Effect of soil processes on the acidification of water by acid deposition
The mechanism whereby acid deposition can cause acidification of surface waters via equilibrium processes in soil solution was investigated using chemical equilibrium models. These models show that for soils with low to moderately low exchangeable bases, the soil solution pH is only slightly affected by CO/sub 2/ partial pressures over the range likely to be found in soils (1-5% CO/sub 2/), but the alkalinity 2(CO/sub 3//sup 2 -/) + (HCO/sub 3//sup -/) + (OH/sup -/) - (H/sup +/) - (Al/sup 3 +/) - (Al(OH)/sup 2/) - (Al(OH)/sub 2//sup 2 +/) of the soil solution increases rapidly with increasing CO/sub 2/ partial pressure. In contrast, solutions that are not in contact with the soil's cation exchange complex maintain alkalinity independently of CO/sub 2/ partial pressure. If alkalinity is positive, the pH in such solutions rapidly increases in response to decreasing CO/sub 2/ pressure. The effect on the soil of precipitation containing H/sub 2/SO/sub 4/ is to increase the SO/sub 4//sup 2 -/ concentration. In acid soils, ion exchange reactions that take place in response to increasing SO/sub 4//sup 2 -/ from 25 to 250 ..mu..mol (e/sup -/) L/sup -1/ can be expected to depress soil solution pH by 0.2 to 0.4 units. This depression is sufficient to cause a switch from positive to negative alkalinity in many soil solutions and when waters with negative alkalinity are released from the soil they remain acid when degassed. This mechanism could easily account for a change in the pH of surface waters from 6.25 to 5.0 or less, while the associated change in soil solution would be < 0.3 units. This mechanism does not depend on soil acidification in the sense of a reduction in base saturation. It is completely reversible and responses to changes in SO/sub 4//sup 2 -/ concentration is instantaneous, so that lags in response to changes in sulfate input levels would be controlled only by processes such as soil sulfate adsorption and biological cycling that tend to buffer changes in SO/sub 4//sup 2 -/ concentration.
- Research Organization:
- Colorado State Univ., Fort Collins
- DOE Contract Number:
- AC05-84OR21400
- OSTI ID:
- 5830835
- Journal Information:
- J. Environ. Qual.; (United States), Journal Name: J. Environ. Qual.; (United States) Vol. 14:1; ISSN JEVQA
- Country of Publication:
- United States
- Language:
- English
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54 ENVIRONMENTAL SCIENCES
ACID NEUTRALIZING CAPACITY
ACID RAIN
ACIDIFICATION
ALUMINIUM
ATMOSPHERIC PRECIPITATIONS
CARBON COMPOUNDS
CARBON DIOXIDE
CARBON OXIDES
CATIONS
CHALCOGENIDES
CHARGED PARTICLES
CHEMISTRY
ELEMENTS
ENVIRONMENTAL EFFECTS
EQUILIBRIUM
HYDROGEN COMPOUNDS
INORGANIC ACIDS
ION EXCHANGE
IONS
METALS
OXIDES
OXYGEN COMPOUNDS
PARTIAL PRESSURE
PH VALUE
RAIN
SALTS
SOILS
SULFATES
SULFUR COMPOUNDS
SULFURIC ACID
SURFACE WATERS
WATER CHEMISTRY