Shenandoah National Park is home to about 90 perennial streams. All of the streams in the park eventually feed into the Chesapeake Bay, the largest estuary in North America. A main feature of these high elevation streams is their high gradient; the quick drop off the mountain includes pools interspersed with riffles, rapids, cascades, and falls.
Water quality in the park is directly affected by air quality. Stream acidification occurs when sulfur and nitrogen oxides that mostly come from fossil fuel burning react with water in the air to form acid compounds. These compounds are deposited by rainwater onto sensitive landscapes and into streams. This is commonly called acid rain. The park’s location downwind from industrial facilities, along with its position along ridgetops has led to high levels of acid rain and stream acidification over many decades.
Despite dramatic reduction in levels of acid rain, stream acidification continues in the park. Simply put, some streams are recovering and others are not because of varying bedrock geology in the park. Watersheds with underlying basaltic or limestone bedrock are recovering or holding steady. Watersheds associated with siliciclastic geology (1/3 of the park) are not recovering because they lack the ability to buffer acids.
Dragonflies: Indicators of Mercury Pollution
Citizen scientists at Shenandoah National Park are working as part of a nationwide study collecting juvenile dragonflies for mercury analysis. Mercury is a toxic pollutant that can harm human health and wildlife health. Mercury often enters the park as air pollution from distant, human-caused sources, such as coal-burning power plants.
Dragonflies live in park streams and wetlands. We usually see the luminescent wings and elongated bodies of the adults zoom across the water’s surface. Juvenile dragonfly larvae live just below the surface, on rocks, plants, and in the sediment. Dragonflies spend most of their lives in the water as larvae.
Dragonfly larvae can live for 5-7 years underwater eating insects and even small fishes. Mercury builds up inside the larvae and can help scientists learn about the health of the waters in which they live. As mercury accumulates up the food chain, fish can be harmed. Fish-eating birds and mammals, as well as people who eat fish from affected streams, can also experience negative effects.
Wild brook trout, or “brookies,” are highly prized by fly fishermen in Shenandoah National Park—but how does air affect brook trout?
Deposition of air pollutants into streams in the park leads to an increase in their acidity. There are 41 fish species living in streams throughout the park, 13 of which are common and widely distributed. These fish differ in their sensitivities to acidic water.
Acidity is measured according to a pH value. Pure water has a neutral pH of 7. pH values lower than 7 are acidic with 1 being the most acidic. pH values higher than 7 are not acidic but are alkaline or basic.
Some species of fish are harmed when pH values in a stream fall below about 6, while others are tolerant of pH values near 5. In addition, the sensitivity of some species to pH changes as they enter different life stages. You might be surprised to learn that even low levels of stream acidification can harm fish reproduction, causing a gradual decline in fish abundance over time. Brook trout are very tolerant of acidic streams, so when a stream loses its “brookies” scientists know the water must be unhealthy.
Many species of small aquatic animals, collectively known as benthic macroinvertebrates, also live in park streams. Much like the fish, they have a wide range of sensitivities to acidic water. Mayflies, caddisflies, and stoneflies are important food sources for brook trout and tend to be very sensitive to acidification. Benthic macroinvertebrates have been monitored in the park since the early 1980’s. They break down leaves and other organic matter in the stream and are an important food source for fish and other animals up the food chain.
Shenandoah Watershed Study
Shenandoah National Park and the University of Virginia began the Shenandoah Watershed Study in 1979 to monitor the acid-base levels of park streams. The program continues today and scientists monitor three watersheds on a weekly basis. The scientific legacy of this project is considerable, mostly because of the record itself. A long data record means scientists can investigate changes that occur in the park over time. For example, if air pollution has been reduced, why are some park streams still acidic?
Thanks to the work of project scientists, we now understand that sulfur from acid rain stays in some park soils for a really long time and is released slowly into streams. As a result, it is taking sensitive watersheds longer to “heal” from the effects of decades of acid rain.
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Rocks Can Impact Stream Recovery from Acid Rain
Shenandoah National Park has 231 headwater streams. Each of these small streams starts near the top of a mountain, usually as a spring or seep on a hill face. As the water flows downhill in the stream, it increases in size as more tributaries and groundwater contribute to it. Because these streams begin with little water, acid rain can cause them to be very acidic at the top of the mountain. In general, the steeper the slope, the more quickly acid rain reaches the stream with less chance of being neutralized by rocks and soils along the way.
The rocks and soils beneath park streams also have a major influence on the effects of acid rain. The three main types of rock in the park are basaltic, granitic, and siliciclastic. Basaltic rocks and soils have lots of base cations available to neutralize acid and are the best at handling acidic inputs. Siliciclastic rocks and soils, on the other hand, have very few base cations and are poor at neutralizing acids. Siliciclastic watersheds in the park may take hundreds of years to recover from acid rain, despite reduced deposition, because there are so few base cations available to change acidity levels. Granitic and basaltic watersheds are recovering much more rapidly.
For these reasons, a small stream near the crest of a mountain, on a steep slope of siliciclastic bedrock, is likely to have the most acidic water in the park. Such streams do not support a diverse population of fish and will take a long time to recover from acidification. The opposite is true of large streams with gentle slopes on basaltic bedrock. These have water that is near neutral acidity, a diverse population of aquatic life, and a speedier recovery from acidification!
Dragonflies: Indicators of Mercury Pollution
Citizen scientists at Shenandoah National Park are working as part of a nationwide study collecting juvenile dragonflies for mercury analysis. Mercury is a toxic pollutant that can harm human health and wildlife health. Mercury often enters the park as air pollution from distant, human-caused sources, such as coal-burning power plants.
Dragonflies live in park streams and wetlands. We usually see the luminescent wings and elongated bodies of the adults zoom across the water’s surface. Juvenile dragonfly larvae live just below the surface, on rocks, plants, and in the sediment. Dragonflies spend most of their lives in the water as larvae.
Dragonfly larvae can live for 5-7 years underwater eating insects and even small fishes. Mercury builds up inside the larvae and can help scientists learn about the health of the waters in which they live. As mercury accumulates up the food chain, fish can be harmed. Fish-eating birds and mammals, as well as people who eat fish from affected streams, can also experience negative effects.
