In the past century, one of the greatest threats to North America’s aquatic ecosystem has been the widespread acidification of hundreds of thousands of waterways. Acid rain has effected plant and animal life within aquatic ecosystems, as well as microbiologic activity by affecting the rates of decomposition and the accumulation of organic matter. What causes this poisonous rain, and what can be done to improve North America’s water quality and prevent future catastrophes? To answer these questions, we must first examine the cause and formation of acid rain, as well as understand ways to decrease or prevent its formation.
Formation of acid rain. Acid deposition, more commonly known as acid rain, occurs when emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) react in the atmosphere with water, oxygen, and oxidants to form acidic compounds. This mixture forms a mild solution of sulfuric and nitric acid which then falls to the earth in either wet (rain, snow, sleet or fog) or dry (gas and particles) form.
Approximately one-half of the atmosphere’s acidity falls back to earth through dry deposition in the form of particles and gases, and are then spread hundreds of miles by winds where they settle on surfaces of buildings, cars, homes, and trees. When acid rain falls, the dry deposited gases and particles are sometimes washed from buildings, trees and other surfaces making the runoff water combine with the acid rain more acidic than the falling acid rain alone. This new combination is referred to as acid deposition. The runoff water is then transported by strong prevailing winds and public sewer systems into lakes and streams.
Although some natural sources such as volcanic eruptions, fire and lightening contribute to the emissions of sulfur dioxide and nitrogen oxides in the atmosphere, more than 90% is the result of human activies such as coal burning, smelting of metals such as zinc, nickel and copper, and the burning of oil, coal and gas in power plants and automobiles.
When does rain become acidic? Scientists determine whether rain or lake water is acidic by measuring its pH (the measure of acidity or alkalinity of a solution on a scale of 0 to 14). A value of 7 is considered neutral, whereas values less than 7 are acidic and values over 7 are alkaline or basic. A change of one unit on the pH scale represents a factor of ten in acidity; for example, a solution with a pH of five is ten times as acid as one with a pH of six (Somerville, 1996, p.174). Normal or clean rainfall–without pollutants–is slighty acidic due to carbon dioxide, a natural gas in the air that dissolves in water to form weak carbonic acid. But rain, snow, or other moisture is not called “acid rain” until it has a pH value below 5.6 (Gay, 1992, p.44). Rainfall in eastern North America is often acidic with a pH of 4 to 5.
Why is North America greatly at risk? Acid rain is more common in the Eastern U.S. and Canada than in the Western U.S. because emissions rise high into the atmosphere and are carried by prevailing winds from the west, falling out with precipitation in the east. Some areas in the U.S. where acid rain is most common include the New York Adirondacks, mid-Appalachian highlands, and the upper Midwest. Canada shows an even greater threat with half of its acid deposition caused by a large amount of metal smelting industries in Ontario and the other half attributed to pollution from combustion in U.S. factories in Ohio, Indiana, Pennsylvania, Illinois, Missouri, West Virginia, and Tennessee.
Most lakes have a pH between 6 and 8; however, some are naturally acidic even without the effects of acid rain. Lakes and streams become acidic (pH value goes down) when the water itself and its surrounding soil cannot buffer, or shield, the acid rain enough to balance its pH level. In areas such as the northeastern United States and parts of Canada where soil buffering is poor, many lakes now have a pH value of less than 5. One of the most acidic lakes reported is Little Echo Pond in Franklin, New York, which has a pH of only 4.2. In New York’s Adirondack region, acid deposition has affected hundreds of lakes and thousands of miles of headwater streams, while 300,000 lakes in eastern Canada are now vulnerable to acid deposition.
How does Acid Rain effect Aquatic Ecosystems? As lakes and streams become more acidic, the amount of fish, aquatic plants and animals that live in these waters decrease. Although some plants and animals can survive acidic waters, others are acid-sensitive and will die as the pH declines. Plants and animals living within an ecosystem are highly interdependent. If acid rain causes the loss of acid-sensitive plants and animals, organisms at all trophic levels within the food chain may be affected which then causes a disruption to the entire ecosystem. In New York’s Adirondack region, the diversity of life in these acidic waters has been greatly reduced. Fish population have disappeared and loons and otters have moved to other lakes where they can find food (Simonin, 1998, p4). In Canada, over 14,000 lakes have been acidified to the point where they have lost significant amounts of fish. The chart below shows that not all fish, shellfish or their foot insects can tolerate the same amount of acid. The shaded bars represent the highest degree of pH balance that animal can tolerate within an acidic lake before it becomes extinct from that lake. For example, frogs seem to be the toughest survivor by being able to tolerate a pH up to 4.0, whereas clams and snails are the weakest only being able to tolerate a pH of 6.0 before it will become extinct. (*Source: United States Environmental Protection Agency; www.epa.gov):
AnimalspH 6.5pH 6.0pH 5.5pH 5.0PH 4.5pH 4.0
There are two patterns that contribute to the disappearance of fish from acidic bodies of water. The first pattern is known as “acid shock”, which is a sudden drop in pH. These pH shocks usually occur in early spring when melting snow releases acidic elements accumulated during the winter into a lake or stream causing a rapid decrease in pH level, which in turn causes fish to die. A second pattern is the gradual decrease in pH level over a prolonged period of time interfering with fish reproduction; therefore, causing decrease in fish population, and a change in size and age of the population. Other animals are affected by acidic water as well. For example, low pH will often stunt the growth of frogs, toads and salamanders.
Changes in pH level have caused alterations in the structure of the aquatic plant life involved in primary production. Reducing the diversity of the plant communities in lakes and streams and disrupting primary production will most likely reduce the supply of food; therefore, the energy flow within the ecosystem will decrease. Changes in these communities also reduce the supply of nutrients. These factors limit the number of organisms that can exist within the ecosystem (Brittenbender, B., et. al., p. 4)
In addition to affecting the plant and animal life, microbiological activity is also reduced affecting the rate of decomposition and accumulation of organic matter. Organic matter plays a central role in the energy flow of a lake’s ecosystem. “The biochemical transformations of detrital organic matter by microbial metabolism are fundamental to nutrient cycling and energy flux within the system, and the trophic relationships within lake ecosystems are almost entirely dependent on detrital structure” (Brittenbender, B., et. al., p. 5). There are two responsible causes for the slowing rate at which organic matter decomposes underwater. First, the disappearance of certain invertebrates such as snails that shred organic debris as they feed; and second, a decrease in the metabolic rate of decomposition bacteria at a low pH level.
Fighting acid rain. There are several ways to treat the acid rain problem. The answers depend heavily upon local politics and global economics. One solution is to use low-sulfur coal as opposed to high-sulfur coal. Unfortunately, high-sulfur coal is far more expensive than low-sulfur coal due to the economics of mining and transporting it. Another solution is to chemically treat high-sulfur coal before burning it. Devices known as scrubbers can be installed on smokestacks to reduce the amount of sulfur dioxide being released into the atmosphere.
The pH levels in lakes can be increased by a technique called liming. This process involves adding large quantities of hydrated lime to the waters in order to increase the alkalinity and pH. Areas that have used this method have had some success; however; liming does not always work because the lake may be too large and therefore economically unfeasible. In other cases, the lake may have a high flush rate, or poor buffering, so they quickly become acidified again after liming. Liming the acidic soils surrounding the lake so that the lime slowly dissolves over time to wash alkalinity into the lake is a more simple answer as well as less expensive.
Although these solutions decrease sulfur dioxide in the atmosphere, nitrogen oxides are still increasing. Reducing nitrogen oxides is more difficult to treat because this type of acidic pollution is mainly caused by automobile exhaust. Although a reduction in number of automobiles used is unlikely, regulating the use of specially designed catalytic converters could control emissions.
Improvements are being made. Thanks to environmental regulations and agreements to control pollution, lakes and streams in North America are beginning to recover from acid rain and life is being restored. In 1995, phase I of the Clean Air Act Amendment was launched. Through this Act, over 400 power plants in the U.S. were instructed to reduce their sulfur dioxide emissions by 3 million tons. Power plants are now instructed to reduce their use of fossil fuels, burn low-sulfur coal or use scrubbers.
In 1991, the United States and Canada established the Air Quality Accord that controls the air pollution that flows across international boundaries. In this agreement, acid deposition causing emissions of sulfur are permanently capped in both countries (13.3 million tons for the U.S. and 3.2 million tons for Canada) and plans were implemented for the reduction of nitrogen oxides.
Phase II of the Clean Air Act will kick off this year, mandating even steeper cuts in sulfur emissions. The National Atmospheric Deposition Program/National Trends Network (NADP/NTN) has 191 sites across the country which measure the emissions of sulfur dioxide. Establishing more organizations such as this will help us understand how and where to combat the acid rain problem.
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