On the basis of isotopic and chemical analyses of 45 spring, well and meteoric water samples from the El Tatio geothermal field in Northern Chile, four main processes giving rise to the formation of a wide range of thermal discharges can be distinguished. (1) Deep dilution of a predominant, primary high chloride (5500 mg/l, 260°) supply water derived from precipitation some 15 km east of El Tatio with local groundwater produces a secondary chloride water. (4750 mg/l, 190°) feeding springs over a limited area. (2) Single step steam separation from these two waters leads to isotopic shifts and increases in chloride contents to 8000 and 6000 mg/l respectively. (3) Absorption of this separated steam and carbon dioxide into local ground water and mixing with chloride waters at shallow levels produces a series of intermediate temperature (160°), low chloride, high bicarbonate waters. (4) Absorption of steam containing H 2S into surface waters leads to the formation of zero chloride, high sulfate waters; the isotopic enrichment observed is governed by a kinetic, steady state evaporation process.