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Effect of Soil Processes on the Acidification of Water by Acid Deposition
Author(s) -
Reuss J. O.,
Johnson D. W.
Publication year - 1985
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq1985.00472425001400010005x
Subject(s) - alkalinity , chemistry , soil water , soil acidification , soil ph , environmental chemistry , cation exchange capacity , saturation (graph theory) , inorganic chemistry , alkali soil , soil science , geology , mathematics , organic chemistry , combinatorics
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 (15% Ca 2+ saturation), the soil solution pH is only slightly affected by CO 2 partial pressures over the range likely to be found in soils (1–5% CO 2 ), but the alkalinity (defined as alkalinity = 2[CO 3 2− ] + [HCO 3 − ] + [OH − ] − [H + ] − [Al 3+ ] − [Al(OH) 2+ ] − [Al(OH) 2 2+ ] of the soil solution increases rapidly with increasing CO 2 partial pressure (the brackets denote molar concentrations). In contrast, solutions that are not in contact with the soil's cation exchange complex maintain alkalinity independently of CO 2 partial pressure. If alkalinity is positive, the pH in such solutions rapidly increases in response to decreasing CO 2 pressure. Waters having positive alkalinity will undergo a rapid rise in pH when released from the soil due to CO 2 degassing, while waters with negative alkalinity (net acidity) remain acid when degassed. The effect on the soil of precipitation containing H 2 SO 4 is to increase the SO 4 2− concentration. In acid soils, ion exchange reactions that take place in response to increasing SO 4 2− from 25 to 250 µ mol (e − ) L −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 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 4 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 4 2− concentration.