4.2 Carbonate Equilibria and Groundwater pH

In groundwater systems, pH is initially established as meteoric water comes into equilibrium with atmospheric carbon dioxide (Langmuir, 1997). This results in the formation of diprotic carbonic acid (a diprotic acid yields two H+ ions per acid molecule), which subsequently undergoes dissociation to form first bicarbonate and then carbonate. As the first deprotonation product of carbonic acid, the amphoteric behavior of bicarbonate (i.e., its ability to react as either a monoprotic acid or base) is particularly important for groundwater pH. The interactions between dissolved inorganic species are intricate; however, the impact of carbonate equilibria on pH can be explored by considering the ion charge balance that would be expected in pristine meteoric water as shown in Equation 24.

\displaystyle [\textup{H}^{+}]=[\textup{HCO}_{3}^{-}]+2[\textup{CO}_{3}^{2-}]+[\textup{OH}^{-}] (24)

Over the pH range of most natural waters, the concentrations of carbonate and hydroxyl ions are much smaller than the concentration of bicarbonate. This simplifies the charge balance to Equation 25.

\displaystyle [\textup{H}^{+}]=[\textup{HCO}_{3}^{-}] (25)

Combining the charge balance with the mass action equilibrium constants for the formation of carbonic acid (KH = 10-1.47) and dissociation into bicarbonate (K1 = 10-6.35) gives the proton concentration as a function of carbon dioxide partial pressure (pCO2 = 10-3.5) as shown in Equation 26.

\displaystyle [\textup{H}^{+}]=\sqrt{K_{1}K_{H}pCO_{2}} (26)

This relationship yields a pH of 5.7 (corresponding to −log[H+]; Equation 2). The equation also shows that an increase in the partial pressure of carbon dioxide will tend to decrease pH, whereas a decrease in carbon dioxide partial pressure promotes an increase in pH.

In groundwater systems, carbon dioxide is produced from the degradation of organic matter by heterotrophic microorganisms, particularly as meteoric water infiltrates through soil and the vadose zone. As a consequence, subsurface carbon dioxide partial pressures are often higher than in the open atmosphere. This serves to increase the formation of carbonic acid, which is the main source of protons for mineral weathering and dissolution reactions in groundwater systems (Kump et al., 2000; Wilson, 2004).

While heterotrophic microbial activity tends to increase carbon dioxide partial pressure, autotrophs have the opposite effect, as they rely on the uptake and reduction of carbon dioxide to produce cellular organic material. If the metabolic demand for carbon is sufficiently high, a drawdown in the partial pressure of carbon dioxide may occur. To compensate, pH increases as protons recombine with bicarbonate to compensate for decreases in carbonic acid concentrations. In the subsurface, the influence of autotrophs on carbonate equilibria and pH is usually not as pronounced as that of heterotrophs, particularly in systems buffered by higher concentrations of dissolved inorganic carbon.


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