[en] Direct measurements of partial pressure of CO₂ (P) on water samples from several CO₂-charged warm springs are significantly higher than P values calculated from field pH and alkalinity (and other constituents). In addition, calcite saturation indices calculated from field pH and solution composition indicated supersaturation in samples which, on the basis of hydrogeologic concepts, should be near saturation or undersaturated. These discrepancies are attributed to uncertainties in field pH, resulting from CO₂ outgassing during pH measurement. Because samples for direct P measurement can be taken with minimal disturbance to the water chemistry, the measured P has been used to back calculate an estimate of the field pH and the carbon isotopic composition of the water before outgassing. By reconstructing water chemistry in this way, generally consistent grouping of delta¹³C, pH, and degree of calcite saturation is found in samples taken from the same source at different . times. This suggests that for very careful geochemical work, particularly on ground-waters much above ambient temperature, P measurements may provide more information on the system and a better estimate of its state of saturation with respect to carbonate minerals than can field measurements of pH. (author)