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Graphical Calculation of Ion Concentrations in Calcium Carbonate and/or Gypsum Soil Solutions¹

ABSTRACT

Water quality specialists and agricultural extension personnel, among others, need estimates of soil solution Ca concentrations when evaluating the soil chemistry of irrigated arid lands. Existing prediction methods require computer models or extensive numerical calculations, which are often not convenient to use by field personnel. A new graphical method for calculating the equilibrium chemical concentration of waters undergoing calcium carbonate (CaCO₃) and/or gypsum dissolution or precipitation is presented. The procedure requires the assumption that the water is in an open system with CO₂, i.e., CO₂ is not affected by carbonate dissolution/precipitation reactions but rather is controlled by respiration and diffusion. The procedure can be utilized to calculate steady-state ion concentrations of soil and drainage waters.

The method of calculation permits corrections to be made for ionic strength (activity coefficient). Simplified numerical methods for correcting for ion complexation are also presented. The graphical procedures allow quick prediction of equilibrium concentration of major species in solutions in the absence of ion exchange. An evaluation of the procedure, based on various western U.S. irrigation waters and different leaching fractions, shows that the graphical method is comparable to computer simulations.

Key Words: soil-water composition • calcium • bicarbonate • sulfate

¹ Contribution of the U.S. Salinity Laboratory, USDA, Riverside, CA 92501.

² Geochemist.

Received for publication June 9, 1981.