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

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 (CaCO,) 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., CO, 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. 'Contribution of the U.S. Salinity Laboratory, USDA, Riverside, CA 92501. Received 9 June 1981. 1 Geochemist. Additional Index Words: soil-water composition, calcium, bicarbonate, sulfate. Suarez, D. L. 1982. Graphical calculation of ion concentrations in calcium carbonate and/or gypsum soil solutions. J. Environ. Qual. 11: 302-308. Existing methods of satisfactorily predicting the chemical composition of waters undergoing precipitation or dissolution of calcite and/or gypsum require computer models or extensive numerical calculations (4, 13, 16). Although computer simulation models yield thermodynamically correct predictions, field personnel require both a satisfactory program and access to computing facilities each time a water is to be evaluated. Such programs are primarily research tools and are often not convenient for use by water quality specialists, engineers, or agricultural extension personnel. Yet there is an acute need for calculations of Ca concentrations in evaluating the soil chemistry of arid regions. Waters percolating through the soil may undergo large concentration changes through evapotranspiration and are in contact with a much higher CO2 partial pressure than is present at the soil surface. 302 J. Environ. Qual., Vol. 11, no. 2,1982