It is believed that early Mars possessed liquid water that is the most essential ingredients for life. However, to create a habitable environment for microorganisms, not only the presence of liquid water alone but also the source of energy is required. Here we introduce our ongoing research revealing potential habitability on early Mars, based on the thermodynamic calculation of Gibbs free energy distribution in mixing where groundwater and lake water may mix. Dissolved ion concentrations available from both groundwater and lake water can be obtained based on the thermodynamic calculations simulating water-rock reactions. Our preliminary Gibbs free energy calculations targeting Fe-oxidizing microorganisms suggest that in the mixing zone between Fe2+-containing anoxic groundwater and lake water, the energy released from Fe-oxidation could be higher than those calculated from terrestrial groundwater discharged area where Fe2+-oxidizing microorganisms are actually living. We propose that further Gibbs free energy calculation targeting various chemolithoautotrophic reactions better constrain the habitability of early Mars.