U K Sinha S V Navada

Isotope Hydrology Section, Isotope Applications Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, India (e-mail: [email protected])

Abstract: Environmental isotope studies of groundwaters carried out in the arid region of Rajasthan, India, suggest that recharge to shallow groundwater occurs as a result of direct infiltration of precipitation and/or through river channels during episodic floods. In many parts of Rajasthan deep fresh groundwater is available, which was recharged in the past when the climatic conditions were more favorable than at present. However, negligible modern recharge and over-exploitation of groundwater resources have led to the deterioration of groundwater quality in the Bhadka-Bheemda area of Barmer district. Buried river courses (palaeochannels) could be potential sources of groundwater since radiocarbon measurements of groundwater along palaeochannels in Jaisalmer district revealed that it was old water recharged a few thousand years ago.

There is evidence that the climate has changed repeatedly during the Earth's history and, in the past, the present deserts were not always so arid, so devoid of life and vegetation (Singh et al. 1974). As groundwater is the only available source of water in most of the desert regions, exploitation of these resources could help to solve the water requirement of these regions. Sound management of these groundwater resources requires data on the source, dynamics and quality of groundwater, which could be obtained using isotope techniques, hydrochemical and hydrogeological studies. Isotope techniques have successfully been used by many investigators to study some of the water resource problems in arid regions (Fontes & Edmunds 1989). A basic problem in arid areas, which often cannot be solved easily with conventional hydrological techniques, is to determine whether a given body of groundwater is actively recharged, i.e. whether it is a renewable resource. Environmental tritium and radiocarbon (14C) can be used to identify recharge pertaining to modern-day precipitation or precipitation in the past (up to 45 000 years) (Sukhija & Rama 1973; Sukhija et al. 1996; Allison & Hughes 1974, 1978; Dincer et al. 1974). Several hydrological studies carried out in arid zones (mostly in the Sahara) using isotope techniques have shown that the deep groundwater (generally >200 m) is generally old, with radiocarbon ages >20 000 years BP (Sonntag et al. 1979; Mabrook & Abdel Shafi 1977; Abdelkader & Zuppi 1999). This was supported by variations of the stable isotopic composition (2H and 18O) in groundwater.

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