Two well-equilibrated Na-Cl geothermal liquids are recognizable in the San Marcos area. Both have the same CI concentration (540 mg/kg) and the same isotopic composition (delta D of -66.5 parts per thousand and delta(18)O of -9 parts per thousand) under reservoir conditions, but they come from two distinct aquifers with different temperatures, i.e. 240 degrees C below La Cimarrona and 185 degrees C below La Castalia. The numerous thermal Na-Cl to Na-Cl-HCO3 springs located in the San Marcos area originate through dilution and boiling of these two geothermal liquids and different degrees of re-equilibration at lower temperatures. Silica and K contents are useful in discriminating between dilution, boiling and re-equilibration phenomena. Thermal Na-HCO3 waters, generated through conductive heat transfer or input of geothermal vapor or gases from below, delineate the extent of the geothermal reservoir(s) at depth.