Groundwater represents a significant global resource, supplying drinking water to billions of people and supporting agricultural and industrial activities. The chemical quality of this subsurface water is crucial for its safe and effective use, and one of the most important chemical properties is its acidity. The acidity, or pH, of groundwater dictates its ability to dissolve rock, its potential to corrode infrastructure, and its safety for human consumption. Understanding why this water source can become acidic involves examining a complex interplay of natural geological processes and human-driven environmental changes.
Understanding the pH Scale and Groundwater
The pH scale is a logarithmic measure used to specify the acidity or basicity of an aqueous solution, ranging from 0 to 14. A pH of 7 is neutral, while values below 7 are acidic and values above 7 are basic, or alkaline. Because the scale is logarithmic, a single unit change represents a tenfold change in the concentration of hydrogen ions (H+) that determine acidity.
For most natural groundwater systems, the pH falls within a range of 6.0 to 8.5. Water is considered acidic or corrosive when the pH drops below 6.5. Low pH levels influence the water’s chemical reactivity, making it chemically aggressive. This increases its capacity to dissolve minerals from the surrounding rock and sediment, as well as from plumbing materials.
The Role of Natural Carbon Dioxide
The most common natural cause of groundwater acidity is the formation of a weak acid called carbonic acid. This process begins with carbon dioxide (CO2), which is readily available in the atmosphere and in higher concentrations within the soil. As rainwater infiltrates the ground, it absorbs atmospheric CO2, forming a dilute carbonic acid solution.
The acidity increases when the water passes through the soil layer above the water table. Here, the CO2 concentration is elevated due to the biological activity of plant roots and the respiration of soil microbes breaking down organic matter. This intensifies the formation of carbonic acid (H2CO3).
As this weakly acidic water percolates deeper into the aquifer, the carbonic acid dissociates, releasing hydrogen ions (H+) that lower the pH. These hydrogen ions drive chemical weathering, reacting with and dissolving solid minerals in the surrounding rock matrix, such as calcium carbonate found in limestone. This mechanism makes most groundwater naturally slightly acidic before it encounters alkaline buffering materials.
Acidity from Mineral Reactions and Human Activity
While the carbonic acid pathway is the most widespread, other natural geological formations can generate stronger acidity. In areas rich in sulfide minerals, such as pyrite (FeS2), exposure to oxygen and water triggers sulfide oxidation. This process generates sulfuric acid (H2SO4), a strong mineral acid capable of dropping the pH to extremely low levels, often below 4.0. This phenomenon, often associated with mining activities, is also a natural weathering process.
Natural organic acids contribute to acidity, particularly in shallow aquifers overlain by peatlands or thick organic soils. The decay of plant and animal matter produces humic and fulvic acids, which leach into the water. These weak organic acids can lower the groundwater pH, especially where the geological material lacks the neutralizing capacity of carbonate minerals.
Human activities exacerbate groundwater acidity through the introduction of strong acid precursors. Acid rain, formed when sulfur and nitrogen oxides from industrial emissions and vehicle exhaust, infiltrates the ground, adding sulfuric and nitric acids to the subsurface environment. Agricultural runoff containing excessive fertilizers and manure can also introduce chemical compounds that upset the natural chemical balance.
How Acidic Water Affects Infrastructure and Health
The corrosive nature of low-pH groundwater has two consequences: damage to infrastructure and risks to human health. Acidic water is aggressive toward metal plumbing systems, accelerating corrosion in pipes, fixtures, and well components. This chemical dissolution can lead to premature failure of metal pipes, causing pinhole leaks and requiring replacement.
The primary health risk associated with acidic water is the mobilization of toxic heavy metals. As the low-pH water dissolves metal pipes, it leaches elements like copper, lead, zinc, and cadmium from plumbing materials into the drinking water supply. In older homes, acidity can dissolve lead from solder or service lines, resulting in dangerous contamination. The concentration of these metals can lead to health issues, including neurological damage from lead exposure and gastrointestinal problems from high levels of copper.