Direct P_co2  measurements on water samples from several CO₂-charged warm springs are significantly higher than P_co2 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. We attribute these discrepancies to uncertainties in field pH, resulting from CO₂ outgassing during pH measurement. Because samples for direct P_co2  measurement can be taken with minimal disturbance to the water chemistry, we have used the measured P_co2  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, we find generally consistent grouping of δ¹³C, pH, and degree of calcite saturation in samples taken from the same source at different times, an observation which we expect based on our understanding of the hydrogeology and geochemistry of the ground-water systems. This suggests that for very careful geochemical work, particularly on ground-waters much above ambient temperature, P_co2 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.