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Reevaluation of Calcite Supersaturation in Soils

USDA-ARS, U.S. Salinity Lab., 4500 Glenwood Dr., Riverside, CA 92501

I. Ibrahim

Soil Salinity and Alkalinity Lab., Bacos, Alexandria, Egypt

*Corresponding author.

ABSTRACT

The calcite saturation status of soil-water systems in semiarid regions was reexamined taking into consideration organic ligand alkalinity and Ca-organic complexation. These factors have been forwarded as the reason for earlier reports of calcite supersaturation in soil-water systems. We examined groundwaters beneath irrigated lands, soil-water suspensions in the laboratory using four soils and two levels of CO₂, and soil water extracted from the root zone of an irrigated, cropped field. Total alkalinity and net inorganic C alkalinity were determined by titration. Calcium activity values calculated from total concentrations using the speciation program WATEQ4F were compared with Ca²⁺ activity values measured with an ion-specific electrode. In all our systems, total alkalinity and inorganic C alkalinity were almost equivalent and thus organic C alkalinity was negligible. Calculated Ca²⁺ activity was generally 5 to 25% greater than Ca²⁺ activity determined with an ion-specific electrode. Although these data suggest the presence of additional Ca complexes not included in the speciation routines, the determined dissolved organic C values are too low for Ca-organic complexes to be the cause of these differences in Ca²⁺ activity. Ion activity products based on either measured Ca²⁺ activity values or calculated Ca²⁺ activities indicated that all systems were supersaturated, with saturation ratios of two to three.

Contribution from the U.S. Salinity Lab., Riverside, CA.

Received for publication July 18, 1991.