Effect of CO₂ on the chemical equilibrium of soil solution and ground water.

Abstract

The equilibrium equations relating dissolved CO2 , HC03- CO3-, H+, solid phase CaC03 , exchangeable H+ and ionic strength were programmed for simultaneous solution on a 7072 IBM digital computer. This routine was combined with an eXisting program which had been successfully used to relate the equilibrium of soluble and exchangeable Ca+ + ,Mg+ + ,Na+; dissolved Cl - , S04- ' and N03-; and solid phase CaS04 •2H20. The final systems analysis model thus developed accounted for most of the dissolved substances normally present in significant quantities in ground waters and soil solutions. This model made possible calculation of the equilibrium concentrations which would result if the concentration of one or more of these constituents were to be arbitrarily changed. This systems analysis model was used to predict the equilibrium concentrations of ionic species in soil solutions obtained from soils at moisture and carbon dioxide levels different from those prevailing in the original analysis. The soils used in this study were vastly different in chemical characteristics, texture, and genesis. Included were soils which were both acid and basic in reaction, calcareous and noncalcareous, gypsiferous and nongypsiferous, and both high and low in organic matter. In general, the chemical predictions obtained were of about the same level of accuracy as the experimental methods used to determine these chemical constituents. Soil cores from strata underlying an irrigated area were collected, and the 1: 1 soil-water extracts of these cores were analyzed for chemical constituents. The systems analysis model developed in this study was then used to estimate the concentrations of the chemical constituents which would have been in solution in the cores at the time they were sampled. The soil solutions calculated to be in most strata were similar to the underlying ground water, thus indicating a probable common origin for the water above and below the water table. It is believed that this systems analysis approach can, with minor modification, be used to predict changes in ground water quality as water percolates through strata of known chemical characteristics.