Reevaluation of Calcite Supersaturation in Soils
- D. L. Suarez ,
- J. D. Wood and
- I. Ibrahim
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.
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