Soil CO2

Processes such as biodegradation of organic matter and respiration of plants and organisms which commonly occur in the subsurface consume O2 and produce CO2. In the soil subsurface, air in the pore spaces cannot readily equilibrate with the atmosphere, and therefore pore space air becomes lower in O2 and higher in CO2 concentrations.

• Oxygen may decrease from about 21% (210,000 ppmv) in the atmosphere to between 15% and 0% (150,000 to 0 ppmv) in the soil.

• Carbon dioxide may increase from about 0.04% (~360 ppmv) in the atmosphere to between 0.1% and 10% (1000 to 100,000 ppmv) in the soil.

When water moves through the subsurface, it equilibrates with soil gases and may become more acidic because of a higher concentration of dissolved CO2. Acidic groundwater has an increased capacity for dissolving minerals. The higher the CO2 concentration in soil air, the lower is the pH of groundwater. Acidic groundwater may become buffered, minimizing pH changes, by dissolution of soil minerals, particularly calcium carbonate. Limestone (calcium carbonate, CaCO3) is particularly susceptible to dissolution by low pH waters. Limestone caves are formed when low pH groundwaters move through limestone deposits and dissolve the limestone minerals.

Rules of Thumb

1. Unpolluted rainwater is acidic, about pH = 5.7, because of dissolved CO2 from the atmosphere.

2. Acid rain has lower pH values, reaching pH = 2.0 or lower, because of dissolved sulfuric, nitric, and hydrochloric acids which result mainly from industrial air emissions.

3. The dissolved carbonate species, CO2(aq) (equivalent to H2CO3), HCO3-, and CO32-, are present in any natural water system near the surface of the earth. The relative proportions depend on pH.

4. At pH values between 7.0 and 10.0, bicarbonate is the dominant dissolved inorganic carbon species in water. Between pH 7.8 and 9.2, bicarbonate is close to 100%; carbonate and dissolved CO2 concentrations are essentially zero.

5. In subsurface soil pore space, oxygen is depleted and carbon dioxide increased, compared to the atmosphere. Oxygen typically decreases from 21% in atmospheric air to 15% or less in soil pore space air, and carbon dioxide typically increases from ~360 ppmv in atmospheric air to between 1000 and 100,000 ppmv in soil pore space air. Thus, unpolluted groundwaters tend to be more acidic than unpolluted surface waters because of higher dissolved concentrations of CO2.

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