What Goes Up Must Come Down!

What's up, Forsyth? Remember—what's "up" must also come down, which is fitting for this blog post on the links between air and water quality. 

What Goes Up Must Come Down: How Today’s Air Pollution Becomes Tomorrow's Water Pollution

When we burn fossil fuels in our vehicles, gas-powered equipment, power plants, or factories, they emit nitrogen oxides (NOx) and sometimes (depending on the type of fuel used) sulfur dioxide (SO2). These gases pollute the air and cause problems for human health, and they also harm our environment. Specifically, many of our water quality issues, including freshwater acidification and eutrophication, can result from emissions of air pollutants. Essentially, the air pollution we create today can become a major source of water quality issues tomorrow. In North Carolina, on-road mobile sources (cars and trucks) are the biggest contributors to NOx; whereas, electric generating units (EGUs), more commonly known as power plants, are the biggest source of SO2. 

Acid Deposition

Acid deposition (often called acid rain) occurs when nitrogen oxides and sulfur dioxide react with water, oxygen, and other chemicals to form sulfuric and nitric acids. These acids mix with water before falling to the ground as acid rain, fog, snow, or hail (See Figure 1). Acid droplets can even be deposited in the absence of moisture, which is more common in desserts or other areas that are dry for much of the year. Although acid rain problems have decreased significantly thanks to the work of scientists and the Acid Rain Program established under the 1990 amendments to the Clean Air Act, in some places in the United States, it still causes damages to buildings and other manmade structures and harms plants and wildlife. Additionally, a significant amount of the mercury found in surface waters comes from atmospheric emissions from coal combustion at EGUs. The North Carolina Department of Environmental Quality estimates up to 98% of mercury deposited in our state is due to air emissions.

Figure 1. This image illustrates the pathway for acid rain in our environment: (1) Emissions of SO₂ and NOx are released into the air, where (2) the pollutants are transformed into acid particles that may be transported long distances. (3) These acid particles then fall to the earth as wet and dry deposition (dust, rain, snow, etc.) and (4) may cause harmful effects on soil, forests, streams, and lakes.  (Image and caption by the United States Environmental Protection Agency)

On land, acid rain especially affects mountainous forests with thin forest soils. The photo below shows acid rain effects in the Great Smoky Mountains, where the average acidity (pH) of rainfall in the park is 4.5—that’s 5-10 times more acidic than normal rainfall (5.0-5.6)! According to the National Park Service, high elevation forests in the Great Smoky Mountains National Park are often among clouds that have pHs as low as 2.0!

Figure 2. Acid Rain Effects in the Great Smoky Mountains. Image credit: "Don't be afraid its only business! The aftermath of acid rain in Great Smoky Mountains, TN, USA" by You are being watched is licensed under CC BY-NC-ND 2.0.

While reducing acid deposition in our forests is important for improving the health of terrestrial ecosystems, the greatest effects are felt by aquatic and marine systems. Critters that live in our ponds, lakes, streams, and wetlands, including snails, macroinvertebrates, and amphibians, are often the most sensitive to changes in the acidity of the water.  See Figure 2 to learn the critical pH levels for different aquatic organisms. (The higher the pH, the less that species can tolerate acidity in its environment). Something to consider is that if one of those critters is the primary food source for another organism, it can have a cascading effect on the entire food web! 

Figure 3. This figure illustrates the pH level at which key organisms may be lost as their environment becomes more acidic. Not all fish, shellfish, or the insects that they eat can tolerate the same amount of acid. (Image and caption by the United States Environmental Protection Agency). 

Eutrophication

Today, eutrophication poses an even greater threat to our freshwater and saltwater ecosystems than acid deposition. Eutrophication occurs when too many nutrients, primarily nitrogen and phosphorus, are added to a waterbody. This can lead to the excessive growth of algae and some other aquatic plants that block sunlight. When the algae decomposes, it depletes dissolved oxygen which the fish and other organisms need to survive. This can lead to “dead zones” and large fish kill events. Eutrophication also promotes the growth of invasive (harmful, non-native) aquatic plants and reduces biodiversity. Figure 4 shows a large algal bloom in Lake Eerie. 

Figure 4. Eutrophication can lead to algal blooms like these, as shown in Lake Erie. Image credit: "Lake Erie HABs Sept 25 2017" by NOAA Great Lakes Environmental Research Laboratory is marked with Public Domain Mark 1.0.

Nitrogen oxides, coming mainly from gas-powered vehicles, are a major source of nitrogen pollution in our waterways. Certain industrial and agricultural processes increase atmospheric ammonia (NH3) concentrations, which also can lead to eutrophication. While the amount of eutrophication linked to atmospheric deposition varies, in heavily studied waterbodies like the Chesapeake Bay, it is estimated that nearly 30% of the nitrogen entering the watershed results from human-caused air pollution. Other sources of excess nitrogen that contribute to eutrophication include agricultural runoff (fertilizers, manure, etc.), wastewater discharge and leaking septic systems, and industrial activities.

In summary, by reducing air pollution in our communities, we can become better stewards of our watersheds. To learn more about how to protect clean air (and water) in your community, please reach out to the Triad Air Awareness Coordinator, Sarah Coffey at coffeyse@forsyth.cc.