Hydrogeochemistry of the Purana formations of eastern Madhya Pradesh in India

The Raipur and Indravati series comprising the Purana formations of Chhattisgarh consist of conglomerates, orthoquartzites, sub-arkoses, shales, limestones and dolomites. The lithofacies in these have considerable lateral variations. Both Mahanadi and Indravati basins are saucer-shaped with centripetal dips; the arenaceous rocks along the fringes have dips up to 15°, while the younger rocks in the centre are almost horizontal. In carbonate rocks, karst is developed on a regional scale with sinkholes of higher topographic areas linked with resurgences of low relief areas. Groundwater in the above formations occurs generally under water table condition, but in karstic limestone also under confined condition. The chemical relationship of groundwater circulating in the various lithofacies are plotted in the trilinear diagrams after Piper. Groundwater within depths of 92m has carbonate hardness (Secondary alkalinity) exceeding 50 per cent with the chemical properties dominated by alkaline earths and weak acids. The cations and anions in water from similar lithofacies of Mahanadi and Indravati basins are distributed in the same field of the trilinear diagram with almost similar scatter of ions suggesting thereby a similar geochemical environment. The quality of groundwater is within permissible limits for irrigation and domestic purposes, while softening of water from temporary bicarbonate hardness may be necessary for certain specific industries. Expansion of groundwater recharge by increasing the ‘area of spreading’ of flood water of the Mahanadi basin and application of the technique in the Indravati basin will enrich the soils. Prevention of disposal of industrial wastes underground, especially in karstic regions will be a primary step in pollution control.