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Effect of Soil Processes on the Acidification of Water by Acid Deposition¹

ABSTRACT

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% Ca²⁺ saturation), the soil solution pH is only slightly affected by CO₂ partial pressures over the range likely to be found in soils (1–5% CO₂), but the alkalinity (defined as alkalinity = 2[CO₃^2–] + [HCO₃^–] + [OH^–] – [H⁺] – [Al³⁺] – [Al(OH)²⁺] – [Al(OH)₂²⁺] of the soil solution increases rapidly with increasing CO² 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 CO₂ partial pressure. If alkalinity is positive, the pH in such solutions rapidly increases in response to decreasing CO₂ pressure. Waters having positive alkalinity will undergo a rapid rise in pH when released from the soil due to CO₂ degassing, while waters with negative alkalinity (net acidity) remain acid when degassed.

The effect on the soil of precipitation containing H₂SO₄ is to increase the SO₄^2– concentration. In acid soils, ion exchange reactions that take place in response to increasing SO₄^2– from 25 to 250 µmol (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 SO₄^2– 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 SO₄^2– concentration.

Key Words: salt effect • mobile anion • alkalinity • CO₂ pressure • cation exchange • aluminum • sulfate

¹ Research sponsored in part by the USEPA/NCSU Acid Precipitation Program (a cooperative agreement) between the USEPA and North Carolina State Univ. (NCSU) and a subsequent agreement (AP-0307-1983) between NCSU and Colorado State Univ., and in part by the USEPA and the Office of Health and Environ. Res., U.S. Dep. of Energy, under Contract no. DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. Publication no. 2417. Environ. Sci. Div., Oak Ridge Natl. Lab., Oak Ridge, TN 37831.

² Professor of agronomy, Colorado State Univ., Fort Collins, CO 80523; and research staff member, Environ. Sci. Div., Oak Ridge Natl. Lab., P.O. Box X, Oak Ridge, TN 37831, respectively.

Received for publication November 16, 1983.