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

vaterite, and huntite among others, were proposed to be the The pH, Caand HCO3relations were investigated in 28 well controlling phase. Enhanced solubility due to Mg substituwaters obtained from beneath irrigated fields in Arizona. The mean tl0n lnto sml calcltes has ° been P^posed— but without [Ca] [C03] ion activity product (IAP) was 11.3 x i0-» (pIAP = 7.95) consideration of ion pairs. Studies by Suarez (1976) mwith a standard error of 0.7 x 10" based on field measurements of dicate that a range of solubility products exists for the pH. As indicated by ionic strength the residence time of the waters in CaCO3 solid phase in any soil, and the values obtained the soil vary widely. The lack of a trend in pIAP vs. ionic strength independ on the experimental conditions. If equilibrium is dicates the kinetics of calcite equilibration (from supersaturation) is approached from undersaturation, the activity product dequite slow in soil-water systems. This conclusion was supported by an pencjs on now mucn material was dissolved, since there is a average pIAP of 7.97 calculated from 35 water analyses obtained from variation in the stability of the solid, probably due to slow wells in the Grand Valley of Colorado and by a pIAP value of 7.96 obrecrystalization of the fresh precipitate. As the most unstatained from a soil water sample in a lysimeter study. Since these pIAP ,. J . . ,. . ,, ,£ ,. , . . , . . , .. values are quite consistent and were obtained undlr a wide variety of We material d.ssolves first (the fresh preapltate) its SO ubllconditions, a pIAP value of 7.95 is recommended for use in predictive >* cannot ^ y obtalned lf * constitutes a small permodels when CaCO., precipitation occurs, centage of the dissolved soil carbonate material. Experiments from supersaturation yield very soluble apparAdditionalIndex Words: CaCO3 solubility, soil waters, precipitation ent stability constants, especially if stirring is minimized. of CaCO3 The present investigation was undertaken to directly de________________ termine the [Ca] [CO3] ion activity product of waters under irrigated lands which contain calcite, rather than assume I MOST INVESTIGATIONS of the solubility of soil calcium equilibrium with the soil CaCO3. The Wellton-Mohawk Ircarbonate, calcareous soils were equilibrated with disrigation District of Arizona was chosen because of the extilled water at various soil-water ratios. The determined ion _____ activity products varied from values similar to that for cal'Contribution from the USDA-ARS, Riverside, Calif. Received 25 cite (Cole, 1957) (without consideration of ion pairs) to valMarch 1976. Approved 13 Dec. 1976. ues 10 times greater. As a result, solid phases including cal92£2 '' Salinity '' '°' Box ' ''' CA cite, aragonite, dolomite, calcium carbonate monohydrate, "Unpublished data, this laboratory. SUAREZ: ION ACTIVITY PRODUCTS OF caco3 IN WATERS BELOW ROOT ZONE 311 tensive network of shallow drainage wells available for sampling. These wells are pumped continuously at rates of 1,000 to 4,000 liters/min, thus CO2 degassing would not be expected, and was demonstrated as not significant. Determining the activity product of CaCO3 in soil drainage waters obtained by pumping from the saturated zone prevents degassing and yields a direct determination of the desired predictive value (in contrast to laboratory studies which give variable results depending on experimental con-