Freshwater acidification happens when acids enter a body of fresh water. This can occur through the weathering of rocks, the entry of acid gases like carbon dioxide, or the reduction of certain anions, such as sulfate and nitrate, in lakes, ponds, or reservoirs. The main causes of freshwater acidification are sulfur oxides (SOx) and nitrogen oxides (NOx), which enter water from atmospheric deposits and soil leaching. Carbonic acid and dissolved carbon dioxide can also enter fresh water through runoff from soils rich in carbon dioxide. Runoff containing these compounds may include hydrogen ions and inorganic aluminum, which can harm marine organisms. Acid rain also contributes to freshwater acidification. A well-known example of this occurred in the Adirondack Lakes in New York during the 1970s. This was caused by acid rain from industrial emissions of sulfur dioxide (SO2) and nitrogen oxide (NOx).

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. 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 such as humic and fulvic acids. These acids build up in water bodies, especially in areas with forests and wetlands. Peatlands and wetlands often produce acidic water because of the high amount of organic matter decomposition. 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 reacts with oxygen and water to form sulfuric acid. This sulfuric acid splits into sulfate ions (SO₄²⁻) and hydrogen ions (H⁺), increasing acidity.

SO₂ + ½O₂ + 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 + ½O₂ → HNO₃; HNO₃ → H⁺ + NO₃⁻

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

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

Buffering Capacity

Ecosystems have the ability to resist changes in pH because of their buffering capacity. In freshwater systems, bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions help neutralize incoming hydrogen ions (H⁺).

The reaction is:
HCO₃⁻ + H⁺ → CO₂ + H₂O

However, areas with low alkalinity, such as those with silicate bedrock, lack the natural materials needed to neutralize hydrogen ions. This causes pH levels to drop quickly. For example, the Atlantic region of Canada has the lowest acid deposition rates in Eastern North America, but its waters are the most acidic on the continent. This is because the region’s bedrock, such as granite and shale, contains very little buffering material. Soils formed from these materials and the water that flows through them are easily affected by acidification, even when acid deposition is low.

Effects on ecosystems

Acidification of freshwater ecosystems can reduce the variety of native species and change how ecosystems function. Small animals without backbones and large animals often experience higher death rates and lower reproduction when water becomes more acidic. In contrast, some algae and mosses, such as Sphagnum, grow better in acidic conditions. These plants may quickly take over habitats, outcompeting other species. Sphagnum is especially common in acidic environments. This moss can exchange hydrogen ions for basic minerals in water. The thick layer of Sphagnum limits the movement of water between the surface and sediment, which reduces the flow of nutrients in the ecosystem. Scientists use aquatic biomonitoring to assess the health of water ecosystems.

Soil that becomes acidic can harm agriculture. Some species can survive in environments with very low pH levels. For example, frogs and fish like perch can live in water with a pH as low as 4, which means they are not harmed by acid deposition. This allows them to survive in these conditions. However, many aquatic species, such as clams and snails, cannot survive in low pH environments. This harms their growth and survival. High acidity weakens the thick shells of these animals, making them more vulnerable to predators.

Minimizing acidification

Agricultural runoff is a major source of nitrogen and phosphorus, which can cause freshwater acidification. Using best management practices (BMPs) in farming, such as reducing chemical fertilizer use, improving manure handling, and using precision agriculture methods, can greatly lower the amount of nutrients that enter water systems. Creating riparian buffer zones—areas of plants along waterways—can also help by trapping pollutants from farmland before they reach freshwater systems. These actions help reduce acidification, prevent excessive nutrient growth (eutrophication), and improve water quality.

Wetlands and peatlands protect freshwater systems by absorbing pollutants and controlling water flow. Restoring wetlands has been shown to make freshwater systems more resistant to acidification and other environmental challenges.

Liming is a common method to fix acidification. This process involves adding calcium carbonate (CaCO₃) to a system to raise pH levels. Increasing pH helps restore habitats to conditions similar to those before acidification occurred.

To reduce acidification linked to mining, methods like passive treatment using natural biological processes and treating drainage with alkaline materials are used. Another important factor in reducing freshwater acidification is everyday choices that protect the environment. Using a circular approach—reducing waste, reusing materials, and recycling—can lower resource use and minimize waste, including reducing water acidity.

Regulations

Controlling human-caused emissions of 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. Scientists play an important role in collecting data to create models that help design effective policies. A plan can be created to address the problem. Governments may also provide financial support to companies to reduce pollution and encourage them to use new production methods that lower greenhouse gas emissions and acidic substances. International cooperation is important to solve acidification. Examples of successful efforts include the Acid Rain Program in the United States, started in 1995, and the Gothenburg Protocol, created by the United Nations Economic Commission for Europe 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. Beginning in the 1970s, these lakes showed signs of acidification because of low levels of acid neutralizing capacity (ANC) and industrial emissions of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), which caused acid rain. Winds carried these pollutants from the Midwestern United States to the Adirondack region, lowering the pH levels of water bodies and surrounding soils. The acidification of the water led to a major decrease in the variety of aquatic life, including the loss of fish and crustacean species.

To improve the environmental conditions of the Adirondack lakes, efforts were made to reduce SO₂ and NOₓ emissions through the Clean Air Act of 1990. Monitoring data shows better water quality, but many ecosystems remain at risk because acid rain has long-lasting effects on soils and watersheds. This case shows how the Clean Air Act has helped reduce human-caused causes of freshwater acidification. However, research indicates that recovery of ecosystems is still difficult due to the long-term damage caused by acid deposition.