Evaluation of the effects of major components of groundwater systems on photocatalytic activity is essential for the development of commercial photocatalytic water purification systems. In this work, we have probed the effects of bicarbonate ion (HCO3-) on the photocatalytic activity of a TiO2 ceramic photocatalyst. Two brands of mineral water, Evian and Contrex, which contain high concentrations of alkaline earth metal ions Mg2+ and Ca2+, were used as models for groundwater. The degradation rate of formic acid was used as an index of photocatalytic activity. The rate of HCO3- degradation and changes in the type and concentration of the HCO3- counterion with the progress of photocatalytic reaction were also observed. After the completion of 15 cycles of 8 h of photocatalytic reaction (total run time 120 h), the surface of the photocatalyst and precipitates formed on it were analyzed by means of X-ray diffraction analysis, Raman spectrometry, scanning electron microscopy, and energy dispersive X-ray spectrometry. We found that the photocatalytic activity was reduced with increasing HCO3- concentration, irrespective of the type of counterion present. In addition, photocatalytic activity remained unchanged despite an increase in accumulation of the precipitate, which we identified as CaCO3, on the photocatalyst surface with the progress of experimental cycles. We also observed that the concentration of formic acid and HCO3- were decreased with photocatalysis when Ca2+ was a counterion, however, no effect in presence of other metal ions, Na+, K+ or Mg2+, was observed. These results show that bicarbonate ion was involved in the photocatalytic reaction when Ca2+ was the counterion, and as a result, the photocatalytic degradation rate of formic acid was decreased and CaCO3 precipitate was formed.