Carbon dioxide (\(\text{CO}_2\)) is a ubiquitous component of all natural waters, including oceans, rivers, and groundwater. This compound is highly soluble, meaning it readily dissolves into liquid water. The presence of dissolved \(\text{CO}_2\) is a fundamental aspect of aquatic chemistry that governs many natural processes. Understanding how this gas interacts with water dictates the water’s chemical characteristics and has widespread implications for life on Earth.
The Chemical Transformation of Dissolved Carbon Dioxide
When carbon dioxide gas enters water, it undergoes a chemical transformation. The initial step is the physical dissolution of \(\text{CO}_2\) molecules into the liquid. A portion of this dissolved \(\text{CO}_2\) then reacts with water molecules (\(\text{H}_2\text{O}\)) to form a weak acid called carbonic acid (\(\text{H}_2\text{CO}_3\)).
This reaction is reversible, and the various forms of carbon exist in chemical equilibrium. Carbonic acid quickly dissociates into hydrogen ions (\(\text{H}^+\)) and bicarbonate ions (\(\text{HCO}_3^-\)). The bicarbonate ions can further dissociate into more hydrogen ions and carbonate ions (\(\text{CO}_3^{2-}\)).
These four components—dissolved \(\text{CO}_2\), carbonic acid, bicarbonate, and carbonate—all exist simultaneously in a dynamic balance. The relative concentration of each form depends heavily on the water’s \(\text{pH}\). In typical ocean water, which is slightly alkaline, the bicarbonate ion is the most prevalent form, holding the majority of the dissolved carbon.
Sources of Carbon Dioxide in Natural Water Systems
Carbon dioxide enters water systems through several distinct pathways. A major source is the direct exchange with the atmosphere at the water’s surface. Since \(\text{CO}_2\) is present in the air, constant diffusion occurs until the gas concentration in the water is in balance with the air above it.
Biological processes within the water are a significant contributor. Aquatic organisms, including fish and zooplankton, release \(\text{CO}_2\) as a waste product of cellular respiration. Similarly, the decomposition of organic matter, such as dead plants and animals, releases \(\text{CO}_2\) into the surrounding water.
Geological sources also play a role, especially in groundwater and deep ocean environments. Water flowing through underground rock formations, particularly those containing organic material or carbonate minerals, can absorb significant amounts of \(\text{CO}_2\). This results in groundwater naturally rich in the dissolved gas, which can then surface in lakes and rivers.
The Role of Dissolved \(\text{CO}_2\) in Water pH and Aquatic Ecology
The formation of carbonic acid introduces hydrogen ions into the water, which lowers the \(\text{pH}\), a process known as acidification. This change has consequences for aquatic life, particularly in marine environments, where it is known as ocean acidification. Since the Industrial Revolution, the ocean’s \(\text{pH}\) has dropped from approximately 8.2 to 8.1, representing an increase in acidity of about 30 percent due to the absorption of atmospheric \(\text{CO}_2\).
This increasing acidity impacts calcifying organisms, which rely on carbonate ions to build and maintain their shells and skeletons. Corals, oysters, mussels, and certain plankton struggle to construct their calcium carbonate structures because the available carbonate ions are bound up by the excess hydrogen ions. At low \(\text{pH}\) levels, the water can become corrosive enough to dissolve existing shells.
While freshwater systems are generally more buffered against wide \(\text{pH}\) shifts than the open ocean, high \(\text{CO}_2\) concentrations still pose a threat. Elevated levels reduce the respiratory efficiency of fish, making it harder for them to take up oxygen at their gills. For sensitive freshwater species, such as trout, concentrations exceeding \(45 \text{ mg/L}\) can be lethal.
Carbon Dioxide and Human Drinking Water Quality
Dissolved carbon dioxide is generally not a health concern in drinking water. Naturally occurring \(\text{CO}_2\) is non-toxic and has no direct physiological effect on humans. Commercially carbonated beverages are intentionally pressurized to dissolve much higher concentrations of the gas, creating the characteristic fizz.
High concentrations of dissolved \(\text{CO}_2\) affect water quality in two ways. The resulting carbonic acid can give the water a slightly acidic or “flat” taste. More significantly, the acidic nature of the water makes it corrosive to plumbing infrastructure, including copper and galvanized pipes.
Municipal water treatment facilities often reduce \(\text{CO}_2\) levels to prevent this corrosion, protecting the public water distribution system and household plumbing. High \(\text{CO}_2\) is prevalent in groundwater drawn from deep wells, sometimes leading to a cloudy appearance as the pressure drops when the water is brought to the surface. The dissolved gas is easily removed during treatment, often by aerating the water to allow the \(\text{CO}_2\) to escape back into the air.