Isotope studies to understand the dynamic changes due to long term exploitation of groundwater systems in Western Rajasthan, India

Environmental isotopes (2H, 18O, 3H and 14C), hydrochemistry and numerical modelling techniques were used to understand the hydrodynamics, recharge conditions and change in flow pattern due to long term exploitation in the arid regions of northern Barmer and Jalore basins of western Rajasthan, India. It is reported that in these areas groundwater levels are declining at a rate of ∼1m/a because of large-scale abstraction for municipal supply as well as irrigational requirements. In the Barmer basin, the effect of long term exploitation on the deep freshwater lens was studied. Analyses of isotope and hydrochemical data reveal that long term exploitation of deep groundwater has resulted in the influx of more and more brackish and isotopically enriched water having 14C content :∼27 pMC, which are probably recharged during an arid phase in the past. Different groundwater recharge episodes could also be delineated from the study. Interaction of an ephemeral river with an unconfined alluvial aquifer in the Jalore area was investigated using environmental isotopes and numerical modelling. A three-dimensional transient groundwater flow model was used to interpret the measured isotope and chemical data collected during 1983 and 2000. Analysis indicate that (a) the shallow aquifer receives recharge through river channels during episodic floods caused by intense rain events (b) increased flood events in the recent years have resulted in lowering the brackishness in some places (c) lowered piezometic levels in the deep aquifers induce leakage of relatively enriched younger water from the intermediate zone. The study also helped to identify the affected areas and delineated potential zones for the safe groundwater abstraction. (author)