Use of a chemical equilibrium model to understand soil chemical processes that influence soil solution and surface water alkalinity

A chemical equilibrium model was applied to soil chemistry data (Spodosols) collected from 30 and 21 forested watersheds in New York and Maine, respectively, during the EPA Pilot Soil Survey. Chemistry data were evaluated between states using lumped series and within New York using three series (Adams, Becket, and Canaan). All New York horizons had soil characteristics that tend to cause lower solution alkalinity in comparison to Maine horizons. Negative alkalinities were produced in all E horizons (? 69 to ? 37 μmol LU^?1) at each of the pCO₂ levels used (0.3 to 2%). All B horizons had negative alkalinities at low PCO₂ levels, which became positive at higher levels, except for the Canaan B and New York Bh horizons, which were negative at all pCO₂ levels. C horizons generated positive alkalinities (1 to 67 μmol L^?1) at most pCO₂ levels. Results indicate the importance of water contact with different horizons and soil series in determining solution alkalinity. Because of degassing effects, solutions with a positive alkalinity will increase in pH after leaving the soil, whereas solutions with a negative alkalinity will remain at low pH (pH < 5.5) and cause the surface water to be acidic. Application of the model to soil chemistry data collected in the northeastern U.S. illustrates the importance of various factors such as pCO₂, Al solubility, base saturation, and exchange coefficients in determining surface water chemistry.