Acid Rain-April 2004

Raindrops Keep Falling on My Head

Soon summer will be here and we can look forward to warm days with occasional cool refreshing showers of pure, clean rain. Right? Wrong! The summer may be warm, and it will rain, but the rainwater is far from pure and clean. Rainwater, by its very nature, is slightly acidic. As raindrops form and fall, the water combines with carbon dioxide in the atmosphere to form a weak acid called carbonic acid, with an average pH of 5.6. This naturally occurring ‘acid rain’ slowly works to break down rocks on the earth’s surface. You can see the effects of this rain in the wearing away of old tombstones or statues that are made from marble (a form of calcium carbonate that easily dissolves in acid). This ‘acid rain’ also seeps into the ground and will dissolve limestone, where it is present in the substrate, to form caves.

What many of us refer to as acid rain, however, is not naturally occurring. This acid forms in a similar way to carbonic acid, but the water combines with sulfur dioxide and nitrogen oxide in the atmosphere to form sulfuric and nitric acids respectively. Sulfur dioxide (SO2) and nitrogen oxide (NOx) are pollutants given off by combustion of fossil fuels in power plants, automobiles, and industry.

Here in western Pennsylvania, we are ‘downwind’ of large coal-burning power plants in the Ohio River valley and major cities such as Chicago, Detroit, and Cleveland. In the Pittsburgh region, we have the most acidic rainfall in North America, with pH values as low as 4.0 to 4.3. This is as acidic as orange juice.

This anthropogenic (human caused) acid rain has major effects on our environment. First, acid rain flowing through soils in a watershed releases aluminum from soils into the lakes and streams. As the pH in a lake or stream decreases, aluminum levels also increase. Both low pH and increased aluminum levels are directly toxic to fish. For example, when the pH of lake water gets as low as 5.0, game fish such as walleye, small mouth bass, and lake trout will die off, and most fish eggs will not hatch. At a pH of 4.7, northern pike, brown bullhead, sunfish, and rock bass disappear. Only hearty perch can survive below a pH of 4.5.

Whether or not acid rain is a problem to streams depends on the geology of a region.   Fortunately, most of western Pennsylvania rocks, bedrock, and soil contain calcium carbonate and magnesium carbonate, which can neutralize the acid.   Acid rain does become a problem in western Pennsylvania streams in major storm events, where surface runoff does not allow the acidic precipitation to come in contact with the buffering geology of the region.  Quick acidic snowmelts can also cause these “acid spikes” in a stream. The stream’s pH will recover eventually from these acid spikes.  

Acid rain can also significantly affect trees and other plants. Acid rain does not usually kill trees directly. Instead, the acidic water dissolves the nutrients and helpful minerals in the soil and then washes them away before trees and other plants can use them to grow. The high rate of sugar maple and red spruce mortality in western Pennsylvania is linked to depletion of soil nutrients caused by acidification. Acid rain also causes the release of substances that are toxic to trees and plants, such as aluminum, into the soil.

Forests in high mountain regions, such as Chestnut Ridge, Laurel Mountain and the Allegheny mountains to our east, often are exposed to greater amounts of acid than local lower-land forests because they are occasionally surrounded by acidic clouds and fog that are more acidic than rainfall. When leaves are frequently bathed in this acid fog, essential nutrients in their leaves and needles are stripped away. This loss of nutrients in their foliage makes trees more susceptible to damage by other environmental factors, particularly cold winter weather and insect pests.

In addition, acid rain damages many building materials, including steel paint, plastics, cement, masonry, limestone, sandstone, and marble, and it damages automobile coatings and corrodes metals such as copper and bronze.

What can you do? Let your national and state representative know that you support continued, and even enhanced, controls on power plant emissions. Also, purchase fuel-efficient automobiles with low emissions, and support research into alternative fuels.

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