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Effect of Small Amounts of Gypsum in Soils on the Solutes in Effluents¹

ABSTRACT

Columns of soil initially containing exchangeable Ca²⁺ and 0.0, 0.5, 1.0, and 2.0% CaSO₄·2H₂O by weight were leached with a solution containing 50 me. NaCl and 50 me. MgCl₂ per liter. The effluents from these columns were collected and analyzed for Ca²⁺, Mg²⁺ and Na⁺. The breakthrough curves for Na⁺ were not affected significantly by the presence of gypsum; however, the curves for Mg²⁺ were dependent on the amount of CaSO₄·2H₂O present in the system. When small amounts of gypsum were present, Mg²⁺ concentration in the effluent increased and then leveled off at some concentration below that of the solution entering the column. The length of the plateau and the concentration at which it occurs are functions of the amount of gypsum present in the system. When the gypsum was leached from the system, the Mg²⁺ concentration increased to a value approaching the concentration of the solution entering the soil.

A general method for calculating the ionic composition of the effluent from initial conditions was developed. It was found that theoretical values calculated by this computer method approximated those found experimentally.

¹ Contribution from the Department of Irrigation, University of California, Davis. This work was supported by funds from the California State Department of Water Resources, Water Resources Center of the University of California, and U. S. Public Health Service (Grant No. FR00009), and was presented before Div. S-1 and S-2, Soil Sci. Soc. Am. at Denver, Colo., Nov., 1963.

² Associate Professor, Department of Agricultural Chemistry and Soils, University of Arizona.

Received for publication May 1, 1964. Accepted for publication June 22, 1964.