Freshwater acidification happens when acids enter lakes, ponds, or reservoirs. This can occur through the breakdown of rocks, the entry of acidic gases like carbon dioxide, or the decrease of certain chemicals such as sulfate and nitrate in the water. The main causes of freshwater acidification are sulfur dioxide and nitrogen dioxide, which enter water from rain and soil runoff. Carbonic acid and carbon dioxide can also enter freshwater through runoff from soils rich in carbon dioxide. Runoff containing these substances may carry hydrogen ions and aluminum, which can harm aquatic life. Acid rain also plays a role in making freshwater more acidic. A well-known example of freshwater acidification occurred in the Adirondack Lakes in New York during the 1970s. This was caused by acid rain resulting from industrial emissions of sulfur dioxide and nitrogen dioxide.

Causes

Carbon dioxide (CO₂) from the atmosphere or from the breakdown of organic matter affects the acidity of freshwater. When CO₂ dissolves in water, it forms carbonic acid (H₂CO₃). This carbonic acid splits into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻), increasing the number of hydrogen ions and lowering the pH level.

CO₂ + H₂O → H₂CO₃; H₂CO₃ ⇌ H⁺ + HCO₃⁻

Microbial activity breaks down organic matter, releasing organic acids like humic and fulvic acids. These acids collect in water bodies, especially in areas with forests and wetlands. Peatlands and wetlands often produce acidic water because of high organic matter breakdown. This creates naturally acidic conditions, which are common in boreal and subarctic regions.

Volcanic activity can release sulfur dioxide (SO₂) and other acidic oxides into the atmosphere. In the air, sulfur dioxide combines with oxygen and water to form sulfuric acid (H₂SO₄). This sulfuric acid splits into sulfate ions (SO₄²⁻) and hydrogen ions (H⁺), increasing acidity.

SO₂ + ½ O₂ + H₂O → H₂SO₄; H₂SO₄ → 2H⁺ + SO₄²⁻

Human activities, such as burning fossil fuels, release sulfur dioxide (SO₂) and nitrogen oxides (NOₓ). These gases react with water and air to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). Like sulfuric acid, nitric acid lowers the pH level by splitting into hydrogen ions (H⁺) and nitrate ions (NO₃⁻).

NOₓ + H₂O + ½ O₂ → HNO₃; HNO₃ → H⁺ + NO₃⁻

This process is especially harmful in areas where water has low natural buffering capacity, as these ecosystems struggle to neutralize added acidity.

Mining can increase freshwater acidity through acid mine drainage. When sulfide minerals, such as pyrite (FeS₂), are exposed to air and water during mining, they react to form sulfuric acid.

Buffering Capacity

The ability of ecosystems to resist changes in pH is called buffering capacity. In freshwater systems, bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions help neutralize hydrogen ions (H⁺) that increase acidity. This reaction is shown below:

HCO₃⁻ + H⁺ → CO₂ + H₂O

In areas with low buffering capacity, such as regions with silicate bedrock, there are not enough bicarbonate or carbonate ions to neutralize hydrogen ions. This leads to quick drops in pH. For example, the Atlantic region of Canada has the lowest acid deposition rates in Eastern North America but has the most acidic waters on the continent. This is because the bedrock in this area, such as granite and shale, contains little buffering material. Soils and water draining from these materials are easily affected by acidification, even when acid deposition is low. Natural organic acids from nearby wetlands also contribute to the acidity.

Effects on ecosystems

Acidification of freshwater ecosystems can reduce the number of native plant and animal species and change how ecosystems work and look. Small animals without backbones and larger animals with backbones often die more easily and have fewer babies when water becomes more acidic. In contrast, some types of algae and mosses, such as Sphagnum, grow better in acidic water. These plants can quickly take over areas, making it hard for other species to live there. Sphagnum is often found in greater numbers in acidic environments. This moss can swap hydrogen ions for other minerals in water. The thick layer of Sphagnum limits the movement of water between the surface and the bottom of lakes or streams, which slows how nutrients move through the ecosystem. Scientists can study living things in water to check how healthy aquatic ecosystems are.

Soil that becomes more acidic can harm farming. Some animals can survive in environments with very low pH levels. For example, frogs and certain fish can live in water with a pH of 4. This helps them stay alive even when acid rain affects their habitat. However, many other water animals, like clams and snails, cannot survive in very acidic conditions. This harms their growth and survival. High acidity weakens the hard shells of these animals, making them easier for predators to catch.

Minimizing acidification

Agricultural runoff is a big cause of nitrogen and phosphorus, which make freshwater more acidic. Using good farming methods, like using less chemical fertilizer, managing manure better, and using technology to apply fertilizers more carefully, can help reduce these nutrients from entering water. Planting areas of plants near water, called riparian buffer zones, can also help by trapping pollutants before they reach freshwater. These steps help reduce acidification, stop water from becoming too rich in nutrients, and improve water quality.

Wetlands and peatlands help protect freshwater by soaking up pollutants and controlling water flow. Restoring wetlands has been shown to make freshwater systems stronger against acidification and other problems.

Adding lime, which is made of calcium carbonate (CaCO₃), is a common way to fix acidification. This process raises the pH level of water, helping the environment return to a state similar to before acidification occurred.

Some methods help reduce acidification from mining, such as using natural processes to treat water and adding materials that make water less acidic. Another important step is making daily choices that protect the environment, like reducing waste, reusing materials, and recycling. These actions help use fewer resources and reduce water acidity.

Regulations

Regulating human-caused emissions, such as sulfur oxides and nitrogen oxides, can greatly reduce acid rain and acidic water. For example, the Canada-United States Air Quality Agreement has reduced acid rain and ozone levels by 78% in Canada and 92% in the United States as of 2020. Supporting scientists to monitor and collect data is important for creating models that help make effective policies. A plan can be put in place to reduce the problem. Governments can also provide funding to help companies lower pollution and encourage them to use new production methods that reduce greenhouse gases and acidic substances. Governments worldwide can work together to address acidification and find solutions through international agreements. Examples of successful efforts include the Acid Rain Program started in the United States in 1995 and the Gothenburg Protocol, created by the United Nations Economic Commission for Europe (UNECE), to reduce acidification.

Case Study: Freshwater Acidification in the Adirondack Lakes, New York

The Adirondack Lakes in New York are a well-known example of freshwater acidification. As early as the 1970s, these lakes showed signs of acidification because of low acid-neutralizing capacity, pollution from factories that released sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), and the formation of acid rain. Winds carried these pollutants from the Midwestern United States to the Adirondack region, lowering the pH level of lakes and soil. Acidification of the water caused many fish and crustacean species to disappear, reducing the variety of life in these ecosystems.

Efforts to improve the environment included reducing sulfur dioxide and nitrogen oxide emissions through the Clean Air Act of 1990. Monitoring shows better water quality, but ecosystems remain at risk because acid rain has long-term effects on soil and watersheds. This example shows how the Clean Air Act helped reduce human-caused acidification. However, recovery is still difficult because acid rain's effects last a long time.