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Origin of a fresh groundwater body in Cholistan, Thar Desert, Pakistan

Abstract The Thar Desert of Pakistan stretches along the border to India and is one of the most densely populated deserts in the world. Brackish to saline groundwater prevails. A locally restricted fresh groundwater resource was discovered by a comprehensive hydrogeological, geophysical, and isotope hydrological survey conducted from 1986 to 1991. The origin, recharge mechanism and age of the fresh groundwater resource were assessed. There is only fossil groundwater and this must be mined. Sodium is the predominant cation. Present groundwater recharge is absent or extremely low as the annual precipitation rate and the potential evapotranspiration rate amount to less than 200 mm/a and about 2700 mm/a, respectively. The investigations comprised a hydrogeological well inventory, electrical resistivity transects on the ground and an air-borne electromagnetic survey, followed by a test-hole drilling programme combined with geophysical borehole logging, aquifer testing, and groundwater sampling for both chemical and environmental isotope analyses. The results of this study delivered a hydrogeological concept on the origin and recharge of the fresh groundwater body. We found that the fresh groundwater was indirectly recharged during flash floods in low lands during the last pluvial period rather than directly replenished in the high mountain areas far in the east.