Acid Rain - Effects Of Acid Deposition Onliving Organisms

An ecosystem is a particular environment and the biological organisms that live there. Ecosystems can be global or tiny. The plants and animals living within an ecosystem are interdependent. For example, frogs eat water insects. If the insects disappear because of acid deposition effects, the frogs may not thrive because part of their food supply has disappeared. Because of the many and varied interconnections among the plants, animals, and microorganisms living in an ecosystem, changes in pH may change the ecosystem's biodiversity or overall health.

The duration of the effects of acid rain on living organisms can vary from a few hours to many years. For example, soils that are depleted of essential nutrients may take decades or even centuries to recover. Once acid rain is reduced to normal levels, the slow process of nutrient buildup in the soil is dependent on the gradual succession over time of plant life. Plants that are tolerant of depleted soils will restore nutrients over time as they grow, die, and decompose, putting essential nutrients back in the soils. In the natural order, these plants will be followed by other plants that require more nutrient-rich soil, and as they grow, die, and decompose they will return more nutrients to the soil. Animals attracted to each stage of plant succession will also add their wastes to the process, bringing in additional nutrients. This process will continue through time until a healthy, balanced population appropriate to the ecosystem is restored.

Aquatic Systems

The effects of acid rain on aquatic systems are varied and many. They include great harm or death to fish, diminished fish populations, loss of a species in a particular water body, and reduction in biodiversity. As acid rain moves through soils in a watershed, aluminum is released from soils into the lakes and streams. As the pH lowers in a water body, the aluminum level climbs. Both low pH and elevated aluminum levels are toxic to fish (aluminum burns the gills of fish and accumulates in organs, causing organ damage). They can also cause chronic stress, which does not immediately kill an individual fish, but impairs its ability to take in the oxygen, salts, and nutrients needed to stay alive.

Freshwater fish need to maintain their osmoregulation to stay alive. Osmoregulation is the process of maintaining the delicate balance of salts and minerals in their tissues. Acid molecules stimulate the formation of mucus in the gills, which interferes with their ability to absorb oxygen. If mucus buildup continues, the fish suffocate. In addition, a low pH disrupts the balance of salts in fish and other aquatic life, interfering with reproduction and maintenance of bones or exoskeleton.

TABLE 9.1
Sulfur dioxide emission trends by source category, selected years 1970–2002

TABLE 9.1
Sulfur dioxide emission trends by source category, selected years 1970–2002 [CONTINUED]

TABLE 9.1
Sulfur dioxide emission trends by source category, selected years 1970–2002 [CONTINUED]
SOURCE: Adapted from "National Emission Trends: Sulfur Dioxide Emissions (Thousand Short Tons)," in 1970–2002 Average Annual Emissions, All Criteria Pollutants, U.S. Environmental Protection Agency, January 2005, http://www.epa.gov/ttn/chief/trends/ (accessed April 14, 2005)

TABLE 9.2
Nitrogen oxide emission trends, by source category, selected years 1970–2002

TABLE 9.2
Nitrogen oxide emission trends, by source category, selected years 1970–2002 [CONTINUED]

TABLE 9.2
Nitrogen oxide emission trends, by source category, selected years 1970–2002 [CONTINUED]

TABLE 9.2
Nitrogen oxide emission trends, by source category, selected years 1970–2002 [CONTINUED]
SOURCE: Adapted from "National Emission Trends: Nitrogen Oxide Emissions (Thousand Short Tons)," in 1970–2002 Average Annual Emissions, All Criteria Pollutants, U.S. Environmental Protection Agency, January 2005, http://www.epa.gov/ttn/chief/trends/ (accessed April 14, 2005)

Some aquatic plants and animals are able to tolerate more acidic waters. For example, frogs can tolerate lower pH than trout, crayfish, or clams. Acid-sensitive species, however, are lost as pH declines. It is usually the young of most species that are the most sensitive to environmental conditions. For example, at less than pH 5, trout and salmon eggs cannot hatch. At lower pH levels (pH 4 to 4.9), some adult fish die. Some extremely hardy fish such as the roach (a type of carp) can survive at a pH as low as 3.5 if the change is gradual and they have time to adjust.

Other effects include:

• Sudden, short-term shifts in pH levels, resulting in acid shock to freshwater ecosystems
• Gradual declines in fish populations and numbers of adult and juvenile fish over time as pH decreases
• Unsuccessful reproduction by many aquatic species, including poor egg production, abnormal eggs, and poor juvenile survival
• Physical impairment in juveniles of some species
• Loss of the ability in salmon to find home streams because of impaired sense of smell
• Loss of important components of the food web, leading to poor nutrition or starvation in species dependent on those components
• Changes in the plant and animal species within an ecosystem

Nitrogen has been shown to play an important role in both episodic and long-term acidification. It is also an important nutrient, but excess nitrogen can cause water quality degradation. Of the nitrogen released into the atmosphere through human activities, 10% to 45% is transported to U.S. oceans and estuaries through air deposition. In Chesapeake Bay, for example, 30% of the nitrogen contributed from human-made sources is atmospheric deposition.

Another area in which excess nitrogen from acid deposition, among other sources, is being watched very carefully is along the coastline. Studies of a phenomenon nicknamed dead zones have been underway for many years. The term dead zone actually refers to a state of hypoxia. A water body that is suffering from hypoxia is one in which excess nitrogen has caused the dissolved oxygen in the water to deplete to the point where the water can no longer support life. The three sources of nitrogen known to cause large dead zones to appear every summer in the Gulf of Mexico are agricultural run-off, industrial waste, and acid deposition.

Forest Systems

Acid deposition can have serious impacts on trees and soils, causing slower growth, injury, or death of forests. Acid deposition has been implicated in forest and soil degradation in the eastern United States, particularly in the high elevations of the Appalachian Mountains from Georgia to Maine, an area including the Shenandoah and Great Smoky Mountain National Parks.

When rain falls to the forest floor, the buffering capacity of the soil may neutralize some or all of its acidity. Differences in soil-buffering capacity is the reason that some areas that receive a lot of acid rain show little damage while other areas that receive the same amount show a lot of damage. The ability of forest soil to resist becoming acidified depends on the thickness of the soil and the type of bedrock below the soil.

When the soils cannot buffer the acid rain, vital nutrients present in the soil, such as calcium and magnesium, are stripped away by the acid-driven reactions. Aluminum, a toxic element present in all soils, is made more available to the trees and taken up by their roots. The combination of toxic aluminum and poor nutrition retard growth, make the trees more vulnerable to infection, and can eventually kill the trees.

In March 1999 the U.S. Geological Survey (USGS), in Soil Calcium Depletion Linked to Acid Rain and Forest Growth in the Eastern United States, reported that calcium levels in forest soils had declined at locations in ten states in the eastern United States. Calcium is necessary to neutralize acid rain and is an essential nutrient for tree growth. Sugar maple and red spruce trees, in particular, showed reduced resistance to stresses such as insect defoliation and low winter temperatures. Although the specific relationships among calcium availability, acid rain, and forest growth are uncertain, Dr. Gregory Lawrence, scientist and coauthor of the report, speculated:

Acid rain releases aluminum from the underlying mineral soil layer, which is followed by the upward transport of the aluminum into the forest floor (the nutrient-rich organic soil layer where root activity is greatest) by root uptake and water movement. The result is that aluminum replaces calcium, and the trees have a harder time trying to get the needed calcium from the soil layer.

Acid deposition can affect trees in other ways. Sulfur dioxide that has not been converted to sulfuric acid has been shown to clog up the leaf stomata (tiny openings in leaves where gases diffuse in and out), impairing plant respiration and photosynthesis. Nitric acid and nitrogen oxide have been shown to stimulate tree growth outside the growing season, leaving trees vulnerable to winterkill. Forests in high mountain regions often are surrounded by acidic clouds and fog that are more acidic than rainfall. Scientists believe that when the tree leaves and needles are frequently wetted in this acid fog, essential nutrients are stripped away. Loss of nutrients in the foliage makes the trees more vulnerable to other environmental threats, particularly winterkill. Winterkill resulting in damage or death is the result of naturally occurring stress caused by cold, wind, ice, and dehydration on trees and other woody plants that have been weakened by insect damage, nutrient deficiency, or drought.

Plants that are found in locations that are susceptible to high acid deposition experience the same fate as trees. The processes causing growth retardation and ultimately death are believed to be the same.

Human Health

Acid rain feels, tastes, and looks just like clean rain. Sulfur dioxide and nitric oxides, the pollutants that cause acid rain, however, can damage human health. These gases interact with particulate in the atmosphere to form aerosols (a mixture of very tiny liquid and solid particles) that can travel long distances transported by winds. When inhaled, aerosols penetrate deep into the lungs and are readily retained. Because of their very fine size, they can also penetrate indoors through ventilation systems.

Air pollution studies have indicated that elevated levels of acidic particles can cause asthma attacks, particularly in adolescents, and can also impair the ability of the upper respiratory tract to remove other potentially harmful particles. Some scientific studies have also established a relationship between elevated levels of fine particles and increased deaths from heart and lung disorders, such as bronchitis and asthma. Other scientists believe that these pollutants may increase the health risks to those over age sixty-five; those with asthma, chronic bronchitis, and emphysema; pregnant women; and those with histories of heart disease.

In "Effects of Acid Rain: Human Health" (Environmental Protection Agency, http://www.epa.gov/airmarkets/acidrain/effects/health.html, November 12, 2003), the EPA reported that sulfate aerosols make up about 25% of fine particles in the air in the eastern United States. Lowering emissions from power plants that contain fine sulfate and nitrate particles should eventually reduce the incidence and severity of the health problems believed related to these pollutants. The EPA estimated that when fully implemented in 2010, the public health benefits of the Acid Rain Program (created by Congress under Title IV of the 1990 Clean Air Act Amendments) would be about $50 billion annually in reduced health care costs because of decreases in emergency room visits, hospital admissions, and number of deaths.

Decreased nitric acid emissions are expected to lower the amount of ozone formed. Ozone is believed to increase the risk of illness or death from lung inflammation, including asthma and emphysema.

An indirect effect of acid deposition on human health is the increased reactivity in acid water of toxic metals and other chemicals. Increased reactivity means that the chemicals and toxic metals in the water are more likely to be taken up in fruits, vegetables, and animal tissue. Air deposition is believed to be the leading source of mercury bioaccumulation in fish. This sort of bioaccumulation has led to advisories against eating certain kinds of fish.

The principal pollutants generated by coal combustion that can cause health problems are particulate, sulfur and nitrogen oxides, trace elements (such as arsenic, fluorine, selenium, and the radionuclides, uranium, and thorium), and organic compounds as a result of incomplete coal combustion. Some of these trace elements have been shown to cause severe health effects in other countries, such as China, Romania, and Bulgaria.

The EPA conducted a detailed study of possible health effects that can come from the exposure to emissions of about twenty potentially toxic substances from coal-burning electric utilities. In this study, the EPA used USGS information on U.S. coal quality to assess the potential health impact of fourteen potentially toxic trace elements that may be mobilized by coal burning. The USGS fact sheet Health Impacts of Coal Combustion (July 2000) reported that, with the possible exception of mercury, there is no compelling evidence to indicate that emissions from U.S. coal-burning electric utilities cause human health problems. The absence of detectable health problems was credited in part to the use in the United States of coals that contain low to moderate amounts of sulfur and other potentially toxic trace elements. Another reason for the absence of detectable health problems was the common use of sophisticated pollution control systems by coal-burning utilities. These systems are specifically designed to reduce the emission of hazardous elements.