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An area of 2000 km(2) in the arid western cordillera of Bolivia was geologically and hydro-geologically surveyed for the purpose of determining locations for borehole drilling in the framework of groundwater reconnaissance. Vertical geoelectrical resistivity soundings were applied to identify areas at depth in which aquifers with a sufficient thickness of fresh groundwater can be expected. A chemical and isotopic inventory of the regionally occurring groundwater revealed the presence of meteorically-recharged fresh and thermal waters as well as highly mineralized waters from fumaroles and a deep reservoir. Due to their chemical and isotopic composition, the latter group shows influences of juvenile water. Carbon fourteen dating performed on the fresh and thermal waters indicates that they were recharged during the last pluvial phase, about 10,500 years ago. The occurrence of these fossil waters explains the discharge of up to 200 l/s from some springs in a now-arid climate with mean precipitation of 100 mm/y and essentially no groundwater recharge. Extremely low contents of tritium of about 0.1 TU confirm the C-14 age determinations.