is lost or leached, usually as nitrate, from the soil and can be detected in adjacent streams or lakes. This may occur following a major disturbance such as fire, logging, land use change, grazing, agriculture, or where atmospheric N deposition or experimental inputs exceed what the ecosystem can assimilate (Fenn and Dunn 1989, Fenn 1991, Fenn et al. 1996, Adams et al. 1997).

Studies in northern Europe (Dise and Wright 1995) found that European forests leached detectable levels of nitrate at inputs of about 10-25 kilograms N per hectare per year (kg N ha^-1yr^-1). Tundra and high-elevation alpine sites may leach N at much lower levels of input. Mountain watersheds in the western U.S. show signs of N leakage at wet deposition levels of 3-5 kg N ha^-1yr^-1 (Eilers et al. 1994; Williams et al. 1996; Williams and Tonnessen, in review). However, even high elevation, poorly vegetated ecosystems with limited soil development can process more than 80% of the atmospheric N input before it reaches the aquatic system (Campbell et al. 1995, Kendall et al. 1995). Although nitrogen leaching has often been used as an indicator of excess N deposition, major changes occur in below- and aboveground biomass, species diversity, and nutrient cycling long before N input levels are sufficient to cause nitrate leaching (NAPAP 1993, Tilman et al. 1997, Vitousek et al. 1997). For example, with ambient deposition rates of 7-10 kg N ha^-1yr^-1, a Minnesota Long-Term Ecological Research (LTER) grassland study observed shifts from native, warm-season grasses to low diversity mixtures dominated by cool-season grasses and a greater than 50% decline in species richness (Wedin and Tilman 1996, Tilman et al. 1997). Significant losses in terrestrial diversity may have already occurred over extensive areas of the U.S., particularly in forest understories, shrublands, grasslands, and in soil microbial communities.

Because significant ecological changes may occur before nitrate loss can be detected, more sensitive indicators than nitrate leaching are needed to evaluate N deposition effects. Such indicators include changes in carbon and N dynamics of litter and soil and biomass (Aber and Driscoll 1997, Magill et al. 1997). With knowledge of inputs and small-scale N fertilization studies, changes in soil organic matter quality and quantity in response to N deposition can be evaluated. Soil microbial communities control the quantity and quality of N available to ecosystems and may be very sensitive indicators of N deposition. Changes in soil microbe functional groups or biomass may provide good estimates of ecosystem critical loads and incremental effects. Soil N mineralization, small root growth, and carbon:nitrogen ratios of soil and microbial biomass are also sensitive to N deposition. Evidence suggests that current deposition rates may alter the production of dissolved organic carbon and organic N compounds in soils, which are important nutrient and energy sources for both terrestrial and aquatic ecosystems. These could also be used as indicators of N deposition effects. However, because there are many other variables that also affect soil processes, it may be very difficult to discern effects on any soil indicators that are solely attributable to N.

Freshwater Ecosystems

AQRVs in freshwater ecosystems include lakes and streams and their associated flora and fauna. Sensitive receptors include water chemistry and clarity, phytoplankton, zooplankton, fish, amphibians, macroinvertebrates, and benthic organisms. Water chemistry indicators that respond to deposition include pH, ANC, conductance, cations and anions, metals, and dissolved oxygen. Physical indicators, such as water clarity, and biological indicators, including species diversity, abundance, condition factor and productivity of fish, amphibians, macroinvertebrates, and plankton can also be used to detect deposition effects in aquatic ecosystems. Much research has been done on the sensitivity of aquatic species to deposition, many of which are discussed in the 1990 National Acid Precipitation Assessment Program (NAPAP) State of Science report (NAPAP 1991a) and the 1998 NAPAP report (NAPAP 1998).

Sulfur is not a limiting nutrient in freshwater ecosystems. However, there are small regions of the U.S., including some FLM areas, where a relatively high percentage of surface water is sensitive to present acidic inputs. In these areas, S deposition can cause decreases in ANC and pH. For these sensitive or low-ANC waters, the best approach to quantify S deposition effects is the procedure currently used, monitoring changes in ANC and pH.

Nitrogen deposition, like S deposition, can cause episodic acidification of surface water in certain sensitive high-elevation ecosystems that have low-ANC headwater lakes and streams. Episodic acidification occurs in these areas when deposition is as low as 3-5 kg N ha^-1yr^-1 (Williams et al. 1996).

Estuarine Ecosystems

AQRV sensitive receptors in estuarine ecosystems include plankton, seagrasses, and water chemistry and clarity. Associated coastal forest and dune soils may also be useful as sensitive receptors. Water and soil nutrient concentrations, phytoplankton species composition and abundance, seagrass health, and dissolved oxygen concentrations can be used to evaluate deposition effects.

In estuaries, S is not a limiting nutrient. In addition, estuarine waters are highly buffered and, therefore, not subject to acidification. However, many coastal forest and dune soils are dominated by sandy soils that are sensitive to leaching of limiting nutrients because of very low cation exchange capacity (Au 1974). Monitoring for change in estuarine areas with high S deposition should therefore focus on soil ion mobility. As soil calcium and magnesium levels are generally adequate because of deposition from marine sources, potassium is likely the only limiting nutrient subject to significant loss by sulfate leaching.

The role of N in estuaries is probably the best-documented example of anthropogenic alteration with a literature record dating back to the 1950s. Production and use of fertilizers, land use changes, and fossil fuel combustion have greatly increased the available N, normally a limiting nutrient, which enters coastal waters. This has increased estuarine production and accelerated the process of eutrophication. Eutrophication can result in dramatic algae blooms, anoxia, the production of toxic hydrogen sulfide gas, and species extirpation in estuarine ecosystems. Human induced eutrophication has been documented for many areas along the Atlantic and Gulf coasts, including the Chesapeake Bay, Tampa Bay, Sarasota Bay, Florida Bay, and Long Island Sound.

A number of FLM areas along the Atlantic and Gulf coasts contain significant coastal waters that may be sensitive to eutrophication. Little is known about excess N effects in most of these areas, although eutrophication is well documented in Florida Bay, located in Everglades National Park. Also, recent evidence indicates that coastal waters in Chassahowitzka Wilderness (Florida) experience N-induced algal blooms (Dixon and Estevez in draft). In most coastal waters, 10-45% of the N entering the system is atmospheric, either from direct deposition to surface water or deposition to the watershed. Complete elimination of atmospheric N inputs would not entirely mitigate ecosystem change due to N because of the substantial contributions from agricultural and urban runoff. However, for most estuaries, any reduction in N input would be beneficial in restoring ecosystem structure and function.

The monitoring procedures recommended, and currently used, in estuaries are similar to those used in freshwater, with emphasis on incremental changes in plankton, aquatic plant, benthic, and invertebrate community composition; species diversity, distribution, and biomass; and ecosystem trophic status.

Significance of Long-Term Monitoring to Evaluate Trends and Validate Modeling

Long-term monitoring is critical to evaluate trends in deposition and deposition effects. Monitoring programs should concentrate not only on areas with high past and/or present sulfate, nitrate, or ammonium deposition, but also in areas that are very sensitive to deposition and in areas where deposition is expected to increase. For selected monitoring sites, the FLM should (1) obtain ion deposition data for the site, as from NADP or CASTNet, (2) identify sensitive AQRVs and appropriate variables to monitor, (3) evaluate the present condition of the sensitive AQRVs, (4) determine the degree to which results from one site can be extrapolated to other FLM areas in the region, and lastly (5) implement a long-term monitoring program, using carefully selected variables.

Long-term monitoring data are also needed to support and validate models used to predict deposition and deposition effects, including the effects of increases or decreases of S and N on ecosystems. Long term studies in both aquatic and terrestrial ecosystems such as Hubbard Brook, Lake Tahoe, and the Experimental Lakes Area have provided useful information for modeling (Bormann and Likens 1967, Holm-Hanson et al. 1976, Likens and Bormann 1977, Leonard et al. 1979, Byron and Eloranta 1984, Schindler et al. 1985, Schindler 1987, Schindler et al. 1990, Jassby et al. 1995). NAPAP and the National Science Foundation LTER program have addressed monitoring to meet modeling needs in both terrestrial and aquatic ecosystems.

Data requirements to support models vary, but the quality of input data will determine the quality of a model's predictions. Modeling is further discussed in the "Other AQRV Identification and Assessment Tools" section of this chapter.

d. Determining Critical Loads

Critical load is defined by FLMs as "the concentration of air pollution above which a specific deleterious effect may occur." Critical loads have been widely accepted in Europe and Canada as a basis for negotiating control strategies for transboundary air pollution (Posch et al. 1997).

In Canada, researchers have estimated the critical loads of S in wet deposition necessary to protect moderately sensitive lakes in eastern provinces. That value, equivalent to 6.7 kg ha^-1yr^-1 of S in wet deposition, was used by Canada to argue for the U.S. to implement the Clean Air Act Amendments of 1990, which call for the initial reduction of sulfur dioxide emissions in the eastern U.S. and later from all electric utilities nationwide. With additional data on lake and stream chemistry available for sensitive systems in Nova Scotia, Ontario, and Quebec, the Canadians are now recommending a more stringent critical load, equivalent to 2.7 kg ha^-1yr^-1 of wet deposition S.

In both European countries and in North America, attention has expanded beyond ecosystem damage caused by S deposition to ecosystem damage caused by N deposition. In some European forests, chronically high N deposition has exceeded the assimilation capacity of local ecosystems, resulting in the release of nitrate into surface waters (Dise and Wright 1995). Watersheds that are leaking nitrate into surface waters during the growing season, are referred to as "N saturated" (Aber et al. 1989). Nitrogen saturation has been linked to forest decline in Europe (Schulze 1989). Based on a set of regional N addition experiments conducted at sites in northern Europe (NITREX), Wright (1995) recommended a N critical load of less than 10 kg ha^-1yr^-1 to protect European forests and freshwaters from N saturation. However, this critical load does not protect ecosystems from the changes caused by N deposition prior to actual N saturation, including shifts in composition and abundance of soil fauna species and alterations in soil chemistry.

In the United States, two states have attempted to set deposition standards or critical loads to protect sensitive ecosystems. In 1982, the State of Minnesota passed the Acid Deposition Control Act to limit wet sulfate deposition to 11 kg ha^-1yr^-1, which is equivalent to 3.7 kg S ha^-1yr^-1. At this sulfate level, precipitation pH was likely to remain above 4.7, which would protect lakes with ANC less than 50 microequivalents per liter (µeq l^-1). This critical load was to be achieved by controls on large sources of sulfur dioxide in Minnesota. As of 1990, monitoring by state officials showed no evidence of lake acidification under the sulfur dioxide control program. However, the efficacy of this control strategy is still uncertain, because as much as 90% of the sulfate deposited in northern Minnesota may have sources outside of the state (Orr et al. 1992).

In 1989, the California legislature adopted the Atmospheric Acidity Protection Act, which required the Air Resources Board (CARB) to "develop and adopt standards, to the extent supportable by scientific data, at levels which are necessary and appropriate to protect public health and sensitive ecosystems from adverse effects resulting from atmospheric acidity" (CARB 1993). An assessment of existing data identified the high elevation watersheds, surface waters, and mixed conifer forests of the Sierra Nevada and the Los Angeles Basin as sensitive ecosystems. CARB analyses suggested that appropriate standards would include a critical load value for inorganic N to protect forests, and critical loads for both N and S to protect poorly buffered lakes and streams. However, no acidity standards to protect human health or critical loads to protect ecosystems have been set in California to date.

The Clean Air Act Amendments of 1990, Title IV, section 404, called on the Environmental Protection Agency (EPA) to prepare a report on the feasibility and effectiveness of setting deposition standards nationwide to protect sensitive aquatic and terrestrial resources. The completed report includes a number of modeling analyses that project the effect of reductions in both S and N deposition in areas studied during NAPAP. EPA concluded that deposition standards could not be set at this time because of 1) the lack of clearly defined policy regarding appropriate or desired goals for protecting sensitive aquatic or terrestrial resources, and 2) key scientific uncertainties, particularly regarding nitrogen watershed processes. In addition, EPA recognized that a national deposition standard might be inappropriate because of differences among ecosystems. However, in response to public comments on the report, EPA stated that "Given an adequate level of monitoring and assessment data, Class I areas could serve as potential targets for standard setting activities." (U.S. EPA 1995)

Critical Loads in FLM Areas

In the Clean Air Act, as amended in 1977, Congress gave FLMs an "affirmative responsibility" to protect AQRVs from the adverse effects of air pollution. Congress' intent was, "…In cases of doubt the land manager should err on the side of protecting the air quality-related values for future generations…" (Senate Report No. 95-127, 95th Congress, 1st Session, 1977). In an effort to ensure AQRV protection, FLMs have established critical loads for many FLM areas. FLMs agree that a critical load should protect the most sensitive AQRVs within each FLM area and should be based on the best science available. As new scientific information becomes available, critical loads should be reviewed and updated. Critical loads should ensure that no unacceptable change occurs to the resource.

FLMs have previously used a combination of approaches to establish critical loads, including national and regional workshops, regional reviews, and site-specific studies (see Appendix H). In all cases, the FLMs have used peer-reviewed scientific literature and expert judgment to make their decisions. For example, the NPS has established critical loads for several national parks through regional reviews that have evaluated existing information on air quality, deposition, and effects on AQRVs in national parks. For these reviews, NPS grouped parks by region and ecosystem type, including the Pacific Northwest, the Colorado Plateau, and the Rocky Mountains, and conducted an empirical assessment of the status of aquatic and terrestrial resources. An analysis of deposition effects was done, using current deposition data for S and N and effects information from field observations and research. In the Pacific Northwest region, this analysis led researchers to recommend guidelines for critical loads of S and N to protect sensitive resources, particularly low-ANC lakes, streams and ponds. These guidelines for critical loads will be available on the NPS AirResources Division web site in the near future at:

http://www.nature2.nps.gov/air

The FWS is also committed to establishing critical load guidelines to protect sensitive resources. These guidelines for critical loads will be available through the FWS National Wildlife Refuges site at:

http://refuges.fws.gov

The USDA/FS has conducted a series of national and regional workshops to establish critical loads and concern thresholds. In the late 1980s, the USDA/FS published prototype methods for evaluating the effects of acid deposition on AQRVs, including A Screening Procedure to Evaluate Air Pollution Effects on Class I Wilderness Areas (Fox et al. 1989) and Guidelines for Measuring the Physical, Chemical, and Biological Condition of Wilderness Ecosystems (Fox et al. 1987). Subsequently, the USDA/FS held regional workshops to develop screening procedures for new air pollutant emissions sources. These workshops were comprised of national and regional USDA/FS land managers, deposition experts from the academic and air pollution research community, and agency air quality professionals. Dependent on the workshop leadership, each regional workshop followed a slightly different process and a variety of outputs and formats resulted. However, all workshops used a collaborative process to determine S and N deposition rates that would pose a risk to the aquatic and terrestrial ecosystems protected in FLM areas, while addressing the scientific uncertainty inherent in ecosystem response to acidic deposition. Critical load guidelines for many USDA/FS Class I areas are published in workshop reports (see Appendix H) and are available at:

http://www.fs.fed.us/r6/aq/natarm.

The USDA/FS is currently adding to and updating this information.

As resources permit, during Phase II of FLAG, the subgroup will develop methods for establishing critical deposition loading values for all FLM areas and recommend critical loads for areas where adequate information exists. For areas lacking sufficient information to determine critical loads, strategies will be developed to obtain needed information.

e. Other AQRV Identification and Assessment Tools

In addition to AQRV monitoring, there are several tools available to the FLM for identifying AQRVs and assessing the response of sensitive AQRVs to pollutant deposition. These include the aquatic effects expert system component of the FWS/NPS Air Synthesis, the Natural Resource Information System - Air Module (NRIS-Air), and deposition models such as the Model of Acidification of Groundwater in Catchments (MAGIC) and MAGIC-With Aggregated Nitrogen Dynamics (MAGIC-WAND).

Air Synthesis

Air Synthesis is an information management and decision-support computer system under development by NPS and FWS. Air Synthesis is designed to assist FLMs in determining potential effects of pollutants on AQRVs. It contains information on air quality and its effects in parks and wildernesses as well as natural resource data and annotated bibliographies of current literature on deposition. An interactive expert system module is under development for inclusion in Air Synthesis to allow FLMs to assess the current status of freshwaters and determine if these resources are likely to be affected by deposition of S or N. The aquatic effects expert system is being developed by regional scientists. This system will allow FLMs to input existing surface water data for lakes and streams to determine: (1) the acidification status of the waters, (2) the likely cause of high concentrations of acid anions (e.g., deposition, land use, organic inputs) and, (3) the sensitivity of the waters to increases in N or S deposition. Results can be displayed in a geographic information system (GIS) image that color-codes the acidification status of lakes and streams. In addition, the expert system evaluates the completeness and the amount of uncertainty in water chemistry data sets. Air Synthesis (now called ARIS) is available through the NPS Air Resources Division website at:

www2.nature.nps.gov/air/Permits/aris/

or the FWS National Wildlife Refuge System web site at:

http://refuges.fws.gov

Natural Resource Information System - Air Module (NRIS-Air)

The Air Module is part of the USDA/FS Natural Resource Information System that integrates various physical, biological and socioeconomic data within a system of database, map-based spatial information, and analytical tools. Version 1.0 of NRIS-Air, released in November 1998, tracks AQRVs, sensitive receptors and indicators for each of the USDA/FS Class I areas. The water submodule provides data storage, reports, and tools for evaluating locally entered water quality and wet deposition data. It also integrates the NADP data set and the entire National Surface Water Survey including the Eastern and Western Lakes Surveys and the National Stream Survey. Future NRIS-Air versions under development will provide the information structure for visibility, flora, fauna, soil, geologic resources, cultural resources, and air quality data, as well as providing an air pollution permit tracking system.

Information from NRIS-Air, including USDA/FS Class I area AQRV information, is available at:

http://www.fs.fed.us/r6/aq/natarm

Deposition Effects Models

A number of watershed process models have been developed and tested in an attempt to simulate the effects of S and N on soils, forests, and surface waters. These models are used by FLMs to predict effects from increases in deposition and vary from detailed, compartment models of watersheds to lumped parameter models that do not track different ions through each soil compartment. For a review of models developed under NAPAP see NAPAP 1991.

A commonly applied watershed model is MAGIC. MAGIC was first developed for eastern U.S. watersheds and then extensively tested and validated throughout Europe and North America (Cosby et al. 1985, 1995, 1996). The model was used by NAPAP in its 1990 Integrated Assessment to project surface water chemistry resulting from various deposition scenarios (NAPAP 1991b). In another application in the eastern U.S., MAGIC has been linked with a simple, empirical, dose/response fish model developed at University of Virginia, that makes it possible to predict changes in fish productivity based on modeled changes in streamwater chemistry.

As a result of NAPAP, there was increased awareness of the potential impacts of inorganic N deposition on watersheds and surface waters. In response, the MAGIC model was updated with a module called With Aggregated Nitrogen Dynamics (WAND). MAGIC-WAND is a process-based model that uses site-specific information on hydrology, soils, and hydrochemistry. The model predicts changes through time in lake or stream chemistry. These time-series of changes in pH and ANC can subsequently be used by FLMs to calculate critical S or N loads for watersheds.

MAGIC-WAND has been extensively tested in the Adirondacks and at watersheds in Maine. For example, the Bear Brook Watershed Manipulation Project uses MAGIC-WAND to predict the effects of experimentally added N and S on a test watershed. MAGIC-WAND has also been applied to watersheds in FLM areas in the Cascades, the Sierra Nevada, the Rocky Mountains, and the Wind River Range in an effort to quantify critical S and N loads to aquatic and terrestrial resources. In the southeastern U.S., MAGIC-WAND is being used under the auspices of the Southern Appalachian Mountains Initiative (SAMI) to predict the effects of future deposition scenarios on FLM areas. Future SAMI modeling efforts will link watershed model results with fish dose/response models. The ultimate goal is to calibrate MAGIC-WAND with landscape level data in order to set regional critical loads.

Other models are also in use. For example, the USDA/FS Rocky Mountain Region recommends using either CALPUFF or ISCST (or other approved models) to estimate S and N deposition. The Screening Methodology for Calculating ANC Change to High Elevation Lakes (USDA Forest Service 2000) summarizes procedures for estimating total deposition of S and N. The document also recommends computations for estimating alkalinity changes in lakes caused by increases in S and N deposition. Another model, the Nutrient Cycling Model (NuCM) has been used in the East to predict the effect of changes in deposition on nutrient concentrations in soils and vegetation.

f. Recommendations and Guidance for Evaluating Potential Effects from Proposed Increases in Deposition to an FLM Area

FLMs often request that proponents of new emissions sources or modifications of existing sources near FLM areas provide sufficient information for the FLM to evaluate the potential effects of emissions increases on AQRVs. FLMs have provided guidance for applicants through guidance documents, correspondence, meetings, and phone consultations. This chapter summarizes current guidance for the evaluation of new emissions on deposition and sensitive AQRVs and includes recommendations for:

the types of data, information, and analysis needed before a permit can be considered complete, including analytical and modeling protocols for a proponent's use in conducting an AQRV impact analysis;
approaches and sources of appropriate values for estimating wet and dry deposition; and
permit conditions to mitigate source impacts.

These recommendations can most easily be described using a flow chart. Figure D-1 summarizes the approaches to be taken to evaluate a proposed action.

The flowchart begins with the question, "Are there currently adverse effects from pollutant deposition to AQRVs in the FLM area?" To answer this question, the FLM needs information on deposition-sensitive AQRVs, deposition loads at which these AQRVs are affected (i.e., critical loads), and the current pollutant deposition rates in the area. In areas where no information is available, information from a nearby, or ecologically similar area, may be used. An adverse effect may be expected to occur if the critical load is exceeded for an area. AQRV and critical load information are discussed earlier in this report. Procedures for estimating current pollutant deposition rates are summarized in the section, "Estimation of Current and Future Deposition Rates." After considering this information, the FLM determines if adverse effects to AQRVs already exist at an area. If adverse effects are present, the FLM may recommend that "a" or "b" or both of Figure D-1 are included as permit conditions. If these recommendations, or some combination of them, cannot be implemented, the FLM is likely to recommend denial of the permit.

If there are no current documented adverse effects from pollutant deposition to AQRVs, or there is a lack of information on deposition and deposition effects in the area (and information from nearby or ecologically similar areas is unavailable), the FLM may ask, "Will the proposed action cause an adverse effect to AQRVs?" The information needed to answer this question includes the information listed above regarding AQRVs, critical loads, and current deposition rates. In addition, an estimate is needed of the future predicted deposition rate. Procedures for this estimate are found in the "Estimation of Current and Future Deposition Rates" section of this report.

With this information, the FLM can determine if the proposed action is likely to cause an adverse effect to AQRVs. If the answer is no, or unknown, the FLM would not object to the action because of potential deposition effects. The FLM may still, however, object to the action for other reasons including an inadequate best available control technology analysis, predicted National Ambient Air Quality Standards violations, predicted Class I increment impacts, or other predicted AQRV impacts. If the available information is insufficient for the FLM to determine if the proposed action will cause an adverse effect to AQRVs, the FLM may ask for deposition and deposition effects monitoring and/or research in the FLM area (i.e., item "b"). If the answer is yes and the proposed action will likely cause an adverse effect to AQRVs, the FLM may recommend permit conditions that ensure mitigation, including stricter emissions controls and effective emissions offsets (i.e., item "a"). If no mitigation is possible, the FLM is likely to recommend denial of the permit.

Available Deposition Monitoring Data

Atmospheric pollutants are deposited to ecosystems primarily through wet deposition and dry deposition. FLMs participate in national monitoring programs to monitor wet and dry deposition, including the National Atmospheric Deposition Program (NADP) and the Clean Air Status and Trends Network (CASTNet). A 1999 report, "The Role of Monitoring Networks in the Management of the Nation's Air Quality," (CENR, 1999) identified these two networks as being critical for characterizing baseline air quality data in the U.S.

Wet Deposition

Wet deposition includes rain, snow, fog, cloudwater, and dew. In most FLM areas, rain and snow are the primary contributors to wet deposition. However, in some high elevation areas, fog, cloudwater, and dew are significant contributors, as discussed below.

Because rain and snow are the primary constituents of wet deposition at most FLM areas, the FLM generally relies on data from NADP to evaluate wet deposition of pollutants. NADP samplers collect rain and snow and NADP has documented deposition for many years in a nationwide network that currently includes over 220 monitoring sites. The network collects data to evaluate spatial and temporal long-term trends in precipitation chemistry. The precipitation at each site is collected weekly and sent to a central analytical laboratory for analysis of hydrogen (acidity as pH), sulfate, nitrate, ammonium, chloride, and base cations, including calcium, magnesium, potassium, and sodium. Data and isopleth maps of pollutant concentrations and deposition are available on the NADP web site at:

http://nadp.sws.uiuc.edu/

FLMs agree that it is preferable to obtain NADP data from the web site, rather than summarizing wet deposition data in this report. In this way, current data can be easily accessed by FLMs and the public.

Approximately 50 FLM areas have NADP samplers in or immediately adjacent to them. Because some of these areas are classified as wilderness, FLMs install sampling equipment in adjacent non-wilderness areas in order to preserve the wilderness character of the area. Ambient air in these adjacent areas is considered representative of air in the wilderness area.

A number of FLM areas do not have an NADP sampler in or adjacent to them. Where possible, the FLM has identified an NADP site whose data may be used to characterize deposition at the area. This information is appended to this Deposition chapter (Table D-2). Deposition rates generally increase with elevation and deposition in high-elevation areas may be difficult to characterize with data from a lower-elevation NADP site. FLM consultation may be necessary to estimate deposition in these areas.

Areas that experience significant deposition from fog and cloudwater or large amounts of snow may need to use alternate sampling methods and data in addition to NADP protocols and NADP data to characterize them. Wet deposition in these areas may need to be sampled with alternate methods, including cloudwater samplers and snowpack sampling or estimated by modeling. At sites where such data or modeled estimates are available, they should be used to calculate total deposition. At mountain sites frequented by clouds and fog, deposition from clouds may equal or exceed that from precipitation. Cloud water is generally more acidic and contains higher concentrations of base cations than rain water; therefore, it can contribute significantly to total loadings of S and N (Hemmerlein and Perkins 1992). Various methods have been developed to measure deposition from cloudwater. The Mountain Acid Deposition Program (MADPro) used automated cloud water collectors to sample at three high-elevation eastern sites (Anderson et al. 1999). Forests covered by fog for significant periods of time may be especially susceptible to injury from acid deposition. Acidic cloud water has predisposed red spruce in the high elevations of the northeast U.S. Appalachians to winter injury and cumulative impacts with other biotic and abiotic stresses have caused mortality. The contribution of clouds and fog to deposition at high elevations may overshadow both deposition from precipitation and dry deposition (Hidy 1998). The U.S. EPA estimated that as a result of cloud cover, high elevation forests might experience four times the amount of total pollutant deposition as lower elevation forests without cloud cover (NAPAP 1991). High elevation lakes are also impacted by fog and clouds, as well as rain and snow. Measurements in high elevation areas that do not include all contributions to wet deposition will result in under-estimates.

Modeling has been used to estimate total wet deposition in some areas. For example, the Southern Appalachian Man and the Biosphere Cooperative (as part of the Southern Appalachian Assessment) has used NADP data, topographical data, and meteorological data to model wet deposition loading at locations in the southeastern U.S.

Dry Deposition

Dry deposition includes gases, aerosols and particles. The primary gases involved with N and S deposition are ammonia (NH₃), nitric oxide (NO), nitrogen dioxide (NO₂), nitric acid (HNO₃), and sulfur dioxide (SO₂), while the primary particles are nitrate (NO₃^-), ammonium (NH₄⁺), and sulfate (SO₄^2-) ions (Hanson and Lindberg, 1991). Ammonia, NO, NO₂ and SO₂ are taken up by plants through stomata, while HNO₃, due to its high deposition velocity, is deposited to plant surfaces in addition to being taken up by stomata. Nitrate, ammonium, and sulfate particles deposit to surfaces (Bytnerowicz and Fenn, 1996).

Dry deposition is much more difficult to estimate than wet deposition. The estimation of dry deposition rates requires information on the ambient concentrations of pollutants, meteorological data, and information on land use, vegetation, and surface conditions, all of which are site-specific. Because of this site-specificity, it is difficult to spatially extrapolate dry deposition data as is often done for wet deposition data.

In general, FLMs rely on data from CASTNet for estimates of dry deposition in FLM areas (http://www.epa.gov/castnet). CASTNet was developed by EPA, as a result of the Clean Air Act Amendments of 1990, and currently includes over 70 sites. These include a combination of former National Dry Deposition Network sites, Park Research and Intensive Monitoring of Ecosystems Network sites (PRIMENet), and others. Dry deposition is measured at 26 NPS areas and 2 USDA/FS areas. FLMs agree that it is preferable to obtain CASTNet data from the web site, rather than summarizing dry deposition data in this report. In this way, current data can be easily accessed by FLMs and the public.

Other methods for measuring dry deposition are available. For example, information on vertical changes in concentrations of major gases and particles of interest over plant canopies can be used for calculation of deposition of these compounds to forests and other ecosystems (Hicks et al., 1987). Models, such as "Big-Leaf" (Baldocchi et al., 1987) allow estimating dry deposition to uniform canopies, such as agricultural crops or lowland forests. However, no models have been developed so far for reliable estimates of deposition of gases and particles to forests and other ecosystems in complex mountain terrain (Bytnerowicz et al., 1997). Therefore, no good large-scale estimates of dry deposition are available for western U.S. forests.

Another approach to evaluating dry deposition is net throughfall technique. By measuring concentrations of ions in throughfall (bulk precipitation) and after subtracting concentrations of the same ions in precipitation in an open area, fluxes of ions such as nitrate, ammonium, and sulfate can be calculated. A branch washing technique is similar to the net throughfall approach and is used when no wet precipitation is present. The pre-washed branches are exposed to ambient air for a certain time period and then carefully rinsed with water (Lindberg and Lovett, 1985). Information about amounts of nitrate, ammonium and sulfate rinsed from branches of a known surface area, time of exposure, and leaf area index of a given forest stand allow the calculation of fluxes of the measured ions to trees. Adding stomatal uptake of gases (calculated from information on gas concentration and stomatal conductance), and estimates of deposition to other landscape forms (such as soils and rocks) allow for quite reliable estimates of dry deposition at a forest stand level (Bytnerowicz et al., 2000). Such estimates have been made for the subalpine zone of the eastern Sierra Nevada and mixed conifer forests on the western Sierra Nevada and the San Bernardino Mountains (Bytnerowicz and Fenn, 1996; Bytnerowicz et al., 1999). Both the net throughfall and branch washing techniques, although providing relatively accurate estimates of deposition to certain ecosystems, cannot be applied to every type of vegetation. These techniques work well for conifers with relatively thick cuticles. For plants with thinner cuticle, extraction of ions from plant interior or transcuticular uptake of deposited ions may not allow for making good estimates of dry deposition to plant surfaces.

Recent developments, such as passive samplers that allow for relatively inexpensive determinations of nitric oxide, nitrogen dioxide, ammonia, nitric acid and sulfur dioxide concentrations, provide some promising opportunities for large-scale estimates of distribution of these pollutants. This, together with information on landscape-level vegetation coverage, leaf area index, and deposition velocity of the monitored pollutants, will allow calculating deposition of the measured gases to various landscape forms. Although this approach would not include deposition fluxes of particulate pollutants, a large portion of dry N and S deposition (gases) would be covered. Information on fluxes of the N and S particulate component (nitrate, ammonium, and sulfate ion concentrations) can be estimated based on their concentrations from annular denuder/filter pack systems or other comparable techniques and literature values of deposition velocities of these ions.

For many FLM areas, detailed site-specific information and monitoring needed for dry deposition measurements are not available. Therefore, the FLM may choose to recommend a reasonable estimate of dry deposition. NAPAP's 1991 summary report concluded that dry deposition of sulfur is 30-60% of the total (wet plus dry) deposition at regionally representative sites; dry deposition of nitrogen is 30-70% of the total (wet plus dry) deposition at regionally representative sites (NAPAP 1991a). An analysis of one year (1991) of NADP, CASTNet, and IMPROVE (Interagency Monitoring of Protected Visual Environments) data from national parks and wildernesses found that wet deposition dominated total deposition in both the East and the West. Dry deposition of sulfur was 20-50% of the total; dry deposition of nitrogen was 30-60% of the total (Hidy 1998). These estimates, and similar ones, have led to the common assumption that dry deposition is approximately 50% of the total deposition. Therefore, for many FLM areas without on-site or nearby representative dry deposition sampling, the FLM may recommend that dry deposition is equal to wet deposition. The FLM recommends this as a "best available estimate," recognizing that in some areas it may result in under- or over-estimating total deposition. Total deposition, which is the sum of wet plus dry deposition, therefore equals twice the wet deposition.

In summary,

Total Deposition = Wet Deposition + Dry Deposition

Or,

Total Deposition = 2 x Wet Deposition, assuming Dry Deposition = Wet Deposition

Table D-2 identifies monitoring stations in or near FLM areas for estimating wet and dry deposition values. For some areas the FLM assumes that dry deposition equals wet deposition, recognizing that this may result in under- or over-estimates of total deposition. The table provides information on the appropriate dry deposition data to use at sites where data are available.

FLMs will continue to participate in monitoring and research to further our understanding of dry deposition dynamics and improve our measurements of dry deposition.

Other Deposition Measurement Methods

Pollutant deposition, particularly in areas where traditional wet and dry deposition sampling is impractical, can also be estimated by other methods. These methods include bulk samplers that collect both wet and dry deposition and snowpack measurements that estimate the total amount of pollutants in the snow column at the time of maximum snow accumulation. Special methods have also been developed for collecting fog and cloud water (Anderson et al. 1999).

In addition, methods are being developed to estimate dry deposition rates from pollutant concentrations obtained by IMPROVE fine particle samplers. IMPROVE samplers are located at many FLM areas and expanded coverage is planned for 1999.

Modeling Deposition Rates

Deposition from existing sources can be estimated from deposition monitoring data, but contributions to deposition from the proposed source and other sources permitted but not yet operating must be modeled.

Modeling should be done in accordance with recommendations developed by the Interagency Workgroup on Air Quality Modeling (IWAQM) Phase 2:

http://www.epa.gov/ttn/scram/7thconf/calpuff/phase2.pdf

IWAQM provides the procedures that can be used to estimate S and N deposition from a proposed source and other sources permitted but not yet operating. The FLMs propose that these procedures be used to estimate S and N deposition. For S deposition, the wet and dry fluxes of sulfur dioxide and sulfate are calculated, normalized by the molecular weight of S, and expressed as total S. For N deposition, IWAQM recommends that the wet and dry fluxes of nitric acid (HNO₃) and nitrate (NO₃^-) and the dry flux of nitrogen oxides (NO_x) be calculated, normalized by the molecular weight of N, and expressed as total N. In addition, the FLMs agree that wet and dry fluxes of ammonium sulfate ((NH₄)₂SO₄)) and ammonium nitrate (NH₄NO₃) should be calculated, normalized by the molecular weight of N, and added to the estimate of total N. Therefore, total N deposition is the sum of N contributed by dry and wet fluxes of HNO₃, NO₃^-, (NH₄)₂SO₄, and NH₄NO₃ and the dry flux of NO_x.

The FLMs recognize that the ammonia (NH₃) in these compounds is derived from both man-made and natural sources. Free gaseous NH₃has a high deposition velocity and tends to deposit quickly. However, if sulfates and nitrates (which are primarily man-made) are present in the atmosphere, free NH₃quickly reacts to form (NH₄)₂SO₄and NH₄NO₃. These compounds, because of their fine particle size and slower deposition velocity than free gaseous NH₃, can be transported long distances and deposited in a FLM area, adding to the total N deposition loading.

An appropriate estimate of ambient free gaseous NH₃is needed for the modeling analysis. IWAQM refers to Langford et al. (1992), who suggest that typical (within a factor of 2) background values of NH₃are: 10 parts per billion (ppb) for grasslands, 0.5 ppb for forest, and 1 ppb for arid lands at 20^oC. Langford et al. (1992) provide strong evidence that background levels of NH₃show strong dependence with ambient temperature (variations of a factor of 3 or 4) and a strong dependence on the soil pH. However, given all the uncertainties in NH₃data, IWAQM recommends use of the background levels provided above, unless better data are available for the specific modeling domain. IWAQM notes that in areas where there are high ambient levels of sulfate, values such as 10 ppb might overestimate the formation of particulate nitrate from a given source, for these polluted conditions. IWAQM further notes that areas in the vicinity of strong point sources of NH_3, such as feed lots or other agricultural areas, may experience locally high levels of background NH₃.

Questions regarding these recommendations should be resolved through consultation with the appropriate FLM and the appropriate State and/or EPA modeling representative. Applicants should provide a modeling protocol to the appropriate FLM prior to conducting modeling analyses.

Estimation of Current and Future Deposition Rates

In order to evaluate a proposed source's contribution to total (wet + dry) deposition in a FLM area, it is necessary to first estimate current pollutant deposition rates. The current rate is a result of deposition from all existing natural and anthropogenic sources. FLMs use two approaches to estimating the current rate of deposition. One approach estimates the current rate by averaging data from an appropriate monitoring site for the pollutant of interest, using all years with complete data records. The second, more conservative, approach assumes that the current rate is equivalent to the highest rate for the pollutant of interest in the data record.

The method for estimating future total deposition rates is:

1. Identify in table D-2 available on-site or representative wet and dry deposition data for the FLM area. Wet deposition data can be obtained through NADP (http://nadp.sws.uiuc.edu/).

http://www.epa.gov/castnet

Table D-2 will indicate if dry deposition is assumed to equal wet deposition for the site. For high-elevation sites, consult with the FLM to determine if deposition from cloudwater, fog, dew, or snowpack should be considered. For sites without on-site data, consult FLM for further guidance.

2. After consulting with the FLM, estimate either:

3. Calculate current total deposition (wet + dry = total).

4. Estimate, using the appropriate dispersion model as described in the "Modeling Deposition Rates" section above, the proposed source's contribution to future total deposition on an annual or seasonal basis.

5. Estimate, using appropriate dispersion model as described in the "Modeling Deposition Rates" section above, the contribution of any sources permitted but not yet operating to future total deposition. This estimate may be available from the State permitting authority.

6. The current pollutant deposition rate plus the proposed source's contribution to deposition plus the contribution from other sources permitted but not yet operating equals the future total deposition rate.

Current + Proposed + Permitted (not yet operating) = Future Total Deposition

This future total deposition rate for a given pollutant can then be used to determine the potential for adverse effects to AQRVs. If appropriate, the change in deposition rate can be used to estimate changes in pH or ANC in an ecosystem. If the future total deposition rate is expected to cause an adverse effect to AQRVs and/or exceeds the critical load established for a FLM area, the FLM may recommend mitigation, as outlined in the flowchart on Figure D-1. If no critical load has been established for the FLM area, the FLM will use the best information available in determining whether to recommend mitigation.

g. Summary

Deposition of S and N has the potential to affect terrestrial, freshwater, and estuarine ecosystems on FLM lands.
The FLM has identified, where possible, AQRVs sensitive to deposition of S and N on FLM lands and the critical loads associated with those AQRVs.
A proponent of a source of new emissions with the potential to contribute to S or N deposition in an FLM area should consult with the FLM to determine what analyses are needed to assess AQRV effects. The FLM may request a deposition impact analysis, described in detail in this chapter and summarized below.

1. Estimate the current deposition rate to the FLM area. A list of monitoring sites providing data to characterize deposition in FLM areas is included in Table D-2.

2. Estimate the future deposition rate by adding the existing rate, the new emissions' contribution to deposition, and the contribution of sources permitted but not yet operating. Modeling of new and permitted but not yet operating emissions' contribution to deposition should be conducted following IWAQM Phase 2 recommendations.

3. Compare the future deposition rate with the recommended screening criteria (e.g., critical load, concern threshold, or screening level value) for the affected FLM area. A list of documents summarizing these screening criteria, where available, can be found in Appendix H. Information for USDA/FS Class I areas is also available at:

http://www.fs.fed.us/r6/aq/natarm

A web site with NPS and FWS Class I area information is currently under development. The web site will be available at:

http://www.nature.nps.gov/air/

http://refuges.fws.gov

The appropriate FLM should be contacted for additional information.

h. Websites for Deposition and Related Information

Clean Air Status and Trends Network (CASTNet) dry deposition data:

IWAQM guidance for deposition modeling:

http://www.epa.gov/scram001

National Acid Precipitation Assessment Program:

http://gcmd.nasa.gov/records/GCMD_EPA0141.html

National Atmospheric Deposition Program (NADP) wet deposition data: