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Ion Activity Products of Calcium Carbonate in Waters Below the Root Zone¹

ABSTRACT

The pH, Ca²⁺, and HCO₃^- relations were investigated in 28 well waters obtained from beneath irrigated fields in Arizona. The mean [Ca] [CO₃] ion activity product (IAP) was 11.3 x 10^–9 (pIAP = 7.95) with a standard error of 0.7 x 10^–9 based on field measurements of pH. As indicated by ionic strength the residence time of the waters in the soil vary widely. The lack of a trend in pIAP vs. ionic strength indicates the kinetics of calcite equilibration (from supersaturation) is quite slow in soil-water systems. This conclusion was supported by an average pIAP of 7.97 calculated from 35 water analyses obtained from wells in the Grand Valley of Colorado and by a pIAP value of 7.96 obtained from a soil water sample in a lysimeter study. Since these pIAP values are quite consistent and were obtained under a wide variety of conditions, a pIAP value of 7.95 is recommended for use in predictive models when CaCO₃ precipitation occurs.

¹ Contribution from the USDA-ARS, Riverside, Calif.

² Geochemist, U.S. Salinity Laboratory, P.O. Box 672, Riverside, CA 92502.

Received for publication March 25, 1976. Accepted for publication December 13, 1976.

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