Water acidity is measured by the concentration of free hydrogen ions (\(\text{H}^+\)) using the pH scale, which runs from 0 to 14. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are basic or alkaline. Because the scale is logarithmic, a decrease of one pH unit represents a tenfold increase in acidity, meaning a small change signifies a major shift in water chemistry. The causes of this shift are varied, stemming from natural processes that slightly reduce pH to human activities that introduce strong acid compounds into the aquatic environment.
The Baseline: Natural Carbon Dioxide Interaction
The most fundamental cause of slightly acidic water is the natural interaction between the atmosphere and water bodies. Pure water has a neutral pH of 7.0, but in nature, water readily absorbs atmospheric carbon dioxide (\(\text{CO}_2\)).
When carbon dioxide dissolves, it reacts with water molecules (\(\text{H}_2\text{O}\)) to form a weak acid known as carbonic acid (\(\text{H}_2\text{CO}_3\)). This chemical process is represented by the equilibrium reaction: \(\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3\). Carbonic acid then partially dissociates, releasing hydrogen ions (\(\text{H}^+\)) into the water, which lowers the pH.
This natural process means that clean, unpolluted rainwater or surface water in equilibrium with atmospheric carbon dioxide is naturally slightly acidic, typically exhibiting a pH around 5.6 to 5.7. This natural acidity provides the baseline against which more severe, human-caused acidification is measured.
Anthropogenic Atmospheric Deposition
Beyond the natural carbon dioxide baseline, human activities introduce pollutants that significantly increase water acidity through atmospheric deposition, commonly known as acid rain. This process is distinct because it involves the formation of strong mineral acids, which are far more potent than carbonic acid. The primary precursors are sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)), released into the atmosphere primarily through the combustion of fossil fuels.
The largest sources of these acidic precursors are electric power generation, particularly from coal-burning plants, and emissions from vehicles and heavy industrial manufacturing. In the atmosphere, these gases undergo complex chemical reactions involving water and oxygen to transform into sulfuric acid (\(\text{H}_2\text{SO}_4\)) and nitric acid (\(\text{HNO}_3\)).
These strong acids then fall to the earth as wet deposition (rain, snow, fog) or dry deposition (acidic particles and gases), drastically lowering the \(\text{pH}\) of surface waters. While natural rain has a \(\text{pH}\) around 5.6, acid rain typically registers values in the range of \(\text{pH}\) 4.0 to 4.5, representing a substantial increase in acidity. The long-range transport of \(\text{SO}_2\) and \(\text{NO}_x\) means that water bodies can be acidified even if they are far removed from the pollution source.
Geologic and Terrestrial Sources
Water acidity can also originate from terrestrial and subsurface processes, independent of direct atmospheric deposition.
Acid Mine Drainage (AMD)
One major localized source is Acid Mine Drainage (AMD), which occurs when water interacts with sulfide minerals exposed during mining operations. Pyrite (\(\text{FeS}_2\)), an iron sulfide mineral, is the most common culprit. When pyrite is exposed to both oxygen and water, an oxidation reaction begins that generates large amounts of sulfuric acid and dissolved iron. This complex chemical cascade releases hydrogen ions (\(\text{H}^+\)), which can lower the \(\text{pH}\) of the water to extremely acidic levels, sometimes below 3.0. This highly acidic water then leaches other toxic heavy metals from the surrounding rock, which are carried into streams and rivers, creating long-lasting pollution.
Organic Decomposition
A different, entirely natural terrestrial source of acidity comes from the decomposition of organic matter, which releases organic acids. In environments like bogs, swamps, and forested wetlands, the slow microbial breakdown of plant material, such as leaves and peat, produces humic and fulvic acids. These substances are complex organic molecules that contain weakly acidic functional groups. These organic acids dissolve in the water, contributing to a naturally low \(\text{pH}\) in these localized water bodies, often giving the water a tea-stained, brownish color. This type of acidity is a characteristic feature of these ecosystems and is generally not considered a form of pollution.