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Deposition occurs when compounds of various types of air pollution are deposited on the earth's surface through rain, clouds, snow, fog, or as dry particles. The amount of deposition received in a given area is affected both by the concentration of pollution in the atmosphere and the way in which it is deposited. General factors such as climate, meteorology and topography influence how much pollution reaches the area from both local and distant sources, as well as how much of that pollution actually impacts the earth's surface via the various wet and dry forms of deposition.

There are several types of ecosystem effects associated with deposition that tie to the pollutant being deposited. These include acidic deposition (acid rain), heavy metals (including mercury) and excess amounts of nitrogen.

Acid Deposition

Acidic inputs from the atmosphere, mainly sulfate (SO4) and nitrate (NO3), can negatively impact aquatic and terrestrial ecosystems. Their acidifying effects contribute to degradation of stream and lake water quality by lowering the acid neutralizing capacity (ANC) which represents the water's natural acid buffering system. As the ANC decreases, the pH decreases as the acid levels in the water increase. In areas such as the central and southern Appalachians, forest streams have acidified to the point where they are no longer capable of sustaining aquatic life such as fish or amphibians. The sensitivity of lakes and streams to the negative effects of acidic deposition are often linked to natural watershed characteristics, most notably the type of bedrock geology. Watersheds containing naturally occurring bedrock that weathers (breaks down easily) and that are made up of minerals containing high levels of base cations like calcium (nutrients that plants need) are less susceptible to negative impacts of acidic deposition. Likewise, watersheds where the soils are derived from bedrock that is resistant to weathering like granite or that contains thin shallow soils are very susceptible to acidic deposition. These same susceptible areas may not only exhibit lake and stream water chemistry changes, but they may also affect soil chemistry causing problems such as nutrient leaching. Nutrient leaching can eventually lead to deficiencies in macro nutrients important for plant growth.

Areas that receive high levels of acidic deposition and have bedrock geology with a naturally low buffering capacity may exhibit nutrient depletion and stream acidification.

Sulfate is the primary component of acid rain in the eastern U.S. with the highest levels of emissions coming from the heavily industrialized Ohio River Valley. In spite of recent reductions across the eastern U.S., sulfate deposition is still higher than the ecosystems of the Appalachian states can tolerate. Nitrogen deposition is more of a factor in acid rain in the mid and western United States.

Nitrogen Effects

In addition to contributing to acid rain, nitrogen can cause other ecosystem impacts by unnaturally fertilizing land and water. These excess inputs of nitrogen, termed nutrient enrichment and eutrophication, can disrupt the natural flora and fauna by allowing certain species that would not naturally occur in abundance to out-compete those that thrive in pristine nitrogen-limited systems. The end result is an unnatural shift in species composition and that in turn may have a subsequent impact on other components of the ecosystem.

Toxic Methylmercury and Atmospheric Deposition

Toxic air contaminants like mercury, are emitted primarily by coal-fired utilities, and may be carried thousands of miles before entering lakes and streams as mercury deposition. Mercury can bioaccumulate and greatly biomagnify through the food chain in fish, humans and other animals. Non-organic forms of mercury are converted to methylmercury by sulfur reducing bacteria in aquatic sediments. Methylmercury is a potent neurotoxin, and has been shown to have detrimental health effects in human populations as well as behavioral and reproductive impacts to wildlife. Almost every state has consumption advisories for certain lakes and streams, warning of mercury-contaminated fish and shellfish. High concentrations of mercury are measured in sediments and fish tissue, even in remote areas of the Arctic. Recently, elevated methylmercury loads have been monitored in upland bird species, calling into question the traditional wisdom that methylmercury contamination is directly linked solely to aquatic systems. The link between sulfur-reducing bacteria and biotic mercury concentrations has led researchers to establish that reductions in sulfur dioxide emissions and a resulting reduction in sulfate deposition will abate mercury concentrations in wildlife. Consequently, as the level of sulfates is reduced in aquatic systems, sulfur reducing bacteria will reduce less sulfur, and this will in turn lead to less inorganic mercury being methylated. For more information, download the Mercury deposition briefing (.pdf 247kb).

Addressing Deposition Impacts in the National Forests and Class I Areas

Forest Service managers directly monitor and use models to measure or estimate the amount of deposition occurring on the national forests and how this deposition is affecting Forest resources. Long-term air quality and resource monitoring on and near the National Forests and Class I areas has helped establish air pollution trends and existing condition of the resources. For more information on monitoring, click here. Based on these existing conditions, and documented cause and effect relationships, the air resource specialists in the Forest Service are identifying critical loads and resource concern thresholds for evaluating potential impacts of new sources of air pollution for each Class I area.

Forest Service air resource specialists are also working to establish critical and target loads for Class I areas. Setting critical and target loads provides an additional tool to help evaluate adverse effects from air pollution. A critical load can be defined as "a quantitative estimate of the exposure to one or more pollutants below which significant harmful effects on specific sensitive elements of the environment do not occur according to present knowledge". A target load is set based on policy and management direction, and depending on whether or not current critical loads values have been exceeded, a target load can be above or below the critical load. In general, the critical load is based on modeled or measured dose-response data, while target load can be based on political, economic, spatial, or temporal considerations in addition to scientific information. Defining the critical and target loads for areas on the Forest helps resource managers communicate the effects of air pollution on resources to Forest Service decision-makers as well as to air regulators. This information will also be used to assess how some management activities may exacerbate air pollution related problems or identify areas where mitigation may be an option for resources that have already been negatively affected. This information can be used in a regulatory context when consulting with and advising air regulatory agencies on effects to Forest resources resulting from new and existing sources of air pollution. For more information on critical loads and the related science, please see: Click here.

For more on acid rain, visit http://www.epa.gov/acidrain/ or download the Ecological effects briefing (.pdf 408kb).