The pH of water measures the concentration of hydrogen ions (\(\text{H}^+\)), determining how acidic or alkaline a substance is on a scale of 0 to 14. A value of 7 is neutral, and numbers below 7 indicate acidity. Because the scale is logarithmic, a drop from pH 7 to pH 6 represents a tenfold increase in acidity. Low pH water is generally corrosive, meaning it can dissolve and leach metals like copper and lead from plumbing. This acidic condition also stresses aquatic life, as most organisms thrive within a narrow pH range, typically between 6.5 and 9.0.
Atmospheric and Carbonic Acid Effects
The primary reason water becomes slightly acidic is the continuous interaction between atmospheric carbon dioxide (\(\text{CO}_2\)) and water molecules. When \(\text{CO}_2\) dissolves into water, it reacts to form a weak acid known as carbonic acid (\(\text{H}_2\text{CO}_3\)). This process happens naturally in all environmental waters, including rivers, oceans, and raindrops.
The chemical reaction (\(\text{CO}_2 + \text{H}_2\text{O} \leftrightarrow \text{H}_2\text{CO}_3\)) releases hydrogen ions, which lowers the pH level. Due to this natural phenomenon, unbuffered rainwater typically measures a pH between 5.0 and 5.5. In large bodies of water, the absorption of increasing atmospheric \(\text{CO}_2\) causes ocean acidification, establishing a baseline acidity that other factors can intensify.
Natural Environmental and Geological Factors
A water source’s susceptibility to low pH is influenced by the surrounding geology and the presence of natural buffering agents. Buffering capacity is the water’s ability to neutralize acids using dissolved alkaline minerals, primarily calcium carbonate (limestone) and magnesium. Water flowing through bedrock rich in these compounds, such as limestone aquifers, is naturally protected and maintains a higher, more stable pH level.
Conversely, water in areas with inert geology, such as granite or quartz, has low buffering capacity. In these environments, small additions of natural acid can cause a substantial drop in pH. Another natural contributor is the decomposition of organic matter (leaves, wood, and peat) common in forested watersheds. This biological breakdown releases humic and fulvic acids, often called tannins, which are weak organic acids that naturally acidify the water, sometimes dropping the pH below 5.0.
Direct Chemical and Pollutant Inputs
Low pH levels result from external inputs of strong acids, often linked to human industrial and agricultural activities. Acid precipitation, commonly called acid rain, forms when atmospheric pollutants like sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)) react with water vapor. These gases, emitted from industrial combustion and vehicle exhaust, dissolve to create sulfuric and nitric acids. This precipitation can dramatically lower the pH of lakes and streams, especially in areas with poor geological buffering.
Acid mine drainage (AMD) is a major source of strong acidity. It occurs when sulfide minerals like pyrite are exposed to oxygen and water during mining operations. This reaction generates highly acidic runoff containing sulfuric acid, which is released into surrounding waterways.
AMD can lower a water body’s pH to extremely acidic levels, sometimes below 4.0, simultaneously mobilizing toxic heavy metals like cadmium and lead. Direct chemical additions, such as runoff from nitrogen-based fertilizers or improper pool balancing acids, also introduce concentrated acid locally.
Practical Steps for Adjusting Acidity
For household water concerns, the first step is accurate measurement using a specialized pH meter or reliable testing kit. This provides a precise baseline reading and confirms the extent of the acidity. A professional water test can also identify any toxic metals that may have leached into the water due to corrosion.
If the water is acidic, remediation involves neutralizing it by introducing an alkaline substance. For whole-house systems, this is often achieved using two primary methods:
Neutralizing Filters
A neutralizing filter uses a contactor tank filled with a medium like crushed limestone (calcite). The acidic water dissolves the calcite as it passes through, raising the pH by adding calcium carbonate.
Chemical Feed Pumps
Alternatively, a chemical feed pump can inject a solution of soda ash (sodium carbonate) or sodium hydroxide directly into the water line. This method achieves a more controlled and consistent pH adjustment.