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Acid Rain Makes Some Shenandoah National Park Streams Unfavorable to Fish


Many streams in Shenandoah National Park are vulnerable to acid rain. Steep slopes, small watersheds, and underlying geology, combined with acid rain make many streams inhospitable to native fish for extended periods of time, according to the U.S. Geological Survey (USGS).

“Because of acid rain, Shenandoah is the third most contaminated park in the national park system,” said Karen Rice, the USGS scientist who led the study. “Many streams in the park have low acid neutralizing capacities for periods lasting from six hours to one week. About 14 percent of the park’s watersheds will have 3-day episodes of acidity sufficient to kill native fish such as brook trout. These high acid episodes occur at least once every two years.”

Acid neutralizing capacity is an important factor in stream-water quality and biologic health. In Shenandoah National Park, the rains are typically ten times more acidic than normal rainfall. The pH of rainfall in the park is usually 4.6, although it has dipped well below 4.0. Normal rain has a pH of 5.6. (pH is a logarithmic scale, therefore each whole-number denotes a 10-fold change.)

While this study did not look at likely impacts to fish populations, the long-term outlook for fish inhabiting streams with the lowest acid neutralizing capacity is not good. Over the next 40-100 years, there is a greater than 90 percent probability in the most vulnerable streams of at least one acid episode every year for four consecutive years.

The USGS, and the University of Virginia, in cooperation with the National Park Service began this study in 2002 to predict stream response to acid rain. They found that the vulnerability of a stream to acid rain was controlled by a combination of factors including watershed size, elevation, steepness of gradient and underlying rock type. Streams with the greatest vulnerability were in small watersheds with a high elevation, steep gradient, and were underlain by silicate bedrock. Those underlain by basaltic rocks have a greater capacity to neutralize acid rain, all other factors being equal.


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