The pH scale measures how acidic or basic water is, ranging from 0 to 14. A pH value of 7 indicates neutrality, while values below 7 signify acidity and values above 7 denote alkalinity. The pH of water measures the relative amount of free hydrogen ions present; more hydrogen ions lead to increased acidity. Each whole pH value represents a tenfold change in acidity or basicity, meaning a pH of 5 is ten times more acidic than a pH of 6. Maintaining a stable water pH is important for aquatic life and ecosystems, as deviations from typical ranges can be harmful.
Absorption of Atmospheric Carbon Dioxide
Atmospheric carbon dioxide (CO2) naturally dissolves into water bodies, a process that becomes more pronounced with increased atmospheric concentrations. When CO2 dissolves in water, it reacts with water molecules to form carbonic acid (H2CO3). The amount of dissolved CO2 directly influences the water’s pH; higher CO2 levels lead to a lower pH.
Carbonic acid partially dissociates, releasing hydrogen ions (H+) and bicarbonate ions (HCO3-) into the water. The increase in hydrogen ions directly lowers the water’s pH.
Ocean acidification is the most prominent example of atmospheric CO2 absorption affecting water pH. Since the industrial era, oceans have absorbed a substantial portion of human-released CO2, leading to a measurable drop in ocean pH. The ocean’s average pH has decreased by about 0.1 pH units, representing approximately a 30 percent increase in acidity due to the logarithmic nature of the pH scale. This rapid change is faster than any known shift in ocean chemistry over the last 50 million years.
Increased acidity impacts marine life, particularly organisms that construct shells and skeletons from calcium carbonate. Calcifying organisms, such as corals, oysters, mussels, and certain plankton, find it more difficult to extract minerals from seawater to build and maintain their protective structures. In severe cases, existing shells can begin to dissolve, threatening these species and potentially disrupting marine food webs.
Acid Rain and Airborne Pollutants
Acid rain, also known as acid deposition, refers to any form of precipitation that exhibits high acidity due to atmospheric pollution. While natural processes like volcanic eruptions can contribute, human activities are the predominant source. The primary acidic components are sulfuric acid (H2SO4) and nitric acid (HNO3).
The formation of these acids begins with the emission of precursor gases. Sulfur dioxide (SO2) largely originates from burning fossil fuels in power plants and industrial processes, and from smelting mineral ores. Nitrogen oxides (NOx) primarily come from fossil fuel combustion in vehicles, power plants, and industrial boilers. Natural sources for NOx include lightning strikes.
Once released, these gases react in the atmosphere with water, oxygen, and other chemicals to form sulfuric and nitric acids. For instance, sulfur dioxide can oxidize to sulfuric acid, while nitrogen dioxide reacts with hydroxyl radicals to form nitric acid. These acids then dissolve into atmospheric moisture and fall to the ground as acid rain.
When acid rain reaches terrestrial and aquatic environments, it lowers the pH of lakes, rivers, and soils. This increased acidity can harm aquatic ecosystems, altering plant and animal assemblages. Some lakes affected by acid rain can become very clear with reduced fish populations, as acidic conditions make the water unsuitable for their survival.
Land-Based Sources and Chemical Discharges
Beyond atmospheric influences, various land-based activities and natural processes can cause water to become acidic. One localized source is Acid Mine Drainage (AMD), which occurs when sulfide minerals, such as pyrite, found in excavated rock, are exposed to air and water during mining operations. This exposure initiates a chemical reaction that produces sulfuric acid, leading to highly acidic water that can contain dissolved heavy metals. The resulting acidic fluid can be toxic and impact aquatic life in mining regions.
Direct discharge of acidic wastewater from industrial processes also contributes to water acidification. Industries like chemical manufacturing, metal processing, and pulp and paper mills can release low pH effluents directly into rivers, lakes, or coastal waters. These industrial discharges can alter the pH of receiving water bodies, affecting local ecosystems.
Natural processes, such as the decomposition of organic matter, can lead to localized water acidification. In environments rich in decaying plant and animal material, like wetlands, bogs, and peaty soils, the breakdown process can release organic acids, including humic and fulvic acids. Additionally, microorganism respiration during decomposition produces carbon dioxide, which can dissolve in local soil water or small ponds to form carbonic acid, further contributing to acidity.
Agricultural runoff can also play a role in localized acidification of surface waters. Certain agricultural practices, particularly the use of specific fertilizers, can contribute to acidity. The process of nitrification in the soil, which converts ammonium to nitrate, can release hydrogen ions, increasing soil acidity that may subsequently leach into nearby water bodies. This effect is typically more localized and less widespread than other causes of acidification.