Stream acidification

Issue 4 > Meet the Managers > Water quality issue: Stream acidification
Clouds carry pollutants from power plants and cars to the Smokies.

Clouds carry acidic pollutants from power plants and cars to the Smoky Mountains.

NPS Look Rock remote photo.

Understanding how pollution comes in from outside the park. The most harmful air pollutants—nitrates and sulfates—cause acid deposition in the form of rain, fog, snow, and dry particles. All of these deposits come from outside the park, carried in on warm air masses that rise and cool to form clouds, rain, and fog over the Smokies’ high peaks.

Nitrates and sulfates are byproducts of burning fossil fuels. Vehicle emissions (what comes from your tailpipe) contribute the most nitrate to the air. Coal-fired power plants, which produce electric energy in the southeast, contribute the most sulfate.

How does acid from the air affect Smokies’ streams?
More precipitation falls at higher elevations (above 4000 feet), which is also the level where the Smokies’ characteristic wisps of clouds and fog hang. Rain, snow, clouds, and fog all carry particles of sulfates and nitrates in their drops of moisture. Clouds and the wind also carry dry particles of pollution, which settle on tree leaves and end up in streams when rain washes them into the water.

Macroinvertebrates collected in a Smoky Mountain stream.

Can you find the critters? This net contains aquatic insects that need pristine water quality to survive.

NPS photo.

Pollution affects acid-sensitive animals more more than it affects non-sensitive animals. Acid-sensitive animals include brook trout, salamanders, mayflies,caddisflies, and more. They start to disappear when a stream’s average pH dips below 5.0. When the pH drops below 4.0, almost all animals disappear. Alarmingly, some of the highest elevation streams are consistently below pH 4.0, and even lower elevation streams routinely drop below 4.0 during storms, when huge amounts of acid rain flush dry-deposited acid from leaves and the ground into waterways.

Acidic water also flushes nutrients and metals out of the soil (see the next section for more details). One metal that washes into streams is aluminum. High levels of aluminum are toxic to fish, so biologists expect to see increased die-offs as this common metal rushes into streams from soil after storms.

Are plants and animals on land affected by acid deposition?
Yes! When more sulfate and nitrate enter the water cycle than are normally there, everything changes: the rocks, the soil, the water, and the plants and animals that live there. There’s a complex relationship between chemicals that fall in rain, the soil, and the streams in the Smokies.

Normal rain is slightly acidic, with a pH of about 6.0, but that’s not enough acid to cause any problems. In fact, scientists are beginning to think that the slight acidity of normal rainwater helps the soil capture the small amounts of acid that normally fall, creating a chemical “sponge” so the soil can hold and buffer low amounts of sulfates.

Snails require calcium in soils for strong shells.

Snails use calcium from the soil to build strong shells.

NPS photo.

Adding more sulfate and nitrate to the nutrient system disrupts the balance. Plants can store a certain amount of nitrates (they need them to grow—this is a common ingredient in fertilizer), and soils can store a certain amount of sulfates, but we have already exceeded this necessary amount. Excess acid then washes straight into the stream, taking with it metals and nutrients such as calcium from the soil. This disrupts natural levels of magnesium, potassium, and sodium. Changes in soil chemistry have serious consequences for plants and animals:

  • Land snails that can't absorb enough calcium from soil can't build strong, spiral shells
  • Birds that rely on a diet of snails to boost their own calcium levels do not receive enough of the nutrient, and in turn their eggshells are weakened
  • Plants require a balance of nutrients in the soil to grow; we are still examining the relationship of changing nutrient levels to long-term plant growth for species such as dogwood, which needs calcium to grow normally

How do we measure what’s in our water?

  • Researchers have collected water samples from all over the Park since 1991
  • They take samples from 43 monitoring sites every other month
  • They also take samples of air pollution, soil chemistry, and water quality at Noland Divide every two weeks
  • Samples are sent to the University of Tennessee for chemical analysis

For more detail and photos of water quality monitoring, click to go to the Partner Profile: Taking the pulse of Smoky Mountain streams.


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