Freshwater acidification

Freshwater acidification occurs when acidic inputs enter a body of fresh water through the weathering of rocks, invasion of acidifying gas (e.g. carbon dioxide), or by the reduction of acid anions, like sulfate and nitrate within a lake, pond, or reservoir. Freshwater acidification is primarily caused by sulfur oxides (SO_x) and nitrogen oxides (NO_x) entering the water from atmospheric depositions and soil leaching. Carbonic acid and dissolved carbon dioxide can also enter freshwaters, in a similar manner associated with runoff, through carbon dioxide-rich soils. Runoff that contains these compounds may incorporate acidifying hydrogen ions and inorganic aluminum, which can be toxic to marine organisms. Acid rain also contributes to freshwater acidification. A well-documented case of freshwater acidification in the Adirondack Lakes, New York, emerged in the 1970s, driven by acid rain from industrial sulfur dioxide (SO₂) and nitrogen oxide (NO_x) emissions.

CO₂ from the atmosphere or the decomposition of organic matter affects freshwater acidity. The CO₂ dissolved in water to form carbonic acid. This carbonic acid dissociated into hydrogen ions (H⁺) and bicarbonate (HCO₃^-), which increases the H+ ions and leads to decrease in pH level.

CO₂ + H₂O → H₂CO₃; H₂CO₃ ⇌ H⁺ + HCO₃^-

Microbial activity breaks down of organic matter releases organic acids such as humic and fulvic acids. These acids accumulate in water bodies, especially those surrounded by forests and wetlands. Peatlands and wetlands often produce acidic waters because of the high levels 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 air, sulfur dioxide converts to sulfuric acid: This sulfuric acid dissociates into sulfate ions (SO₄^2-) and hydrogen ions (H⁺), increasing the acidic condition.

SO₂ + 1⁄2 O₂ + H₂O → H₂SO₄; H₂SO₄ → 2H⁺ + SO₄^2-

Human activities can significantly accelerate freshwater acidification. In addition to carbon dioxide, the combustion of fossil fuels sulfur dioxide (SO₂) and nitrogen oxides (NO_x). These gases react with water and air to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). Similar to sulfuric acid, nitric acid also decrease the pH level by dissociates into hydrogen ions (H⁺) and nitrate ions (NO₃^-).

NO_x + H₂O + 1⁄2 O₂ → HNO₃; HNO₃ → H⁺ + NO₃^-