Bicarbonate’s Origins: How It Gets Into Water
Bicarbonate is created when carbon dioxide ($CO_2$) dissolves in water, forming carbonic acid ($H_2CO_3$), which then dissociates into hydrogen ions ($H^+$) and bicarbonate ions ($HCO_3^-$). In natural water systems, this process is influenced by several factors, including the atmosphere, surrounding soil, and bedrock. The dissolved carbon dioxide and water react with carbonate-rich minerals in the earth's crust, such as limestone, which enriches the water with bicarbonates. This geological weathering is a primary reason why certain regions have water sources naturally high in this compound.
Natural Sources of Bicarbonate
Mineral Water and Artesian Springs
Mineral waters, especially those sourced from areas rich in limestone or dolomite, are well-known for their high bicarbonate content. As water filters through these rock layers, it absorbs minerals, including calcium and magnesium bicarbonates. The label on a bottle of mineral water will list its mineral composition, and anything with over 600 mg of bicarbonate per liter is considered rich in this compound. Naturally effervescent mineral waters, like those from springs in Germany and Bulgaria, are a prime example of this geological process at work.
Examples of high-bicarbonate natural mineral waters:
- Gerolsteiner Sparkling ($>1,800$ mg/L)
- Vichy Catalan ($>2,000$ mg/L)
- Puits St Georges ($>1,400$ mg/L)
Groundwater and Geological Influence
For many, the most common source of bicarbonate water is what comes directly from the tap. The alkalinity of tap and groundwater is a direct result of the local geology. In areas where aquifers are located within limestone or chalk formations, the water naturally dissolves these minerals, leading to elevated levels of bicarbonate. Conversely, regions with granite or volcanic rock formations typically have water with lower alkalinity.
Seawater and the Carbon Cycle
Bicarbonate is the most dominant form of dissolved inorganic carbon in seawater and plays a crucial role in the ocean's carbon cycle. It serves as a major buffer that prevents drastic shifts in ocean pH due to the absorption of atmospheric carbon dioxide. The concentration of bicarbonate in the ocean is a key factor in understanding the impact of ocean acidification on marine life.
Commercial and Treated Water Sources
Artificially Carbonated Water (Soda Water)
While some sparkling waters are naturally carbonated, others have carbon dioxide and, in many cases, sodium bicarbonate added artificially. This process creates soda water, which contains dissolved carbon dioxide and sodium bicarbonate. The addition of bicarbonate helps to regulate the water's pH and can give it a slightly salty flavor. It’s important to distinguish between naturally sparkling mineral water and commercially produced soda water, as their mineral and bicarbonate origins are different.
Alkaline Bottled Water
Some bottled water brands market their products as "alkaline water." While some of these are naturally alkaline due to mineral content, others achieve a higher pH through the addition of alkaline compounds like sodium bicarbonate or through an ionization process. For those concerned about added salts, it is wise to check the label for the specific mineral content and processing method.
The Role in Municipal Water Treatment
Water treatment plants sometimes add sodium bicarbonate to municipal water supplies to correct the pH and control corrosion. This is particularly important for water with low natural alkalinity, which can be corrosive to pipes. The added bicarbonate raises the water's alkalinity, making it less acidic and protecting infrastructure from damage.
Comparison of Bicarbonate Sources
| Water Type | Bicarbonate Source | Typical Concentration | Key Characteristics |
|---|---|---|---|
| Natural Mineral Water | Dissolution of limestone/dolomite layers during natural filtration. | Varies widely; some exceed 1,800 mg/L. | Contains natural mineral salts; taste and composition depend on geological origin. |
| Tap Water/Groundwater | Weathering of local carbonate-rich geology, atmospheric $CO_2$. | Highly variable, depends on location; affects water 'hardness'. | Contains minerals but concentration is highly location-dependent. |
| Artificially Carbonated Water | Added sodium bicarbonate and pressurized carbon dioxide. | Variable; depends on the manufacturer's recipe. | Can have a slightly salty or soapy taste; not derived from natural springs. |
| Alkaline Bottled Water | Added sodium bicarbonate, ionization, or natural mineral content. | Variable; engineered to achieve a specific pH. | Lacks the diverse minerals of natural water unless sourced naturally. |
| Seawater | Dissolved inorganic carbon from atmosphere and rock weathering. | Very high concentrations; average of ~140 mg/L chloride. | Important for marine ecosystems; not for human consumption. |
The Chemical Difference: Bicarbonate vs. Carbonate
Bicarbonate ($HCO_3^-$) and carbonate ($CO_3^{2-}$) are closely related but distinct chemical species. The pH of the water determines the ratio of these two forms. In the typical pH range of most natural drinking water (6.3 to 10.3), bicarbonate is the dominant form. Above a pH of 10.3, carbonate becomes more prevalent. Bicarbonate is highly soluble in water, whereas many carbonates are insoluble. This chemical distinction is important for processes like brewing and understanding water hardness.
How Bicarbonate Content is Measured
Bicarbonate concentration is typically measured as part of a total alkalinity test. The process usually involves a titration method using an acid and pH indicators. For field tests or commercial applications, specialized equipment like pressure gauges and sealing vessels can also be used. Results are often reported as parts per million (ppm) or milligrams per liter (mg/L). Accurate measurement is essential for municipal water treatment and scientific studies, but for the average consumer, the mineral label on bottled water is the simplest indicator. The USGS provides detailed information on groundwater chemistry, including bicarbonate levels, for various regions across the United States.
Conclusion: Understanding Your Water's Bicarbonate
Bicarbonate is a common component of many water sources, from natural mineral springs shaped by geological history to treated tap water and commercially prepared beverages. Its presence is the foundation of water's alkalinity and buffering capacity, influencing everything from taste to health effects. By understanding the diverse sources of bicarbonate—both natural and artificial—consumers can make informed choices about the water they drink. Whether naturally present due to the water's geological path or added during processing, bicarbonate plays an important role in the chemistry of water.
