Surface WatersWater in lakes and streams can fall into the body of water directly, through precipitation, or it arrives from runoff. The runoff can be a particular problem because the water dissolves additional acid from the dry deposition on surfaces. Acidification is most important for lakes located on soils with a low buffering capacity.
Most surface water is slightly basic but some, particularly water in contact with acidic minerals is slightly acidic. The pH of natural waters varies between 6 and 8. Acid rain can greatly increase the acidity of surface water. The National Surface Water Survey investigated acidic deposition in over 1,000 lakes larger than 10 acres and in thousands of miles of streams. Acid rain caused acidity in 75 percent of the acidic lakes and about 50 percent of the acidic streams. The Adirondacks and Catskill Mountains in New York state, the mid-Appalachian highlands along the east coast, the upper Midwest, the mountainous areas of the Western United States, and the Northeastern United States region. One of the most acidic lakes is Little Echo Pond in Franklin, New York with a pH of 4.2 units.
SoilAcid deposition has chemically altered soils and damaged acid-sensitive ecosystems. The soils lose their ability to neutralize more acid and provide poorer growing conditions for plants.
The nitric and sulfuric acids react with basic calcium compounds in soils. This produces water soluble products [Ca][NO3]2 and Ca[SO4] that are washed from the soil. The calcium, then, is not available for growing plants.
After calcium salts are used, aluminum compounds react with the acids. The soluble aluminum compounds that are produced and washed into ground water and surface water can be toxic to fish and aquatic plants (see below).
Animals and PlantsAquatic animals are susceptible to acid rain. It causes a cascade of effects that kill individual fish and other animals, reduce population numbers, completely eliminate some species from a waterbody, and decrease biodiversity. As acid rain flows through soils in a watershed, aluminum is released from soils into the lakes and streams located in that watershed. Both low pH and increased aluminum levels are directly toxic. The chart below shows pH conditions under which different aquatic animals can survive.
Nitric acid and nitrate salts can act as fertilizers for algae and other aquatic plants. This causes another problem: eutrification. When these plants die, the biomass sinks and decays. This uses up the dissolved oxygen in the water and kills aquatic animals.
Human HealthThe oxides of both nitrogen and sulfur, as we have discussed previously are also toxic. The pH level of acid rain and the acidification of surface water caused by acid rain are not a direct health concern to humans. However, high levels of nitrates in drinking water are toxic.