Field vs. lab pH in remote wells, Konza Prairie LTER site, and implications for limestone weathering

摘要

Ground waters from shallow, horizontal, thin limestones in the central USA are Ca-HCO_3 type waters with moderately high alkalinity ( > 5 mmol) and low SO_4 ( < 0.5 mmol). Because of their remote location, wells are bailed to collect samples, making accurate pH measurements difficult. This 13-month study compares field-measured pH with lab-measured pH that was corrected for controlled degassing of CO_2. Lab pH was not always higher than field pH, and the difference between the two was greatest in the winter to spring months, when calculated P-CO_2 values were lowest, dissolved ions concentrations were lowest, and meteoric precipitation was lowest. Saturation index calculations show that the ground water from the well used for this study was oversaturated with respect to calcite during late winter to spring months, despite the lower dissolved ions concentrations, and close to saturation or undersaturated during the summer months. These findings suggest that equilibrium with calcite is accelerated during periods of high P-CO_2 and impeded during periods of low P-CO_2, and that most of the weathering of the limestone aquifers takes place during the summer months.