Groundwater recharge studies

Direct recharge of precipitation to the groundwater was studied during 1982-1984 and 1990-1992 in Jodhpur and Barmer districts of western Rajasthan using artificial 3H tracer as tritiated water (HTO) in the unsaturated zone (Shivanna et al. 1994). The results showed negligible groundwater recharge due to low rainfall (c. 200-300 mm) during the study periods. Since the artificial 3H tracer method determines recharge for the study period only, other methods such as environmental H, 36Cl and chloride methods, need to be employed to determine long-term recharge (Gaye & Edmunds 1996; Cook et al. 1994). In the Barmer district of Rajasthan, mean annual recharge of c. 14 mm was obtained using environmental chloride and natural tritium methods (Navada et al. 2001).

In arid areas, indirect recharge through wadis (river channels) could be an important mechanism for groundwater recharge (Darling et al. 1987). Such an observation was made in the Jalore area of western Rajasthan (Navada et al. 1993) using isotope studies of groundwater, which showed that shallow groundwater near Sukri river is recharged from river channels during flash floods. Buried river courses (palaeochannels) are important as they have good groundwater potential. In Jaisalmer district of Rajasthan, a Holocene age freshwater channel superimposed on older groundwaters was identified. Direct head water connection to the groundwater in the study area from present-day Himalayan sources appeared to be remote (Nair et al. 1999).

An environmental isotope study in the Bhadka-Bheemda area in Barmer district, Rajasthan (Navada et al. 1996), showed that the deep ground-water (depth >100 m) has depleted 82H and 818O compared to the shallow groundwater. The deep groundwater has negligible 3H and the 14C model (Pearson) ages range from 4000 to 9500 years BP, hence it is palaeowater recharged in the past. Palaeontological studies have proposed the following dry and wet periods in Rajasthan (Das 1968):

10 000-6000 years BP moderately humid, rainfall greater than present

5000-3000 years BP humid period 3000-1100 years BP low rainfall 1100 BP-Present dry conditions.

Hence the observed 14C ages of groundwater in the Bhadka-Bheemda area represent recharge during humid periods.

Over-exploitation of groundwater resources in arid regions with limited water resources reveals the adverse effects of rapid lowering of the water table and deterioration of water quality. For example, in Bikaner town in Rajasthan the aquifers are being used extensively, resulting in declining groundwater levels. An environmental isotope (18O, 2H and 3H) study (Navada 1988) showed mixing of shallow and deep zone waters due to heavy exploitation of groundwater in the area. The effect of over-exploitation of groundwater was also studied in the Bhadka-Bheemda area (Nair et al. 2007) in Barmer district, and the limestone belt of the Jodhpur-Nagaur district (Nair et al. 1993) using isotope techniques.

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