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Acid Rain Overview |
Acid rain or acidic deposition is the most widely acknowledged form of atmospheric deposition. This is the process whereby precipitation (rain, snow, fog etc.), airborne particles, and gases move from the atmosphere to the surface of the Earth.
These particles and gases reach the surface in both wet and dry deposition and yield from a variety of air pollution sources which can adversely affect the environment and public health. Most of us are only familiar with "acid rain" but the deposition of toxic compounds and excessive nutrients are also important.
Let's take a graphical look at the process before we go further.  Source: EPA 1994
The chemical compounds that we are most interested in are sulfur dioxide (SO2), nitrogen oxides (NOX), trace elements, and organic compounds. These compounds are released into the atmosphere from both anthropogenic (man-made) and natural sources. Anthropogenic includes fossil-fuel power plants, vehicles, and factories. Natural sources include fires and volatilization from forests and marshes.
 Both of these sources can be further classified as being "point" or "non-point" sources meaning that point sources are in a fixed location and non-point sources release substances from larger, non-defined areas such as from the agricultural spraying of pesticides and fertilizers or vehicle emissions.
 Once these chemical compounds have been released, the wind transports them from their source to other locations. While in transit, the compounds undergo physical and chemical changes, reacting with sunlight, water vapor, and other atmospheric gases and particles to produce sulfates and nitrates. These changes can create a variety of compounds which will affect how quickly acidification will happen within a watershed.
 Research has shown that surface waters are most vulnerable to the effects of acidic deposition but how it responds depends on many other factors. One of which is the buffering capacity of the water body. Usually when a body of water is most sensitive to acidification, it has a relatively low to moderate pH (< 6.0) and acid neutralizing ability (ANC < 200 µeq/l). This yields a higher hydrogen ion concentration which is represented by a lower pH. A pH of 5.0 has ten times the hydrogen ion content (H+) than a pH of 6.0 and is thus ten times more acidic.
 Now we will examine the adverse effects acidification causes.
The low pH and ANC in Maryland's affected streams can be directly toxic to organisms or cause other disruptions to the ecosystem. Scientific evidence indicates that the number of aquatic taxa in an ecosystem usually declines with increasing acidity. Because relatively higher numbers of invertebrate taxa are sensitive to acidification, detrimental effects on food chains may occur well before direct toxicity to larger species is evident. Sometimes the loss of a species is compensated by an increase in numbers of acid-tolerant species, resulting in little or no depletion of overall biomass. However, recent studies on the acidification of low-order streams suggest that long-term chronic depression of stream pH results in biological simplification and a reduction of detrital processing and a decrease of energy and material input into food webs.
Although acid precipitation affects nearly every aspect of the environment, fish are among the most seriously affected. As the pH level of water drops, the normal ionic salt balance within the fish is disturbed and the fish begins to lose body salts to the surrounding water. If the salt losses exceed intake, fish go into shock, lose equilibrium and eventually die. Although dramatic fish kills from such direct effects have been reported from eastern states including Maryland, the loss of entire fish populations in abnormally acidic streams or lakes usually occurs because of successive failures in the reproductive cycle. Acidic waters inhibit the development of fish reproductive organs and facilitate the development of a mucous that suffocates eggs and fry.
The highest acidity in Maryland streams have been recorded in the spring, when many economically important anadromous fish species of the Chesapeake Bay enter the streams to spawn. Juvenile fish are even more susceptible to the deleterious effects of low pH than are adults so the unusually high spring acidities may effectively prevent reproduction and be partly responsible for recent precipitous declines in the Chesapeake population of many important fish species, such as striped bass, white perch, yellow perch and American shad. Acidification is having a detrimental effect on fish populations in Maryland.
Studies have indicated that Maryland and its watershed and airshed receive some of the highest concentrations of sulfate and nitrate deposition in the United States. This research has also looked into the sources of deposition in Maryland and have concluded that these sources reach as far away as South Carolina, Indiana and even Canada. Limiting the sources within Maryland would have a beneficial effect on the water quality in the Chesapeake Bay. This does not give us the right to sit back and blame the rest of the country, we have the responsibility to demonstrate that drastic changes can be made with a little effort on everyone's part.
Source Material:
Atmospheric Deposition in Maryland: Assessment of Status, Trends, and Environmental Effects, a publication of the Maryland Power Plant Research Program by the Maryland Department of Natural Resources. |
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