Abstract:
The yield characteristics of shallow aquifers in hard consolidated rocks, which underlie a major part of India and form important aquifer systems, are severely limited by their considerable heterogeneity and extremely low capabilities for storing and transmitting economic quantities of groundwater. These constraints, coupled with the ever-increasing need for water to satisfy the requirements of a spiraling population, has resulted in the proliferation of groundwater abstraction structures without due regard to spacing norms and environmental considerations. The adverse environmental consequences accompanying this phenomenon, such as de-saturation of shallow aquifer zones and drying up of wells, are often further aggravated by the increasing groundwater extraction from a large number of medium and deep bore wells tapping fracture zones in hydraulic connection with the weathered zone. The properties, that limit the yield of aquifers, also impose severe constraints on the scope for augmentation of resources in depleted aquifers through artificial recharge techniques.
An overview of challenges of groundwater management in the hard rock terrain in the southern Indian State of Tamil Nadu is described in this paper. About 73 percent of the total geographical area of the State is underlain by hard rocks of the Archaean crystalline metamorphic complex. Available surface water resources have already been almost fully developed. Ground water is the major source of water supply for various sectors, accounting for about 50 percent of the net area irrigated. Development of groundwater has traditionally been through large diameter open wells, though a large number of bore wells have become operational in recent years on account of the open wells ceasing to be sustainable due to decline in groundwater levels. The number of open wells in the State has increased from 15.10 lakhs during 1980-81 to 16.6 lakhs during 1999-
2000, whereas the number of bore/tube wells has increased from 0.20 lakhs in 1970-71 to 1.63 lakhs during 1999-2000. The decline of groundwater levels due to the over-exploitation of groundwater is considered to have put about 16,000 wells out of use during the nineties. It has also been reported that the average area irrigated by bore/tube wells in the state has declined from 2.10 ha during 1980-81 to about 1.37 ha during 1999-2000 for the same reason. A critical analysis of the assessment of the dynamic groundwater resources of Tamil Nadu based on GEC 1997 methodology indicates that 17 out of 29 districts of the State, underlain almost entirely by hard consolidated rocks, accounted for about 66 percent of the total annual groundwater recharge and about 75 per cent of the gross groundwater draft as in January 1998. This imbalance between the recharge and draft is indicative of the over-exploitation of the limited groundwater resources in the hard rock terrain, which is further substantiated by the fact that more that 70 per cent of the over-exploited and critical blocks are located in these districts.
The large-scale heterogeneity in the properties of aquifer materials even within short distances, lack of availability of source water in sufficient quantities when and where it is required and the lack of proper understanding of hydraulic connection between the weathered zone and fractures, pose serious challenges in attempts to augment the groundwater resources of depleted aquifer zones in the hard rock terrain of the State. The increasing abstraction around recharge sites makes it difficult to quantify the benefits of such schemes realistically. In view of the limitations imposed by nature on the scope for recharge augmentation in hard rock terrain, a two-pronged strategy involving regulation of groundwater abstraction on a regional scale, coupled with cost-effective recharge augmentation at the local level, is likely to be the most effective management strategy for ensuring long-term sustainability of the limited groundwater resources of the State.
